BVD strikes at heart
As more dairy herds expand,
BVD poses an increasing
threat to herd health and
productivity. Sue Rider visits
one herd £6000 worse off
VACCINATING against bovine viral diarrhoea is now routine on one Leics dairy unit – the disease cost it £68 a cow last year due to poor fertility and lost milk production.
The Hancocks 90-cow herd tested negative for BVD in April 1997, but one year later more than 65% of animals at their 170ha (420 acre) Lodge Farm, Blaby, had been exposed to the virus.
For Roger and Christine Hancock, who had improved herd pregnancy rates from 46% in 1996 to 60% in 1997, BVD brought embryo loss, knocking pregnancy rates back to 45% (see table), reduced yields, and lowered resistance to other infections. Mastitis, which had run at two cases a year, increased five-fold as a consequence of the disease.
The virus struck when six in-calf heifers came into the otherwise closed herd to boost numbers. On arrival in August 1997 they were blood tested by Peter Orpin of the Park Vet Group in Whetstone, Leicester.
"Three tested antibody negative, and three positive, meaning they had been exposed to infection but were no longer infectious," says Mr Orpin. All six were quarantined before entering the herd.
First signs of BVD infection were irregular returns which increased three-fold from 7% between August 1996 and May 1997 to 17% for that period the next year (see table). "This was a sign cows were losing embryos," says Wendy Botterill, also of the Park Vet Group.
Farm experience was backed up by DAISY records which showed a gradual reduction in fertility over four months from January to April 1998.
"In April a bulk milk sample confirmed the herd had gone from being totally negative to 65% BVD positive. We vaccinated the entire herd, including heifers, put a bull in and fertility started improving," explains Mr Orpin. So how did the disease get in?
Nose-to-nose contact with a neighbouring herd and infection from a bull were ruled out.
"More probably is that the BVD came from one of the new heifers calves," says Mr Orpin.
"Its the Trojan horse scenario. An infected calf carried by one of the purchased heifers would be difficult to pick up. All their calves were sold within three weeks but a naive, unprotected herd would have absolutely no protection against a persistently infected calf."
Such calves are born when a non-immune animal is infected in early pregnancy. The BVD virus may cause abortion but when it doesnt, the foetus, exposed at an early age, will consider BVD part of itself. The calf never produces antibodies against the virus and remains permanently infected with BVD – known as persistently infected (PI).
"PIs excrete high levels of virus for the rest of their lives, and spread infection quickly to a naive herd.
"When the naive animal is in the first 90 days of pregnancy, BVD causes embryo loss and poor fertility, while exposure at 90-120 days of pregnancy can cause abortion. Infection at a later stage may result in abortion or birth of brain-damaged or mummified calves."
PIs usually die of mucosal disease before theyre two years old, says Mr Orpin, but a significant number can survive and look healthy, themselves giving birth to PI calves.
"Faced with this scenario, we decided the Hancocks best option was to vaccinate the whole herd. Although initial vaccinations cost £10-£12/cow, annual boosters are about half that cost." They felt the outlay was cheap compared with the potential losses from BVD.
Despite exposure for just four months the herds Fertex score (financial assessment of fertility performance) fell from being +£32 a cow to -£36 a cow. The £68 a cow loss results from extended calving intervals, a higher cull rate, lost milk sales and lost calf revenue.
Vaccination and then running a bull with cows afterwards to pick up repeats meant the cull rate of just two cows was lower than would otherwise have been the case. But these losses, and the fertility difficulties, meant some high cell count cows were kept on.
"Our rolling average cell count, usually less than 100,000 went up to 400,000," Mrs Hancock says.
Bacteriology on 10 high cell count cows pin-pointed the types of infection; two older cows were culled, the rest treated. Cost of the outbreak was at least £100/cow treated.
She feels BVD lowered the cows resistance last winter, contributing to the increased mastitis. "The cows didnt thrive and yields, normally 6800-7000 litres, fell 200 litres a cow."
But pregnancy rates post-vaccination are now back at 60%, adds Mr Orpin, and there has been no evidence of embryo loss this year.
Fertility data at Lodge Farm
Aug 96-May 97 Aug 97-May 98
First service pregnancy rate (%) 56 49
All service pregnancy rate (%) 60 45
Services/conception 1.6 2.2
Irregular returns (26-36 days) (%) 7 17
Number reabsorbing foetus after
scanning as positive 1 9
• Buying in.
• Nose to nose contact.
• Hired or shared bull.
• Mixing naive and infected stock.