Emerging Zoonotic Agents of Concern in Agriculture

RICKY LEE LANGLEY AND CARL JOHN WILLIAMS

Key words: zoonoses, hepatitis E, hendra, manangle, lyme disease, erhichia, transmissible spongiform encephalopathy (TSE), hantavirus

Throughout the world, we are seeing unprecedented changes in our economic, social, and ecological systems that are having adverse impacts on plants, animals, and humans. These changes are leading to the resurgence of old diseases and the emergence of new ones. The landscape and diversity of animals in many regions are changing due to overgrazing and deforestation. Increasing pollution of water bodies by nitrogen-rich waste-water, fertilizers, and soil runoff and loss of wetlands and mangroves due to development and aquacul- ture, diking, and drilling is promoting growth of marine and freshwater algal blooms. These algal blooms may be toxic to animals and humans. Monitoring the patterns of temperature, wind, precipitation, and biodiversity has enor- mous implications for surveillance of disease vectors and reservoirs (1). Human communicable diseases can be classified according to the source of infection as:

1. Anthroponoses: source is an infected human 2. Zoonoses: source is an infected animal 3. Sapronoses: source is an abiotic substrate (nonliving environment) (2)

A characteristic of most zoonoses and sapronoses is that once transmitted to a human, the epidemic chain is usually broken. However, a limited number of zoonoses are sometimes communicable from one person to another. Zoonotic diseases can be classified as either synanthropic zoonoses with an urban or domestic cycle in which the sources of infection are domestic animals or exoan- thropic zoonoses with a sylvatic (feral and wild) cycle in nature. However, some zoonotic diseases can circulate in both urban and natural cycles (2). There are greater than 200 known zoonotic diseases in the world and more are being found as people move into or change environments that were

393 394 R.L. Langley and C.J. Williams previously uninhabited by humans, thus exposing them to new vectors. Zoonotic agents have a major economic impact on agriculture, especially in third world countries. In many areas, vaccines are not available to prevent diseases in domestic animals, thus the infected animal suffers and humans may either starve if there are large die-offs or may contract a zoonotic illness. In many cases where a zoonotic disease is found, whole herds are slaughtered to prevent the spread of disease outside the affected area. Mass slaughter has had a major economic impact on farmers in both developed and undeveloped nations. For example, thousands of cows in Great Britain were destroyed due to “mad cow disease.” In Southeast Asia, millions of chickens have been slaughtered to prevent the spread of avian influenza. In Singapore and Malaysia, thousands of pigs have been killed to prevent the spread of Nipah virus (2). This chapter covers a few of the emerging zoonotic diseases in developed nations, especially the United States. We have chosen hepatitis E, Hendra virus, Nipah virus, Menangle virus, hantavirus, Lyme disease, ehrlichiosis, and transmissible spongioform encephalopathies as emerging zoonotic agents to present in detail.

Hepatitis E History Hepatitis E, formerly known as enterically transmitted non-A, non-B hepatitis, is a viral infection with clinical and epidemiological features of acute hepatitis. It is a principal cause of acute hepatitis in many developing nations and has been increasingly seen in industrialized countries. It is endemic in third world countries that have poor sanitary practices. Most cases in developed countries have been traced to travel to endemic areas; however, sporadic cases do occur in patients with no history of travel, suggesting possible reservoirs in developed countries (3).

Agent Hepatitis E is a small (32 to 33 nm), nonenveloped, positive sense, single- stranded RNA virus with icosahedral symmetry. It has a 7.2kb genome that is capped and polyadenylated. There are 4 currently recognized genotypes and 1 serotype (4).

Clinical Signs and Symptoms Signs and symptoms resemble other types of viral hepatitis. Abdominal pain, anorexia, dark urine, fever, hepatomegaly, jaundice, malaise, nausea, and vomiting are seen. Less common manifestations include arthralgias, diar- 29. Emerging Zoonotic Agents of Concern in Agriculture 395 rhea, pruritis, and urticaria. Fulminant hepatitis may develop, especially in pregnant women. The overall mortality ranges from 1% to 3% but may be as high as 25% (higher in some studies) during pregnancy. Death of the fetus, abortion, premature delivery, or death of a live-born soon after birth are common complications of hepatitis E infection during pregnancy. Vertical transmission has been reported to occur in 33% to 100% of cases (5–9).

Reservoirs and Occurrence Humans and animals with subclinical hepatitis E infection may serve as reservoirs. Antibodies against or hepatitis E RNA has been detected in cows, sheep, pigs, deer, monkeys, rats, and even a cat. Nucleotide sequence studies show high if not 100% identity between human and animal hepatitis E RNA (10–12).

Mode of Transmission Hepatitis E is excreted in the feces and is transmitted by the fecal-oral route. Transmission is usually by ingestion of contaminated drinking water. Rare cases of person-to-person transmission have been recorded. There is a possibility that transmission by blood transfusion may occur. In the United States where no outbreaks of hepatitis E have been reported, a low prevalence of antibodies to hepatitis E (<2%) in healthy populations is found (5,13). Some, but not all, studies in sewage workers have shown a higher prevalence of antibodies to hepatitis E when compared to a control population. Studies in swine farmers and swine veterinarians have also shown antibodies to hepatitis E in 10.9% and 23% respectively. There are reports of cases of hepatitis E developing after ingestion of raw deer or wild boar meat, again suggesting a zoonotic infection (14–19).

Incubation Period and Communicability The incubation period averages 40 days with a range of 15 to 60 days. The period of infectivity is not known but the virus can be found in the stool 14 days after illness onset. There is no known chronic form of the disease (5).

Methods of Control and Management Education on sanitary disposal of feces and handwashing after defecation and before handling food is paramount. While there is no specific treatment for hepatitis E, a vaccine is under development. Where animal or human manure is used for fertilization of crops, potential contamination of produce or shell- fish (from runoff) with viral agents is of concern. This may be a means of intro- ducing hepatitis E into new areas of the world by increasing globalization of food markets. Proper washing and cooking of food should be practiced (4,5). 396 R.L. Langley and C.J. Williams

Hendra Virus History In the Brisbane suburb of Hendra, authorities in Queensland, Australia, were advised of acute respiratory disease in horses at a stable in September 1994. By the end of September, 13 horses had died. The sick horses were anorexic, depressed, usually febrile, had an elevated respiratory rate, and became ataxic. Head pressing was occasionally noted and a frothy nasal discharge occurred before death. Two humans working with the horses also developed respiratory illness with fever and myalgia. One man died, and the other remained ill for 6 weeks (20).

Agent Hendra virus, formerly known as equine morbillivirus, is a member of a new genus, Henipaviruses, within the family Paramyxoviridae. It is a single- stranded, enveloped RNA virus. It varies in size from 38 to 600 nm and is covered with 10 nm and 18 nm surface projections. It contains herringbone nucleocapsids that are 18 nm wide with a 5-nm periodicity (20).

Clinical Signs and Symptoms In humans only a few cases have been documented, and two thirds of those had a respiratory illness with severe flu-like signs and symptoms. Two out of three cases in humans resulted in death, one died from acute respiratory illness and one from an encephalitis. In horses, respiratory disease characterized by dyspnea, vascular endothelial damage, and pulmonary edema may occur. Nervous signs may also occur. Following experimentally induced infections, cats and guinea pigs have developed fatal respiratory illness (21,22). Histopathological studies show Hendra virus induces syncytial cells in vascular tissues and is primarily vasotropic and neurotropic, generating interstitial pneumonia or encephalitis (23).

Reservoir and Occurrence Fruit bats, especially flying foxes of the genus Pteropus, appear to be the reservoir in nature (24).

Mode of Transmission The virus is thought to be transmitted to horses by bats and then from horses to humans. There is evidence of horse to horse transmission via nasal secretions, saliva, and/or urine. Some evidence exists that the Australian tick, 29. Emerging Zoonotic Agents of Concern in Agriculture 397

Ixodes holocyclus, may transmit Hendra virus from flying foxes to horses and other mammals (25).

Incubation Period and Communicability Incubation period is unknown since so few cases have been reported, but it appears to be approximately 7 days in humans. The virus is not known to be communicable among humans (25).

Method of Control and Management Early recognition of disease in horses is important to prevent spreading to other horses and humans. Reduce exposure to fruit bats. Wear personal protective equipment and use good sanitation practices when it is necessary to contact potentially infected animals. The drug ribavirin has been shown to be effective in in-vitro studies. The clinical usefulness of this drug is not known (26).

Menangle Virus History In August 1997, an outbreak of reproductive disease occurred in a piggery in Australia. Two humans working at the piggery were infected. The disease was associated with a flu-like illness with a rash in the workers. The virus causes embryonic mortalities, stillbirths, mummified fetuses, and congenital abnor- malities in the pigs (27).

Agent Menangle virus is a single stranded, pleomorphic, enveloped RNA virus. Surface projections 17+ nm long have been noted on the envelope. It is a new member of the genus Rubulavirus within the family Paramyxo- viridae (28).

Clinical Signs and Symptoms Fever, chills, rigors, and drenching sweats characterized patients’ illness, in addition to headaches, myalgias, and photophobia. About 4 days later, a spotty, red nonpruritic rash developed on the torso. Spleen enlargement occurred in one patient. Both eventually recovered after a few weeks (29). In pigs, mummified fetuses, stillborn piglets with arthrogryposis, craniofacial deformities such as brachygnathia, occasional fibrinous body cavity effusions, and pulmonary hypoplasia were found. Degeneration of the brain and spinal cord has been noted along with nonsuppurative myocarditis in some piglets (30). 398 R.L. Langley and C.J. Williams

Reservoir and Occurrence Menangle virus appears to a virus of fruit bats (flying foxes) of the genera Pteropus. Tests on birds, cattle, sheep, cats, and a dog around the affected piggery were seronegative. In piggeries where the virus is detected, neutralizing antibodies are found in a high percentage (up to 95%) of pigs by slaughter age (30,31).

Mode of Transmission The mode of transmission among pigs is unknown, but respiratory, fecal, or urinary excretion is postulated. The mode of spread from pigs to humans is unknown (30,31).

Incubation Period and Communicabilty Incubation period and communicability are unknown in humans. Heavy occupational exposure appears to be needed for transmission from pigs to humans.

Method of Control and Management Serologic testing and segregation of positive pigs used to eradicate menangle virus from one piggery. Prevention of exposure of pigs to fruit bats may possibly be beneficial (31).

NIPAH Virus History In 1998 and 1999 a new disease that spread among pigs, characterized by respiratory and neurologic symptoms and sometimes accompanied by sud- den death of sows and boars, occurred in Malaysia and Singapore. The original name proposed for this new pig disease was porcine respiratory and neurological syndrome or “barking pig syndrome.” The disease occurred in close association with an epidemic of encephalitis in pig farmers. In Malaysia, more than 265 encephalitis cases in humans occurred with a mortality rate of approximately 40%. A new paramyxovirus was found and named Nipah virus. The outbreak stopped after pigs in the affected area were destroyed. In Singapore in 1999, 11 workers in an abattoir developed an encephalitis or pneumonia resulting in one death. Importation of pigs from Malaysia was banned. The virus was found to be transmitted to other animals including dogs, cats, and horses. An outbreak in 2004 in Bangladesh has been recently reported with a 60% to 70% mortality rate. 29. Emerging Zoonotic Agents of Concern in Agriculture 399

The mode of transmission in the Bangladesh outbreak has not been determined. Because of its high mortality rate and spread to domestic animals, this agent has the potential to be considered an agent of bioterrorism (32–37).

Agent Nipah virus is a single stranded, enveloped RNA virus. The virions are enveloped particles composed of a tangle of filamentous nucleocapsids and measure as large as 1900 nm in diameter. The nucleocapsid has the characteristic helical and herringbone structure of paramyxoviruses. The Nipah virus belongs to a new genus Henipaviruses within the family Paramyxoviri- dae. The Nipah virus genome is 12 nucleotides longer than the Hendra virus genome, and both have identical leader and trailer sequence lengths and hexamer-phasing positions for all their genes (38,39).

Clinical Signs and Symptoms The disease presents as an acute encephalitis with symptoms of fever, headache, and giddiness followed by coma. Distinctive clinical features include segmental myoclonus, areflexia and hypotonia, hypertension, and tachycardia. Neurologic relapse occurred in a few patients after initial mild disease. Several of the survivors had relapsed or late-onset encephalitis. In the Singapore outbreak, 3 patients presented with atypical pneumonia, one later developed hallucinations and evidence of encephalitis. Cerebrospinal fluid was abnormal in 75% at presentation. Magnetic resonance imaging shows the presence of discrete high-signal-intensity lesions disseminated throughout the brain. The main histopathologic findings include a systemic vasculitis with extensive thrombosis and parenchymal necrosis, especially in the central nervous system (40–44).

Reservoir and Occurrence Fruit bats, especially Pteropus sp., appear to be the natural reservoir of Nipah virus. Viruses related to Nipah and Hendra appear to be more widespread in Southeast Asia than previously thought (45,46).

Mode of Transmission The virus appears to be transmitted from fruit bats to pigs and from pigs to humans. Respiratory secretions and urine of infected pigs have been shown to contain Nipah virus and may be vehicles of transmission. Nipah virus has been detected in respiratory secretions and urine of patients. Thus it is possible to be infected from secretions of infected patients, but epidemiologic data do not suggest human-to-human transmission is common (47). 400 R.L. Langley and C.J. Williams

Incubation Period and Communicability The mean incubation period appears to be 10 days. While the virus has been detected in human urine and respiratory secretions, human to human transmission, if it occurs, appears to be rare (48).

Method of Control and Management Limiting exposure of pigs to fruit bats is important. Culling infected pigs may help stop spread within the herd. Wear personal protective equipment including goggles. The use of ribavirin appears to reduce the mortality of encephalitis cases (49,50).

Hantavirus History There are currently more than 20 recognized sero/genotypes of hantaviruses. The different hantavirus types are associated with different types of diseases. Two major diseases are recognized: hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS). HFRS is primarily a disease of Europe and Asia while HPS is only recognized in the Americas. This discussion will be limited to HPS. The initial outbreak of illness from Hantavirus in the United States. was described in 1993, however the earliest case of a serologically confirmed infection was in a person that developed an HPS-like illness in 1959. Since 1993, cases in North, Central, and South America have been recognized. The sero/genotype of the virus appears to be different by country although clinical manifestations are similar (51,52). The first outbreak in the United States appeared in June and July 1993 in the Four Corners (Arizona, Colorado, Utah, New Mexico) region. In the United States since then, as of September 1, 2004 there have been 379 cases noted in 32 states. The overall case fatality in the United States has been 36% (52).

Agent Hantaviruses are negative sensed, single stranded, enveloped viruses belong- ing to the Bunyaviridae family of viruses. There are at least 13 viruses known to cause HPS. The first virus isolated from the Four Corners area was given the name Sin Nombre virus. Other hantaviruses in the United States that have caused disease in humans include Bayou virus, Black Creek Canal virus, New York, and Monongahela virus. Most HPS in the United States is caused by Sin Nombre virus. Several other viruses are known to cause HPS in Cen- tral and South America including Andes virus, Bermejo virus, Juquitiba 29. Emerging Zoonotic Agents of Concern in Agriculture 401 virus, Laguna virus, Lechiguanas virus, Oran virus, and Choclo virus. Numerous other hantaviruses have been found in rodents on both continents but have not been shown to cause disease in humans at the present time. This may be related to the lack of contact with the rodents or possibly less viru- lence of the virus (53).

Clinical Signs and Symptoms Patients initially present with a nonspecific febrile illness. In addition, fever, muscle aches, headache, chills, dizziness, non-productive cough, nausea, and vomiting are noted. In about half of the patients malaise, diarrhea, and light- headedness are reported. Patients may report shortness of breath. Less fre- quent reports of arthralgias, back pain, and abdominal pain are noted. Cough and tachypnea develop around day seven. Once the cardiopulmonary phase develops, the disease progresses rapidly (53,54). Laboratory findings include an elevated white cell count with a marked left shift of the myeloid cells. Atypical lymphocytes are frequently present. In the majority of cases, the platelet count is low. A rapid fall in the platelet count may herald the development of a pulmonary edema phase. The hematocrit may be elevated in about 50% of the cases. In severe cases of HPS, disseminated intravenous coagulation may occur, but this is rare. Proteinuria, mild elevation of liver enzymes, amylase, CPK, and creatinine has been reported. Within 24 hours of initial evaluation, hypotension and progressive evidence of pulmonary edema and hypoxia occur (53,54). HPS has a characteristic radiologic evolution that begins with minimal changes of interstitial pulmonary edema progressing to alveolar edema with severe bilateral involvement. Pleural effusions are common (53). HPS has been reported during pregnancy. Symptoms and signs, physical findings, and laboratory values were similar to nonpregnant patients, although fever was lower. No evidence of transplacental transmission was found (54). Histologic evaluation of infected tissue show that viral antigens are dis- tributed primarily within the endothelium of capillaries throughout various tissues. Histopatholgic lesions are mainly seen in the lungs and spleen. Immune complexes have been detected in the sera and may be responsible for the increased capillary permeability, vascular injury, platelet lysis, and kidney damage. Individuals with HLA-B*3501 have an increased risk of developing severe HPS, suggesting that CD8(+) T cell responses contribute to pathogenesis (55,56).

Reservoir and Occurrence All hantaviruses known to cause HPS are carried by New World rats and mice in the family Muridae, subfamily Sigmodontinae. It appears that each virus has a specific rodent host. The deer mouse, Peromyscus maniculatus, is 402 R.L. Langley and C.J. Williams the host of the Sin Nombre virus. The white footed mouse, Peromyscus leucopus, is the reservoir for New York virus. Black Creek canal virus is hosted by the cotton rat, Sigmodon hispidus, and the Bayou virus is hosted by the rice rat Oryzomys palustris. Various other sigmodontine rodents in Central and South America are hosts for hantaviruses in these regions. In the United States, studies of people in endemic areas show the prevalence of antibodies to SNV among healthy people is low (0.3%). Studies in mammalogy field workers show a prevalence of antibodies to hantaviruses of slightly higher than 1%. In South America, seroprevalence surveys show rates as high as 30% in some populations engaged in farming (56,57).

Mode of Transmission Inhalation of the virus, which is shed in rodent urine, feces, and saliva, is felt to be the main method of disease transmission in man. Cases have been reported after a rodent bite, and researchers think that people may become infected if they touch some object that has been contaminated with rodent excretions and then touch their nose or mouth. People may possibly be infected if they eat food contaminated with rodent excretions. No evidence of person to person transmission has been documented in the United States, however in South America, the Andes virus can be spread by person to person. When exposure information was analyzed, 70% of cases of HPS were closely associated with peridomestic activities, such as cleaning, in homes that showed signs of rodent infestation (58). In the United States, farm animals, dogs, cats, ticks and biting insects have not been shown to transmit disease to humans. However a recent study has found hantavirus-specific RNA in chiggers and an ixodid tick parasitizing wild rodents in Texas (59).

Incubation Period and Communicability The incubation period of HPS after exposure to rodents is 9 to 33 days with a median of 14 TO 17 days. Nosocomial transmission and person to person spread have not been noted in the United States, but the Andes virus has been shown to be transmitted by these routes in South America (60).

Method of Control and Management Methods to eliminate or minimize contact with rodents at home, in the work- place and in recreational areas are important. Seal up holes and gaps in the home and garage. Remove any food sources that rodents may access. The Centers for Disease Control and Prevention has recommendations for prop- erly cleaning up areas infested by rodents. There is no specific treatment, cure, or vaccine for hantavirus infection. If there is a suspicion of hantavirus infection, immediate transfer to an intensive care unit is mandatory. Ribavirin 29. Emerging Zoonotic Agents of Concern in Agriculture 403 does not appear to be effective. Passive immunotherapy using plasma from patients that have recovered from HPS may be beneficial, but no studies of clinical effectiveness have been conducted (52,61,62).

Lyme Disease History Borrelia burgdorferi, a spirochete, is the causative agent of Lyme disease, which is a multisystem disorder. Skin lesion(s), erythema migrans (EM), followed in some patients by rheumatologic, neurologic, and cardiac abnormal- ities are the most common clinical manifestations (63). The disease was first characterized almost 30 years ago when Steere and colleagues investigated an unusual cluster of illnesses resembling juvenile rheumatoid arthritis that occurred in 1975 near Lyme, Connecticut. B. burgdorferi was identified as the etiologic agent of Lyme disease and Ixodes scapularis was identified as the principal vector of the spirochete (63).

Agent The genus Borrelia is in the order Spirochetae, which contains other genera that are pathogenic to humans and animals, such as Leptospira and Treponema.It is a spiral shaped gram negative bacteria and contains a single linear chromo- some. The organism is readily killed by drying and by exposure to disinfectants. Other Borrelia species may cause illness in humans, but only B burgdorferi is recognized as a cause of Lyme borreliosis in the United States (63).

Clinical Signs and Symptoms The clinical manifestations of Lyme disease occur in three stages, generally appearing in sequence. Stage one is a rash, erythema migrans (EM), which occurs at the site of tick attachment. Cardiac and acute neurologic, and other dermatologic signs constitute stage two, or early disseminated infection. Arthritis and chronic neurologic disease comprise stage three, or late disseminated infection (64). Stage one is a localized infection. Following delivery of B. burgdorgeri from the tick to the host skin the bacteria will spread and replicate locally in the dermis and epidermis, which results in the EM rash. This lesion typically appears within one month of the tick bite. However, in many patients the rash goes unrecognized or does not occur. The rash is frequently accompanied by influenza-like symptoms. Appropriate treatment during this phase is essential to preventing subsequent stages of disease from developing (64). Within days to weeks of the tick bite, bacterial dissemination via the car- diovascular or lymphatic system may occur. A wide range of symptoms may 404 R.L. Langley and C.J. Williams result, depending on the site of deposition of the bacteria. Objective signs of acute neuroborreliosis occur in about 15% of untreated patients. Manifesta- tions can include lymphocytic meningitis, Bell’s palsy, and cerebellar ataxia. Additionally, in untreated patients, atrioventricular block may occur. (65–67) In the third stage of disease patients may have recurrent attacks of arthritis beginning months after the initial infection. Up to 60% of untreated patients will have intermittent attacks of joint swelling and pain, particularly in large joints such as the knee. After several attacks of arthritis, some patients may experience persistent joint inflammation (67). Diagnosis is usually based on the recognition of characteristic clinical signs, a history of exposure in an area where the disease is endemic, and an antibody response to B. burgdorferi. When serologic testing is indicated, CDC recommends testing initially with a sensitive first test, such as an enzyme- linked immunosorbent assay followed by testing with the more specific West- ern immunoblot test to corroborate equivocal or positive results obtained with the first test (68). Serodiagnostic tests are insensitive during the first few weeks of infection. During this period, up to 30% of patients will have a positive IgM response. By convalescence (two to four weeks later), up to 80% of patients have a positive IgM response. In contrast, after one month, almost all patients will have a positive IgG response (69). Following antibiotic therapy, antibody levels decrease gradually, but IgG and IgM titers may persist for years after therapy. Therefore, an IgM response cannot always be interpreted as a demonstration of recent infection unless clinically compatible characteristics are also present (69,70).

Reservoir and Occurrence B. burgdorferi is maintained in nature in a cycle that involves hard ticks of the Ixodes genus as vectors and small mammals as reservoir hosts. Ixodes ticks attach to a host and take a blood meal at each stage of life (larva, nymph, adult) then drop off the host and molt in the environment. All three stages can feed on people but the nymph stage is most often implicated in transmission of disease to persons (71).

Mode of Transmission Certain species of ticks in the genus Ixodes feed on field mice in their larval stage. Mice, who serve as the reservoir for B. burgdorferi, are asymptomati- cally infected and carry a high number of these spirochetes in their blood- stream. The white-footed mouse (Peromyscus leucopus) is a key reservoir species for B. burgdorferi in the United States. After feeding on an infected mouse, the newly infected Ixodes larva will harbor B. burgdorferi for the remainder of its life. Larval ticks develop into nymphs following this blood 29. Emerging Zoonotic Agents of Concern in Agriculture 405 meal from a mouse. Immature ticks typically require two to four days of attachment to the host to complete a blood meal. The nymph must take another blood meal prior to molting into an adult, and this feature of the tick life cycle places persons at risk. The peak feeding time for nymphs is from May through late summer, when human outdoor activity is at a peak (70,71). Adult ticks will take a final blood meal, usually in late fall or winter, prior to laying eggs. White-tailed deer are the preferred hosts for adult ticks, but serve as poor reservoirs for B. burgdorferi. White-tailed deer thus serve to maintain the population of ticks, and not that of B. burgdorferi. Ixodes scapularis are not found in geographic regions where deer are absent, and tick abundance appears to be directly related to deer abundance. B. burgdorferi is transmitted trans-stadially from larvae to nymph to adult, so even adult tick bites pose a risk to people. Importantly, a tick must be attached to its human host for at least 24 hours for Borrelia transmission to occur (64,71,72,73). Lyme disease is acquired from the bite of an infected tick. Ixodes scapularis is the dominant vector of B. burgdorferi. Although B. burgdorferi has been detected in other blood-feeding arthropods such as the American dog tick (Dermacentor variabilis), mosquitoes, fleas, and tabanid flies (deer flies, horse flies), the presence of the spirochete in these arthropods is transient, and they are unlikely to transmit the spirochete to new hosts. There is no evidence to support person-to-person transmission. Transplacental transmission has been reported, but it appears that adverse birth outcomes are rare. Transmis- sion of B. burgdorferi by the transfusion of blood obtained from a spiro- chetemic donor has never been reported (74–78).

Incubation Period and Communicability Incubation typically ranges from 3 days to one month for EM lesion. How- ever, if this stage is unrecognized the patient may present with late manifestations up to months later. Lyme disease is not communicable from person to person (79,80).

Method of Control and Management Most persons treated for Lyme disease have an excellent prognosis. However, treatment is highly variable depending upon the clinical presentation of the patient. Treatment generally consists of oral or parenteral antibiotic therapy (79,80). Prevention of infection through the use of personal protective measures is ideal. Using DEET on exposed portions of skin and treating clothing with permethrin are effective at preventing not only tick-borne diseases, but also mosquito and other arthropod-borne diseases. Persons should also conduct tick checks after exposure to tick habitat (79,80). 406 R.L. Langley and C.J. Williams

Ehrlichiosis History Ehrlichiosis is one of a number of bacterial tickborne diseases that occur in the United States. The most common by far is Lyme disease, but others include Rocky Mountain spotted fever, tularemia and ehrlichiosis. In the United States ticks can also be responsible for transmitting parasites and viruses, which may cause illness (81). There are numerous ehrlichial species that can infect humans and animals worldwide, five of which are known to infect humans. However, this section will focus on those species that cause clinical illness in humans in the United States. Ehrlichia sennetsu, the cause of sennetsu fever in the far east, is not known to exist in the United States. Ehrlichia canis, the cause of canine monocytic ehrlichiosis, does occur in the US, but has only rarely been documented to infect persons in other countries (81). Ehrlichiosis is considered an emerging zoonotic pathogen based on the fact that both conditions affecting humans in the United States are newly identified and not fully characterized. Ehlichiosis was initially characterized as a condition of dogs in the 1930s. E. canis was identified as a pathogen causing illness in military working dogs in Vietnam in the 1960s. Human ehrlichiosis is a newly recognized disease in the United States and was first identified in 1986. The agent of human monocytic ehrlichiosis (HME), Ehrlichia chaffeensis, was first identified in 1991. The name is derived from Fort Chaffee Arkansas, where the Ehrlichia species was first isolated from an ill soldier. Human granulocytic ehrlichiosis (HGE) was first recognized in a series of patients from Minnesota and Wisconsin in the early 1990s (82–84).

Agent Members of two genera can cause clinical ehrlichiosis in the United States. Ehrlichia and Anaplasma species belong to the family Anaplasmat- aceae. Ehrlichia and Anaplasma are gram negative, obligate intracellular bacteria that replicate in the vacuoles of eukaryotic cells. In humans, the ehrlichial infections are named based on the type of white blood cell that is infected, i.e., monocytic or granulocytic. Anaplasma phagocytophilum, known prior to 2001 simply as the HGE agent, is the causative organism of HGE. Also, several cases of human ehrlichiosis in Missouri have been attributed to infection with E. ewingii, the causative agent of granulocytic ehrlichiosis in dogs (85,86).

Clinical Signs and Symptoms HME and HGE represent two clinically indistinguishable yet epidemiologically and etiologically distinct diseases. Infection generally results in acute, 29. Emerging Zoonotic Agents of Concern in Agriculture 407 influenza-like illness with fever, headache, malaise and frequently low white blood cell and thrombocyte counts. Nausea, vomiting, and a rash may be present in certain cases. Intracytoplasmic bacterial aggregates (morulae) may be visible in the white blood cells of some patients. Because the symptoms are relatively non-specific, a definitive diagnosis depends on development of a clinically compatible illness in conjunction with supportive laboratory results (87).

Reservoir and Occurrence The agents of ehrlichiosis are maintained in wildlife hosts and are transmitted between animals through the bites of infected ticks. White tailed deer are important reservoirs for both E. chaffeensis and E. ewingii. Humans and domestic animals such as dogs are thought to be largely accidental hosts and are unlikely to play an important role in the natural maintenance of these pathogens (88–90).

Mode of Transmission E. chaffeensis and E. ewingii are transmitted among reservoir species and to accidental hosts by the lone star tick (amblyomma americanum), which occurs widely throughout the southeastern and south central United States. In the northeastern and midwestern United States, A. phagocytophilum is maintained in white tailed deer and small rodents. Transmission to humans occurs through the bite of the black legged tick (Ixodes scapularis). Tick transmission is believed to be the only epidemiologically important means of acquiring infection (90–93).

Incubation Period and Communicability The incubation period ranges from 7 to 21 days for both conditions. Neither condition is communicable from person to person (93).

Method of Control and Management If ehrlichial infection is suspected based on clinical findings or history of tick exposure, initiation of antimicrobial treatment should not be delayed. The tetracycline class of antibiotics is the drug of choice for treating ehrlichiosis (93). Prevention of infection through the use of personal protective measures is ideal. Using DEET on exposed portions of skin and treating clothing with permethrin are effective at preventing not only tick-borne diseases, but also mosquito and other arthropod-borne diseases. Persons should also conduct tick checks after exposure to tick habitat (93). 408 R.L. Langley and C.J. Williams

Transmissible Spongiform Encephalopathis History Transmissible spongiform encephalopathies (TSEs) constitute a rare group of neurodegenerative disorders. They are invariably fatal and affect humans and animals. TSEs in animals include transmissible mink encephalopathy, scrapie (affecting sheep and goats), bovine spongiform encephalopathy (BSE or mad cow disease), and chronic wasting disease (CWD) of deer and elk. TSEs in humans include Creutzfeld-Jakob disease (CJD), new variant CJD, fatal familial insomnia, Gertsman-Straussler-Scheinker syndrome, and kuru. Each of the TSEs is unique and apparently has a very limited host range, yet they all share characteristics that allow them to be grouped together (94).

Creutzfeld-Jakob Disease Creutzfeld-Jakob Disease (CJD) was first described in Europe in 1920 and 1921, though it probably occurred prior to that and was not recognized. This disease occurs at a rate of about one case per million persons worldwide and generally affects only persons aged 65 years or more. It presents as a rapidly progressive dementia terminating in death roughly four to five months after symptom onset. CJD can have an incubation period of up to 20 years. Diag- nosis is based on observation of clinically compatible symptoms, and a definitive diagnosis can only be determined postmortem through histopathologic examination of central nervous system biopsy specimens. As mentioned previously, most cases of CJD appear spontaneously (94,95). In 1995 a new variant of CJD (nvCJD) was identified in the United Kingdom. Clinically, this variant was very similar to classic CJD but affected much younger persons, generally in their thirties. From 1995 through June 2002, a total of 124 human cases of vCJD were reported in the United Kingdom, 6 cases in France, and 1 case each in Ireland, Italy, and the United States. The case-patients from Ireland and the United States had each lived in the United Kingdom for more than 5 years during the UK BSE epidemic. The discovery of nvCJD following the BSE outbreak in the UK is very important because it appears that the agent causing BSE is capable of crossing the species barrier and causing illness in humans in the form of nvCJD (95,96).

Bovine Spongiform Encephalopathy Beginning in 1986 an epidemic of bovine spongiform encephalopathy (BSE) was identified in numerous European countries, most notably in the UK. Although it has not been definitively determined, this epidemic apparently arose via feeding ruminant-derived protein contaminated with the 29. Emerging Zoonotic Agents of Concern in Agriculture 409 scrapie agent to cattle in the United Kingdom. It is suspected that render- ing plant procedural changes in the 1970s resulted in the failure to inacti- vate the scrapie agent. Transmission by contaminated feed appears to have been the only mechanism by which cattle became infected. No evidence of horizontal spread from animal to animal has been documented in the BSE outbreak (96). The BSE epidemic in the United Kingdom peaked in 1992 with over 3,500 new cases per month in cattle. Beginning in 1988, the United Kingdom instituted a number of control measures beginning with a ban prohibiting the feeding of ruminant-derived protein to ruminants. In 1990 this ban was extended to prohibit the feeding of specified bovine offal (brain, spinal cord, thymus, tonsil, spleen) to other ruminants, and in 1996 the mammalian meat and bone meal prohibition was instituted. The restrictions have worked to the extent that through the first half of 2004 only 233 cases have been reported to the Dept. of Environment, Food and Rural Affairs, the UK equivalent to the USDA. In calendar year 2003, 457 cases were reported (95–98). In the United States, the feeding of rendered cattle products to other cattle has been prohibited since 1997, and the importation of cattle and cattle products from countries with BSE or considered to be at high risk for BSE has been prohibited since 1989. These measures have minimized the potential exposure of animals and humans to the BSE agent. Nonetheless, on December 23, 2003, the USDA made a preliminary diagnosis of BSE in a single nonambulatory dairy cow in Washington State. The BSE interna- tional reference laboratory in Weybridge, England, subsequently confirmed this diagnosis. This was the first time BSE has been identified in the United States (98,99).

Potential Transmission of BSE to Humans Epidemiologic and laboratory evidence suggests that the BSE agent has been transmitted to humans via consumption of BSE-contaminated cattle products, causing nvCJD. However, the risk for acquiring vCJD from consumption of BSE-contaminated product is low, presumably because of a species barrier that provides some degree of protection against development of nvCJD. BSE is the only TSE of animals that has ever been linked with human disease. In the United Kingdom, where an estimated 1 million or more cattle probably were infected with BSE, cases of nvCJD continue to be reported; however, the number of cases of nvCJD remains small, with 147 probable and confirmed vCJD cases identified as of August 2004, including those of three persons residing in Ireland, Canada, and the United States who are believed to have been exposed to BSE in the United Kingdom. No cases of nvCJD have been identified where the patient did not have exposure within a country where BSE was occurring (96–100). 410 R.L. Langley and C.J. Williams

Chronic Wasting Disease Although CWD was first identified as a syndrome in the 1960s and the etiologic agent was found to be a spongiform encephalopathy in 1978, there has been a growing public health concern about the condition recently as it has been identified in new areas. CWD was first identified in Colorado and Wyoming over 25 years ago. Since 1996 CWD has been found in Kansas, Montana, Nebraska, Oklahoma and South Dakota in captive elk herds. Additionally, it has been identified in wild deer in Nebraska, South Dakota and Wisconsin. Given the popularity of deer hunting there is concern that CWD could pose a risk to human health as BSE did in cattle over 10 years ago. To date, only three species of mammals are known to be naturally susceptible to CWD: mule deer (Odocoileus hemionus), white tailed deer (Odocoileus virginianus), and elk (Cervus elaphus). Cattle and other livestock seem to be resistant (97,100–102). To date no association has been made between CWD and neuropathologic illness in humans. A 2003 report of fatal neurologic illness in men who participated in wild game feasts concluded that there was no association between CWD and CJD type disease, though continued surveillance for both diseases is warranted. Nonetheless, it is currently advised that animals with evidence of CWD should be excluded from human and animal food chains due to the possibility that the CWD prion could cross the species barrier (103).

Agent In 1982 Prusiner first described the concept of the prion to characterize the agent that causes TSEs. Prions are small proteins (253 amino acids in humans) that are encoded on different chromosomes in different species. The exact function of the prion protein is unknown, but it is believed to be involved in neuronal copper metabolism and synaptic transmission. Normal cellular prion protein (PrPc) is susceptible to degradation by protease. How- ever, in certain instances PrPc is converted to a protease resistant form of the protein (PrPres) that accumulates in neural tissue, inevitably resulting in degenerative disorders and death (104–107). PrPc and PrPres are identical in terms of their primary amino acid sequence and differ only in conformational changes. It is theorized that a post-transla- tional, conformational change of PrP alpha-helices into beta-sheets is the pathologic mechanism causing change from PrPc to PrPres. Following intro- duction of PrPres into the mammalian body, PrPres promotes conversion of PrPc to PrPres through direct contact, resulting in a toxic accumulation of PrPres in neurologic tissue (102–107).

Acquisition and Communicability The origin of PrPres in any given mammal is thus critically important. There are several ways by which it may arise. In humans, PrPc may spontaneously 29. Emerging Zoonotic Agents of Concern in Agriculture 411 change into PrPres, or inheritance of a defective gene that codes PrPc may cause the prion protein to be abnormally shaped. Finally, acquired cases may occur when PrPres from an infected mammal is introduced into a susceptible mammal through contaminated central nervous system tissue. In animals the origin of PrPres is thought to be acquired only. In humans or animals, there is a dynamic pathogenic process that occurs for acquired cases. The process can be broken down into distinct phases of infection and peripheral replication, CNS neuroinvasion, and neurodegeneration (94,96).

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