The effect of Rift Valley fever virus clone 13 vaccine on semen quality in rams

Abstract

Rift Valley fever (RVF) is an arthropod-borne viral disease of significant importance in both livestock and humans. Epidemics occur periodically in domestic ruminants, typically after heavy rains, which encourage rapid multiplication of mosquito vectors. Clinical symptoms in livestock vary from inapparent infection to abortions and peracute deaths. The disease has significant zoonotic potential. People in contact with infected livestock may develop disease that varies from mild flu-like symptoms to severe neurological and haemorrhagic disorders and death. An important way of controlling the disease is through vaccination of susceptible livestock. Rift Valley fever virus (RVFV) clone 13 is a relatively new livestock vaccine against RVF that is derived from an avirulent natural mutant strain of RVFV. This vaccine has been shown in previous studies to confer protective immunity against infection with live virus. The effect of this vaccine on semen quality in male animals has never been tested. The purpose of the current trial was to determine whether RVFV clone 13 vaccine had any effect on semen quality in rams. The hypothesis tested was that animals 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. A test/control model was used to evaluate the effect of this vaccine on semen quality. A group of peripubertal ram lambs were tested for antibodies to RVFV using a serum neutralisation test (SNT). Animals without detectable antibodies (n=23) were then randomly allocated to either a test group (n=12) or a control group (n=11). Daily rectal temperature measurements were taken and weekly semen evaluations were conducted. Blood samples were drawn weekly to assess serum antibody titres. Seven animals were subsequently eliminated from the statistical analysis because of potential confounding factors. Of these seven, five animals had extremely poor semen quality at the start of the trial, one animal was found to have a persistent febrile response commencing at the start of the trial, and one animal had seroconverted to Rift Valley fever virus in the period between the initial screening and onset of the trial. 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. It was found that no correlation existed between treatment group and values obtained for the semen quality parameters measured. There was no statistically significant post-vaccination temporal 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. Based on the data from this trial, the hypothesis was not rejected. Despite this finding, it should be stated that the elimination of animals from the analysis had some effect on the statistical power of the study. A repeat of the trial with a larger sample size and a more comprehensive pre-screening process to avoid the inclusion of animals with poor semen quality may be indicated.