Abstract:
Rift Valley fever (RVF) is an arthropod-borne viral disease of importance in livestock
and humans. Epidemics occur periodically in domestic ruminants. People in contact
with infected livestock may develop disease that varies from mild flu-like symptoms
to fatal viraemia. Livestock vaccination may assist in disease control. Rift Valley fever
virus (RVFV) Clone 13 is a relatively new vaccine against RVF, derived from an avirulent
natural mutant strain of RVFV, and has been shown to confer protective immunity
against experimental infection with RVFV. The hypothesis tested in the current trial was
that rams vaccinated with RVFV Clone 13 vaccine would not experience a reduction
in semen quality (measured by evaluating the percentage progressively motile and
percentage morphologically normal spermatozoa in successive ejaculates) relative to
unvaccinated control animals. Ram lambs were screened for antibodies to RVFV using a
serum neutralisation test. Animals without detectable antibodies (n = 23) were randomly
allocated to either a test group (n = 12) or a control group (n = 11). Animals in the test group
were vaccinated with RVFV Clone 13 vaccine. Daily rectal temperature measurements
and weekly semen and blood samples were taken from all animals. Seven animals were
eliminated from the statistical analysis because of potential confounding factors. Logistic
regression analysis was performed on data gathered from the remaining animals to determine whether an association existed between animal group, rectal temperature and
semen quality parameters. No correlation existed between the treatment group and values
obtained for the semen quality parameters measured. There was no statistically significant
post-vaccination decline in the percentage of live morphologically normal spermatozoa,
or the percentage of progressively motile spermatozoa, either when assessed amongst all
animals or when assessed within individual groups. A repeat study with a larger sample
size and a more comprehensive pre-screening process may be indicated to avoid the
inclusion of unsuitable animals.
