N ew World Came lids and TB: the current position in the UK

4 June 2005

Paul Roger and Mary Marshall

Animal Health Resources Ltd.

This paper is set out as instructed by the British Camelid Association (BCA) to explore an initial position with specific regard to the rare occurrence of bovine tuberculosis in New World Camelids (llamas, alpacas, guanacos and vicunas) in the U.K.

The authors are not tending this as a review article to the standards expected by scientific journals, but rather a lay interpretation of the salient factors involved so that the BCA can decide on a position that they would like to pursue.

A . Bovine tuberculosis in livestock

Bovine tuberculosis (caused by Mycobacterium bovis) is an increasingly worrying disease in so far as its prevalence is increasing across the U.K. It is usually detected at abattoir in carcasses rather than easily seen as a fulminant disease. Cobner (2003) reviewed practical prevention and control in cattle.

Infection may be spread by aerosol, sputum, faeces, milk, urine, vaginal and uterine discharges and from open infected lymph nodes. Clinical signs are not seen until at least 90 days post infection. The organisms may remain viable in the environment for 8 weeks or more under suitable conditions (Radostits et al., 1994), for up to 2 years in badger setts, and up to 11 months in dark, damp corners when suspended in badger urine (King, 2004).

Bovine TB is found in descending order of prevalence in cattle, pigs, sheep and goats, horses, and may be found also in elk, farmed and wild deer, water buffalo, camelids, bison, monkeys and other wild fauna and birds. In New Zealand, the bush tail possum is thought to aid the spread of the disease. Most veterinary authorities in the U.K. ascribe a similar role to the badger. According to the President of the British Veterinary Association: “It is a scientific fact that the two major animals in the persistence and spread of TB are the badger and the bovine. Government also accepts this fact. Logic dictates that if a disease is to be controlled then the major reservoirs of infection must b e addressed…The infected badger must be controlled and removed … ” (McCracken, 2005).

The zoonotic implications of transmission are almost completely eradicated by pasteurisation of milk, making cattle to human transmission an unlikely event in the developed world. Stockspeople and their families and veterinarians are the most at risk categories.

Clinical findings of the disease have been widely recorded and are part of the observant clinician’s differential diagnosis in endemic areas.

The tests commonly in use are:

1) Single intradermal test (S.I.D) – allergic skin test

An injection of 0.1- 0.05 ml of purified protein derivative (ppd) is given into a skin fold and read between 48-96 hours post injection. Maximum sensitivity is at 48-72 hours and maximum specificity at 96 hours.

2) Short thermal test (STT) – allergic skin test

An injection of 4ml of ppd is given under the skin of the neck where the animals temperature is equal or <39 oC. This is read at 4, 6, or 8 hours and is positive if the temperature is >40 oC. The test is usually read at 6-8 hours and positives are usually >41 oC.

3) Stormont test (ST) – allergic skin test

A repeat injection of ppd is given at the same site after 7 days. This is examined after 24 hours and an increase of 5mm in skin thickness is positive.

4) Comparative intradermal skin test (CIST) – allergic skin test..

Ppd from both M.bovis and M.avium is injected intradermally in the neck at sites 12 cm apart. This is read at 72 hours and any changes in skin thickness are noted. The results are interpreted on normal or severe levels depending on the local levels of prevalence and herd history, at the discretion of divisional veterinary managers.

5) γinterferon test (γit) – blood test

This test is not as sensitive as the CIST, but it is very specific (Buddle et al., 2003). Samples need to be rapidly transported to the laboratory and the test is therefore difficult logistically. It is not a substitute for the comparative ppd test, but is recognised in the U.K. as a valuable adjunct to the interpretation of the results.

A number of cautions need to be explained:

i) Site of injection. The sensitivity varies with different skin sites. If the skin along the back is taken as the index as 1, then the upper flank is 1.75 and the lower flank is 2.5, but the skin of the neck is 2.75-3 and is much more sensitive than the caudal fold.

ii) Potency. In farmed deer the potency of ppd should be ca 2mg/ml.

iii) Postparturient desensitisation. For about 4-6 weeks post calving and possibly for a similar period prior to calving, there may be an increase of up to 30% in false positives. Calves may be positive for up to 3 weeks postpartum although they are not infected.

iv) Anergy. This is a failure to react to the tests and can be seen where there is extensive pulmonary involvement and also in very old cattle.

So, in summary:

In cattle:

False Positives (FP)-- From sensitisation to M.avium or M.paratuberculosis

or to non pathogenic Mycobacteria.

Occasionally from other sources such as chronic

Infection, e.g. Nocardia farcinica .

False Negatives (FN)--Advanced cases or very early cases (up to 42 days).

Cows due to calve or up to 6 weeks after calving.

Cattle already tested in the last 8-60 days.

Old cattle.

In pigs: Maximum sensitivity is seen 3-9 weeks post infection.

In horses: There is a lack of detailed information.

In sheep and goats: The single intradermal test is relatively inaccurate, but an

increase in skin thickness of 5mm is still taken as positive.

Complicating factors

The badger became a protected species in 1973. There has been a lack of detailed post-mortem surveillance despite requests to DEFRA, but those limited surveys that have been carried out have shown that ca. 50% of badgers found dead from natural causes have lesions of TB. Spread between badger populations has been shown to be most common via infected individuals migrating. Strategic culling policies have helped to limit spread.

Infection has been recognised in indigenous wild life and recently in rural cats.

Vaccination trials are useful, but reliance on flawed trials which have been interrupted by the FMD epidemic and have a low efficiency have cost ￡ millions that could have been better used on surveillance.

Effective control is not possible without culling diseased wild animal populations. There is definitive evidence of transmission from badgers to cattle (Gallagher and Clifton-Hadley, 1995; Griffin et al., 2005).

In continental Europe, the increasing wild boar population may play a role in the transmission between wild boar, cattle and deer. A high prevalence was seen in free living wild boar shot in southern Spain (Gortazar et al, 2003).

The independent scientific group (ISG) have identified a way forward to include (a) the refinement of the γIFN test to increase sensitivity and to combine this with high specificity so that it may replace the comparative skin test and (b) the development of an effective vaccine for badgers and for cattle.

Recently it has been shown that although γIFN test on whole blood is a practical confirmatory test (Ryan et al., 2000), a refinement in using a combination of 2 mycobacterial antigens (esat-6 and cfp10) resulted in significantly fewer false positives (Buddle et al., 2003).

B. Tuberculosis in camelids

Camelids have different blood chemistries, immune system responses and behavioural patterns to other farmed livestock species. There may also be differences between Old World Camelids (OWC) and New World Camelids (NWC). It should also be noted that NWCs are seldom, if ever, milked for commercial production.

Camelids do not appear to be highly susceptible to TB (Fowler, 1998), but infection (both natural and experimental) has been described in the literature. The risk assessment would depend on knowing the prevalence of TB in the 3 categories below on both a local and a national basis, which is currently unknown, as surveillance is minimal and intermittent.

In the U.K. exposure to Mycobacteria could occur from:

1) cattle;

2) endemically infected wildlife, including transmission from an infected carcass to carrion predators;

3) other (co-habiting) infected camelids.

The major problems faced by camelids in the U.K. with reference to TB arise because the prevalence of the disease in the native herd is not known. There are about 4,000 camelids in the U.K. and little testing has been done. In the U.S., where more testing has been done, there are now about 300,000 camelids, although the number would have been smaller in 1999 when the paper on prevalence was written (Fowler and Frost, 1999).

Testing regimes

While there is no testing regime for Old World Camelids (personal communication, Wernery 2005), testing regimes for New World Camelids have been agreed upon in the U.K., U.S., Sweden, and New Zealand.

In Australia, routine testing of cattle is only every 4 years, due to the TB free status that has been achieved. Australian imports of New World Camelids undergo a 12 month quarantine period prior to their release onto farms. Johnes disease, another disease caused by a Mycobacterium, is more problematical to eradicate. The relative importance given to these diseases and to their eradication depends on a number of risk factors such as the zoonotic potential, the economic impact and the perceived effect on welfare.

The agreed testing regime for NWCs in the U.K (personal communication, J Clark, SVS) uses the CIST skin test at the caudal axillary space. The injection is given and read 72 hours later in the same site. The right side is used for avian ppd, the left side is used for bovine ppd and vernier calipers used for deer to measure the skin thickness and reaction. We have been informed that in the US the same side is used for both avian and bovine injections and measures in camelids.

A summary of the actions the UK State Veterinary Service (SVS) would expect to take if TB is suspected in a camelid is described below:

§ TB is not considered to be a major health problem in camelids

§ Currently only Article 15 (movement restrictions) of the TB legislation applies

§ Testing can be performed if flock is shown to be infected, a co-located bovine herd is infected or other information indicates high risk

§ Testing is by single intradermal comparative cervical test (SICCT) as in cattle - deer callipers should be used and injections given on the R (avian) and L (bovine) side of the thorax behind the elbow - "extra severe" interpretation is used

§ Tracings would be undertaken with a 90 day incubation window used.

There are two levels of interpretation of reaction, a normal and a severe. In the U.K. a particularly severe interpretation of reaction is used for camelids. We believe that this extra severe interpretation is used because of the low prevalence and uncertainties of the epidemiology within the UK.

With the extra severe interpretation, an increase of more than 1mm on the bovine reading over the avian reading or an increase of 2mm or more at the bovine site is regarded as positive. If found positive, tracings would be undertaken over the 90 day incubation period. Thousands of these tests have been done, but the sensitivity and specificity of the test has not been validated as no population of infected camelids has been found.

All of these testing regimes may show false positives on occasion, as well as for the reasons noted above. However, regulations for the implementation of the tests may differ, between and within countries. In Texas, for example, TB tests are required for the health papers needed for shows and for interstate transport; the cost for the procedure (i.e., injecting the tuberculin and reading the site 72 hours later, and filling out the form) is paid by the producer.

Epidemiology

Camelids are unlikely to be a factor in the spread of M.bovis (Frost, 2001) and have never been shown to be involved in the spread of M. bovis to date.

Movement of camelids for showing or breeding purposes is a common feature of camelid husbandry in the U.K. and is a possible route of spread of a wide range of diseases, although less, if at all, for TB which is not spread by casual association. If responsible husbandry measures and good biosecurity arrangements are in place, i.e. maintaining cleanliness (and - depending on the level of risk - disinfection of trailers) and providing isolation pens for newly acquired animals and those just returned from shows or outside breeding, then potential disease transmission and risks to other domestic species are reduced to a minimum.

Regular testing of cattle herds and other camelids, combined with good biosecurity and sensible feeding management, should reduce factors 1) and 3) to a very low level.

The transmission from endemically infected wildlife is more difficult to avoid. In areas of the country where there is a prevalence of cattle TB, then it would be reasonable to expect local wildlife populations also to be infected and to be possible vectors. This risk can be reduced by careful feeding management, i.e. troughs lifted off ground and cleaned prior to use, water troughs situated at sites unlikely to be conducive to wildlife visits, protection of bedding and food stores to prevent faecal/urine contamination, and other measures.

A proper identification system which is able to last the lifetime of the animal and to be linked to a national database would be a reasonable starting point. Electronic micro-chips could be used, and the failure rate of these is very low. These chips are accepted in the pet passport scheme and for the identification of horses and are easily applied. The adoption of one method would be a progressive step.