Rift Valley fever (RVF) is an acute, fever-inducing, viral disease that primarily impacts livestock (such as cattle, buffalo, sheep, and goats) and humans. Originating in Africa, the mosquito-borne disease has lately caught the attention of U.S. health officials because of its likeness to the West Nile virus as a public health concern.
There are important differences between West Nile virus and RVF infections. 80% of West Nile virus infections are asymptomatic, and about 1infection in 150 cases will result in severe disease. (Ref http://www.cdc.gov/ncidod/dvbid/westnile/resources/fact_sheet_clinician.htm) The fatality rate in humans is less than 1 in 1000. (Ref http://www.westnilefever.com/west_nile_virus_symptoms.htm) In contrast, up to 30 percent of RVF-infected livestock and 14 percent of people seriously ill in past outbreaks have died from RVF. Infections in animal fetuses are almost always fatal. The fatality rate for RVF in humans is less than 1 in 100. Moreover, RVF has a greater likelihood of transmission because the virus is capable of being carried by at least 30 species of mosquitoes and perhaps other bloodsucking insects. The stability of the virus in aerosol form also makes Rift Valley fever a candidate for an agri-terror agent.
The RVF virus was first isolated in a 1931 sheep outbreak in Kenya’s Rift Valley in eastern Africa. Subsequent outbreaks have occurred since in sub-Saharan and North Africa. An RVF epidemic in Kenya and Somalia in 1997-98 resulted in the deaths of at least 300 people and large numbers of animals. In September 2000, the first RVF cases reported outside of Africa were confirmed in Saudi Arabia and Yemen, raising concerns of virus expansion to other parts of Asia and Europe.
No new cases of Rift Valley fever have been reported outside of Africa since 2001.
Rift Valley fever is spread mainly to people by the bites of infected mosquitoes. Contact with the blood, other body fluids or organs of infected animals can also lead to infection in humans. Exposure to RVF can occur by:Slaughtering or handling infected animals
Touching contaminated meat during food preparation
Inoculation (e.g., if the skin is broken, or through a wound from an infected knife)
Ingesting raw milk
Aerosol inhalation (usually through contact with laboratory specimens containing the virus)
While most cases of RVF are relatively mild, a small proportion of patients experience a much more severe disease course. Generally, this presents itself as one of three recognizable syndromes:Eye disease
Meningo-encephalitis (inflammation of the brain and surrounding tissue, which can lead to headaches, coma, or seizures)
Hemorrhagic fever (which is characterized by bleeding and can lead to shock)
About 0.5-2% of patients develop eye disease, and less than 1% of patients progress to meningoencephalitis and hemorrhagic fever.
Fever and other mild symptoms may accompany eye disease, which usually results in inflammation of the retina (nerve layer that senses light and creates impulses that travel through a nerve to the brain). Consequently, approximately 1% to 10% of patients may suffer some permanent vision loss. Eye disease usually occurs one to three weeks after the first symptoms appear.
The onset of meningo-encephalitis in some patients is also normally one to three weeks after the first symptoms appear.
Most RVF fatalities occur in patients who manifest hemorrhagic fever. Two to four days after the first symptoms appear, these patients develop severe liver disease and jaundice, and may vomit blood, pass blood in the feces, develop a reddish-blue skin rash that doesn’t blanch when pressure is applied (purpura), and bleed from the gums. 50% of RVF patients who develop hemorrhagic fever perish.
Overall, approximately 1% of humans that become infected with RVF die of the disease. People who die from RVF are usually malnourished, have poor access to medical care, or are suffering from other diseases.
Those at risk for contracting RVF include:People who sleep outdoors at night in outbreak areas, thereby exposing themselves to mosquitoes
Animal herdsmen, slaughterhouse workers, veterinarians and others who handle infected animal tissue in areas where the virus is present
International travelers who visit areas where the virus is present during an outbreak or epidemic