Cattle practitioners now have the simplest, most cost-effective method of testing for IBR, BVD and leptospirosis in dairy herds: bulk milk testing.
Yet as Intervet/Schering-Plough Animal Health’s DairyCheck service reveals that many herds continue to test positive for these three key diseases, it appears that it remains hard to persuade clients of the benefits of vaccination and implementing a control programme.
When a disease is endemic, the temptation for farmers is to cut back on costs and declare that their herd probably has sufficient immunity anyway, so there is no point in vaccinating. But speaking at a series of Intervet/Schering-Plough CPD meetings, vets made it clear that these endemic diseases can cause huge losses in dairy herds.
Furthermore, routine testing coupled with health planning, including vaccination programmes, are the key to eradicating disease and improving farm profits.
Bulk milk test results can give an indication of a dairy herd’s health status and are, therefore, the first step in allowing the appropriate course of action to be established. Regular testing and monitoring should be part of any control plan to maintain a disease-free status.
However, Wilts-based Keith Cutler, of Endell Veterinary Group, told fellow practitioners that he advocates testing as part of a five-point plan (see Table 2) for BVD eradication. “And missing a single PI is disastrous, so sensitivity of the test is more important than specificity,” he added.
Although more than 90% of herds are now endemically infected, herd owners need to realise it causes losses in a variety of ways which may not be immediately obvious. “Although leucopaenia was reported in 1946, the significance of BVD infection on the immune system wasn’t fully appreciated until the mid 1980s. Both B and T lymphocyte numbers are reduced – over 90% below baseline levels with highly virulent strains,” he said.
“This means that high cell counts, increased incidence of calf pneumonia and more cases of foul can all be the result of BVD affecting a cow’s immune system. Other herd diseases are therefore more likely to have severe implications.”
Benefits of vaccination
With DairyCheck results showing that 65% of herds are infected with IBR, some dairy farmers may consider the cost benefits of vaccination as borderline. John Fishwick from the RVC reminded practitioners that vaccination can still protect cows in a herd that has already been exposed to the virus. It reduces both virus circulation in the herd and the level of infection, which may in turn reduce the creation of new carrier animals.
This is important because of latent infection. “BHV-1 becomes latent following infection and resides in the Trigeminal Ganglion. This can last indefinitely. It can also recrudesce, reactivate and start shedding again after a very long period – usually associated with stress,” he said.
“Reactivators also include transport, mixing, calving, mating with carrier bulls and use of corticosteroids. Remember: latent infection can’t be reliably detected. Animals may be sero-negative despite carrying latent IBR infection. So all cattle from infected herds must be considered as potentially infected. Consider the group as a whole.”
Mr Fishwick said that regular bulk milk testing for antibody titres can help establish a herd’s status, but he also recommended monitoring abortions and investigating all pneumonia cases. “Use your local VLA for surveillance; check all abortions not just for brucellosis, but also BHV-1.”
Bought-in cows, heifers and stock bulls are the biggest risk for bringing virus onto the farm. Trading between IBR-free herds will help reduce this. “Don’t forget that any replacements or dry cows sent off farm pose a risk as they may come into contact with other livestock. Manage this by suggesting the farm uses a combination of quarantine and isolation.
“We all know that this is difficult and sometimes impractical, but not doing it puts herds at risk. So at the very least, suggest that bought-in animals are kept and milked last as a separate group.
Biosecurity control for leptospirosis needs to focus on operating a closed herd or sourcing replacements from a leptospirosis-free herd, according to Ruth Vernon, livestock adviser for Intervet/Schering-Plough, manufacturer of Bovilis BVD, Leptavoid-H and Bovilis IBR Marker Live.
“Quarantine and test all newcomers; don’t allow farmers to let stock graze alongside sheep; and advise them to fence off streams and rivers,” she said.
Mrs Vernon revealed that the disease picture has changed over the years, which means that vets must assess the risk factors for individual farms. In the UK, both strains – hardjo bovis and hardjo prajitno – are important. They leave carrier animals which shed the leptospires in their urine and milk, and from their uterus. Infected stock bulls can also shed in their semen.
Leptospires survive best under moist, warm (28ºC) conditions. They survive at grass for up to six months, which makes spring turnout the key risk period for infection. Other risk factors include watercourses, flood plains and drainage water. Cows can also pick up infection from wildlife, dogs and cats, and grazing with sheep.
“Textbook leptospirosis, where a naïve herd buys an infected animal and sees abortion storms, weak calves, RFMs and milk drop in up to half of cows (total herd yield can fall by 30%), is rare these days. Most signs of leptospirosis in a herd are subclinical: infertility, depressed yield, with abortion in heifers, or older cows with waning immunity,” she explained.
DairyCheck figures show that 69% of milk samples are now testing positive for leptospirosis – three out of four herds have been exposed. “So it’s financially expensive, just not highly visible,” she added. Furthermore, Mrs Vernon cited fertility data as proof of the hidden losses faced by infected herds.
Cows with leptospirosis take longer to get back in calf (133 days calving to conception) compared with clean cows (95 days). They have lower conception rates to first service (30% compared with 46%). All of this can add up to £70/cow/year lost in poor performance.
“One study showed that vaccinated cows had a conception rate to first service of 51% whereas it was just 34% in unvaccinated cows. But remember that heifers need to finish their primary course (two injections, 4-6 weeks apart) of a leptospirosis vaccine, such as Leptavoid-H, at least two weeks before turnout, the main risk period.”