In many parts of the United States, infection of dogs with the pathogen Ehrlichia ewingii is far more common than infection with the better-known Ehrlichia canis. Although these two ehrlichial pathogens share a phylogenetic similarity, there are many important differences, including
tick vectors, host-cell tropisms, disease manifestations, geographic restrictions, and zoonotic potential. In fact, E. ewingii produces morulae in granulocytes rather than monocytes, is transmitted by a highly prevalent environmental tick (Amblyomma americanum, the lone star tick) as compared with the localized kennel tick (Rhipicephalus sanguineus, the brown dog tick), and often causes acute lameness or polyarthritis as a primary component of the dog's clinical presentation.
It is important for veterinarians practicing in E. ewingii-endemic regions of the south central and southeastern United States to be familiar with tick transmission, disease consequences,
diagnosis, treatment, and the zoonotic implications that are particular to E. ewingii.
Transmission and geographic distribution
The only proven competent vector for the trans mission of canine granulocytic ehrlichiosis is Amblyomma americanum, the lone star tick. Although E. ewingii infection has also been documented in dogs from both South America and Africa, the tick species responsible for natural transmission
in those regions remains to be determined.1,2 Transstadial transmission in the lone star tick assures that ticks can be infected during all three stages of the life
cycle (larva, nymph, and adult) and remain competent to transmit E. ewingii at their next blood meal.3,4
White-tailed deer, the major host species for A. americanum, apparently serve as the major reservoir for E. ewingii. Not surprisingly, E. ewingii infections occur most often in the southeastern and south central United States where both whitetailed deer and lone star
ticks are plentiful (Figure 1).5-8 In fact, dogs are more likely to be seropositive for E. ewingii than for either E. canis or Ehrlichia chaffeensis in Missouri, Oklahoma, Arkansas, Kansas, Georgia, Alabama, Mississippi, Tennessee, Florida, Virginia, and New Jersey.9 The average seroprevalence for E. ewingii is 8.5% in these states.9 Lone star ticks are aggressive ticks that commonly feed on dogs, people, and numerous wildlife species.10,11 As a result, serologic evidence supporting exposure to E. ewingii is very common in A. americanum-endemic states. Antibodies against E. ewingii have been detected in 26% to 44.8% of randomly sampled dogs in Oklahoma, Missouri, and Arkansas, with 9% to 20% testing positive
for E. ewingii DNA by polymerase chain reaction (PCR).9,12,13
Figure 1. This map shows the distribution of Amblyomma americanum (the lone star tick; inset) in the United States.