Caseous Lymphadenitis in Sheep: Abscess Disease Signs and Control
- Caseous lymphadenitis, often called CL or abscess disease, is a chronic contagious bacterial infection caused by Corynebacterium pseudotuberculosis.
- The most common sign is a firm swelling or abscess near a lymph node, especially under the jaw, in front of the shoulder, or near the udder or flank.
- Some sheep have internal abscesses instead of visible lumps. These animals may lose weight over time, cough, or seem unthrifty.
- A visible abscess should be treated as potentially contagious until your vet confirms the cause with culture or other testing.
- Control usually focuses on isolation, careful wound handling, testing, biosecurity, and in some flocks, strategic culling and vaccination.
What Is Caseous Lymphadenitis in Sheep?
Caseous lymphadenitis, or CL, is a chronic infectious disease of sheep caused by the bacterium Corynebacterium pseudotuberculosis. It most often causes abscesses in lymph nodes near the skin, but it can also affect internal lymph nodes and organs. That internal form is especially important in sheep because it may show up as long-term weight loss and poor thrift rather than an obvious lump.
The pus in CL abscesses is often thick and pasty, and the disease tends to recur over time. A sheep may look well between flare-ups, which is one reason CL can spread quietly through a flock. Once the bacterium gets established, elimination can be difficult and usually takes a flock-level plan rather than treatment of one animal at a time.
For pet parents and small flock managers, the big concern is not only the affected sheep but also contamination of equipment, housing, and handling areas. Your vet can help you decide whether the goal is short-term control, long-term reduction, or full flock cleanup.
Symptoms of Caseous Lymphadenitis in Sheep
- Firm lump or abscess near a lymph node
- Abscess that enlarges, softens, or ruptures
- Scars or old drainage tracts
- Weight loss despite eating
- Chronic cough or noisy breathing
- Poor body condition or reduced thrift
- Fever or depression
Any new lump on a sheep should be taken seriously until your vet examines it. CL is not the only cause of abscesses, but it is common enough that isolation is wise while you wait for answers.
Contact your vet promptly if an abscess is close to rupturing, has already started draining, or if the sheep is losing weight, coughing, or falling behind the flock. Those signs raise concern for internal disease and for spread to other animals.
What Causes Caseous Lymphadenitis in Sheep?
CL is caused by the bacterium Corynebacterium pseudotuberculosis. The organism usually enters through breaks in the skin or mucous membranes. Common entry points include small wounds from shearing, ear tagging, tail docking, castration, fencing injuries, rough feeders, or other skin trauma.
The most common source of infection is contact with pus from a ruptured abscess. That material can contaminate wool, pens, chutes, feeders, water sources, clothing, and tools. Biosecurity guides note that the bacterium can survive for days in water, weeks in feed, and months in soil or on equipment, which helps explain why flock control can be challenging.
Sheep can also spread infection during routine management. Shared clippers, handling equipment, and crowded processing areas all increase risk if they are not cleaned and disinfected between animals. In sheep with internal CL, bacteria may also be spread through respiratory secretions when abscess material is coughed up.
How Is Caseous Lymphadenitis in Sheep Diagnosed?
Your vet will start with the sheep's history, the location of any lumps, and a hands-on exam. An abscess in a typical lymph node area is suspicious, but it is not enough to confirm CL because other bacteria can cause similar swellings.
The most definitive test for an external abscess is bacterial culture of material from an intact suspected lesion. Diagnostic labs also offer blood testing that looks for antibodies to the organism. These tests can be useful for flock screening, but they have limits. A positive result may reflect past exposure, recent vaccination, or active disease, and false negatives can happen early after exposure or in sheep with chronic walled-off abscesses.
If your vet is worried about internal CL, additional work may include ultrasound, radiographs, or sampling from the respiratory tract in sheep with pneumonia-like signs. Diagnosis often works best when test results are interpreted alongside flock history, vaccination status, and whether other sheep have had abscesses.
Treatment Options for Caseous Lymphadenitis in Sheep
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Farm or clinic exam
- Immediate isolation from the flock
- Basic abscess assessment
- Protected drainage plan only if your vet advises it
- Disinfection guidance for pens, tools, and handling areas
- Short-term monitoring plan for flock mates
Recommended Standard Treatment
- Exam and flock history review
- Isolation and biosecurity plan
- Culture of abscess material or other diagnostic sampling
- Serology when appropriate for flock screening
- Vet-directed wound care or lesion management
- Discussion of culling versus retention
- Cleaning and equipment disinfection protocol
Advanced / Critical Care
- Comprehensive veterinary workup
- Culture plus additional lab testing
- Ultrasound or radiographs if internal disease is suspected
- Repeated flock testing or targeted screening of contacts
- Detailed segregation, vaccination, and culling strategy
- Surgical excision or more intensive lesion management when appropriate
- Written herd health and biosecurity plan
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Caseous Lymphadenitis in Sheep
Bring these questions to your vet appointment to get the most out of your visit.
- You can ask your vet whether this lump is in a typical CL lymph node location or if other causes are more likely.
- You can ask your vet which test is most useful right now: culture, serology, imaging, or a combination.
- You can ask your vet how to isolate this sheep safely and how long separation should continue.
- You can ask your vet how to handle drainage material, bedding, and contaminated tools without spreading infection.
- You can ask your vet whether flock mates should be tested now or monitored first.
- You can ask your vet whether vaccination makes sense for your flock's current status and goals.
- You can ask your vet whether this sheep is a reasonable candidate to keep, treat, segregate, or remove from the flock.
- You can ask your vet what management changes during shearing, tagging, and other procedures would lower future risk.
How to Prevent Caseous Lymphadenitis in Sheep
Prevention starts with biosecurity. New sheep should come from sources with known health histories whenever possible, and they should be quarantined and examined carefully for lumps, scars, or drainage tracts before joining the flock. Your vet may also recommend testing during quarantine, especially if you are trying to maintain a clean flock.
Daily management matters too. Disinfect clippers, tagging tools, and other instruments between animals. Sheep with suspicious lesions should be handled last, and any draining abscess should be managed in an area that can be cleaned well. Removing sharp wire, rough feeders, exposed nails, and other injury risks can reduce the skin wounds that let the bacterium enter.
Vaccination can be part of control in some infected flocks, especially for young replacement animals, but it does not clear infection from sheep that already have CL. In flocks with no history of CL, Merck notes vaccination is not recommended. The best prevention plan depends on whether your flock is currently clean, already affected, or somewhere in between, so it is worth building that plan with your vet.