Occurrence: Worldwide. Age affected: All ages, particularly older pigs. Causes: Bacterium which can also cause Progressive Atrophic Rhinitis. Effects: Often no signs, may be cough, inappetence, fever pneumonia and death. Tropical strains cause serious rapidly fatal disease.

Pasteurella.multocida type A is most commonly isolated from the lungs, trachea and nasal passages of infected pigs. It is commonly found in small numbers in normal animals and appears to colonise damage caused by other agents in the respiratory tract. Some strains, but not all, produce a toxin (like type D, the cause of progressive atrophic rhinitis). It commonly colonises enzootic pneumonia lesions and can make them more severe, and is found in large numbers in the later stages of respiratory viral infections. The strains causing acute and severe haemorrhagic disease in the tropics often belong to type B.

Serum antibodies develop to the somatic antigens on P. multocida and can be demonstrated within 12 days of infection. They do not appear to be protective. Antibodies to the toxin can be demonstrated in some cases.

Pasteurellosis is transmitted by aerosol, contact and by ingestion and usually occurs in finishing pigs or breeding stock in herds in which enzootic pneumonia is also present and it is commonest under poor husbandry conditions, e.g. overcrowding, dusty ammoniacal atmosphere, etc.

Clinical signs may occur after mixing, weighing, transport or some other stress. P. multocida is readily killed by heating to 60°C and survives for less than an hour in aerosols at low humidity but survives for longer at high humidity and low temperatures. Organisms survive for up to 14 days in water, 6 days in slurry and for up to 7 weeks in nasal washings at room temperatures. Transmission between farms is by carrier pigs. The organism can be isolated from rodents and birds, but rodent strains may differ from those found in pig pneumonias.

P. multocida can be found in the nasal passages of pig workers and pig strains have been recovered from bronchopneumonia in humans. Other animal species are usually more important sources for human infection than pigs, but where there is occupational or local exposure, pigs may be a source.

In acute pasteurellosis affected animals may be depressed and show dyspnoea, with laboured abdominal type breathing, coughing, slight nasal discharge and fever of 40-41.5°C, 104°-106°F. Mouth breathing and cyanosis of extremities may be seen. Lung sounds are often loud. The clinical signs usually last for 5-10 days and may end in recovery or death but may continue for 3-5 weeks. Recovered animals often remain thin.

Less severe degrees of pneumonia associated with coughing and fever may occur and last for 3-5 weeks. Humane slaughter should be carried out on animals which have collapsed, are lethargic and severely congested with white froth on the lips and low rectal temperatures. Animals which are thin with severe respiratory distress may also be destroyed.

The clinical signs of fever, dyspnoea, and cyanosis without enteric involvement suggest the acute pneumonic or septicaemic condition and this can be confirmed by the post‑mortem examination of dead animals. The presence of chronic coughing, reduced daily weight gain and fever may suggest that chronic infection is complicating enzootic pneumonia and the organism plays a part in the Porcine Respiratory Disease Complex (PRDC) sometimes seen in finishing pigs.

Lesions of enzootic pneumonia are almost always present and lesions of Pasteurella pneumonia are superimposed on these. Greyish‑pink lung with reddish sunken areas is found in the anterior lobes and, in severe cases, in the diaphragmatic lobes as well. Lesions often have a yellowish outline and a fibrinous pleurisy is often present. Congestion of the carcase is frequently seen and froth is commonly present in the trachea. Oedema of the cut lung tissue is evident. In more chronic cases of pasteurella pneumonia, the cut surface has a granular whitish or greyish appearance.

Aerobic culture of heart blood and lung lesions usually gives a pure culture of P. multocida.

Severely affected animals should be treated parenterally with an antimicrobial such as ceftiofur, penicillin, streptomycin, tetracycline, trimethoprim sulphonamide, ampicillin, amoxycillin, marbofloxacin, tulathromycin or spectinomycin for 3-5 days or as appropriate.

Parenteral treatment may be supplemented by medication of the drinking water of the affected batch with water soluble derivatives of the above injectable compounds and doxycycline. Tilmicosin can be used in feed in addition to those antimicrobials listed above. If feed medication is used, severely-affected animals should be treated parenterally. Long-acting oxytetracycline injections may also be used. All pigs in the same air-space should be treated and the space filled on an all-in, all-out basis. Antibiotic resistance may occur.

Batches of animals may be treated in feed upon entry to an air space. Control is most efficient if all-in, all-out husbandry is practised. Disinfection rapidly kills P. multocida and the organism dies out rapidly in dry conditions. Reduction in predisposing diseases such as enzootic pneumonia by medicating with tiamulin at 40 ppm in feed or by vaccination against enzootic pneumonia, influenza or PRRS, may reduce the prevalence and severity of disease.

Vaccines may be available, but are not generally registered, because of the difficulty of reproducing the disease experimentally to prove efficacy.

Husbandry conditions should be improved and dust and ammonia levels reduced.

Pasteurella multocida may be recovered from bronchopneumonia in humans and contribute to the condition, but this relatively rare.