Foot and Mouth Disease: The Special Concern with Pigs

Dr. Terry Whiting, Veterinary Services Branch

Very few viruses can infect more than one species of animal. Notable exceptions are rabies and the influenza group which both can affect animals and man. Foot and Mouth disease in comparison is very promiscuous in being able to infect all even toed animals, all the common split hoofed farm animals, wild deer and like animals and elephants and rhinos. It is not considered infective to man.

With the different strains of foot and mouth disease there is generally a preference for a particular species. In 1997 the Taiwan outbreak was a swine adapted strain of FMD and no ruminants were affected with the disease. In the current outbreak in the United Kingdom primarily sheep and cattle are being affected. In general, when FMD affects pigs the disease is hardest to control.

Pigs infected with FMD are tremendous sources of virus production. Pigs are able to generate infective aerosols up to 3000 times greater in virus concentration than cattle. Pig farms can generate an infective cloud, which can predictably be blown 60 km and infect other farms.

Animals become ill between 2-14 days after infection. Infected animals are infectious to others for several days prior to the onset of clinical signs. On average cattle excrete virus from the pharynx for 2.5 days prior to the development of blisters and pigs excrete on average for 5 days (2-10) prior to the development of clinical signs.

The highly infectious nature of this agent and the fact that all species shed virus for considerable lengths of time prior to the development of clinical signs results in two dangerous situations. Firstly, several areas of the country may be involved as is currently the situation in the UK and was the situation in the North American US-Canada-Mexico computer simulation reported in the January Issue of the Cattleman Magazine. Secondly if the response does not include effective pre-emptive slaughter, that is the killing of animals prior to the development of clinical disease, you are unlikely to be able to control the outbreak.

Canada has had significant experience in controlling tuberculosis and brucellosis of cattle. Both of these diseases are poorly infective compared to FMD or hog cholera. Response to foreign animal disease is important to minimizing the risk to our production systems. Prevention is by far the most economical risk management choice.