New evidence from a New Zealand study supports the use of Bovine Viral Diarrhoea (BVD) calf ear notch testing to detect persistent infection (PI) in calves at any age. Identifying and culling these animals, who remain carriers of the disease throughout their lives, minimises the spread of the disease and the unnecessary culling of acutely infected animals.
BVD is a Pestivirus that spreads rapidly and has been found in about 80% of New Zealand’s dairy and beef herds. Its cost to NZ dairy farmers is $127 million ($70,000 per average herd).
The disease causes reduced fertility, abortion, congenital defects, reduced milk production and increases susceptibility to other infectious diseases due to immune suppression, particularly in young calves, which can lead to increased calf deaths.
But voluntary control is entirely achievable through a community effort to identify and remove infected animals from dairy herds.
There are three types of infection – acute, fetal, and persistent.
A first-time exposure to the virus in naïve, susceptible animals results in acutely infected (known as transiently infected or TI) animals that recover but shed low levels of the virus.
Infected dams will be at risk of causing fetal infections due to the virus’s transmission across the placenta.
When fetal infection occurs before the development of a fully functioning immune system, persistently infected (PI) cattle are born. A PI calf will always carry the BVD virus and will never generate antibodies specific to the virus because its immune system does not recognise the virus as “foreign” at the time of infection. PI cattle are the leading cause of new acute and fetal infections owing to their shedding of enormous amounts of the virus via all body fluids.
Current estimates place the prevalence of PI animals in BVD-infected herds in New Zealand at less than two percent, and identifying and appropriately managing PI animals is the primary focus of BVD control programmes.
WHY IT IS ESSENTIAL TO TEST NEWBORN CALVES AND HOW TO DO IT
Since the effects of BVD can look similar to those of other diseases, knowing the farm BVD status is crucial in the fight to achieve control. Comprehensive diagnostic testing plans are fundamental to making informed decisions on controlling and preventing the spread of the BVD virus.
BVD control consists of two critical mechanisms that, together, break the within-herd transmission cycle: the early identification and removal of PI animals; and strict herd biosecurity to prevent the reintroduction of the BVD virus from outside sources. Detecting PI animals is, and should remain, the primary focus for farmers working to remove BVD from their properties.
Previous studies have demonstrated that an enzyme-linked immunosorbent assay (ELISA) test using a small bit of ear tissue (ear notch) is an accurate, economical, and feasible way of detecting PI calves in New Zealand.
A 2019 research trial by Cognosco Animal Health in New Zealand was designed to assess the sensitivity and specificity of ear notch samples of calves analysed using a specific antigen ELISA test and real-time PCR testing at four time points after birth.
The trial results demonstrated an economical breakthrough, said Andrew MacPherson, IDEXX Medical Affairs Veterinarian.
“We now have the perfect test to focus our efforts on identifying only PI animals, saving unnecessary culling of acute animals and enabling farmers to maximise their return on the calves born each year. ”
Adopting a simple and easily understood test for every calf born is a significant opportunity for the New Zealand dairy industry to control BVD voluntarily.
Farmers’ willingness to test all calves to identify PI animals and remove them from their farms, combined with improved biosecurity, will deliver economic benefits for all farmers and lead to significant improvements in New Zealand’s BVD status.
