Oral infection of Aedes aegypti with yellow fever virus: geographic variation and genetic considerations.

Twenty-eight populations representing a worldwide distribution of Aedes aegypti were tested for their ability to become orally infected with yellow fever virus (YFV). Populations had been analyzed for genetic variations at 11 isozyme loci and assigned to one of 8 genetic geographic groups of Ae. aegypti. Infection rates suggest that populations showing isozyme genetic relatedness also demonstrate similarity to oral infection rates with YFV. The findings support the hypothesis that genetic variation exists for oral susceptibility to YFV in Ae. aegypti.

Selection for susceptibility and refractoriness of Aedes aegypti to oral infection with yellow fever virus.

Artificial selection on strains of Aedes aegypti showing susceptibility and refractoriness to oral infection with yellow fever virus (YFV) suggests that there is a significant genetic component to this trait. Using a population with an average susceptibility of 15%, inbreeding of isofemale lines followed by individual selection produced susceptible (29% infected) and refractory (11% infected) lines. The difference between lines was largely apparent before individual selection, which failed to increase/decrease susceptibility significantly. The findings suggest that very few loci with a major bearing on the trait segregated genetic variation in the original population sample, and that non-genetic factors also play a major role in determining whether or not Ae. aegypti females become infected with YFV.

Arthropod studies with rabies-related Mokola virus.

A cell culture-adapted variant of the rabies-related Mokola virus was demonstrated to replicate in inoculated Aedes aegypti mosquitoes. Replication was slow compared to many arboviruses in their vectors. Maximum titers were not obtained until after approximately 6 weeks of extrinsic incubation. Mokola virus underwent nine mosquito-mosquito passages at approximately monthly intervals and was thus maintained in insects for 340 days before terminating the study. Virus antigen was detected by immunofluorescence in a variety of mosquito tissues and organs, including salivary glands, but primarily in nervous tissue. Irrefutable virus transmission by bite could not be demonstrated because of equivocal results. Transovarial passage of virus was observed in the mosquito. Viremia in baby mice was demonstrable. Ornithodoros moubata nymphal ticks were exposed to viremic mice but failed to become infected.

The effect of colonization upon aedes aegypti susceptibility to oral infection with yellow fever virus.

Two colonies of Aedes aegypti were established from two independent collections from Vero Beach, Florida. Eleven sequential generations of the first colony were tested for variation in oral susceptibility to infection with yellow fever virus (YFV). Each generation was also assayed for genetic variability at seven enzyme loci using electrophoretic techniques. Significant differences in infection rates were detected between some generations. These differences were significantly correlated with genetic variation at the malate dehydrogenase locus. Seven generations from the second colony were examined simultaneously for variation in susceptibility to YFV. Significant differences were also detected between some of these generations. The results suggest that colonization may have an effect on the genetic and phenotypic variation in a mosquito strain, and that genetically based variation for susceptibility to infection with YFV occurs in populations of Ae. aegypti.

Dengue-2 vaccine: oral infection, transmission, and lack of evidence for reversion in the mosquito, Aedes aegypti.

The dengue-2 vaccine virus (S-1), and its parent virus (PR-159), were compared for their ability to infect orally, to replicate in, and subsequently to be transmitted by Aedes aegypti mosquitoes. The vaccine virus was markedly less efficient in its ability to infect mosquitoes orally. After ingesting infectious bloodmeals containing 3, 7 to 8.2 log10MID50/ml of the respective viruses, 56% (220/396) of the mosquitoes became orally infected with the parent virus contrasted with 16% (66/397) for the vaccine virus. None of the 16 infected mosquitoes transmitted the vaccine virus, while 14% (3/22) of the mosquitoes transmitted the parent virus. The vaccine virus remained temperature-sensitive (restrictive temperature 39 degrees C) after orally infecting and replicating in Ae. aegypti mosquitoes.

Avian pox in birds from Trinidad.

Over a 7-year period in Trinidad, 9,514 birds were examined for avian pox and four species were found infected: the golden-headed manakin, Pipra erythrocephala (7% infected), the white-bearded manakin, Manacus manacus (5%), the violaceous euphonia Euphonia violacea (1%), and the bare-eyed thrush, Turdus nudigenis (less than 1%). The elaborate courtship displays of manakins may have a bearing on a "common source" type of infection. The apparently abrupt appearance of the disease at three localities in Trinidad in 1964 perhaps indicates introduction of the virus by migratory birds.

Transovarial transmission of yellow fever virus in Stegomyia mosquitoes.

Transovarial transmission of two strains of yellow fever virus was demonstrated in three colonized geographical strains of Aedes aegypti following infection by intrathoracic inoculation. Infected progeny were detected in F1 offspring from only the first three ovarian cycles (OVC). The overall minimum filial infection rate for the first three OVC was 1:596; rates were loser in the second and third OVC. Virus survived in eggs for longer than 4 months and was recovered from progeny of three immersions of first OVC eggs. Infected progeny averaged a longer time to pupation than noninfected siblings. Transovarial transmission of virus was also demonstrated in first OVC progeny of Aedes mascarensis.

Transovarial transmission of yellow fever virus by mosquitoes (Aedes aegypti).

Female Aedes aegypti mosquitoes infected with yellow fever virus by intrathoracic inoculation transmitted the virus to a small percentage of their F1 progeny. Infected offspring were obtained from surface-sterilized as well as from untreated eggs, indicating that the virus was transovarially transmitted. Vertical transmission of yellow fever virus in mosquitoes may be an alternative mechanism for biological survival of the virus during adverse periods or in the absence of susceptible vertebrate hosts.

Aedes aegypti strain fitness for yellow fever virus transmission.

Three geographical strains of Aedes aegypti from Thailand (Amphur), East Africa (Kampala), and the West Indies (Santo Domingo) were compared for susceptibility to infection with low-passage yellow fever virus (French viscerotropic) as well as for ability to transmit virus by bite at varying extrinsic incubation periods. Santo Domingo strain appeared the most competent and Kampala the least when mosquitoes were exposed to a low level virus-infecting blood meal; at higher virus levels, a similar trend was noted but differences were less evident and in no case were the differences statistically significant. All three strains were infected with and transmitted yellow fever virus.

Pacui virus, phlebotomine flies, and small mammals in Brazil: an epidemiological study.

Pacui virus, originally obtained from forest rodents, was isolated 100 times from 61,437 specimens (658 pools) of the phlebotomine fly Lutzomyia flaviscutellata, collected from rodent-baited traps in the forests of Belem, Para, Brazil in the period October 1968 through September 1970. Isolations were made from engorged and unengorged females and from males (3 strains), and occurred in all 24 months. Pacui virus also was isolated from the blood of two wild rodents (Oryzomys), but not from 424 L. infraspinosa, 12,000 mosquitoes, or sentinel mice. Pacui virus neutralizing antibodies were detected in serum of six bait animals after exposure to biting flies in the forest, in 30% of wild rodents surveyed (including two from Amapa Territory), and in 10% of marsupials, but were absent in human survey sera and in bats. Low-passage Pacui virus produced viremia in and was lethal to infant mice by the subcutaneous route. L. flaviscutellata was most abundant in the dry season, in which period Pacui virus isolations increased. This fly is strongly attracted to rodents close to the ground. L. flaviscutellata also yielded single strains of Guama, Icoaraci, and BeAr 177325 viruses.