Rift Valley fever (RVF) is an acute viral haemorrhagic fever that is most commonly seen in domesticated animals like sheep, goats, cattle, camels, and buffalo.
The Rift Valley fever virus is a member of the Phlebovirus genus. It is a viral zoonosis that primarily affects animals but can also infect humans.
It is transmitted by mosquitoes and blood-feeding flies. When livestock are infected, the disease can cause significant economic losses due to high mortality rates in young animals and waves of abortions in pregnant females.
Rift Valley fever was first identified in 1931 in a sheep epidemic on a farm in the Rift Valley of Kenya. Although it is generally found in regions of eastern and southern Africa where sheep and cattle are raised, it exists in most countries in sub-Saharan Africa, including West Africa and Madagascar. In 2000, it was also reported for the first time in Saudi Arabia and Yemen.
Flooding seems to contribute to an increase in Rift Valley fever because more virus-infected mosquitoes buzz about after heavy rainfalls.
Three major epidemics occurred in South Africa during 1950-1951, 1974-1976, and 2010-2011. These primarily affected the Free State and Northern Cape provinces but were widespread during the most recent two outbreaks involving the Northern, Eastern, and Western Cape provinces.
Smaller sporadic animal outbreaks with concurrent human cases have been documented in the periods between these major epidemics. The last major outbreak in South Africa, in 2010-2011, was preceded by smaller isolated outbreaks in 2008 and 2009 in the northeast and northwest parts of the country.
In 2018, an isolated outbreak occurred on a sheep farm in the Free State after heavy rainfall during 2017-2018, followed by a drought period.
Rift Valley Fever in Livestock
Clinical signs vary depending on the species of animal affected, age, and whether the animal is pregnant. Pregnant sheep and cattle affected by this disease will almost always abort (80-100%).
Young lambs of sheep and goats (kids) are considered to be the most susceptible with mortality rates of 70% to 100%. Sheep and calves are highly susceptible with mortality rates of 20% to 70%. Goats, African and domestic buffalo, Asian monkeys, and humans are regarded as moderately susceptible with mortality rates of less than 10%. Other species can be infected but are resistant and do not show any clinical signs.
Since the symptoms of the disease are non-specific, it is difficult to recognise individual cases during epizootics. However, the occurrence of numerous abortions and mortalities among young animals, as well as an occurrence in humans, are characteristic.
The disease in susceptible animals can vary in severity and is characterised by fever, listlessness, severe weight loss, reluctance to move, abortions, and high morbidity and mortality rates in neonatal animals.
In livestock, it causes severe illness and nearly 100% of abortions of livestock pregnancies. Young animals that become infected will mostly die, while the mortality rate among adult animals is significantly lower. These losses lead to loss of trade and income.
In animals, the incubation period is between two and six days and is characterised by fever and weakness. Definitive diagnosis requires laboratory testing of blood and tissue samples.
The virus is most commonly spread by the Aedes and Culex mosquitoes but varies by region. Environmental conditions, particularly heavy rainfall and flooding, are most often linked to the spread of the disease as it allows more mosquito eggs to hatch.
As a listed animal disease with the World Organisation for Animal Health (WOAH) an outbreak must be reported to the WOAH. Because of its long inter-epizootic intervals, it is also regarded as a re-emerging disease.
Different types of vaccines are available for use in animals. Consult your veterinarian or veterinary extension officer. It is better to inoculate in time rather than suffer the losses.
Pregnant livestock usually abort when they are infected with Rift Valley fever. (Source: Pixabay)
The transmission cycle may look like this:
- The virus can spread from female mosquitoes to their offspring through the eggs;
- In the eggs, the virus remains viable or infectious for several years during dry conditions;
- Excessive rainfall allows more mosquito eggs to hatch;
- As mosquito populations increase, the potential for the virus to spread to animals and people increases;
- RVFV outbreaks in animals, most commonly livestock, lead to increased handling of infected animals, which then increases the risk of exposure to the virus for people.
Rift Valley Fever in humans
In humans, the disease ranges from a mild flu-like illness to severe haemorrhagic fever that can be lethal.
People usually get Rift Valley fever through contact with blood, body fluids, or tissues of infected livestock during slaughter or butchering, but also through caring for sick animals or eating raw or uncooked animal products. The virus can also be transmitted to humans by infected mosquitoes and biting flies, but this happens rarely, and it has not been documented that it is spread from person to person. Herders, farmers, slaughterhouse workers and veterinarians are at higher risk of infection. Humans can also be infected by ingesting the uncooked or unpasteurised milk of infected animals.
Most people with RVF either have no symptoms or a mild condition including fever, weakness, back pain, and dizziness. Only 10% may develop serious symptoms including haemorrhage and encephalitis, or eye disease. No vaccines are currently available for vaccination in people.
Prevent infection in areas prone to the virus:
- Avoid contact with blood, body fluids, or tissues of infected animals;
- Wear appropriate protective clothing, gloves, boots and a face shield when working with animals that may be infected;
- All animal products, including meat, milk, and blood, must be thoroughly cooked before consuming it;
- Protect yourself against mosquitoes and other bloodsucking insects by using insect repellents and bed nets and wear long-sleeved shirts and long pants to cover exposed skin.
Treatment for people
Since most cases of RVF are mild and self-limiting, there is no specific treatment for RVF in humans. Symptoms of mild illness such as fever and body aches can be managed with standard over-the-counter medications. Most often, people will get better within two to seven days after the onset of the symptoms. Treatment for more serious cases may require hospitalisation for supportive care.
A vaccine has been developed but it is not yet licensed or available. Rapid case detection, including prompt laboratory testing of people with symptoms, has limited recent outbreaks.
The disease is, however, listed along with eight others as having the potential to cause epidemics when jumping from animals to humans. Other include the Nipah virus, Crimean-Congo haemorrhagic fever, Lassa fever, Zika, Ebola and Marburg virus disease, MERS (Middle East respiratory syndrome), and SARS (severe acute respiratory syndrome).
Countries reporting endemic disease and substantial outbreaks of Rift Valley fever include Egypt, the Gambia, Kenya, Madagascar, Mauritania, Mozambique, Namibia, Saudi Arabia, Senegal, South Africa, South Sudan, Sudan, Tanzania, Yemen, Zambia, and Zimbabwe.
Countries reporting few cases, periodic isolation of the virus, or serologic evidence of RVF infection include Angola, Botswana, Burkina Faso, Cameroon, Central African Republic, Chad, Democratic Republic of Congo, Ethiopia, Gabon, Guinea, Mali, Niger, Nigeria, Republic of Congo, Somalia, and Uganda.