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Field and Laboratory Studies on Eastern Encephalitis in Warm- and Cold-Blooded Vertebrates^*

Richard O. Hayes, Joan B. Daniels, Herbert K. Maxfield AND Ralph E. Wheeler

A survey was made in Massachusetts during a nonepidemic period to determine whether wild mammals, amphibians, or reptiles served as hosts for eastern encephalitis (EE) virus. None of 276 small mammals tested was positive for virus, and only a single eastern cottontail rabbit was positive for EE antibody. None of the blood samples obtained from 123 amphibians and 190 reptiles was positive for virus. One snapping turtle was positive for EE antibody. The results of the survey suggest that neither small wild mammals nor cold-blooded vertebrates were frequently infected with EE in nature during the period of the investigation.

In the studies with experimentally infected animals, the birds reacted in the expected manner and virus circulated in their blood 1 to 4 days following inoculation. The amphibians and the mammals inoculated were relatively refractory to infection with EE virus, but most of the reptiles were found to be highly susceptible to infection. Virus in high titer was detected in reptiles following inoculation, and viremias lasting 2 and 3 weeks were observed in snakes and in turtles, respectively.

Both hemagglutination-inhibition (HI) and neutralizing antibody were found in each animal tested a week after viremia. With few exceptions, the neutralizing antibody persisted until the animal died or the study terminated. The HI was more transient. In a few animals preinoculation sera showed HI without neutralizing antibody, yet these animals developed viremia when injected with the virus. This finding suggests that results from HI tests alone may be a poor index of immune status.

An inoculated garter snake held in an outdoor cage and three inoculated spotted turtles held in a refrigerator served as overwintering reservoir hosts of EE virus. These reptiles were inoculated in November 1961 and still had viremia when bled 6 months later.

Further study is needed to determine whether reptiles, shown to be susceptible to EE virus and capable of overwintering it, are involved in the EE virus cycle as either enzootic or epidemic reservoir hosts, or as both.

^* This investigation was part of a joint study by the Disease Ecology Section, Technology Branch, Communicable Disease Center, Public Health Service, U. S. Department of Health, Education and Welfare and the Institute of Laboratories, Massachusetts Department of Public Health. It was supported in part by research grant E-3788(C1) from the National Institute of Allergy and Infectious Diseases, National Institutes of Health, U. S. Department of Health, Education, and Welfare.

Taunton Field Station, Virology Section, Laboratory Branch, Communicable Disease Center, Taunton, Massachusetts.

Virus Section, Diagnositic Laboratory, Massachusetts Department of Public Health, Boston, Massachusetts.

Department of Bacteriology and Preventive Medicine, Tufts University School of Medicine, Boston, Massachusetts.