Links between BVD and TB are becoming clear
By Marianne Curtis
UNUSUAL factors in some bovine tuberculosis breakdowns have led vets to suspect there may sometimes be a link between the disease and bovine viral diarrhoea.
Speaking at an Intervet-sponsored Wood Vet Group client meeting, Chris Watson – a vet at the Glos-based practice – produced evidence to back his hunch that BVD infection may predispose a cattle herd to TB.
When called to an apparent case of pneumonia in a two-year-old bought-in replacement beef heifer, Mr Watson became suspicious. "She seemed too old for pneumonia and undersized, so I ran a BVD test."
He found the heifer was persistently infected (PI) with BVD (see panel). When TB testing her source herd at a later date, Mr Watson noted some unusual factors. "Usually when TB moves into a herd, a number of adult reactors are seen. But by the 60-day follow-up test the herd is clear. However, this herd had previously had TB problems and in the latest outbreak, four reactors were less than six months old."
Running a BVD test on the four TB reactor calves, Mr Watson discovered two were PI and the other two were BVD antibody positive, meaning they had encountered BVD.
Evidence for a link between the two diseases seemed to grow when BVD testing a number of dams in the same herd at a later date, he discovered five PIs. "One had no calf, three had PI TB-reactor calves and one had a calf die of mucosal disease, which afflicts PI animals."
Signs of BVD were also apparent in calves on a farm which experienced the first calf TB outbreak Truro Vet Investigation Officer Bob Monies had seen in his 30-year career. "While doing a post-mortem on a calf, I found TB lesions, which was unusual in an animal so young," he added.
Cohorts were slaughtered and of 22 calves under six weeks old, 13 had TB lesions. Three calves showed signs – cataracts, a short tail and wobbly legs – they had been infected with BVD in the womb. "It was difficult to reliably test how many were PIs because in young calves, colostrum derived antibodies can interfere with results."
While conducting a BVD antibody bulk milk sampling trial in a dairy herd for Intervet, Mr Watson made a further discovery which seemed more than a coincidence.
"I began sampling in October 1998. The herd remained BVD-antibody negative until July 1999, when levels began to shoot up. In June 1999, Mr Watson discovered the first TB reactor in the herd for 20 years.
"I calculated that BVD had entered the herd by March 1999 because we later discovered a September 1999-born PI calf which must have been infected with BVD between days 90 and 126 of gestation."
The dairy herd continued to have growing numbers of PI calves and persistent TB problems for the next two years, said Mr Watson.
So why does there seem to be a link between the two diseases? "BVD suppresses the immune system. This may make cattle more susceptible to mastitis, pneumonia and fertility problems as well as TB. The relationship between TB and BVD is not one to one, but it is likely they are associated in some situations," he suggested.
"Vaccinating against BVD wont stop TB from occurring, but it will improve herd health and reduce the chance of BVD causing immuno-suppression." *
• BVD is immuno-suppressive.
• May allow TB to enter herd.
• Consider BVD vaccination.
What are PIs?
When a foetus becomes infected with BVD between days 90 and 126 of pregnancy, while its immune system is developing, it will fail to recognise the virus as foreign and never produce antibodies to it. This animal – known as a PI – will shed BVD virus for the whole of its life, posing a risk to herd mates.
PIs are often underweight for age and most die of mucosal disease before reaching two years of age. However, when they survive long enough to calve, their own calf will also be a PI.