New swine influenza viruses in humans

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swineA new strain of swine influenza virus has been recently isolated from seven persons in the US. Is it time to break out the swine flu vaccine of 1976?

Last week the CDC reported that swine influenza virus had been isolated from two children with respiratory illness in California. The cases were not linked and the children recovered from the illness. The virus was identified as a swine influenza H1N1 strain, similar to viruses that have circulated in American pigs for the past ten years. However some of the viral genes are derived from Eurasian swine influenza viruses. The isolates are new because this particular combination of swine influenza virus RNAs has not been observed before among swine or human viruses.

A similar virus was subsequently identified in five additional individuals in Texas. It’s curious that one of the California children had traveled to Texas before becoming ill, but whether or not the cases are related has not been revealed.

What is the origin of these new swine viruses? None of the people who were infected had known contact with pigs. Others must have acquired the virus from pigs, who then passed it on – demonstrating that the virus can be transmitted among humans.

At the moment these infections don’t seem to be cause for alarm. Because influenza virus surveillance is more intense than ever before, it is likely that new viruses will always be detected. Furthermore, respiratory disease caused by these new viruses has not been very severe. Another mitigating factor is that the influenza season is nearly over – viral transmission wanes when the weather becomes warmer and more humid.

It is believed that swine influenza originated in 1918-19, when pigs became infected with the pandemic influenza virus strain. Since that time, the H1N1 swine virus has been transmitted back to humans. The hypothesis for the origin of swine influenza is supported by the finding that pigs can be experimentally infected with the human 1918 pandemic influenza virus strain. Furthermore, other human influenza virus strains are known to infect pigs. For example, in the early 1970s, a human H3N2 subtype entered the European swine population.

Pigs can be infected with both human and avian influenza virus strains because the cells of their respiratory tract bear receptors for both kinds of viruses. Based on this observation, it has been suggested that influenza viruses pass from birds through pigs on their way to infecting people. For example, if a pig is infected with avian and human influenza A viruses, reassortment of the viral RNAs occurs, leading to new virus strains to which humans are not immune. The 1957 and 1968 human pandemic viruses were reassortants of human and bird strains, although there is no evidence that these viruses arose in pigs. The role of pigs as a ‘mixing vessel’ for influenza virus has been questioned in view of the recent transmission of avian influenza viruses directly to humans.

Swine influenza viruses probably routinely pass among humans and swine; in this case they were detected as a consequence of heightened surveillance. Gerald Ford won’t be rolling over in his grave over this incident.

Weingartl, H., Albrecht, R., Lager, K., Babiuk, S., Marszal, P., Neufeld, J., Embury-Hyatt, C., Lekcharoensuk, P., Tumpey, T., Garcia-Sastre, A., & Richt, J. (2009). Experimental Infection of Pigs with the Human 1918 Pandemic Influenza Virus Journal of Virology, 83 (9), 4287-4296 DOI: 10.1128/JVI.02399-08

de Jong, J., Smith, D., Lapedes, A., Donatelli, I., Campitelli, L., Barigazzi, G., Van Reeth, K., Jones, T., Rimmelzwaan, G., Osterhaus, A., & Fouchier, R. (2007). Antigenic and Genetic Evolution of Swine Influenza A (H3N2) Viruses in Europe Journal of Virology, 81 (8), 4315-4322 DOI: 10.1128/JVI.02458-06

Van Reeth, K. (2007). Avian and swine influenza viruses: our current understanding of the zoonotic risk Veterinary Research, 38 (2), 243-260 DOI: 10.1051/vetres:2006062

16 thoughts on “New swine influenza viruses in humans”

  1. ET

    I was too little to remember the last time this happened, and given my interest and understanding (due in very large part to TWiV-so thank you for that) about this topic, I am wholly and utterly mesmerized by this entire course of events. Will our over vigilance in treating this lead to its becoming resistant quicker? Should this be considered a prime canidate for next winters flu season?

    And most importantly, are we going to get a TWiV out of this?

  2. G. Hayduke

    “We also need to be cautioned by the lessons of history. Back in 1976 an emerging swine flu virus appeared to be responsible for the death of a military recruit at Fort Dix (this later turned out to be erroneous), sparking a massive public hysteria fueled by Center for Disease Control press conferences.”

    The above is from an article (see link below). I have tried,without success, to find confirmation of his assertion that the swine flu was not responsible for the death of the soldier at Fort Dix. In searching the web I came upon your blog and thought I would put the question to you.

    http://foxforum.blogs.foxnews.com/2009/04/25/dr…

  3. facebook-118321

    I haven't been keeping up with the news recently. (I have no idea why that is!) I received a few panicked phone calls this weekend regarding this “new” flu strain. My first thought was to wonder if this particular species crossing frequently occurs, because it seems likely that it should. Now we are better able to identify strains (faster, cheaper, better technology) than in the past and maybe this news is no cause for alarm. It seems that my virology hunches weren't too far off. I knew where to go for an expert opinion. 🙂

  4. Pingback: Swine flu returns

  5. profvrr

    TWiV 29 was recorded just before this epidemic hit the fan, so we'll have a more in depth look this week. Unfortunately antivirals will be essential in controlling any epidemic, as there is no vaccine yet available, and that will only lead to resistance, since only two drugs are licensed. As for next season – I presume you mean in the northern hemisphere – it depends upon what happens in the next week or two. If the virus A/H1N1/Mexico/09 spreads in South America and Australia, then it's likely to come back here in the fall. A vaccine could be made in time for the fall season if it's begun in the next month.

  6. profvrr

    All the information I have seen indicates that H1N1 swine flu virus was isolated from the recruit who died, and also from other ill soldiers at Fort Dix in 1976. See this 2006 article for an example: http://bit.ly/8cWEw

  7. Brandon

    Y dont we take a fast pick at it and wear breathing masks(plastic cup type thing) so every one is protected from the Virus.I mean virus get worse depending who the person is so fair.

  8. G. Hayduke

    I emailed Dr. Siegel, the author of the blog article and ask him this:

    Are you indicating that the recruit did not die of or have the swine flu virus? And if so can you direct me to the source of that information.

    He responded as follows: “Actually swine flu was isolated but the few who died were also found to have norovirus which in retrospect was the more likely cause of death”

  9. Alain Gagné

    Just a short reflection on the severity of swine influenza

    It is too early to fully understand the current situation, But I would nevertheless draw your attention to the possibility that the very important pollution level and the high altitude of Mexico City (2,250 meters) can be partly responsible for the severity of swine influenza cases reported in Mexico. Remember that to date, all cases reported elsewhere in the world seem rather benign, or at least non-fatal. If this hypothesis is true, the severity of a pandemic outbreak may well be less severe than that suggests by the seriousness of the cases observed in Mexico City. Indeed, polluted cities such as Mexico City or located more than 2,000 meters are not numerous. I obviously do not know the nature of complications that resulted in the death of Mexican patients, but this knowledge could lead to a better understanding of how pollution and altitude should be considered as aggravating factors in the evolution of infection. Obviously, it is possible that the global death toll increase, but if, in the coming days, the severity of cases seen outside Mexico remains similar to what it is today and if it is confirmed that it is really the same viral strain, pollution and altitude could be important explanations of the current differential severity observed.

    Alain Gagné,
    biologist

  11. Dante

    Is this normal for flu viruses to mutate into new strains (seemingly) every couple of years? I know it gives conspiracy kooks something new to accuse “The New World Order” of concocting to cause panic. Anyone?

  12. Anonymous

    Dear Sirs, Our Federal governments believe that the IMO or the EPA are capable of policing the discharges of ballast water, by allowing foreign sea captains,to decide the water quality is safe, is putting people at risk for the next waterborne flue or pathogen to mutate and be passed around the world by ballast systems intentionally or unintentionally. African Swine Virus, cholera are examples of pathogens that thrive in marine brown algae. The day of designer algae,s is already here. One can only wonder what pathogens they could carry, if released into the environment.
    Politicians will not address this issue because of economic domination due to lobbying, by the countries that supply our country with their manufactured products. These countries are keeping our largest employers(being retail) supplied, while holding our countries national debt along with large stakes in banks holding Americans mortgages. (are we free?) It must be noted that the Chinese. Koreans,and the Europeans have all established programs and goals to become the world leaders in shipping. This has been happening while our 2008 senators that were running for president were not able to use their influence to consider ballast dumping a federal issue rather than a states rights issue. I guess they think pathogens can understand the lines man draws on maps. Unfortunately many of the technologies used to clean ballast water could be altered to do more harm than good. Interesting that the house of representative voted 395-7 in favor of federal regulation. One can only wonder how much money and death can be associated with sickness and seasonal flues from pathogens in ballast water. It is paramount to treason that our politicians ignore and allow destruction to our countries environment, and peoples health, as a policy of economic globalization is pursued. Sincerely,
    Don Mitchel

  14. Don Mitchel

    Dear Sir, Having read dialog from discussion by the house of representatives the day before h.r.3619 passed, it is a bad day for our country when they describe how ballast water was omitted from this bill because they were cognizant, that in the previous Congress the Senate was not able to address ballast water. Unfortunately this is true because in the previous Senate they considered shipping, being allowed to dump untreated water in our country, with human pathogens,virus,invasive,and the possibility of terrorist use as a states rights issue. Scientist know that new microbes will be released from Arctic ice melting and from mining and exploration in the deep Sea Bed, along with the new technologies able to create designer algae s capable of acting as hosts to pathogens and virus. Still our politicians do not mention this as a human health problem. It has been reported that the Senate is going to address the Law of the Sea Treaty soon. To not address these issues before they consider ratification will be locking our country in an agreement that will not protect our health and environment from destruction, but will allow for economic globalization to continue.
    FROM the web site of the house- ” Last year, I worked closely with Chairman Oberstar to include a title on Ballast Water Management in the Coast Guard bill, which would have created a uniform national standard for ballast water treatment. The goal was to have no living organisms in ballast water discharged by ships after 2013.

    Although I would have liked this bill to once again include a provision on ballast water management, I am cognizant that this provision may be one of the reasons this bill has been held up in the Senate.”

    Please let the Coast Guard know that although we are happy with their interest in ballast dumping, not to portray their forum as being representative of what the American people want. The will of the American people can only be expressed in our democracy through legislative efforts by our elected officials.
    Sincerely,
    Don Mitchel

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