Caseous Lymphadenitis of Sheep and Goats
Testing for Caseous Lymphadenitis (CL, occasionally abbreviated CLA) is a bacterial infection caused by Corynebacterium pseudotuberculosis resulting in superficial or internal abscesses and recurrent development of abscesses. In severe cases, wasting can occur due to internal abscesses interfering with normal organ function. 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.
1. What are the major means of spread of caseous lymphadenitis (CL)?
Corynebacterium pseudotuberculosis is spread from animal to animal primarily through contact with material from abscesses (pus) or fomites (inanimate objects) contaminated with abscess material. 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 to prevent transmission via close contact.
The organism can survive several months in the soil and environment, which could be a source of infection.
2. What tests are available for CL?
There are two testing methods offered at WADDL: 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 bacterial 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. May an owner send the pus or serum directly to the lab?
The diagnostic laboratory provides services primarily to veterinarians. Although we will test samples mailed directly from an owner, we strongly encourage animal owners to work with a veterinarian in developing a CL control program. We will send results to the veterinarian, and also to the owner if requested.
4. What samples do I submit?
We recommend working with your veterinarian to obtain appropriate samples.
For bacterial culture, collect abscess material in a sterile, leak-proof 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).
For serology, blood should be collected into a five or ten ml "red-top" clot tube or serum separator tube. Blood should be collected into a 5 or 10 ml "red-top" clot tube or serum separator tube. Leave the blood at room temperature for at least 30-60 minutes to allow clot formation. We recommend centrifugation of the blood sample, separation of the serum into a sterile twist top tube and storage at refrigeration temperature prior to shipping for best results. A minimum of 0.5 ml of sera is required. We will accept uncentrifuged blood tubes, but it is not ideal.
Label the tube with the animal name/number and the owner name or number the tubes consecutively to match a key sheet with the animal names.
5. How should I ship samples?
Pack the container or tubes in a plastic sealable bag with absorbent material and put another plastic bag around the first. If submitting blood tubes, the best method is to use padded pouches designed for transporting blood tubes. If you do not have access to these, we recommend using a thick rubber band and grouping your tubes tightly into groups of 7-10 tubes. If you alternate the direction of the tubes, they will stay tightly packed. Pack so the box can be dropped from a four-foot height without breaking any tubes! An ice pack is recommended for all bacterial culture and for sera/blood if the shipment is expected to take several days in warm weather.
Note that CL testing is normally performed on Mondays and Wednesdays. This means samples must arrive no later than Friday morning for testing on Monday, or Tuesday morning for testing on Wednesday to ensure they will be tested that week. Samples received after Tuesday morning will be tested the following week.
6. What documentation should I send with the blood samples?
Please fill out an accession form found at (https://waddl.vetmed.wsu.edu/forms).
Alternatively, you can use our online ordering system. If you don’t have an online account, please go to: https://w3.vetmed.wsu.edu/newaccount.php
Once approved, you’ll receive an email with a link to submit your order, or you can go directly to the WADDL Client Portal.
7. 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 abscesses should be quarantined until the abscess has completely healed and the causative organism is determined. 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.
8. How long does it take to get CL results?
Serology: CL serum (SHI) tests are generally run twice a week on Mondays and Wednesdays. Samples must arrive by Friday morning to be tested on Monday. Samples must arrive by Tuesday morning to be tested on Wednesday.
Culture: Bacterial cultures for CL are set up on the day received in the lab, and results are typically available within a week.
9. 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.
10. 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.
11. 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 as vaccinated herds may test positive on the blood test (serology). We recommend working with your veterinarian on the pros and cons of vaccination for CL.
12. How can I manage CL positive animals on my farm?
We recommend working with a veterinarian to develop an on-farm disease control and testing plan. Generally, 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 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.
13. Can humans be infected with 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, and has been reported.
14. Can other animals acquire the infection?
Generally, transmission between small ruminants and other species (horses, cattle, camelids) is rare. It is still recommended to separate affected sheep/goats from these species as there have been documented cases of transmission.
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.
Again, animals of any species with active abscesses should be quarantined away from other animals, and appropriate fly control and disinfection of potential fomites should be practiced.
15. Biosecurity Screen
A biosecurity screen is available to monitor the status of a herd and for new animals entering the herd. This screen includes Small Ruminant Lentivirus (SLRV/CAE and OPP), Johne’s disease, and Caseous Lymphadenitis.
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Page revised June 2022