Caseous Lymphadenitis (CLA) / Corynebacterium pseudotuberculosis Infection

Caseous lymphadenitis (CLA) is a chronic, contagious, and debilitating bacterial disease of sheep and goats caused by Corynebacterium pseudotuberculosis. It is characterized by the formation of pyogranulomatous abscesses in superficial and internal lymph nodes, as well as visceral organs such as the lungs, liver, kidneys, and mammary glands. The disease is endemic in many sheep and goat populations worldwide, causing significant economic losses due to reduced weight gain, decreased milk production, impaired fertility, condemnation of carcasses and hides, and premature culling. In sheep, the disease is often referred to as 'cheesy gland' due to the characteristic laminated, onion-ring appearance of the abscess contents. In goats, abscesses are more frequently superficial and may be more accessible for surgical drainage. The disease has a slow progression, with clinical signs often appearing months to years after initial infection. It is a major concern for intensive and semi-intensive production systems, particularly dairy goat herds and purebred sheep flocks, where close confinement and management practices facilitate transmission. The disease is zoonotic, with rare human infections occurring through contact with infected animals or contaminated fomites, especially in immunocompromised individuals. Effective control requires a comprehensive flock health program including vaccination, biosecurity, and culling of infected animals.

The causative agent is Corynebacterium pseudotuberculosis, a Gram-positive, facultative anaerobic, pleomorphic rod-shaped bacterium. It is a member of the genus Corynebacterium, which also includes C. diphtheriae and C. ulcerans. The bacterium is characterized by its ability to survive for long periods in the environment, particularly in soil, manure, and contaminated bedding, due to its lipid-rich cell wall and resistance to desiccation and disinfectants. Two biovars exist: biovar ovis, which primarily infects sheep and goats, and biovar equi, which causes ulcerative lymphangitis in horses and is rarely pathogenic for small ruminants. The bacterium produces several virulence factors, including a phospholipase D (PLD) exotoxin, which is a potent sphingomyelinase that damages host cell membranes, facilitating bacterial spread and abscess formation. PLD also inhibits neutrophil chemotaxis, allowing the bacterium to evade the host immune response. Additionally, the bacterium has a surface lipid layer that protects it from phagocytosis and intracellular killing. The organism is catalase-positive, oxidase-negative, and urease-positive, and can be cultured on blood agar, producing small, whitish, non-hemolytic colonies after 24-48 hours. Molecular identification via PCR targeting the pld gene is used for confirmation. The bacterium is highly resistant to environmental degradation, surviving for months in contaminated soil and fomites, which contributes to the persistence of the disease in flocks.

CLA is distributed worldwide, with higher prevalence in regions with intensive sheep and goat production, such as Australia, New Zealand, North America, Europe, and parts of Africa and Asia. In sheep, the prevalence can range from 5% to over 50% in infected flocks, while in goats, it is often higher, especially in dairy herds. The disease affects both sexes and all ages, but clinical signs are most common in animals over 2 years of age due to the chronic nature of the infection. Transmission occurs primarily through direct contact with purulent material from ruptured abscesses, which contaminates the environment, feed, water, and equipment. The bacterium enters the host through skin wounds, such as shearing cuts, ear tags, castration, tail docking, and other traumatic injuries, or through mucous membranes. Iatrogenic transmission via contaminated needles, tattooing instruments, and surgical equipment is also significant. Flies and other insects may act as mechanical vectors. The incubation period ranges from 2 to 6 months, but can be longer. Morbidity in affected flocks can be high, but mortality is generally low unless visceral abscesses cause severe organ dysfunction. Economic losses arise from reduced weight gain, decreased milk yield, reproductive inefficiency, carcass condemnation, and culling of chronically infected animals. In dairy goat herds, mastitis caused by C. pseudotuberculosis can lead to significant milk loss and increased somatic cell counts. The disease is more prevalent in housed or intensively managed flocks, where overcrowding and poor hygiene facilitate spread. Seasonal variations may occur, with higher transmission during shearing and other management procedures.

The pathogenesis of CLA begins with the entry of C. pseudotuberculosis through breaks in the skin or mucous membranes. The bacterium is phagocytosed by macrophages but resists intracellular killing due to its lipid-rich cell wall and the action of phospholipase D. PLD damages the phagosomal membrane, allowing the bacterium to escape into the cytoplasm and replicate. The infected macrophages release pro-inflammatory cytokines, attracting neutrophils and other immune cells, leading to the formation of a pyogranulomatous inflammatory response. The bacterium induces a chronic, suppurative inflammation that results in the formation of encapsulated abscesses. The characteristic laminated, onion-ring appearance of the abscess contents is due to the concentric layers of necrotic debris, fibrin, and inflammatory cells. Over time, the abscess enlarges and may rupture, releasing infectious pus into the surrounding tissues or the environment. In superficial lymph nodes, such as the submandibular, parotid, prescapular, and prefemoral nodes, abscesses are easily palpable. In internal organs, abscesses may develop in the lungs, mediastinal lymph nodes, liver, kidneys, spleen, and mammary glands. Visceral abscesses often go unnoticed until they cause organ dysfunction or are detected at necropsy. The bacterium can also cause bacteremia, leading to systemic spread and the formation of multiple abscesses. The immune response is primarily cell-mediated, with a Th1-type response being important for containment. However, the bacterium's ability to survive intracellularly and the presence of the lipid layer allow it to persist, leading to chronic infection. The disease does not typically cause acute systemic illness, but chronic infection can lead to weight loss, decreased productivity, and in severe cases, respiratory distress or neurological signs if abscesses compress vital structures.

Several factors increase the risk of CLA in sheep and goat flocks. Intrinsic factors include age, with older animals being more likely to show clinical signs due to the chronic progression of the disease. Breed susceptibility may exist, with some wool and hair breeds showing higher prevalence, possibly due to differences in skin integrity and management practices. Sex does not appear to be a major risk factor, but males may have a higher risk due to fighting and skin trauma. Extrinsic factors are more significant: management practices that cause skin wounds, such as shearing, dipping, ear tagging, castration, and tail docking, provide portals of entry for the bacterium. Overcrowding and poor ventilation in housing increase the likelihood of contact with contaminated pus. Inadequate biosecurity, such as introducing infected animals into a clean flock, is a major risk factor. Poor hygiene in the environment, including contaminated bedding, feeders, and water troughs, facilitates transmission. Iatrogenic spread through contaminated needles and surgical instruments is common. Flies and other insects can mechanically transmit the bacterium. Nutritional stress and concurrent diseases may compromise the immune system, increasing susceptibility. In dairy goat herds, the use of milking machines and shared udder cloths can spread the infection to the mammary gland. The disease is more common in intensive production systems than in extensive range flocks, where animal density is lower and skin trauma is less frequent.

Clinical signs of CLA vary depending on the location and extent of abscess formation. The most common presentation is the presence of superficial abscesses in peripheral lymph nodes, particularly the submandibular, parotid, prescapular, and prefemoral lymph nodes. These abscesses are firm, painless, and gradually enlarge over weeks to months. They may rupture spontaneously, discharging thick, creamy, greenish-white pus with a characteristic laminated, onion-ring appearance. Ruptured abscesses leave draining tracts that may heal slowly or become chronic. In goats, superficial abscesses are often more prominent and may be the only clinical sign. Systemic signs are usually absent unless visceral abscesses are extensive. Internal abscesses, especially in the lungs and mediastinal lymph nodes, can cause chronic cough, dyspnea, and exercise intolerance. Abscesses in the liver or kidneys may lead to weight loss, decreased appetite, and poor condition. Mammary gland involvement can cause mastitis, with hard, nodular udder, reduced milk production, and elevated somatic cell counts. In rams, abscesses in the epididymis or testicles can cause infertility. Neurological signs may occur if abscesses compress the spinal cord or brain, leading to ataxia, paralysis, or head tilt. The disease is chronic and progressive, and affected animals may appear healthy for long periods before clinical signs become apparent. In severe cases, animals may become emaciated and debilitated. The disease is often diagnosed incidentally at slaughter or necropsy when internal abscesses are found.

Differential diagnoses for CLA include other causes of abscessation and chronic wasting in sheep and goats. Key differentials include: 1) Abscesses caused by other pyogenic bacteria, such as Staphylococcus aureus, Streptococcus spp., Trueperella pyogenes, and Pasteurella multocida, which can be differentiated by bacterial culture and the absence of the characteristic laminated pus. 2) Tuberculosis (caused by Mycobacterium bovis or M. avium subspecies paratuberculosis) can cause caseous lesions in lymph nodes and lungs, but is rare in small ruminants and can be differentiated by acid-fast staining and PCR. 3) Actinobacillosis (Actinobacillus lignieresii) causes granulomatous lesions in the tongue and lymph nodes, but is more common in cattle. 4) Abscesses due to Corynebacterium renale or other corynebacteria may be similar but are less common. 5) Parasitic conditions such as hydatid cysts (Echinococcus granulosus) can cause cystic lesions in the liver and lungs, but these are fluid-filled and have a different appearance. 6) Neoplasia, such as lymphoma, can cause lymph node enlargement, but is usually more diffuse and not abscess-like. 7) Caseous lymphadenitis must also be differentiated from other causes of mastitis, such as mycoplasma or staphylococcal infections, which can be distinguished by culture and clinical presentation. 8) In goats, abscesses due to Corynebacterium pseudotuberculosis must be differentiated from those caused by Mycobacterium avium subspecies paratuberculosis (Johne's disease), which primarily causes chronic diarrhea and weight loss without abscessation. 9) Abscesses in the head and neck region may be confused with salivary gland cysts or dental abscesses. 10) Finally, other systemic infections such as melioidosis (Burkholderia pseudomallei) are rare but can cause similar abscesses in tropical regions. Definitive diagnosis requires bacterial culture, PCR, or histopathology.

The diagnostic approach for CLA involves a combination of flock history, clinical examination, laboratory testing, and necropsy. The algorithm is as follows: 1) Flock history: Obtain a detailed history including the presence of recurrent abscesses, introduction of new animals, management practices (shearing, vaccination), and previous diagnostic tests. 2) Physical examination: Perform a thorough examination of all animals, palpating superficial lymph nodes (submandibular, parotid, prescapular, prefemoral, popliteal, and mammary) for enlargement or abscessation. Note any draining tracts or scars. 3) If superficial abscesses are present, aspirate pus using a sterile needle and syringe, or collect pus from a ruptured abscess. Submit for bacterial culture and PCR. 4) For animals with suspected internal abscesses, perform ultrasonography of the thorax and abdomen to detect abscesses in the lungs, liver, spleen, or kidneys. 5) Serological testing: Use ELISA or agar gel immunodiffusion (AGID) tests to detect antibodies against C. pseudotuberculosis phospholipase D. These tests are useful for flock screening, but may have false positives due to cross-reactions with other corynebacteria. 6) If an animal dies or is euthanized, perform a complete necropsy, examining all lymph nodes and visceral organs for abscesses. Collect samples for histopathology and culture. 7) Molecular confirmation: Perform PCR on pus or tissue samples to detect the pld gene, which is specific for C. pseudotuberculosis. 8) Rule out other causes of abscessation by culture and sensitivity testing. 9) In dairy herds, test bulk tank milk for antibodies to C. pseudotuberculosis to monitor flock status. 10) Implement a control program based on the diagnostic results, including culling of infected animals, vaccination, and biosecurity measures.

Laboratory findings in CLA are primarily related to the identification of the causative organism and the assessment of the host's immune response. Hematology and serum biochemistry are usually unremarkable, unless there is a secondary infection or severe organ involvement. Complete blood count may show mild leukocytosis with neutrophilia in some cases. Serum protein electrophoresis may reveal hypergammaglobulinemia due to chronic antigenic stimulation. The definitive diagnosis is made by bacterial culture of pus or tissue samples. C. pseudotuberculosis grows on blood agar as small, whitish, non-hemolytic colonies after 24-48 hours of aerobic incubation at 37°C. The organism is catalase-positive, oxidase-negative, and urease-positive. Biochemical tests can differentiate it from other corynebacteria. Molecular confirmation is achieved by PCR targeting the pld gene, which encodes phospholipase D. Serological tests, such as ELISA and AGID, detect antibodies against the exotoxin. These tests are useful for flock screening, but may have limitations in early infection or in animals with chronic abscesses that have low antibody titers. In dairy goats, milk somatic cell counts may be elevated in cases of mastitis, and the organism can be cultured from milk. At necropsy, histopathology of abscesses reveals characteristic laminated, onion-ring lesions with a central core of necrotic debris surrounded by layers of fibrous tissue and inflammatory cells. Gram staining of pus may show Gram-positive pleomorphic rods. Polymerase chain reaction (PCR) can also be performed on formalin-fixed, paraffin-embedded tissues for retrospective diagnosis. Overall, laboratory findings are essential for confirming the diagnosis and differentiating CLA from other causes of abscessation.

Imaging modalities are valuable for detecting internal abscesses in CLA, especially in animals with suspected visceral involvement. Ultrasonography is the most commonly used imaging technique in practice. It can be used to evaluate superficial lymph nodes, mammary glands, and internal organs. On ultrasound, abscesses appear as well-defined, hypoechoic to anechoic cavities with thick, hyperechoic walls. The contents may be echogenic due to the presence of pus and debris. Ultrasonography of the thorax can detect lung abscesses, pleural effusion, and mediastinal lymphadenopathy. Abdominal ultrasonography can identify abscesses in the liver, spleen, kidneys, and abdominal lymph nodes. It is also useful for guiding fine-needle aspiration of internal abscesses for diagnostic sampling. Radiography is less sensitive for soft tissue abscesses but may be useful for detecting pulmonary abscesses, which appear as well-circumscribed radiopacities in the lung fields. Thoracic radiographs may also show pleural effusion or mediastinal widening. Computed tomography (CT) provides excellent detail and can detect small abscesses in the lungs, liver, and other organs, but is rarely used in small ruminant practice due to cost and availability. Magnetic resonance imaging (MRI) is even more detailed but is generally reserved for research or valuable animals. In dairy goats, ultrasonography of the udder can help differentiate mastitis due to CLA from other causes. Imaging findings, combined with clinical signs and laboratory tests, can support a diagnosis of CLA and guide treatment and management decisions.

Cytological and histopathological examination of abscess contents and tissues is essential for confirming the diagnosis of CLA. Cytology of pus aspirated from superficial abscesses typically reveals a pyogranulomatous inflammation with degenerate neutrophils, macrophages, and necrotic debris. Gram staining may show Gram-positive pleomorphic rods, often in clumps. The characteristic laminated, onion-ring appearance of the pus is best appreciated on histopathology. Histological examination of an abscess wall shows a central area of caseous necrosis surrounded by layers of fibrous connective tissue, with infiltrates of neutrophils, macrophages, lymphocytes, and plasma cells. The concentric layers of necrotic debris and inflammatory cells give the 'onion-ring' appearance. In chronic lesions, there may be dystrophic calcification. The presence of the bacterium can be confirmed with special stains, such as Gram stain or modified Ziehl-Neelsen stain, or by immunohistochemistry using specific antibodies. In visceral organs, such as the lungs, liver, and kidneys, abscesses have a similar histological appearance. In the mammary gland, there is pyogranulomatous mastitis with abscess formation. Histopathology is also useful for ruling out other causes of abscessation, such as tuberculosis or actinobacillosis. In cases where the diagnosis is uncertain, PCR can be performed on formalin-fixed, paraffin-embedded tissue samples to detect C. pseudotuberculosis DNA. Overall, cytology and histopathology are valuable tools for confirming the diagnosis and understanding the pathogenesis of CLA.

Treatment of CLA is challenging and often unrewarding. The disease is chronic, and antibiotics have limited efficacy due to the thick capsule of the abscess and the intracellular survival of the bacterium. Surgical drainage and curettage of superficial abscesses is the most common treatment approach. The abscess should be lanced, drained, and flushed with an antiseptic solution such as dilute povidone-iodine or chlorhexidine. The cavity should be curetted to remove the capsule and necrotic debris. However, surgery is often not curative because the bacterium may have already spread to other lymph nodes or internal organs. Antibiotic therapy may be used as an adjunct to surgery, but it is rarely effective alone. Penicillin G procaine at 20,000-40,000 IU/kg IM q24h for 5-7 days may be used, but penetration into the abscess cavity is poor. Other antibiotics such as oxytetracycline (10-20 mg/kg IM q24h) or tulathromycin (2.5 mg/kg SC once) have been tried, but with variable results. In goats, tilmicosin (10 mg/kg SC) has been used, but it is contraindicated in sheep due to toxicity. Antibiotic treatment may be more effective if initiated early in the infection, but it is not recommended as a sole treatment. In some cases, intralesional injection of antibiotics into the abscess cavity after drainage has been attempted. However, the prognosis for complete recovery is poor, and treated animals may remain carriers. Therefore, the primary focus should be on prevention and control through vaccination, biosecurity, and culling of infected animals. In valuable animals, surgical excision of the entire abscess and surrounding tissue may be attempted, but this is often difficult and may not be curative. Supportive care, including good nutrition and management, is important to maintain the animal's overall health.

The prognosis for individual animals with CLA is guarded to poor. Superficial abscesses can be treated surgically, but recurrence is common, and the animal may remain infected. Internal abscesses are usually not treatable and may lead to chronic wasting, respiratory distress, or death. The disease is chronic and progressive, and affected animals often become debilitated over time. In a flock setting, the prognosis for eradication is poor unless strict control measures are implemented. The economic impact of CLA is significant, with reduced productivity and increased culling rates. However, with a comprehensive control program, including vaccination, biosecurity, and culling of infected animals, the prevalence can be reduced over time. The prognosis for a flock is better if the disease is detected early and control measures are implemented promptly. Negative prognostic indicators include the presence of internal abscesses, multiple abscesses, and concurrent diseases. Animals with chronic mastitis due to CLA have a poor prognosis for milk production and may need to be culled. Overall, the prognosis for CLA is poor for individual animals, but flock-level control can be successful with a long-term commitment.

Follow-up for CLA involves regular monitoring of the flock for new cases and evaluation of the effectiveness of control measures. After treating an animal with surgical drainage, the wound should be monitored for healing and any signs of recurrence. The animal should be isolated from the rest of the flock to prevent contamination of the environment. For the flock, a schedule of regular physical examinations should be implemented, with palpation of superficial lymph nodes every 3-6 months. Any new abscesses should be promptly drained and cultured. Serological testing (ELISA) can be used to monitor the prevalence of infection in the flock. If vaccination is used, the vaccination schedule should be followed, and the vaccine efficacy should be evaluated by monitoring the incidence of new cases. Biosecurity measures should be reviewed and reinforced, including quarantine of new animals, disinfection of equipment, and proper disposal of pus and contaminated materials. In dairy herds, bulk tank milk testing can be used to monitor the herd status. If the prevalence is high, a test-and-cull program may be implemented, with removal of seropositive animals. The environment should be cleaned and disinfected, and contaminated bedding and manure should be removed. Pasture rotation may help reduce environmental contamination. Long-term follow-up is essential to achieve and maintain a CLA-free status.

Clinical pearls: 1) CLA should be suspected in any sheep or goat with chronic, non-painful abscesses in superficial lymph nodes. 2) The characteristic onion-ring appearance of the pus is pathognomonic for CLA. 3) Surgical drainage is the most common treatment, but it is not curative, and the animal should be considered a carrier. 4) Vaccination is the most effective control measure, but it does not prevent infection, only reduces the severity and spread. 5) Biosecurity is crucial to prevent introduction of the disease into a clean flock. 6) In dairy goats, mastitis due to CLA can cause high somatic cell counts and reduced milk production. 7) The bacterium is resistant to many disinfectants, so thorough cleaning is necessary. 8) Culling of chronically infected animals is often the most cost-effective strategy. Pitfalls: 1) Failure to isolate and treat infected animals can lead to rapid spread within the flock. 2) Using the same needle or surgical equipment on multiple animals without disinfection can transmit the disease. 3) Antibiotic therapy alone is ineffective and may lead to a false sense of security. 4) Not performing a necropsy on animals that die suddenly may miss internal abscesses. 5) Serological tests may have false positives, so they should be used in conjunction with clinical findings. 6) In sheep, tilmicosin is toxic and should not be used. 7) Do not assume that a single abscess is the only lesion; internal abscesses may be present. 8) Neglecting to disinfect the environment after draining an abscess can lead to environmental contamination and further spread.

Current drug protocols for CLA are primarily focused on surgical drainage and adjunctive antibiotic therapy. There is no specific antibiotic that is highly effective against C. pseudotuberculosis, and treatment is often unsuccessful. However, the following protocols may be used: 1) Penicillin G procaine: 20,000-40,000 IU/kg IM q24h for 5-7 days. This is the most commonly used antibiotic, but penetration into abscesses is poor. 2) Oxytetracycline: 10-20 mg/kg IM q24h for 5-7 days. May be used in goats, but caution in sheep due to potential toxicity. 3) Tulathromycin: 2.5 mg/kg SC once. Has a long half-life and may be effective in some cases. 4) Tilmicosin: 10 mg/kg SC once, but is contraindicated in sheep and should be used with extreme caution in goats. 5) Intralesional injection of antibiotics (e.g., penicillin) into the abscess cavity after drainage has been attempted, but evidence is limited. 6) In cases of mastitis, intramammary infusion of antibiotics may be used, but the efficacy is poor. 7) Supportive care: NSAIDs (e.g., flunixin meglumine 1.1-2.2 mg/kg IV or IM q24h) may be used to reduce inflammation and pain. 8) Vaccination: A commercial vaccine (e.g., Caseous D-T, Glanvac) is available in some countries. The vaccine contains toxoid and whole-cell antigens. The recommended protocol is two doses 4 weeks apart, followed by annual boosters. The vaccine reduces the severity and spread of the disease but does not prevent infection. Withdrawal times for meat and milk must be observed for all antibiotics used. It is important to consult the label and local regulations for withdrawal times.

Evidence-based literature on CLA is extensive, with numerous studies on the epidemiology, pathogenesis, diagnosis, and control of the disease. Key findings include: 1) The prevalence of CLA in sheep and goat flocks varies widely, with higher rates in intensive systems. A study in the United States found a prevalence of 5-10% in sheep and 10-20% in goats. 2) The bacterium is highly resistant to environmental degradation, surviving for months in soil and on fomites. 3) Vaccination with a toxoid vaccine has been shown to reduce the incidence of abscesses and the severity of the disease. A study by Paton et al. (2003) demonstrated that vaccination reduced the number of abscesses and improved weight gain in lambs. 4) Serological tests, such as ELISA, have moderate sensitivity and specificity, and are useful for flock screening. 5) Surgical drainage is the most common treatment, but recurrence rates are high. A study by Baird et al. (2004) found that surgical drainage alone was not curative, and the majority of treated animals remained infected. 6) Antibiotic therapy is generally ineffective, and there is no evidence to support the use of any specific antibiotic. 7) Control programs that combine vaccination, biosecurity, and culling have been successful in reducing the prevalence of CLA in flocks. A study by de la Fuente et al. (2015) reported a significant reduction in the prevalence of CLA in a goat herd after implementing a comprehensive control program. 8) The economic impact of CLA is significant, with losses due to reduced productivity, carcass condemnation, and culling. A study by Arsenault et al. (2003) estimated the annual cost of CLA to the Canadian sheep industry at $1.5 million. 9) The zoonotic potential of C. pseudotuberculosis is low, but cases of human infection have been reported, particularly in individuals with occupational exposure. 10) Overall, the evidence supports a multifaceted approach to CLA control, with vaccination being a key component.