Ehrlichia ewingii Q&A (Sponsored by IDEXX)

What is Ehrlichia ewingii?

Ehrlichia ewingii is a tick-transmitted member of the alpha Proteobacteria that can induce intra-neutrophilic infection in dogs and acute febrile illness in people.

What are the clinical presentations of the disease that it causes?

Clinical signs can be highly variable among dogs. Following tick-transmission of E. ewingii, clinical signs will most often develop within 7 to 14 days. Clinical manifestations include fever, lethargy, lameness, polyarthropathy, and neurologic manifestations including ataxia, paresis, proprioceptive deficits, and vestibular dysfunction.

How is Ehrlichia ewingii different from other species of Ehrlichia, and what problems does that present?

In North America, dogs can be infected with E. ewingii, Ehrlichia canis, and Ehrlichia chaffeensis. Although the clinical presentations associated with infection with each Ehrlichia species can be similar with overlapping disease manifestations, E. ewingii appears to be somewhat unique in its ability to cause a neutrophilic polyarthritis. Historically the world literature relative to canine ehrlichial disease is biased toward E. canis; only recently, with the advent of PCR testing, have clinicians been able to definitively determine which Ehrlichia species is causing disease in their patients.

What role does the lone star tick play as a vector of Ehrlichia ewingii, and what other diseases (particularly human diseases) does it carry?

Although the most prevalent tick species to infest cats, dogs, people, and many wild animal species in the southern and central United States, Amblyomma americanum (the lone star tick) has been the least studied as a vector for canine and human disease. It is important to realize that the distribution of this tick species continues to move northward, with increasing sightings as far north as New York and Maine. A. americanum transmits E. ewingii and E. chaffeensis to dogs and people and Cytauxazoon felis to domestic and wild cats. The lone star tick also carries Borrelia lonestari and Rickettsia amblyommii, but the pathogenic potential for each of these organisms for dogs and people remains unclear.

What do you do with a dog that tests positive for Ehrlichia ewingii?

With the addition of an E. ewingii-specific peptide to the Ehrlichia spot on the SNAP® 4Dx® Plus Test (IDEXX Laboratories), veterinarians practicing in regions of the country with high numbers of A. americanum should anticipate an increase in the number of Ehrlichia positive SNAP results they see in both healthy and sick dogs. Because the SNAP platform is designed as a screening test, it is important to realize that antibody levels may not be detectable during the acute phase of infection (i.e., the first 7 to 14 days following tick transmission). However, the SNAP Test can be used to confirm seroconversion following an acute infection in dogs. Optimally, when a dog tests positive for Ehrlichia antibodies using the SNAP 4Dx Plus Test, the veterinarian should recommend a complete blood count and a serum protein measurement. If abnormalities are noted (anemia, thrombocytopenia, neutropenia, pancytopenia, hyperglobulinemia) a PCR test can be performed to confirm active infection with an Ehrlichia species or the dog can be treated with doxycycline (5 mg/kg every 12 hours or 10 mg/kg once daily for 4 weeks). Western blot testing is not available for E. ewingii testing because the organism has never been cultured. Immunofluorescence assay (IFA) testing is less specific to differentiate among species, but can be used to detect and quantify antibodies or to confirm seroconversion, if acute and convalescent sera are tested. To document seroconversion by SNAP Test or IFA, an acute serum sample should be collected and the dog should be retested in about 3 weeks. Seroconversion requires a fourfold increase in the IFA titer or a change from a negative to positive Ehrlichia SNAP Test result.