For any business it’s good practice to look at the cost base and see where efficiencies can be made.
In vaccinated herds, where disease has not been seen for some time, it can be easy to forget why you started vaccinating, or to believe vaccination is no longer required.
Stopping vaccination, however, may not be the quick and easy route to reducing farm costs, as the following case study shows.
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 two years this was reduced to vaccinating only the incoming in-calf heifers, and after a further three years, in 2007, a decision was taken to stop vaccinating against IBR altogether. During the five 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 three years the heifer milk tests were negative. The strategy was working.
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, taken from each lactation group, 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, which had tested free of IBR on entering the herd two years previously, also tested seropositive. Fortunately, the bulling heifers, reared away from the milking herd, 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. IBR is caused by a herpes virus, and just like the human cold sore virus, once a cow is infected it becomes a lifelong carrier of the virus, intermittently shedding it during periods of stress. 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.
IBR Vaccination Strategy
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.
Studies have shown that Inactivated IBR vaccines are better than Live IBR vaccines at reducing shedding from infected animals, whilst 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 six 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 adult cows would receive an annual booster dose of the Rispoval IBR Marker Inactivated vaccine, and the replacement heifers would receive a dose of Rispoval IBR Marker Live, followed by a dose of Rispoval IBR Marker Inactivated pre-bulling. This ensured the heifers were protected before being served, and the timing fitted well with the overall farm management.
Since introducing the Live-Inactivated vaccine programme there has to date been no evidence of clinical IBR on this farm.
Continued monitoring of the bulling and in-calf heifers has also shown that the programme has been effective at reducing exposure to the IBR virus, such that the heifers have not become carriers of IBR even after entering the milking herd and mixing with the older cows.
This has meant that with the normal herd culling policy, in just three years, from 2012 to 2015, the percentage of IBR ‘carrier’ animals in this herd fell from 99% to just 45%. Continued vaccination and adherence to biosecurity protocols has meant that the risks and losses from both clinical and subclinical IBR have been minimised for this herd.
Take Home Message
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, such that if there is a trigger (usually some type of stress) which causes the older cows to start shedding the IBR virus, this can trigger a disease outbreak.
Vaccination acts by both increasing the overall immunity of the herd, and reducing shedding of virus from carrier animals, helping protect the whole 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.