Aedes furcifer and other mosquitoes as vectors of chikungunya virus at Mica, northeastern Transvaal, South Africa.

From 1977 to 1981, studies were conducted on a farm at Mica where Aedes furcifer had been a vector during an epidemic of chikungunya virus in 1976 to determine whether the virus persisted in this mosquito, the likelihood of vertical transmission, and whether any other Aedes species could have been vectors. Aedes furcifer/cordellieri was the only prevalent tree hole Aedes which fed readily on monkeys and humans and occurred through the summer until the onset of winter. Virus was not isolated from 7,241 females and 4,052 males of this group, which were largely Ae. furcifer and which included a sample of the first post-epidemic population. Five additional Aedes species were prevalent in bamboo pots, 3 of which (Ae. aegypti, Ae. fulgens and Ae. vittatus) were shown to be competent laboratory vectors. Virus was not isolated from a sample of 13,029 such newly emerged mosquitoes representing the first post-epidemic population. It is concluded that Ae. furcifer is an epidemic-epizootic vector which does not maintain the virus at Mica and that no other mosquito species could have been important vectors.

A bionomic study of adult Aedes (Neomelaniconion) circumluteolus in northern Kwazulu, South Africa.

Sixteen monthly visits were made to the Ndumu game reserve when resting Aedes circumluteolus mosquitoes were collected on the ground. The population density reached exceedingly high levels 7 or more days after the Usutu and/or Pongola rivers inundated their flood plains, and was related mainly to river flooding and far less to local rainfall. During dry months male Ae. circumluteolus disappeared while females persisted at very low levels. Blood-feeding and ovarian development occurred throughout the year and precipitin tests showed the preferred host was almost exclusively antelope (Bovidae), the dominant large mammal present. It was concluded that Ae. circumluteolus is a floodwater mosquito. Owing to the low density of females during dry months and low infection rates with arboviruses it is considered unlikely that Ae. circumluteolus would support viral transmission throughout the year. It is more likely that viruses survive in overwintering eggs and are transovarially transmitted.

Experimental assessment of the vector competence of Culex (Culex) neavei Theobald with West Nile and Sindbis viruses in South Africa.

Culex univittatus is the maintenance vector of West Nile (WN) and Sindbis (SIN) viruses on the inland plateau of South Africa and also transfers infection to man. Both viruses have frequently been isolated from this mosquito and human immunity is high on the plateau. In the Natal lowlands where Cx univittatus is replaced by Cx neavei, apparently the main vector there, immune rates are low and few isolations have been made from this species. Vector competence experiments were carried out with Cx neavei to compare with those previously done with Cx univittatus. It was thought that a difference in such competence might explain the contrasting epidemiological situation in the two regions. After infective feeds on chicks with high viraemias of each virus, Cx neavei showed a high transmission rate. However, the 50% infection thresholds determined by exposing birds with differing viraemias to mosquitoes of this species, were 4.4 logs per ml (WN) and 5.8 logs (SIN). These thresholds are both higher than those for Cx univittatus. It is concluded that Cx neavei is a poorer vector of both viruses which accounts for the lower viral activity, including the lower incidence of human infection, in the Natal lowlands.

Laboratory vector studies on six mosquito and one tick species with chikungunya virus.

The tick Ornithodoros savignyi and the mosquitoes Culex horridus, Culex quinquefasciatus, Aedes fulgens, Ae. furcifer and Mansonia africana were tested for infection rates and ability to transmit chikungunya virus. O. savignyi and Cx quinquefasciatus did not become infected and Cx quinquefasciatus failed to transmit the virus between vervet monkeys, Cercopithecus aethiops. Only one of 17 Cx horridus feeding on a blood-virus mixture became infected which included infection of the salivary glands. Ae. fulgens had a high infection rate and transmitted the virus between Mystromys albicaudatus rodents. Ae. furcifer and Ma. africana both transmitted virus between vervet monkeys: the 50% infection threshold and the transmission rate were less than 4.5 logs and 25% respectively for Ae. furcifer and c. 5.5 logs and 29% for Ma. africana. In a further test, Ae. furcifer transmitted virus from a monkey to hamsters at a transmission rate of 32%. Attempts to demonstrate transovarial transmission of the virus in Ae. aegypti and Ae. furcifer were unsuccessful. It is concluded that Ae. furcifer is fitted for its suspected role as epidemic vector and that Ma. africana could also act as an important epidemic vector in southern Africa.

Rift Valley fever in humans in South Africa.

During an epizootic of Rift Valley fever in South Africa in 1974/1975, mainly affecting sheep and cattle, a large number of human cases occurred. The majority of the patients, mostly farmers, farm labourers and veterinary surgeons, acquired the infection while handling the carcasses of animals which had died of Rift Valley fever. Some gave no history of contact with infected animals and it is presumed that they were infected by mosquitoes. Complications were common. Retinitis clinically associated with defective vision occurred in about 20% of patients investigated. In others, meningo-encephalitis developed; 1 patient died and the brain showed perivascular cuffing and round-cell infiltration. In 110 cases the diagnosis was confirmed in the laboratory, in 17 by isolation of the virus and in 93 by the finding of an antibody response in serological tests. In 3 fatal cases the virus was isolated from the liver, and in a further 4 fatal cases in which virus isolation was not attempted, there was clinical and epidemiological evidence of the diagnosis; in each case a haemorrhagic state associated with hepatitis had developed. In 1978, follow-up studies showed that immune rates among residents on affected farms varied from 10% in children to 17,1% in adult males, with an overall immunity rate of 14,5%.

A survey of the mosquito and Culicoides faunas at two localities in the Karoo region of South Africa with some observations of bionomics.

The mosquito and Culicoides faunas were surveyed at Bethulie and Luckhoff in the arid Karoo region, south Orange Free State, to determine which species occurred, their relative prevalence and the effects of rainfall. The feeding preferences of these insects were also investigated by means of baited catches. Twenty-three mosquito species and 16 Culicoides species were collected. The commonest mosquito species, with their feeding preferences, if known, were as follows: Culex (Culex) univittatus Theo and Culex (Culex) pipiens Linnaeus, which are strongly ornithophilic and poorly anthropophilic; Culex (Culex) theileri Theo, which feeds on sheep and man avidly but is only moderately ornithophilic; Aedes (Neomelaniconion) luridus McIntosh, Aedes (Neomelaniconion) lineatopennis (Ludlow), Aedes (Ochlerotatus) caballus (Theo) and Aedes (Ochlerotatus) juppi McIntosh, all of which feed on sheep and man readily and which can aestivate as eggs for up to 20 months but only appear in numbers after rain; Anopheles (Cellia) listeri De Meillon, Anopheles (Cellia) squamosus Theo, Culex (Culex) quinquefasciatus Say and Culiseta (Akllotheobaldia) longiarelata (Macquart). By far the commonest Culicoides at both localities was Culicoides pycnostictus Ingram & Macfie, which is strongly ornithophilic and also feeds on sheep. The following 5 species were also prevalent: Culicoides similis Carter, Ingran & Macfie, Culicoides spec. nov. 1., Culicoides schultzei (Enderlein), Culicoides ondersteportensis Fiedler and Culicoides nivosus De Meillon. The last species is strongly ornithophilic.

Methohexitone and etomidate for bronchoscopy.

General anaesthesia for bronchoscopy has been provided with either methohexitone or etomidate, with and without fentanyl. Recovery was faster following methohexitone and there was no statistically significant difference in the incidence of complications. Inclusion of fentanyl in the technique did not confer any benefits with either drug.

Isolation of an arenavirus closely related to Lassa virus from Mastomys natalensis in south-east Africa.

Five unidentified virus strains were recovered from the multimammate mouse ,Mastomys natalensis, during the course of studies on arbovirus infections in Mozambique. These agents were found to be morphologically and immunologically related to Lassa virus. Four of 19 sera from Mastomys captured in the study area had antibodies to both Lassa virus and one of the unidentified strains. Although not definitive, the differences noted in results of complement fixation and indirect immunofluorescent tests suggest that these viruses from south-east Africa are not identical to West African Lassa virus.