The primary risk factor for feline ehrlichiosis is exposure to vectors. Affected cats range from 1 to 14 years of age, although most are young.10 There is no obvious sex predisposition and most affected cats are domestic shorthairs.⁹

Although feline ehrlichiosis occurs worldwide, there is a poor knowledge of prevalence. A study in Brazil found a prevalence of 8.6% in 93 cats with PCR testing, while 45% of those cats were seropositive.¹¹ A seroprevalence of 17.9% was found in urban stray and pet cats in Spain.¹² However, seroprevalence studies may overestimate prevalence as antibodies from exposure to other pathogens (e.g., Bartonella henselae¹³ and other Neorickettisa and Anaplasma spp.¹⁴) may cross-react with E. canis antigens.

Owners may or may not report a history of tick exposure and a history of purchasing ectoparasite preventatives does not rule out ehrlichiosis. Clinical signs are nonspecific and include anorexia, lethargy, and weight loss. In a report of three cats, physical examination abnormalities thought to be secondary to ehrlichiosis included fever, splenomegaly, joint pain, lameness, myalgia, lymphadenopathy, pallor, and petechial hemorrhage.9

Laboratory abnormalities in cats with evidence of ehrlichiosis (based on detecting E. canis-like DNA) include thrombocytopenia, severe bicytopenia, marked pancytopenia, marked neutrophilic leukocytosis (with or without a left shift), non-regenerative anemia, positive anti-nuclear antibody titer, and polyclonal hyperglobulinemia.9,11,15 In a report of three cases, one cat was diagnosed with neutrophilic polyarthritis and a pancytopenic cat had bone marrow cytology changes consistent with myelodysplasia or myeloid leukemia.⁹ A cat with anemia and thrombocytopenia had cytologic and histopathologic bone marrow changes that included marked erythroid and megakaryotic hypoplasia. Lymph node cytology findings included reactive hyperplasia and pyogranulomatous inflammation. All three cases were complex and all physical and laboratory abnormalities could not be attributed definitively to E. canis-like infection, although most have been reported in dogs with ehrlichiosis.

A combination of PCR, serology, and close examination of a blood smear for morulae (Fig. 38.1) are recommended in cats when a vector-borne illness such as ehrlichiosis is suspected. Identification of morulae in feline monocytes should always lead to submission of blood for PCR evaluation for VBP. Unfortunately, even in severely ill animals, intracellular morulae are uncommon. When performed by experienced reference laboratories, PCR is sensitive for diagnosis of infection in patients that are rickettsemic and a positive result is consistent with a current infection. However, a negative PCR result does not rule out ehrlichiosis. Negative results are more likely in chronic infections or if patients have received prior antibiotic therapy, particularly tetracyclines.

The primary risk factor for infection is likely exposure to the vectors that transmit A. phagocytophilum. In the United States, anaplasmosis has only been diagnosed in cats living in the Northeast; the tick vector is Ixodes scapularis.17 In Europe, the vector for A. phagocytophilum is Ixodes ricinus. Most cats with a PCR-based diagnosis have access to outdoors; use of ectoparasite preventatives does not prevent infection in all cats. Anaplasmosis has been diagnosed in cats from 4 months to 13 years of age, with a median age of 2 years.¹⁷ There is no sex or breed predisposition.

Anaplasma phagocytophilum has been detected in cats in many countries, such as the United States, some European countries, Korea, and Brazil. However, the prevalence of exposure to A. phagocytophilum is higher than the prevalence of clinical disease. ¹⁷

Owners may or may not report exposure to ticks. Historical findings are often nonspecific. The most common clinical signs are lethargy and fever.17–19 Clinical signs are usually acute and may occur 3 days to 3 weeks after tick removal. Duration of clinical signs is 1 to 14 days.

Physical examination abnormalities include joint swelling and pain, conjunctivitis, ocular discharge, abdominal discomfort, ataxia, and pallor.^17–19 Tachypnea and increased lung sounds have also been described.

Borreliosis (Lyme disease) is caused by bacteria in the B. burgdorferi complex found in much of the world, including the United States (especially the upper Midwest, Northeast, mid-Atlantic, and parts of northern California) and Europe (Fig. 38.3). This bacterium can infect a range of mammals, including cats. In the United States and Europe, borreliosis is the most common vector-borne illness of humans. In North America, B. burgdorferi is transmitted by Ixodes scapularis and Ixodes pacificus ticks. In other countries, tick species such as Ixodes ricinus, Ixodes hexagonus, and Ixodes persulcatus are involved in transmission. Wildlife reservoirs for B. burgdorferi include mice, voles, shrews, chipmunks, squirrels, hedgehogs, and many bird species. Ticks acquire B. burgdorferi when they feed on a reservoir host and transmit the infection when they attach to a cat to feed. In endemic areas, cats are sometimes seropositive for B. burgdorferi, and the infection can be transmitted experimentally.^22,23

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