Outbreak: Rift Valley Fever in Kenya

SPECIAL COVERAGE

An outbreak of rift valley fever has killed more than 300 people and countless ruminants in the Garissa District of Kenya. Rift Valley fever is caused by an enveloped RNA virus that is a member of the genus Phlebovirus, family Bunyaviridae. A virus called Zinga appears serologically identical to Rift Valley fever virus by neutralization testing and is, in fact, a strain of this virus (Centers for Disease Control. MMWR Morb Mortal Wkly Rep 1983;32:90-92).

Three to seven days after infection with this virus, illness begins abruptly with fever, headache, arthralgias, myalgias, and photophobia. More than 90% of cases undergo self-limited undifferentiated febrile disease without complication. Many of the remainder develop macular and perimacular retinitis with vasculitis, which may cause permanent blindness. In perhaps 1% of cases, frequently fatal fulminant illness manifest by coagulopathy, with bleeding and hepatic failure, may develop, often after 4-6 days of illness (Laughlin LW, et al. Trans R Soc Trop Med Hyg 1979;73:630-633). Encephalitis may also occur. Ribavirin has been reported to have activity in vitro and in animal models against Rift Valley fever virus (Huggins JW, et al. Antimicrob Agents Chemother 1984;26:476-480; Kende M, et al. Antimicrob Agents Chemother 1987;31:986-990).

The main route of transmission to humans is by the bite of infected Aedes and Culex mosquitoes. It also may be transmitted directly, possibly via aerosol, from affected domestic livestock. Person-to-person transmission does not ordinarily occur.

Other mosquitoes, especially ones that breed in natural depressions on the savanna that become watering holes after rains, transmit the virus between other vertebrates. Human activity, such as dam construction, may also potentially lead to epidemics (Walsh J. Science 1988;240:1397-1399).

The virus appears to be maintained in the endemic areas by transovarial transmission in certain species of Aedes mosquitoes, while additional mosquito genera become important during epizootics and epidemics. During outbreaks, which tend to occur after infrequent heavy rains, sheep and cattle serve as "amplifiers" because of the high level of viremia achieved in these domestic ungulates (Lithicum KJ, et al. J Hyg 1985;95:197; Easterday BC. Adv Vet Sci 1965;10:65).

This virus received its name because it was first isolated from an infected host in the Great Rift Valley of Kenya in 1931. The central highlands of Kenya are bisected by this distinctive terrestrial feature, which runs in a generally north-to-south direction through the country. This fissure in the earth’s crust spans a distance of 6500 km from the Dead Sea to Beira on the coast of Mozambique. It was formed more than 25 million years ago by a process (rifting) in which a block of the earth’s crust sinks between two parallel diverging lithosphere plates.

The currently affected part of Kenya, the Garissa District, is a low-lying area remote from the usual tourist centers of Nairobi, Mombassa, and the game parks, and is not part of the Rift Valley. The western border of the district is comprised of the Tana River, its only permanent surface water and along which most sedentary district agriculture occurs. (A detailed map of Kenya can be view on the Internet at http://www.city.net/countries/ kenya/maps/kenya.html.)

This arid portion of Kenya is sparsely populated, with many of the inhabitants practicing nomadic pastoralism—their movements determined by the availability of water. The climate is hot and dry with unreliable torrential rains which, when they occur, generally do so in April and October. The infrequent rains are harvested in man-made earth pans, which become watering holes for the camels, cattle, sheep, and goats—the maintenace of which are central to the existence of the human population. These infrequent collections of water bring to life dormant transovarially infected eggs, and the resultant mosquitoes find a ready source for blood meals in the thirsty livestock.

As a consequence, outbreaks of Rift Valley fever usually occur during periods of greatest rainfall. Recently, Southwest Somalia and the Northeastern Province of Kenya, of which the Karissa District forms the southern portion, suffered from heavy flooding, with waters first beginning to recede at the beginning of November 1997. Such outbreaks may prove predictable based on knowledge of relevant ecological parameters, particularly ground moisture and flooding of mosquito larval habitats (dambos) associated with outbreaks by monitoring with satellite remote sensing imagery (Linthicum KJ, et al. Science 1987;235:1656-1659; Pope KO, et al. Remote Sensing of Environment 1992;40:185-196).

In addition to flooding caused by rainfall, man-made flooding has also been associated with epidemics of Rift Valley fever as illustrated by an outbreak in the Senegal River Valley of Mauritania. The existence of this virus in this region was already known at the time the Diama Dam was completed near the mouth of the Senegal River in 1986. In fact, environmental impact studies had predicted the "amplification of the virus" as a consequence of the upriver flooding (Digoutte JP, Peters CJ. Res Virol 1989;140:27-30). A 1977 outbreak of Rift Valley fever in Egypt, the first evidence of this disease north of the Sahara, may also have been the consequence of the inundation of 800,000 hectares by the Aswan High Dam between 1970 and 1977. It is estimated that 200,000 human cases with almost 600 deaths resulted from this outbreak in the Nile delta. Another outbreak in the Aswan Governate began in 1993 and led the United Nations to provide 1.5 million doses of vaccine to be used in the livestock population.

Which of the following is correct?

a. Rift Valley fever is caused by a DNA virus.

b. Outbreaks of Rift Valley fever generally occur after heavy rains in otherwise arid or semi-arid areas.

c. Ticks of the genus Hyalomma are responsible for the transmission of Rift Valley fever virus.

d. Rift Valley fever virus is commonly transmitted by aerosol between humans.