NAIROBI, 22 December (IRIN) - An outbreak of Rift Valley Fever (RVF), a rare contagious hemorrhagic fever, has killed 11 people in Kenya's northeastern districts of Garissa and Ijara, health officials have said.

"The dead are mainly livestock herders," Charles Nzioka, the head of Communicable Disease Control at the Ministry of Health, said on Friday in Nairobi.

Four more people have been admitted at the Garissa Provincial Hospital's isolation unit, Nzioka added.

It is thought, however, that the number of those affected could be higher "because the symptoms of fever, headache, and muscle pain are also common in other diseases such as flu and malaria," he said.

RVF is endemic in Africa, naturally occurring in livestock but occasionally affecting humans. Animals are infected with the RVF virus by the Aedes mosquito.

The disease, characterised by bleeding from all orifices, is spread to humans in a number of ways: from infected mosquitoes; through contact with blood or other body fluids; or from the organs of infected animals.

Nzioka said samples from the patients were tested at the Kenya Medical Research Institute, which has highly specialised laboratories.

"The disease is only severe in about one percent of all human cases," Nzioka said. "In other cases, the infection usually disappears before patients succumb."

Currently, there is no vaccine available for the treatment of the viral disease in humans, although one is available for animals.

RVF mainly occurs following periods of heavy rainfall. In 1998, an outbreak claimed at least 200 lives in the Northeastern Province following the El Nino rains. The province also experienced heavy rainfall in November which led to massive flooding, affecting hundreds of thousands of people.

There was a major RVF outbreak in Kenya and Somalia in 1997 and 1998 with the first cases of RVF outside Africa being reported in September 2000 in Saudi Arabia and Yemen.

"We have been alert since the onset of the flooding," Nzioka said, "We have also heightened our surveillance of all infectious diseases."

Due to the flooding, there has also been an increased risk of waterborne diseases such as cholera. The number of malaria cases has also risen.

To cope with the outbreak, the government has mobilised additional medical and veterinary personnel to the affected district.

"We are also asking the people living in flood affected areas to seek medical assistance if they experience any of the symptoms," he said.

Symptoms of RVF include fever, general weakness and malaise, cough and diarrhoea, with severe cases progressing to show facial swelling and bleeding from the orifices. The incubation period (interval from infection to onset of symptoms) of RVF varies from two to six days.

"We are also asking livestock keepers to be on the look-out for abortions in livestock," he said.

According to the United Nations World Health Organisation (WHO), an epidemic of RVF is usually first manifested as a wave of unexplained abortions among livestock.

With the disease mainly occurring among livestock, the government has also imposed a control on the movement of livestock in the affected areas.

Meanwhile, Nzioka said, there are plans to merge RVF livestock vaccinations along with an existing campaign to vaccinate against Foot and Mouth disease.

"We are conducting wide scale insecticide spraying to reduce the numbers of mosquitos breeding in the stagnate waters following the floods," he said.

The Ministry of Health is also involved in the distribution of insecticide-treated mosquito nets.

Since 1930, when the virus was first isolated amongst sheep on a farm in the Rift Valley of Kenya, there have been outbreaks in sub-Saharan and North Africa, according to the WHO.

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