The cows most likely to be infected with Johne's disease can now be identified automatically as part of a herd screening service by NMR.
The new auto 30-cow selection facility has been developed by NMR and uses results from a study carried out by the University of Reading that looked at the correlation between Johne's test results and cow health and production records. This herd-level service is an option for initial screening and for surveillance of proven low-risk herds with minimal or zero prevalence of Johne's disease.
"We analysed almost 14,000 cow records and linked these to their Johne's test results," says James Hanks from the University of Reading. "We saw there were strong statistical links between Johne's status and high SCC, lactation number, recorded cases of mastitis, lameness and some fertility parameters."
One of the strongest correlations was with high cell counts in the six months proceeding the test date, which were almost twice as likely to occur in Johne's-positive cows. The relative risks of high cell count cows having Johne's has been used in the development of the new software.
"Including cows in the high 'relative risk' groups in the 30-cow surveillance screen will increase the chance of identifying any Johne's disease in the herd," adds Dr Hanks.
"For example, cows that had four or more somatic cell count recordings above 200,000 cells/ml or two above 500,000 cells/ml in the latest six recordings were twice as likely to give a positive milk test for Johne's as cows with consistently lower cell counts.
"We also found about double the prevalence of Johne's-positive cows in lactations three to six compared to both younger and older cows. This implies that Johne's infected cows struggle to survive in the herd as they get older, succumbing to culling for production and health-related reasons.
"So by factoring in lactation number in the auto 30-cow selection software the chance of correctly identifying Johne's disease in the herd will increase even more."
NMR, in initiatives with milk buyers and vets, has been using 30-cow screening tests for the past two years as an initial assessment of the presence of Johne's disease within a herd. "Groups of cows from more than 1,000 herds have been screened and results have identified the presence of Johne's in just less than 70% of the screens," says NMR manager Steve West. "This has proved to be a far more effective method of herd-level testing than the use of a bulk milk test."
The 30-cow screening test has been the first stage in developing a Johne's control programme for many herds. It can also be used in low-risk herds where no initial evidence of Johne's has been identified and where there are efficient biosecurity measures in place.
"Selection of the 30 cows for screening has, in the past, been carried out by the vet and the producer, who have identified those animals they think most likely to be affected by the disease," adds Mr West. "Based on an accurate database of Johne's test results, milk records and work carried out at Reading, an accurate calculation has been developed to automatically select the most likely 30 cows to be affected by Johne's. And this selection procedure is now consistent within and between herds."
Available to all NMR recording herds, the new 30-cow selector will be used in the company's ad-hoc and HerdTracker Johne's herd screening services, drawing on the cow's latest six months' production and health information.
The auto selection facility brings additional advantages. Milk samples are collected routinely for milk recording and these can be used for the Johne's test, so no further sampling is required. Those herds using HerdTracker for quarterly 30-cow screening will be scheduled into the system and results will be sent to the producer and vet as requested.
"The auto 30-cow selection calculation will be reviewed and updated as more analyses are performed," adds Mr West. "This service is a valuable addition to our Johne's disease control programme for herd surveillance in low-risk herds where there is minimal or zero prevalence of Johne's.
"It must be remembered that those herds with average or high levels of Johne's are advised to adopt individual cow-level tests and risk-based management using quarterly testing for the whole herd. Producers should work closely with their vets in developing the best control programme for their own herds."
The screening service is expected to cost about £85 as an ad-hoc option or £280 a year in the HerdTracker routine surveillance service, which includes four 30-cow screens.
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