Tuberculosis (TB) in Animals

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Tuberculosis (TB) in Animals

Tuberculosis (TB) in animals is also known as Consumption, Pthisis, Pearl’s disease and Scrofula.

Tuberculosis (TB) in animals is a chronic, contagious, granulomatous disease characterized by development of tubercle nodules followed by caseation and calcification, debility and muscle wasting. The disease is transmissible between animal and human beings.

Tuberculosis (TB) in a Cattle

Etiology

As based on species of genus mycobacterium, affecting animals, human and birds, it is classified majorly into four types:

  • Bovine type: Animals and human
  • Human type: Human and animals
  • Avian type: Fowls, pigs, horses and dogs
  • Volebacillus: Murrines species

Different species of mycobacterium affecting different animals and human beings:

  • Cattle: M. bovis and M. tuberculosis
  • Human: M. tuberculosis and M. bovis
  • Horses: M. bovis (majorly), M.tuberculosis and M. avium
  • Dogs: M. bovis, M. tuberculosis and M. avium
  • Pigs: Bovine, human and avian type
  • Birds: M. avium

Mycobacterium is a slender, rod shaped organism, appears as single or pair or in group, resembles bundle of faggots.

The cell wall of the organism is rich in mycolic acid (waxy coat) and sulphur rich glycolipids, which facilitate the bacteria to take up certain dyes such as Aniline and Ziehl Nielsen stain.

During heating due to relaxation of the cell wall, the stain taken up inside the cell is retained in itself and thereby help resisting acid alcohol decolourisation. It is a characteristic phenomenon of mycobacterium hence why it is ascribed as acid fast organism.

Physical and Chemical features

The organism is a slow grower in laboratory media. It is sensitive to heat. The organism is killed by creosote and sodium orthopheno phenoate. Resistant to chlorine and 6% NaOH. Remain viable in organic materials.

Epidemiology

Prevalence of infection

Tuberculosis is prevalent worldwide both in animals and human beings. It is endemic in India both in animals and human beings.

Economic impact Apart from the mortality and  productive loss.

Disease is relatively benign in pigs. Discarding and condemning infected carcasses at slaughter.

Predisposing factors

  • Overpopulation in small area
  • Purely intensive rearing
  • Inter current infection
  • Poor sanitation
  • Inadequate ventilation
  • Vitamin A and C deficiency
  • Young age groups and malnourishment

Source of infection

  • Exhaled air
  • Feces
  • Infected meat
  • Nasal discharge, sputum and tracheal mucus
  • Contaminated inanimate and animate objects
  • Reproductive discharges
  • Feed, water and soil

Reservoirs and carriers of infection South West England

Badgers (Meles Meles), During their nocturnal visits in the farm buildings, excrete organism through urine and cattle in the same premises pick up the infection.

New Zealand

Brush tailed possum (Trichosarus vulpecula)- Cattle infected when get along with possums in pasture. North America : Mule deer (Odocoileus hemianus), whilte tailed deer, elk, (O. viriginianus and Cervus elaphus Canadensis), Bison (Bison Bison). Great Britain : Red deer is playing very important role. South Africa : Buffaloes (Syncrus caffer) and water buffaloes (Bubalis bubalis) acts as maintenance hosts. India: Infected animals and human being acts as source of infection.

Transmission

  • Nasal discharge and tracheal mucus.
  • Ingestion: Communal use of feed and water troughs.
  • Ingestion of tuberculosis infected animals milk and their products
  •  Direct contact with infected animals.
  • Inhalation of droplet nuclei from aerosol.
  • Congenital infection by vertical transmission.

Host Affected

  • Cattle are the primary host highly susceptible for the infection.
  • Pure bred and cross bred are highly affected as compared to zebu cattle.
  • Sheep and goats more or less resistant to infection.
  • Badgers, possums, pigs, dogs, primates, large cats and elephants other than cattle are affected and acts as maintenance host.
  • Infected cattle are the main source for human infection.

Pathogenesis

Mechanism to develop infection The cells wall of the mycobacteria is constituted by 40% of the lipid which is of trehalos-6, 6’dimycolate and sulphur containing glycolipids.

Virulent tubercle bacilli destroy the phagosome and causes failure of phagolysosomal fusion and so there is survival of mycobacteria within the phagocytic cells.

Primary Tubercle Complex or Ghon’s focus The mycobacteria which gain entry  in the host by inhalation get lodged in the alveolar surface of the lung and its bronchial and thoracic lymph nodes. By ingestion the organism reaches pharyngeal and mesenteric lymph nodes get locked up there. Within the corresponding lymph nodes the mycobacteria undergoes multiplication and develop small tubercles. The focus of such infection in the lymph nodes given the condition “ Primary complex of infection” or “Ghon’s focus”.

Post Primary dissemination and Stage of late generalization Through draining of lymph nodes, bacteria reaches the blood circulation and spread into other body cavities due to drop in immune response. As a result the bacteria overcome the killing effect by host immune mechanism and causes infiltration and necrosis of neutrophils. Together, the bacteria and the dead immune cells are surrounded by multi epitheloid cell layers which is called as Langhan’s giant cells or granulomas and further this is surrounded by lymphocytes and fibrous connective tissues.

Clinical Signs

General form

  • Affected animals become docile, lethargic but seems to be bright and alert.
  • Cows with prominent miliary tubercle lesions are clinically normal.
  • Progressive emaciation.
  • Capricious appetite, fluctuating body temperature and rough / sleek hair coat. In spite of good appetite animal does not put up weight.
  • All these general signs are pronounced following calving.

Respiratory form

  • Metritis and inflammation of placenta leads to infertility, abortion and failure in conception.

Nervous form

  • In horse painful cervical osteomyelitis causes stiffness of neck and difficulty to eat.

Sheep and Goats

  • Bronchopneumonia and terminal dyspnea
  • Intestinal ulceration with diarrhea which is rare in cattle
  • Enlarged lymph node of alimentary tract.
  • Kids more progressive to early death.

Pigs

  • TB lesion is cervical lymph nodes.

Horses

  • Cervical vertebrae show osteomyelitis, and causes stiffness of neck.

Zoonosis

Tuberculosis (TB) is an important zoonotic disease. There is an increase incidence with an increase spread from animal to human. M. bovis is responsible for 5-10% of human tuberculosis. Children getting infected via drinking of infected breast milk.

Necropsy Findings

Cattle, Sheep and Goats

  • Granulomatous lesion in bronchial, retropharyngeal and mediastinal lymph nodes.
  • Firm and enlarged pharyngeal lymphnode, swelling in dorsum of the pharynx.
  • Miliary abscesses in lung.
  • Pus is characteristic creamy and cheesy.
  • TB nodule present in pleura and peritoneum.
  • TB lesions are covered with fibrous capsule.
  • Lesions in placenta with chronic purulent material.
  • Orchitis
  • Enlargement of supramammary lymphnode: TB lymphomatosis
  • Heart – miliary nodules – cattle Peritonium – miliary nodules – cattle
  • In chronic cases: Discrete, nodular, thick yellow to orange caseous material followed by calcification covered by a thick fibrous capsule.

Pigs

Enlarged miliary TB localized in tonsils, submaxillary, cervical, hepatic, bronchial, mediastinal, mesentric lymphnode with white caseous calcified material surrounded by a strong fibrous capsule.

Horses

TB lesion in intestinal wall, mesenteric lymphnode and spleen. Cut surface of the LNs appears fleshy and resemble neoplastic tissue. Mineralisation and tissue necrosis also noticed.

Diagnosis

  • Based on clinical signs and necropsy findings.
  • Palpation of supramammary lymphnode essential for suspected TB mastitis cases.
  • Isolation and identification of organism by culture or acid fast staining.

The following are the tests for detection of cell mediated immunity:

Single intradermal test

PPD injection is given at caudal fold at base of the tail in sheep and goats / lateral center portion of the neck in cattle is highly sensitive. Initial skin thickness measured with a Vernier Caliper.

0.1 ml herds with unknown status of sensitivity. 0.2 ml herds with known infected herds. Final skin thickness and inflammatory changes at 72 hours post injection should be taken.

Increasing skin thickness more than 4mm is taken as positive. Demerits Lack of specificity. Non visible lesions reactors (NVLs).

Short thermal test

Intradermal tuberculin 4ml injected s/c in the mid neck of cattle. Initial rectal temperature 39 ° C (102 ° F) at the time of injection. If the temperature increase in 4, 6, 8hrs to above 40 ° C animal is classed as positive reactor.

Stormont test

First intradermal injection of 0.1 PPD at middle of the neck followed second injection should be given 0.1 after 7 days at the same site. Increase in skin thickness of 5mm or more at 24 hrs after this second injection is considered positive. Cattle with M. avium does not give positive reaction but skin TB cases do. It is more accurate than SID.

Comparative test

  • For testing the animals for TB, along the side of PPD tuberculin injection, 12cm apart, M.avium PPD should be given and the test result should be compared.
  • Test results read after 72 hrs.
  • This test is not used for initial screening.
  • But can be used as a primary test when there is a high incidence of Avian TB and JD.
  • False positive
  • No gross lesions reactors.
  • Non pathogenic mycobacteria
  • Anergic animals are those with visible lesions of TB.
  • But do not react to intradermal DTH test.
  • Early cases until 6 wks after injection.
  • Cows which have calved within preceding 6 weeks.
  • Gamma Interferon Assay: This test measures the amount of gamma interferon released from the antigen stimulated whole blood cells.
  • Elisa for detection of antibody.
  • Molecular diagnosis by PCR: Sputum, feces, milk, blood and tracheal secretions must be collected for the diagnosis.

Differential Diagnosis

Treatment

There is no antibiotic treatment succesful in control tuberculosis. Supportive treatment to enhance the immune response can be done.

Prevention

All animals over six months and above should be tested by tuberculin test. Positive reactors disposed of according to local legislation. Suspicious cases must be retested by comparative test.

Retesting: If incidence is higher, retesting should be done in 45-60 days after desensitization of intradermal injected animals. Annual testing of all cattle, quarantine, test positive herds should be slaughtered.

Identification of individuals and wild life reservoir. BCG vaccination is available for calf hood vaccination. But vaccinated animals react positive to the skin test. Immunity is not strong and long lasting Most probably cell mediated immunity is predominant.

Accredited areas set up : All cattle in this area are tested and reactors removed and monitored until next generation off-springs become free from infection by tests.

Control

Disinfection of utensils with 5% hot phenol and cresol. Feeding of calves with milk free from infection. Test and introduce new stock. Education public about the significance of the disease.

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