[Isolation of a Tahyna-like virus (Bunyaviridae, Bunyavirus, California encephalitis complex) on northern Sakhalin Island]

The strain LEIV-11483 Sak of Tahyna virus or a virus closely related to it antigenically was isolated from Aedes spp. mosquitoes (predominantly A. communis) collected August 25, 1985, in the light coniferous subzone of the mid-taiga landscape zone in the north of Sakhalin Island (54 degrees N, 142 degrees E). Altogether, 17.8 thousand mosquitoes were examined which were collected in June-August, 1985, in the light and dark coniferous subzones of mid-taiga. In the human population, virus-neutralizing antibodies to Tahyna virus were found in 7-18%, to Batai virus in 3%, to Uukuniemi virus in none.

California serogroup and Powassan virus infection of cats

One hundred and seventy five sera from cats in Ontario, Canada, were tested for hemagglutination inhibition (HI) antibodies to three arboviruses; namely, Powassan (POW) of the Flavivirus serogroup, and Snowshoe hare (SSH) and Jamestown Canyon (JC) viruses of the California (CAL) serogroup. All sera were negative for antibodies to POW virus. Twelve cats possessed CAL serogroup antibodies including 3 with antibodies to SSH alone, 6 with antibodies to JC alone, and 3 with antibodies to both SSH and JC antigens. POW virus was inoculated into seven cats, one intracerebrally and six intravenously. Neurologic signs were not detected in any of the cats. Histologic lesions of a nonsuppurative encephalitis and encephalomyelitis were observed in the intracerebrally inoculated cat and in one of the intravenously inoculated cats, respectively. POW virus was not isolated from the brain or spinal cord of either of these two cats. HI antibodies were detected in the sera of all inoculated animals. HI antibodies were not detected in the CSF of any animal.

Production and use of a hemagglutinin for detecting antibody to Jamestown Canyon virus

A procedure was developed for producing a hemagglutinin for the California serogroup (family Bunyaviridae, genus Bunyavirus) virus Jamestown Canyon, a human pathogen. Serum samples from humans putatively infected with this virus or with La Crosse virus were tested by hemagglutination inhibition. Each antigen detected antibody to the respective virus, with little cross-reactivity. These results suggest that both antigens should be used when the hemagglutination inhibition test is applied to the diagnosis of human infections with California serogroup viruses in North America.

Isolation of Cache Valley virus and detection of antibody for selected arboviruses in Michigan horses in 1980

Blood samples collected in September and November 1980 from 87 horses in southwestern Michigan were examined for virus isolation and for plaque-reduction neutralizing antibody against selected arboviruses. Cache Valley virus was isolated from the blood of a clinically normal horse in St Joseph County in September. The age-specific antibody prevalence for Cache Valley virus indicated enzootic transmission in the study area. The high antibody prevalence and the lack of age-specific antibody prevalence indicated sporadic, but intense, exposure to Jamestown Canyon virus. Low prevalences of antibody were detected for Highlands J, snowshoe hare, St Louis encephalitis, and trivittatus viruses.

Serological evidence of California serogroup virus activity in Oregon

We wished to demonstrate evidence of the presence of California serogroup viruses in Oregon and to test for the presence of certain other arboviruses in large ungulates. Blood samples from black-tailed deer (Odocoileus hemionus columbianus), mule deer (O. hemionus hemionus), and Roosevelt elk (Cervus elaphus roosevelti) from nine counties in Oregon were tested by serum-dilution plaque reduction neutralization for antibody to California serogroup viruses, including snowshoe hare, California encephalitis, and Jamestown Canyon, as well as to Cache Valley (Bunyamwera serogroup) and Klamath, an ungrouped rhabdovirus. Of 132 samples tested, 60 (46%) were found to be seropositive at a dilution of greater than or equal to 1:10 for at least one of the five different arboviruses. Forty (30%) samples contained antibody to more than one arbovirus, and 15 samples (11%) contained antibody to all five. Of these 15, 14 were from 75 black-tailed deer sera collected in Lincoln County, Oregon. Seropositivity rates for black-tailed deer ranged from 23% to 35%, with all five arboviruses represented. Positive reactions for all five arboviruses were represented among mule deer sera at rates from 5% to 29%. Elk sera were found to be positive for four of the viruses (none for Klamath virus). Although Cache Valley and Klamath viruses have been reported from Oregon, these data represent the first evidence of a California serogroup virus in the state.

A G1 glycoprotein epitope of La Crosse virus: a determinant of infection of Aedes triseriatus

Arthropod-borne viruses (arboviruses) have specific vector-vertebrate host cycles in nature. The molecular basis of restriction of virus replication to a very limited number of vector species is unknown, but the present study suggests that viral attachment proteins are important determinants of vector-virus interactions. The principal vector of La Crosse (LAC) virus is the mosquito Aedes triseriatus, and LAC virus efficiently infects the mosquito when ingested. However, a variant (V22) of LAC virus, which was selected by growing the virus in the presence of a monoclonal antibody, was markedly restricted in its ability to infect Ae. triseriatus when it was ingested. Only 15% of the mosquitoes that ingested V22 became infected and 5% of these developed disseminated infections. In contrast, 89% of the mosquitoes that ingested LAC became infected and 74% developed disseminated infections. When V22 was passed three times in mosquitoes by feeding, a revertant virus, V22M3, was obtained that infected 85% of Ae. triseriatus ingesting this virus. In addition, V22M3 regained the antigenic phenotype and fusion capability of the parent LAC virus. These results suggest that the specificity of LAC virus-vector interactions is markedly influenced by the efficiency of the fusion function of the G1 envelope glycoprotein operating at the midgut level in the arthropod vector.

Seroconversion rates to Jamestown Canyon virus among six populations of white-tailed deer (Odocoileus virginianus) in Indiana

The annual seroconversion of fawns, yearlings, and adult white-tailed deer (Odocoileus virginianus) to Jamestown Canyon virus (California group) was followed at six Indiana sites from 1981 through 1984. In all, sera from 1,642 deer (515 fawns, 618 yearlings, and 509 adults) were tested for neutralizing antibody to three California serogroup viruses: Jamestown Canyon, La Crosse, and trivittatus. Virtually all deer with specific neutralizing antibody showed evidence of a prior infection with Jamestown Canyon virus; only three deer showed evidence of a prior infection with only La Crosse virus and none showed evidence of an infection with only trivittatus virus. While there were no significant differences in antibody prevalence to Jamestown Canyon virus between yearling and adult deer at any site, fawns had significantly lower antibody prevalences than either of the two older age groups. Significant differences in antibody prevalence were found between northern versus southern populations of white-tailed deer in Indiana, however, no significant differences were found among the four northern populations or between the two southern populations. The mean antibody prevalences in the two southern fawn, yearling, and adult populations were 15%, 38%, and 41% respectively, while the prevalences in the four northern fawn, yearling, and adult populations were 5%, 67%, and 67% respectively. These different prevalences (northern vs. southern) correlate with the higher Jamestown Canyon virus antibody prevalence in human residents of northern Indiana (2-15%) compared to residents of southern Indiana (less than 2%) found in other studies. The significantly lower prevalence of antibody to Jamestown Canyon virus in fawns is attributed to maternal antibody protecting them from a primary infection their first summer. Yearling deer showed high rates of seroconversion following their second summer of life. These results suggest that infection of white-tailed deer in Indiana with Jamestown Canyon virus is a common phenomenon.

Infection of white-tailed deer (Odocoileus virginianus) in Michigan with Jamestown Canyon virus (California serogroup) and the importance of maternal antibody in viral maintenance

Sera collected from a captive population of white-tailed deer (Odocoileus virginianus) penned in the lower peninsula of Michigan were assayed over a 29-mo period for neutralizing antibody to California serogroup viruses. In all, 130 individual white-tailed deer were bled one to 22 times between June 1983 and November 1985. Of the 130 sampled after active transmission had ceased, or passage of maternal antibody in colostrum had occurred, only one (0.8%), a newborn fawn, had no serum neutralizing antibody to California group viruses. All 18 1-yr-old does sampled acquired specific neutralizing antibody to Jamestown Canyon (JC) virus within a 6-wk period in 1984 and within a 10-wk period in 1985 indicating the prevalence of infection in this nonimmune age group was 100% for 2 successive yr. All 32 2- to 7-yr-old adult does and eight bucks sampled between June 1983 and June 1985 had specific neutralizing antibody to JC virus. No white-tailed deer had specific neutralizing antibody to trivittatus or La Crosse/snowshoe hare viruses at this study site. In 1984 and 1985, 78% and 63% of the adult does respectively exhibited significant anamnestic responses; all 19 adult does sampled over two seasons (between October 1983 and June 1985) showed a significant anamnestic response during at least 1 of the 2 yr. One-third of adult does with significant springtime antibody titer increases apparently experienced reexposure prior to the emergence of aedine mosquitoes, suggesting an alternate vector may overwinter at this site and transmit viruses in early spring. Specific neutralizing antibody was detected in 98% (66/67) of nursing fawns bled within 5 wk of birth in May-June 1984 and 1985, including three of three nursing fawns bled within 24-96 hr of birth. Of the 66 newborn fawns with specific neutralizing antibody to JC virus in June 1984 and 1985, 95% (54/57) of the surviving fawns lost maternal antibody and had no measurable titer when sampled 20-24 wk after birth, however. Serum antibody titers in 25 newborn (1984-cohort) fawns and their mothers and titers in 38 newborn (1985-cohort) fawns and their mothers were significantly correlated at the 5% and 1% levels respectively, suggesting that maternal antibody rather than a naturally acquired infection was the source of immunity in these suckling fawns.(ABSTRACT TRUNCATED AT 400 WORDS)

Prevalence of neutralizing antibody to Jamestown Canyon virus (California group) in populations of elk and moose in northern Michigan and Ontario, Canada

Blood samples were collected from free-ranging elk (Cervus elaphus) harvested in Michigan's northern Lower Peninsula, from moose (Alces alces) relocated from Ontario's Algonquin Provincial Park to Michigan's Upper Peninsula, and from moose from Michigan's Isle Royale National Park. Sera were tested by serum dilution neutralization tests in Vero cell culture for neutralizing antibody to California serogroup viruses, in particular Jamestown Canyon (JC), La Crosse/snowshoe hare (LAC/SSH), and trivittatus (TVT) viruses. Specific neutralizing antibody to JC virus was detected in 71% of 31 and 65% of 20 moose from Algonquin and Isle Royale, respectively. An additional six moose from Algonquin and five from Isle Royale showed evidence of multiple infection. One juvenile moose from Isle Royale had specific neutralizing antibody to TVT virus. Specific neutralizing antibody to JC virus was detected also in 54% of 50 elk from Michigan; 20 of the 50 elk showed evidence of multiple infection. While no single serum sample showed specific neutralizing antibody only to LAC/SSH virus, its presence in sera from some animals may have been masked by the high prevalence of antibody to JC virus.

Serodiagnosis of La Crosse virus infections in humans by detection of immunoglobulin M class antibodies

Sera from 92 humans with illnesses clinically compatible with those caused by California serogroup virus infections were tested for antibody to La Crosse (LAC) virus by using the immunoglobulin M (IgM) antibody-capture enzyme-linked immunosorbent assay (MAC ELISA), the IgG ELISA, and the hemagglutination inhibition (HI), complement fixation and serum dilution-plaque reduction neutralization tests. On the reported day of onset of illness in 18 individuals, 94% had IgM antibody, 50% had neutralization antibody, 33% had HI antibody, and 11% had IgG antibody. Neutralization, HI, and IgG antibody prevalence rates increased thereafter, whereas IgM antibody prevalence remained high (92% 2 or more weeks after the onset of illness). It was concluded that the MAC ELISA is a sensitive test for the presence of antibody to LAC virus. The sensitivity of the MAC ELISA and the rapidity with which it can be performed appear to provide a powerful tool for the clinically relevant serodiagnosis of LAC virus infections in humans.

Jamestown Canyon virus (California serogroup) is the etiologic agent of widespread infection in Michigan humans

In a sample population of 780 Michigan residents tested for neutralizing antibodies to California serogroup viruses, 216 (27.7%) had specific neutralizing antibody to Jamestown Canyon virus. An additional eight (1.0%) had specific neutralizing to trivittatus virus; none had specific neutralizing antibody to La Crosse virus. Significantly more male residents than female residents of the Lower Peninsula had antibody to Jamestown Canyon virus. The frequency of neutralizing antibody titers fits the Poisson distribution, suggesting that Jamestown Canyon virus infections occur endemically in residents of Michigan. Among 128 sera with specific neutralizing antibody to Jamestown Canyon virus, only two (1.6%) were found to have significant hemagglutination-inhibiting antibody titers with La Crosse virus, while 23 of 44 (52%) had significant titers with Jamestown Canyon virus; a single serum had significant antibody by complement fixation tests with both La Crosse and Jamestown Canyon viruses. This study confirms earlier speculation that complement fixation and hemagglutination-inhibition tests with La Crosse virus (the only tests for California serogroup virus infections performed by most state diagnostic laboratories) fail to detect antibody to Jamestown Canyon virus. ASPEX computer-drawn maps demonstrated that the distribution of persons with antibody to Jamestown Canyon virus and residing in Michigan's Lower Peninsula is closely correlated with the estimated distribution of white-tailed deer in that part of the state, further supporting the hypothesis that white-tailed deer are the primary vertebrate host for Jamestown Canyon virus.

Two monoclonal antibodies against La Crosse virus show host-dependent neutralizing activity

Mammalian and arthropod cell cultures were used to assess the neutralizing activity of six monoclonal antibodies specific for the G1 glycoprotein of La Crosse virus. Four antibodies, two neutralizing and two non-neutralizing, showed no host-dependent differences, giving similar results when post-treatment infectivity was determined using either Aedes albopictus cells or BHK-21 cells. For two other antibodies, however, dissimilar activities were observed between the vertebrate and invertebrate cell lines. One of these antibodies was positive when BHK-21 cells were employed as the post-treatment host and negative when mosquito cells were used; the other antibody was the converse. The epitope for this last antibody was present on all California serogroup viruses examined, which suggests that it may have a special significance in the natural life-cycle of the virus.

Rapid separation of IgM from whole serum using spun column chromatography

Spun columns were used to separate IgM from serum samples by exploiting molecular weight and size difference between IgM and other serum antibodies. Miniature gel chromatography columns were prepared in 3 ml syringe barrels. A polyacrylamide liquid chromatography gel, Bio-Gel P-200, was chosen to exclude proteins with molecular weights greater than 200,000. IgM, 900,000 MW, was excluded while IgG, 150,000 MW, was retained in the gel column. Centrifugation of the serum-loaded columns in test tubes accomplished the separation in one step by eluting a void volume equal to the sample volume from each column into a holding tube. IgM recovery in the eluent exceeded 96% of the total serum IgM in the pre-column sample. No IgG was detected in the eluent. IgM separated from La Crosse encephalitis immune human serum retained immunological activity in a viral neutralization test. Spun column chromatography is eminently suitable for diagnostic laboratories as more than 100 sera may be fractionated in one day using inexpensive materials and a low-speed centrifuge equipped with a swinging-bucket rotor.

Rapid diagnosis of LaCrosse encephalitis: detection of specific immunoglobulin M in cerebrospinal fluid

An immunoglobulin M (IgM) antibody capture enzyme immunoassay (MAC-EIA) was developed for the rapid and early diagnosis of LaCrosse (LAC) virus infections. The MAC-EIA was a sensitive and specific technique for the detection of IgM antibodies to LAC virus in cerebrospinal fluid specimens and in acute-phase serum specimens. In a retrospective study, cerebrospinal fluid and acute-phase serum paired samples from 108 patients were tested by the MAC-EIA and by an IgM immunofluorescence assay. The results were compared with the original diagnosis, which was made by using a variety of classical serological tests including serum neutralization, hemagglutination inhibition, and complement fixation. Thirty patients were confirmed as having LAC virus infections; of these, 30 (100%) were diagnosed as positive by serum MAC-EIA, and 27 (90%) were positive by cerebrospinal fluid MAC-EIA. The MAC-EIA was more sensitive than the IgM immunofluorescence assay. Two patients who were not previously confirmed as positive cases were diagnosed as having LAC virus infections by the MAC-EIA. One patient who was subsequently diagnosed as having a Jamestown Canyon virus infection and two patients who were previously infected with Jamestown Canyon virus were not falsely identified as having LAC virus infections by the MAC-EIA.

[Isolation of the Tahyña bunyavirus in the Arctic]

Tahyna virus (Bunyaviridae, Bunyavirus, the California encephalitis complex) was isolated from Aedes communis complex mosquitoes collected at the border of the north-taiga landscape zone (in latitude 68 degrees North and longitude 33 degrees East) at the Kolsky peninsula (the Murmansk region). The LEIV-9843 Mur strain was isolated from 2.4 thousand mosquitoes collected there (altogether 3.8 thousand mosquitoes had been collected in the Murmansk region). This is the first isolation in the USSR of a California complex virus in the Arctic and the northernmost site of Tahyna virus isolation in the world. 18% of the human population residing near the site of the virus isolation had virus-neutralizing antibody to Tahyna virus.

Oral infection and transmission of La Crosse virus by an enzootic strain of Aedes triseriatus feeding on chipmunks with a range of viremia levels

The susceptibility of Aedes triseriatus to oral infection with La Crosse (LAC) virus resulting from feeding on chipmunks with viremia titers of 0.6 to 4.6 log10SMICLD50/0.025 ml was determined. Results indicated that viremia titers must exceed 3.2 log10SMICLD50/0.025 ml before a significant proportion (greater than or equal to 50%) of mosquitoes are infected and capable of transmitting LAC virus. Mosquitoes which fed on chipmunk blood-LAC virus mixtures through a membrane feeder had significantly lower infection rates at virus titers of 1.8 to 4.4 log10SMICLD50/0.025 ml and transmission was also significantly reduced. Application of these data to LAC viremia titers measured in chipmunks in an earlier study indicate that viremias sufficiently high to ensure transmission by the mosquitoes becoming orally infected average only about 1 day per infective bite delivered to the susceptible portion of the amplifier population. Oral infection and transmission rates were also determined for Ae. triseriatus feeding on chipmunk blood containing LAC virus neutralizing (N) antibodies and for Ae. triseriatus feeding on deer blood containing Jamestown Canyon (JC) virus N antibodies. Infection rates were similar to those observed in mosquitoes imbibing blood free of N antibody at the virus titers tested, but, oral transmission was reduced in females feeding on chipmunk blood-LAC virus mixtures containing LAC N antibodies and there was no transmission by females feeding on deer blood-LAC virus mixtures containing JC N antibodies. These data suggest that high LAC antibody prevalences in chipmunk populations and high LAC or JC antibody prevalences in deer populations may be antagonistic to horizontal LAC virus transmission.

California serogroup virus infection in a horse with encephalitis

A 4-fold or greater seroconversion to the snowshoe hare serotype of the California serogroup of viruses in a horse with acute encephalitis was demonstrated by hemagglutination-inhibition, complement-fixation, and neutralization tests. The horse had a mild fever, was ataxic, had a head tilt, and was observed to circle. Chloramphenicol, dexamethasone, and B complex vitamins were administered and the horse recovered. The snowshoe hare virus is a recognized human pathogen, but it has not been associated with disease in horses. It is unknown whether horses play a role as amplification hosts for the snowshoe hare virus in nature, and further studies appear indicated.

Mosquito-borne arboviruses in Norway: further isolations and detection of antibodies to California encephalitis viruses in human, sheep and wildlife sera

Seven virus strains antigenically related to the California encephalitis (CE) virus group were isolated from Norwegian Aedes spp. mosquitoes collected in 1976. So far CE viruses have been isolated from five different Aedes spp. in Norway. Furthermore, two virus strains related to the Bunyamwera group were isolated from Anopheles claviger. Antibodies to CE viruses were demonstrated in 22% of 1014 military recruits tested. Among 91 soldiers who were monitored by monthly blood samples during the mosquito season, seroconversions were detected in 11 individuals. Specific IgM antibodies were found in seven of them. Disease symptoms in connexion with the CE virus infections were not seen. The prevalence of CE antibodies in patients with CNS or respiratory infections was not higher than in control groups. Seroconversions were not seen in any of the groups. Screening of sheep sera from six different areas in northern Norway indicated significantly different degrees of CE virus activity. Passerine birds may be important CE virus hosts, while small rodents seem unimportant. Specific IgM antibodies were detected in the sera of one of three hares and one of two squirrels. Of the methods used, single radial haemolysis (SRH) and immuno-electro-osmophoresis (IEOP) seemed to be well suited for serological screenings. However, an indirect immunofluorescence antibody test (IFAT) which was used may be an attractive alternative if high-quality anti-species conjugates are available. The haemagglutination-inhibition (HI) test used gave a high number of false positive results.

California serogroup viruses in New York State: the role of subtypes in human infections

The roles of various subtypes of the California serogroup viruses as infectious agents and as neuropathogens were evaluated by using the plaque reduction neutralization test. Sera from 394 patients with central nervous system (CNS) infections during 1971-1982 and from 501 persons without CNS manifestations were studied. Jamestown Canyon (JC) and La Crosse (LAC) viruses were found to have been common infectious agents in New York State for at least 16 years. JC virus was the prevalent indicated agent in patients with antibody to California serogroup viruses in screening tests (62 of 93 cases), followed by LAC virus (11 cases), snowshoe hare (2 cases), and trivittatus (1 case). In the remaining 17 patients the subtype was undetermined. LAC virus appears to be more pathogenic for children and to produce more serious illness, as judged by the frequent clinical diagnosis of encephalitis. JC virus affects mainly adults, and meningitis was the most common diagnosis. JC virus appears to cause a stronger neutralizing antibody response than does LAC virus, with a longer persistence of high levels of antibody. Some cases of JC virus infection may have been missed in the past due to the choice of a LAC-like isolate from New York State as the sole antigen in hemagglutination-inhibition (HI) screening tests. Comparison of the HI test and a single-dilution neutralization assay for screening for the two major subtypes, JC and LAC, indicated that the latter procedure is more broadly reactive and is less likely to miss cases if only one test antigen is used.

Mechanisms of bunyavirus virulence. Comparative pathogenesis of a virulent strain of La Crosse and an avirulent strain of Tahyna virus

To analyze mechanisms of virulence in the California serogroup bunyaviruses, the virulent La Crosse/original (LAC/original) strain was compared with the avirulent Tahyna/181-57 strain. In suckling mice, both viruses were lethal upon intracerebral injection but differed markedly in their neuroinvasiveness following subcutaneous injection; 20 and 20,000 plaque-forming units, respectively, were equivalent to 1 subcutaneous LD50. The sequential course of infection was followed after subcutaneous injection of 700 plaque-forming units; LAC/original replicated in striated muscle, caused a high titer plasma viremia, invaded the central nervous system, and killed all mice; the same dose of avirulent Tahyna/181-57 failed to replicate in extraneural tissues, did not invade the central nervous system, and caused no apparent illness. Immunofluorescent examination of peripheral and central nervous system tissues showed the same distinctions between virulent and avirulent viruses and pinpointed striated muscle as the major extraneural target of virulent LAC/original virus. Paradoxically, after intracerebral injection of suckling or adult mice, Tahyna/181-57 virus killed more quickly than LAC/original. This difference was correlated with replication differences; Tahyna/181-57 multiplied marginally faster in the brain than did LAC/original virus.

Serological evidence of infection with California serogroup viruses (family Bunyaviridae) in residents of Long Hua, suburb of Shanghai, People's Republic of China

Sera from 126 residents of Long Hua, a suburb of Shanghai, in the People's Republic of China, were studied. Sera were tested for haemagglutination inhibiting antibodies to alphavirus (eastern equine encephalitis, western equine encephalitis), flavivirus (St. Louis encephalitis, Powassan, dengue) and California serogroup (snowshoe hare) antigens. Flavivirus antibodies were found in 14 (11.1%) and California serogroup antibodies in 5 (3.9%) individuals. Neutralizing antibodies with highest titres to snowshoe hare virus were demonstrated in 3 of the 5 California serogroup reactors. We believe this to be the first report of California serogroup virus antibodies in Chinese residents and the first evidence to suggest that California serogroup viruses may be circulating in the Orient.

Lower rates of oral transmission of La Crosse virus by Aedes triseriatus venereally exposed after engorgement on immune chipmunks

Transmission rates of La Crosse (LAC) virus observed in Aedes triseriatus females that had engorged on chipmunks with antibody to LAC and had been mated by infected males 5-11 days later (24%, 69/288) were 40% lower than in those fed on chipmunks without antibody (38%, 112/293). Similar results were obtained in three separate trials using males infected 1) by inoculation with prototype LAC virus, 2) transovarially with a field strain, or 3) transovarially with the field strain following passage through a viremic chipmunk. Similar rates were also observed in trials with F2 and F3 progeny of several strains of Ae. triseriatus collected from LAC-endemic and non-endemic areas. Reduction of oral transmission by venereally infected females mated by transovarially infected males following engorgement of antibody in chipmunks or other vertebrates could be important in the natural control of LAC virus, since most adult chipmunks sampled in endemic areas have antibodies neutralizing LAC. Ten-fold higher rates of venereal infection found in females mated by infected males 5 or more days after engorgement on LAC antibody-negative chipmunks than in those mated without prior engorgement extend previous findings of higher rates of transmission after engorgement on laboratory mice to include the natural vertebrate host.