Investigations of the vertebrate hosts of eastern equine encephalitis during an epizootic in Michigan, 1980.

The study focuses on a detailed examination and exploration of the natural hosts for the eastern equine encephalitis virus during an outbreak that took place in Michigan in 1980. It emphasizes the role of horses, birds, and small mammals that were sampled from the affected regions of St. Joseph and Kalamazoo counties in Michigan.

Sampling Process

• The researchers collected samples from a variety of vertebrate hosts (horses, birds, and small mammals) from specific areas in St. Joseph and Kalamazoo counties in Michigan over the period from 25 August to 5 September 1980.
• Blood samples from four horses sick with encephalitis were obtained and tested, as well as samples from 16 of 39 horses who shared pastures with the sick animals.
• Samples were also taken from 24 draft horses from a different site in St. Joseph County, as well as 28 small mammals captured at sites where cases of encephalitis were occurring among horses.
• A large number of blood samples from 401 wild birds representing 42 species were taken from eight different sites in both counties, and samples from five species of domestic birds, mostly chickens and ring-necked pheasants, were also taken.

Detection and Analysis

• Neutralizing (N) antibody against EEE virus was present in the serum of the four acutely ill horses and 16 of their pasture mates, which accounted for 46.5% of all sampled horses. No EEE virus was detected in these samples, but a strain of Cache Valley virus was found in the blood of one of the 24 draft horses.
• The study found no viruses or EEE virus antibodies in the blood samples from small mammals. In contrast, six strains of EEE virus, five of Highlands J virus, and one of Flanders virus were detected from the blood of wild birds. Furthermore, about 29.9% of the wild birds had EEE antibody.
• The presence of EEE virus was confirmed in six strains isolated from 152 ring-necked pheasants, with three of these isolates coming from the brains of dead birds. About 13% and 9% of the pheasants from two small flocks in backyard pens and a large game farm respectively showed presence of the EEE virus antibody.

Major Findings of the Research

• The exact source of the EEE virus and the key factors that contributed to this outbreak of the virus are still not clearly known. The major point of the epizootic seems to be an explosively expanded transmission of the virus from some endemically existing baseline.
• The study further confirmed the presence and active transmission of EEE virus in horses, and wild and domestic birds in the counties under study, whereas no presence of the EEE virus was detected in small mammals.