Outbreaks of Rift valley fever (RVF) seen in the past 3-4 years have resulted in both economic losses due to mortalities of farm animals and human losses who have contracted the disease. Heavy rains that we have experienced in the last few weeks might be a very good source for vectors to increase possible outbreaks. We will need to be more aware of what the disease is, the clinical signs and how to prevent it.

What is Rift Valley Fever (RVF)?

Rift Valley Fever (RVF) is an acute viral disease that causes severe disease in domestic animals (Cattle, sheep and goats) and humans. Disease in these species is characterized by fever, severe illness, abortions and high morbidity and mortality rate.

Where is the disease found?

RVF is mainly found in countries of sub-Saharan Africa, the Middle East and Madagascar; however we have seen outbreaks in North East Africa in countries such as Sudan. In the past, outbreaks of the disease occurred in Africa at 5-15 year intervals. These outbreaks occur when areas that are typically dry experience a period of heavy rainfall and/or flooding. The 2007 outbreaks occurring in Africa coincided with heavy rains associated with El Niño.

How is the disease transmitted and spread?

Many different species of mosquitoes are vectors for the RVF virus and are most commonly encountered during years of unusually heavy rainfall and subsequent flourishing of mosquito populations. Mosquitoes feed on infected (virus circulating in the bloodstream) animals and then transmit the virus to other animals on which they
subsequently feed. Some species of mosquitoes (Aedes, for example ) are capable of transmitting the virus from infected female mosquitoes to offspring via its eggs. This contributes to the survival of the virus in the environment. Mosquito eggs may survive during prolonged periods (up to several years) in dry conditions. During periods of high rainfall and/or flooding the eggs hatch and there is an increase in the infection of animals on which these mosquitoes feed.

What is the public health risk associated with this disease?

RVF is a zoonotic disease (a disease which primarily affects animals, but causes disease in humans). Humans are highly susceptible to the RVF being bitten by infected mosquitoes, through contact with blood, other body fluids or tissue during killing, skinning and cutting of infected animals, or by consumption of raw milk or uncooked meat from infected animals. Humans working in slaughter facilities, laboratories or hospitals are at risk of acquiring infections.

What are the clinical signs of the disease?

Clinical signs depend on the species of animals affected and conditions such as age and pregnancy. During epidemics the occurrence of numerous abortions and mortalities among young animals, together with disease in humans, is characteristic. Pregnant sheep and cattle affected by this disease will almost always abort (80-100%). Young lambs and calves develop a fever, become weak and die very suddenly. The mortality rate in all young animals is very high whereas mortality in adult sheep is also high. Mortality in adult sheep is about 20% and about 10% in adult cattle. Adult sheep and cattle may have nasal discharge, excess salivation and loss of appetite, weakness or diarrhea.

How is the disease diagnosed?

It is highly advisable for any suspected case to be reported immediately to the local state veterinarian. They will collect samples for proper investigation; laboratory tests are required to confirm the diagnosis.

How to prevent or control this disease?

It important to always report any suspected case to local state authorities. Controlling the vector (mosquito) population through spraying and management of mosquito breeding areas has also been an effective mechanism. Vaccination can be used for prevention of RVF on animals in areas where the disease is endemic. An OBP modified live vaccine is available that requires only one dose and produces adequate immunity, but is not recommended for use in pregnant animals, due to the risk of abortion. OBP also has an inactivated RVF vaccine which is widely and has been successfully used without causing unwanted side effects. However, this vaccine is more expensive to produce and require multiple vaccinations to produce protective immunity.

Can we use RVF vaccine during an outbreak?

In areas known to be of high risk, an inactivated vaccine should be used on all animals including pregnant animals. However a booster vaccination should be given 3 to 4 weeks after the first injection. One needle per animal should be used to avoid spreading the disease through the use of contaminated needles.

In areas of low risk, it is advisable to use a live RVF vaccine to vaccinate non pregnant and young animals to induce a high level of herd immunity. Farmers should be cautious of using the live vaccine in pregnant animals as it may cause abortions.

How does it affect humans?

Unfortunately no vaccine is licensed or available commercially for humans. Natural immunity will develop in humans who have contracted RVF and recovered from the disease. Personal protective clothing, such as long shirts and trousers, the use of mosquito nets and insect repellent, avoidance of outdoor activity at peak biting times of the vector species, are all effective measures. Care should be taken when handling sick animals or human patients, their tissue and samples.

For more information on the use of vaccines please contact;
Dr Jacob Modumo at
Jacob@obpvaccines.co.za
or 012 522 1500