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Caseous Lymphadenitis (CLA) in Animals

Introduction

Caseous lymphadenitis (CLA) is a chronic, contagious bacterial disease of small ruminants characterized by abscess formation in lymph nodes and internal organs. It is caused by Corynebacterium pseudotuberculosis and is of major economic importance in sheep and goat production due to reduced productivity, carcass condemnation, and culling losses.

Etiology
Causative Agent

The disease is caused by Corynebacterium pseudotuberculosis, a Gram-positive, facultative intracellular bacterium.

Characteristics
  • Non-spore forming, pleomorphic rod
  • Produces phospholipase D (major virulence factor)
  • Survives for long periods in the environment
Epidemiology
Species Affected
  • Sheep and goats (primary hosts)
  • Occasionally cattle, horses, and other species
Transmission
  • Entry through skin wounds (shearing cuts, abrasions)
  • Direct contact with pus from abscesses
  • Contaminated equipment, housing, or environment
Risk Factors
  • Poor shearing practices
  • Overcrowding
  • Presence of sharp objects causing skin injuries
  • Inadequate hygiene
Pathogenesis
Mechanism of Infection

The bacterium enters through skin wounds and spreads via lymphatic vessels to regional lymph nodes, where it establishes infection.

Lesion Development
  • Formation of abscesses in lymph nodes
  • Characteristic "onion ring" layering due to repeated cycles of necrosis and encapsulation
Systemic Spread

In some cases, the organism disseminates to internal organs (lungs, liver, kidneys), leading to visceral CLA.

Clinical Signs
External Form
  • Enlarged superficial lymph nodes (e.g., submandibular, prescapular)
  • Firm swellings that may rupture and discharge thick, caseous pus
Internal (Visceral) Form
  • Weight loss
  • Chronic ill-thrift
  • Respiratory signs (if lungs are involved)
Chronic Nature
  • Slow progression
  • Recurrent abscess formation
Summary for Practitioners

CLA should be suspected in small ruminants with chronic abscesses in lymph nodes. The presence of thick, caseous pus is highly suggestive. The disease is difficult to eradicate once established in a flock.

Postmortem Findings
Gross Lesions
  • Abscessed lymph nodes with thick, greenish-white caseous material
  • Layered ("onion ring") appearance on cut surface
  • Abscesses in internal organs in visceral form
Microscopic Lesions
  • Central necrosis surrounded by fibrous capsule
  • Chronic inflammatory response
Diagnosis
Clinical Diagnosis

Based on characteristic abscesses and herd history.

Laboratory Diagnosis
  • Bacterial culture of abscess contents
  • PCR testing
  • Serological tests (e.g., ELISA)
Differential Diagnosis
  • Other causes of lymph node abscesses
  • Actinobacillosis
  • Staphylococcal abscesses
  • Tuberculosis
Summary for Practitioners

Laboratory confirmation is recommended for flock-level diagnosis. Serological testing is useful for identifying subclinical infections.

Treatment
Management of Abscesses
  • Drainage of abscesses in controlled conditions
  • Proper disposal of infectious material
Antibiotic Therapy
  • Limited effectiveness due to encapsulation of lesions
Culling
  • Often recommended for chronically infected animals
Summary for Practitioners

Treatment is generally unrewarding. Focus should be on containment and prevention. Infected animals act as long-term sources of infection.

Control and Prevention
Hygiene Practices
  • Disinfect shearing equipment between animals
  • Maintain clean housing
Flock Management
  • Isolate infected animals
  • Cull chronic cases
Vaccination
  • Vaccines are available in some regions
  • Reduce severity but do not eliminate infection
Biosecurity
  • Introduce animals from CLA-free flocks
  • Quarantine new additions
Summary for Practitioners

Control requires a long-term, integrated approach combining biosecurity, culling, and management practices. Eradication is challenging once the disease is established.

Zoonotic Importance

Corynebacterium pseudotuberculosis can occasionally infect humans, causing localized lymphadenitis. Proper handling of infected material is essential.

Economic Importance

CLA leads to significant economic losses due to reduced wool and meat quality, decreased productivity, carcass condemnation, and increased management costs.

Summary

Caseous lymphadenitis is a chronic, contagious disease characterized by abscess formation in lymph nodes and internal organs. It is difficult to treat and control, making prevention and biosecurity essential in affected flocks.