Influenza in Pigs
History. You are called to examine a group of 3-month-old pigs in an intensively managed fattening house. The group of 20 pigs is in a pen, and there are multiple similar pens within the barn.
In the last 2 days in this particular pen of pigs, the animals have been reluctant to eat and have started huddling together. The owner has observed that the outer pigs in the huddle continually try to burrow toward the center of the pile of pigs and that they appear to be trembling. At this time the remaining pigs in the barn are not affected. When you enter the barn and the pigs are disturbed, they begin sneezing and coughing, and some are reluctant to move.Clinical Examination. Three pigs are caught, and the rectal temperature is found to be 41oC (normal, 39.2oC). There is a nasal discharge, and the conjunctiva and nasal mucosa are congested. You treat the pigs with antibiotics, and over several days, the pigs recover; however, the disease spreads progressively through the remaining pens in the fattening house. All pigs show the same clinical signs, and no pigs die from the disease. Blood samples arc taken for virus neutralization tests from the acutely affected pigs 2 weeks after they have recovered.
The diagnosis from the viral neutralization test is swine influenza, which has a high morbidity rate but a low mortality rate.
Comment. The clinical signs produced by this disease are caused largely by the development of fever. The pigs that were examined had an elevated body temperature because the infection had raised the set point of their thermoregulatory centers to a high value. To raise body temperature to this new value, the pigs huddled together, and the pigs on the outside shivered to generate metabolic body heat. Once the infection is overcome and pyrogens are metabolized, the behavior of the pigs changes; they need to lose heat, so they separate and move around the pens more freely.