25 May 2023

by Fredalette Uys

All livestock farmers have been left to their own mercy if Rift Valley fever should break out as Onderstepoort Biological Products (OBP) has once again produced too few vaccines this year.

This is a further reason for the private sector to get involved in the production of essential vaccines, says Dr Stuart Varrie, a vet consultant at Cape Wools.

According to Dr Varrie, farmers could not vaccinate their animals against Rift Valley fever this season as OBP simply did not have the ability to make the necessary volume of vaccines. “Farmers certainly could not vaccinate sufficiently, and it is not their fault.”

He says the problem is not limited to the current season. The industry has been giving warnings for the last couple of years about the damage that a shortage of vaccines and a subsequent outbreak can cause to the industry.

It is estimated that South Africa suffered a direct loss of close to R2,253 billion after the big Rift Valley fever outbreak in 2010 and 2011. Besides economical losses, farmers also suffered genetic and reproductive losses due to livestock deaths. 

If Rift Valley fever is reported in a district, a ban on the export of wool will be placed on the entire district for a period of 90 days after the last positive case has been diagnosed, says Leon de Beer, general manager of the National Wool Growers Association. 

Dr Varrie says the outbreak of diseases also has the potential to damage South Africa’s biosecurity reputation as a safe export partner for animals and animal products. 

According to Dr Varrie, private vaccine manufacturers, who have played a limited role in vaccine supply thus far, will become important partners for the livestock industry in the coming years. There are encouraging signs that this is indeed happening, and he calls on farmers to support these manufacturers. 

“We depended on only one source for many of our essential vaccines for too long and it again shows how at risk the farmers, their animals and the national economy is.”

Immunisation

According to Dr Varrie, the current immunity status of the South African herd against Rift Valley fever is alarmingly low. It has been more than 10 years since South Africa had a big outbreak and is moreover human nature for farmers to become overconfident. “It means there are many animals that have either never been vaccinated or did not receive the necessary follow-up doses.”

There are various factors that play a role for an outbreak to occur. It includes the presence of a vector and a disease. Rift Valley fever is endemic to South Africa, which means it is always present to some extent. Furthermore, the right climatic conditions must be in place, as it is this year. The right climatic conditions lead to the hatching of larger numbers of mosquitoes (vectors), which are the carriers of the disease.

The outbreak usually starts to spread on a low level in a susceptible herd, which means the sickness is present but not on the level of an epizootic outbreak.

If there are enough susceptible animals, vectors will spread the disease from one animal to another at an increasing rate. When the herd’s immunity is low, the virus can aggressively multiply within an animal. Therefore, there will be more virus particles in circulation that could be spread by the vector. The vector is also easily infected due to the increased viral load of the host animal. This way the disease is spread more and more easily and quickly to more animals with weak immunity. Eventually, the cycle gets out of control, which leads to an epizootic outbreak.

Since there has been good rainfall over large parts of the small livestock production areas, the industry fears a possible outbreak as pans of water in certain areas provide the ideal conditions for the carrier mosquitoes to hatch. According to Dr Varrie, farmers should be grateful that a major outbreak has not yet occurred.

However, he warns that although winter is approaching, there is still a chance of an outbreak.