Reference Point

Recognition of the threat of Ehrlichia ruminantium infection in domestic and wild ruminants in the continental United States

Thomas R. Kasari, dvm, mvsc, dacvim, dacvpm; Ryan S. Miller, msc; Angela M. James, phd; Jerome E. Freier, dvm, phd

isease attributable to Ehrlichia ruminantium (for- Abbreviation Dmerly Cowdria ruminantium) infection was first described in South Africa in 1838 as a nervous condition FAD Foreign animal disease of sheep that developed after a substantial infestation of ticks.1 It is an infectious but noncontagious tick-borne trophils and endothelial cells and replicates by binary disease (ie, heartwater) that affects domestic and wild fission and, less frequently, by budding. The number of ruminants. Infection often causes the death of clinically replicated organisms inside these cells can range from 1 affected ruminants. In 2009, many sub-Saharan African to several thousand.4,6 countries and the island of Guadeloupe in the Carib- Different strains of E ruminantium may not induce bean reported clinical cases of heartwater in their do- homologous or heterologous cross-protection, with this mestic ruminant populations.2 Additionally, livestock lack of immune induction potentially attributable to have died of heartwater on the islands of Marie-Galante antigenic diversity.4 Antigenic diversity also impacts the and Antigua in the Caribbean.3 degree of pathogenicity of various strains of E ruminan- On the basis of the proximity of the United States tium in Africa and the Caribbean islands. Whereas some mainland to the Caribbean islands, the ultimate concern strains are highly virulent, others appear to be non- for animal health officials, members of the livestock in- pathogenic.7,8 Remarkably, a less pathogenic strain of dustry, and other stakeholders may not be what should E ruminantium, which is referred to as Panola Mountain be done to keep heartwater from being introduced into Ehrlichia, may exist in the central, southeastern, and the continental United States, but rather what can be eastern United States; preliminary characterization of done to limit the extent of an outbreak when this in- this pathogen by investigators indicates that this strain cursion transpires. A key element to controlling the is genetically and antigenically more closely related to extent of a disease outbreak is early recognition of the E ruminantium than to any other Ehrlichia spp.9,10 In a disease in an animal population. Therefore, the pur- goat with experimentally induced disease,9 the Panola pose of the information reported here is to describe the Mountain Ehrlichia bacteria caused a transient illness salient features of heartwater and to promote increased with a mild febrile condition that was followed by a disease awareness among veterinarians, livestock own- chronic latent infection. The lack of reports of heartwa- ers, wildlife biologists, and other stakeholders. It is ter or heartwaterlike disease in domestic or wild rumi- our hope that this awareness will translate into a more nants from areas in which Panola Mountain Ehrlichia rapid detection of disease and the ability to limit the organisms have been detected provides further indirect extent of an outbreak should it occur in the continental support that this bacterium has low pathogenicity in United States. infected animals.10 Ehrlichia ruminantium does not survive for long Pathogen Characteristics periods outside of a host. Blood exposed to sunlight will lose infectivity in ≤ 5 minutes.11 In a dead animal, Ehrlichia ruminantium is an aerobic, gram-negative, 12 nonmotile, coccoid- to ellipsoidal-shaped organism in the organism will typically begin to die in ≤ 6 hours. 4 However, infectivity may be prolonged for as long as 72 the order Rickettsiales and family Anaplasmataceae. 13 This organism is typically transmitted to a susceptible hours under cold conditions (4°C [39.2°F]). Ehrlichia 5 ruminantium is also susceptible to treatment with anti- host through infective blood. Once inside the host, this 5 obligate intracellular agent typically resides inside in- microbials, particularly tetracycline derivatives. tracytoplasmic inclusions (diameter, ≤ 4.0 µm) in neu- Epidemiology From the National Surveillance Unit (Kasari) and Center for Ani- Tick vector—Amblyomma spp ticks must be pres- mal Health Information and Analysis (Miller, James, Freier), ent in a geographic area to transmit E ruminantium to Centers for Epidemiology and Animal Health, Veterinary Ser- susceptible ruminant hosts during and after an out- vices, APHIS, USDA, 2150 Centre Ave, Building B, Fort Collins, CO 80526. break of heartwater. This tick species is categorized as Address correspondence to Dr. Kasari ([email protected]. a 3-host tick; therefore, each life stage of the tick (ie, gov). larva, nymph, and adult) feeds on a different host and

520 Vet Med Today: Reference Point JAVMA, Vol 237, No. 5, September 1, 2010 may require as few as 5 months or up to 4 years to com- water buffalo [Bubalus bubalis]) are also susceptible to plete the tick life cycle.14,15 Infection can be transmitted disease.26 Sheep and goats typically have more severe by each life stage of the tick.16 Infective organisms can clinical disease than do cattle. Bos indicus breeds of cattle endure in these various life stages for an extended pe- typically are more resistant to infection than are B taurus riod (eg, ≤ 15 months).15,17 breeds.27 Similarly, indigenous breeds of goats and sheep In Africa, there are 5 native species of Ambly- in heartwater-endemic areas appear to be more resistant omma spp ticks (Amblyomma variegatum, Amblyomma to disease, compared with resistance of other nonindig- hebraeum, Amblyomma lepidum, Amblyomma astrion, enous breeds of goats and sheep.27 Additionally, young and Amblyomma pomposum) considered natural vec- lambs and kids during the first week after birth and tors of heartwater. These 5 species have transmitted calves during the first month after birth typically have E ruminantium under field conditions to susceptible ani- resistance to overt clinical disease after infection.28–30 mal species, and transmission has caused clinical signs Although heartwater has been the suspected cause of disease.18,19 In experimental conditions, 5 other spe- of death in many species of ungulate ruminant wild- cies of Amblyomma spp ticks (Amblyomma cohaerens, life, only 12 species indigenous to Africa and 3 non– Amblyomma gemma, Amblyomma tholloni, Amblyomma African-origin species are proven to be susceptible to in- sparsum, and Amblyomma marmoreum) indigenous to fection by E ruminantium26,31–42 (Table 1). All 15 species Africa are capable of transmitting E ruminantium to ru- are present in the continental United States as captive (eg, minants.18 Three Amblyomma spp ticks (Amblyomma farmed) or free-ranging populations.33–35 Although these maculatum, Amblyomma cajennense, and Amblyomma species can function as a sentinel species for heartwater, americanum) indigenous to North America are sus- this disease is likely to be observed first in white-tailed ceptible to infection with E ruminantium,19,20 but only deer (Odocoileus virginianus) or axis deer (Axis axis). A maculatum appears to be efficient at transmitting White-tailed deer are an important sentinel species be- heartwater to susceptible ruminants.19,20 Amblyomma cause of their ubiquitous geographic distribution36 and americanum also serves as a natural vector for the E ru- potential interaction with heartwater-infected domestic minantium–like Panola Mountain Ehrlichia organisms9,10 ruminants. Although they lack the extensive range of initially identified in the southeastern United States.9 white-tailed deer, there are free-ranging populations of Amblyomma dissimile, which was introduced into Florida axis deer in Texas, California, and Washington35,38; axis and typically parasitizes reptiles and amphibians,21,22 has deer are a popular farmed species in many southern and also transmitted E ruminantium to goats in experimental midwestern states.33–35,37 Several other ungulate rumi- conditions.23 Despite being found occasionally on cattle nant wildlife species have been introduced into the con- in natural conditions, researchers23 believe it is unlikely tinental United States as captive and free-ranging popu- that A dissimile plays a substantial role in the transmis- lations33–35,37–42 and are suspected to be susceptible to sion of heartwater among ruminants. E ruminantium infection.31,32 However, studies are need- The tropical bont tick (A variegatum) and bont tick ed to confirm the susceptibility of these introduced spe- (A hebraeum) are considered to be the most important cies to E ruminantium infection. vectors of heartwater because of their adaptation to domestic ruminant livestock, ease with which they trans- Transmission—The typical life cycle of E ruminan- mit E ruminantium among ruminants, and geographic tium includes a phase in which the organism is transmit- distribution.24,25 However, given the widespread distri- ted between successive life stages of Amblyomma spp ticks bution of the tropical bont tick in the Caribbean islands, before transmission to a susceptible vertebrate host. Ad- this tick may be the arthropod vector that is eventually ditionally, the organism may be transmitted iatrogenically. responsible for the first outbreak of heartwater in the United States. Consequently, veterinarians, livestock Tick-to-tick transmission owners, and wildlife biologists should be familiar with Miniscule amounts of E ruminantium–infected the phenotypic characteristics of this tick and be on blood (0.001 to 0.02 mL) from a febrile animal are nec- alert for infestations on domestic and wild ruminants (Figure 1). Other tick species (Rhipicephalus evertsi, Boophilus decoloratus, Ornithodo- ros savignyi, Rhipicephalus appendicula- tus, Hyalomma truncatum, Amblyomma nuttalli, A americanum, Amblyomma neumanni, and Amblyomma imitator) are not capable of transmitting E ruminantium.16,19 Mechanical transmission of E ruminantium by arthropods has not been confirmed or verified.16 Ruminant host susceptibility—Do- mestic cattle (Bos taurus and Bos indicus), sheep (Ovis aries), and goats (Capra hircus) are target species for heartwater, but other less familiar domestic rumi- Figure 1—Male (left) and female (right) tropical bont ticks (Amblyomma variegatum). nants in the family Bovidae (eg, Asian (Photographs provided by USDA heartwater scientists.)

JAVMA, Vol 237, No. 5, September 1, 2010 Vet Med Today: Reference Point 521 Table 1—Summary of susceptibility to Ehrlichia ruminantium (heartwater) infection26,31,32 in captive and free-ranging populations of wild ungulate ruminants (Order, artiodactyla; suborder, suminantia) in the continental United States.32–42

Experimentally induced Natural infection† Population type Ruminant infection* African buffalo (Syncerus caffer) Yes — Captive Axis deer (Axis axis) — Yes Free-ranging, captive Barbary sheep (Ammotragus lervia) — Suspected Free-ranging, captive Bison (Bison spp) — Suspected Free-ranging, captive Black wildebeest (Connochaetes gnu) Yes — Captive Blackbuck antelope (Antilope cervicapra) — Suspected Free-ranging, captive Blesbok (Damaliscus dorcas phillipsi) Yes — Captive Blue wildebeest (Connochaetes taurinus) Yes — Captive Eland (Taurotragus oryx) Yes — Captive Fallow deer (Dama dama) — Suspected Free-ranging, captive Giraffe (Giraffa camelopardalis) Yes — Captive Greater kudu (Tragelaphus strepsiceros) Yes — Captive Himalayan tahr (Hemitragus jemlahicus) — Suspected Captive Lechwe (Kobus leche kafuensis) — Yes Captive Mouflon Ovis( aries) — Suspected Free-ranging, captive Nilagai antelope (Boselaphus tragocamelus) — Suspected Free-ranging, captive Rusa (Timor) deer (Cervus timorensis) — Yes Captive Sable antelope (Hippotragus niger) Yes — Captive Scimitar-horned oryx (Oryx dammah) Refractory — Captive Sitatunga (Tragelaphus spekii) — Yes Captive Springbok (Antidorcas marsupialis) — Yes Captive Steenbok (Raphicerus campestris) — Yes Captive White-tailed deer (Odocoileus virginianus) Yes — Free-ranging, captive *A yes response for experimental heartwater infection indicates infection as a result of ticks or inoculation of infected blood or infected cell culture material into these species; infection was confirmed by simultaneous control infections in small ruminants or evaluation of brain smears via light microscopy. A refractory response indicates that no clinical signs of disease were observed in this species after exposure to E ruminantium; however, no follow-up diagnostic tests were performed to detect rickettsemia to exclude the potential for subclinical infection. †A yes response for natural infection indicates that natural infection was confirmed via examination of brain or tunica intima smears and supportive evidence of disease on the basis of pathological or epidemiological (or both) findings. A suspected response indicates that the clinical signs of disease were compatible with heartwater in these species, but no diagnostic tests were performed to confirmE ruminantium infection. — = Not applicable. essary to initially infect an Amblyomma spp tick with migratory birds. Immature A variegatum, A maculatum, E ruminantium.16 Then, development of the bacteria be- and A cajennense are frequently found on migratory gins in the intestinal epithelial cells and continues in birds. Although birds may be resistant to clinical infec- the hemolymph and salivary glands of infected ticks. tions, infections among these tick species are possibly Infections render the tick as an infected carrier for the caused by cofeeding on infected birds. These 3 tick spe- entirety of its life stage.43 Whether during molting of cies exist within the Caribbean islands (A variegatum the tick from a larva to a nymph or from a nymph to and A cajennense) and Venezuela (A maculatum and an adult, the organism is typically transferred between A cajennense). Thus, the potential for interaction be- life stages (ie, transstadial transmission) because of the tween these tick species is plausible, particularly if the persistence of infection in the tissues and fluids of the geographic distribution of A variegatum continues to E ruminantium–infected ticks.44 Transovarial transmis- expand into South America via the migration of egrets sion, which is characterized by a disease agent being or other migratory bird species. transferred from an infected female to her eggs, may rarely occur, but this has been reported1,45,46 only in Tick-to-vertebrate host transmission bont ticks. Six to 12 weeks may pass before an infected tick is In field settings, the proportion ofAmblyomma spp able to transmit E ruminantium to a susceptible host.49,50 ticks that become infected with E ruminantium varies. However, only a portion of infected ticks will transmit This variability is a consequence of the carrier state of E ruminantium during feeding.51 Furthermore, tick ruminant hosts, carrier state of wild hosts, inherent dif- transmission of E ruminantium to a susceptible host ferences between Amblyomma spp and E ruminantium during feeding is not instantaneous and requires 2 to 3 strains, and concurrent infection of a batch of newly days for nymphs and ≤ 4 days for adults.46,52 hatched tick larvae by an E ruminantium–infected rumi- In heartwater-endemic areas, the prevalence of sub- nant.16 For example, only 1% to 4% of adult A variega- clinical vertebrate carriers predominates over hosts with tum ticks on the Guadeloupe islands were infected with clinical disease, which may be attributable to the repro- E ruminantium.46 In another study,47 E ruminantium induction of E ruminantium in capillary endothelial cells fection rates were 0% to 91%. with intermittent release of organisms into the blood- Some evidence48 suggests that Ehrlichia spp can stream.53 Consequently, the amount of rickettsemia in also be transmitted among ticks that are cofeeding on carrier animals is generally less than that of clinically

522 Vet Med Today: Reference Point JAVMA, Vol 237, No. 5, September 1, 2010 affected animals. As a result, ticks have lower infection other islands as determined on the basis of serologic rates for E ruminantium54,55 when feeding on carrier ani- evidence of infection.2,3,63,64 mals than when an infection is acquired from a febrile Trade of live ruminants between heartwater- animal.52 endemic countries and the United States appears to have been curtailed.a Therefore, animals and birds that Vertebrate host-to-vertebrate host transmission are otherwise resistant to E ruminantium infection are Vertical transmission of E ruminantium from cows likely to increase in importance as transporters of in- to neonatal calves purportedly occurs under natural fected ticks into the United States from endemic coun- conditions. Transmission is thought to be caused by in- tries. Support for this belief is provided in a review65 in gestion of E ruminantium–infected leukocytes in colos- which the importation of exotic ticks into the United trum produced by the infected dam.56 States on reptiles (eg, alligators, iguanas, monitor lizards, snakes, tortoises, and turtles), wild animals (eg, Iatrogenic transmission Cape buffalo, elephants, rhinoceroses, and zebras) or The SC, IM, and IP inoculation of infective blood their skins, birds (eg, ostrich), and humans is sum- into susceptible animals can also initiate infection.57 marized. Six heartwater-susceptible Amblyomma spp Therefore, it is important to recognize management ticks (ie, A dissimile, A gemma, A hebraeum, A lepidum, procedures that involve the use of blood-contaminated A marmoreum, A sparsum, A tholloni, and A variegatum) hypodermic needles or other instruments that may be were described66 among these imported ticks, and some fomites for the transmission of this disease to suscep- ticks were infected with E ruminantium. tible ruminants. Given their susceptibility to infestation by A variegatum ticks and the adaptation of their feeding biol- Geographic distribution—Heartwater can exist ogy almost exclusively to ungulate hosts,67–69 the cattle only where tick vectors are present. The geographic egret (Bubulcus ibis) may have the most potential for distribution of Amblyomma spp ticks is predicated on a the transport and introduction of A variegatum ticks suitable habitat for tick survival. into new geographic regions of the United States. The cattle egret is firmly established in the Caribbean is- Traditional range lands and, because of their tendency to migrate ex- Amblyomma spp ticks are generally found at eleva- tensively, is thought to be partly responsible for the tions from sea level to 2,590 m (8,497 feet). These spe- dissemination of A variegatum ticks to many of the cies are more abundant during the rainy season and in Caribbean islands.59,68–70 environments typified by continuous high relative hu- Additionally, populations consisting of 750,000 to midity. However, Amblyomma spp ticks cannot survive 1.5 million cattle egrets are established in the United when rainfall is < 25.0 cm/y (9.8 inches/y) or > 280.0 States.71,72 Year-round breeding colonies exist in Florida, cm/y (110 inches/y).14,16 Because the seasonal reproduc- and these birds frequently migrate to the Greater Ca- tive activity of these ticks is regulated by rainfall and ribbean Basin, including Guadeloupe.68,73,74 Seasonally warm temperatures, heartwarter-endemic regions are variable populations of cattle egrets are also found in found in a variety of habitats in sub-Saharan Africa and every state in the continental United States, and breed- from wooded savanna to bushland to wooded grassland ing colonies exist in every state except Montana, New that comprise tropical, humid temperate, dry subhumid, Hampshire, Washington, and West Virginia.69 As native semiarid, and arid climates. Outside of Africa, suitable vegetation is converted into grazing lands, these birds habitat for A variegatum ticks can be found in the Carib- are expected to continue to colonize new areas.75 bean islands, southeastern United States, and Latin America.58

Introduction to new geographic regions Because E ruminantium cannot persist outside a host for more than a few hours, the principal mode of release of E ruminantium into a new geographic location is via carrier animals, transport on a host, or both. Cattle transported from Senegal to Guadeloupe were be- lieved to have introduced A variegatum ticks into the Caribbean islands in 1828.1,59 This tick has slowly spread to at least 16 of the 27 Caribbean islands or island groups in this region after introduction (Figure 2).59–62 Subse- quently, heartwater has been diagnosed in domestic ruminants on 3 of these islands (ie, Guadeloupe, Marie-Galante, Figure 2—Geographic distribution of the tropical bont tick (Amblyomma variegatum)59–62 and Antigua) and perhaps as many as 7 and Ehrlichia ruminantium2,3,63,64 in the Caribbean islands.

JAVMA, Vol 237, No. 5, September 1, 2010 Vet Med Today: Reference Point 523 Cattle egrets use 3 distinct spring and fall migration Unquestionably, A variegatum is the most important corridors in North America (Figure 3).69,76,77 These cor- vector for introducing heartwater into the United States. ridors include an eastern route from Newfoundland that However, given the presence of 3 indigenous Amblyom- follows the Atlantic coast states to Florida and Mississip- ma spp (A maculatum [Gulf Coast tick], A cajennense pi, through the Greater Caribbean Basin to the southern [Cayenne tick)], and A americanum [Lone Star tick]) portion of Mexico, and then through Central America that feed on ruminants, it may not be necessary for and into Colombia; a central route from Kansas east to A variegatum ticks to become established in some areas Alabama and then south to Panama; and a Pacific Coast of the United States to sustain an outbreak of this disease. route from southern Arkansas to west-central Mexico.69 For example, transstadial transmission of E ruminantium Records76 of the US Geological Survey Bird Band- from larva to nymph and from larva to nymph to adult ing Laboratory for North American and Caribbean has been confirmed.20 More importantly, that same study adult cattle egret populations have revealed migrations revealed that A maculatum nymphs and adults that were > 3,100 km (1,926 miles). Juvenile birds will also infected in the larval stage transmitted E ruminantium migrate over great distances (eg, ≤ 5,000 km [3,107 to susceptible goats. Another study19 has also shown miles]) from their birthplace.78 These distances are crit- that E ruminantium–infected A maculatum nymphs were ical because the feeding periods for A variegatum larvae able to successfully transmit this pathogen to suscep- and nymphs on cattle egrets are 6 to 13 days and 5 to 10 tible sheep. The efficiency of A maculatum as a vector days, respectively.79 Thus, these ticks may survive long was similar to that of A variegatum, the most important enough on cattle egrets that migrate from the Great- natural vector of heartwater.19 Thus, A maculatum has er Caribbean Basin to North America that they could the potential to play a future role in the establishment eventually feed on a susceptible population of domestic and maintenance of heartwater, if the disease were to be or wild ruminants in the United States. introduced into the United States. In contrast, A cajen- The high probability of tick-ruminant interaction is nense and A americanum appear to be of less concern as attributable to the feeding characteristics of cattle egrets; a potential indigenous vector of heartwater. These same these birds forage almost exclusively in close association studies verified the inability of E ruminantium–infected with cattle or other ruminants. Additionally, cattle egrets A cajennense and A americanum nymphs to transmit in- synchronize their feeding and resting periods with those fection when allowed to feed on susceptible goats20 and of cattle and oftentimes are found on or near resting cattle. sheep.19 Transstadial transmission of E ruminantium In some instances, attempts are made by the cattle egrets to from larva to nymph and from larva to nymph to adult rouse resting cattle by making flights near these cattle.80,81 also does not appear to be important in A cajennense.20 Consequently, this close association to both feeding and Another Amblyomma spp found in the United States, resting cattle may provide the opportunity for mechanical A dissimile (American reptile tick) has transmitted E ru- transport of ticks from cattle egrets to other hosts and to minantium to a goat in experimental conditions.23 How- environments frequented by resting cattle; this allows for ever, because this tick species usually feeds on snakes, the opportunity for introduced ticks to find new hosts.82–85 iguanas, and lizards, it is not considered to have a major role in the transmission of heartwater between ruminants.23 Larvae and nymphs of A maculatum feed on small rodents and ground-dwell- ing birds.86 Nymphs also readily feed on cattle, whereas adults primarily feed on larger mammals (eg, cattle, horses, and white-tailed deer).87 The original distribution of this tick seldom exceeded > 161 km (100 miles) inland along the Gulf coast and on the Atlantic coast in Florida, Georgia, and the southern portion of South Carolina.88 Currently, A maculatum can be found in Ala- bama, Arizona, Arkansas, Florida, Georgia, Indiana, Kansas, Kentucky, Louisiana, Mississippi, Missouri, North Carolina, Oklahoma, South Carolina, Tennessee, Texas, Virginia, and West Virginia. This expansion in geographic distribution is hypothe- sized to be caused, in part, because this tick feeds on many bird species.89

Disease Characteristics Incubation period—After transmis- Figure 3—Geographic distribution and known migratory paths of cattle egrets (Bubul- sion of E ruminantium to a susceptible cus ibis) in the greater Caribbean Basin and the Americas.69,76,77 ruminant host, the organism is seques-

524 Vet Med Today: Reference Point JAVMA, Vol 237, No. 5, September 1, 2010 tered in regional lymph nodes where replication be- peracute form is relatively rare and has usually been gins.90,91 A febrile stage develops between 3 and 10 days observed in European breeds of cattle (B taurus), goats, after transmission, at which time organisms are located and sheep introduced into heartwater-endemic areas of in plasma cells, neutrophils, and, occasionally, other Africa. Furthermore, sudden death is a characteristic granulocytes.57,92 After E ruminantium transmission by of the peracute form. However, livestock owners that infected ticks during feeding under field conditions, closely and frequently observe their animals may find susceptible animals typically have overt clinical signs that high fever, respiratory distress, terminal convul- within 1 to 5 weeks (mean, 2 weeks).44 sions, and diarrhea may precede death during the peracute form. Immunity and carrier state—In heartwater- The acute form is most often detected and is typi- endemic areas, populations of indigenous cattle (par- fied by a fever (up to 42°C [107°F]) that will subside ticularly B indicus–type breeds), goats, and sheep after 1 or 2 days but will recur and persist for an ad- typically are more resistant to infection than are non- 30,93,94 ditional 3 to 6 days. Inappetance, signs of depression, indigenous breeds. This suggests that genetic listlessness, moist cough, and bronchial rales typically factors are important in these ruminants for the de- accompany fever. Cyanosis of mucous membranes and velopment of immunity against heartwater. Immunity dyspnea may be observed. In most animals with acute is probably maintained best in sheep; in contrast, im- 44 disease, clinical signs of CNS disease (eg, rapid blink- munity is of the shortest duration in cattle. Calves, ing of the eyes, protrusion of the tongue, twitching of kids, and lambs also have natural resistance against muscles, hypersensitivity to touch, a staggering and of- heartwater during the neonatal period, and resistance ten high-stepping gait, circling, abnormal postures, and appears to be unrelated to the immune status of the 28–30 aggression) are next to appear. Signs of CNS disease respective dam. typically increase in severity and end with the animal Following recovery from clinical disease, some do- in lateral recumbency, with opisthotonos, nystagmus, mestic ruminants can become reinfected with E rumi- galloping movements, chewing movements, frothing at nantium, resulting in another bout of clinical disease or the mouth, and convulsions shortly before death. In in- even death.50 Some animals may also have a transient 95 stances in which marked clinical signs of CNS disease carrier state. For example, in 1 report, E ruminan- are absent, a profuse and often hemorrhagic diarrhea is tium was recovered from the blood of cattle, goats, and observed. The acute form of disease is typically fatal in sheep for up to 40 days following spontaneous or drug- < 1 week from the onset of clinical signs. induced (oxytetracycline) recovery from clinical dis- 53 The subacute form of disease is rarely encoun- ease. In another study, cattle and sheep maintained tered in animals with heartwater and is characterized a carrier state for 5 to almost 9 months after recovery by prolonged fever, coughing, and mild incoordina- from clinical disease. In the Caribbean islands, cattle tion. Affected animals recover or die within 1 to 2 and Creole goats were transient carriers of E ruminan- 96 weeks after the onset of the subacute form of disease. tium for 2 and 11 months, respectively. A mild or subclinical form of heartwater is character- A carrier state has been detected in both clinically ized by transient fever and can develop in partially im- normal wild ungulate ruminants that are free-ranging 97 mune cattle or sheep, calves < 3 weeks old, antelope, in Africa and in eland (Taurotragus oryx), giraffe (Gi- and some indigenous breeds of cattle and sheep with raffa camelopardalis), greater kudu (Tragephalus strep- natural resistance to the disease. Similarly, susceptible siceros), blue wildebeest (Connochaetes taurinus), and wild ruminants may have inapparent and transient African buffalo (Syncerus caffer) with experimentally 53,98 infections or overt signs of clinical disease and may induced disease. The latter 5 species with experi- subsequently die. mentally induced disease are located in the continental United States as part of captive populations. If infected Pathophysiologic and clinicopathologic chang- with E ruminantium, there is the possibility that these 5 es—The tropism of E ruminantium for endothelial cells and perhaps other wild ungulate ruminants could be- results in damage to capillaries of the host. Thus, the come a reservoir for E ruminantium infection and con- resulting increased capillary permeability caused by tribute to the spread of heartwater. damage to epithelial cells is particularly detrimental to Morbidity and mortality rate—In heartwater- cardiac and respiratory function. In severely affected endemic areas, morbidity varies considerably and is animals and animals with advanced stages of disease, a substantial decrease in cardiac output is observed dependent on the strain of E ruminantium that causes 99 the outbreak and the intensity of management practic- in addition to a decrease in diastolic blood pressure. es (eg, tick control and vaccination). Mortality rate in Changes in lung function are variable and are depen- clinically affected animals also varies considerably; this dent on the stage of disease. Respiratory alkalosis that is particularly dependent on when treatment is initiated is evident during the early febrile stage of disease may during the course of the disease. Mortality rate in indig- transform to respiratory acidosis during more advanced enous breeds may be 5% to 10%, but is typically greater disease stages. Furthermore, endotoxin release may cause a portion of these observed changes in cardiac in nonnative populations. For example, mortality rates 100 of 40%, 60%, and 90% have been observed in calves, and respiratory function. adult cattle, and Angora goats, respectively.27 Primary clinicopathologic changes include progres- sive anemia, fluctuations in total and differential WBC Clinical signs—Clinical signs depend on the stage counts (ie, neutropenia, eosinopenia, and lymphocy- of disease of the animal at the time of examination (ie, tosis), hyperbilirubinemia, a decrease in concentra- peracute, acute, subacute, or subclinical form).15,27 The tions of serum total proteins (hypoalbuminemia), and

JAVMA, Vol 237, No. 5, September 1, 2010 Vet Med Today: Reference Point 525 azotemia.99,101 Effusion into body cavities is commonly detected in most fatal instances of disease and is characterized by a transparent or slightly turbid light yellow fluid that will often coagulate after exposure to air because of high albumin and globulin concentrations.101 Hypocalcemia may also develop because of the effects of hypoalbuminemia.102

Postmortem lesions—Gross lesions attributable to endothelial damage are typically observed in cattle, goats, and sheep that die as a result of heartwater; however, there are instances in which no discernible lesions are detected.15 Effusion of body cavities is an extremely common finding in most fatal cases of the disease.102 Hydropericardium is a common lesion, particularly in Figure 4—Photomicrograph of a section of cerebral cortex ob- goats and sheep, which prompts the use of the term tained from a goat. Notice the elementary body inclusions (arrow) heartwater to describe this disease. In some cases, of E ruminantium (Mali 1 isolate) within capillary endothelial cells. hydrothorax, which consists of several liters of fluid in Giemsa stain; bar = 10 mm. (Photomicrograph courtesy of Dr. Linda cattle, 0.5 L in sheep, and rarely > 20 mL in goats, with Logan, USDA.) minimal hydropericardium may be the most promi- nent finding detected during necropsy.102 Additionally, dark blue appearance, as opposed to the violet color ascites, pulmonary edema, perirenal edema, cerebral of the nucleus of the host cell.4,104 In other instances, edema, and splenomegaly are typically evident.15,102 homogeneous purple masses of initial bodies that are 1 Congestion, edema of abomasal mucosa, or both are to 2 µm in diameter may be observed in the cytoplasm commonly observed in cattle and less frequently obser- of affected cells.104 Apart from cocci and initial bod- eved in goats and sheep.102 Subendocardial petechial, ies, bulky dark blue forms with diameters of up to 2 to submucosal, and subserosal hemorrhages are typically 4 µm can be evident in the shape of horseshoes, rods, evident elsewhere in the body.15,102 irregular masses, and rings. Reports on the pathological changes in wild rumi- Heartwater can also be diagnosed in ruminants nants that have died are limited, but findings appear via the testing of tissues (eg, blood; brain, lung, and similar to those observed in domestic ruminants.102 kidney tissues; and thoracic fluid) with a PCR assay15 Generalized congestion, hydrothorax, hydropericar- or the isolation of E ruminantium in cultured endothe- dium, ascites, lung edema, and splenomegaly are the lial cells inoculated with infected blood.15 Isolation of most commonly observed gross postmortem findings. E ruminantium from Amblyomma spp ticks that were re- Histopathologic lesions appear similar to the changes trieved from animals with clinical signs of heartwater described in domestic ruminants.102 would also provide supportive evidence for a diagnosis of heartwater.15 Unfortunately, available serologic tests Diagnosis cross-react with antibodies against related Ehrlichia and Anaplasma spp,103 as well as Rickettsia spp, Neorickettsia Detection of the previously described diverse range spp, and Coxiella burnetii,4 resulting in many false-posi- of characteristic clinical signs of disease and postmor- tive results. Thus, serologic tests should not be used as tem lesions in domestic and wild ruminants and the the sole method for the establishment of a definitive presence of Amblyomma spp ticks on an animal or a diagnosis of heartwater in areas endemic for these other history of tick exposure provide support for a presump- pathogens.15 tive diagnosis of heartwater. A definitive diagnosis of heartwater can be established in animals that died of Treatment the disease by use of light microscopy to detect the characteristic E ruminantium organisms in the cyto- In outbreaks of heartwater, tetracycline anti- plasm of cerebral cortical or hippocampal capillary microbials (ie, chlortetracycline, doxycycline, oxy- endothelium103 (Figure 4). A small piece (5 X 5 X 5 tetracycline, and tetracycline) have been used to mm) of cerebral cortex or hippocampus should be successfully treat heartwater in clinically affected crushed between 2 microscope slides until the tissue domestic ruminants105 and white-tailed deer.106 Sul- has a soft, pasty consistency.103 The tissue should then fonamide antimicrobials (ie, sulfadiazine, sulfadimi- be moved to the end of the horizontally oriented bot- dine, sulfanilamide, sulfapyradine, sulfaguanidine, tom slide. Next, the top slide, angled at approximately and sulfathiazole) are also effective, but they often 45° to the bottom slide, is used to drag the tissue along are not used because of the superior effectiveness of the bottom slide in a manner to produce both a thick tetracycline antimicrobials.103,105 and thin smear. This procedure stretches the capillar- For tetracycline antimicrobials to be effective ies linearly and facilitates microscopical detection. The against heartwater, it is important to initiate treat- slide is allowed to air-dry, then fixed in absolute metha- ment during the early stages of infection when the nol or ethanol, stained with a Romanowsky-type stain only clinical sign is fever.30,107 If signs of CNS disease (eg, giemsa),103 and examined via light microscopy by are evident at the time of treatment, prognosis is poor. use of oil immersion at 100X magnification.b If present, Typically, the minimum therapeutic dose for short-act- the cytoplasmic vacuoles of E ruminantium will have a ing tetracycline antimicrobials is between 10 and 20

526 Vet Med Today: Reference Point JAVMA, Vol 237, No. 5, September 1, 2010 mg/kg (22 and 44 mg/lb) administered as a single IM Panola Mountain Ehrlichia, which may yet be injection; alternatively, half of the dose can be admin- proven to be another strain of E ruminantium, might istered IV and the remaining half administered IM.103 also emerge as a cause of zoonotic disease in the United Experience has shown that a single treatment is not States within the geographic distribution of its tick vec- reliable and a second injection is often needed to bring tor, A americanum. Recently, a human was infected with about a return of normal body temperature and to this disease agent after a tick bite.118 prevent death.27 Consequently, treatment is typically repeated 24 hours later. During the advanced stages Vigilance for the Detection of FADs of heartwater, larger dosages, multiple injections, or both may be necessary.107 Long-acting formulations of Animals and animal by-products are being trans- oxytetracycline can also be used, but they do not ap- ported globally at record rates. Therefore, an incursion of an FAD into the United States is more plausible than pear to be any more effective than the aforementioned 119 short-acting tetracycline antimicrobials as a treatment ever. Consequently, veterinarians, livestock produc- for heartwater.27 ers, or anyone involved with wildlife must not discount Other treatments to counteract or treat peripheral the possibility that an FAD could be encountered in vascular collapse and increased capillary permeabil- their locality. Education about which FADs can affect ity should be considered when treating animals with these animals must be a priority. heartwater. Furthermore, the administration of drugs Everyone in the agriculture and wildlife industries that are active for reducing edema (eg, diuretics), sta- must gain knowledge of the clinical signs of FADs and bilizing membranes (eg, corticosteroids), and blocking vigilantly monitor for their emergence. The 3 hallmark vasoactive compounds (eg, corticosteroids and NSAIDs) signs of heartwater are clinical signs compatible with a may be beneficial.105 Additionally, IV administration of CNS disorder in multiple animals, high mortality rate fluids and electrolytes and provision of nutritional sup- in clinically affected animals, and postmortem findings port should be considered. related to increased vascular permeability (ie, hydropericardium, hydrothorax, pulmonary edema, and splenomegaly); all 3 emerge concurrently among a group of Prevention and Control ruminants infected with E ruminantium. For anyone that After a viable population of E ruminantium– has not observed an outbreak of heartwater, it is conceiv- infected Amblyomma spp ticks becomes established in able that the aforementioned 3 hallmark signs of heart- a geographic area, prevention of heartwater through water may be attributed to more common diseases of the eradication of tick vectors is likely to be a chal- domestic animals, such as oak poisoning,120 hemoncho- lenging process. Consequently, livestock owners in sis,121 Clostridium perfringens type D enterotoxemia,122 heartwater-endemic areas have implemented strategic and ionophore toxicosis.123 Additionally, it should be tick control practices and vaccination to control this mentioned that theileriasis (East Coast Fever),124 babesi- disease.30 Routine administration of oxytetracycline is osis (Babesia bovis),125 and trypanosomosis126 are 3 FADs also used as a metaphylactic control strategy to pro- with clinical signs and postmortem and histologic le- tect susceptible animals that are to be introduced into sions similar to those of heartwater in infected animals. endemic areas.108 Strategic tick control practices are A comprehensive list of differential diagnoses for more effective in cattle than in goats or sheep109; the endemic diseases should also include FADs that in- objective of these practices is to minimize tick burden duce similar clinical signs. This preparation will keep without drastically interfering with natural transmis- the clinical signs and characteristics of FADs at the top sion of heartwater, which is responsible for the devel- of a diagnostician’s list of causes when an FAD may be opment of immunity in infected neonates and for the encountered. Enhanced awareness of these diseases by maintenance of immunity against heartwater in older veterinarians and producers should lead to early recog- herdmates. In field conditions, animals vaccinated with nition of an incursion and reduce the economic and bio- inactivated or preferably live-attenuated vaccines have logical impact of the disease. If a veterinarian suspects been protected after exposure to E ruminantium.110–115 that domestic ruminants are affected by heartwater or Recombinant DNA vaccines do not appear to be as ef- any other FAD, the state veterinarian or federal area vet- fective in protecting susceptible animals against heart- erinarian in charge should be contacted immediately; it water under field conditions.114,115 is the responsibility of these animal health authorities or their designee to initiate all FAD investigations.127 Live- Public Health Implications stock owners can also contact these officials directly or defer this responsibility to their attending veterinarian. Despite the ability of E ruminantium to infect hu- The USDA-APHIS Veterinary Services, Wildlife Services man endothelial cells in vitro,116 humans are thought National Wildlife Disease Program operates a surveil- to be resistant to E ruminantium infection.15 However, lance and emergency response system. If heartwater or fatal E ruminantium infection has been reported117 in 4 any other FAD is suspected in wildlife, veterinarians humans in South Africa. All 4 of these people initially and others may contact local wildlife disease biologists; had clinical signs consistent with viral encephalitis. alternatively, they may also contact the APHIS National Ehrlichia ruminantium infection was strongly suggested Wildlife Disease Surveillance and Emergency Response on the basis of DNA-sequence evidence for this organ- Program. If an FAD is observed in wildlife, coordinators ism obtained from all 4 people and the presence of of the national wildlife disease surveillance and emer- brain lesions that are typical of those seen in domestic gency response system will send wildlife biologists to animals infected with this rickettsia. evaluate this type of emergency situation.128

JAVMA, Vol 237, No. 5, September 1, 2010 Vet Med Today: Reference Point 527 Conclusions 12. Cowdry EV. Studies on the etiology of heartwater. I. Observa- tion of a Rickettsia, Rickettsia ruminantium (n. sp.), in the tissues The introduction of heartwater into a nonendemic of infected animals. J Exp Med 1925;42:231–252. area is a considerable threat to the United States main- 13. Logan LL. Cowdria ruminantium: stability and preservation of land. Despite operations to eradicate this tick and, thus, the organism. Onderstepoort J Vet Res 1987;54:187–191. this disease from the Western Hemisphere through the 14. Petney TN, Horak IG, Rechav Y. The ecology of the African 59 vectors of heartwater, with particular reference to Amblyomma Caribbean Amblyomma Program, A variegatum ticks hebraeum and Amblyomma variegatum. Onderstepoort J Vet Res and E ruminantium continue to be found in the Carib- 1987;54:381–395. bean islands. Consequently, surveillance of domestic 15. Mahan SM. Heartwater. In: Brown C, Torres A, eds. Foreign ani- ruminants for the early detection of heartwater must mal diseases. 7th ed. St Joseph, Mo: United States Animal Health be conducted by veterinarians and livestock owners. Association, 2008;287–296. Scrutiny at the local level is the first line of defense 16. Camus E, Barré N, Martínez D, et al. III. Epidemiology. In: Ca- mus E, Barré N, Martínez D, et al, eds. Heartwater (cowdrio- against heartwater becoming an established disease in sis): a review. 2nd ed. Paris: Office International des Epizooties, the United States. If heartwater is introduced into the 1996;45–63. continental United States, all critical factors are present 17. Barre N, Camus E. The reservoir status of goats recovered from (ie, tick vectors, environmental factors, and domestic heartwater. Onderstepoort J Vet Res 1987;54:435–437. and wild ruminant hosts) to sustain this disease after 18. Walker JB, Olwage A. The tick vectors of Cowdria ruminantium an outbreak. Environmental conditions in the southern (Ixodoidea, Ixodidae, genus Amblyomma) and their distribution. Onderstepoort J Vet Res 1987;54:353–379. United States are favorable for the long-term survival of 19. Mahan SM, Peter TF, Simbi BH, et al. Comparison of efficacy of A variegatum ticks and currently support populations of American and African Amblyomma ticks as vectors of heartwa- domestic A maculatum ticks. Because this tick species ter (Cowdria ruminantium) infection by molecular analyses and is also a competent vector for E ruminantium, domes- transmission trials. J Parasitol 2000;86:44–49. tic and wild ruminants in this geographic area may be 20. Uilenberg G. Experimental transmission of Cowdria ruminan- most at risk for heartwater infection. tium by the Gulf Coast tick Amblyomma maculatum: danger of introducing heartwater and benign African theileriasis onto the American mainland. Am J Vet Res 1982;43:1279–1282. a. Import Tracking System database. Fort Collins, Colo: USDA- 21. 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