Rift Valley Fever (RVF) – Vaccinating maiden ewes each year
By Dr Mackie Hobson BSc(Agric),BVSc

A Rift Valley Fever (RVF) outbreak can have devastating consequences on a livestock production system. Due to the irregular outbreaks, sometimes decades apart, farmers often seem reluctant to vaccinate their livestock.

See the article on the SAMGA website ‘Should I vaccinate my Angora goats against Rift Valley Fever’.  https://www.angoras.co.za/page/rift_valley_fever#33

“If you are reluctant to vaccinate your Angora goats then I would suggest you at least vaccinate your maiden ewes each year (with the OBP live attenuated RVF vaccine)”

The question then is “If we are just vaccinating our maiden ewes each year what sort of ‘immunity’ (anti-body titre) would the vaccinated ewes still have after a few years?” Remember true immunity can only be checked by a live virus challenge.

To try answer this question we blood sampled ewes 1 to 4 years after they were first vaccinated as maiden ewes. The field study was a very small sample size (only 3 goats sampled from each year) so the average level is only a small window into the flock situation. Thanks here to Jordi and Gay van Hasselt for helping SAMGA do the field study.

4 years after vaccinating the maiden ewes with the OBP live attenuated RVF vaccine the IgG titre averaged 92. Year 1 and 2 average was below year 4. Possible reasons include the small sample size and different vaccine batches?

Yes, but what should we expect the Antibody levels to be?

Comparing these results to the SAMGA study at Ordonnantsie  Graaff-Reinet (Richie Herold) in 2017. The average antibody titre of 10 goats sampled 21 days after vaccination with OBP RVF (Smithburn) Batch 123 in table below.

See the website for the seroconversion study https://www.angoras.co.za/article/rift-valley-fever-seroconversion#238

So these results 4 years post vaccination were the same as the antibody titres 3 weeks after vaccinating in the Ordonnantsie 2017 study.

Comparing these results with trials done in the past to determine seroconversion (Paweska et al 2005) (Van Vuren et al 2006) and (Paweska et al in 2003).

Superimposed on Paweska’s graph are:

Average 92    (Ordonnantsie 2017).

Year 4: 92      (Van Hasselt- 2019)

 Mean±S.D. immunological responses in sheep  (n = 10) vaccinated with liveattenuated

Smithburn strain of Rift Valley fever virus.

This small field study suggests that 4 years after vaccinating the maiden ewes with OBP live RVF vaccine that the ewes are still maintaining antibody titres as per the licensed vaccine expectations.

Angora goats can be vaccinated at any age except kids from vaccinated ewes should not be vaccinated before they are six months old as 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 1 ml subcutaneously (under the skin). Follow manufacturer’s guidelines.

Dr Mackie Hobson

LITERATURE CITED

Janusz T Paweska, Elizabeth Mortimer, Patricia A. Leman, Robert Swanepoel 2005

An inhibition enzyme-linked immunosorbent assay for the detection of antibody to Rift Valley fever virus in humans, domestic and wild ruminants

Paweska, J.T., Burt, F.J., Anthony, F., Smith, S.J., Grobebellaar, A.A., Croft, J.E., Ksiazek, T.G., and Swanepoel, R. 2003

IgG-sandwich and IgM capture enzyme-linked immunosorbent assay for the detection of antibodies to Rift Valley fever virus in domestic ruminants.

Petrus Jansen van Vuren, Abraham C. Potgieter, Janusz T. Paweska, Alberdina A. van Dijk. 2006

Preparation and evaluation of a recombinant Rift Valley fever virus N protein for the detection of IgG and IgM antibodies in humans and animals by indirect ELISA