Fowl Play

Animals, chicken principally, serve as mixing vessels for the flu virus

By Lucio C. Victor Jr.

In March 1997, 6,800 chickens died in three Hong Kong farms due to avian influenza. Two months later, the first reported case of bird flu (Influenza AH5N1) was seen in a three-year-old boy who died soon after.

    By year's end, the US Centers for Disease Control and Prevention (CDC) ordered the slaughter of chickens to stop the spread of the virus. It was revealed that roughly 20 percent of all the chickens that died were positively infected with AH5N1.

    Luckily, the carnage and gore involved more fowl than human flesh as only 18 persons were reported to have succumbed to the bird flu. By January of 1998, the bird flu was considered contained. In March 1999, another new strain surfaced, again in Hong Kong. Influenza AH9N2 was isolated from two children-a one-year-old and a four-year-old-who later on fully recovered. Just when the fear of bird flu was again on the rise, another strain, H6N1 loomed.

Karma Chameleon

    As much as it is amazing, it is also annoying to know that Influenza A has the capacity for genetic reassortment due to the nature of its eight-segment RNA strand. Aside from this, possible point mutations can crop up any time along the amino acid sequence of the viral genome. In both instances, changes in the genome known as genetic shift and genetic drift are induced, giving rise to at least 135 probable combinations based on influenza A's nine neuraminidase (NA) and 15 hemaglutinin (HA) surface antigens. Aside from the 135 possible subtypes, an innumerable number of strains can crop up anytime from single to numerous point mutations.

    Although there are three existing types of influenza viruses, only influenza A is capable of infecting vertebrates other than man. Influenza B and C strictly infect humans with the latter being an insignificant health threat.

    Dr. Veronica Chan, head of the WHO National Collaboration Center for Influenza, says that the characteristics of influenza A make it a serious health concern. Dr. Chan, emeritus professor of microbiology at the University of the Philippine College of Public Health, notes that the capability of the flu virus to vary and adapt according to the immune defenses set up by man makes it difficult, if not impossible, to eradicate the disease anywhere in the near future.

    She explains that during an infection, the human immune system is able to produce antibodies against invading pathogens by recognizing it through antigens present on their cell surface and elsewhere. This is why producing antibodies versus HA and NA once a person has already been infected is supposedly enough to offer immunity to the flu virus. However, this only holds true for influenza B and C infections. Influenza A has nine types of NA and 15 types of HA. An infection with one subtype will not necessarily confer immunity to another subtype.

    It has been reported that the influenza A virus affecting humans may have only HA 1, 2 or 3, and NA 1 or 2 as surface antigens while avian species may be infected with all its possible subtypes. Swine, on the other hand, may be infected with flu viruses having HA 1 or 3 and NA 1 or 2 as surface antigens. Although flu also infects aquatic vertebrates and horses, avian species are the principal carriers of the flu virus. Transmission is greatly spread by migratory birds and this is why flu is manifest in both temperate and tropical regions of the globe. Flu in migratory birds can infect livestock, fowl, aquatic vertebrates, and humans as the virus is shed through the birds' excrement. Influenza viruses in horses and aquatic vertebrates are not known to infect other animal species, but it has been clearly established that the flu viruses in swine, fowl, and man can jump species and transfer across any of these vertebrate groups.

The Animal Lab

    The Spanish Flu of 1918 was thought to have originated from viruses affecting swine. Dr. Chan says that during the pandemic, the swine served as a mixing vessel for a human flu virus. With the two different subtypes simultaneously infecting the swine, it was inevitable for the two viruses to merge genetic material resulting in a new subtype or strain.

    The Spanish Flu was fist reported on March 4, 1918 at Camp Funston, Kansas. The subtype at the time was AH1N1. Four decades later, another pandemic, the Asian Flu, broke out. The subtype identified was AH2N2. Unlike the Spanish Flu that lasted nearly a decade, the Asian Flu persisted for only two years and killed less compared to the 20 million downed by AH1N1. A decade after the Asian Flu, a third pandemic arose as the Hong Kong Flu with AH3N2 as the subtype responsible.

    Dr. Chan points out that the flu pandemics of the 20th century were mainly due to the increase in international travel across nations. Since the flu virus is highly infectious and transmission is nearly a hundred percent, no border was able to keep it at bay.

    In 1976 to 1977, Influenza AH1N1 reemerged as the Russian Flu. Younger populations who were not previously exposed to the Spanish Influenza were severely infected. Although the Russian Flu did not trigger a pandemic, it confirmed that previous strains and subtypes can reemerge and cause as much havoc as before. Last year, the Philippines suffered its share of AH1N1 reemergence when several schoolchildren were infected with a flu virus that closed down several schools for days. The epidemic, however, was also due to a circulating Influenza B to which the younger previously uninfected children were susceptible.

The Collaborators

    The Philippines is one of the 83 nations that house the 110 WHO Collaborating Centers for flu. Dr. Chan has already been part of the National Collaborating Center since it first operated in 1978. Aside from the Collaborating Centers, there are also four Centers for Virus Reference located in Melbourne, Tokyo, London, and Atlanta where isolated viral samples cultured in embryonated chicken eggs are sent for confirmation and identification.

    Dr. Chan says that based on the isolated cultures and surveillance studies, recommendations are made so that flu vaccines for the following year will include the predominant circulating virus strains and subtypes. She reveals that the recommended vaccines cover strains from Influenza A and B they evaluate would be the most probable culprit in the coming year. She also points out that, although flu can exist throughout the year, the surveillance networks in individual countries have identified certain peaks in the incidence of flu. She correlates this with the time of the year when most persons are observed to be crowding in closed spaces such as during a snowstorm or floods.

    In the Philippines, primary (end of June and the beginning of July to September) and secondary (early November to early February) peaks have been observed. Based on these data, she says that it is recommended to have flu shots during the summer when the incidence of the circulating virus is at a trough so that by the time the rainy season kicks in, seroconversion has already been achieved.