Kvet Forum
Well-known member
Rift Valley fever (RVF) is an acute arthropod-borne viral disease that can cause severe disease in domestic animals, such as cattle, goats, sheep, camels and buffalo. Rift Valley fever is also an important zoonosis that can cause severe disease in humans.
Aetiology
Rift Valley fever virus is a single-stranded RNA virus of the family Bunyaviridae, genus Phlebovirus.
The virus remains viable in aerosols at 25° C for 1 hour or longer but can survive in serum at 4° C for several months.
Infected sheep plasma can retain its infectivity over years with storage and shipment under a variety of refrigeration conditions. Infectious material thus presents a potential source of infection for laboratory personnel or veterinarians for a prolonged period of time.
Epidemiology
Rift Valley fever (RVF) was first recognized in 1930 in the Rift Valley in Kenya but now exists and occurs as epizootics throughout Sub-Saharan Africa, with recent extensions into Egypt, Mauritania, and Madagascar.
The disease has great potential for spread to other countries either through legal or illegal movement of infected livestock or the encroachment of mosquitoes into new areas. Epidemics are inherently linked to regional climate variability, particularly to rainfall patterns.
Risk Factors
1. Animal Risk Factors
Rift Valley fever is an infectious disease primarily affecting ruminants but to which other species, including humans, are also susceptible. Lambs, goat kids, puppies, and kittens extremely susceptible, with mortality rates between 70% and 100%. Adult sheep and calves have been categorized as highly susceptible, with mortality rates between 20% to 70%. Camelids, equids, pigs, and adult dogs and cats are considered resistant to clinical disease, with infection being inapparent in these species.
2. Environmental Risk Factors
Outbreaks of RVF have been associated with above-normal rainfall and climatic conditions favorable to competent vectors. Epidemics have been reported in:
- Valleys near a river in where outbreaks occur following heavy rainfall events.
- In artificially irrigated areas the permanent availability of water favors the persistence of a vector population.
- Temperate and mountainous areas where transmission of RVF virus in results from vector-borne transmission associated with livestock movement.
Method of Transmission
In ruminants RVFV is transmitted between animals through primary and secondary arthropod vectors. Several different species of mosquitoes are competent vectors for the RVF virus, and rises in incidence of RVF most commonly occur after periods of heavy rainfall which lead to an abundance of mosquitoes. Some species of mosquito (Aedes, for example) are capable of transmitting the virus from infected female mosquitoes to offspring via their eggs. This contributes to the survival of the virus in the environment.
People usually get Rift Valley fever through contact with blood, body fluids, or tissues of infected animals, mainly livestock such as cattle, sheep, goats, buffalo, and camels. This direct contact can occur during slaughter or butchering, while caring for sick animals, during veterinary procedures like assisting an animal with giving birth, and when consuming raw or undercooked animal products.
Spread from person to person has not been documented.
Economic Importance
The disease causes significant morbidity and mortality in calves and lambs and has been associated with abortion storms in adult ruminants, with pronounced health and economic impacts.
Clinical Findings
The clinical presentation of RVF varies by species and age. The disease is most severe in young ruminants, particularly lambs. After an incubation period of between 12 and 36 hours, anorexia, weakness associated with fever, and lymphadenopathy become apparent. Hemorrhagic diarrhea with abdominal pain may be seen. In calves, icterus is a common clinical finding. Mortality rates are high and can reach 90% to 100% in lambs and 70% in calves.
In adult sheep and cattle, abortion is the outstanding and in many cases only clinical sign. Abortion storms affecting up to 100% of ewes and 85% of cows can occur. In clinical cases in cattle and adult sheep there is febrile disease, with anorexia, weakness, and a drop in milk production that can be associated with hemorrhagic diarrhea. In severe cases the mortality rate in adult sheep may be as high as 25% and 10% in cattle. Goats show a febrile reaction but few other clinical signs.
Clinical signs can be unspecific when considering an individual animal, but RVF should be suspected whenever high abortion rates in adult ruminants and high mortality rates in neonatal ruminants occur in combination with flu-like disease in humans who had contact with sick ruminants.
Samples for Confirmation of Diagnosis
- Virology—liver, spleen, brain (virus isolation, fluorescence antibody test, PCR)
- Histology—liver, spleen, brain (light microscopy, immunohistochemistry)
Treatment
Little attention has been given to the aspect of treatment of the disease, and no known treatment is of any value.
Control
Measures to control Rift Valley fever include the following:
- Control of livestock movement. Over long and short distances.
- Vector control. Larvicides are used in mosquito breeding sites. Limitations of this approach are that breeding sites must be clearly identified and must have a limited surface to be manageable.
- Vaccination. Vaccines are not recommended for pregnant animals because they may result in abortions.