An outbreak of the highly contagious and potentially fatal Rift Valley fever killed scores of people in Kenya, Somalia and Tanzania between December 2006 and mid-2007. The most vulnerable included thousands of refugees fleeing the Somali conflict to an area near the border with Kenya.
- The virus is spread through mosquito bites and contact with infected animals
- Outbreaks are more likely after heavy rainfall
- In a few cases people vomit blood or bleed to death
People catch the virus through infected mosquitoes, or by contact with body fluids such as blood and untreated milk, and with organs of infected animals. Sheep, goats, camels and cattle are all susceptible to the virus.
Outbreaks are more likely during a period of heavy rainfall when mosquito eggs hatch. The virus can lie dormant for years in mosquito eggs which survive long periods of drought before hatching in the rainy season.
Most human cases are mild, and people usually recover within a week. Symptoms include fever, headache, muscle pain, phobia to light, and vomiting.
But a few can develop haemorrhagic fever and bleed to death. This usually affects less than 1 percent of cases, according to the U.N. World Health Organisation, but has had a much higher death rate in the 2007 epidemic.
Symptoms include severe liver disease, jaundice, vomiting blood, passing blood in the faeces, developing a rash caused by bleeding in the skin, and bleeding gums.
There is no cure yet, although human vaccines are being developed. Animals can already be vaccinated against the virus, and this is the best form of prevention along with wearing protective clothing when handling sick animals.
The virus was first isolated in 1930 in the Rift Valley of Kenya, and outbreaks have been detected across the continent since then. The first cases reported outside Africa were in Saudi Arabia and Yemen in 2000.
The last major outbreak was in Kenya and Somalia in 1997, when thousands were infected and hundreds died.