Wesselsbron disease

Wesselsbron disease

R SWANEPOEL AND J A W COETZER

Introduction

Wesselsbron disease (WSL) is an acute arthropod-borne flavivirus infection of sheep, cattle, and goats. The virus causes relatively high mortalities in new-born lambs and kids, subclinical infection in adult animals, occasional abortion in ewes, congenital malformations of the central nervous system accompanied by arthrogryposis of the ovine and bovine foetus, and hydrops amnii in ewes. In humans it causes a non-fatal influenza-like illness.

The history of WSL in southern Africa is interwoven with that of Rift Valley fever (RVF), and this has had a marked influence on the manner in which the disease is perceived. In 1955 neonatal deaths and abortions occurred in a flock of sheep in the Wesselsbron district of the Free State in South Africa on a farm where pregnant sheep had been inoculated withRVFvaccine two weeks previously. It was feared that the problem was associated with the use of the vaccine, but a new virus, named after the district of its origin, was isolated from the brain and liver of an eight-day-old lamb.88 Within a month, the same virus was isolated from the blood of a febrile field worker employed by a medical team that was investigating arthropodborne viruses in the northern part of KwaZulu-Natal, and from mosquitoes.72 The virus was also isolated from a lamb in the Kroonstad district, close to Wesselsbron, in 1957. Transmission by mosquitoes was soon demonstrated experimentally,61 and the new agent was identified as a flavivirus.18, 72

In 1956 and 1957 outbreaks of WSL complicated by geeldikkop, a hepatotoxic poisoning associated with the plant Tribulus terrestris,46, 82 and enzootic icterus, a chronic form of copper poisoning,13 occurred on three farms in the Middelburg district of the Northern Cape Province.14, 54 During these outbreaks WSL virus (WSLV) was isolated from the tissues of a number of sheep as well as from mosquitoes and an entomologist.37, 52

In South Africa, the virus was subsequently isolated from a moribund cow in KwaZulu-Natal in 1973,58 the organs of two calves in 1974 and 1975 in the Mpumalanga and Gauteng provinces,20 two new-born lambs in 1975 in the central Free State,24 two lambs in 1988 (one animal originating from Upington in the Northern Cape Province and the other from Clocolaan in the eastern Free State), a lamb in 1994, and mosquitoes and lambs in 1996 (both the latter two incidents occurring in the Free State).20, 42, 83

In Zimbabwe, WSLV was isolated from wild-caught mosquitoes during the 1969–70 RVF epidemic and from the organs of a cow that had died during an epidemic of RVF in 1978.15 There have been several isolations of the virus from mosquitoes, humans and a camel in West Africa and from mosquitoes in Thailand.68, 75

It is widely accepted that WSLV infection constitutes an important disease of sheep in South Africa, but this assessment is not supported by published and field evidence, nor has a livestock disease problem attributable to this virus been identified elsewhere in Africa. On the other hand, misuse of the modified live (attenuated) vaccine in pregnant sheep has resulted in significant economic losses as a result of abortions and foetal malformations.75

Aetiology

The WSLV has not been well characterized, but it has the properties typical of a haemagglutinating flavivirus.65 The investigations which proved that it was a new flavivirus18, 72 were performed with the second strain (H177) to be isolated, which originated from the blood of a laboratory field worker in 1955.72 Consequently, strain H177 has been widely regarded as the prototype of WSLV78 rather than the original Van Tonder strain.88 Strain YM310.66, isolated from a mixed pool of aedine mosquitoes in Cameroon in 1966, shows quantitative differences in cross-serological tests with WSLV and is regarded as a subtype of the virus.66 Evidence that WSL, yellow fever, Banzi and Uganda S viruses form a close antigenic group or complex within the flaviviruses15, 35, 73, 78 has not been apparent in all studies.12, 26

Primary isolation of WSLV is most easily achieved by the use of mice. Similar titres are reached when the virus is titrated by intracerebral or intraperitoneal inoculation of new-born mice or intracerebral inoculation of weaned mice, but mice over three weeks of age are resistant to intraperitoneal infection.12, 88 Hamsters develop viraemia after the subcutaneous inoculation of virus.58 Titration of virus infectivity and demonstration of virus neutralization have been performed most frequently by intracerebral inoculation of new-born or weaned mice, but there is a trend towards using cytopathic effect or plaque formation in cell cultures for these purposes.12, 21, 15, 25, 26, 45, 74, 84 The virus replicates with cytopathic effects or formation of plaques in a variety of cell cultures including Vero and baby hamster kidney-cell lines and primary lamb kidney cells.15, 45, 88

Epidemiology

Wesselsbron virus has been isolated from vertebrates or arthropods in South Africa, Zimbabwe, Uganda, Kenya, Nigeria, the Central African Republic, Senegal, Cameroon, Ivory Coast and Thailand.75, 60 In addition, the results of antibody surveys provide evidence for the occurrence of the virus in Mozambique, Botswana, Namibia, Angola and possibly Madagascar.75 Information is lacking for many countries in Africa, but from the distribution of the...

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