Andrew Swallow

Two risk factors stand head and shoulders above all others when it comes to the risk of picking up Mycoplasma bovis, New Zealand Veterinary Association chief veterinary officer Dr Helen Beattie says: prolonged contact with an infected animal, and ingestion of infected milk by calves.

“Everything else pales in significance in comparison,” she told Country-Wide.

Dr Helen Beattie, New Zealand Veterinary Association chief veterinary officer.

The minuscule, wall-less bacteria doesn’t survive more than a few hours outside the animal or milk, hence the emphasis on prolonged contact for transmission. Saleyards, where an animal might go in a pen after another animal, or be in a neighbouring pen, or walk the same races, are a potential but less likely transmission route, she believes.

“There’s definitely the potential for infection via those routes. But they are much less concerning than animal movements and subsequent prolonged contact with new animals, and the ingestion of infected milk. In some infected animals you can swab the cheeks and get an M bovis isolate, and from some animals you’ll get it from a swab from the back of the throat… so in theory, yes, it could be picked up.”

‘A whole bunch of people are going to have to work extra hard for a very long time. And it relies on everybody doing the right thing.’

However, stock are in saleyard pens for a limited time and not all infected animals are permanently shedding, she says. Also, as MPI tracks the incursion and imposes movement restrictions, only animals from farms believed to be free of the disease should be reaching saleyards. That said, saleyards are not a great idea from a disease-control perspective in general, not just from an M bovis one, she says.

Eradication is “definitely feasible”, Beattie believes, but it is going to be a big job and not easy to achieve because M bovis is “such a tricky little bug”. Infected animals are frequently subclinical and even when disease develops, the agent, M bovis, is hard to find and test for.

“It [eradication] is bigger than Ben-Hur,” Beattie says, referring to the 1959 movie which smashed all previous records for movie budgets, casts and sets. “A whole bunch of people are going to have to work extra hard for a very long time. And it relies on everybody doing the right thing.”

Surveillance during the dairy calving season, when stress could provoke shedding and clinical signs from previously undetected M bovis infection, will be key.

Having worked in the tail-end of the United Kingdom’s 2001 foot-and-mouth disease epidemic, and the aftermath, Beattie says incursion fatigue, where vigilance and standards slip over time, is a real danger.

The NZVA is also concerned farmers will attempt to treat unidentified M bovis cases with antibiotic, and may only seek veterinary advice when treatment fails.

“During the incursion, it is really important to seek early veterinary intervention for any unusual or non-responsive cases, particularly of pneumonia in calves, mastitis and arthritis,” she stresses.

Testing is only of limited help in preventing introduction of the disease to a herd due to the difficulty of obtaining a sample containing M bovis from an infected animal or, in the case of antibody (serology) tests, the unknown period during which antibodies are present. Research shows varying periods of antibody persistence – as little as a week, or up to two months, or potentially more.

“Before you hang your hat on a clear result you have to understand what that test result is really telling us… What is useful, is understanding your risks to M bovis infection. A focus on testing as the panacea isn’t helpful.”

Key points

  • Animal movement and infected milk main risks.
  • Check stock origins before buying and quarantine.
  • Feed calves onfarm milk only, wherever possible
  • Recognise limits of tests available.
  • Spring surveillance pivotal for eradication plan.