Method of infection does not alter response of chicks and house finches towestern equine encephalomyelitis and St. Louis encephalitis viruses

The effects of method of infection and virus dose on the viremia and antibo dy responses of 1-wk-old chicks and after-hatching-year house finches to in fection with western equine encephalomyelitis (WEE) and St. Louis encephali tis (SLE) viruses were studied under laboratory conditions. Using a capilla ry tube technique, females from 2 strains of Culex tarsalis Coquillett mosq uitoes were estimated to expectorate from 1.0 to 1.7 log(10) plaque forming units (PFU) of WEE and from 1.9 to 2.2 log(10) PFU of SLE. Based on the pr oportion of parenterally infected females that transmitted and the number t hat blood fed during each experiment, virus doses per bird were estimated t o be 1.0-1.9 log(10) PFU for WEE and 1.4-2.3 log(10) PFU for SLE. When infe cted with comparable doses of WEE by subcutaneous inoculation, there was no significant difference in the duration or magnitude of the viremia respons e between birds infected by mosquito bite or syringe; few birds developed a viremia response after infection with SLE, precluding analysis. In chicken s, increasing the syringe dose of WEE from 0.3 to 1.7 log(10) PFU/0.1 ml sh ortened the time when viremia first appeared from 3 to 1 d postinfection an d increased the duration of the viremia period from 1 to 3 d, but did not a lter the maximum viremia titer. In house finches, increasing the syringe do se of WEE from 2.6 to 3.3 log(10) PFU/0.1 ml did not alter markedly the vir emia response. Most birds developed antibody detected by enzyme immunoassay (EIA) or plaque reduction neutralization test (PRNT). In chickens, WEE EIA levels and PRNT titers were higher for birds infected by syringe than by m osquito bite, whereas in house finches the pattern was reversed. For birds infected with SLE, there was overlap among groups infected by mosquito bite or syringe. These results indicate that subcutaneous syringe inoculation p rovides a biologically sound mode of infection that did not alter viremia a nd antibody responses when compared with infection by mosquito bite.