Climate change and Rift Valley fever - Has global warming increased the risk of an outbreak?

Global warming has become a reality that is well recognized by scientists and together with this, according to Dr Gideon Brückner of the OIE, the threat of tropical diseases is moving to new areas. One of these is Rift Valley Fever.

Picture 1. From presentation by Dr Gideon Brückner, Deputy Director General at OIE, presented at the IDF Dairy Farming Summit on Climate Change in June 2008.

History of the disease

Rift Valley Fever (RVF) was named after the Rift Valley in Kenya, over a century ago, as a cause of severe mortalities in newborn lambs. The epidemiology was described in 1930, when the virus, which is transmitted by mosquitoes, was isolated. RVF causes epidemics of abortions in sheep and goats with acute deaths in lambs and kids. It has also been described in wild ruminants, cattle and camels. During an outbreak of RVF in South Africa in the 1950-51 rainy season, 100 000 sheep died and over half a million aborted. Many of those veterinarians and animal health officers who were involved in diagnosing and controlling the outbreaks in 1950/51 and again in 1976-78, in South Africa were, as a direct result, partially blinded by retinal detachment and some died.

In 1977, six years after the Aswan dam was built, more than 200 000 Egyptians were infected with RVF and 598 died. So many livestock were affected that a meat shortage resulted. Mosquitoes breeding in the large new dam were the main vectors and the disease affected both rural and urban communities along its banks. This was when RVF took on the aspect of an emerging zoonosis and the World Health Organisation became involved. Between 1997 and 1998, there was a major outbreak in East Africa and in 2000, RVF spread to Saudi Arabia and Yemen, in the Middle East, sparking fears of a possible spread into Asia and Europe.

Between October and December 2006, the rainfall in Kenya was three times the average for that period and 13 times greater than the same period in 2005. Severe flooding resulted and, as the water drained away, leaving muddy pools, mosquitoes multiplied. According to Pub Med, by the 26th of January 2007, 404 human cases of RVF had been reported with 118 deaths. The most important risk factors for transmission to humans were: drinking unpasteurised milk (72%), living within 100 metres of a swamp (70%), being in contact or owning a sick animal (67%), drinking milk from a sick animal (59%), working as a herdsman (50%), being in contact with a dead animal (50%) and slaughtering an animal (42%). Although 9% of those affected said they had been in contact with a sick person, transmission from human to human is thought to be unlikely. Two patients from Nairobi were suspected to have become infected by handling and eating partly raw or undercooked meat.

Picture 2. From presentation by Dr Gideon Brückner, Deputy Director General at OIE, presented at the IDF Dairy Farming Summit on Climate Change in June 2008.

Symptoms in people

RVF is a highly infectious zoonosis. In earlier outbreaks the symptoms seen in people were usually mild and were sometimes mistaken for influenza. Within three to seven days after contact with an infected animal, patients showed fever, often accompanied by headaches, muscle pains and nausea. Light sensitivity and watery eyes were described as an early sign of retinal detachment, which could lead to partial blindness. Severe cases showed blood in the stool, vomit or sputum or unexplained bleeding from the gums, nose or eyes (haemorrhagic fever). In Egypt and South Africa, hemorrhagic fever, encephalitis and necrotic hepatitis were seen in patients that died of the disease in the late 1970’s.In these outbreaks, the death rate was estimated at between 0.5 and 1% of those who had the disease. However, during the outbreak in East Africa in 2007, the symptoms described in patients were more severe and the mortality rate in affected patients rose to 29%. The haemorrhagic form was common with blood-stained vomit and stools, bleeding gums and noses. The increase in mortality was possibly due to the very heavy rains and flooding, so the dose per patient was probably higher due to many mosquito bites. The presence of HIV/AIDS in the population could also be a factor in the increased risk of mortality. There is no vaccine currently available for RVF in people.

Symptoms in Livestock

Abortion storms and mortality in young animals are considered to be the most common symptoms of RVF. Haemorrhages can sometimes be seen under the skin and in the mucous membranes of the eyes, mouth and vulva and, in the latest cases in South Africa, calves showed a bloody diarrhoea. Blood clotting also appeared to be impaired in dairy cows. On post mortem examination acute deaths can result in bleeding throughout the carcase with bloodstained organs and small haemorrhages in the muscles and organs (See plates 1-3)

[Images 1-3: photographs of organs of dairy cattle that died from RVF, Courtesy of Prof Neil Duncan, Veterinary Faculty of Veterinary Science at the University of Pretoria, South Africa].

RVF in dairy cattle – a case study from South Africa

Although RVF is regarded as a disease that occurs sporadically in wild ruminants, there has not been a spill-over to livestock in nearly 40 years in Southern Africa. However, climate change resulted in unexpected heavy rainfall in the region and in January 2008, the National Institute for Communicable Diseases reported that Rift Valley Fever (RVF) had broken out near Komatipoort in Mpumalanga Province, in South Africa. The disease was confirmed in a Cape buffalo cow and aborted buffalo foetuses in a Cape buffalo breeding unit as well as in several calves. Previously, RVF was considered mainly a disease of sheep, although cattle are known to be susceptible. Two people, one of them the veterinarian who performed the post mortem examination on the buffalo cow, were hospitalised. In March, further cases were confirmed in Limpopo Province in a buffalo–breeding unit near Phalaborwa and also in a group of seven calves that died acutely with haemorrhagic diarrhoea on a cattle farm in Bela-Bela, previously known as Warmbaths. The farmer, as well as one of his workers, who had been in contact with the calves, later showed symptoms of RVF. The carcasses of five of these calves were sent to Onderstepoort Veterinary Faculty and between 3 and 5 days later, four students who performed the necropsies were hospitalized with symptoms of RVF; two workers who assisted them, also showed mild symptoms. Liver damage was evident from blood samples taken from these patients during the febrile stage. All the human cases were confirmed by reverse transcription-polymerase chain reaction (RT-PCR). This is a relatively new technique that is only done in South Africa by the Special Pathogens Unit of the National Institute for Communicable Diseases in Johannesburg. Previously, diagnosis was only possible from serology, which meant that only recovered cases with antibodies against RVF could be confirmed 14-21 days after infection.

A month later, newspapers and the radio reported an outbreak in a dairy herd near Pretoria. There were abortions and at least one cow died acutely. Although the virus is destroyed by pasteurisation, the public was alarmed due to its potential for transmission to consumers. The state veterinary authorities reacted rapidly with heightened surveillance and vaccination of susceptible animals. Fortunately RVF is a seasonal disease and once the frosts arrived in May, the outbreak ended.

Epidemiology

Blue Tongue, another tropical livestock disease transmitted by flying insect vectors is spreading through dairy herds in Europe, probably because global warming is making the climate more suitable for Cullicoides immitus, a tiny biting midge that transmits the disease. RVF has a similar potential, if one looks at the situation in South Africa. It is transmitted by several strains of mosquito. Mosquitoes multiply in the pools left after heavy rainfall and bite both livestock and humans. In East Africa, large populations of Aedes mosquitoes are the first to appear, as their eggs can lie buried up to 6 cm deep in dried out pans until washed out by floods. Shortly thereafter they are followed by Culex spp. In Kwa-zulu Natal in South Africa in 1981, virus was also isolated from Ochlerotatus spp. mosquitoes. Although the epidemiology is not yet well understood, RVF was isolated from mosquito larvae collected in the field in Kenya and it was suggested that the virus may remain in dormant mosquito eggs, surviving a number of dry seasons.

Prevention and control

The OIE strongly supports vaccination, rather than culling out, as a control measure for RVF in livestock. Because of the sporadic nature of the outbreaks, however, annual vaccination is not practiced and consequently, there is usually a shortage of vaccine when an outbreak occurs. The two currently used RVF vaccines have disadvantages. The live Smithburn vaccine can be produced rapidly, but causes problems in pregnant ewes, while the killed vaccine, which is a safe product, takes much longer to produce in an emergency. Pamela Hunter and Michele Bouloy have recently described a new vaccine for livestock, made from a highly attenuated natural RVF mutant, designated as Clone 13, that may solve these problems.

Conclusion

As the risk of RVF has increased due to global warming, farmers should be aware of this disease and be very careful when handling aborted material and foetuses. Milk from all cows should be pasteurised or boiled if an outbreak occurs in an area as it can be infective for humans, even before a cow shows symptoms of the disease. Abortion storms in cattle could be mistaken for brucellosis. A state veterinarian should be contacted immediately if an outbreak is suspected and all livestock vaccinated as soon as possible.

University of Pretoria

University of Pretoria