Simulation studies of St. Louis encephalitis and West Nile viruses: the impact of bird mortality

West Nile virus (WNv) has spread through much of the eastern United States following its introduction in 1999, and arrived in Florida in 2001. Prior to its arrival, we anticipated that its transmission cycle was likely to be similar to that of St. Louis encephalitis virus (SLEv). However, high levels of avian mortality have been reported for WNv in the northeastern United States, and it was unknown how this would impact the transmission dynamics of WNv. Simulation models were used to compare the two viruses by considering the impact of bird mortality on the transmission dynamics of arboviruses in south Florida. Transmission models without disease-induced mortality (SLEv) were compared with models including disease-induced mortality (WNv). Disease-induced mortality depressed transmission, eliminating epizootics in two of 14 simulations that were epizootic without the additional mortality. In both models, the most important factor in the likelihood of epizootics was mosquito population size; the mosquito mortality rate was also important. The additional avian mortality altered the factors most important in the size and timing of epizootics, although it did not always directly affect the outcome of the simulations. In some cases, low-level transmission occurred prior to the epizootic peak. When disease-induced avian mortality was included in the simulations, appreciable numbers of dead birds occurred prior to high levels of infection in mosquitoes. This has implications for the use of dead birds as a surveillance tool monitoring the spread and transmission of WNv.

St. Louis encephalitis: clinical and epidemiologic aspects, Mississippi, 1974

During July, August, and September 1974, 23 confirmed or presumptive cases of St. Louis encephalitis occurred in Mississippi. Attack rates were highest in the northwest corner of the state and among persons over 70 years of age. Rural and urban residents had similar attack rates. Nine persons have not resumed their pre-illness level of activity because of residual neurologic impairment six months after their illness. Additional cases of St. Louis encephalitis are expected in the next few years.

The epidemiology of St. Louis encephalitis in Dallas, Texas, 1966

An epidemic of St. Louis encephalitis (SLE) occurred in Dallas, Texas, in the summer of 1966. A total of 545 suspected cases within Dallas city and county were reported, of which 145 were laboratory-confirmed as SLE virus infection. The greatest concentration of cases occurred in lower socioeconomic areas of the central part of the city in black populations. The attack rate and mortality rate increased markedly with age. The overall attack rate was 15.2 per 100,000, with a case fatality rate of 9.7%. During the course of the epidemic, most of the county was sprayed aerially with an ultra-low volume (ULV), high-concentration malathion mist. The effects of this treatment cannot be adequately assessed from the human epidemiologic aspect alone, but the spraying clearly reduced the number and infection rate of the vector mosquitoes.

The epidemiology of St. Louis encephalitis in Corpus Christi, Texas, 1966

In the summer of 1966, an epidemic of St. Louis encephalitis occurred in Corpus Christi, Texas, coincident with one occurring in Dallas about 563 km to the north. Among the 76 cases confirmed in Corpus Christi, there were two deaths; the attack rate was 41.0 per 100,000. In contrast with a concurrent outbreak in Dallas and the 1964 outbreak in Houston, attack rates were much higher in populations of the upper socioeconomic districts. This distribution may have resulted from the combined effects of an unusual concentration of vector mosquito breeding sites in storm sewers in the upper socioeconomic districts and a higher degree of residual immunity in the residents of the lower socioeconomic areas.

Influence of sex and age on natural resistance to St. Louis encephalitis virus infection in mice

A difference was observed in susceptibility of adult male and female mice to St. Louis encephalitis (SLE) virus as measured by the death rate after intravenous challenge. Female mice that had susceptibility similar to that of males at 2 months of age had increased resistance to SLE virus at 3 and 4 months of age. The increased resistance occurred after sexual maturity, indicating that the resistance factor possibly was related to an aging process in the female. The susceptibility of male mice remained unchanged over the 2- to 4-month period. Neither pregnancy nor castration had any effect on resistance of adult mice to St. Louis encephalitis virus.