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Eight enzootic strains of western equine encephalomyelitis (WEE) virus isolated from Culex tarsalis or Aedes melanimon collected in several geographic areas of California were evaluated for their virulence in suckling mice, adult mice, and one-day-old baby chickens. The epidemic Fleming strain and the cloned B628(Cl 15) variant were used as virulent and avirulent control viruses, respectively, in adult mice. Enzootic strains of WEE virus were grouped into three phenotypes on the basis of their neurovirulent and neuroinvasive properties in adult mice. Three strains possessed high neurovirulence and were neuroinvasive; three strains had intermediate neurovirulence and lacked neuroinvasiveness; and two strains had low to nil neurovirulence and were non-neuroinvasive. In fact, five of the eight enzootic strains lacked neuroinvasiveness. Interestingly, highly virulent enzootic strains of WEE virus were isolated from Cx. tarsalis collected in the Sacramento Valley during 1994 and 1995 in the absence of identified human disease. The Fleming strain, the B628(Cl 15) variant, and four enzootic strains from the Sacramento Valley were virulent for baby chickens following subcutaneous inoculation. Thus, inoculation into baby chicks cannot discriminate between WEE viruses that are virulent and avirulent for adult mice.

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.

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.

We conducted a study to look for a simian counterpart of human T-lymphotropic virus (HTLV) in wild-caught monkeys in the Republic of Panama. Serum specimens were obtained from 102 monkeys (Ateles fusciceps, n = 75; Alouatta villosa, n = 18; and Cebus capucinus, n = 9) captured in Panama's Darien rain forest in 1979-1980. Specimens were screened for HTLV antibody by enzyme-linked immunosorbent assay, and reactive specimens were further tested by Western blot. None of the 102 specimens were seropositive for HTLV. Our findings provide no evidence for an HTLV-like virus in New World primates from Panama, but the sample size was small, and further studies are warranted.

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.

Eight previously untyped Bunyamwera serogroup bunyaviruses that had been isolated from mosquitoes collected in California and Oregon between 1969 and 1985, were identified by cross-neutralization tests. Four viruses from Anopheles freeborni and a virus from Aedes sierrensis collected in Butte County in the Central Valley of California in 1970-71 were shown to belong to the Northway serotype. The existence of a Northway serotype virus in California had been inferred from previous serologic surveys of deer and horses, but these are the first Northway serotype viruses to be identified from the contiguous United States or from An. freeborni. This is also the first isolation of any virus from Ae. sierrensis. Two viruses from Culiseta inornata collected in Umatilla County, Oregon in 1969 may represent a new serotype in the Bunyamwera serocomplex of the Bunyamwera serogroup. The name "Stanfield" is proposed for this serotype, which is closely related to the Cache Valley and Northway bunyavirus serotypes. A virus from Ae. taeniorhynchus collected in San Diego County, California in 1985 was confirmed to belong to the Main Drain serotype. Previously, Main Drain serotype viruses in California have been associated principally with Culicoides variipennis.

More than 28,000 mosquitoes in four genera were collected from high elevation (greater than or equal to 1,000 m) areas of California during 1988-89 and tested for virus by plaque assay in Vero cells. Viruses were serogrouped by enzyme immunoassay and serotyped by cross-neutralization. Six strains of Jamestown Canyon virus in the California serogroup were isolated from three species of boreal Aedes in the Aedes communis group of the subgenus Ochlerotatus. All isolates were from mosquitoes collected in Alpine County at approximately 2,300 m elevation in the Sierra Nevada. These included one virus from a pool of male Aedes cataphylla collected in immature stages, which is evidence for vertical transmission; four viruses from adult female Ae. communis (sens. lat.); and one virus from adult female Aedes hexodontus. These are the first isolations of viruses from boreal Aedes mosquitoes in California and the first reported isolations of Jamestown Canyon virus from Ae. cataphylla or Ae. hexodontus.

Preliminary studies found that 9% of Guaymi Indians from Bocas del Toro province have antibody to human T cell lymphotropic virus (HTLV-I/II). The present study enrolled 317 (21% of the population) Guaymi Indians from Changuinola, the capital of Bocas del Toro province and 333 (70% of the population) from Canquintu, an isolated rural village. Demographic information and family relationships were ascertained and subjects were screened for neurologic diseases. Serum specimens were screened by an enzyme-linked immunosorbent assay for HTLV-I/II antibody and positives were confirmed according to U.S. Public Health Service criteria. Twenty-five (8%) Guaymi residing in Changuinola and 7 (2.1%) from Canquintu were confirmed seropositive. In Changuinola, antibody was virtually limited to residents greater than or equal to 15 years of age (24 [16%] of 153) and rates were slightly higher in males than in females; in Canquintu, antibody rates did not increase significantly with age and appeared higher in females than in males. In Changuinola, there was no evidence for household clustering of infection. In contrast, HTLV antibody among Canquintu residents clustered significantly by household. HTLV-associated neurologic disease was not detected in either population. The atypical seroepidemiology observed in both locations might be explained if the virus endemic to the Guaymi differed from HTLV-I previously described in the Caribbean basin and Japan.

The human T-lymphotropic virus (HTLV) and associated diseases, adult T cell leukemia and spastic paraparesis, appear to be endemic in southwestern Japan and the Caribbean. This cross-sectional population-based study was conducted to describe the seroepidemiology of HTLV in the Republic of Panama. HTLV antibody was measured by first generation and commercial ELISA tests and confirmed by competitive binding ELISA, a radioimmunoassay for anti-p 24, and Western blot. Of 3,231 subjects greater than or equal to 15 years of age, 135 (4.2%) had antibody detected in ELISA screening tests, but because only 20% were confirmed positive, HTLV seroprevalence varied from 0.2-2% throughout the Republic. Infection with HTLV clustered in Guaymi Indians living in Bocas del Toro province (9.9% prevalence rate). With the exception of Guaymi Indians, no major geographic, urban/rural, male/female or racial differences in antibody prevalence were observed; specifically, HTLV infection rates were not elevated in black Panamanians. Clustering of infection in an isolated Amerind population must be further investigated. The small proportion of screen-positive sera which confirmed positive illustrates the importance of strict uniform criteria for seropositivity.

Neutralization tests were done on sera from 141 horses from high elevation regions of California. Antibody prevalences to Jamestown Canyon, snowshoe hare, and California encephalitis viruses in the California serogroup and Northway virus in the Bunyamwera serogroup were 55%, 43%, 18%, and 46%, respectively. In 51 horses from rural low elevation regions, seroprevalences were 31%, 35%, 35%, and 37%, respectively. Twenty-four horses from a suburban lowland area were seronegative, except for a single horse with a low titer to snowshoe hare virus. Seroprevalence to Jamestown Canyon and snowshoe hare viruses was associated with increasing age. Only 2 of 177 rodents from the Sierra Nevada had antibodies to Northway virus; none had antibodies to Jamestown Canyon or snowshoe hare viruses.

Neurocysticercosis was not been reported from Panama until 1984. The first documented case was a 5-year-old male who lived with his family in a typical subsistence agriculture environment lacking all sanitary amenities. Pigs and other animals ranged freely in and around the home. This report concerns clinical studies of the patient and his family conducted 4 years later. The index case had neurologic sequelae and was strongly seropositive. A cerebral CT scan revealed multiple intracerebral calcifications. Three family members had antibody to Cysticercus detected by immunoblot assay. A 7-year-old seropositive sister had an intracerebral calcification detected by CT scan. All 6 family members had completely normal neurologic examinations.

California encephalitis (CE) virus was transmitted transovarially by its natural vectors, Aedes dorsalis and Aedes melanimon following infection by intrathoracic inoculation. Virus was recovered from adult Ae. melanimon reared from eggs that were stored for up to 19 months and exposed to repeated freezing and thawing. Neither time since oviposition nor storage conditions affected infection rates in surviving embryos. Survival rates were highest in eggs stored at 4 degrees C. Transovarial infection with CE virus did not affect survival of embryos, larvae or adults. However, transovarially infected larvae took longer to develop than did their uninfected siblings.

Colonized and field-collected female Culex tarsalis, infected with St. Louis encephalitis, (SLE) virus by intrathoracic inoculation or by feeding on a viremic host, transmitted virus to their F1 adult and/or larval progeny when reared at 18(+/- 1) degree C but not when reared at 27(+/- 1) degrees C. The minimal infection rates (MIR) for different populations of Cx. tarsalis ranged from 1:32 to less than 1:250 (mean = 1:121) for larval progeny and from 1:32 to less than 1:1, 989 (mean = 1:1,571) for adult progeny. SLE virus also was transmitted transovarially by colonized and field-collected populations of Culex pipiens (mean MIRs = 1:340 and 1:1,815 for larval and adult progeny, respectively) and by a field population of Culex quinquefasciatus (MIR = 1:500 and less than 1:246 for larval and adult progeny, respectively), but not by colonized strains of Cx. quinquefasciatus and Culex peus. SLE virus was not recovered in tests on 5,522 Cx. tarsalis and 4,798 Cx. quinquefasciatus that were collected as larvae or pupae from field sites in Southern California and reared to adults at 18 degrees C in the laboratory. Transovarial transmission of SLE virus by Aedes epactius was confirmed and extended to a closely related species, Aedes atropalpus. Efforts to demonstrate transovarial transmission of SLE virus by Aedes melanimon, Aedes sierrensis, and Aedes triseriatus were unsuccessful. Aedes dorsalis, Cx. peus, and Toxorhynchites amboinensis were equally sensitive hosts for viral isolation when inoculated with suspensions of larvae transovarially infected with SLE virus.

After prolonged selection, two hybrid strains of Culex tarsalis were evolved that were highly resistant to infection following ingestion of western equine encephalomyelitis virus. These strains were greater than 25,000-fold more resistant than the most susceptible parental strain when fed on viremic chicks. Resistance was associated with a mesenteronal barrier since both refractory and parental strains were equally susceptible to infection by intrathoracic inoculation. Susceptibility was dominant, possibly incompletely dominant, over resistance. Inheritance was probably polyfactorial but this could not be determined with certainty since a small proportion of individuals appeared to become infected by nongenetic mechanisms.

Plaque reduction-serum dilution neutralization was used to evaluate the status of bunyavirus activity in deer in mountainous areas of California. Antibodies against 9 bunyaviruses were measured in 337 mule deer (Odocoileus hemionus hemionus, O. hemionus californicus, and O. hemionus inyoensis) and black-tailed deer (O. hemionus columbianus). More deer from high mountainous areas had neutralizing antibodies against Jamestown Canyon virus than did deer from low mountainous areas (23% vs. 9%; P less than 0.01). This finding is consistent with transmission by snow pool Aedes mosquitoes. Results for Jerry Slough virus were nearly identical to those for Jamestown Canyon virus, which is further evidence that these are strains of the same virus. Neutralizing antibodies against Northway virus were present in 26% of deer from high mountainous areas and 23% of deer from low mountainous areas, suggesting the involvement of a widespread vector, such as Culiseta inornata. Northway virus is not known to occur outside of Alaska and northwestern Canada. Low prevalences of antibodies were detected in deer to California encephalitis, La Crosse, and snowshoe hare viruses of the California serogroup; and Cache Valley, Lokern, and Main Drain viruses of the Bunyamwera serogroup.

California encephalitis (CE) virus was transmitted transovarially by its natural vector, Aedes melanimon. Vertical transmission rates ranged from 13-26% in geographical populations of Ae. melanimon infected with CE virus by intrathoracic inoculation. No consistent pattern of transmission rates was detected for location or time of year mosquito collection. Vertical transmission rates ranged from 9-29% in Aedes dorsalis inoculated with CE viral strains isolated from Ae. melanimon collected in California. The month or year of viral isolation had no effect on the efficiency of vertical transmission. However, a viral strain isolated from the Owens Valley was less efficiently transmitted than strains from the Sacramento Valley, and strains from the San Joaquin Valley were intermediate in efficiency. Filial infection rates were highest in first ovarian cycle progeny and declined with increasing ovarian cycles in both Ae. dorsalis and Ae. melanimon.

St. Louis encephalitis (SLE) virus, an arbovirus, and Rio Bravo (RB) virus, a non-arthropod-borne virus, are flaviviruses which cross-react in neutralization tests. Several of their biological properties were compared. Viral growth curves revealed that greater than 99% of infectious SLE (Parton) virus remained cell-associated in Vero cells but was released slowly into the medium of infected BHK-21 cells. In contrast, RB (M-64) virus was released upon maturation into the fluids of Vero and BHK-21 cell cultures. Fortyfold more SLE virus than RB virus adsorbed to monolayer cultures of Aedes dorsalis cells. SLE but not RB virus replicated and reached high titers (10(8) pfu/ml) in Ae. dorsalis cell cultures maintained in media which were expected to enhance recovery of RB virus. Further, SLE but not RB virus replicated in cultured bat epithelial cells and primary duck embryo cells incubated at 42 degrees C. Clearly, the prototype strains of RB and SLE viruses are distinct viruses. Both the classification of RB virus in the genus flavivirus and the antigenic relationship between RB and SLE viruses require further clarification.