Tuberculosis

Introduction

Tuberculoisis is a general term for diseases caused by the tuberculous group of bacteria, Mycobacteria species. Mycobacterium tuberculosis, Mycobacterium bovis, Mycobacterium avium are host adapted to humans, cattle and birds respectively. All three types can produce disease in other species; Mycobacterium tuberculosis is the most host specific strain and rarely causes progressive disease in species other than humans, Mycobacterium bovis causes disease in cattle but can affect a number of other species, see Bovine Tuberculosis. Mycobacterium avium is the only type that causes disease in birds but it can also affect mammalian species, see Avian Tuberculosis.

Infection is usually via inhalation of infective droplets, but can also be by ingestion of contaminated feed or water, and occasionally via coitus or milk. The primary focus of infection is determined by the route of infection. Characteristic granulomatous tubercles develop. These become necrotic, shedding the infective organism which disseminates to other body systems initiating secondary foci.

Clinical Signs

Location and severity depend on which body systems are affected and the advancement of the disease. Due to the testing and slaughter policy most cases in the UK are identified before development of clinical signs.

Respiratory form

• Chronic cough- soft and productive
• Tachypnoea
• Dyspnoea
• Dull areas on auscultation of the lungs in advanced cases

Alimentary form

• Few clinical signs
• Diarrhoea
• Bloat

Laboratory Tests

An ELISA test has been developed but is not widely used. The gamma interferon test can also be used for diagnosis of the condition.

Pathology

Findings at post mortem depend on the route of entry of the organism, whether it became generalised or not and the stage of the disease. One or more lymph nodes will display the characteristic granulomatous tubercles. In the respiratory form the mediastinal and bronchial lymph nodes are affected, with lesions in the lungs.

If the mycobacteria disseminated from the primary complex then lymph nodes in other regions will also be affected and there will be multiple small foci of infection on other organs.

Microscopically there are epithelioid cells, with large vesicular nuclei and pale cytoplasm and giant cells, formed by the fusion of macrophages, are found at the centre of tubercles. Surrounding this, there is a narrow layer of lymphocytes, mononuclear cells and plasma cells, more advanced cases show peripheral fibroplasia and central necrosis.

Treatment

Control measures depend on the species affected and the prevalence of the disease. The BCG vaccine provides protection for humans, if disease occurs treatment is with antibiotics. Chemotherapy has also been attempted in elephants and primates.

The test and slaughter policy is the only method of eradicating TB, positive reactors to the intradermal tuberculin test are culled and testing of the herd is undertaken every 3-6 months.

References

• Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial
• Radostits, O.M, Arundel, J.H, and Gay, C.C. (2000) Veterinary Medicine: a textbook of the diseases of cattle, sheep, pigs, goats and horses Elsevier Health Sciences