Swine Flu, Seasonal Flu, and You

Posted on October 1, 2009

Written By Bradford P Smith, DVM, DACVIM

On September 22 the New York Times reported “On Monday, China became the first country in the world to begin a national swine flu vaccination campaign…” The Times also reported that the National Institute of Health had announced that a single dose of vaccine is sufficient to protect most children and adults, but children under 9 need two doses at 21 day intervals. Both an injectable vaccine and a modified live nasal mist vaccine have been developed. There are two different influenza vaccines, one for the seasonal flu and one for H1N1 swine flu.

You have no doubt been hearing a lot about the H1N1 swine flu for the last year, and as a veterinary student, you should be conversant about this problem. The H1N1 swine influenza virus has been in U.S. swine for many years, and a vaccine for swine is currently in use.

Influenza viruses of pigs pose a public health concern because humans can catch new strains from pigs. Pigs in the U.S. normally circulate H1N1 and H3N2 subtypes, and these subtypes are currently found in most vaccines licensed for use in swine in the U.S. At least six manufacturers produce these killed virus vaccines.

The subtypes are named for the type of hemagglutinin (H) and the type of neuraminidase (N), two glycoproteins that are on the surface of the viral envelope. There are 16 H types and 9 N types that can be identified. Of the two, the hemagglutinin appears to be the most important because it binds the virus to the host cell. Antibodies directed against this antigen can be protective.

If we already know about the H1N1 subtype and even have a vaccine against it, why should anyone be concerned about it or any other subtype? Influenza viruses can re-assort RNA and morph into new virus strains that evade the immune system and can be more pathogenic. The segmented nature of the influenza genome allows two viruses that co-infect a single host to exchange RNA segments during replication.

How does that exposure happen? Influenza viruses in waterfowl and shorebirds are highly host adapted and rarely cause illness in these species. All 16 H subtypes and all 9 N subtypes are found in water birds, and they are excreted in feces. Consequently aquatic birds provide a constant source of influenza viruses, and phylogenetic data indicates that aquatic birds are the progenitors of mammalian influenza viruses. So if a bird virus infects a pig and the virus rearranges, a novel variant can emerge, infect humans or other mammals and start a new pandemic.

Some nations (Egypt most recently) have killed all their swine in a panic. Others with more advanced swine housing systems urge that swine be isolated from aquatic birds. Governments and international organizations such as OIE (Office International des Epizooties) will continue to monitor swine flu viruses in an effort to stay ahead of the curve, and to be aware of new viruses that emerge as potential public health problems.

In the meantime, you should see your physician and receive the seasonal and swine flu vaccines as soon as they are available. Students or those who work with large groups of people are likely to be exposed. Vaccine should reach most locations by October.