Rift Valley Fever is a zoonotic infection, caused by a phlebovirus of the Bunyaviridae family, which was first identified in Kenya in 1931. It has since been found in Egypt, South Africa, Madagascar, and the Middle East, but is mostly confined to the Great Rift Valley from which it takes its name. It infects livestock such as sheep, cattle, and goats, though it can be transmitted to humans through mosquitoes and handling the corpses of infected animals. Its effects are mainly economic, as it kills up to 45% of sheep and 30% of cattle and goats; it can also cause mass abortions throughout a flock. Its effects in humans are less severe (resembling a nasty flu with slight photophobia), though the virus may cause hemorrhagic fever, permanent blindness, and encephalitis. There is no cure.
Rift Valley Fever was first found in the Great Rift Valley area of Kenya in 1931, isolated from the blood of a newborn lamb. Since then, the disease has been found from South Africa to Egypt, Yemen and Saudi Arabia to Senegal. It has spanned all of Africa and, in 2000, began to spread into the Middle East. At last count, 33 countries (31 in Africa and 2 in the Middle East) showed signs of the disease.
Vectors and transmission
The virus can be transmitted through contact with infected fluids, as might occur during butchery of an infected animal; it can also be spread as an aerosol, which has led to speculation that it might be used as a bioweapon. However, mosquitoes must take the blame for most outbreaks. The life cycle is fairly simple; when a mosquito1 sucks the blood of an infected animal, it and all its offspring will carry the virus, as it is passed on from mother bug to baby bug. In normal weather, the disease remains dormant; when a heavy rainfall floods the land and provides massive new breeding grounds for mosquito larvae (recall that mosquitoes lay their eggs only in stagnant water), the plague spreads2. All major outbreaks recorded to date have coincided with heavy rains or floods.
Notable outbreaks to date
- Kenya, 1930-1931. This was the first recorded instance of RVF. Imported European breeds of sheep were found to be particularly susceptible to the virus.
- Kenya, 1951. Over 100,000 sheep died.
- South Africa, 1951. Estimated livestock deaths were over 1 million.
- Nigeria, 1967. The disease began to march westward.
- Sudan and Egypt, 1977-1978. The disease spread north, killing hundreds of thousands of animals and approximately 600 people.
- Mauritania, 1987, following the opening of the Diama Dam on the Senegal River. 200 human deaths.
- Egypt, 1993, after opening the Aswan Dam.
- Kenya and Somalia (just what they needed, eh?), 1997. Heavy rains gave rise to a surge in mosquito population. 300 recorded deaths.
- Saudi Arabia and Yemen, fall and winter of 2000. Fewer than 200 deaths, but led to a ban on meat from African countries.
Rift Valley Fever is more severe and widespread among domestic animals than it is amongst humans. The three main herds of the regions (sheep, goats, and cattle each react differently to the virus. Sheep are hit the worst, while goats fare best. Incubation time ranges from 12 hours in young animals to 6 days in adult humans. Any animal that catches the disease can expect the following symptoms:
This last is the first warning sign of the impending plague. After this initial "abortion storm" (grisly image, eh?), a similar percentage of young animals will die off. Recovery rates among adults are somewhat higher: 20-45% mortality among sheep, 10-30% among goats and cattle. In human beings, RVF mortality is around 1%. The typical case will appear similar to a nasty flu; fever, nausea, vomiting, muscle aches, headaches, and sometimes hypersensitivity to light. These usually go away after 2-14 days. More severe cases can cause blindness from damage to the retina (about 2% of patients), encephalitis/meningitis (about 1% incidence), or a hemorrhagic fever similar to Lassa or Ebola (also a 1% chance). The encephalitis is not usually fatal, while the hemorrhagic fever kills approximately half of its victims. Children and the elderly, as usual, are more likely to die from the infection.
Anyone living in an outbreak-prone region is likely to catch the disease. The risk is higher for those who sleep outside, exposed to the mosquitoes (herdsmen, for example) and those who come in contact with contaminated body fluids (butchers, veterinarians, and others who handle the corpses of infected animals). There is some indication that it is spread through the milk of diseased animals as well.
The wave of spontaneous animal abortions will be the first inkling that anything is wrong. Afterwards, animals will exhibit the symptoms listed above. In humans, it usually appears as a flu, and in most cases never progresses further; the photosensitivity is the only unusual symptom. The hemorrhagic fever version causes jaundice from liver damage, subcutaneous bleeding, and massive gastrointestinal hemorrhages. Doctors can confirm a diagnosis, if need be, by testing the patient's blood for the virus. They may use the enzyme-linked immunoassay (ELISA) test to detect the antigens of the virus, or they may detect the virus directly in blood and dead tissue.
None. There have been experiments with the antiviral drug ribavirin, but no conclusive results.
There are two animal vaccines currently available, one live and one inactivated. The live version has caused birth defects and abortions in sheep, but is effective in slowing the spread of the disease. The inactive vaccine requires multiple doses, which can be difficult and expensive. There is also an inactive vaccine for human beings which has not been approved for broad use, though it is sometimes given to laboratory workers in high-risk situations. Standard precautions against mosquitoes (protective clothing, netting, repellents and insecticides) help reduce the spread of the virus via mosquitoes. Respiratory masks and rubber gloves reduce contact with infectious aerosols and fluids. Taken together, these precautions can help slow the spread of the disease.
RVF as a bioweapon
Just a minor side note: The United States Army, before shutting down its biological warfare program in 1969, developed RVF as a bioweapon. They discovered that 50 kilograms, properly dispersed, could kill or incapacitate over 35,000 people. Furthermore, it would settle into the animal herds and be nearly impossible to eradicate, thus contaminating any targeted country's food supply for the foreseeable future. Any enemy so attacked would be crippled. Fortunately, all such research was abandoned in 1969.
1 Three known species of mosquito carry the virus, though others are certainly capable of doing so: Aedes cumminsii, Aedes circumluteolus and Aedes mcintoshi.
2Another theory holds that the virus is carried in wild animals, then transmitted to livestock and human beings. Unfortunately, no plausible candidate animal has been found (though the antelope look mighty suspicious), so the "mosquito" theory is the most commonly accepted.
"Rift Valley Fever." Encyclopædia Britannica 2003 Encyclopædia Britannica Premium Service.
06 Feb, 2003 .
CDC Office of Communication: http://www.cdc.gov/od/oc/media/fact/riftvall.htm
CDC Special Pathogens Branch, http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/rvf.htm
Tara K. Harper, TKH Virology notes: http://www.tarakharper.com/v_rift.htm
WHO Fact Sheet, http://www.who.int/inf-fs/en/fact207.html
Glenn E. Schweitzer and Carole C. Dorsch, Superterrorism: Assassins, Mobsters, and Weapons of Mass Destruction (New York: Plenum Trade, 1998).
Iowa State University, College of Veterinary Medicine, http://www.vetmed.iastate.edu/services/institutes/iicab/fad/riftvalleyfever/whatisrvfdetail.htm
This writeup complies with the standards put forth in E2 FAQ: Diseases.