A unique case study of a herd infected by pestivirus has found that the disease's biggest impact was not fertility, as traditionally believed, but calf health.
As part of a Charles Sturt University project, the introduction, impact and subsequent elimination of Bovine Viral Diarrhoea Virus (BVDV) was monitored in a self-replacing, 800 to 900 head, Angus herd between 2013 and 2018.
The Graham Centre's Professor Bruce Allworth, who oversaw the study, explained BVDV was first detected in the herd in 2013.
He said while the herd was infected they saw normal fertility levels in both heifers and cows but believed there was an increase in calf losses from calving to marking.
He explained while most animals built immunity when exposed to the virus, calves exposed to BVDV during the first trimester could become infected.
"The effect is that the virus is recognised as part of self so they never develop antibodies and can't resist the infection," Prof. Allworth said.
"Foetuses infected in the first trimester may survive the infection but that infection makes their whole immune system weaker and when they're born they're more susceptible to any health problems."
He said once an infected calf was born it could expose other cattle in the herd to the disease, the result being the herd develops a strong immunity to the virus.
"If all cows are immune, no problems occur and it can become self-eliminating," Prof. Allworth said.
Self-elimination of the disease is what the Prof. Allworth believes eventually occurred in their case study.
"An elimination program was set up in 2017 and required all the animals on the farm to be ear notch tested for BVDV," he said.
"However, we didn't find any animals with BVDV, the infected animals had died or been sold out of the herd."
He said despite this, the case-study highlighted the need of extensive testing and additional biosecurity measures if if BVDV was to be eliminated quickly, with infected cattle often showing no obvious symptoms.
"You can't just test animals and think you'll be right," Prof. Allworth said.
"If you successfully eliminate it and then get it back in again, you've wasted the investment you made in testing."
He said biosecurity was important to prevent the re-introduction of pestivirus.
"In the case study no cows were introduced. The two main risks to address were potential contact with neighbouring cattle and the introduction of bulls," Prof Allworth said.
"Any bulls introduced to the herd need to be tested as negative to the virus and then a buffer should be maintained between neighbouring herds."
Antibodies test proves eradication
Charles Sturt University honours student, Rachael Long, found that a blood test looking for the presence of antibodies was an efficient way to show pestivirus had been completely eliminated from a herd.
Ms Long explained animals were usually tested for pestivirus with an antigen ear notch test that determined whether the animal had any viral proteins.
She said to eliminate the virus, any animals found to be positive for the disease would need to be removed.
Then to check the disease was no longer present, producers' could do antibody testing on a small sample of young cattle to see whether these cattle had been exposed to the disease and therefore mounted an immune response.
"In theory if we had been successful with eliminating the disease, these young animals shouldn't have been exposed and they should be negative on the test," Ms Long said.
"This is an easier process than re-doing an antigen ear-notch test with every animal in the herd and can provide greater than 99 per cent probability that pestivirus is not present in the herd."