29 December 2024

Trends in TB, reducing risk for dairy herds

At the National Dairy Conference 2024, Damien Barrett, Niamh Field, and Derek O’Donoghue, presented the latest trends in bovine TB and practical strategies to reduce the risk for dairy herds, ensuring healthier livestock and a stronger industry.

National trend data on herd bovine TB incidence and reactor numbers has shown an unwelcome increase across 2023 and 2024 (Figure 1, available at www.bovinetb.ie). An increasing proportion of TB-restricted herds are dairy herds. There are several potential factors contributing to the relatively higher risk of infection in dairy herds, including larger herd sizes and the closer air-space proximity of animals during daily milking. These factors may contribute to greater animal-to-animal transmission and residual infection risks. Additionally, there is significant regional variation in TB incidence rates, with notable clusters of infected herds in the North East, South West, and South Midlands.

Figure 1. Trends in Bovine TB Incidence Rates in Ireland, 2023-2024

How Accurate Is the TB Skin Test?

The accuracy of any disease diagnostic test can be defined by two separate but related metrics:

• Sensitivity: The ability of the test to correctly identify a truly infected individual as having the disease.
• Specificity: The ability of the test to correctly identify a non-infected animal as not having the disease.

Therefore, a test with high sensitivity has a low ‘false negative’ rate, while a test with high specificity has a low ‘false positive’ rate.

For the TB skin test, the sensitivity under good testing procedures is approximately 80%, while the specificity is around 99.5%. In other words, for every 100 infected animals, the skin test will identify 80 as truly infected, while 20 will go undetected (false negatives). On the other hand, only about 1 in 5,000 skin reactors will be a false positive in real terms. This counters the commonly held belief that there is a high proportion of ‘false positives’ on the skin test, which is often linked to the absence of detected lesions in the reactor animal at the factory. However, the absence of detected lesions does not necessarily mean the absence of disease.

In a herd breakdown situation, the sensitivity of the TB skin test means there is a risk of leaving residual infection behind, which can lead to within-herd spread and prolonged, more severe stock losses over time. Repeating the skin test and using a complementary blood gamma interferon (GIF) test will increase the aggregate sensitivity of the testing schedule to over 95%. The specificity of the blood test is lower than that of the skin test, meaning a somewhat higher false positive rate. However, in a breakdown scenario, this is often a necessary trade-off to improve the detection of truly infected animals who have not shown as reactors on the skin test. This explains why multiple clear tests are required to restore a herd’s trading status. Farmers are advised to be aware of the risk of false negative animals in their herd during a breakdown and to take all necessary steps to identify and remove residual infection quickly.

Current Research on TB

Teagasc is currently leading three new research projects on bovine TB. The single intradermal comparative cervical tuberculin (SICCT) test (skin test) is used to screen all cattle herds for infection annually, with the gamma-interferon test being used in certain cases to increase sensitivity. Both of these tests rely on the animals’ immune system to detect infection.

Physiological stress can lead to immune suppression in animals, and it is hypothesized that this may impact TB test performance. In cattle, calving and early lactation are stressful events, which may increase susceptibility to disease during this period and reduce the response to diagnostic tests. The objectives of the BoviTB project are:

1. To determine whether there is an association between the stage of lactation and an animal’s bTB test result.
2. To determine if there is an association between an animal’s health and metabolic status and its bTB test outcome.

The MetabTB project follows a similar theme. Nutrient deficiencies have significant impacts on immune function, which can increase susceptibility to infection, thereby raising the risk of bovine tuberculosis (bTB) herd breakdowns and overall bTB incidence. One potential method for reducing TB incidence is improving animals’ immune status through optimal nutritional management. The objective of the MetabTB project is:

1. To assess the immune, nutritional, and metabolic status of herds affected by a bTB breakdown (case subjects) and pair them with matched bTB-free herds (control subjects). This will allow researchers to understand the potential benefits of enhancing immunity in the national herd through better nutritional and metabolic management at key times of the year.

Outcomes from these projects will be directly applicable to the national eradication program.

Summary of Actions Farmers Can Take to Reduce the Risk of TB in Dairy Herds

Farmers can take steps to protect their herds from TB by addressing key risk factors for a breakdown. These include:

• Cull any cattle that were inconclusive at a previous test no later than the end of the current production cycle. Inconclusive cattle are 12 times more likely to become reactors at a subsequent test.
• Cull any cattle that had bovine reactions at a previous TB test, as these are 4 times more likely to become reactors at a subsequent test.
• Consider culling any older cattle that were present during a previous restriction, particularly age cohorts of infected cattle.
• When sourcing breeding replacements, choose cattle from herds that have not been restricted in recent years, and ensure cattle have a recent test date. Cattle exposed to TB recently may have undetected infection and could bring the disease into your herd.
• Ideally, breed your own replacements to avoid introducing disease.
• There is significant genetic variation in TB susceptibility within the dairy cattle population. Select herd sires with high breeding values for TB resistance. Information on individual bulls is available on the ICBF website or from your adviser when selecting bulls.
• If you haven’t already done so, locate all badger setts on your farm and notify the Department of Agriculture, Food and the Marine of any setts you find.
• Fence off the setts and latrines to prevent cattle from consuming any grass potentially contaminated with TB bacteria (Mycobacterium bovis).
• Raise water and feed troughs to prevent badgers from accessing them.
• Do not feed concentrates on the ground, as badger saliva can contaminate the area when they eat any leftover feed.
• When selecting bulls for breeding, choose those with greater genetic resistance to TB, based on the health traits sub-index of the EBI.
  • If your herd is restricted for TB, this can reduce the number of cattle exposed to infection.
  • Many bulls with historically high EBI figures have inferior TB resistance figures, so it is important to improve genetic resistance to TB within the national herd.
  • There are still bulls with both high EBI and good TB resistance figures. Identify these bulls to increase both the genetic merit and TB resistance in your herd.
• Ensure good-quality testing facilities and assistance are available for TB tests, so the test is carried out properly. Each animal must be identified and have its skin measured on both test days.
  • If the test is not carried out properly, infection may be missed, allowing the disease to spread widely before it is detected at a later test, resulting in many more reactors than if the disease had been detected early.
• Wash and disinfect any machinery and facilities shared with neighbors, as TB bacteria can survive in the environment and lead to new infections.
• Ensure boundary fences are well maintained to avoid mixing with cattle from other herds.
• If you have your young stock contract-reared, ask the rearer to take steps to reduce the risk of TB and have a contingency plan for a TB breakdown in either herd.

Freedom from TB is important for international trade and individual farm profitability. However, controlling bovine TB remains a challenge for the dairy industry. Each TB restriction brings significant financial and emotional challenges for those involved. Multiple risk factors contribute to any TB outbreak, making the disease more challenging to manage. By addressing each of these risk factors, the likelihood and extent of TB breakdowns can be reduced.

Read the full Dairy Conference 2024 Proceedings