Pathogenesis and clinical disease
Ehrlichia ewingii is a small, obligate intracellular bacterium, and, following the bite of an infected tick, it invades granulocytes forming
membrane-bound, intracytoplasmic colonies of organisms known as morulae.14 The time required from tick attachment to pathogen transmission is unknown. Morulae can be observed within granulocytes
in as little as 12 days after experimental inoculation.3,15,16
Clinical illness associated with canine granulocytic ehrlichiosis is most often an acute febrile condition associated with
musculoskeletal signs. Reluctance to stand or walk, lameness, a stiff or stilted gait, and joint effusion are common findings
in E. ewingii-infected dogs and may be quite severe.12,17-19 Lethargy, anorexia, and central nervous system signs (e.g., head tilt, tremors, and anisocoria) may also be present.19,20 Rarely reported findings have included hemorrhage, weight loss, organomegaly, uveitis, pruritus, vomiting, and diarrhea.12,19-21 Onset of clinical signs generally occurs within 7 to 14 days following infection.
It is quite likely that many dogs infected with E. ewingii either remain clinically healthy or develop a brief, self-limiting illness, as has been reported in experimentally infected
dogs.15,16 The often mild pathogenicity following infection is further supported by the high seropositive rates among dogs lacking
historical evidence of clinical disease attributable to canine granulocytic ehrlichiosis, and the lack of reported mortality
due to E. ewingii infection.3,9,15 Immunosuppression may exacerbate or potentiate disease manifestations in infected dogs, as it apparently does in E. ewingii-infected people.22-24 Experimental infection with E. ewingii has been more successfully attained in dogs receiving either cyclophosphamide or glucocorticoids.3,16,25 Similarly, co-infection with other tick-transmitted pathogens may worsen disease manifestations in infected dogs.26
Evidence to date suggests that canine and human granulocytic ehrlichiosis causes only an acute illness in dogs and people.
This association is in contrast to the pathogenesis of canine monocytic ehrlichiosis, caused by E. canis, where the acute illness typically resolves spontaneously, after which the dog enters a period of subclinical infection,
followed in some cases by the development of chronic disease manifestations that can be quite challenging to treat.27-29 Due to the acute nature of canine granulocytic ehrlichiosis, disease manifestations have only been identified when vector
lone star ticks are active, predominantly during the late spring and summer.19,30 At the University of Missouri Veterinary Medical Teaching Hospital, from 32 dogs with granulocytic morulae and clinical
illness compatible with E. ewingii infection, only two were identified outside of the months May to August (in September and November) (Cohn, L: Unpublished data). Despite
the fact that illness documented to result from E. ewingii infection is acute in nature, actual infections can be persistent.15,19 It is possible that chronic E. ewingii infections may contribute to as-of-yet unrecognized disease manifestations or pathophysiologic consequences.