Eastern equine encephalitis in the United States, 1971: Past and prologue

During 1971, surveillance for equine encephalitis in the United States was increased due to an epizootic of Venezuelan equine encephalomyelitis. Of 1,982 specimens from 1,551 equines, 76 isolates of eastern equine encephalitis (EEE) virus were recovered from 67 individuals. The virus was isolated from 50/176 brains, 8/74 spleens, 14/1,127 sera, and 4/147 whole bloods from infected equines in 12 of the 31 states bounded by or east of the Mississippi River and in Texas and Iowa; no specimens were received from 9 of these 31 states. Thus, EEE virus was isolated from equines in 12 of 22 of these states. Determinations of antibody to EEE and western equine encephalitis (WEE) viruses indicated a relatively high prevalence of infection with EEE virus in the eastern USA and similarly high prevalence of antibody to WEE virus in the western USA. These data indicate that equine infections with EEE virus in the eastern USA are considerably more common than previous surveillance data have suggested. Increased surveillance and submission of specimens to diagnostic laboratories for diagnosis of EEE virus infections in equines are suggested so that a greater proportion of the thousands of unspecified equine encephalitis cases occurring in the United States each year can be laboratory confirmed.     

The 1952 outbreak of encephalitis in California; epidemiologic aspects

For the most part, epidemiologic phenomena observed in the outbreak of encephalitis in 1952 accorded with patterns that had been apparent in previous years. Ninety-seven per cent of the 414 laboratory-confirmed cases of western equine and St. Louis encephalitis in humans occurred in the 20 Central Valley counties. The cases of western equine encephalomyelitis in horses were generally scattered over the state. In the Central Valley most of the cases in horses were in animals less than two years of age; elsewhere the incidence was higher in older horses.There were no laboratory-confirmed cases of western equine or St. Louis encephalitis in humans earlier than June or later than October. In 1952 there were far more cases of western equine than of St. Louis encephalitis-a departure from the pattern in the previous seven years when there were about as many of one as of the other. No known satisfactory index is available for the prediction of the extent or type of outbreaks in humans. Approximately one-third of the cases of western equine encephalitis were in patients less than one year of age, whereas there were no cases of the St. Louis disease in patients that young.The incidence of western equine encephalitis in persons under 5 years of age was about the same for girls as for boys. In higher age brackets, males with western equine encephalitis outnumbered females 2 to 1. The corresponding ratio for St. Louis encephalitis was only 1.2 to 1.     

The 1952 Outbreak of Encephalitis in California (A Symposium): EPIDEMIOLOGIC ASPECTS

For the most part, epidemiologic phenomena observed in the outbreak of encephalitis in 1952 accorded with patterns that had been apparent in previous years. Ninety-seven per cent of the 414 laboratory-confirmed cases of western equine and St. Louis encephalitis in humans occurred in the 20 Central Valley counties. The cases of western equine encephalomyelitis in horses were generally scattered over the state. In the Central Valley most of the cases in horses were in animals less than two years of age; elsewhere the incidence was higher in older horses.There were no laboratory-confirmed cases of western equine or St. Louis encephalitis in humans earlier than June or later than October.In 1952 there were far more cases of western equine than of St. Louis encephalitis—a departure from the pattern in the previous seven years when there were about as many of one as of the other. No known satisfactory index is available for the prediction of the extent or type of outbreaks in humans.Approximately one-third of the cases of western equine encephalitis were in patients less than one year of age, whereas there were no cases of the St. Louis disease in patients that young.The incidence of western equine encephalitis in persons under 5 years of age was about the same for girls as for boys. In higher age brackets, males with western equine encephalitis outnumbered females 2 to 1. The corresponding ratio for St. Louis encephalitis was only 1.2 to 1.     

Arbovirus infections in man in British Columbia

During the summer of 1971, the first laboratory-proved cases of acute encephalitis in man due to any of the known arboviruses occurred in the south-central region of British Columbia. Five human cases of encephalitis with two deaths were diagnosed; three of these patients, including one of the fatalities, were proven in the laboratory to have contracted western equine encephalitis.During 1968 and 1969, a human serum survey undertaken in approximately 2000 life-long residents of the province discovered low levels of hemagglutinin-inhibiting and/or complement-fixing as well as neutralizing antibodies for western equine encephalitis, St. Louis encephalitis, Powassan encephalitis, California encephalitis and Colorado tick fever. Evidence of recent sub-clinical infection was detected in some cases.     

Arbovirus-related encephalitis.

Arthropod-borne virus encephalitis in the U.S.A. includes LaCrosse, St Louis, western equine, eastern equine, Venezuelan equine, and Powassan in that order of frequency. Diagnosis can be aided by the history of seasonal occurrence, climate, geographic location, exposure to vectors, and age of the patient. The definitive diagnosis is usually made by serological tests such as neutralization, complement-fixation, hemagglutination-inhibition, and immunofluorescence, the radioimmune assay and the enzyme-linked immunosorbent assay show promise of future utility. These diseases are prevented by vector control. It is unlikely that vaccines or anti-viral agents will have application in the near future.     

California encephalitis virus,a newly described agent

In three cases of encephalitis in humans that occurred in the area where the newly described California virus was isolated from mosquitoes, serological evidence seemed to indict the California virus as the etiological agent. In the case of an infant with very severe disease, the serological evidence was convincing; the evidence was almost as strong in the case of a seven-year-old boy; the results in an adult were equivocal. Inapparent infection in man is quite common as indicated by neutralization tests on the sera of nearly 600 residents of California, but encephalitic manifestations of infection are extremely rare. In Kern County, California, where the virus was discovered, approximately 11 per cent of the population has been infected. Infection rates are higher in adults than in very young children. Absence of neutralizing antibodies from 64 specimens of blood from persons in Japan, Washington, and other states supports the specificity of the neutralization test in man and suggests that this virus is absent or is not being similarly transmitted in some areas. Serological evidence from serial bleedings of two sick horses suggested, but did not definitely establish, that this virus leads to a naturally acquired encephalomyelitis in horses. Serological tests with the viruses of western equine and St. Louis encephalitis did not lead to any other etiological diagnosis in the sick animals studied. Results of neutralization tests on the sera of eight horses and three cows in Kern County suggested extremely high infection rates, and an immunity rate of 18 per cent was noted in rabbits and ground squirrels. In the natural biological cycle rabbits and ground squirrels are suspected as the possible counterpart of birds in the St. Louis and western equine virus cycles. There is no evidence from field or laboratory to indicate that birds become infected with the California virus. Sera from 33 mammals other than man were collected from Northern California and Washington. All were free from neutralizing antibodies, again supporting the specificity of positive findings from Kern County.     

CALIFORNIA ENCEPHALITIS VIRUS—A Newly Described Agent

In three cases of encephalitis in humans that occurred in the area where the newly described California virus was isolated from mosquitoes, serological evidence seemed to indict the California virus as the etiological agent. In the case of an infant with very severe disease, the serological evidence was convincing; the evidence was almost as strong in the case of a seven-year-old boy; the results in an adult were equivocal.Inapparent infection in man is quite common as indicated by neutralization tests on the sera of nearly 600 residents of California, but encephalitic manifestations of infection are extremely rare. In Kern County, California, where the virus was discovered, approximately 11 per cent of the population has been infected. Infection rates are higher in adults than in very young children.Absence of neutralizing antibodies from 64 specimens of blood from persons in Japan, Washington, and other states supports the specificity of the neutralization test in man and suggests that this virus is absent or is not being similarly transmitted in some areas.Serological evidence from serial bleedings of two sick horses suggested, but did not definitely establish, that this virus leads to a naturally acquired encephalomyelitis in horses. Serological tests with the viruses of western equine and St. Louis encephalitis did not lead to any other etiological diagnosis in the sick animals studied.Results of neutralization tests on the sera of eight horses and three cows in Kern County suggested extremely high infection rates, and an immunity rate of 18 per cent was noted in rabbits and ground squirrels. In the natural biological cycle rabbits and ground squirrels are suspected as the possible counterpart of birds in the St. Louis and western equine virus cycles. There is no evidence from field or laboratory to indicate that birds become infected with the California virus.Sera from 33 mammals other than man were collected from Northern California and Washington. All were free from neutralizing antibodies, again supporting the specificity of positive findings from Kern County.     

Serologic survey for selected arboviruses and other potential pathogens in wildlife from Mexico.

During 1988 and 1989, a serologic survey of wildlife was conducted in northeastern Mexico to determine the presence, prevalence, and distribution of arboviruses and other selected disease agents. Eighty mammal specimens were tested. Antibodies to vesicular stomatitis-Indiana, Venezuelan equine encephalitis-Mena II, Rio Grande virus, and vesicular stomatitis-New Jersey were detected predominantly in small mammals. Deer and mouflon (Ovis musimon) had antibodies to bluetongue and epizootic hemorrhagic disease. Two species had serologic evidence of recent exposure to Francisella tularensis. A white-tailed deer (Odocoileus virginianus) had antibodies to Anaplasma marginale. All specimens tested for antibodies against Yersinia pestis and Brucella abortus were negative. Sera from 315 birds were tested for antibody against five equine encephalitis viruses and six avian pathogens. During 1988, antibodies to Venezuelan equine encephalitis-Mena II, Venezuelan equine encephalitis-TC83, St. Louis encephalitis, eastern equine encephalitis, and western equine encephalitis were detected in birds of several species. Antibodies to Pasteurella multocida and Newcastle disease virus were also detected. Birds from five species presented antibodies to Mycoplasma meleagridis. Specimens tested for M. gallisepticum, M. synoviae, and Chlamydia psittaci were negative. To the best of our knowledge, this survey represents the first serologic evidence of bluetongue, Cache Valley virus, epizootic hemorrhagic disease, Jamestown Canyon virus, vesicular stomatitis-Indiana, vesicular stomatitis-New Jersey, Rio Grande virus, and tularemia reported among wildlife in Mexico.     

Western equine and St. Louis encephalitis in man,California, 1945-1950

A large group of etiologic agents, some known and some unknown, produce in man a clinical syndrome now labeled "infectious encephalitis."The separation, from this larger group, of single disease entities which cause similar clinical symptoms is possible, but calls for diagnostic acumen plus supporting laboratory evidence. Two etiologically specific entities, western equine encephalitis and St. Louis encephalitis, are frequently encountered in rather well-defined areas of California, the Central Valley and Imperial Valley; and there is a definite seasonal pattern of occurrence-June through October. There are certain guides that are helpful in differential diagnosis. Establishing a diagnosis on the basis of clinical evidence is difficult. Laboratory studies are of great importance not only for diagnosis in the individual case but for advancement of etiology. Specimens are worthless unless taken at proper intervals and submitted by methods described. It is probable that encephalitides caused by still unknown agents exist in California. The isolation and identification of new encephalitogenic viral agents will depend in large measure upon the submission by physicians of suitable specimens from patients with central nervous system disease in which the cause is obscure.     

Neutralising antibodies for Mayaro virus in Pantanal, Brazil

The Pantanal hosts diverse wildlife species and therefore is a hotspot for arbovirus studies in South America. A serosurvey for Mayaro virus (MAYV), eastern (EEEV), western (WEEV) and Venezuelan (VEEV) equine encephalitis viruses was conducted with 237 sheep, 87 free-ranging caimans and 748 equids, including 37 collected from a ranch where a neurologic disorder outbreak had been recently reported. Sera were tested for specific viral antibodies using plaque-reduction neutralisation test. From a total of 748 equids, of which 264 were immunised with vaccine composed of EEEV and WEEV and 484 had no history of immunisation, 10 (1.3%) were seropositive for MAYV and two (0.3%) for VEEV using criteria of a ≥ 4-fold antibody titre difference. Among the 484 equids without history of immunisation, 48 (9.9%) were seropositive for EEEV and four (0.8%) for WEEV using the same criteria. Among the sheep, five were sero- positive for equine encephalitis alphaviruses, with one (0.4%) for EEEV, one (0.4%) for WEEV and three (1.3%) for VEEV. Regarding free-ranging caimans, one (1.1%) and three (3.4%), respectively, had low titres for neutralising antibodies to VEEV and undetermined alphaviruses. The neurological disorder outbreak could not be linked to the alphaviruses tested. Our findings represent strong evidence that MAYV and all equine encephalitis alphaviruses circulated in the Pantanal.     

Acute neuropathogenicity with experimental infection of equine herpesvirus 9 in common marmosets (Callithrix jacchus)

BACKGROUND: Equine herpesvirus 9 (EHV-9) is a new neurotropic equine herpesvirus which induced encephalitis in a variety of animals. However, there was no information on the susceptibility of EHV-9 in primates. METHODS: To assess the infectivity of EHV-9, four common marmosets (Callithrix jacchus) were inoculated by the nasal route with 10(6) plaque-forming units of EHV-9. RESULTS AND CONCLUSIONS: All of the inoculated animals exhibited various neurological signs progressing to collapse. Histologically, the affected animals had severe encephalitis characterized by neuronal degeneration and necrosis with intranuclear inclusion bodies, which extended from the olfactory bulb to the rhinencephalon and piriform lobe. Immunohistochemistry revealed EHV-9 antigens in degenerating neuronal cells. The nasal cavity had severe necrotizing rhinitis with prominent intra-nuclear inclusion bodies in the olfactory mucosa. These findings indicate that the marmosets are susceptible to EHV-9.     

Purification of the envelope glycoproteins of western equine encephalitis virus by glass wool column chromatography.

Glass wool column chromatography was used for separation of the two glycoproteins of western equine encephalitis virus. Cross-contamination of each protein separated was confirmed to be negligible by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.     

WESTERN EQUINE AND ST. LOUIS ENCEPHALITIS IN MAN,CALIFORNIA, 1945-1950

A large group of etiologic agents, some known and some unknown, produce in man a clinical syndrome now labeled “infectious encephalitis.”The separation, from this larger group, of single disease entities which cause similar clinical symptoms is possible, but calls for diagnostic acumen plus supporting laboratory evidence. Two etiologically specific entities, western equine encephalitis and St. Louis encephalitis, are frequently encountered in rather well-defined areas of California, the Central Valley and Imperial Valley; and there is a definite seasonal pattern of occurrence—June through October. There are certain guides that are helpful in differential diagnosis. Establishing a diagnosis on the basis of clinical evidence is difficult. Laboratory studies are of great importance not only for diagnosis in the individual case but for advancement of etiology. Specimens are worthless unless taken at proper intervals and submitted by methods described.It is probable that encephalitides caused by still unknown agents exist in California. The isolation and identification of new encephalitogenic viral agents will depend in large measure upon the submission by physicians of suitable specimens from patients with central nervous system disease in which the cause is obscure.     

An outbreak of Eastern equine encephalitis virus in free-ranging white-tailed deer in Michigan

Eastern equine encephalitis (EEE) virus has been recognized as affecting horses and humans in the eastern United States for 70 yr. Evidence of exposure with EEE virus has been reported in a variety of free-ranging wild birds and mammals but cases of clinical disease are much less commonly reported. In Michigan, reports of outbreaks of EEE virus in equine species extend back more than a half century. We report diagnosis of EEE virus infection of multiple free-ranging white-tailed deer (Odocoileus virginianus) from three Michigan counties during late summer of 2005. Infection was confirmed in seven of 30 deer collected based on reported neurologic signs and results from immunohistochemistry, polymerase chain reaction, and/or virus isolation. One of the deer also was infected with West Nile virus and an eighth deer had microscopic lesions in the cerebrum consistent with those reported for EEE. To our knowledge, this is the first report of multiple cases of EEE in free-ranging white-tailed deer, and highlights several issues of significance to wildlife managers and public health officials.     

Venezuelan Equine Encephalitis Virus Activity in the Gulf Coast Region of Mexico, 2003–2010

Venezuelan equine encephalitis virus (VEEV) has been the causative agent for sporadic epidemics and equine epizootics throughout the Americas since the 1930s. In 1969, an outbreak of Venezuelan equine encephalitis (VEE) spread rapidly from Guatemala and through the Gulf Coast region of Mexico, reaching Texas in 1971. Since this outbreak, there have been very few studies to determine the northward extent of endemic VEEV in this region. This study reports the findings of serologic surveillance in the Gulf Coast region of Mexico from 2003–2010. Phylogenetic analysis was also performed on viral isolates from this region to determine whether there have been substantial genetic changes in VEEV since the 1960s. Based on the findings of this study, the Gulf Coast lineage of subtype IE VEEV continues to actively circulate in this region of Mexico and appears to be responsible for infection of humans and animals throughout this region, including the northern State of Tamaulipas, which borders Texas.     

Experimental inoculation of three arboviruses in black-bellied whistling ducks (Dendrocygna autumnalis).

Wild-caught, immature black-bellied whistling ducks (Dendrocygna autumnalis) were inoculated with eastern equine encephalitis (EEE), St. Louis encephalitis (SLE), or western equine encephalitis (WEE) virus. Susceptibility, duration and titer of viremia, and antibody response to these arboviruses were determined. Birds from all inoculated groups became viremic. Higher virus titers occurred in the EEE group but overall mean titers were not significantly different among experimental groups. All birds inoculated with EEE and SLE viruses developed antibodies, and six of seven ducks receiving WEE virus were seropositive. All seropositive ducks had antibodies for at least 59 days, when the study was terminated. The EEE group had significantly more seropositive ducks during more days than the WEE and SLE groups. Geometric mean antibody titers were significantly smaller in the WEE group when compared to the EEE and SLE groups. Control ducks did not develop viremia or antibodies. Gross and histopathologic lesions compatible with viral encephalitis were absent in all of nine ducks necropsied. Black-bellied whistling ducks can develop low and short-term levels of viremia sufficient to infect mosquitoes, but probably cannot contribute significantly to the transmission of EEE and SLE. They may serve as good indicators of virus activity.     

Serologic survey for selected microbial agents in mammals from Alberta, 1976

Blood samples were taken from humans and several species of free-ranging wild mammals from five different geographic areas of Alberta, Canada. Sera were tested for antibody to eastern equine encephalitis (EEE) virus, western equine encephalitis (WEE) virus, St. Louis encephalitis (SLE) virus, Powassan (POW) virus, the snowshoe hare (SSH) strain of the California group (CAL) of viruses, Northway (NOR) virus, Klamath (KLA) virus, infectious bovine rhinotracheitis (IBR) virus, and two bacteria, Brucella abortus and Francisella tularensis. CAL antibody was found in 63% of 11 snowshoe hares (Lepus americanus), 33% of 167 black bears (Ursus americanus), and 19% of 55 humans (Homo sapiens). NOR antibody was found in 0.4% of 258 hares, 11% of 9 bighorn sheep (Ovis canadensis), 20% of 44 moose (Alces alces), 4% of 56 bears, 14% of 22 woodland caribou (Rangifer tarandus), and 2% of 50 humans. IBR antibody was detected in 14% of 14 moose. B. abortus antibody was found in 1% of 283 bears. F. tularensis antibody was detected in 2% of 52 humans. These findings represent extension of: (1) the natural host range of IBR, CAL, and NOR; (2) the geographical distribution of NOR infection in North America; and (3) the geographical distribution of CAL infection within Alberta.     

A serologic survey of mule deer and elk in Utah.

Sera from mule deer (Odocoileus hemionus) and elk (Cervus canadensis) in central and northern Utah were tested for the prevalence of antibodies to 11 diseases communicable to man or domestic livestock. Antibodies to Francisella tularensis (at 1:20) were found in 47 of 88 (53.4%) elk and 1 of 89 (1.1%) deer. A screening slide agglutination test for titers to Brucella (at 1:20) showed two reactors in elk but none in deer sera. No positive antibody titers were obtained in tests for anaplasmosis, Colorado tick fever, Rocky Mountain spotted fever, Q-fever, psittacosis, Powassan, western equine encephalitis, St. Louis encephalitis and California encephalitis.     

The 1952 Outbreak of Encephalitis in California (A Symposium): LABORATORY METHODS FOR ETIOLOGIC DIAGNOSIS

The general procedures used in the diagnosis of neurotropic viral diseases are outlined and are discussed with specific reference to western equine encephalitis.Cerebrospinal fluid is considered practically worthless as a starting material, in attempts to isolate the causal agent. The material of choice in attempting to recover the virus is central nervous system tissue, available only in instances of fatal infection. In the usual case, the diagnosis depends upon serologic or immunologic methods. These methods are aimed at detecting the presence of specific antibodies and of increases in the content of antibodies in the blood during the course of the illness.The in vitro complement fixation test is considered a better diagnostic tool than the in vivo neutralization test, since rises in titer are more readily detectable by the former technique than by the latter.     

Inhibition by Agaricus blazei Murill fractions of cytopathic effect induced by western equine encephalitis (WEE) virus on VERO cells in vitro

Anti-viral activities of Agaricus blazei Murill were investigated. The water extracts of the cultured mycelia and fruiting bodies were fractionated with different concentrations of ethanol. To several viruses which have cytopathic effects (CPE) on VERO cells, inhibition of these effects by the ethanol fractions was tested. Strong inhibition of CPE induced by western equine encephalitis (WEE) virus was observed in the mycelial fractions but not those of fruiting bodies.