Rift Valley fever virus activity in East Africa in 1989.
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Biomedical subjects
Publications and source records attributed to F G Davies.
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A survey was carried out in horse, zebra, elephant, camel, sheep and goat and wild carnivore sera for virus-serum neutralising antibody to the nine type strains of African horse sickness virus. Antibody was found amongst the horse, zebra and elephant sera to all nine different strains. No antibody was detected in any sera from camels, sheep and goats. None was found in sera from hyaena and jackals in this series but had been detected earlier.
An apparently new strain of bluetongue virus was first isolated in Kenya in 1965 and since, has been obtained on 7 further occasions from diseased sheep during clinical outbreaks of disease. It proved to be serologically different from the 16 bluetongue virus strains then held at this laboratory. The virus was modified by passage in embryonated hens eggs to produce a live virus strain suitable for inclusion in a polyvalent vaccine. Recent neutralisation tests, carried out with 24 guinea pig immune sera prepared at Pirbright against the currently known World serotypes, have confirmed the earlier results and show that it is different from any of the existing serotypes.
One hundred and seven consecutive patients attending the outpatient epilepsy clinic at a teaching general hospital were assessed by clinical interview for a history of sexual abuse. Questionnaires dealing with overall psychiatric symptomatology i.e., (SCL-90), (TSC-40) and depression (ZSRDS) were also used. The majority of subjects were single (60%), living at home (76.6%) and had an average age of 29 years. The mean duration of epilepsy was 18.8 years and the seizures were controlled with medication in 65.2% of patients. Ten (9.3%) of the subjects had been sexually abused. This frequency of sexual abuse is lower than in the general population and among psychiatric patients. The specific form of sexual abuse consisted of sexual intercourse (n = 4), fondling (n = 4) and oral sex (n = 2). The sexually abused subjects had significantly higher scores on the anxiety subscale of the SCL-90 and depression score on the ZSRDS than non-abused subjects.
An hypothesis that there was an annual emergence of Rift Valley fever virus in Zambia, during or after the seasonal rains, was examined with the aid of sentinel cattle. Serum samples taken during 1974 and 1978 showed evidence of epizootic Rift Valley fever in Zambia, with more than 80% positive. A sentinel herd exposed from 1982 to 1986 showed that some Rift Valley fever occurred each year. This was usually at a low level, with 3-8% of the susceptible cattle seroconverting. In 1985-6 more than 20% of the animals seroconverted, and this greater activity was associated with vegetational changes--which could be detected by remote-sensing satellite imagery--which have also been associated with greater virus activity in Kenya.
During an outbreak of Rift Valley fever (RVF) in livestock near Lake Naivasha, Rift Valley Province, Kenya, 61,347 mosquitoes (1,287 pools) collected in CO2-baited light traps yielded seven viral isolates. Five isolates of RVF virus were recovered from 18,831 Culex zombaensis Theobald and one from 14,439 Mansonia africana (Theobald). One isolate of a Bunyamwera group virus was recovered from 1,175 Aedes quasiunivittatus (Theobald).
Ephemeral fever occurs in Kenya in epidemics which have been associated with periods of unusually heavy and prolonged rainfall. The epidemics persist for 2-3 years and involve most of the country. They occur in the same years as epizootics of Rift Valley fever, which is a mosquito-borne Phlebovirus infection. Sporadic outbreaks of the disease have been associated with local heavy rains; though one outbreak occurred during a dry period. Culicoides spp. were generally present in large numbers at the site of the outbreaks. Seroconversions were detected in sentinel cattle, when there had been no apparent clinical disease.
Measurements of green-leaf vegetation dynamics recorded by the advanced very high resolution radiometer instruments onboard National Oceanic and Atmospheric Administration satellites 7 and 9 were used to derive ground moisture and rainfall patterns in Kenya and monitor resultant flooding of mosquito larval habitats (dambos) likely to support Rift Valley Fever virus vector mosquitoes (Aedes and Culex spp.). Satellite-derived data from mid-1981 to December 1988 have been analysed with corresponding rainfall, flooding and vector population data as they relate to Rift Valley Fever virus ecology. Single (7 x 7 km) and multiple grid-cell image analysis (200 x 300 km) in small localized areas and large geographical regions, respectively, of vegetation data were used to quantify the potential for flooding of mosquito breeding habitats. The ability to detect accurately parameters, such as ground moisture, that determine flooding could provide local officials with sufficient warning to allow for implementation of specific mosquito control measures before a disease outbreak.
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Eleven virus isolations were made from the blood of 45 free living healthy African buffaloes by long term cocultivation of their leucocytes with bovine thymus or spleen cells. The isolates were indistinguishable from each other or from herpesviruses isolated from a severely ill buffalo calf and from a dead buffalo. These viruses possessed the characteristics of the bovine herpesvirus-3 (BHV-3) group and were indistinguishable by serology and restriction endonuclease analysis from the BHV-3 type strains Movar 33/63 and DN599. There was a 93.6 per cent prevalence of indirect immunofluorescent antibody to BHV-3 in the sera of 94 buffaloes in the sample population. No clinical signs or viraemia were detected in five cattle inoculated with 10(8.7) log10 TCID50 of the isolate from the sick buffalo calf. Two of three cattle hyperimmunised with this virus resisted challenge with malignant catarrhal fever herpesvirus, which proved fatal for the other immunised animal and for three control cattle.
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A Kenyan sheep and goat pox virus was attenuated by serial passage in bovine foetal muscle cell cultures. The pathogenicity of the strain was lost between the 15th and 20th passages. Serum-neutralizing antibody developed after inoculation with passages tested up to the 50th. These passages appeared to protect animals against laboratory challenge by intra-dermal titration. The 18th passage was successfully and extensively used to control the disease under field conditions.
Serum neutralizing antibody to camel pox virus was found in 5 out of 6 camel herds in Kenya. This was not related to recently observed clinical disease in the herds.
A total of 134 876 Diptera collected in Kenya during a 3-year period were tested in 3383 pools for Rift Valley fever (RVF) virus. Nineteen pools of unengorged mosquitoes were found positive for RVF. All isolations were made from specimens collected at or near the naturally or artificially flooded grassland depressions that serve as the developmental sites for the immature stages of many mosquito species. The isolation of virus from adult male and female A. lineatopennis which had been reared from field-collected larvae and pupae suggests that transovarial transmission of the virus occurs in this species.
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