Saturday, March 30, 2024

New tools in Johne’s fight

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Farmers concerned about Johne’s disease in their herd are being advised to take a multi-pronged approach to dealing with it that includes a new bull index.
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Johne’s disease (JSD) susceptibility is a new genomic estimate of how susceptible a bull’s progeny will be to the wasting disease – a disease that’s most often picked up by calves but not usually obvious until cattle are much older.

A JSD breeding value has been developed by LIC with funding from DairyNZ and the Johne’s Disease Research Consortium (JDRC) and is being made available to all genetics companies supplying NZ dairy farmers.

LIC’s latest Alpha Sire Catalogue includes the index and gives a breeding value that’s either negative or positive to indicate how susceptible progeny will be to the disease.

LIC chief scientist Dr Richard Spelman said the research project to develop the index took five years and has resulted in breeding values that have so far ranged from about -50 to +50.

A value of -10 indicated calves resulting from that bull would be 10% less likely to contract the disease than the average animal whereas a value of +20 indicates progeny would be 20% more likely.

Spelman said heritability appeared to be high at about 22%. Milk production was 25-30% heritable.

However, reliability of the JSD breeding value was relatively low at about 30% compared with 80-85% for milk production in daughter-proven sires.

The low reliability stemmed from the limited number of animals tested. It meant while the JSD breeding value was a robust indicator it should be used as just one tool in a wider control programme.

“It’s not a silver bullet so it has to be used in combination with a more comprehensive Johne’s disease health plan,” he said.

The research programme looked at 2000 animals that tested positive for Johne’s out of herds identified through bulk milk samples. Those animals were genotyped and blood typed and compared using genomic techniques with genotypes of about 100,000 animals on LIC’s database.

Scientists found a combination of gene markers could be used to identify animals more susceptible to the disease.

Former DairyNZ chief scientist Dr Eric Hillerton said a new Johne’s Disease management book produced in conjunction with the JDRC was available from DairyNZ and detailed how the disease was spread and what farmers could do to minimise the risk of spreading the infection.

Because most transmission of the disease was to calves or heifers from bacteria shed in infected cow faeces, as well as through colostrum or milk from clinically infected cows, a test and cull strategy was advised, along with strategies that minimised calf exposure to cow faeces or potentially infected milk.

Any cows presenting with clinical signs of the disease should be culled immediately to prevent further shedding of the bacteria and in herds where the disease was a problem, cows should be tested before calving, with animals that tested positive culled.

The bacteria, Mycobacterium avium paratuberculosis or MAP, could survive in effluent for some months, in water and on pasture.

Ten to 40% of calves born to cows with advanced infections would be infected before birth. The bacteria caused intestines to become inflamed so nutrients couldn’t be absorbed.

The clinical signs, which were more likely to occur in cattle seven years or older, included loss in body condition and ill thrift, profuse diarrhoea and swelling under the jaw.

A survey of 457 New Zealand herds found 54% reported up to 6.2% of the herd had clinical signs of Johne’s disease. In the South Island 68% of herds reported having animals with clinical signs while in the North Island that figure was 44%. Tests for the disease are widely available and can be done by blood test or faeces test, or the herd could be screened by bulk milk testing.

Whole herd milk or blood screening that showed a negative result didn’t guarantee the disease is not present but does indicate prevalence is low.

Johne's Disease management guide

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