“The Angora farming areas of the Eastern Cape are at high risk of Rift Valley fever (RVF) outbreaks,” says Dr Mackie Hobson, who is regularly asked by producers if they should vaccinate their Angora goats against RVF. Dr Hobson believes there will be an outbreak of RVF in the Karoo again. He says we must learn from past outbreaks to combat the disease in the future.
Dr Hobson writes:
RVF increases with rain
Due to the extent of the outbreak seen during 2010/11, it is very likely that most of our Angora goats would have been naturally exposed to the virus and so became naturally ‘vaccinated’ and will have immunity to the disease.
It is now six years after the last outbreak and over the years, however, the immune Angora ewe flocks would have been largely replaced by maiden ewes joining each year and the older ewes being culled. As the proportion of immune goats in our flocks decline, the possibility of an outbreak will increase if the rainfall conditions trigger an explosion of vector numbers in late summer.
Remember these facts:
- Outbreaks continue to occur for several years after the epidemic when conditions favour the vectors (mosquitoes). This would suggest that we need to be aware that it is still possible to have more outbreaks over the next few years, if we get excessively high rainfall patterns over the later summer months.
- Studies in Africa indicated that the RVF virus activity was more likely after cumulative rainfall, rather than heavy precipitation over a short period.
- Most outbreaks occurred from January onwards.
- Where outbreaks occurred before January, it often followed outbreaks late during the previous season.
We should be vaccinating our Angora ewe flocks. If, as a farmer, you are reluctant to vaccinate your Angora goats, then I would suggest that you at least consider vaccinating just your maiden ewes each year with the live attenuated RVF vaccine.
This should be done before they enter your ewe flock and all your ewes should be vaccinated again if above average rainfall occurs. Repeating the vaccine will likely stimulate a better immune response than a single live vaccination.
Vaccinating at the time of an outbreak with the live RVF vaccine is not recommended. Only a ‘dead’ vaccine should be used during an outbreak.
The required dosage
Angora goats can be vaccinated at any age, except kids from vaccinated ewes, which should not be vaccinated before they are six months old – maternal antibodies may block the vaccine response. Pregnant Angora ewes should not be inoculated with this vaccine as it can cause abortion or foetal malformation. The dose is 1ml subcutaneously.
How effective is the RVF vaccine?
As an advisor to Cape Wools SA in 2011, we conducted a trial due to perceived vaccine failure of the Onderstepoort Biological Products (OBP) RVF vaccines to determine zero-conversion after vaccination.
The tests showed a negative zero-conversion response in 67,6% of animals vaccinated with the live RVF (Smithburn) vaccine 14 days post-vaccination, with all remaining animals showing antibody titres no greater than 1:10. Similar results of 66,7% of sheep showed no immune response with the ELISA test. So clearly there was a loss in confidence that the vaccine was having any effect.
The South African Mohair Growers Association (SAMGA) plans to do a small trial to check whether the current live RVF vaccine being produced by OBP is causing effective zero-conversion. I will inform mohair producers about the outcome of this trial.
What happens to the virus between outbreaks?
Several possibilities exist:
- Trans-ovarial transmission in certain mosquitoes may occur (Aedes spp) (Linthicum et al, 1985; Pepin et al, 2010). The eggs lay dormant on the edges of pans and hatch when conditions of high rainfall occur. However, the three major epidemics were associated with a different RVF virus lineage (Grobbelaar et al, 2011), indicating that long-term survival of the virus in dormant mosquito eggs is unlikely to have been the main mechanism for virus survival between these epidemics.
- Another possibility is the low-level circulation of the virus between animals and mosquitoes, without resulting in clinical signs or severe outbreaks (Pepin et al, 2010). Serological evidence of low-level circulation of the RVF virus in African buffalo has been found in the Kruger National Park and antibodies have been found in several wildlife species.
- The transmission of vectors from possible endemic areas, either within South Africa or in neighbouring countries, is also a potential mechanism for the initiation of an outbreak.
For further enquiries, contact Dr Hobson on 082 860 0406 or firstname.lastname@example.org. – Mohair Media & Communications