Practical Diagnosis of Caseous Lymphadenitis in Small Ruminants

Caseous lymphadenitis (CLA) is a chronic infectious disease in sheep and goats that can be difficult to diagnose, particularly when infected animals lack obvious external lesions. While superficial abscesses often raise clinical suspicion, visceral infections and subclinical carriers may remain unnoticed, allowing the disease to persist within a flock. A systematic diagnostic approach is therefore essential to confirm infection, identify carrier animals, and support effective flock health management¹. 

Begin with Clinical Evaluation 

The diagnostic process starts with a thorough clinical examination. Animals presenting with enlarged superficial lymph nodes or characteristic abscesses should be considered potential CLA cases. Commonly affected lymph nodes include the mandibular, parotid, prescapular, popliteal, and supramammary nodes. However, the absence of visible swellings does not rule out infection, particularly in animals with the visceral form of the disease². 

Although clinical examination provides valuable initial information, laboratory confirmation is necessary because several conditions can produce similar lesions, and subclinical infections cannot be identified through physical examination alone. 

Bacterial Isolation Remains the Diagnostic Standard 

Isolation of Corynebacterium pseudotuberculosis from abscess material remains the definitive method for confirming CLA². 

Cytological examination of aspirated material using Giemsa or Gram staining can support the diagnosis by demonstrating the organism. However, interpretation may become more challenging when samples are obtained from older or calcified abscesses². 

For bacterial culture, samples are incubated on sheep blood agar at 37°C for 48–72 hours under aerobic or anaerobic conditions. Colonies typically appear small, white, dry, and are surrounded by a narrow zone of β-haemolysis. Growth in Brain Heart Infusion (BHI) broth is characterized by abundant yellowish-white sediment². 

Commercial biochemical identification systems, such as the Analytical Profile Index Coryne kit, can further assist in confirming bacterial identity through biochemical profiling². 

Serological Testing Helps Detect Hidden Infections 

Because infected animals may not develop visible abscesses, serological testing plays an important role in flock investigations. The enzyme-linked immunosorbent assay (ELISA) is widely used to detect infection in live animals by measuring the humoral immune response. 

Among the available assays, ELISA targeting phospholipase D (PLD) has demonstrated a specificity of 98% and a sensitivity of 87%². Despite its usefulness, veterinarians should recognize that some infected animals may still produce false-negative results because diagnostic sensitivity is not absolute². 

Interferon-gamma (IFN-γ) ELISA provides another valuable option. In sheep, this assay has shown greater sensitivity than conventional antibody ELISA and is not influenced by vaccination status, making it particularly useful during flock screening^2,3. 

Molecular Methods Support Epidemiological Investigation 

Molecular diagnostic techniques provide additional information, particularly when investigating disease transmission or characterising bacterial isolates. 

Methods such as PCR-RFLP, PFGE, RAPD, multiplex PCR, and ERIC-PCR can differentiate the two biovars of C. pseudotuberculosis. Among these, ERIC-PCR offers good discriminatory ability for epidemiological investigations, although its repeatability and standardisation remain limitations^4,5,6. 

For more consistent strain characterisation, multilocus sequence typing (MLST) and multilocus sequence analysis (MLSA) are preferred. Whole-genome sequencing (WGS) provides the highest level of resolution by identifying genes associated with virulence, antimicrobial resistance, and environmental adaptation, making it particularly valuable for detailed epidemiological investigations^7,8. 

Practical Clinical Insights 

No single diagnostic method is sufficient for every case of CLA. Diagnostic decisions should be guided by the clinical presentation, flock history, disease prevalence, vaccination status, available laboratory facilities, and the purpose of testing. Combining clinical examination with appropriate laboratory techniques improves diagnostic accuracy and increases the likelihood of identifying subclinical infections that could otherwise maintain disease transmission within the flock. 

References 

1. Kaba J, Czopowicz M, Mickiewicz M, Witkowski L, Moroz-Fik A, Biernacka K, Szaluś-Jordanow O, Nalbert T, Bereznowski A, Potârniche AV, Mālniece A. Herd-level true seroprevalence of caseous lymphadenitis and paratuberculosis in the goat population of Poland. Preventive Veterinary Medicine. 2024 Sep 1;230:106278. https://www.sciencedirect.com/science/article/pii/S0167587724001648 

2. Dopuđ M, Reil I, Zdelar-Tuk M, Špičić S, Duvnjak S. Caseous Lymphadenitis in sheep and goats–“Cheese Glands”. Veterinarska stanica. 2025;56(3):303-16. https://hrcak.srce.hr/file/462890 

3. Dorella F, Pacheco LG, Oliveira S, Miyoshi A, Azevedo V. Corynebacterium pseudotuberculosis: microbiology, biochemical properties, pathogenesis and molecular studies of virulence. Veterinary research. 2006;37(2):201-18. https://hal.science/hal-00903023/document 

4. Dorneles EM, Santana JA, Ribeiro D, Dorella FA, Guimarães AS, Moawad MS, Selim SA, Garaldi AL, Miyoshi A, Ribeiro MG, Gouveia AM. Evaluation of ERIC-PCR as genotyping method for Corynebacterium pseudotuberculosis isolates. PloS one. 2014 Jun 5;9(6):e98758. https://journals.plos.org/plosone/article/file?id=10.1371/journal.pone.0098758&type=printable 

5. Schlicher J, Schmitt S, Stevens MJ, Stephan R, Ghielmetti G. Molecular characterization of Corynebacterium pseudotuberculosis isolated over a 15-year period in Switzerland. Veterinary sciences. 2021 Jul 30;8(8):151. https://www.mdpi.com/2306-7381/8/8/151 

6. El Damaty HM, El-Demerdash AS, Abd El-Aziz NK, Yousef SG, Hefny AA, Abo Remela EM, Shaker A, Elsohaby I. Molecular characterization and antimicrobial susceptibilities of Corynebacterium pseudotuberculosis isolated from caseous lymphadenitis of smallholder sheep and goats. Animals. 2023 Jul 18;13(14):2337. https://www.mdpi.com/2076-2615/13/14/2337 

7. Costa WL, Alves JT, Dias LM, Araújo CL, Morais E, Silva AG, Andrade SS, Ramos RT, Silva A, Folador AR. Whole-genome sequence of Corynebacterium pseudotuberculosis PA04, isolated from the lymph node of a sheep in the Amazon, Brazil. Genome announcements. 2017 Apr 20;5(16):10-128. https://journals.asm.org/doi/pdf/10.1128/genomea.00202-17 

8. Markova J, Langova D, Babak V, Kostovova I. Ovine and caprine strains of corynebacterium pseudotuberculosis on Czech farms—A Comparative study. Microorganisms. 2024 Apr 27;12(5):875. https://www.mdpi.com/2076-2607/12/5/875