Caseous lymphadenitis (CLA), so called because of the chronic abscesses which occur in the superficial lymph nodes is a worldwide disease of sheep, goats, cattle, horses and more rarely man. Caused by Corynebacterium pseudotuberculosis, it is significantly more prevalent where intensive husbandry is practised (On a worldwide basis the disease is most prevalent in Australia where husbandry is very extensive). Sheep and goat industries worldwide suffer significant economic loss due to the culling of infected animals, carcass condemnation and decreased wool production. As European border controls have become less stringent and livestock are moved freely between nations, countries previously free from CLA have reported outbreaks. CLA was first reported in the UK CLA was first reported in goats that had been in contact with goats imported from Germany. Despite stringent restriction orders on the movement of livestock and the tracing of in contact animals, the first occurrence of CLA in UK sheep was reported in 1991. Since then cases have been identified in sheep flocks in England, Scotland and more recently Northern Ireland. UK national statistics indicate that twice as many new outbreaks of CLA occurred in 1998 as 1997.
This information sheet has been issued after a CLA discussion forum, which was held at the SVS Autumn meeting, Carmarthen 10th – 13th September 2000. The aim is to provide information on the current state of knowledge about the disease, along with some practical advice about preventing and controlling infections in flocks, as well as some thoughts on possible future eradication strategies. The contents will hopefully be of benefit to vets, farmers and scientists and is intended to increase awareness about this potentially devastating condition.
Implications of CLA for UK Sheep Farming
- Production losses may occur when internal lesions go undetected and cause poor condition, milk production, wool production and reproductive performance.
- Other losses may result from the disruption of shearing time to treat a ruptured lesion or disinfect shearing equipment.
- Fleece value can be reduced if it becomes contaminated with abscess contents. · Market trading may be compromised both internally and internationally.
- Drastic effects on the income of a pedigree flock owners selling breeding animals
- The unsightly nature of abscesses will restrict the exhibition of show animals.
- The spread of CLA in the terminal sire breeds might be accelerated by widespread distribution through sale or loan of these sires throughout the UK.
- The downgrading and condemnation of carcasses due to lesions, may cause slaughter losses.
- Although they are rare, there may be a zoonotic implication as human cases have been linked to occupational exposure.
Keeping CLA Infection Out Of Clean Flocks
- Maintain a CLOSED FLOCK management policy wherever possible, ideally this should also include rams
- ISOLATE all bought in sheep, including rams, until they are likely to be ‘safe’. This means two months quarantine at least and even then, the presence of undetected and resolved lesions may remain a potential source of infection, possibly for the life of the infected sheep.
- Individual animal screening by serology is not possible. Tests currently at the most advanced stages of development lack the necessary sensitivity. So THOROUGH CLINICAL EXAMINATION at time of purchase is vital. Care is necessary as abscesses may partially resolve, fluctuate in size or have been treated prior to sale. Internal infections, e.g. in the lung or internal lymph nodes, cannot be detected by clinical examination. (A clinical examination based scheme may be a way forward but the potential limitations of the clinical examination as an accurate means of detection could cause problems.)
- There is no data to support the usefulness of antibiotic treatments after purchase. Once abscesses are present poor bioavailability in the centre of the lesions usually limits the therapeutic effect. Treatments may be more effective if given towards the end of the incubation phase but it is unlikely that this stage of the disease process can be determined with any degree of accuracy. Use of antibiotics raises concerns with regard to the development of antibiotic resistance.
- C. pseudotuberculosis has not been found in semen, ARTIFICIAL INSEMINATION is therefore a possible means of increasing genetic merit without the risks associated from introducing new rams.
Treatment and Control in CLA Infected Flocks
- PUS or SWABS can be used to confirm the diagnosis by bacteriology and help to rule out other possibilities such as Actinobacillosis, Arcanobacterium pyogenes infections and Staphylococcal dermatitis.
- TREATMENTS include surgical drainage and impaction of lesions with appropriate long-term systemic antibiotic administration. However, complete bacteriological cure cannot be guaranteed and may lead to active infections being overlooked at subsequent clinical examinations. There are no data to support the effectiveness of such treatment. Lancing abscesses may lead to contamination of the environment.
- Once detected, Infected animals should be SEPARATED, thus forming CLEAN and DIRTY FLOCKS. However, it should be remembered that “clean” flocks might contain subclinically infected animals.
- Infected stock should ideally be CULLED, although it is accepted that widespread culling is usually not financially viable, especially in pedigree flocks. Lambs may be SNATCHED AT BIRTH from infected ewes but the potential husbandry and welfare implications of this management procedure need to be appreciated.
- The disease is less common in juveniles. KEEP YOUNG RAMS SEPARATE FROM OLDER ANIMALS AND ALWAYS BUY FROM A KNOWN SOURCE IF POSSIBLE.
- C. pseudotuberculosis can persist in purulent discharges for up to eight months under optimal environmental conditions. Contaminated buildings should be thoroughly cleaned to remove all organic material using HOT WATER and/or STEAM. The building should then be disinfected using PROPRIETARY DISINFECTANTS SUCH AS 1% CHLOROHEXIDINE OR 20% CALCIUM HYPOCHLORITE. There is no data available to allow estimates for survival times on grass under UK conditions but it is probably reasonable to assume that most pastures will be safe after 6 months without infected sheep.
- SHEARING (BECAUSE OF CONTAMINATED EQUIPMENT) and DIPPING (BECAUSE THE ORGANISM PENETRATES WET SKIN) are highrisk transmission factors. GATHERING SHEEP AND HOLDING THEM ALSO FACILITATES THE POTENTIAL FOR TRANSMISSION BY AEROSOL. Appropriate hygienic precautions, including thorough equipment disinfection, should be taken at shearing and dipping, and sheep should not be housed together for longer than necessary when handling for routine procedures. (Note that prolonging the interval between shearing and dipping has not been shown to be a useful means of controlling CLA despite suggestions to the contrary.). It is also recommended that sheep be handled roughly in order of age within a “clean” flock (eg. replacement stock first then older animals), and the “dirty” flock last. The “dirty” flock includes all previously and currently affected sheep.
Although rare, CLA Iin man can be a SERIOUS DISEASE requiring hospitalisation, prolonged antibiotic therapy and even surgery. Clients should be made aware of this and advised to take appropriate precautions when handling infected sheep.
Possible Eradication Strategies
CLINICAL EXAMINATION AND ROUTINE POST MORTEM EXAMINATION OF CARCASSES
Although relatively easy to implement, and feasible using current technology, difficulties associated with the accuracy of diagnosis due to lesion variability are likely to cause problems when it comes issuing certificates. The approach may be most useful for monitoring control strategies in flocks where infection is already known to occur. Problems may arise with subclinically infected sheep remaining undetected, particularly those with lung lesions, which may be important sources of infection to others in the flock.
SEROLOGICAL TESTING AND SEPARATION OR CULLING OF REACTORS
ELISAs based on the detection of antibody to C. pseudotuberculosis exotoxin have been developed in the Netherlands and Canada. However, with current sensitivity levels of only 80% it is unlikely that these assays will be of use when it comes to testing and certifying individuals, such as tups, which traditionally pose a high risk for disease transmission between flocks. The assays might be of more value if it came to the development of whole-flock screening programmes but in many cases the requirement to cull or separate large numbers of sero-positive animals may prove impossible for financial or management reasons.
Given the failure of initial attempts to control the UK spread of CLA, through testing and movement restriction, the development and implementation of suitable vaccines may be another control option. Such strategies are currently practiced in Australia where CLA components are incorporated in commercially available multivalent clostridial vaccines. However, phenotypic differences in the clinical presentation of infections in Australia and the UK suggests that, without further efficacy data, the existing Australian vaccines may not be wholly protective or appropriate for use here. Even in Australia, use of the vaccine does not result in elimination of disease from a flock, but only a reduction in prevalence and severity. Concerns have also been raised about the possibility of inducing carrier status in vaccinated animals whilst vaccination may interfere with the subsequent development of serological testing programmes required for UK eradication or export requirements.
The Sheep Veterinary Society would like to thank Dr. Willie Donachie and Kathleen Connor for allowing material from the Moredun Foundation CLA Newsheet to be used in this document. Thanks also to Mr. Graham Baird and Dr. Sarah Binns for their comments and input.