Test your calves - the sooner after birth the better

Testing calves for BVD as soon as possible after birth using either Antigen-ELISA or PCR on ear notch samples can help you remove PI animals from the herd before they have a chance to infect susceptible pregnant cattle

Background

One of the major cornerstones of BVD control is identifying and removing PI animals from the herd before they have the chance to infect pregnant dams. With the highly seasonal nature of cattle production systems in New Zealand, this ideally means testing calves in the short time window between the start of calving and the start of mating while the breeding herd is empty.   In many other countries with BVD eradication programmes, testing is often performed at the same time that calves get their national identification tags using devices that collect labelled ear notch biopsy samples as the tag is placed.

Previous Recommendations

In New Zealand, it has previously been recommended to wait until calves are over 35 days of age before testing with an antigen ELISA because of the diagnostic gap, which is the potential for samples from young PI calves to return negative results on antigen-ELISA due to interference from maternal antibodies ingested through colostrum. However, this can create logistical challenges for farmers since the most convenient time to collect samples is often at tagging or disbudding. The additional inconvenience of arranging a separate sampling event at a later date or collecting blood samples instead of ear notch samples may deter some farmers from testing their calves.  The results from real-time PCR are not affected by maternal antibodies since the test looks directly for the viral genetic material. PCR can therefore safely be performed on blood and ear notch samples from calves of any age.

What has changed?

Since the original recommendations around calf testing were made, there have been changes to the antigen ELISA methodology that have increased its reliability for testing ear notch samples in young calves. Most European countries with national BVD control programmes have been successfully testing calves from birth using the antigen ELISA on ear notch samples for more than a decade and recent preliminary field data from New Zealand (IDEXX BVD Antigen-ELISA Ear Notch Trial) suggests that the antigen ELISA is likely to perform similarly under New Zealand farming conditions.

Bringing New Zealand in line with international practices for testing calves, the national BVD Steering Committee has released a new Position Statement on Testing Calves Under 35 Days of Age for BVD changing the recommendation to indicate that calves in New Zealand can be tested any time after birth using PCR or antigen ELISA on ear notch samples and PCR only on serum/blood samples.

Cautions

It should be noted that although both test methods have excellent sensitivity and specificity for detecting BVD virus in cattle, there is always the potential for false positive and false negative results to occur. We therefore strongly recommend that farms have additional monitoring programmes in place to detect any potential breakthroughs before they cause significant problems in the herd. This includes annual bulk milk testing (PCR and ELISA) for dairy herds and youngstock antibody screening tests for beef or dairy herds.