A case-control study of an outbreak of Eastern Equine Encephalomyelitis in Michigan (USA) equine herds in 1991

Abstract

Eastern Equine Encephalomyelitis (EEE) is a potentially fatal arboviral disease affecting equids, humans, commercially raised birds, and other vertebrate species. In Michigan, outbreaks of the disease in equids have been reported sporadically from as early as 1942. In 1991, 55 equids were reported to be affected by EEE. A retrospective case control study of the 1991 outbreak was conducted to identify specific management and environmental risk factors that may have been associated with the occurrence of EEE in Michigan equine herds. The case group included equine herds having positive or suspect EEE affected equids during 1991. The control group included equine herds having no reported EEE affected equids in 1991. Data were collected from 100 Michigan equine operations between May and November 1992. Management risk factors assessed included operation type, herd size, vaccination and insect repellent use, and stabling methods. Environmental risk factors focused on the operation setting, including types of land cover on the operation and in the surrounding area. Multiple logistic regression was used to calculate odds ratios (OR) for management and environmental risk factors on a herd level. Annual vaccination against EEE (OR = 0.14; P < 0.003) and the use of insect repellent methods (OR = 0.04; P = 0.02) decreased the risk of EEE occurring in Michigan equine herds. In contrast, when wooded land (OR = 3.70; P = 0.03) and swamp land (OR = 2.38; P = 0.14) were found on the operation, the risk of EEE occurring increased.