Cost effectiveness of three arbovirus surveillance methods in northern California.

We compared the cost effectiveness of enzootic arbovirus surveillance in northern California by antibody detection in sentinel chickens, virus isolation from mosquitoes, and antibody detection in wild avian hosts. Total and annual recurring costs were determined for each method based on estimated personnel and actual material and travel costs for biweekly surveillance at 3 sites in the Sacramento Valley from May 1 through mid-October 1997 and 1998. Serologic detection of antibodies in wild birds was the most expensive method. Total costs associated with sentinel chickens and mosquitoes combined were less than half of those for the wild bird program. Recurring annual costs for the wild bird and mosquito methods were only slightly less than expenses for those methods during the 1st year of operation, which included nonrecurring setup costs. Recurring costs for sentinel chickens were reduced approximately 40% from total costs during the 1st year of the program and were <14% of recurring costs for wild bird serology. Exceptions and caveats of our analysis are discussed. When considering data from a companion paper on detection of enzootic virus transmission using the 3 methods, we concluded that the current system that combines sentinel chickens and virus isolation from mosquitoes is the most cost-effective and efficient surveillance program and should be retained. Future research efforts should investigate the costs and surveillance efficiency of modifications in the frequency of specimen collection and the placement of chicken flocks and mosquito traps.

The first reported case of California encephalitis in more than 50 years.

A recent case of California encephalitis, a rare mosquito-borne viral disease, represents only the fourth ever reported and the first since the initial three cases in 1945. This case was diagnosed retrospectively on the basis of a rise in antibody titer between acute- and convalescent-phase serum samples.

Patterns of avian seroprevalence to western equine encephalomyelitis and Saint Louis encephalitis viruses in California, USA.

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-seeking Culex tarsalis Coquillett 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 222,455 Cx. tarsalis females 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 where Cx. tarsalis host-seeking females hunt most frequently. These serosurveys provided important background information for planned host competence and chronic infection studies.

Response of house finches to infection with sympatric and allopatric strains of western equine encephalomyelitis and St. Louis encephalitis viruses from California.

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.

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

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.

Genetic and morphological characterization of the Aedes (Ochlerotatus) dorsalis (Diptera: Culicidae) group in North America.

An examination of the electrophoretically detectable variation among the North American members of the Aedes (Ochlerotatus) dorsalis group revealed large genetic differences among all 4 species. At least 9 of 18 loci examined (50%) were diagnostic for each species pair. However, morphological variation observed among species was low. Only Aedes canadensis (Theobald) was separated readily from the other members of this group [Aedes dorsalis (Meigen), Aedes melanimon Dyar and Aedes campestris Dyar & Knab] in all life stages. Characters traditionally used to separate the remaining 3 species were less reliable. In the adult female, Ae. melanimon may be distinguished from Ae. campestris by the scaling patterns of the wings and abdomen, but Ae. dorsalis could not be distinguished reliably by these characters. Adults of Ae. dorsalis may be separated reliably from those of Ae. campestris and Ae. melanimon only by the length of the subapical tooth relative to the length of the tarsal claw. Ae. melanimon was identified in the larval stage by the short mesothoracic hair 1. Eight larval characters differed between Ae. dorsalis and Ae. campestris. However, the ranges of these characters overlapped and no character was truly diagnostic. Genetic variation within species was low as measured by average heterozygosity and Nei's genetic distance coefficients. No allozymes were diagnostic for coastal and inland populations of Ae. dorsalis, and the pattern of genetic differentiation within this species did not correspond to the geographic location of the populations examined. Therefore, the genetic data did not support the hypothesis that Ae. dorsalis represents a complex of 2 or more cryptic species.

Temporal and spatial genetic variation within and among populations of the mosquito Culex tarsalis (Diptera: Culicidae) from California.

The genetic structure of 11 populations of Culex tarsalis Coquillett from California and 1 population from Nevada was examined at 18 loci using polyacrylamide gel electrophoresis. Six populations from northern and southern California were sampled repeatedly to determine if the genetic structure of Cx. tarsalis changes seasonally. Significant differences in allele frequencies at 13 different loci were seen in 3 populations over time as determined by contingency chi-square tests. Nei's genetic distance coefficients among different sampling dates was consistently < 0.025. The number of alleles per locus in these populations ranged from 1.6 to 2.7, whereas the average heterozygosity ranged from 0.086 to 0.228. No single locus was found to vary in a consistent pattern within all populations that were sampled repeatedly. These results indicate that Cx. tarsalis populations are genetically stable over time and that temporal variation is due to fluctuations in population size or immigration of genetically distinct individuals. In contrast, Cx. tarsalis did exhibit some differences in genetic structure that were related to geographical features including the Sierra Nevada and the Tehachapi Mountains of southern California. Genetically differentiated populations occurred in Nevada, southern and northeastern California, and the Central Valley of California. Little differentiation was observed among populations located in the Central Valley of California and those located at high elevations in the Sierra Nevada. Thus, in the populations examined, mountain ranges or arid conditions that limit the number of larval development sites appeared to be important barriers to the dispersal of Cx. tarsalis.

The distribution of species of the Aedes increpitus complex in the western United States.

Maps of the California and Oregon distribution of members of the Aedes increpitus complex (Aedes clivis Lanzaro and Eldridge, Aedes increpitus Dyar, and Aedes washinoi Lanzaro and Eldridge) are presented that are based on collections reported by Lanzaro and Eldridge (1992) and new collections from various sites, many in the Central Valley of California. Analysis of individually reared specimens by polyacrylamide gel electrophoresis and conventional morphological methods confirmed the diagnostic value of isozymes for these species and of larval head hairs for distinguishing Ae. clivis from other members of the complex. Other larval characters and pupal hairs did not appear to have diagnostic value. An additional site was discovered where apparent hybrids between Ae. washinoi and Ae. increpitus occur, and a single possible hybrid between Ae. washinoi and Ae. clivis was found at a site where these species had previously been reported to occur sympatrically.

Ecology of Jamestown Canyon virus (Bunyaviridae: California serogroup) in coastal California.

This paper reports the first isolation of Jamestown Canyon (JC) virus from coastal California and the results of tests for antibody to JC virus in mammals living in coastal California. The virus isolation was made from a pool of 50 Aedes dorsalis females collected as adults from Morro Bay, San Luis Obispo County, California. The virus isolate was identified by two-way plaque reduction-serum dilution neutralization tests done in Vero cell cultures. Sera from the mammals were tested for antibody to JC virus by a plaque-reduction serum dilution neutralization method. A high prevalence of JC virus-specific antibody was found in horses and cattle sampled from Morro Bay. This finding is additional evidence for the presence of a virus antigenically identical or closely related to JC virus in Morro Bay and indicates that the vectors of the virus in Morro Bay feed on large mammals. A high prevalence of virus-specific antibody was also found in horses sampled from Marin and San Diego counties. This finding suggests that viruses antigenically identical or closely related to JC virus are geographically widespread in coastal California.

Geographic distribution and serologic and genomic characterization of Morro Bay virus, a newly recognized bunyavirus.

More than 75,000 immature mosquitoes in three genera were collected from coastal California, reared to the adult stage, and tested for virus by plaque assay in Vero cell cultures. Twenty-six strains of Morro Bay (MB) virus, a newly recognized member of the California (CAL) serogroup, were isolated from Aedes squamiger, a pestiferous salt marsh mosquito species restricted to intertidal salt marshes in coastal California and Baja California. The geographic distribution of the isolates was 10 from San Luis Obispo County, one each from Santa Barbara and Orange Counties, and 14 from San Diego County. No virus isolations were made from 23,157 Ae. squamiger collected north of San Luis Obispo County (midpoint in the geographic range of this species in California). Thus, MB virus infection in Ae. squamiger appears to be restricted to the southern range of this species in California. Serum dilution neutralization tests indicated that MB virus represents a novel subtype of the California encephalitis (CE) serotype within the CAL serogroup. Comparative analyses of genomic sequence data from four geographically distinct MB virus isolates indicated that the isolates are genetically similar to each other and distinct from other CE serotype bunyaviruses. Phylogenetic analysis of nucleocapsid protein gene sequence data indicated that MB virus represents a distinct lineage within the CE serotype and thus supports the serologic classification of MB virus as a distinct CAL serogroup virus.

Natural vertical transmission of western equine encephalomyelitis virus in mosquitoes.

The mechanism by which western equine encephalomyelitis (WEE) virus and other mosquito-borne alphaviruses (Togaviridae) survive during periods of vector inactivity is unknown. Recently, three strains of WEE virus were isolated from adult Aedes dorsalis collected as larvae from a salt marsh in a coastal region of California. This provides evidence of vertical transmission of WEE virus in mosquitoes in nature. Vertical transmission in Ae. dorsalis and closely related mosquito species may be an important mechanism for the maintenance of WEE virus in temperate regions in North America where horizontal transmission of the virus is seasonal.

Isolations of Jamestown canyon virus (Bunyaviridae: California serogroup) from mosquitoes (Diptera: Culicidae) in the western United States, 1990-1992.

Nearly 80,000 immature and adult mosquitoes in three genera were collected in high-elevation (> 1,000 m) areas of California (68,229), Nevada (3,721), Oregon (5,918), and Washington (1,629) during 1990-1992 and tested for virus as adult males or females in 1,799 pools. Collections comprised primarily alpine Aedes in the Aedes communis (De Geer) group of the subgenus Ochlerotatus. Thirteen strains of Jamestown Canyon (JC) virus were recovered by plaque assay in Vero cell culture from three members of the Ae. communis group: 10 from Aedes tahoensis Dyar, 2 from Aedes cataphylla Dyar, and 1 from Aedes hexodontus Dyar. All isolates came from collections made in Alpine, Sierra, Tulare, or Tuolumne counties in the Sierra Nevada of California. Vertical transmission of JC virus in all three mosquito species was demonstrated by the isolation of virus from adult males or females reared from field-collected larvae or pupae. The prevalence of infected Ae. tahoensis was significantly higher in field-collected adult females than in reared adult males and females in Alpine County, which indicated that JC virus was being amplified by horizontal transmission. This study further incriminated Ae. tahoensis, Ae. cataphylla, and Ae. hexodontus as natural vectors of JC virus in California and greatly extended the known geographical range of this virus in the Sierra Nevada.

Vector competence of selected mosquito species (Diptera: Culicidae) for California strains of Northway virus (Bunyaviridae: Bunyavirus).

Selected mosquito species from Central Valley, coastal, and alpine habitats of California were evaluated for their vector competence for Northway (NOR) virus. Culiseta incidens Thomson, Culiseta inornata (Williston), and Anopheles freeborni Aitken were the only competent vectors when fed virus. Aedes sierrensis (Ludlow), as well as alpine snow pool Aedes (i.e., Ae. cataphylla Dyar, Ae. hexodontus Dyar, Ae. increpitus Dyar and Ae. tahoensis Dyar), Ae. melanimon Dyar, Ae. washinoi Lanzaro & Eldridge, Culex erythrothorax Dyar, and Cx. tarsalis Coquillett were highly refractory to peroral infection. Alpine snow pool species were poor vectors even following parenteral infection, but 21-35% of intrathoracically inoculated Ae. sierrensis from several lower elevation localities and 39-46% of parenterally infected Cx. tarsalis were able to transmit NOR virus per os. Perorally infected Cs. inornata transmitted NOR virus vertically to their F1 adult progeny at a low rate (minimal filial infection rate, 1:248).

Vector competence of alpine, Central Valley, and coastal mosquitoes (Diptera: Culicidae) from California for Jamestown Canyon virus.

Mosquitoes collected from alpine, Central Valley, and coastal habitats in California were evaluated for their vector competence for four strains of Jamestown Canyon (JC) virus. Three of the viral strains examined were isolated from alpine Aedes species collected in California, and one, the prototype JC virus, was isolated from Culiseta inornata (Williston) collected in Colorado. Alpine Aedes tahoensis Dyar, Ae. cataphylla Dyar, Ae. hexodontus Dyar, Ae. increpitus Dyar, Ae. clivis Lanzaro and Eldridge, and coastal Aedes washinoi Lanzaro and Eldridge were variably susceptible to alpine strains of JC virus. Infection rates ranged from 22 to 77%, and peroral transmission rates of the infected females ranged from 0 to 26%. The differences were related to both mosquito species and viral strain. Coastal populations of Cs. inornata, Ae. washinoi, and Ae. sierrensis (Ludlow) were incompetent vectors when fed an alpine strain of JC virus, whereas Ae. squamiger (Coquillet) and Ae. dorsalis (Meigen) were competent vectors. Peroral transmission rates following parenteral infection of females of most species were about twofold higher than those for perorally infected females. A population of Cs. inornata from the Central Valley was highly susceptible when fed an alpine strain of JCV and transmitted virus both horizontally and vertically. Alpine strains of JC virus also were transmitted vertically by Ae. tahoensis, Ae. washinoi, and Ae. squamiger following parenteral infection of females.

Vector competence of California mosquitoes for California encephalitis and California encephalitis-like viruses.

Mosquitoes collected from coastal, inland valley, and alpine locations in California were evaluated for their experimental vector competence for two viruses in the California serogroup (Bunyaviridae:Bunyavirus). Aedes squamiger, a coastal salt marsh mosquito, was an efficient vector of a California encephalitis (CE)-like virus isolated from its habitat (89% of the pledget-fed females became infected and 61% transmitted virus). Aedes dorsalis, a coastal mosquito, and Ae. melanimon, an inland valley mosquito, were competent vectors of prototype CE virus (98% and 100% of the pledget-fed females became infected and 56% and 30%, respectively, transmitted virus). Aedes squamiger and Ae. dorsalis transmitted both viruses vertically to one or more of 20 of their progeny. Culiseta inornata was susceptible to infection with both viruses, but 5% or less transmitted virus perorally. Alpine mosquitoes, Ae. cataphylla, Ae. increpitus, and Ae. tahoensis, became infected with both CE and CE-like viruses, but 3% or less transmitted virus. All species of mosquitoes were more efficient vectors of both viruses following intrathoracic inoculation than following pledget feeding, suggesting the presence of mesenteronal barriers.

Patrick Manson and the discovery age of vector biology.

There have been few scientists who have had a greater impact on the history of vector biology than Sir Patrick Manson (1844-1922). By demonstrating that mosquitoes became infected with microfilariae in the process of taking a blood meal, he became the first to prove an association between insects and pathogens causing human and animal diseases. He also contributed substantially to the discovery of mosquito transmission of malaria parasites and was a principal force behind the founding of the London School of Tropical Medicine and the Royal Society of Tropical Medicine and Hygiene. Manson's career is reviewed in historical context as well as in relation to modern concepts of vector biology.

Seroepidemiology of California and Bunyamwera serogroup bunyavirus infections in humans in California.

Several human populations in California were surveyed cross-sectionally and longitudinally for neutralizing antibodies to selected arthropod-borne bunyaviruses in the California and Bunyamwera serogroups. Overall, the prevalence of antibodies to California serogroup viruses was 6.4% in 702 individuals sampled during 1963-1988. Comparative antibody titers in individual sera indicated that 4.1% and 1.6% of these infections were caused by viruses similar or identical to Jamestown Canyon and California encephalitis, respectively. Evidence of prior infection with the Jamestown Canyon serotype was found in 10% of 118 humans employed outdoors in high elevation areas and sampled in 1988, including 5 of 16 persons (31%) employed as rangers patrolling in remote forests and meadows. This probably reflects increased exposure to bites of boreal mosquitoes that breed in pools of melted snow. Antibodies to Bunyamwera serogroup viruses, including the Northway serotype, which was recently shown to be enzootic in California, were found in only 2 of 702 humans studied. No seroconversions were detected to selected California or Bunyamwera serogroup viruses in paired samples from 392 humans, including 349 patients with acute central nervous system disease or undifferentiated febrile illnesses who were sampled during 1963-1988, and thus these viruses are currently unconfirmed as human pathogens in California.

Conservation of the names Culex stigmatosoma and Culex thriambus.

As a result of action by the International Commission on Zoological Nomenclature, the name Culex peus Speiser is suppressed and never to be used as a valid name, and the names Culex stigmatosoma Dyar and Culex thriambus Dyar are conserved for the western banded foul-water mosquito and the related southwestern U.S. species, respectively.

Occurrence and evolutionary significance of a California encephalitis-like virus in Aedes squamiger (Diptera: Culicidae).

More than 12,000 Aedes increpitus Dyar and 4,600 Aedes squamiger (Coquillett) were tested for the presence of arboviruses to test the hypothesis that there is a coevolutionary relationship between Aedes (Ochlerotatus) mosquitoes and California serogroup viruses. Five strains of a California encephalitis-like virus were isolated from adults reared from larvae of Ae. squamiger collected in January 1989 from a coastal salt marsh at Morro Bay, San Luis Obispo County, California. Viruses were isolated in Vero cell cultures and serotyped by cross-neutralization tests. These isolates represent the first arboviruses isolated from this species. On the basis of morphology, Aedes squamiger has been included in the Aedes stimulans group of the subgenus Ochlerotatus. Other species within the Ae. stimulans group are vectors of California (CAL) serogroup viruses elsewhere in North America. Analysis of isozyme variability supports the inclusion of Ae. squamiger in the Ae. stimulans group and suggests that coastal populations of Ae. increpitus are the closest California relatives of Ae. squamiger. Recovery of virus from Ae. squamiger reinforces the relationship between CAL serogroup viruses and Aedes (Ocherlotatus) mosquitoes. However, the failure to isolate virus from large samples of Ae. increpitus from coastal and low elevation inland habitats suggests a complex evolutionary history involving both vertical and horizontal transmission mechanisms.