TB monitoring

Olivia Franks, The Farm Vets at Hampden Veterinary Hospital, underlines the importance of herd monitoring for bovine tuberculosis to detect and contain infection. Preventing the disease can seem like a challenge but free advice is available.

Bovine tuberculosis (bTB), the infectious disease caused by Mycobacterium bovis, is endemic in cattle and wildlife in some areas of the UK, and causes severe disease in alpacas. Its complex biology makes it a difficult disease to detect.

However, it is imperative that alpaca owners undertake proactive monitoring for bTB in their herds. bTB infection in alpacas can take a long time to develop to the point of showing clinical signs, which can be vague and non-specific when they do appear. Meanwhile, those animals can be seeding infection through the herd, so that on discovery it is already widespread. Proactive monitoring is key to early detection and containment of infection.

Clinical signs

Alpacas infected with bTB can show a wide variety of clinical signs, which in turn are rarely specific to bTB. They can appear fit and healthy despite quite extensive lung disease, due to their adaptation to tolerate low oxygen conditions at high altitude. However, signs to look out for include:

• Breathing difficulties

• Coughing

• Lethargy and weakness

• Weight loss, which can be rapid

• Lack of appetite

• Enlarged lymph nodes (glands)

• Sudden death

Many of these clinical signs are more often caused by other diseases, such as gastrointestinal parasitism, than bTB, but bTB should be considered as a possible diagnosis. Isolating animals showing suspicious clinical signs away from the main herd is a sensible precaution while awaiting diagnosis (with a companion for welfare reasons). Regular body condition scoring will enable early detection of animals losing condition unexpectedly.

Postmortem examination (PME)

Many cases of bTB in alpacas are detected following post-mortem examination of a sick animal that has either died or been euthanased. The presence of characteristic lesions will be followed up with laboratory testing to determine if bTB infection was the cause of disease. PME can be a valuable source of information about many herd level diseases,

including gastrointestinal parasitism, and alpaca owners should consider routinely submitting animals that die for PME, especially if the death is unexplained. Your vet should be able to advise you how to arrange this.

Surveillance testing

This term refers to tests performed on animals in whom there is no suspicion of bTB. This might be as part of routine annual whole herd surveillance, or as a pre-movement test; for example before a sale or mating.

Surefarm Ltd provide a voluntary private system for herd surveillance, facilitating annual testing of the whole herd, or in larger herds testing a proportion of the herd. For information on this scheme, visit www.surefarm.co.uk/camelid-diagnostics. Whether using this scheme or not, camelid owners must obtain permission from the Animal and Plant Health Agency (APHA) to test their animals for bTB (country specific rules apply). Several tests are available.

The skin test detects infection by measuring the skin reaction to injection with tuberculin (a sterile, purified protein extract from Mycobacterium avium and M bovis bacteria). A positive reaction indicates that the animal’s immune system has already been exposed to M bovis, the M avium tuberculin being used as a control. Skin testing is usually carried out by your private vet. The skin test is considered to have a low sensitivity – that is, the likelihood that an infected animal will test positive is low. For this reason, blood tests are also recommended for surveillance.

The blood tests, Enferplex (2-spot or 4-spot), DPPVetTB and IDEXX, detect antibodies in the blood to a set of M bovis antigens. Antigens are molecules on the bacteria that stimulate an immune response from the host. Looking for a set of different antibodies improves the probability of detecting an infection and improves confidence in the result. These tests can be read and combined in various ways to improve their performance and depending on the context in which they are being used. For surveillance testing, high specificity test options are used which minimise the probability of false positive results.

APHA recommends that alpacas are boosted with bovine tuberculin 10-30 days before blood sampling for antibody testing, using either a skin test, or injection with bovine tuberculin only (the rules for this differ between England/Scotland and Wales). This step is recommended to boost levels of pre-existing specific antibodies to M bovis, which may otherwise be too low to detect; thus improving the likelihood of detecting infected animals. It does not induce antibodies in uninfected animals.

If a positive bTB result is identified through testing or suspected from PME, the animal will be culled (if applicable) and movement restrictions placed on the holding pending further testing. This can cause reluctance among keepers to test animals or submit for PME for fear of finding a ‘false positive’; however, the risk of allowing an unidentified infected animal to spread this fatal disease on a holding, or in the national herd, is arguably far more significant.

I recommend visiting tbhub.co.uk/tb-in-non-bovine-animals/camelids/ for more information on bTB in camelids, along with detail about the different testing methods available, and regional differences in testing practices.

Preventing bTB entering a herd is much easier than eradicating an infection, but it can seem like an overwhelming challenge. The TB Advisory Service is a DEFRA funded project that offers free, bespoke, practical and cost-effective advice to all farmers of camelids in England to help reduce the risks associated with TB. The advice can be provided over the telephone or in a series of farm visits – and all it will cost is your time!

About the author

Oli is a farm vet based in Buckinghamshire who enjoys all aspects of farm work, with particular interests in sustainable agriculture and camelid medicine. She enjoys the variety that camelid work offers at both the individual and the herd level.

For more information: www.tbas.org.uk info@tbas.org.uk