Western Encephalitis Virus Research Articles

The Madariaga virus follows the footsteps of the Chikungunya and Zika viruses

Viruses transmitted by arthropods are the cause of important encephalitis. Several equine encephalitis viruses belong to the family Togaviridae genus Alphavirus, besides Chikungunya and Mayaro, there are the Western encephalitis viruses (WEEV), Venezuelan equine encephalitis (VEEV) and Eastern equine encephalitis (EEEV). EEEV was previously classified into four different lineages / subtypes (I to IV) (1,2). The South American strains (SA) (lineages II-IV), which include isolates from South and Central America, are genetically different in contrast to the strains of North America (NA) that belong to lineage I (3). Due to genetic divergence and significant differences in ecology and pathogenesis, the South American isolates of EEEV were recently classified as a different species that was named Madariaga virus (1).

Pathogenesis of a Venezuelan encephalitis virus strain lethal for adult white rats.

AbstractThe pathogenesis for adult rats of a Venezuelan encephalitis virus (VEE) strain, V‐198, was studied. This virus strain was chosen following virulence‐testing of 12 prototype alphavirus strains, selected from the Semliki Forest, Venezuelan, Eastern, and Western encephalitis virus complexes; only VEE strain V‐198 killed all adult rats when inoculated subcutaneously (sc) with 6.0–6.5 log10 plaque‐forming units (PFU). Lower doses of strain V‐198 by the same route were also lethal: 5.0 and 3.0 log10 PFU killed 100%, and 1.0 log10 PFU killed 70% of rats. Viral infectivity titrations of tissues obtained from V‐198 straininfected rats demonstrated that virus replicated early in thymus and spleen, but did not replicate in bone marrow or liver. Following a brief viremia, virus infectivity titers peaked in brain, but did not persist there. A biphasic neutrophilia also occurred. Lower doses of inoculum virus affected the timing but not the degree of neutrophil fluctuations, virus replication, and clinical disease. Rats, moribund seven days after inoculation of 4.3 log10 PFU, exhibited central nervous system signs and histologic evidence of encephalomyelitis. Significant proportions of rats, otherwise lethally infected with V‐198 virus, were protected by a single intraperitoneal inoculation of hyperimmune V‐198 antiserum, administered 3 or 4, but not 5 days after virus.Since the rat has been well characterized with respect to metabolic alterations following other infectious diseases, it is anticipated that this model for lethal virus‐induced disease will be useful for defining more precisely the functional relationships between viral growth, tissue destruction, metabolic alterations, and clinical course of disease, and that it may suggest specific approaches for effective treatment of viral diseases in general.

Equine viral encephalitis.

The most important neurotropic viral infections of the horse are the arthropod-borne encephalitides. These include Venezuelan encephalitis (VE), eastern encephalitis (EE) and western encephalitis (WE), which are found in the Americas, and Japanese B encephalitis which occurs in the Far East. All the viruses cause encephalitis in man. Between 1969 and 1972 an epidemic of VE occurred in Central America. In 1971 the disease was reported in Texas, where it was brought under control by the vaccination of susceptible horses with an attenuated live virus vaccine and by the reduction of the mosquito population with insecticides sprayed from aircraft. A high titre viraemia occurs with VE virus in the horse and epidemics are maintained by a mosquito/horse cycle; infection of man and other species is incidental. EE and WE have been recognised as separate diseases since 1933 and in the U.S.A. horses are protected by routine vaccination. Epidemics of these diseases are routine vaccination. Epidemics of these diseases are now uncommon. In contrast with VE, both EE and WE viruses are maintained by a bird/mosquito cycle. The viraemia in the horse is generally considered insufficient to infect mosquito vectors; the horse is a "dead end host". Several species of mosquito can act as vectors of VE, WE and EE. The extension of other arthropod-borne diseases to areas originally outside their geographical distribution (e.g. bluetongue in sheep) serves to illustrate the potential of VE, WE and EE to cause disease on other continents.

Enhanced detection of Western equine encephalitis virus plaque variants in crowded cultures and plaque progeny--potential use in genetic studies.

Enhanced detection of large-plaque variants of Western equine encephalitis virus in the presence of large numbers of small-plaque Western encephalitis virus was achieved by using higher bicarbonate concentrations in the agar-overlay medium and increasing concentrations of CO(2) in an enclosed environment during the time plaques were developed with neutral red. By this technique we were able to detect large-plaque mutants in cloned small-plaque suspensions for the first time and to show an increase in detectable plaque progeny in plaques developed by neutral red. Further increase in the number of progeny per plaque was determined by assaying extracts of agar overlaying cultures containing known numbers of plaques. The significance of these findings for potential use in mutational studies is discussed.

Enhanced Detection of Western Equine Encephalitis Virus Plaque Variants in Crowded Cultures and Plaque Progeny—Potential Use in Genetic Studies

Enhanced detection of large-plaque variants of Western equine encephalitis virus in the presence of large numbers of small-plaque Western encephalitis virus was achieved by using higher bicarbonate concentrations in the agar-overlay medium and increasing concentrations of CO(2) in an enclosed environment during the time plaques were developed with neutral red. By this technique we were able to detect large-plaque mutants in cloned small-plaque suspensions for the first time and to show an increase in detectable plaque progeny in plaques developed by neutral red. Further increase in the number of progeny per plaque was determined by assaying extracts of agar overlaying cultures containing known numbers of plaques. The significance of these findings for potential use in mutational studies is discussed.

Nesting Activity of the House Sparrow in Hale County, Texas, During 1968

Nestling House Sparrows, Passer domesticus, are involved in the amplification of western encephalitis (WE) virus activity in Hale County, Texas, during the epidemic season (Hayes et al., 1961 Holden et al, 1973a). Experimental studies (Holden et al., 1973b) have shown that nestlings of this species are admirably suited for this role since they may circulate virus in the bloodstream for several days at concentrations sufficient for infecting vector mosquitoes. Information on the nesting activity and reproductive capacity of the House Sparrow in Hale County is requisite to an understanding of the seasonal dynamics of WE virus activity in the area; therefore, the present study was undertaken.

Known mosquito hosts of western encephalitis virus in Saskatchewan.

Seven species of Saskatchewan mosquitoes have been found infected with the virus of western encephalitis (WE) in nature. The majority of WE virus isolations from mosquitoes were made during July and August, the months when epidemics begin. In 2 epidemic years the majority of the isolations from mosquitoes were from Culex tarsalis which had higher seasonal infection rates and generally larger infected populations than any other species in 5 collecting areas. In 1 interepidemic year, isolations of WE virus from species of mosquitoes other than C. tarsalis predominated and in at least 1 locality 1 of these species had a high seasonal infection rate and a relatively large infected population. Based on ecological and epidemiological evidence, it is concluded that, in Saskatchewan, C. tarsalis is the principal epidemic transmitter of the WE virus and that other species maintain the enzootic status of the disease. On similar grounds, Culiseta inornata may be a significant transmitter of the virus in epidemics of WE-in horses in Saskatchewan. Infected mosquitoes have been found in most of the agricultural area of the province except the Southwest, but testing of larger numbers of specimens from the Southwest would probably reveal infected mosquitoes in that area.

SEROLOGIC EVIDENCE OF ARBOVIRUS INFECTIONS IN WILD RUMINANTS1

Trainer, D. O. and R. P. Hanson (Dept. Veterinary Science, University of Wisconsin, Madison, Wis. 53706). Serologic evidence of arbovirus infections in wild ruminants. Amer. J. Epid., 1969, 90: 354–358.—Almost 1,500 sera from wild ruminants of North America, including bighorn sheep, prong horn antelope and whilte-tailed deer, were tested for neutralizing antibodies to 7 viruses: eastern, western, St. Louis, Venezuelan and California encephalitis, vesicular stomatitis and encephalomyocarditis. Among the bighorn sheep sampled, neutralizing antibodies were detected for western encephalitis virus (12%) and California encephalitis virus (12%); among prong horn antelope 16% were positive for California encephalitis. In white-tailed deer, serologic evidence of Venezuelan encephalitis was restricted to deer from Texas, while deer from Nebraska and Texas had antibodies to western encephalitis virus, and deer from Wisconsin and Texas had antibody to St. Louis encephalitis virus. California encephalitis antibodies were detected in almost every species and geographic site examined, and the largest number of reactors occurred in deer from Wisconsin (26%) and Texas (50%). Approximately 8% of the Texas deer sera were serologically positive for vesicular stomatitis and 4% for encephalomyocarditis. Results of this study suggest that wild ruminants and white-tailed deer in particular could serve as a valuable indicator for the activity of arboviruses—a “natural” sentinel system.

Epidemiology of western encephalitis in Alberta: response of natural populations of mosquitoes to avian host.

The response of natural populations of mosquitoes to avian host was determined under field conditions in dryland and irrigated areas in Alberta. Caged flocks of chickens were used as avian host. Females of Culex tarsalis, Culiseta inornata, Aedes flavescens, Aedes dorsalis, Aedes spencerii, Aedes vexaus, Aedes campestris, Aedes cataphylla , and Aedes nigromaculis were attracted to and readily fed on the chickens although other hosts were available in the areas studied. Bird-biting mosquitoes, which are in continuous supply throughout the spring and summer, were observed to feed on nestlings of Franklin gulls, red-winged blackbirds, Brewer's blackbirds, yellow-headed blackbirds, and ringed-neck pheasants. The interrelations of mosquito and bird populations are believed to be essential factors in the epidemiology of Western Encephalitis virus in the study area.

Effects of ultra-low volume applications of malathion in Hale County, Texas. I. Western encephalitis virus activity in treated and untreated towns.

Ultra-low volume (ULV) aerial applications of technical malathion at the rate of 214 g/ha (3 oz per acre) were applied routinely in 3 Hale County, Texas towns (Plainview, Abernathy, Hale Center) to reduce arbovirus transmission by mosquitoes during the summer of 1967. One untreated town, Petersburg, and 3 rural premises sites were selected for comparison. Used to evaluate the effectiveness of treatment were vector population indices before and after treatment, western encephalitis (WE) virus infection rates in mosquitoes and nestling house sparrows, WE antibody conversion rates in sentinel chicken flocks, and subclinical infection rates in school children. A significant reduction in the Culex tarsalis Coq. population level was evident only during the 48 hr immediately following treatment. Infiltration of adult mosquitoes from outside the treated areas was postulated to account for this rapid posttreatment population build-up. Seasonal WE virus infection rates in mosquitoes collected in treated and untreated towns did not differ significantly. Infiltration of infected adults into the treated areas may have contributed to the uniform infection rates. The WE virus infection rate was significantly higher in nestling house sparrows from untreated Petersburg as compared to Plainview and Hale Center, but not to Abernathy. Sparrow infection rates also were significantly lower at untreated rural sites as compared to Petersburg; therefore, factors other than ULV treatment must have been operative in keeping the infection rates at a low level. The WE antibody conversion rate in sentinel chicken flocks was significantly lower in Plainview than in Petersburg; however, observed differences in conversion rates between Petersburg and the 2 other treated towns and 3 rural sites were not significant. There was no confirmed human case of arboviral encephalitis in Hale County during 1967; however, few human cases were reported elsewhere in Texas or other parts of the United States during the year. The effectiveness of the ULV treatments in reducing the number of sub-clinical human infections was suggested, but the data were not conclusive. In order to control C. tarsalis in Hale County towns sufficiently to reduce WE virus activity, it was concluded that it would be necessary to make barrier treatments around the populated areas to prevent infiltration of infected mosquitoes.

Studies on the role of mammals in the natural history of western encephalitis in Hale County, Texas.

Following an epidemic of western encephalitis (WE) in Hale County, Texas during the summer of 1963, studies were initiated to determine if small mammals were naturally infected with WE virus in that area. An antibody survey in December 1963 revealed that more than half of the rabbits and only 2% of the rodents sampled were positive for WE antibody. Collection and testing of rabbits continued through April 1964, and 52% of the 137 tested had antibody to WE. Simultaneous collecting and testing of 518 rodents and other mammals for detection of overwintering virus gave negative results. In 1965 and 1966, mammal trapping studies revealed Mus musculus and Sigmodon hispidus to be the most abundant rodent species in rural study sites. The rabbit, Lepus californicus, was commonly the most abundant Lagomorph. During May-November 1965, 572 small mammals were tested for virus and in July, WE virus was isolated from 5 of 249 Lepus californicus, 2 of 12 Gynomys ludovicianus, 1 of 44 Citellus tridecemlineatus, and 1 of 103 Sigmodon hispidus. Virus was not isolated from 197 small mammals collected during April, September, and October 1966, but HAI antibody tests of 115 specimens collected during autumn 1966 again indicated high WE antibody rates among rabbits. The results of these studies indicate that small mammals are not basically involved in the maintenance of WE virus. However, small mammals may become infected during periods of high levels of enzootic transmission, and Lepus californicus appears to be the one most frequently infected during such periods.

Arbovirus activity in southern Arizona. Isolations of virus and observations of sentinel chickens.

Abstract This study was undertaken to define the nature and extent of arbovirus activity in the environment of Tucson, Arizona; especial interest was centered in the prevalence of arthropod-borne encephalitis. Field collections of mosquitoes, ticks, birds, and small mammals were made in the selected study areas along the valleys of the Santa Cruz River and the Rillito in or near Tucson. Flocks of sentinel chickens were maintained during the hot seasons in the same areas. The virus of St. Louis encephalitis (SLE) was isolated 12 times from mosquitoes in three separate summer seasons. The virus of Western encephalitis (WE) was recovered in the summer of 1966 in three different lots of mosquitoes. The results of serologic tests on the chickens maintained as sentinels in the study areas confirmed that active transmission of SLE and WE viruses had occurred during the summers that the isolations had been obtained. There was a relative scarcity of Culex tarsalis in the areas studied. The explanation for this is not clear. As a result of this study it appears clear that arthropod-borne encephalitis poses a continuing threat to the population of southern Arizona.

Isolation of western encephalitis virus from mosquitoes in Alberta.

Mosquitoes were collected in Alberta during the spring, summer, and early fall of 1965. Western encephalitis (WE) virus was isolated from pools collected between August 11 and August 25.Two positive pools, one of Aedes vexans (Meigen) and one of Culiseta inornata (Williston) were collected by sweep nets in irrigated areas of southern Alberta near places where horses were reported sick with WE virus. Seven other positive pools, three of C. inornata and four of Culex tarsalis Coquillett, were collected in southern Alberta from mammalian burrows, which are hibernating habitats for C. inornata, C. tarsalis, and Anopheles earlei Vargas. This indicates that C. tarsalis and C. inornata can enter hibernation infected with WE virus. None of the A. earlei pools tested were positive for WE virus.About 50% of the C. inornata and 9% of the C. tarsalis collected from mammalian burrows were blood-engorged at the time of collection. None of the A. earlei collected from mammalian burrows were blood-fed.

Seasonal dynamics of western encephalitis virus.

Seasonal dynamics of western encephalitis virus.

Standardization of the Fluorescent Antibody Technique for the Detection of Avian Respiratory Viruses

7 Fothergill, L. D., and J. H. Dingle. A fatal disease of pigeons caused by the virus of the eastern variety of equine encephalomyelitis. Sci. 88: 549-550, 1938. 8. Hayes, R. 0., J. B. Daniels, and R. A. MacCready. Western encephalitis virus in Massachusetts. Proc. Soc. Exptl. Biol. Med. 108: 805-808, 1961. 9. Hayes, R. 0., L. D. Beadle, A. D, Hess, O. Sussman, and M. J. Bonese. Entomological aspects of the 1959 outbreak of eastern encephalitis in New Jersey. Am J. Tropical Med. Hyg. 11(1): 115-121, 1962,, 10. Kissling, R. E. Equine encephalomyelitis virus in birds. Chapt. 24, "Diseases of Poultry." 4th ed. (H. E. Biester and L. H. Schwarte, eds.). Iowa State University Press, 572-574, 1959. 11. Luginbuhl, R. E., S. F. Satriano, C. F. Helmboldt, A. L, Lamson, and E. L. Jungherr. Investigation of eastern equine encephalomyelitis. II. Outbreaks in Connecticut pheasants. Am. J. Hyg. 67: 4-9, 1958. 12. Reardon, J. P. Personal communication. Division of Communicable Diseases, Massachusetts Department of Public Health, 1964. 13. Tyzzer, E. E., A. W. Sellards, and B. L.. Bennett. The occurrence in nature of "equine encephalomyelitis" in the ring-necked pheasant. Sci. 88: 505-506, 1938.

Natural Outbreak of Eastern and Western Encephalitis in Pen-Raised Chukars in Florida

Eastern encephalitis (EE) and Western encephalitis (WE) have not been previously recognized as disease problems in pen-raised chukar partridges in Florida. Stamm (7) reported that 52 species of birds are known to be susceptible to EE virus and 51 species are known to be susceptible to WE virus by demonstrating a viremia resulting from natural or laboratory infections. Moulthrop and Gordy (6) isolated EE virus from 1 pen of chukars in Maryland in 1960 without gross lesions, and described clinical signs as those shown in crazy chick disease. Kissling (5) described clinical signs of EE in ring-necked pheasants as a central nervous system involvement with leg paralysis, torticollis, and tremors. Gross heart lesions from which EE was isolated have been reported in pheasants by Gainer (3). Dougherty and Price (2) reported that White Pekin ducks over 18 days are not susceptible to natural infection with EE. Vaccination of pheasants in New Jersey with a bivalent (EE, WE) vaccine indicates that mortality from natural infections with EE is significantly curbed (8). This paper describes the occurrence of encephalitis in 5 rather widely separated flocks of pen-raised chukars in Florida in the summer of 1963; the EE virus was isolated and specifically identified in 4 flocks; the WE virus was isolated and specifically identified in the 5th flock. In all flocks, clinical signs and gross lesions were consistent. Brief experimental infections were induced in chukars; follow-up hemagglution

Arbovirus Neutralizing Substances in Avian Plasmas

Summary Arboviral neutralizing (N) substances appearing in avian plasmas after shooting and cardiac puncture were found sometimes to be nonspecific and at other times to be specific antibody. N substances which selectively neutralized western encephalitis (WE) and not St. Louis (SLE) or Japanese encephalitis (JE) viruses, and possibly also those that coexisted with HI substances in acetone-precipitated plasmas could be interpreted as being viral antibodies. N substances neutralizing both WE and SLE viruses were considered to be nonspecific. Neutralization of SLE or JE but not WE virus was an uninterpretable finding because this result could readily be produced by bile in plasma as well as by specific antibodies. Trauma to abdominal and thoracic viscera from shooting was probably more important in the appearance of N substances in avian plasmas than was cardiac puncture per se, since cardiac puncture of living, netted birds failed to engender appearance of N substances. Nevertheless, in birds dying or dead after shooting, it seemed likely that probing of the chest and abdominal cavities during cardiac puncture sometimes resulted in contamination of blood with bile or other body fluids containing nonspecific N substances. These findings make qualitative interpretations of results of serologic surveys of shot birds for arboviral N antibodies impossible unless each plasma with N substance is studied for specificity for the neutralized virus.

EXPERIMENTAL INFECTION OF CULEX NIGRIPALPUS THEOBALD WITH THE VIRUS OF ST. LOUIS ENCEPHALITIS.

Summary Experimental St. Louis encephalitis (SLE) virus infection and transmission studies with Culex nigripalpus revealed that all became infected and nearly 100 percent transmitted. These findings, together with the virus isolations from field-collected individuals in an epidemic area, establish this species as a proven vector of SLE. It was found to be relatively refractory to infection with eastern and western encephalitis viruses.

Relation of Temperature to Activity of Western and St. Louis Encephalitis Viruses

SummaryA review of the geographic distributions of all recorded human outbreaks of arthropod-borne encephalitis in the United States revealed that most western encephalitis (WE) outbreaks have occurred at or above the 70°F June isotherm, whereas St. Louis encephalitis (SLE) outbreaks generally have occurred at or below this isotherm.Average SLE and WE antibody rates in avian sentinel flocks (mostly domestic chickens) at 11 sites in the western United States from 1954 through 1960 indicated that high WE transmission rates occurred at all latitudes, whereas maximum SLE transmission rates were limited to the warmer latitudes at or below the 70°F June isotherm.SLE and WE virus transmission rates in avian sentinel flocks near Greeley, Colorado, during 1954 through 1960 showed marked annual variations and these variations were found to be closely related to spring temperatures. The highest SLE rates and the lowest WE rates occurred during years when the April–June period was unusually warm. the lowest SLE rates and the highest WE rates occurred during years when the April–June period was unusually cold. A similar relationship between temperature and WE virus transmission rates was apparent for shorter periods of observation at nine other sentinel sites in the western states and for a 10-year period of observation near Bakersfield, California. The relationship between SLE virus transmission rates and temperatures at these additional sites was similar to that at Greeley, except for the three southernmost sites where temperature was apparently not a limiting factor.Indices were developed for use in predicting enzootic activity levels of SLE and WE viruses. The index for SLE is the date when 10 day degrees above 75°F are first accumulated, and the index for WE is the date when 10 or 50 day degrees above 70°F are first accumulated. The possible use of these indices for forecasting human outbreaks is discussed.

Selection of a Variant of Western Encephalitis Virus of Low Pathogenicity for Study as a Live Virus Vaccine

Summary Western encephalitis virus strains obtained from naturally infected birds by inoculation into chick embryos, young chicks and hamster kidney tissue culture were subcultured by various methods in an effort to develop variants of low pathogenicity. Prolonged passage of the virus in chick embryos and chicks did not reduce the neuro-pathogenicity of the virus for mice and hamsters. A variant of low pathogenicity was obtained by cloning on chick embryo cells. Clones were at first selected on the basis of pathogenicity of the progeny of the cloned virus for young adult mice inoculated intracerebrally. Later clonal selections were done on the basis of the relative average survival time (in days) (AST) in infant mice inoculated intracerebrally and intraperitoneally, looking for variants having a long AST or the same AST for both routes of inoculation. The clone 15 variant was selected for study as a live virus vaccine because the neurotropic character of the virus was either markedly reduced or eliminated and animals vaccinated with the virus were immune to subsequent intracerebral challenge with the natural virus as obtained from wild birds.