Herd Health: What you should know about swine flu

Tuesday, June 2, 2009

How does it affect pigs and humans and what can be done to prevent it spreading?

by BETH YOUNG and CATE DEWEY

Swine influenza, also known as swine flu, is a respiratory disease caused by the type A influenza virus. This classical H1N1 swine influenza virus is common in Canadian swine herds and may cause outbreaks of clinical disease, in the fall and winter.

Swine flu viruses do not normally infect people, but occasional human infections with the classical H1N1 swine influenza have been reported. These cases happen in people who have had direct exposure to pigs. There are 50 reported cases world-wide since the early 1970s.

What happens to pigs? Swine flu can affect pigs of any age and, in an outbreak situation, most pigs in a herd suddenly become ill. Signs of the disease are similar to those experienced by humans with the flu and include fever, weakness, decreased feed intake, coughing, and discharge from the eyes and nose. However, as in humans, pigs can become infected with the virus and not show any clinical signs.

Although many pigs in a herd may be affected, if the disease is not complicated by other respiratory diseases, very few will die from swine influenza. Infected pigs can spread the virus to other pigs when they cough. Pigs only shed the virus for two to five days after they get infected. Sick pigs will begin to recover around the sixth day and the entire herd will recover from an outbreak in one to two weeks. 

In addition to classical swine influenza, pigs can also become infected with human and avian (bird) influenza viruses. If a pig is infected with two different types of influenza virus at the same time, the viruses can "swap" genetic material. This can lead to the development of new strains of flu viruses that could infect pigs or humans.

What happens to people? Many people infected with swine influenza virus don't develop symptoms and therefore never know that they have been infected. However, they can still shed the virus and infect others. Some people develop symptoms that look like those suffered by people infected with a human influenza virus. These include fever, muscle aches, headache, loss of appetite, coughing and runny nose. Rarely, people contracting swine flu develop a severe pneumonia that leads to death.

What can be done to prevent it spreading? To reduce the risk of pigs becoming infected with avian flu viruses, domestic poultry and waterfowl should not be housed on the same premises as pigs and swine facilities should be bird-proof. Vaccination of pigs against swine influenza virus can reduce the levels of virus shed by infected animals, which reduces the potential for exposure for people working with pigs.

People can only get infected with the swine influenza virus during the five days when pigs are shedding the virus. This only happens with close contact, by breathing the same air. People working with pigs during a swine flu outbreak may choose to wear masks.

People working with pigs should frequently wash their hands and avoid touching their face to minimize the risk of becoming infected with swine influenza virus. Barn boots and clothes should be removed before leaving a barn to prevent carrying swine flu virus off a farm.  Pork cannot be infected with the swine influenza virus. While there is no vaccine to protect people from swine flu, people working in swine facilities are encouraged to get an annual flu vaccine as it may provide partial protection against certain swine influenza viruses. It may also reduce the chances of people infecting pigs with human influenza viruses.

Swine farm workers are encouraged to stay home from work if they are experiencing flu-like symptoms to prevent the spread of human influenza to pigs. People who work with pigs and seek medical attention for flu-like illness should let their health care provider know that they have been in close contact with pigs. BP

Beth Young is a swine extension veterinarian at the University of Missouri.  Cate Dewey is a professor in the department of Population Medicine, Ontario Veterinary College, University of Guelph.