Bovine Viral Diarrhoea Risks In Naive Herds

The Fragile Progress Against BVD

Significant strides have been made in the effort to eliminate bovine viral diarrhoea (BVD). Since 2013, the number of persistently infected (PI) calves has seen a dramatic decline, falling from over 16,000 to 485 in 2024. Data from 2025 confirmed 644 BVD-positive calves, though early 2026 figures show a positive trend, with 139 cases recorded by week 11 compared to 219 during the same period in 2025.

Despite these improvements, specialists warn that the progress is fragile. The success of the eradication programme has inadvertently created a growing population of naive herds across the country. Because the virus is less prevalent, many farmers have stopped vaccinating their livestock. This lack of immunity means that if a single PI calf enters a herd today, the resulting outbreak could be far more severe than those seen in previous years. Some veterinary professionals describe this situation as a ticking time bomb, noting that they have seen more BVD in the previous three years than in the six or seven years prior.

Understanding Infection Types And Risks

It is critical for producers to distinguish between persistently infected (PI) and transiently infected (TI) animals. A PI animal is a calf born to a dam that was infected during pregnancy; these animals act as virus factories, shedding massive amounts of the pathogen. In contrast, a TI animal is infected after birth. While a TI animal can eventually clear the virus, it remains immunosuppressed and sheds the virus for approximately three weeks, making it highly vulnerable to secondary infections and facilitating the spread of the disease within the herd.

Rapid detection is the primary defense against these outbreaks. Veterinary professionals advise tagging calves as early as possible and sending samples to the laboratory without delay, ideally several times a week, to ensure any positive cases are identified and isolated immediately. This speed is essential because the virus can spread rapidly through a naive population.

The Hidden Threat And Prevention

One of the most dangerous aspects of BVD is its ability to remain silent. Symptoms such as reduced conception rates, poor calf performance, and increased respiratory disease are common, but they do not exclusively signal BVD without diagnostic testing. The virus can be introduced through various channels, including neighbouring herds, the mixing of cattle at marts, purchased livestock, transport equipment, and personnel.

To mitigate these risks, experts recommend a strict biosecurity protocol. This includes buying livestock only from known sources, staying informed about the health status of the purchasing area, and always quarantining incoming animals. Checking monthly programme maps before purchasing livestock is also highly recommended to avoid introducing the virus into a clean herd. This awareness of a herd’s BVD status can be the difference between a safe purchase and a full-scale outbreak.

The Role Of Vaccination

Vaccination remains a fundamental pillar of herd protection, particularly for breeding females. Protecting the foetus against transplacental infection is essential for preventing the birth of PI calves. A standard vaccination course typically involves two injections four weeks apart, with boosters administered before the next breeding cycle at intervals no longer than 12 months. The final dose must be completed at least four weeks before breeding begins.

In cases where a positive result is confirmed, vaccinating all breeding females and bulls aged 12 months or older becomes mandatory. This process is conducted under veterinary supervision and is funded by the Department of Agriculture, Food and Marine. Experts suggest that if a neighbour has had a positive case, vaccination is definitely required to protect the following year’s calves.

The danger of complacency was highlighted by a case in Co Clare involving a 100-cow herd that purchased six in-calf heifers. Four of the resulting calves were PI. Following a four-week circuit breaker and blood testing, 80 animals in the herd tested positive as transiently infected. The immunosuppressive effects of the virus led to the loss of 60 animals due to various secondary diseases, demonstrating the devastating financial and emotional impact a single introduction can have on a business.