Bruce F. Eldridge

Patterns of Avian Seroprevalence to Western Equine Encephalomyelitis and Saint Louis Encephalitis Viruses in California, USA

Abstract Temporal and spatial changes in the enzootic activity of western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses were monitored at representative wetland study sites in the Coachella, San Joaquin, and Sacramento valleys of California from 1996 to 1998 using three methods: (1) virus isolation from pools of 50 host-seekingCulex tarsalisCoquillett females, (2) seroconversions in flocks of 10 sentinel chickens, and (3) seroprevalence in wild birds collected by mist nets and grain baited traps. Overall, 74 WEE and one SLE isolates were obtained from 222,455Cx. tarsalisfemales tested in 4,988 pools. In addition, 133 and 40 seroconversions were detected in 28 chicken flocks, and 143 and 27 of 20,192 sera tested from 149 species of wild birds were positive for antibodies to WEE and SLE, respectively. WEE was active in all three valleys, whereas SLE only was detected in Coachella Valley. Seroconversions in sentinel chickens provided the most sensitive indication of enzootic activity and were correlated with seroprevalence rates in wild birds. Avian seroprevalence rates did not provide an early warning of pending enzootic activity in chickens, because positive sera from after hatching year birds collected during spring most probably were the result of infections acquired during the previous season. Few seroconversions were detected among banded recaptured birds collected during spring and early summer. Age and resident status, but not sex, were significant risk factors for wild bird infection, with the highest seroprevalence rates among after hatching year individuals of permanent resident species. Migrants (with the exception of mourning doves) and winter resident species rarely were positive. House finches, house sparrows, Gambel’s quail, California quail, common ground doves, and mourning doves were most frequently positive for antibodies. The initial detection of enzootic activity each summer coincided closely with the appearance of hatching year birds of these species in our study areas, perhaps indicating their role in virus amplification. Bird species most frequently positive roosted or nested in elevated upland vegetation, sites whereCx. tarsalishost-seeking females hunt most frequently. These serosurveys provided important background information for planned host competence and chronic infection studies.

Impact of climate variation on mosquito abundance in California.

ABSTRACT Temporal variation in the abundance of the encephalitis virus vector mosquito, Culex tarsalis Coquillet, was linked significantly with coincident and antecedent measures of regional climate, including temperature, precipitation, snow pack, and the El Niño/Southern Oscillation anomaly. Although variable among traps, historical records that spanned two to five decades revealed climate influences on spring and summer mosquito abundance as early as the previous fall through early summer. Correlations between winter and spring precipitation and snow pack and spring Cx. tarsalis abundance were stronger than correlations with summer abundance. Spring abundance was also correlated positively with winter and spring temperature, whereas summer abundance correlated negatively with spring temperature and not significantly with summer temperature. Correlations with antecedent climate provide the opportunity to forecast vector abundance and herefore encephalitis virus risk, a capability useful in intervention decision support systems at local and state levels.

Method of Infection Does Not Alter Response of Chicks and House Finches to Western Equine Encephalomyelitis and St. Louis Encephalitis Viruses

Abstract The effects of method of infection and virus dose on the viremia and antibody responses of 1-wk-old chicks and after-hatching-year house finches to infection with western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses were studied under laboratory conditions. Using a capillary tube technique, females from 2 strains of Culex tarsalis Coquillett mosquitoes were estimated to expectorate from 1.0 to 1.7 log10 plaque forming units (PFU) of WEE and from 1.9 to 2.2 log10 PFU of SLE. Based on the proportion of parenterally infected females that transmitted and the number that blood fed during each experiment, virus doses per bird were estimated to be 1.0–1.9 log10 PFU for WEE and 1.4–2.3 log10 PFU for SLE. When infected with comparable doses of WEE by subcutaneous inoculation, there was no significant difference in the duration or magnitude of the viremia response between birds infected by mosquito bite or syringe; few birds developed a viremia response after infection with SLE, precluding analysis. In chickens, increasing the syringe dose of WEE from 0.3 to 1.7 log10 PFU/0.1 ml shortened the time when viremia first appeared from 3 to 1 d postinfection and increased the duration of the viremia period from 1 to 3 d, but did not alter the maximum viremia titer. In house finches, increasing the syringe dose of WEE from 2.6 to 3.3 log10 PFU/0.1 ml did not alter markedly the viremia 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 mosquito 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 provides a biologically sound mode of infection that did not alter viremia and antibody responses when compared with infection by mosquito bite.

Seasonal Abundance of Culex tarsalis and Culex pipiens Complex Mosquitoes (Diptera: Culicidae) in California

ABSTRACT Large-scale patterns in the seasonal abundance profiles of the arboviral mosquito vectors Culex tarsalis Coquillett and the species of the Culex pipiens complex were described based on a decade of counts from 868 New Jersey light traps located throughout the urban and agricultural areas of California. Mean seasonal abundance profiles varied markedly among hydrologic regions. For all species, abundance increased earlier and declined later in drier, warmer southern regions, but variation could not be explained fully by latitude. The observed patterns may be driven by temperature, availability of larval habitats, and for the Cx. pipiens complex, the taxonomic composition of local populations.

Response of House Finches to Infection with Sympatric and Allopatric Strains of Western Equine Encephalomyelitis and St. Louis Encephalitis Viruses from California

Abstract Adult house finches from Kern County were inoculated subcutaneously with recent sympatric and allopatric isolates of western equine encephalomyelitis and St. Louis encephalitis (SLE) viruses made from Culex tarsalis Coquillett collected in Kern County and Coachella Valley, CA, respectively. Virulence, as measured by the amplitude of the viremia response during days 1 and 2 postinfection, varied significantly among strains, but independently of geographic origin. The intensity of the immune response, as measured by an enzyme immunoassay and a plaque reduction neutralization test, seemed to be independent of virulence, especially for SLE where some strains failed to produce a detectable viremia but elicited a strong antibody response. Our preliminary data indicated that strain virulence may be associated with the level of enzootic activity during the year of isolation.

Temporal connections between Culex tarsalis abundance and transmission of western equine encephalomyelitis virus in California.

Definition of targets for vector control requires an understanding of the relationship between vector abundance and the intensity of arbovirus transmission. Using an extensive surveillance dataset with observations from sentinel chicken flocks and mosquito traps paired in time and space, hierarchical autoregressive logistic regression models were developed to predict the probability of seroconversion in chickens for western equine encephalomyelitis virus (WEEV) based on the relative abundance of the principal vector, Culex tarsalis. After adjustments for confounders, the abundance of Cx. tarsalis 29-42 d before the date of chicken sampling was credibly associated with the risk of WEEV transmission in both the Central and Coachella Valleys, and a doubling of relative Cx. tarsalis abundance was associated with a 58% increase in the odds of seroconversion. The critical time windows identified in our study highlight the need for surveillance of vector populations and forecasting models to guide proactive vector control measures before the detection of transmission to sentinel chickens.

Chapter 172 – Mosquitoes

Publisher Summary This chapter discusses the mosquitoes. They are small flying insects and are related to other members of the order Diptera, the “two-winged flies.” The immature stages of larvae are aquatic and live in stagnant water sources in every biogeographic region of the world. Adult female mosquitoes of most species feed on blood of vertebrates, including humans, and this habit has resulted in great economic and public health significance for this group of insects. There are well over 3000 species and subspecies of mosquitoes in the world. They occur in a variety of habitats, ranging from deserts at or below sea level to high mountain meadows at elevations of 3000 m or more. Adult mosquitoes are terrestrial flying insects; immature stages are aquatic. Larvae and pupae of the various species can be found in ponds, ditches, puddles, swamps, marshes, water-filled rot holes of trees, rock pools, axils of plants, pools of melted snow, water in discarded tires, tin cans, and other artificial containers. Some species are most active in the warmest part of the year, whereas others are adapted to cool temperatures. Many species of mosquitoes are rarely encountered and seldom pose a threat to the health or well-being of humans and domestic animals. However, other species are abundant, frequently encountered, and readily attack people, their pets, and their livestock. Some of these species transmit microbial organisms that cause malaria and encephalitis, and other severe diseases of humans and other vertebrates. Mosquitoes are also commonly used as research objects in a wide range of biological studies.

WILLIAM EMERY HAZELTINE II: REBEL WITH A CAUSE1

Today we honor a man who made a difference. He was a medical entomologist who had a varied career in the field of mosquito biology and control, but he will forever be remembered as a man who fought in the trenches of the pesticide controversy from roughly 1960 until the end of his life, and who made the safe and efficient use of pesticides in public health a personal crusade. Some of you may wonder why I was selected to give this lecture. I certainly have very little background in the chemical control of mosquitoes. Actually, Bill and I had much in common. We were both born and raised in San Jose, California, and both graduated from San Jose State University and Purdue University. At Purdue, we had the same major professor, Leland Chandler. Because Bill was a few years older than me, we were never classmates. We first met when I arrived at Purdue in 1963, and Bill was in Lafayette, Indiana, to put the finishing touches on his Ph.D. dissertation. The annual meeting of the Entomological Society of America was held in Philadelphia that year, and Leland Chandler, known to all as ‘‘Chan,’’ offered the two of us, plus another student, a ride from Lafayette to Philadelphia for the meeting. The trip started off quietly enough, until one of us brought up the subject of evolution. That launched a vigorous debate that lasted until we were to the Philadelphia city limits. The other student that rode along refused to ride back with us to Lafayette. After I joined the University of California faculty in 1986, Bill and I had frequent interactions, and although I didn’t always agree with him, I grew to have considerable respect for his professional accomplishments and for his honesty and conviction. Mosquito abatement agencies in California benefited from his work, and so did members of the American Mosquito Control Association (AMCA). I also valued Bill’s friendship. I think that to Bill, family and friends were at the top of his priorities. Bill Hazeltine spent the majority of his professional career in the field of mosquito abatement in California. He was an advocate for the use of mosquito control to protect people from mosquitoes and the disease agents they transmit, and he believed chemical control to be a necessary part of the