From the laboratory diagnostician's point of view, Corynebacterium pseudotuberculosis, the causative agent of pseudotuberculosis, is a perfect parasite. Once successfully established in the host animal, the bacterium successfully escapes the immune system and causes chronic, usually lifelong infections that are rarely lethal. If infected animals remain undetected in a herd, this leads to a progressive contamination of the entire herd.
In addition to its ability to evade the body's own defences, the pathogen's excellent ability to survive in the outside world further favours the risk of infection. The complex cell wall structure of this gram-positive rod bacterium consisting of mycolic acids (lipids), arabinogalactan and peptidoglycan is responsible for these properties. C. pseudotuberculosis is inactivated by direct sunlight within 24 hours. However, protected from direct sunlight, the pathogen remains infectious in feces, water, soil, and straw for several days to weeks, depending on environmental temperatures. Temperatures of 70 °C and all common disinfectants also lead to pathogen inactivation. The natural infection spectrum of C. pseudotuberculosis includes sheep, goats and horses as individual occurrences as well as cattle and humans.
In addition, it should be noted that the clinical picture of pseudotuberculosis in rodents, rabbits and hares is an independent disease and is caused by the bacterium Yersinia pseudotuberculosis, which is not clinically relevant for ruminants.
From the point of view of animal welfare, animal health (reduced animal performance, treatment costs, eradication due to an unfavourable prognosis) and food hygiene (milk, dairy products and meat), this infectious disease is of outstanding importance and consequently requires sustained control and monitoring.
While prevalence studies on the occurrence of pseudotuberculosis have already been carried out in some countries such as Australia or New Zealand, the actual distribution of C. pseudotuberculosis in Austria - as in most other EU countries - cannot be estimated due to the lack of reliable data. However, experience from Great Britain has shown that after the first appearance of this disease pattern in 1989, which is referred to as "caseous lymphadenitis" in the English-language literature, a steady spread in sheep and goat flocks was observed. The lack of clinical symptoms in the early stages of infection and the partial lack of knowledge of the disease among livestock keepers, in conjunction with uncontrolled animal traffic or trade, favoured the spread of the disease.
Due to the strong increase in herd sizes, especially in dairy goats and dairy sheep, and the increased purchase of breeding animals from Germany and abroad, the risk of introducing infectious diseases is increasing. In addition to the tests for maedi-visna, CAE and brucellosis, which are currently mostly carried out during the purchase of animals, tests for pseudotuberculosis should also be carried out in future. The development and introduction of a standardised ELISA now makes it possible to detect this disease at an early stage and to clarify corresponding suspected clinical cases.
The pathogen is primarily excreted through the exudate of abscessed lymph nodes approx. 2-6 months post infection or is transmitted aerogenically or via sputum. Animals usually become infected via skin lesions (shearing, horn bumps, tattooing, ear tagging) or the mucous membranes. The infection is supported by the enzyme phospholipase D produced by the pathogen, which destroys cell membranes, increases blood vessel permeability and, as a so-called exotoxin, is responsible for the spread in the organism. In particular, contaminated feed, wooden housing, grooming equipment, fences and hoof care tools are precursors of infection. There are relatively few reports of C. pseudotuberculosis in humans in the literature, but contact with infected animals can also lead to granulomatous, necrotising lymphadenitis in humans, particularly in the neck, axilla and groin. Since antibiotic treatment is unsatisfactory in most cases, surgical excision of the affected lymph nodes is usually indicated. Farmers, shearers, slaughterhouse workers and veterinarians in particular should be aware of the risk of infection and therefore maintain appropriate protective and hygienic measures when handling small ruminants.
The typical leading symptom of this chronic infectious disease is a caseating and necrotizing inflammation of the lymph nodes, and two forms of progression are distinguished on the basis of the localization of the pathological changes:
In the external form, the clinical changes are limited to the external body lymph nodes. The affected subcutaneous lymph nodes on the head, shoulder, knee crease and udder are sometimes greatly enlarged and may abscess.
In the internal form, the pathological changes occur in the lungs and lymph nodes in the thoracic cavity. Occasionally, after hematogenous spread, abscesses are also observed in the area of the liver or other organs or lymph nodes in the abdominal cavity.
Depending on the location and degree of the changes, swallowing or respiratory problems occur, as well as digestive and also febrile general disorders. These two clinical pictures can also occur in combination in one animal.
The primary objective in the control of pseudotuberculosis is to catch infected animals at an early stage and remove them from the herd. As these are usually life-long persistent infections, each antibody-positive animal is to be regarded as a potential carrier and should therefore be eradicated or at least kept separate from the unsuspected animals in the herd. Since infected dams pass on the infection to lambs or fawns, motherless rearing should be used to break the vertical chain of infection.
In order to determine the herd status via the presence of pseudotuberculosis in the flock, a representative sample of the animals can be tested for C. pseudotuberculosis specific antibodies by ELISA, depending on the flock size. Preference should be given to older animals and clinically suspect animals.
The following table shows the number of animals to be sampled in relation to the herd size (all animals over 6 months of age in a herd are taken into account).
|Herd size (number of animals)||Sample size|
|less than 20||all|
Therapy and vaccination
Although the pathogen is highly sensitive to various chemotherapeutic agents, attempts at therapy usually fail because a sufficient concentration of active substance cannot be achieved in the encapsulated foci of infection and extirpation of the altered lymph nodes is not a promising practical method. Due to the considerable economic damage caused by carcass complaints and reduced performance, there have been considerable efforts to develop suitable vaccines to combat pseudotuberculosis, especially in countries with intensive sheep and goat farming such as Australia or South Africa. However, the use of vaccines is discussed very controversially, since vaccination is not expected to eradicate the pathogen, but only to reduce clinically manifest forms of the disease. A specific vaccine is currently not licensed in Austria or in any other member state of the European Union.
Due to the usually complex sanitation, the most urgent aim is to minimise the risk of pseudotuberculosis entering the herd by taking appropriate measures. The most important measure is the controlled movement of animals or the purchase of animals from known unsuspicious farms. In particular, breeding rams that are used for mating in different farms represent a risk factor in the spread of the disease. General principles of hygiene such as disinfection of equipment (instruments, sheep shearing equipment and the like), cleaning and disinfection of boots and work clothes as well as the change of work clothes in the flock serve to protect not only against pseudotuberculosis but also against a large number of other infectious diseases.
The classical standard method in the diagnosis of pseudotuberculosis is the bacteriological detection of the pathogen by means of culture tests from abscess material. In the meantime, PCR methods have also been developed, but these do not improve the detection of subclinically infected animals or the internal course of the disease, as abscess material is also required for detection. Since direct pathogen detection only detects late stages of infection and an exclusive manifestation of the internal organs or lymph nodes is not detected, there are considerable efforts to detect clinically inapparently infected animals by determining C. pseudotuberculosis-specific antibodies. In the meantime, a standardized antibody (Ak) ELISA is available, with which clinically inconspicuous animals can also be detected. In this indirect ELISA the recombinant form of the important C. pseudotuberculosis specific virulence factor phospholipase D (PLD) is used as antigen. Furthermore, a PLD-specific Western blot for the detection of antibodies in serum samples from sheep and goats was developed at the Institute of Veterinary Medicine Linz.
In addition to laboratory diagnostics, clinical examination of the external lymph nodes makes an important contribution to the detection of animals suspected of having pseudotuberculosis. Differentially, infections with Actinobacillus lignieresi and "ubiquitous opportunistic pathogens" such as Stapylococcus aureus or Arcanobacterium (syn. Actinomyces) pyogenes should be considered.
Institut für veterinärmedizinische Untersuchungen Linz
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