Entomological studies at an enzootic Venezuelan equine encephalitis virus focus in Guatemala, 1977-1980.

The ecology of several potential mosquito vectors of Venezuelan equine encephalitis (VEE) alphavirus was studied in an enzootic focus of that virus on the Pacific coast of Guatemala over a four-year period. Four species-Culex taeniopus, Mansonia titillans, Culex nigripalpus and Aedes taeniorhynchus-were most prevalent during the wet season when transmission normally occurs. However, only Cx. taeniopus yielded VEE virus. The bloodfeeding patterns of these species revealed that Ae. taeniorhynchus and Ma. titillans fed almost exclusively on bovine and equine hosts. Conversely, Cx. nigripalpus was highly ornithophilic but occasionally fed on mammals. Cx. taeniopus exhibited a wide host range, utilizing both large and small mammals as well as birds and, rarely, reptiles. The versatility in feeding pattern displayed by this mosquito coupled with its ability to become infected with relatively low levels of enzootic VEE virus suggests that vertebrates other than rodents may serve as amplifying hosts in this habitat. Nepuyo virus was also isolated from Cx. taeniopus, suggesting that this mosquito might be an endemic vector of this rodent-associated bunyavirus. A single isolate of St. Louis encephalitis virus was made from Cx. nigripalpus.

Ecologic observations of Venezuelan encephalitis virus in vertebrates and isolations of Nepuyo and Patois viruses from sentinel hamsters at Pacific and Atlantic habitats in Guatemala, 1968-1980.

La Avellana and Puerto Barrios, two enzootic foci of Venezuelan encephalitis (VE) virus on the Pacific and Caribbean lowlands (respectively) of Guatemala have been studied over a 13-year period. Data from sentinel hamsters and guinea pigs and wild and domestic vertebrates are reported. VE virus strains were isolated from hamsters each period they were exposed during the rainy seasons 1968-1980 and at the end of the dry season 1974. Rates of isolation of VE virus ranged from 0.2%-5.7% hamster/days/exposure. All strains tested were free of epizootic virions. Although virus was isolated from sentinel guinea pigs, their deaths were not attributable to infection with VE virus. Antibody titers in 26 of 28 terrestrial mammals bled at La Avellana in 1971 were higher to enzootic than to epizootic VE strains. Thirty-seven percent of 109 residents of Puerto Barrios had antibody to VE virus. In 13 of 20 tested, antibodies were engendered by the enzootic strain. Nepuyo and Patois viruses were isolated from sentinel hamsters at both La Avellana and Puerto Barrios.

Enfermedad humana causada por el virus Nepuyo, un bunyavirus de mesoamerica transmitido por mosquitos. / Human diseases caused by Nepuyo virus, mosquito-borne bunyavirus of Middle America

En el curso de estudios sobre el virus de la encefalitis venezolana realizados en La Avellana, departamento de Santa Rosa, en la costa del Pacifico de Guatemala se contrajeron dos infecciones por virus Nepuyo, un bunyavirus del complejo del grupo C de la zona neotropical. Un caso se presento en agosto de 1972 y el otro en agosto de 1977. En ambos casos aparecio una enfermedad febril leve, parecida al dengue, con viremia, comienzo subito, cefalea, mialgias y postracion. Se encontraron anticuerpos inhibidores de la hemaglutinacion y neutralizantes, pero no anticuerpos fijadores del complemento. Probablemente los vectores selvaticos sean mosquitos del complejo Culex (Melanoconion), mientras que como huespedes vertebrados amplificadores de los arbovirus del grupo C actuan roedores, marsupiales y seres humanos. En todo caso, convendria realizar estudios ecologicos detallados

Mesenteronal infection threshold of an epizootic strain of Venezuelan encephalitis virus in Culex (Melanoconion) taeniopus mosquitoes and its implication to the apparent disappearance of this virus strain from an enzootic habitat in Guatemala.

Culex (Melanoconion) taeniopus is a vector of Venezuelan encephalitis (VE) virus at a marsh focus in Guatemala and has low mesenteronal thresholds for infection by and transmission of two enzootic strains of VE virus. In contrast, samples of natural populations and subsequent F2 and F4 generations of these mosquitoes have a high mesenteronal threshold for infection by an epizootic VE strain isolated at the same marsh during the end of the 1969 VE epidemic-equine epizootic. The resistance of Cu. (Mel). taeniopus to mesenteronal infection by this VE strain probably represents a key factor in the apparent disappearance of epizootic VE virus from the marsh focus following the 1969 outbreak.

Intestinal threshold of an enzootic strain of Venezuelan encephalitis virus in Culex (Melanoconion) taeniopus mosquitoes and its implications to vector competency and vertebrate amplifying hosts.

The minimal intestinal dose of an enzootic strain of Venezuelan encephalitis (VE) virus for Culex (Melanoconion) taeniopus mosquitoes caught at a marsh habitat of VE virus in Guatemala was less than five plaque forming units (pfu) of virus. Ingestion of this dose of virus in blood of viremic hamsters resulted in transmission of virus to other hamsters. This low intestinal threshold of an enzootic strain of VE virus indicates that the natural Guatemalan population of Cu. (Mel.) taeniopus can acquire VE virus from vertebrates that have viremia levels as low as 1,000-5,000 pfu/ml of blood, provided other factors do not limit virus interchange between mosquitoes and vertebrates.

Transmission of Venezuelan encephalitis virus by naturally infected Culex (Melanoconion) opisthopus.

During August 1977 two of 975 Culex (Melanoconion) opisthopus collected from an enzootic marsh habitat on the Pacific coast of Guatemala transmitted VE virus to hamsters. Eight VE strains were isolated from Cu. opisthopus. The minimal level of VE infection in this species during July-August 1977 at La Avellana, Guatemala was 1/128 (8/1,021), and the prevalence of Cu. (Mel.) opisthopus transmitting VE virus was 1/487 (2/975). This mosquito was the predominant species attacking humans at that time, suggesting that Cu. opisthopus is a vector of VE virus to man as well as a vector in enzootic cycles in Guatemala. These studies establish Cu. opisthopus as the third proven enzootic vector of VE virus.

Further observations on infections of guinea pigs with Venezuelan encephalitis virus strains.

Guinea pigs from a Guatemalan colony died after subcutaneous inoculating of moderately small doses of equine-benign strains of Venezuelan encephalitis (VE) virus of hemagglutination-inhibition subtype I-E from enzootic habitats in Mexico and Guatemala. Thus these guinea pigs were unlike English short hair and inbred 13 guinea pigs, which usually survive infections with equine-benign VE strains of subtype I-E. We therefore caution others that not all strains of guinea pigs can be used to evaluate the potential equine virulence of VE viruses.

Search for persistent epizootic Venezuelan encephalitis virus in Guatemala, El Salvador and Nicaragua during 1970-1975.

Evidence was sought during 1970-1975 of persistence of equine-virulent Venezuelan encephalitis (VE) virus in regions of Central America that were heavily involved in the epidemic-equine epizootic of 1969. (a) Four sentinel horses were exposed in an arid, upland region of the Atlantic drainage of Guatemala during August-October 1970, but no horse became infected. (b) The epicenter region of the 1969 outbreak, in southwestern Guatemala and southwestern El Salvador, was studied during July 1970-February 1974; no antibody developed in sentinel horses, sentinel hamsters did not die, mosquitoes yielded no virus, wild rats had no detectable VE virus HI antibody. Unexplained decreases in populations of wild terrestrial mammals possibly limited maintenance of VE virus. However, mosquitoes were plentiful and present in the same species composition found at a focus of enzootic VE virus about 35 km northwest of the epicenter region. (c) In studies at two Guatemalan ranches near the epicenter, where horses died in 1969, VE viruse infected sentinel horses along one of three lakes on one ranch during the wet season of 1972 but not during the dry or wet seasons of 1973; the titers of neutralizing antibodies in these four horses were higher against an enzootic strain of VE virus than against an epizootic strain. During 1970 and 1971, VE virus was isolated from sentinel hamsters exposed at a marsh on the other ranch, but Vero plaque characteristics were those of enzootic VE virus. (d) The only epizootic activity of VE virus discovered in Central America in 1970-1975 occurred in Nicaragua between April and June 1972. Several hundred horses died, and N antibody, like that engendered by epizootic virus, was found in two young, unvaccinated horses. Whether this represented persistence of epizootic VE virus or reintroduction of virus is unknown.

Search for epizootic-like Venezuelan encephalitis virus at enzootic habitats in Guatemala during 1969-1971.

Seventy-four strains of Venezuelan encephalitis (VE) virus recovered from sentinel hamsters or mosquitoes at enzootic habitats in Guatemala in the two years following the 1969 epidemic-equine epizootic were examined for ability to produce small plaques in Vero African green monkey kidney cell cultures, like isolates obtained during the epizootic. (a) One strain recovered from a sentinel hamster in late October 1969 at an enzootic habitat near the epicenter of the hemagglutination-inhibition (HI) and equine-virulence properties like epizootic virus; this strain retained its small plaque characteristic after inoculation and recovery from bloods of three horses. (b) None of the other 73 strains produced uniformly small plaques, but 31 formed a few small plaques among large ones. Virions from small plaques of five strains were cloned twice in Vero cell cultures. Four clones produced uniformly small plaques after one more passage in Vero cells; three had hemagglutination-pH properties compatible with epizootic virus or intermediate between epizootic and enzootic virus, but HI tests with these three hemagglutinins or with antibody to the fourth cloned strain showed them to be like Central American enzootic virus. One of three cloned strains tested in horses produced encephalitis and death in one of four horses; another strain produced encephalitis with recovery in one of two horses. (c) Thus these small Vero plaque clones resembled Central American enzootic strains of VE virus in HI and equine-virulence tests, and the small Vero plaque characteristic was not a satisfactory marker for consistently isolating equine-virulent, epizootic VE virions. Nevertheless, this technic led to recognition of one epizootic strain isolated at an enzootic habitat in Guatemala at the end of 1969 outbreak. Whether this strain was there before the outbreak or subsequently penetrated the habitat is uncertain. During the next two years, this strain did not become dominant in that enzootic focus.

Ecologic studies of Venezuelan encephalitis virus and isolations of Nepuyo and Patois viruses during 1968-1973 at a marsh habitat near the epicenter of the 1969 outbreak in Guatemala.

Ecologic studies of Venezuelan encephalitis (VE) virus at a marsh habitat near the epicenter of the 1969 outbreak in Guatemala revealed that the virus was enzootic there. VE virus was isolated yearly during 1968-1973 from sentinel hamsters exposed during the rainy seasons and from mosquitoes collected during July and August 1970. Hamsters yielded 41 strains of VE virus and virus was detected within 2 km of the edge of the marsh, in its interior, and at its western extreme 18 km from the central study site at La Avellana. One strain of virus came from a hamster that died in the dry season of January 1970. Culex mosquitoes yielded 20 strains of VE virus and Mansonia and Aedes one each. Culex (Melanoconion) and Aedes taeniorhynchus were most prevalent near the marsh. Hemagglutination-inhitibion (HI) and neutralization antibody tests of sera showed that wild terrestrial mammals (opossums and rodents), humans, and dogs, but not wild birds, were frequently infected. Seven of 16 susceptible residents of villages at the edge of the marsh developed antibodies without symptoms during an 18-month period between September 1971 and February 1973. Only 1 of 5 sentinel rabbits, and none of 30 sentinel chickens developed VE HI antibody during August-September 1971, a period when virus activity was readily detected by the use of sentinel hamsters. Five strains of group C arbovirus (one identified as Nepuyo) were recovered from sentinel hamsters during 1968 to 1970, and one strain of Nepuyo virus was isolated from the blood of a person with a febrile illness during 1972. Two strains of Patois group arboviruses were isolated from Culex mosquitoes during 1970.

Studies of possible movement of Venezuelan encephalitis virus from an enzootic focus in Guatemala during 1971-1974.

During the wet seasons of 1972 and possibly 1971, sentinel horses became infected by Venezuelan encephalitis (VE) virus in a temporally and geographically progressive manner inland from an enzootic marsh focus of virus on the Pacific couast of southeastern Guatemala. During the wet seasons of 1972 and 1973, VE virus was detected by sentinel horses (and a sentinel hamster in 1972) in a small woods 10 km north of the marsh, but virus was undetectable there during the dry seasons of 1973 and 1974 and the wet season of 1974. Culex (Melanoconion) mosquitoes were found in this woods and at the marsh during August 1973. These observations are compatible with movement of VE virus from the marsh habitat during some wet seasons. However, virus activity in this region adjacent to the marsh was quantitatively unpredictable on a yearly basis and occurred in only very focal habitats during 1971 to 1974. Mechanisms of VE virus movement from the marsh are currently unknown, but bats are under study as a likely possibility.

Studies of Patois group arboviruses in Mexico, Guatemala, Honduras, and British Honduras

Patois, Zegla, and Shark River-like arboviruses in the Patois group were isolaed in Mexico, Guatemala, Honduras, and British Honduras during 1963 to 1968. Twenty-three Patois viruses came from mosquitoes, sentinel hamsters, and suckling mice, 21 Zegla viruses from sentinel hamsters and mice and from a wild cotton rat, and two Shark River-like viruses from sentinel hamsters. Hemagglutination-inhibition(HI) and neutralizing antibody tests with Patois virus were positive with human sera from each country, and HI antibody tests were positive with sera from pigs in Mexico, Guatemala, and British Honduras and from small wild mammals, wild birds, and cattle in Mexico.(AU)