Studies on the North American arthropod-borne encephalitides. II. Eastern equine encephalitis in horses.

The research explores the potential role of horses in the transmission cycle of Eastern equine encephalitis (EEE), a mosquito-borne virus. The findings are drawn from observing incidences of EEE in horses and human beings, and by conducting experimental infection of horses, monitoring their response, and attempting to transmit the disease between horses.

Context of the Study

• The research was grounded on the premise that horses may be a vital link in the ecological cycle of the EEE virus. The speculation arises from previous patterns of the disease, where outbreaks in horses seemed to happen prior to human cases and appeared to spread from specific initial points. This pattern, however, was noted to not always be consistent.

Direct Observations of EEE Infections

• Epizootics, or disease events in nonhuman animals, of EEE in Louisiana during 1947 and 1949 represented some crucial case studies. The infection in horses appeared to expand from specific localized zones. However, the areas affected in the 1949 outbreak were not contiguous.
• In instances involving human infections, peaks in human cases followed those in horses by about one to two weeks. However, during an epidemic in the Dominican Republic in 1948-1949, incidents of the disease in humans coincided with those in horses, suggesting that observations of a staggered pattern may not always be significant.

Experimental Studies on Horses

• Evidence of EEE virus was found in the blood of naturally infected horses, even before clinical symptoms appeared. The virus was then successfully transmitted from an infected horse to a healthy one through Aedes solicitan mosquitoes.
• However, the virus was only found in blood samples for a short timeframe. Moreover, no traces of the virus were found in mosquitoes collected near infected horses during epizootics in Massachusetts in 1938 and in Louisiana in 1947 and 1949.
• The research also involved a meticulous examination of the response of horses to the EEE virus, introduced either through peripheral injection or infected mosquito bites. The researchers closely observed the clinical progression, duration, and the amount of circulating virus. The production of specific antibody and terminal pathologic changes in the central nervous system were also part of their observation parameters.
• During the inoculation studies, small amounts of virus were used to closely replicate the dosage that might be delivered in the field by vectors.