The herd in question, a previously closed 220 cow Holstein-Friesian dairy herd, had suffered a clinical outbreak of IBR in 2002 after a one off introduction of bought-in cows. Subsequently all cows in the herd were vaccinated with a modified live IBR vaccine.
After 2 years this was reduced to vaccinating only the incoming in-calf heifers, and after a further 3 years, in 2007, a decision was taken to stop vaccinating against IBR all together.
Essentially during the 5-year period when an IBR vaccination programme was in place there had been no clinical IBR in the herd. As the herd was closed, it was decided that the risk of infection was low and that vaccination could cease. Instead the first lactation heifers would be routinely monitored using a milk antibody test to ensure IBR wasn't actively circulating in the herd. For 3 years the heifer milk tests were negative. The strategy was working.
Then in 2012 a fresh-calved, first-lactation heifer developed clinical signs of IBR and died. In the six months prior to this diagnosis the herd had experienced sporadic cases of adult respiratory disease, abortion and poorer than average fertility but no 'classical' signs of IBR had been noted.
Pooled milk samples were taken from each lactation group which confirmed all age groups had been exposed to wild IBR virus.
Individual cow milk sampling revealed 99% of the cows were seropositive (and therefore carrying wild virus). The bull, who had tested free of IBR on entering the herd 2 years previously, also tested seropositive. Whilst the findings indicated widespread exposure to IBR in the main herd, the bulling heifers, reared away from the milking herd, with the exception of one, tested free of IBR.
As this herd was closed the disease breakdown was most likely due to reactivation and shedding of virus from the older latently infected or 'carrier' cows. The heifers, having never received a vaccine or been exposed to infection, were most at risk, and this is where the first clinical case was seen.
As the source of the outbreak was most likely the older carrier cows, the vaccination programme needed to both protect the naïve animals from disease, and reduce viral shedding from the older animals. Studies1 have shown that inactivated IBR vaccines are better than live IBR vaccines at reducing shedding from infected animals. Live IBR vaccines generally provide a faster onset of protection, and are favoured in protecting naïve animals. It was therefore decided to vaccinate the herd using the Rispoval® IBR Marker Live/Inactivated vaccine programme.
Initially all cows received a dose of Rispoval IBR Marker Live, followed within 6 months with a dose of Rispoval IBR Marker Inactivated. Incorporating both a live and an inactivated IBR vaccine into this programme ensured both protection from disease, and effective reduction in viral shedding from the carrier animals.
Subsequently the cows received an annual booster dose of the inactivated vaccine, and each year the replacement heifers start the vaccine programme pre-bulling. Monitoring of the young heifers has shown they remain free from infection until they join the herd, therefore vaccinating pre-bulling ensures heifers are protected against IBR before being served, and this fits well with the overall farm management.
Since introducing the Live-Inactivated vaccine programme there has been no evidence of clinical IBR on this farm.
And, as shown by continued antibody monitoring of the bulling and in-calf heifers, the programme has also been effective at reducing exposure to the IBR virus, such that the heifers have not become carriers of IBR (carrier animals will test antibody positive) even after entering the milking herd and mixing with the older cows, a high proportion of which were carriers. This has meant that with the normal herd culling policy, in just 3 years, from 2012 to 2015, the percentage of 'carrier' animals in this herd fell from 99% to 45%.
Based on the current results, continued vaccination and adherence to biosecurity protocols means that the risks and losses from both clinical and subclinical IBR have been minimised for this herd.
There are a few lessons to be learned from the experience of this herd.
Even in closed herds, with no recent signs of clinical IBR disease, if there are carrier animals within the herd, and the herd is not vaccinated, the risk of disease is still there. Over time herd immunity reduces, and then all it takes is a trigger (usually some type of stress) which causes the older cows to start shedding the virus, which can trigger a disease outbreak. Vaccination increases the overall immunity of the herd, and reduces shedding of virus from carrier animals, protecting the herd from a potential disease breakdown.
Therefore before you stop vaccinating because you haven't seen disease for a while, consider carefully the risks, and make sure you discuss them thoroughly with your vet.