As discussed last week, Rift Valley fever (RVF) is spread by the bite of infected mosquitoes, and often results in significant economic losses due to death and abortion among livestock. Sheep are more susceptible than cattle or goats. The rate of abortion among pregnant, infected ewes is almost 100%. RVF is also a zoonotic disease: it can be transmitted from animals to humans.
Signs & symptoms
The disease is non-specific and can be confirmed only with a laboratory test. However, there are signs to look out for. These include abortions, fever and death in ruminants after heavy, prolonged rain. The severest reactions occur in newborn lambs and kids, which usually die within 36 hours of infection. If the animal is less than a week old, there is a 90% chance of mortality.
The onset of RVF is accompanied by a high fever, which subsides sharply before death. Most pregnant, infected sheep, cattle and goats abort their foetuses. The incubation period (the time between exposure to the virus and when symptoms start appearing) for RVF is up to three days in sheep, cattle and goats. In newborn animals, it can be as short as 12 hours.
Symptoms of RVF vary with species and age. In lambs, they include fever, reluctance to move or feed, rapid breathing, bleeding from the nose and bloodstained diarrhoea. As mentioned, the animal usually dies within 36 hours. Adult sheep and goats get a high fever, and are weak, listless and breathe quickly. Some regurgitate rumen contents and have bloodstained nasal discharge, foul-smelling diarrhoea and black faeces.
Cattle may have a fever, shed large tears continuously, salivate a great deal, and have a discharge from the nostrils. They may also have blood-stained diarrhoea and jaundice. Abortion may occur. In 10% to 20% of cases, death occurs within two to eight days.
How can RVF be controlled?
There is no specific treatment for RVF, but the disease can be controlled or prevented through sound hygiene and veterinary procedures. Sanitary measures include:
- Control of animals’ movements to limit the spread of the disease.
- Control at abattoirs to limit exposure to RVF.
- Draining standing water to eliminate or reduce mosquitoes.
- Spraying methoprene or burning grass where mosquito larvae are found. (Methoprene is an insecticide that prevents mosquito larvae from growing into adults.)
Although these measures may help to prevent the spread of the disease in the first place, they will be largely ineffective during widespread outbreaks. Equally, moving livestock to higher altitudes is not usually practical, and is often done too late to be of any value. Immunisation remains the only effective way to protect livestock.
Three types of vaccine are available for immunising sheep and cattle:
- Inactivated whole RVF virus vaccine. This requires a booster vaccination and annual re-vaccination.
- Live-attenuated Smithburn vaccine. This has several advantages over the inactivated vaccine: it lasts longer, is more effective and is cheaper. However, due to residual virulence, it can cause abortion and foetal malformations when given to pregnant animals.
- RVF Clone 13 – a live attenuated vaccine derived from the Clone 13 virus. Unlike the Smithburn vaccine, it is avirulent.
Ultimately, the best way to control RVF is to ensure that animals have adequate herd or flock immunity, and this requires annual vaccination. The essence of this immunity is safety in numbers: if more animals in the flock or herd are immune to RVF, either through vaccination or natural exposure, more animals will be protected from disease even if they are not immune.
Signs vs symptoms
At first glance, the ‘signs’ and ‘symptoms’ of a livestock disease might seem like the same thing. They are not. The sign of a disease is something that can be seen, such as a rash. A symptom is something that the sufferer experiences, such as a high fever. It’s important to keep a close eye on animals for signs of the disease, as they are of course unable to communicate their symptoms to us. The animal may already be seriously ill by the time the first signs, or outward appearances, of the disease appear.
Why diagnosis is important
RVF is a non-specific disease, which means its symptoms are similar to those of other diseases. A quick response and confirmed diagnosis are therefore important to containing an outbreak. Contact your local vet, who will collect samples and send them to an authorised laboratory for analysis. It is important to make sure the samples are secure to prevent the disease from spreading.
Samples of the blood, liver, spleen, kidney, lymph nodes, brain and dead foetuses should be taken. Take care to avoid exposure to the disease. At the minimum, a face mask that filters virus particles, and rubber gloves, should be worn. Carcasses should be buried or burnt. RVF symptoms are similar to those of bluetongue, heartwater and other diseases. Wesselsbron disease and RVF are particularly alike and often occur simultaneously. They can usually be distinguished only in the laboratory.
For more information, contact Onderstepoort Biological Products on 012 522 1512, or email [email protected].