Caseous Lymphadenitis of Sheep and Goats
Testing for Caseous Lymphadenitis (CL, occasionally abbreviated CLA) is recommended as part of the small ruminant herd biosecurity screen offered through the Washington Animal Disease Diagnostic Laboratory (WADDL) at Washington State University. This infection is a potential health threat to sheep or goat herds. WADDL receives numerous inquiries about CL, how to test for it, and most importantly, how to take steps to control the infection in herds and flocks. We have taken some of the most frequently asked questions and presented them along with some short answers. If you have further questions, please call the main WADDL office at 509-335-9696 and ask to speak with the consulting microbiologist.
Dr. Tessa LeCuyer
Dr. Jim Evermann
Dr. Tom Besser
1. What are the major means of spread of caseous lympadenitis (CL)?
Corynebacterium pseudotuberculosis, the bacterium that causes the disease CL, is spread from animal to animal primarily through contact with material from subcutaneous abscesses (pus) or fomites (inanimate objects) contaminated with abscess material. The organism can survive several months in the soil and environment, remaining a source of infection. Though much less common than visible subcutaneous abscesses, internal abscesses may also form in the lungs and abdominal organs as a result of spread of the organism within the animal via blood or lymph. When abscesses are present in the lungs, the organism may be transmitted through respiratory secretions (nasal discharge or coughing). In rare cases, C. pseudotuberculosis may be present in the milk. Although CL is not sexually transmitted, it is recommended to avoid natural breeding of animals with abscesses in order to prevent transmission via close contact.
2. What tests are available for CL?
There are two testing methods for CL offered at WADDL: bacterial culture to detect the bacterial organism in abscess material, and serology to detect C. pseudotuberculosis-specific antibodies in sheep and goat blood samples. For animals with visible subcutaneous abscesses, it is best to submit abscess material for culture since this is the most direct and definitive method to diagnose CL in an individual animal. It is recommended that all abscesses be cultured regardless of serology test results.
The serological test is the best method of “herd-level diagnosis” (screening herds). WADDL runs the Synergistic Hemolysin Inhibition (SHI) test, which measures the antibody response to an exotoxin produced by the organism. No CL serological test is sufficiently reliable to confidently detect infection in individual sheep or goats, therefore the serology results for an individual animal test should be interpreted with caution. Furthermore, although a positive serologic test result alone may indicate active infection, it also may indicate past infection that has resolved. The SHI test specificity and sensitivity for individual animals may not be high in some herds, however, the prevalence of positive tests within a herd usually reflects the herd prevalence of infection reasonably well.
3. What samples do I submit?
We recommend working with your veterinarian to obtain appropriate samples. If submitting for a single animal, complete a WADDL General Accession Form for each submission. If sending blood samples for multiple animals, fill out the WADDL General Accession Form for Multiple Animals. Number the tubes consecutively to match a key sheet with the animal names. Use the Shipping address listed at the top of the forms. WADDL received deliveries from FedEx, UPS, and USPS.
For bacterial culture, collect abscess material in a sterile container (red top tube, for example) or with a bacterial culture swab. If an abscess is lanced, be sure not to contaminate the environment (see #4 for more information). Please include ice packs with samples intended for culture, and always enclose the primary container in a sealed bag or secondary container.
For serology, blood should be collected into a five or ten ml. "red-top" clot tube or serum separator tube. Leave the blood at room temperature for at least 1 hour to allow clot formation. We do not recommend separating the serum from the clot prior to shipment. Ice packs are not required for shipping blood samples unless the weather is very hot. Use bubble wrap to pad the tubes and pack the box so that no tubes break when dropped from a height of four feet.
4. What if the goat/sheep has an abscess?
Until proven negative by culture, all abscesses should be treated as if they were CL. Bacterial culture is the most reliable test for determining the CL status of an animal with abscesses. It is possible for infected animals with active abscesses to test negative on serology due to a delay in antibody production (there is a delay in antibody production after initial infection). Many environmental bacteria can cause abscesses via traumatic wounds, but unlike CL these are sporadic and not readily transmitted from animal to animal. Animals with CL abscesses should be quarantined until the abscess has completely healed or be culled. If an abscess is lanced, it should be over a hard surface that can be disinfected (concrete) or thrown away (tarp). If an abscess ruptures in a pasture, the organic material (soil, grass) is contaminated, and the pasture should be rested for a prolonged period of time; CL has been shown to survive in soil for 8 months.
5. How long does it take to get CL results?
Serology: CL serum (SHI) tests are generally run once a week. Samples must arrive by Tuesday afternoon for results to be reported on Friday. If samples arrive Wednesday-Friday, they will be held until the next week’s run.
Culture: Bacterial cultures for CL are set up on the day received in the lab, and results are typically available within a week.
6. What does a positive or negative blood test mean?
The serology test is best used as a screen to find out if a herd or flock has been infected, rather than to diagnose an individual animal with CL. An individual animal positive CL serology test does not necessarily mean an animal is infected with C. pseudotuberculosis or has CL (serology can also detect past, resolved infections). Furthermore, the test cannot distinguish between natural exposure and vaccination, therefore vaccinated herds may test positive. Nonetheless, herds with a high proportion of animals with positive SHI tests are very likely to contain C. pseudotuberculosis infected animals, whereas herds with few or no SHI positive animals are less likely to contain infected animals. Animals within a positive herd are at risk for developing abscesses, and the herd should be monitored for visible subcutaneous abscesses. Titers in an individual animal do not correlate well with risk of abscess development. A negative serologic result on an individual animal does not definitively rule out infection by C. pseudotuberculosis. The confidence in a negative result is enhanced if most or all herd mates also test negative.
7. How often should I test my animals by serology?
When acquiring new animals, testing the herd of origin (10 or more animals) is the preferred approach to determining the status of the new additions. If testing the herd of origin is impossible, new additions should be quarantined and tested twice (30 days apart) before introduction into the negative herd. Testing only the new additions provides less confidence in negative tests than does testing the herd of origin. The frequency for testing an established herd or flock should be based on previous test results, eradication strategies, and the risk of exposure to other herds or flocks.
8. Is there a vaccine available?
There is a vaccine available for use in sheep, as well as a conditionally licensed C. pseudotuberculosis bacterin vaccine available for goats 3 months of age or older. Because this product is conditionally licensed, the vaccine is regulated by each state's veterinary agency. In Washington, the vaccine is currently only available to veterinarians. The sheep vaccine should not be used in goats. If a herd or flock is vaccinated, then serologic screening is no longer a useful method for detecting natural infection, and therefore it is generally recommended that vaccination only occur in herds of flocks in which CL is already present. Vaccinated herds may test positive on the blood test (serology).
9. How can I manage CL positive animals on my farm?
Because CL is a chronic infection, efforts should be directed toward preventing spread to uninfected animals. The first step is to identify infected animals within a herd or flock, which can be done through a combination of palpation for external abscesses, with confirmation by bacterial culture (see #3), and serological screening. Animals with CL abscesses should be either culled or quarantined until the abscesses have completely healed. Serological screening can assist in determining the prevalence of CL within a herd. Frequency of screening should be based on the prevalence within the herd (from previous herd tests) and the risk of outside exposure (level of biosecurity for new animals entering the herd, and animals attending outside events). Animals with signs of respiratory or wasting disease in a known CL positive herd or flock should also be quarantined, as these may be signs of abscesses in the lungs or abdominal organs. Any animals dying of respiratory or wasting disease should be necropsied by a veterinarian, and any abscesses cultured, to identify the cause of death. Flock owners should purchase and disinfect their own shearing equipment to prevent introduction of CL from outside farms, and be sure to disinfect feed bunks and stanchions, which may become contaminated by abscess material. Keep new additions in a separate pen until either the herd of origin tests negative, or the animals test negative on two tests 30 days apart. See #6 for more information on testing new additions.
10. Is it okay to drink raw milk containing C. pseudotuberculosis?
Human infections with this bacterium are rare, but when found are often associated with occupational exposure to sheep and goats. Drinking raw milk is a potential source of human infection, especially in immunosuppressed individuals. There are other more serious zoonotic pathogens (infectious agents transmitted from animals to humans) that are regularly transmitted to humans through raw milk. Consult your veterinarian regarding the public health hazards of consuming raw milk.
11. Can horses acquire the infection from small ruminants?Generally, transmission between horses and small ruminants does not occur. Pigeon Fever in horses is caused by a different biovar (strain) of C. pseudotuberculosis. However, the equine biovar can cause abscesses in goats when injected subcutaneously experimentally. Animals of either species with active abscesses should be quarantined away from other animals of any species, and appropriate fly control and disinfection of potential fomites should be practiced.
12. Biosecurity Screen
We recommend a basic biosecurty screen for establishing the status of a herd, new animals entering the herd, and animals producing milk for human consumption. This screen includes small ruminant lentivuruses (CAE and OPP; see the CAE FAQ for more information), Johne’s disease, and caseous lymphadenitis. Additionally, Q-Fever and Brucella testing are recommended, and may be required in your area, for animals producing raw milk for human consumption. Please keep in mind that there are many pathogens that can be transmitted in raw milk that cause severe human disease; it is not possible to test for every pathogen potentially transmitted to humans through raw milk, and that negative tests do not guarantee raw milk is safe or pathogen-free.
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