Most published serological surveys pestivirus show that about 80-90 % of herds show evidence of exposure to pestivirus. In 2006, I found that thirteen of 17 mobs of heifers had evidence of exposure to pestivirus (76%). One mob had less than 20% exposure, five mobs had 20-50% exposed, four mobs had 50-80% exposed and in three mobs, more than 80% of heifers had been exposed to pestivirus (Watt 2007).
In 2008 a subsequent survey of three age groups of cattle from nine herds (unpublished) I found that while only two thirds of heifer mobs had evidence of exposure to pestivirus, all herds had evidence of exposure in older mobs of cows.
It is only possible to give appropriate advice on pestivirus management to cattle producers with information on the risk of pestivirus. The owner needs to consider this advice while considering his own attitude to risk. For larger cattle producers serological testing of 8-10 heifers well before joining gives some information on the antibody status of what is usually the most at risk mob. Serological testing of at least three age groups gives a better indication of the pestivirus status and so risk of the property especially if neighbours and enterprise type are taken into consideration. For small producers however the cost of testing may exceed the cost of vaccination.
Options for various producers include for:
As it costs at least as much to test 10 heifers as to vaccinate 20 small producers are better off to simply vaccinate depending on their risk, attitude to risk, value of their herd and availability of labour.
Larger producers with high levels of exposure to pestivirus as determined by serological testing.
Vaccination is clearly not warranted as the cattle have previous exposure and so lifelong immunity. However the subsequent drop of heifers may not be exposed to the virus before joining so my recommendation is to test a sample of the next drop well before joining.
Large producers with low to moderate levels of exposure.
Vaccination is more likely to be warranted in these herds as a large proportion of the herd is at risk and the virus is present in the herd. As the losses from pestivirus may in some cases approximate the cost of vaccination the owner needs to make an informed decision based on his own tolerance of risk.
Producers with no exposure to pestivirus.
As these producers are vulnerable they have two options; to commence a vaccination program or to maintain strict biosecurity. Maintaining strict biosecurity is an option for isolated properties but we need to be reasonably confident that the herd is genuinely free of the virus rather than the virus being present at a low prevalence or confined to other untested mobs. If producers intend buying cattle or plan to take or receive agistment cattle or have risky neighbours biosecurity is threatened.
These producers have a more valuable herd so vaccination is warranted. However bull producers must also avoid selling persistently infected animals. While this is unlikely the odds clearly increase for each bull sold and so over time represent a substantial risk to larger producers. While persistently infected bulls represent just a poor investment to producers with exposed herds they represent a potential disaster if sold into naïve herds or mobs. The possibility of subsequent litigation represents a significant risk to the viability of the enterprise. At least in the short term bull producers need to also test their sale bulls to detect and cull persistently infected animals. This is not an expensive option.
Producers selling to feedlots.
Persistently infected animals represent a significant risk and loss to feedlots as not only is their own performance affected but they also affect the performance of their cohorts. Producers may however elect to wait until they are paid a premium before opting to vaccinate specifically for this purpose.
Assessing the cost of pestivirus is a challenge. In the previously mentioned 2006 heifer survey I also asked producers about the reproductive performance of their cattle and whether they noticed any ill-thrifty or stunted calves. Most producers reported very good reproductive performance in the cow herd and also reported virtually no poor calves. One producer reported reproductive losses due to dystocia and a number commented that heifers sometimes produced poorer calves but they attributed this simply to poorer milk production.
Therefore, in spite of the high prevalence of pestivirus it was very difficult to demonstrate any losses. However, the presence of pestivirus comes at some cost and the absence must come at some risk. Pestivirus is essentially only perpetuated through persistently infected animals and ever if they happen to spread the virus to all cattle in the herd before they reach a vulnerable stage of their pregnancy, they will usually perform poorly and die prematurely. One would expect however in herds where the virus is present that at least some cattle will become infected for the first time when vulnerable and so therefore abort or produce affected calves.
In herds with a moderate prevalence and imperceptible losses a full vaccination program may be difficult to justify. An intermediate option may be to simply vaccinate heifers twice before their first joining. I think that an evaluation of this option is warranted.
In my experience while losses from pestivirus are imperceptible in many herds, occasional disasters occur when PI animals are introduced to naive herds. Therefore producers with herds with no evidence of pestivirus need to be alerted of this risk. They may choose to protect themselves with strict bio-security or by vaccination.