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The prevalence of antibodies to Ehrlichia equi in horses from the foothill regions of northern California and from the Sacramento valley (non-foothill area) was determined, using an indirect fluorescent antibody test. Horses from foothill regions had a higher prevalence of seropositivity (10.4%) and higher titer (1:10 to 1:80) than did those from non-foothill regions (3.1%; titer ≤ 1:10). Fifty percent of healthy horses on a foothill farm enzootic for E equi had titer to E equi, suggesting that infection with E equi can be subclinical. Six veterinarians surveyed from northern California diagnosed clinical E equi infection in 38 horses during 1985-1986 based on clinical signs of infection and observation of E equi inclusion bodies in neutrophils on blood smears.

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in Journal of the American Veterinary Medical Association


The original objective was to determine seroprevalence of Ehrlichia risticii antibody among horses in California. On the basis of the unexpected results of the survey, an investigation into the accuracy and reproducibility of results of the indirect fluorescent antibody ( ifa) test for E risticii was carried out.


Prospective, seroprevalence study.


Healthy horses (n = 655) and horses with clinical signs of equine monocytic ehrlichiosis ( eme; n = 514) from various regions of California.


The ifa test was performed. Results were compared with results of an elisa and with results of western immunoblot analysis.


Overall, 104 of 655 (15.9%) healthy horses had evidence of an antibody response. However, 84 of 514 (16.3%) horses with clinical signs of eme also had positive test results, and of the 8 seropositive diseased horses for which paired (acute and convalescent) samples had been submitted, only 1 had a rise in antibody titers between the acute and convalescent samples. Comparison of results for the ifa test, elisa, and western immunoblot analysis revealed a high rate of false-positive results for the ifa test. Subsequent studies suggested that routine vaccination of horses with non-E risticii vaccines may have contributed to the false-positive reactions.

Clinical Implications—

The data failed to provide conclusive evidence of E risticii infection among California horses. Owing to the high percentage of false-positive test results, caution is advised when using the ifa test to diagnose eme in horses or to determine the necessity for E risticii vaccination.

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in Journal of the American Veterinary Medical Association


Objective—To determine susceptibility of cattle to infection with Ehrlichia equi and the agent of human granulocytic ehrlichiosis (HGE).

Design—Experimental disease and prevalence survey.

Animals—6 cattle, 2 horses, and 2,725 serum samples from healthy cattle.

Procedure—2 cattle and 1 horse were inoculated with E equi, 2 cattle and 1 horse were inoculated with the HGE agent, and 2 cattle served as sham-inoculated controls; inoculated animals were evaluated via clinical, hematologic, serologic, and real-time polymerase chain reaction tests. Prevalence of antibodies against E equi in 2,725 healthy cattle was determined by use of an indirect immunofluorescent technique.

Results—No abnormal clinical or hematologic findings or inclusion bodies within granulocytes were observed in the cattle after inoculation, and results of all polymerase chain reaction tests were negative. Seroconversion in inoculated cattle developed 10 to 12 days after inoculation (reciprocal titers, 160). Both horses developed clinical signs of ehrlichiosis. Five of 2,725 (0.18%) cattle were seropositive for E equi, with titers ranging from 20 to 80. All seropositive cattle originated from the same tick-rich region in the Sierra Nevada foothills.

Conclusions and Clinical Relevance—Results suggest that cattle are not susceptible to infection with E equi or the agent of HGE and that prevalence of exposure to E equi in healthy cattle is low. Therefore, E equi and the agent of HGE are likely of negligible importance for cattle in North America. (J Am Vet Med Assoc 2001;218:1160–1162)

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in Journal of the American Veterinary Medical Association