Development and duration of antibody response against Ehrlichia equi in horses.

The research sought to determine and examine the development and duration of antibodies against an infection called Ehrlichia equi in horses. The study found that antibodies against the infection rise anywhere between 19 to 81 days after the onset of the disease, indicating that the testing for antibodies may not be a helpful method of early diagnosis.

Study Methods

• The researchers carried out a prospective study involving 13 horses confirmed to have acute E equi infection.
• In order to identify horses with naturally acquired equine granulocytic ehrlichiosis (EGE), a condition caused by E equi, sequential serum sampling was carried out in Connecticut and New York in 1995 and 1996.
• The horses included in the study exhibited clinical signs of EGE, such as fever without respiratory involvement.
• The infection was confirmed through polymerase chain reaction (PCR) detection of E equi DNA and by observing a minimum fourfold increase in total antibody count using indirect fluorescent antibody staining methods. DNA sequencing further corroborated the infection.

Study Results

• At the beginning of the disease, 11 out of 13 horses did not have any detectable antibodies against the infection in their serum samples.
• Seroconversion, the period during an infection when the antibodies begin to be detectable, was noticed in samples taken between 19 to 81 days after the disease began.
• The researchers found that the median time to peak antibody response was 46 days after disease onset and a median titer of 1:320 was recorded.
• For 11 of the 13 horses, antibody titers fell to less than or equal to 1:40, approximately 215 days after disease onset.

Study Conclusions

• The infection, E equi, was found in horses in the northeastern United States and was determined to be causing EGE.
• The concentration of antibodies against E equi was found to increase within 19 to 81 days of the onset of the disease but remained low during the early weeks of infection, suggesting that antibody detection may not be an effective method for early diagnosis.
• The researchers suggested the possibility that disease may develop from reinfection, and retrospective serologic studies carried out to determine exposure to E equi may reflect a disproportionate number of negative reactions.