Cause
The cause of tuberculosis is a group of aerobic, non-motile, non-spore-forming bacteria, called Mycobacteria. They are resistant to heat , pH and routine disinfectants. However, they are susceptible to sunlight and 5% phenol, and they are quite susceptible to 5% household bleach.

Several different Mycobacteria have been associated with infection in animals :

• Mycobacterium avium-intracellulare complex - Dogs and cats are rarely infected with this organism because they are resistant to it, but pigs and poultry are most often infected. When dogs or cats have been reported to be infected it has been through eating infected meat or direct or indirect contact with infected faeces from birds. The organism causes granulomas in various body organs. This organism is quite resistant and can survive for over 2 years in the environment but there is no evidence of spread to other animals or to people.
• Mycobacterium bovis - The main host for this organism is cattle, and the organism can be transmitted to humans (rarely) and other animals, including cats and dogs. The route of infection is by drinking infected, unpasteurised milk, or by eating infected, uncooked meat. It causes disease mainly in the gastrointestinal tract (cats)  and respiratory tract (dogs and people). M.bovis does not survive long in the environment (less than 28 days ). In some countries wild life reservoirs of infection eg badgers in the UK, may have a role in the transmission of this disease between animals.
• Mycobacterium tuberculosis - Humans are the main reservoir host for this organism but dogs and cats are susceptible to it as well. Cats and dogs contract the infection from humans, and there has not been any reported cases of transmission from pets to people, so this is an inverse zoonosis. Infected people and dogs have infected sputum and spread of the disease is by aerosol or direct contact.
• A Variant of M.tuberculosis-bovis - This organism occurs in the UK and it has been found in cats and is thought to originate in rodents which are their natural prey.

Human infection is usually by one of the following routes :

• Inhalation
• Contact with infected lesions

The most common signs are :

• Dogs - respiratory signs mainly
  • Fever
  • Weight loss
  • Cough
  • Inappetance
  • Increased salivation
• Cats - gastrointestinal signs mainly, 
  • Weight loss
  • Diarrhoea
  • Vomiting
  • Anaemia
  • Swelling of lymph nodes throughout the body
  • Ascites (dropsy) - fluid accumulation in the abdomen
• Cattle - usually show local signs, but also :
  • Weight loss
  • Variable appetite
  • Fluctuating body temperature
  • Enlarged lymph nodes
  • Cough - if lungs involved
  • Increased breathing rate and dyspnoea
  • Diarrhoea if intestine involved
  • Mastitis - presents a human health risk, and risk of spread to suckling calves.
• Horses
  • Often get a stiff neck due to osteomyelitis of the spine
  • Anorexia - due to difficulty eating
  • Polyuria (increased urine production)
  • Cough 
  • Enlarged lymph nodes
  • Nasal discharges
  • Fluctuating body temperature
• Pigs
  • Discharges from enlarged lymph nodes in the neck
  • Similar to disease in cattle, but may affect joints and meninges of the brain more commonly
• Sheep and Goats 
  • Cough due to bronchopneumonia is the most common sign
  • Goats may get diarrhoea
  • May get rapid death in young kids

In addition to these signs, as a granuloma (mass) gets bigger it involves local organs, also the organisms can spread to other parts of the body and a wide variety of non-specific signs may occur including :

• Heart failure
• Eye disease - uveitis
• Bone fractures 
• Poor healing skin ulcers
• Jaundice
• Neurological signs
• Sudden death can occur in some cases

In humans signs include :

• Respiratory signs - most common
• Fever
• Weight loss
• Granulomas in the skin - most often the hands

Complications
Numerous complications can occur due to spread of the organism throughout the body. Because humans are susceptible to tuberculosis pets and other in-contact animals should be screened whenever there is an outbreak of human disease.