Anything that causes irritation to the respiratory tract of your pig can cause her to cough. Respiratory infections with viruses, bacteria or parasites are the most likely causes, but non-infectious irritants like poor air quality, inhaled dust or allergies can result in coughing as well. Pigs raised on large commercial farms will be more susceptible to respiratory infections than pet pigs. Less commonly, older pigs may develop a cough secondary to heart disease or cancer.
Respiratory Bacteria and Viruses
There are a variety of bacteria and viruses that infect the respiratory tracts of pigs and result in coughing. These respiratory infections will be more common in large commercial pig farms, but can still infect the small backyard herd or individual pet pig. Swine influenza, porcine respiratory and reproductive syndrome and porcine respiratory corona virus are the most common respiratory viruses in pigs. Bacterial infections like Bordatella bronchiseptica, Mycoplasma hyopneumoniae and Actinobacillus pleuropneumonia are also quite common. Respiratory infections in pigs frequently occur as a mixture of both viral and bacterial infections -- porcine respiratory disease complex is the name given to this combination of viral and bacterial respiratory infections, and is the most important infectious respiratory disease problem in today's pig.
Pigs that spend time outdoors will pick up internal parasites from their environment. Ascaris suum is a parasite of pigs that spends part of its life cycle in the lungs of the pig. The pig ingests the eggs of the worm, which then mature into larvae in the pig's intestines. Once the larvae have matured, they will migrate through the pig's body, often passing through the liver and the lungs of the pig before returning to the intestines where they mature into the adult worm. When the larvae pass through the lungs, they can cause inflammation and secondary bacterial infections. All pigs who are housed outside need to be on a regular deworming program to prevent parasites.
Inhaled Airway Irritants
The combination of poor ventilation and large amounts of manure and urine in pig housing can lead to toxic levels of ammonia in the air. Ammonia is a known respiratory irritant in all species -- humans and pigs included. Chronically high levels of ammonia can directly damage cells in the pig's airway and can also make them more prone to respiratory infections. Inhalation of dust and other small particles can also damage the respiratory tract of pigs. This can occur with poor ventilation in dusty barns, but may also happen if their feed material has been ground into a fine powder.
Other Causes of Cough
Pigs may also develop a cough due to other less common causes. Heart failure is known to cause coughing in other species -- as the heart fails to pump blood efficiently, fluid accumulates in the lungs, leading to respiratory signs that include coughing. Pigs are susceptible to heart disease, especially if they are overweight. Pigs have also been reported to develop lymphoma, a type of cancer that can invade the lungs and may initially present with a cough as a symptom. Always consult an experienced veterinarian regarding the health and treatment of your pigs.