Rift Valley fever

Rift Valley fever
Classification and external resources
TEM micrograph of tissue infected with Rift Valley fever virus
ICD-10	A92.4
ICD-9	066.3
DiseasesDB	31094
MeSH	D012295

Rift Valley fever (RVF) is a viral zoonosis (affects primarily domestic livestock, but can be passed to humans) causing fever. It is spread by the bite of infected mosquitoes, typically the Aedes or Culex genera. The disease is caused by the RVF virus, a member of the genus Phlebovirus (family Bunyaviridae). The disease was first reported among livestock in Kenya around 1915, but the virus was not isolated until 1931.^[1] RVF outbreaks occur across sub-Saharan Africa, with outbreaks occurring elsewhere infrequently, but sometimes severely. In Egypt in 1977-78, an estimated 200,000 people were infected and there were at least 594 deaths among hospitalized patients.^[2] In Kenya in 1998, the virus claimed the lives of over 400 Kenyans. In September 2000, an outbreak was confirmed in Saudi Arabia and Yemen). On 19 Oct 2011, a case of Rift Valley fever contracted in Zimbabwe was reported in a Caucasian female traveler who returned to France after a 26-day stay in Marondera, Mashonaland East Province during July and August, 2011^[3] but later classified as 'not confirmed.'^[4]

Virology

The virus belongs to the Bunyavirus family. This is a family of enveloped negative single stranded RNA viruses. All Bunyaviruses have an outer lipid envelope with two glycoproteins - G(N) and G(C) - required for cell entry. They deliver their genome into the host-cell cytoplasm by fusing their envelope with an endosomal membrane.

The Rift Valley fever virus G(C) protein has a class II fusion protein architecture similar to that found in flaviviruses and alphaviruses.^[5] This structural similarity suggests that there may be a common origin for these viral families.

Transmission

The virus is transmitted through mosquito vectors, as well as through contact with the tissue of infected animals. Two species - Culex tritaeniorhynchus and Aedes vexans - are known to transmit the virus.^[6] Other potential vectors include Aedes caspius, Culex pipiens, Culex antennatus, Culex perexiguus, Culex zombaensis and Culex quinquefasciatus.^[7][8] Contact with infected tissue is considered to be the main source of human infections.^[9] The virus has been isolated from two bat species: the Peter's epauletted fruit bat (Micropteropus pusillus) and the aba roundleaf bat (Hipposideros abae), which are believed to be reservoirs for the virus.^[10]

Clinical signs and diagnosis

In humans, the virus can cause several syndromes. Usually, sufferers have either no symptoms or only a mild illness with fever, headache, myalgia and liver abnormalities. In a small percentage of cases (< 2%), the illness can progress to hemorrhagic fever syndrome, meningoencephalitis (inflammation of the brain), or affecting the eye. Patients who become ill usually experience fever, generalized weakness, back pain, dizziness, and weight loss at the onset of the illness. Typically, patients recover within two to seven days after onset.

Distribution of Rift Valley fever in Africa: Blue, countries with endemic disease and substantial outbreaks of RVF; green, countries known to have some cases, periodic isolation of virus, or serologic evidence of RVF

About 1% of human sufferers die of the disease. In livestock, the fatality level is significantly higher. Pregnant livestock infected with RVF abort virtually 100% of fetuses. An epizootic (animal disease epidemic) of RVF is usually first indicated by a wave of unexplained abortions.

Other signs in livestock include vomiting and diarrhoea, respiratory disease, fever, lethargy, anorexia and sudden death in young animals.^[11]

Diagnosis relies on viral isolation from tissues, or serological testing with an ELISA.^[11]

Animal vaccination

Several animal vaccines have been made to protect against RVF infection. The first one to be developed was a live vaccine. When administered to mice, the results were promising; this vaccine provided immunity for three years. However, a problem was encountered: administration to pregnant ewes on many occasions led to abortion. Since then, attenuated vaccines have been developed. Although they are protective and do not cause adverse effects, these results are only achieved after multiple inoculations, which may prove problematic, especially in areas where RVF is endemic.

2006/07 outbreak in Kenya and Somalia

In November 2006, a Rift Valley fever outbreak occurred in Kenya. The victims are from the North Eastern Province and Coast Province of Kenya, which had received heavy rain in recent months, causing floods and creating breeding grounds for mosquitoes, which spread the virus of the fever from infected livestock to humans.

By 7 January 2007, about 75 people had died and another 183 were infected.^[12] The outbreak has forced the closure of livestock markets in the North Eastern Province, affecting the economy of the region.^[13]

The outbreak was subsequently reported to have moved into Maragua and Kirinyaga districts of Central Province of Kenya.^[14]

On 20 January 2007, the outbreak was reported to have crossed into Somalia from Kenya and killed 14 people in the Lower Jubba region.^[15]

As of 23 January 2007, cases had started to crop up at the Kenyan capital, Nairobi. An estimated large number of businesses were supposedly suffering large losses as customers were shunning the common meat joints for the popular nyama choma (roast meat), as it was believed to be spreading the fever.

In December 2006 and again in January 2007, Taiwan International Health Action (TaiwanIHA) began operating missions in Kenya consisting of medical experts assisting in training laboratory and health facility personnel, and included donations of supplies, such as mosquito sprays. The United States Centers for Disease Control has also set up an assistance mission and laboratory in Kenya.

By the end of January, 2007, some 148 people had died since the outbreak began in December.

As at 14 March 2007, the Kenyan government declared RVF as having diminished drastically after spending an estimated 2.5 million in vaccine and deployment costs. It also lifted the ban on cattle movement in the affected areas.

As of 2 November 2007, 125 cases, including 60 deaths, had been reported from more than 10 localities of White Nile, Sinnar, and Gezira states in Sudan. Young adult males are predominantly affected. More than 25 human samples have been found positive for RVF by PCR or ELISA.^[16]

2010 South Africa outbreak

As of 8 April 2010, the Ministry of Health South Africa had reported 87 human cases infected with Rift Valley fever (RVF), including two deaths in Free State, Eastern Cape and Northern Cape provinces.^[17] Most of these cases reported direct contact with RVFV-infected livestock and or were linked to farms with confirmed animal cases of RVF. The human cases are: farmers, veterinarians and farm workers. All cases were confirmed with RVF by test conducted at the National Institute of Communicable Diseases (NICD) in Johannesburg, South Africa.

An ongoing outbreak of Rift Valley fever virus (RVFV) infection is affecting sheep, goats, cattle and wildlife on farms within Free State, Eastern Cape, Northern Cape, Western Cape, Mpumalanga, North West, and Gauteng provinces. As of 29 March 2010, about 78 farms reported laboratory-confirmed animal cases, with extensive livestock deaths.

Outbreak investigations by the Department of Health and the Department of Agriculture, Forestry and Fisheries are ongoing, and are being supported by the South African Field Epidemiology and Training Programme and NICD. The Department of Health and the Department of Agriculture are taking measures to enhance disease surveillance among cattle and in managing the control of the disease outbreak.

Sporadic cases of RVFV infection in animals have been documented in South Africa in recent years. The last major outbreak of the disease in humans occurred between 1974 and 1976, where an estimated 10,000 to 20,000 cases were recorded.^[18]

The disease claimed the life of Springbok rugby player Juan Smith's father Giel.^[19]

Use as a biological weapon

Rift Valley fever was one of more than a dozen agents that the United States researched as potential biological weapons before the nation suspended its biological weapons program in 1969 due to possible bomb threats that might cause a panic in the United States.^[20][21]

References

External links

• Official CDC RVF Information Page
• WHO Rift Valley fever