RIFT VALLEY FEVER UPDATE DAFF-Dr Grietjie de Klerk:
History and background
RVF was first recorded in South Africa in late 1950 when a severe outbreak occurred in the North Eastern part of the country. A second major outbreak occurred in 1974/1975. Outbreaks have always been associated with above average rainfall at irregular intervals of 5 to 15 years. Major outbreaks occurred in 1950/51, in 1974/1975 and in 2010. Less severe and localized outbreaks of RVF or sporadic isolation of virus were recorded in the inter-epidemic periods.
Maps and Figures:
Figure 1: 1950/51 RVF outbreak
Figure 2: 1974/75 RVF outbreak
A localized outbreak occurred in 1999 in a buffalo herd kept in a boma at Skukuza during a research project. Results of work done by Dr. Roy Bengis, the then State Veterinarian in the Kruger National Park, indicate that RVF appears to have been circulation at low levels of activity in buffalo in the Park. This happens in the absence of any RVF epidemics elsewhere in South Africa, where RVF occasionally surfaces in years with high specific rainfall patterns.
Figure 3: RVF outbreak in 1999
After almost no outbreaks during the 1990’s and early 2000’s, Rift Valley Fever reoccurred in South Africa in 2008 in Mpumalanga, Gauteng and North West Province. These outbreaks were all localised and mild.
Figure 4: RVF outbreak in 2008
During 2009, localised outbreaks occurred in Kwazulu-Natal, Mpumalanga and in the Northern Cape.
Figure 5: RVF in KwaZulu Natal and Mpumalanga in 2009
Figure 6: RVF in the Northern Cape in 2009
In February 2010, RVF was detected in the Free State Province in the Bultfontein area. Outbreaks were thereafter detected in the rest of the Free State, the Northern Cape Province, Eastern Cape Province and the Western Cape Province. Localised outbreaks were also detected in the Babsfontein area of Gauteng and in the area of Mpumalanga, bordering the Free State. Cattle, sheep, goats, alpacas and some game species were affected. This major outbreak only came to an end with the start of the 2011 winter season.
A total of 489 outbreaks have been reported in 2010, with more than 14 300 animal cases and more than 8 800 animal deaths. Approximately 3% of sheep and 1% of cattle have died on the infected farms. Sheep are by far the most affected species, with some cattle and a few goats also affected.
Apart from the three major outbreaks, lesser outbreaks of RVF or sporadic isolations of virus were recorded to date.
The nature of RVF
The extent and distribution of the 2010/11 RVF outbreak mirrored almost exactly the severe outbreaks seen during 1974-76. This reoccurrence of RVF was a result of the increased rainfall patterns that have been observed. The increased levels of rainfall lead to the formation of standing pools or pans of water. This leads to the hatching of previously infected mosquito eggs as well as a general increase in the mosquito population, which multiplies the disease. During a severe outbreak, other bloodsucking insects play a secondary role in the spread of the outbreak over larger areas.
Recent outbreak in the Free State
Seven years after the last severe outbreak, a single outbreak was detected in the Free State near Jacobsdal at the end of May 2018. Although this was an isolated incident without consequence for the national herd, the individual farmer lost his lamb crop. The colder winter temperatures and the first frost, together with decreased rainfall at the end of the summer season, lead to this isolated outbreak. Whether the outbreak will reoccur next year, depends on unpredictable weather and environmental factors, as well as the immunity status of the national herd.
Rift Valley Fever is a notifiable disease, but not a controlled disease according to the Animal Diseases Act, Act 35 of 1984. The reason is that there are no practically implementable and scientifically valid control measures that can be used to prevent the spread of a disease that is transmitted by flying insect vectors. The only measure that has been advised and would be of help is regular vaccination of livestock. Attenuated live vaccine (Smithburn strain) was introduced in the period July 1954 to June 1955 and the inactivated vaccine in the period July 1974 to June 1975.
The most commonly used and freely available live RVF vaccine (produced and distributed by Onderstepoort Biological Products (OBP)) produces very good immunity and if all susceptible livestock species were vaccinated at least once in their lives, no large outbreaks would occur or spread. During an outbreak, however, the inactivated vaccine (also produced and distributed by OBP) has to be deployed under strict biosecurity precautions. Please take care when vaccines are administered after outbreaks have been detected. Use a clean needle for each animal as the virus can be spread between animals if the animals are in the incubation period. Animals in the incubation period will have a virus infection but will not show symptoms of the disease, yet. Live vaccine can only be used in non-pregnant animals as the live vaccine can cause abortions. Only dead (inactivated) vaccine must be used in pregnant animals. The inactivated vaccine can be produced in small amounts only and would therefore not be available on short notice. It is important to keep the herd immunity at an acceptable level by including RVF vaccine as part of the vaccine schedule to provide protection against stock losses with the sudden appearance of a disease cycle.
RVF in humans
Humans can be infected with Rift Valley Fever if they come into contact with the blood and other body fluids of an infected animal or an aborted fetus. Care should be taken when handling possibly infected material. Symptoms in humans are flu-like symptoms and a few individuals can develop more serious complications. If someone suspects that they might have become infected with Rift Valley Fever, they must consult with a doctor.