Role of poikilothermic animals in overwintering western equine encephalitis virus

Female Culex tarsalis mosquitoes were shown to feed on snakes in a large, outdoor cage containing wood and rock piles and natural vegetation. Feeding activity commence in the late summer when environmental temperatures begin to decline. Feedings activity also seemed to depend on environs where close association between mosquitoes and snakes can occur, such as in wood or rock piles. In anther study C. tarsalis was shown to feed on radioactive snakes contained in a large cage constructed on a dike in a isolated marshy area. The cage contained large wood an rock piles, tunnels and natural vegetation; one are of the cage extended into the water. The cage was enclosed by one-fourth inch hardware cloth which retained the snakes, but allowed mosquitoes to enter and leave at will. Eleven radioactive mosquitoes were collected from this cage during the period July 28 to August 12. During this period mosquitoes were collected approximately every three days. It was noteworthy that the engorged radioactive mosquitoes and snakes were always found closely associated in either the wood or rock piles and that these mosquitoes fed on these reptiles in spite of the presence of a large bird population in the area. Another study showed that one C. tarsalis infected with Western equine encephalitis (WEE) virus could transmit enough virus to a snake to cause a viremia. Four out of eight randomly selected snakes exhibited virus in their circulating blood after being fed upon by one infective mosquito; three out of four snakes were infected by the bite of two to three of these mosquitoes. Virus was detected in the blood of these reptiles at varying times following feeding of the mosquitoes. Five of the seven of the infected snakes developed blood virus titers greater than 10(5) plaque forming units (PFU) per ml. The duration of viremia in these animal also varied considerably; however, virus was detected in the blood of three out of seven of the infected snakes for fourteen days or longer; one snake demonstrated viremia for more that twenty-seven days. WEE virus was isolated on three different occasions from the blood of naturally infected snakes. Most of these isolations were made prior to the emergence of C. tarsalis from hibernation. This virus was also isolated form the blood of neonatal snakes born to naturally infected mothers; thus, demonstrating in this instance, that WEE virus can traverse the fetal membranes in snakes (Thamophis). When virus was no longer detectable in the blood of naturally infected adult snakes, eight of them were placed into a refrigerator of 4?C for one month; and following hibernation and three weeks' exposure to room temperatures, virus was detected in the blood of three out of eight of these reptiles. No virus wad detected in the blood of the numerous non-hibernated snakes during this time. In conclusion, evidence was presented that further indicated snakes may be the overwintering reservoirs of WEE virus; since it was shown that snakes may serve as blood sources for C. tarsalis in nature, that the feeding of infective C. tarsalis on a snake can cause a viremia in these reptiles, and that virus could be isolated from the blood of naturally infected snakes and their offspring.