Bovine Viral Diarrhoea: Why a 100% Foetal Protection Vaccine Matters

Bovine viral diarrhoea (BVD) poses a significant health and economic challenge for cattle farms worldwide. This insidious disease weakens immune systems, increases susceptibility to secondary infections, and causes reproductive failures, including abortions, stillbirths, and weak calves. The persistence and transmission of BVD within herds are primarily driven by animals that continuously shed the virus, known as Persistently Infected (PI) animals.

Without vaccination, BVD will continue to spread, leading to chronic herd health issues and substantial financial losses.

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What Is a Persistently Infected (PI) Animal in Bovine Viral Diarrhoea?

Persistently infected (PI) animals become infected in utero between 30 and 125 days of gestation. At this stage, their immune systems are not yet fully developed, meaning they do not recognise the virus as a foreign pathogen. As a result, they remain infected for life: they never eliminate the virus. These animals act as “viral reservoirs”, perpetuating the disease within the herd.

Controlling BVD therefore hinges on managing PI animals. This involves:

• Detecting and removing existing PI animals.
• Preventing the birth of new PI calves, an effective strategy for eradication.

Vaccination of dams is critical to achieving this goal.

Why Is 100% Foetal Protection Essential for a BVD Vaccine?

A BVD vaccine must provide complete (100%) foetal protection to be fully effective. Partial protection is insufficient, as even a small risk of PI calf birth can undermine disease control. The following case studies illustrate the consequences of partial versus full protection.

Case Study 1: Year-Round Calving Herds

Consider a naïve herd of 100 cows with calvings distributed evenly throughout the year. At any given time, 75 cows are pregnant, of which 25 are in the critical 30–125-day window, the period when PI calves may develop.

If the BVD virus enters the herd in mid-August, for example, cows bred between April and July are at risk. Without vaccination, 25 PI calves could be born. If the herd is vaccinated with a 100% foetal protection vaccine, no PI calves will be born, ensuring full protection. However, if a vaccine with partial protection, such as 94%, is used, 6% of the 25 at-risk cows remain unprotected, meaning 1–2 PI calves could still be born.

Case Study 2: Seasonal Calving Herds

Now consider a seasonal calving herd from November to April. The risk window shifts: no cows are in the 30–120-day danger zone between December and February. However, if the virus arrives in August, during the Northern Hemisphere’s pasture season, 50 cows are at risk.

• Without vaccination: 50 PI calves, a catastrophic outcome for the herd.
• With 100% foetal protection: zero PI calves.
• With partial protection (94%): 3 PI calves, enough to reignite an outbreak.

Why Isn’t a Partial Foetal Protection Vaccine Enough to Control Bovine Viral Diarrhoea?

Partial protection still allows some PI calves to be born. Because each PI animal continuously spreads the virus, even one infected calf can maintain disease transmission within the herd for years.

At first glance, a vaccine efficacy of 94% may seem robust. However, in BVD control, partial protection offers a false sense of security. Here’s why:

• PI calves act as multipliers: a single PI calf can infect dozens of others, including pregnant cows, before detection.
• Research by Fountain et al. (2021) found that a single viral incursion into a large herd of 800 animals carries a 23% risk of the pathogen persisting for over 15 years, thwarting eradication efforts. Even in smaller herds of 50 animals, the virus may take eight years to self-eliminate.
• A 6% failure rate means the disease remains entrenched in the herd, jeopardising eradication efforts.

Key Messages

• Vaccination is a cornerstone of effective BVD control.
• Only a 100% foetal protection vaccine can permanently break the disease cycle.

References

Fountain, J., Hernandez-Jover, M., Kirkeby, C., Halasa, T., Manyweathers, J., Maru, Y., & Brookes, V. (2021). Modelling the Effect of Bovine Viral Diarrhea Virus in Australian Beef Herds. Frontiers in Veterinary Science, 8, 795575.