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An Overview of Canine Ehrlichiosis

Lauren Bockino, DVM; Paula M. Krimer, DVM, DVSc; Kenneth S. Latimer, DVM, PhD; and Perry J. Bain, DVM, PhD Class of 2003, Ross University, School of Veterinary Medicine, St. Kitts, West Indies (Bockino) and Department of Pathology (Krimer, Latimer, Bain), College of Veterinary Medicine, University of Georgia, Athens, GA 30602-7388

Veterinary Clinical Pathology Clerkship Program http://www.vet.uga.edu/VPP/clerk/Bockino/

Introduction

The Ehrlichiae are a group of small, gram-negative, pleiomorphic, obligate intracellular cocci that infect different blood cells in various animal species and in humans. There has recently been a reclassification of the family Anaplasmataceae to which the Ehrlichiae belong.8 According to this new classification there are two leukotrophic diseases in dogs that are caused by bacteria in the genus Ehrlichia, namely, Canine Monocytic Ehrlichiosis (caused mainly by Ehrlichia canis) and Canine Granulocytic Ehrlichiosis (caused by Ehrlichia ewingii). It should be noted that cross-reactivity and co-infection is common among the ehrlichiae.7 Classically, canine ehrlichiosis presents as a rather non-specific multisystemic disorder with the primary complaints being depression, lethargy, mild weight loss, vomiting, diarrhea, and anorexia, with or without hemorrhagic tendencies. Furthermore, patients may present with uveitis and/or retinal petechiae, polymyositis, polyarthritis, and central nervous system signs.2 Hematologic abnormalities most commonly associated with canine ehrlichiosis include nonregenerative anemia and thrombocytopenia. Serum chemistry commonly reveals hyperglobulinemia (monoclonal or polyclonal gammopathy), hypoalbuminemia, and low albumin-globulin ratio.5

Canine Monocytic Ehrlichiosis (Ehrlichia canis)

Canine Monocytic Ehrlichiosis (CME), caused by E. canis, is an acute to chronic disease of monocytes, and is the ehrlichial disease most extensively studied. This organism is primarily transmitted by Rhipicephalus sanguineus, the brown dog tick. It is seen mostly in the southeastern and southwestern United States, although it is recognized in all states and worldwide. Amblyomma and Dermacentor ticks have also been implicated in transmission of this disease.3 Dogs may present with variable clinical signs, but thrombocytopenia with bleeding tendencies is the most consistent presenting complaint in dogs in both the acute and chronic stages of the disease.1 During the acute stage, splenomegaly and lymphadenomegaly are common. In the chronic stage, widespread hemorrhage and increased mononuclear cell infiltration of organs may also be evident. Hematologic changes include nonregenerative anemia, An Overview of Canine Ehrlichiosis University of Georgia

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thrombocytopenia, and leukopenia. Pancytopenia may occur as a result of hypoplasia of all bone marrow precursor cells, more commonly in the severe chronic phase.4 Some dogs may develop a secondary immune-mediated hemolytic anemia (IMHA) and have an acute hemolytic crisis, and, thus, a positive direct antiglobulin (Coombs') test.1

Canine Granulocytic Ehrlichiosis (Ehrlichia ewingii)

Canine Granulocytic Ehrlichiosis (CGE) caused by Ehrlichia ewingii, is a disease of neutrophils and, rarely, eosinophils. CGE classically presents with mild signs including fever, lethargy, anorexia, weight loss, vomiting, diarrhea, severe but transient thrombocytopenia, and transient mild nonregenerative anemia with ineffective erythropoeisis. Commonly, the major presenting clinical signs associated with E. ewingii include lameness and joint swelling due to polyarthritis. This form of ehrlichiosis is generally seen in the southern and mideastern United States.1,4 Ticks including Ixodes pacificus, Dermacentor variabilis, Rhipicephalus sanguineus, Amblyomma americanum (especially in North Carolina), and Ixodes scapularis (damminni) have been implicated as vectors.3,6

Pathogenesis of Ehrlichiosis

The pathogenesis of infection with E. canis is the most extensively studied; therefore this discussion will focus on this particular species.

Infection occurs through salivary secretions of the tick at the attachment site during ingestion of a blood meal or through blood transfusions. If the adult Rhipicephalus sanguineus engorges on the dog during the acute stage, it can transmit the disease to other dogs for at least 155 days following detachment.1 Transmission by Rhipicephalus sanguineus is transstadial: the tick acquires the bacteria by feeding on an infected dog in either the larvae or nymph form and the tick transmits the disease to another dog as either the nymph or adult form. The life cycle of Ehrlichia is not yet completely understood but it is thought that it occurs in three intracellular forms. The initial bodies are small spherical structures (1-2 micrometers in diameter) which are believed to develop into larger multiple membrane-bound units known as morulae. The morulae are inclusions within the cytoplasm of the leukocyte as seen in Figure 1. This morula is thought to then dissociate into small granules called elementary bodies.

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Figure 1. Ehrlichia canis seen in a membrane-bound inclusions (morulae) within the cytoplasm of a monocyte (buffy coat smear, Wright stain).

After an incubation period of 8-20 days, the acute phase of infection occurs which lasts 2-4 weeks. At this time, the organism multiplies within circulating mononuclear cells and the mononuclear phagocytes within the liver, spleen, and lymph nodes. The infected cells are then transported in circulation to the rest of the body, with a predilection for the the lungs, kidneys and meninges. Cells infected with ehrlichia adhere to the vascular endothelium and induce a vasculitis and subendothelial tissue infection. This subsequently leads to platelet consumption, sequestration, and destruction that results in the thrombocytopenia seen during this acute phase. Variable leukocyte counts and anemia may also develop progressively during this stage.1 After 6-9 weeks, dogs will either eliminate the parasite (if immunocompetent) or develop a parasitemia in which clinical signs absent to mild to severe. This stage is also characterized by variable persistence of thrombocytopenia, leukopenia, and anemia. Dogs that cannot mount an effective immune response will become chronically infected.1

Diagnosis

Definitive diagnosis of CME requires visualization of morula within monocytes on cytology, detection of E. canis serum antibodies with the indirect immunofluorescence antibody test (IFA), polymerase chain reaction (PCR) amplification, and/or gel blotting (Western immunoblotting).

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On cytology, ehrlichiae stain dark blue to purple with Romanowsky stain. The morulae are well-defined, round to oval, eosinophilic to basophilic bodies found in host membrane-lined vacuoles within the cytoplasm of the mononuclear cells.1

In dogs experimentally infected with E. canis, the IFA test has detected serum antibodies as early as 7 days after initial infection, although some dogs do not become seropositive until 28 days post-infection. If ehrlichiosis is highly suspected clinically in a seronegative dog, serology should be repeated in 2-3 weeks. In the past, titers of IgG antibodies of >1:80 have been considered diagnostic,1 but the most recent research has indicated that titers <1:80 should be deemed suspect and serology should be repeated in 2-3 weeks or a PCR or Western immunoblotting should be considered. A diagnosis should be made and treatment instituted when clinical signs and clinicopathological abnormalities consistent with canine ehrlichiosis are found.2

There are a few potential downfalls of using the IFA test for the diagnosis of E. canis infection. One major concern exists in endemic areas with dogs that are chronically infected and have a positive titer, but are otherwise healthy or show non-specific clinical signs. In these dogs, a positive antibody titer does indicate past exposure to E. canis, does not prove that ehrlichiosis is necessarily an active infection or the cause of the presenting clinical signs. In dogs with non- specific clinical signs, a repeat IFA test after 1 or 2 weeks may be beneficial to differentiate between primary E. canis infection and another secondary disease. Antibody titers to E. canis should increase with active infection. Furthermore, one must consider co-infection with multiple tick-borne diseases caused by agents such as other Ehrlichiae, Rickettsia species, Bartonella species, and Babesia canis. Disease caused by any of these agents may be clinically, hematologically, and serologically indistinguishable from each other. In addition, the immunodominant proteins of E. canis have been shown to serologically cross- react with those of E. chaffeensis (the agent that causes Human Monocytic Ehrlichiosis). Studies have shown that serologic testing by IFA could not consistently distinguish between infections of these two species. Interpretation of E. canis serology should include the consideration of the disease process, cross- reactivities with other ehrlichial species, the possibility of multiple tick-borne infections, and persistent IFA antibody titers post-treatment. Antibody titers be used to gauge the success or failure of treatment of CME. Treatment success should be based on remission of clinical signs, a decline in E. canis antibody titers and a concurrent decrease in gammaglobulin concentrations.7

PCR amplification is also a sensitive method for the detection of acute E. canis although there are currently several potential limitations. It is recommended that this method be used in addition to serology for the initial diagnosis of ehrlichiosis, not instead of it.2

The diagnosis of CGE differs from that of CME as E. ewingii has not yet been cultivated in an in vitro system, therefore antigens have not been available for An Overview of Canine Ehrlichiosis University of Georgia

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comparative serological testing. Diagnosis of CGE requires visualization of morula within neutrophils in peripheral blood (Figure 2), joint effusions, and PCR or Western immunoblot.3 In a study using Western immunoblots, sera from dogs that were experimentally infected with E. ewingii were tested on E. canis antigens. Although there were no reactions with the dominant E. canis antigens, the sera produced binding patterns similar to those of anti-E. canis sera with high molecular proteins. This also may help with the diagnosis of CGE.7

Figure 2. A segmented neutrophil from a dog that contains morula of a granulocytic species of Ehrlichia, most commonly Ehrlichia ewingii (blood smear, Wright stain).

Treatment and Prevention

The mainstay of prevention of canine ehrlichiosis is tick control. The drug of choice for treatment for all forms of ehrlichiosis is doxycycline for at least one month. There should be dramatic clinical improvement within 24-48 hours following initiation of treatment in dogs with acute-phase or mild chronic-phase disease. During this time, platelet counts begin to increase and should be normal within 14 days after initiation of treatment.1,2 Polyarthritis associated with E. ewingii may be self-limiting.3 Previous infection does not confer lifelong immunity, and dogs can become reinfected with the same or other ehrlichial species after re-exposure to infective ticks.

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References

1. Ettinger SJ, Feldman EC: Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat, vol. 1, 5th ed. W.B. Saunders Co., Philadelphia, 2000, pp. 402-406.

2. Neer TM, Breitschwerdt EB, Greene RT, Lappin, MR: Consensus statement on ehrlichial disease of small animals from the Infectious Disease Study Group of the ACVIM. J Vet Intern Med 16:309-315, 2002.

3. Goldman EE, Breitschwerdt EB, Grindem CB, Hegarty BC, Walls JJ, Dumler JS: Granulocytic ehrlichiosis in dogs from North Carolina and Virginia. J Vet Intern Med 12:61-70, 1998.

4. Neer TM: Canine monocytic and granulocytic ehrlichiosis. In: Greene CE (ed): Infectious Diseases of the Dog and Cat, 2nd ed. W.B. Saunders Co., Philadelphia, 1998, pp.139-147.

5. Varela F, Font X, Valladares JE, Alberola J: Thrombocytopenia and light-chain proteinuria in a dog naturally infected with Ehrlichia canis. J Vet Intern Med 11:309-311, 1997.

6. Wolf L, McPherson T, Harrison B, Engber B, Anderson A, Whitt P: Prevalence of Ehrlichia ewingii in Amblyomma americanum in North Carolina. J Clin Microbiol 38:2795, 2000.

7. Waner T, Harrus S, Jongejan F, Bark H, Keysary A, Cornelissen A: Significance of serological testing for ehrlichial diseases in dogs with special emphasis on the diagnosis of canine monocytic ehrlichiosis caused by Ehrlichia canis. Vet Parasitol 95:1-15, 2001.

8. Ehrlichia Research Laboratory, College of Veterinary Medicine, The Ohio State University, Columbus, OH

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