Rift Valley fever

Rift Valley fever

R SWANEPOEL AND J A W COETZER

Introduction

Rift Valley fever (RVF) is a peracute or acute disease of domestic ruminants in Africa and Madagascar, caused by a mosquito-borne virus and characterized by necrotic hepatitis and a haemorrhagic state, but infections are frequently inapparent or mild. The disease is most severe in sheep, cattle and goats, producing high mortality in new-born animals and abortion in pregnant animals. It is a zoonosis and humans become infected from contact with tissues of infected animals or mosquito bite. Infection in humans is usually associated with mild to moderately severe influenza-like illness, but severe complications such as ocular sequelae, encephalitis and haemorrhagic disease, occur in a small proportion of patients. Outbreaks of the disease occur when particularly heavy rains favour the breeding of the mosquito vectors. In 2000 and 2001 Rift Valley fever virus (RVFV) escaped from the African region to cause a major outbreak of disease on the Arabian Peninsula.358

An acute and highly fatal disease of lambs associated with heavy rains and accompanied by reports of illness in humans was first recognized in the Rift Valley in Kenya at the turn of the century, but the causative agent was not isolated until 1930.82, 272, 374 Major outbreaks of the disease affecting sheep and cattle were recorded in Kenya in 1930–31, 1968, 1978–79, 1997–98 and lesser outbreaks at irregular intervals during the intervening years.94, 95 258, 446

The disease was first recorded in southern Africa in the late 1950s when a large epidemic occurred in the western Free State, southern Gauteng and adjacent North West and Limpopo provinces of South Africa, although it was only recognized as RVF early in 1951 when humans became ill after assisting at a necropsyona bull near Johannesburg.8, 294 Sheep farming dominates in the affected area and it was estimated that 100 000 sheep died and 500 000 aborted in the epidemic, with smaller losses occurring in cattle.347 Lesser outbreaks of the disease or sporadic isolations of virus were recorded in South Africa in 1952–53, 1955–59, 1969–71, 1981 and 1996.21, 29, 244, 248, 253, 425, 438

A severe outbreak affecting mainly sheep occurred occurred in Namibia in 1955, and a second major epidemic occurred in South Africa in 1974 and 1975 following exceptionally heavy rains, with a few cases continuing to be recorded in 1976.29, 63, 342, 438 No estimates were made for total losses in South Africa in 1974–76, but the disease was far more widespread than it had been in the 1950–51 epidemic, extending to the Angora goat farming areas of the Eastern Cape Province, and into Namibia where 230 000 fewer Karakul pelts were exported in 1974 compared to 1973.342, 343

It was recognized from the outset in Kenya that the influenza- like illness in humans could be accompanied by transient loss of visual acuity, but the occurrence of serious ocular sequelae was first reported in the 1950–51 epidemic in South Africa.143, 156, 344, 345 Human deaths following natural infection were first recorded in South Africa during the epidemic in 1975 when seven patients died of encephalitis and haemorrhagic fever associated with necrotic hepatitis. 155, 154, 247, 254, 428 Subsequently, human deaths were recognized during outbreaks in several countries, as recorded below, including a large epidemic which affected the normally arid north-eastern region of Kenya and adjacent part of Somalia after heavy rains in 1997–98.446

Further extensive outbreaks of the disease in southern Africa, involving mainly cattle, occurred in Zimbabwe in 1955, 1957, 1969–70 and 1978,59, 360, 378, 381 in Mozambique in 1969 244, 246, 316, 424 and in Zambia in 1973–74, 1978 and 1985.6, 181 During the 1978 epidemic in Zimbabwe virtually all pregnant cows aborted on some properties and up to 3 per cent died, but serological evidence indicated that on average only 30 per cent of cattle became infected in the affected areas and, taking into account livestock census figures and assuming a 50 per cent fertility rate, it was estimated that there had been 60 000 abortions in the outbreak but that less than 10 000 cattle had died.381 Losses were thought to have been similar in the 1969–70 epidemic in Zimbabwe.381 Human deaths were recorded during the 1978 outbreak.390

In Madagascar, RVFV was first isolated from mosquitoes and a laboratory worker in 1979, but no naturally occurring vertebrate infections were recognized until large outbreaks of abortion in cattle were recorded in 1900 and 1991, together with disease and one death in humans.283, 284, 285, 287, 288

The furthest north that outbreaks of the disease were reported prior to 1977, was in the Sudan in 1973 and 1976.258, 312 During 1977 and 1978, however, a major epidemic occurred along the Nile delta and valley in Egypt, causing an unprecedented number of human infections and deaths, as well as numerous deaths and abortions in sheep and cattle and some losses in goats, water buffalo (Bubalus bubalis) and camels. Estimates of the number of human cases range from 18 000 to more than 200 000 with at least 598 deaths occurring from encephalitis and/or haemorrhagic fever.256– 258, 312, 357 There was a marked decline in the occurrence of RVF in Egypt after 1978, but isolated foci of infection were detected up to 1981, with the virus being associated with meningitis in humans...

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