Rift Valley Fever
Rift Valley Fever
Disease History, Characteristics, and Transmission
Introduction
Rift Valley fever (RVF) is a viral disease usually associated with outbreaks among livestock animals, but also affects humans. It is caused by a virus from the family Bunya-viridae and is endemic in areas of Africa, but can spread to surrounding regions.
The virus is passed to animals and humans via mosquitoes. The virus is naturally occurring in some mosquitoes and the highest incidence of infection is associated with periods of heavy rain and flooding when mosquito populations are at peak numbers. Mortality rates among animals are high, but typically only 1% of human cases prove fatal. Symptoms generally resolve within a week of illness onset and include fever, headache, and weakness. In some cases, complications occur including inflammation of the eyes, meningoencephalitis, and hemorrhagic fever.
There are currently no preventative treatments available. However, researchers are developing vaccines and treatments for the disease among humans and animals. Increasing evidence of human fatalities has led to Rift Valley fever being termed an emerging virus and one that is considered to pose significant potential threat to communities.
Disease History, Characteristics, and Transmission
Rift Valley fever (RVF) is a viral disease primarily affecting domestic livestock (such as cattle, sheep, buffalo, goats, and camels), but one that can also be passed on to humans. The disease is caused by the RVF virus, which is a member of the genus Phlebovirus in the family Bunyaviridae. RVF was first reported among livestock in Kenya around 1915. The Rift Valley virus was first isolated in Kenya in 1931.
The Rift Valley fever virus is naturally occurring in some species of mosquitoes such as the Aedes. The virus may lay dormant in the eggs, which are capable of surviving for several years in dry conditions. During periods of heavy rain and flooding, these eggs will hatch and cause a significant increase in the mosquito population. The mosquitoes transfer the virus to the animals on which they feed. The disease can be transmitted to humans via mosquitoes, or by exposure to the blood or organs of infected animals.
In humans, the virus is symptomatic in 90% of those infected. Incubation of the infection is usually 2 to 6 days, after which symptoms commonly present as a flulike illness including fever, headache, muscle pain, generalized weakness, dizziness, and weight loss. In less than 2% of cases, the illness progresses and develops into a severe form. In less than 2% of cases, eye disease occurs and involves ocular swelling and retinal inflammation. This can lead to permanent vision loss, including blindness. In less than 1% of cases, RVF leads to meningoencephalitis. The most severe complication is hemorrhagic fever, and this occurs in less than 1% of cases. The hemorrhagic fever complication is responsible for most RVF deaths, with around 50% of cases of hemorrhagic fever proving fatal.
Scope and Distribution
Rift Valley fever was initially limited to the regions of eastern and southern Africa where sheep and cattle are raised. Prior to 2000, RVF was limited to Africa. In late 2000, cases of Rift Valley occurred in Saudi Arabia and Yemen. This spread of the virus indicates a potential threat of the virus spreading further into Europe or Asia.
People at risk of contracting the disease include people in contact with animals such as animal herdsman, veterinarians, and abattoir (slaughterhouse) workers. Frequent exposure to mosquito bites in areas where outbreaks occur will also increase risk of contracting the virus. Travelers to areas where the Rift Valley fever virus is endemic are also under threat of infection, particularly during times of viral outbreak.
Epidemics are almost always associated with heavy rainfall periods and localized flooding, which creates breeding grounds for the mosquitoes. The first RVF outbreak was reported in Egypt in 1977–1978 where human infection rates in some parts were as high as 35% and 598 deaths resulted from hemorrhagic fever. An epidemic occurred in 1987 in West Africa due to flooding caused from construction of the Senegal River Project. In 1997, Kenya and Somalia suffered an epidemic that resulted in 300 human fatalities with much higher rates for livestock. In 2006, an epidemic occurred in Kenya following flooding.
Treatment and Prevention
Diagnosis of Rift Valley fever in humans is performed through laboratory blood analysis identifying antibodies to the virus. In the majority of cases, the causative agent is obvious due to the epidemic nature of outbreak. There is no treatment available for the infection except for supportive therapy for the symptoms. Researchers are investigating the potential for the use of an antiviral drug in humans. However, as of 2006, it is still in developmental stages.
WORDS TO KNOW
ENDEMIC: Present in a particular area or among a particular group of people.
EPIDEMIC: From the Greek epidemic, meaning “prevalent among the people,” is most commonly used to describe an outbreak of an illness or disease in which the number of individual cases significantly exceeds the usual or expected number of cases in any given population.
EPIZOOTIC: The abnormally high occurrence of a specific disease in animals in a particular area, similar to a human epidemic.
Many animal vaccines have been developed to protect against RVF infection, but are often subject to limitation. One such vaccine was found to deliver immunity to mice for up to three years, but led to spontaneous abortion when administered to pregnant ewes. It was found that multiple doses of vaccines may be required to provide immunity. This would prove problematic in areas of endemnicity where successful immunity would be subject to resource availability. Human vaccines are also under trial, but as of 2006, are in the early phases and require significant testing.
Preventative measures may be taken by people to avoid contracting the disease. These include reducing possible contact with mosquitoes. This may be achieved by wearing protective clothing such as long pants and long sleeved shirts in addition to the use of insect repellents and bed nets while sleeping. People in contact with animal blood or tissue can avoid infection by wearing gloves and other protective equipment.
Impacts and Issues
Rift Valley fever poses significant economical impacts on communities due to the fatality rates among livestock and the permanent threat of epidemics in certain areas. In an outbreak of RVF in Kenya in the 1950s, over 100,000 sheep were killed. This devastated the community and it took several years to recover. In pregnant livestock, infection by this virus results in abortion of almost all fetuses. This raises the issues of herd sustainability and growth. The mode of transmission of the Rift Valley fever virus makes it virtually impossible for farmers to protect their herds or themselves. With the natural occurrence of the virus in some mosquitoes, a rainy season or flood will almost certainly leave some communities devastated.
The complications associated with infection from RVF among humans can be quite severe and as such, this disease can also be considered a high risk. Spreading of the disease from Africa to Yemen and Saudi Arabia has raised concern that the disease could spread to new areas. It is considered that stock, mosquitoes, and travelers could all potentially act as carriers of the virus and introduce it into new regions. Various species of mosquitoes act as vectors for the RVF virus, suggesting that the virus could be maintained once in a new region. This may potentially cause animal and human epidemics.
In addition to being spread by mosquitoes, the virus can also be spread by aerosols. This suggests that the virus could be introduced to a new area and spread rapidly within the area. These concerns have led the United States to list Rift Valley fever as a significant biological warfare threat.
Rift Valley fever remains a health threat, especially in Africa's developing nations and at-risk areas following natural disasters. From December 2006 through February 2007, an epidemic of Rift Valley in Kenya fever killed 155 people. There were nearly 700 suspected cases associated with the outbreak. The epidemic hit most acutely in several regions already strained by severe flooding and food shortages. Health officials instituted a quarantine of animals and humans in disease-affected areas, but the disease spread across national borders, infecting 90 and killing 19 in neighboring Tanzania.
See AlsoAirborne Precautions; Antiviral Drugs; Bioterrorism; Emerging Infectious Diseases; Viral Disease.
BIBLIOGRAPHY
Books
Fong, I.W., and K. Alibek. Bioterrorism and Infectious Agents: A New Dilemma for the 21st Century. New York: Springer Science, 2005.
Mandell, G.L., J.E. Bennett, and R. Dolin. Principles and Practice of Infectious Diseases. Vol. 2. Philadelphia, PA: Elsevier, 2005.
Web Sites
Centers for Disease Control and Prevention (CDC). “Rift Valley Fever Fact Sheet.” <http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/Fact_Sheets/Rift_Valley_Fever_Fact_Sheet.pdf> (accessed March 9, 2007).
Directors of Health Promotion and Education. “Rift Valley Fever.” 2005 <http://www.dhpe.org/infect/rift.html> (accessed Mar. 9, 2007). World Health Organization (WHO). “Rift Valley
Fever.” September, 2000 <http://www.who.int/mediacentre/factsheets/fs207/en/> (accessed March 9, 2007).