Herd Health: The facts about H1N1 Influenza A virus

Tuesday, August 4, 2009

Apart from one Alberta herd, this new virus is not found in our pig population, but rather is circulating among humans. And even if pigs did catch it, it is not present in pork

by S. ERNEST SANFORD

I  was well into preparation of an article for this issue of Better Pork when the H1N1 Influenza A virus epidemic erupted in Mexico and spilled out across North America, threatening a pandemic as it spread to distant continents. When the virus was incorrectly named "swine flu," it had an immediate and devastating impact on our industry just at the exact time that market hog prices were expected to start recovering.

As the media frenzy escalated, I decided to put aside that article and replace it with an accounting of what swine influenza virus (SIV) does in pigs and where it and influenza in humans intersect. No doubt the media frenzy will be long gone by the time this article comes out and the media will have moved on to their next big "hot" story.

History of SIV in Canada
We get swine influenza outbreaks in our swine herds every fall and winter. In the United States, these outbreaks have been endemic, occurring annually since 1930, when the first SIV was isolated.

Swine influenza has occurred annually in Canada since the winter of 1981, when we had our first epizootic (disease in an animal population) outbreaks in Quebec, Ontario and Manitoba, and retrospectively probably also in the Maritimes. Prior to that, there had been one documented case in a herd in Manitoba in the fall of 1967. Unlike the outbreaks that have occurred since 1981, that Manitoba case was confined to a single herd and never spread further.

We had known that SIV had been circulating in our swine population without active clinical signs for many years prior to 1981. Federal testing of slaughterhouse blood samples had shown significant numbers of seropositive swine influenza samples over the years, but no clinical outbreaks had been recorded.

As the media frenzy continued during the last week of April, I did a quick cross-Canada survey of provincial diagnostic laboratories and veterinary colleges to determine just how many cases of swine influenza had been diagnosed in the last few years and especially since the start of 2009.

The answers came back over the next 24-48 hours. Through the laboratory systems in the major pig-producing provinces, swine influenza diagnoses increased eight to tenfold in the 2005-2007 period, then decreased over the last two years. The increases were due to an epizootic outbreak that went through most of the Canadian swine population in those years. Interestingly, the outbreaks were due to an H3N2 (not an H1N1).

In the June 2009 newsletter of the Animal Health Laboratory of the University of Guelph, the authors pointed out that their diagnoses of swine influenza were running below normal in the first four months of 2009 and there were no cases of the new variant H1N1 (Mexican) influenza A virus in the few cases they had diagnosed in 2009.

Clinical signs of SIV
Clinical signs of swine influenza are similar to those in people with human influenza and include anorexia, marked respiratory distress, sudden onset of persistent, unrelenting coughing, and lethargy leading to total prostration.

The cough in pigs is a distinctive "barking" cough reminiscent of a dog's cough and may occur in any age or stage of production of pigs, but is most often and most consistently seen in pigs in the finishing barn.

What starts out seeming like a dire situation with the apparent pending death of large numbers, perhaps all, of the pigs in the barn, subsides after about four to five days and all clinical signs recede. The cough stops, the pigs get up, start eating heartily and are back to normal as abruptly as they started into their illness. Mortality is minimal and, unless there are other complicating diseases or undue stresses, can be expected to be zero.

Treatment and control
Unlike the situation in humans where Tamiflu and other anti-influenza medications are administered to combat flu, this is not done in pigs. Antibiotics can be given if secondary bacterial infections are present or suspected, but we do not treat pigs for influenza itself. We let it run its course, which it does after a few days of illness.
Vaccines are available and can be used prior to an outbreak. In herds with endemic influenza, the sow herd is vaccinated and passive maternal antibodies protect the piglets through to at least the end of the nursery phase. Above all, swine herds are protected by following strict biosecurity procedures.

Biosecurity measures, especially those used to keep the PRRS virus out of barns, are the same measures used to keep SIV out of pig herds. These include:

– Maintaining minimum Danish entrance requirements to the pig barn;
– Changing outer clothing and boots to farm-specific boots and coveralls;
– Controlling and restricting visitors' access to your herd, especially those with respiratory signs;
– Quarantining incoming replacement animals;
– Maintaining rigorous sanitation in barns;
– Routinely monitoring herd health, as per the herd veterinarian's guideline;
– Wearing an N95 mask, especially if respiratory signs are present in pigs (or people).

Zoonotic potential of SIV
Influenza viruses are shared between people and animals. Influenza A viruses can infect pigs, birds, humans and several other animal species. Pigs can infect people and people can infect pigs, as was apparently the case in Alberta.

Infection is spread between pigs and people by infectious droplets passing from one individual to another. This can come about by close contact between an infected individual shedding the virus and passing it to an uninfected person or pig. Overall, however, influenza viruses do not usually jump from one species to another very easily.  The potential is there, but it happens infrequently.

Although it has been said many times by national and international authorities, it bears repeating that influenza viruses do not get into pig muscles and hence do not get into pork. The virus invades the lungs and respiratory tract of pigs and people. It does not travel in the blood. So, even if pigs did catch this new H1N1 influenza A virus, it does not end up in the pork. 

In summary, influenza viruses are common in pigs and people. Swine influenza has been endemic in our Canadian swine herds for decades, with outbreaks occurring every fall and winter. Clinical signs include high fever, coughing, lethargy and prostration over a period lasting usually four to five days, with uneventful recovery unless other diseases are also present in the pigs at the same time.

Morbidity is high but mortality is very low. Apart from the single swine herd in Alberta, infected by someone in the community who had returned from Mexico, the new H1N1 influenza A virus is not in our pig population, it is circulating in the human population.

S. Ernest Sanford, DVM, Dip. Path., Diplomate ACVP, is a swine specialist with Boehringer Ingelheim Vetmedica (Canada) in Burlington. Email: ernest.sanford@boehringer-ingelheim.com