Summary
On each of three farms in New South Wales experiencing significant OJD losses (5 to 15% per annum), 200 Merino lambs (age 1-4 months) were vaccinated with Gudair™, and 200 lambs were sham vaccinated with saline. Animal assessments and sample collections were conducted twice yearly until 4 or 5 years of age. We examined the impact of vaccination on mortality rate, faecal shedding of M. a paratuberculosis (by pooled and individual faecal culture), lamb growth, condition score and wool productivity, vaccine injection site lesions and cellular (BOVIGAM™) and humoral (PARACHEK™) immunity. Experimental sheep that were culled for normal flock management reasons, died or showed clinical signs of OJD, were assessed at necropsy or slaughter.
Results:
Clinical disease and mortality - By project end there had been only seven confirmed OJD mortalities in vaccinates compared to 80 from the controls (a significant reduction of about 90%), and the mortalities in the vaccinated sheep were delayed by at least 12 months. Vaccination reduced mortalities over the productive life of the sheep and did not simply delay the onset of mortalities. All the vaccinates, and 88% of the controls that died of OJD had multibacillary disease. Sheep with multibacillary lesions can excrete enormous numbers of organisms and the “breakdown” of a single vaccinated animal can have a disproportionate effect on transmission of the infection and the persistence of disease in the flock.
Subclinical disease – Among both the hogget culls, and the sheep that went to slaughter at trial end, there was a significant reduction in the proportion of vaccinates with subclinical OJD compared to controls. The magnitude of this reduction was, however, less than that for clinical disease – from 49% in controls to 17% in vaccinates for the hoggets, and from 28 to 15% respectively at final slaughter.
M. a. paratuberculosis excretion - The results from farms P1 and P3 clearly demonstrated a delay and reduction in faecal excretion of M. a. paratuberculosis, with maintenance of the protection for the economic life of the sheep. On these farms, there was no detectable excretion of M. a. paratuberculosis by the vaccinated groups until 18 months pv (about 21 months of age), compared to 6 or 8 months pv (9 -11 months of age) in the controls. The reduction in prevalence of excreters averaged about 90%. On P2, the property with the highest prevalence of infection, excretion of M. a. paratuberculosis by vaccinates was also delayed, but only by about 4 months. At 12-36 months pv, reduction in prevalence in the vaccinates was also about 90%. At 42 and 48 mths pv there was little difference in prevalence between the vaccinated and control groups, but the control group was much smaller due to the large number of OJD mortalities. The peak prevalence of excreters over the life of the sheep was much lower and delayed in the vaccinated group. Across all farms, the numbers of M. a. paratuberculosis excreted by the vaccinated groups were reduced by at least 90% at most sampling times. However, the actual level of excretion was sometimes very much higher than that suggested by prevalence data alone, a reflection of the large contribution that a single (or a few) multibacillary infected sheep may make.
Immunological responses - The vaccine stimulated both cell-mediated and humoral immune responses in a high proportion of vaccinated lambs which declined over time, accompanied by a significant increase in the proportion of unvaccinated animals with positive immune reactions, presumably reflecting an increasing prevalence of OJD in this group. Among vaccinated sheep, positive IFN-γ response was negatively associated with subsequent shedding of M. a. paratuberculosis and with subsequent infection status. No significant association with OJD-mortality was found, but there were very few vaccinated sheep that died of OJD. Positive ELISA response in vaccinates was negatively associated with shedding, with infection and with OJD-mortality. Thus, although humoral responses are not considered to be protective, it appears that pv ELISA response may be a suitable marker for “vaccination take”. The presence of maternal antibodies was negatively correlated with pv CMI responsiveness, but no significant effect of maternal antibody on later infection status or OJD-mortality was found.
Injection site lesions - Vaccine injection site lesions were detected in almost 50% of sheep 2 months pv, and these persisted for at least 4 years in 20-25% of vaccinates. The presence or absence of a pv lesion was not associated with subsequent infection or excretion of M. a. paratuberculosis, so was not a useful marker for vaccination take. The presence of lesions in a proportion of the flock, however, may be a useful indication that the flock has been vaccinated, for up to 3 years after vaccination. Post vaccinal lesions did not result in losses or downgrading of carcasses on the slaughter floor.
Production measurements - These were conducted on surviving sheep at each sampling time. Thus the findings for vaccinates vs controls do not include prior losses due to OJD mortality, and the findings with regard to infection status are for subclinically infected sheep. A small, but consistent and statistically significant reduction in the weight gains of vaccinates over the first 12 months pv was seen; there were not consistent differences among adults. Vaccinates cut slightly more wool than controls at the 2003 shearing, but there were no differences at other shearings, nor in fibre diameter at any shearing. With regard to OJD infection status, subclinically infected sheep were lighter and had lower condition scores than uninfected sheep between 18 and 42 months pv. No differences in weight or condition score due to infection status were detected in growing sheep, nor in the fleece parameters at any time. Reproductive parameters were not studied.
