By Kelly A. Reynolds, MSPH, PhD
As concern mounts over the potential for avian influenza A to
gain the genetic tools necessary for transmission between humans, response networks prepare for the worst. Identifying how people are exposed and what environments need to be managed in the event of an outbreak is an important part of minimizing the spread of any microbial disease. Recently, evidence has surfaced linking water (drinking and recreational) to the spread of avian flu. The good news is that, at least for drinking water, there are many effective options for control.
Avian influenza basics
There are three types of influenza viruses (A, B and C). While influenza A infects humans, pigs, horses and other animals, birds are the natural host but they can cause pandemic flu outbreaks. Humans are also infected by B and C types, which present much milder effects. The viruses are further classified into subtypes based on the characteristics of their viral proteins, haemagglutinin (H) and neuraminidase (N).
Avian influenza A subtype H5N1 causes a highly contagious and devastating disease in birds and rare, but often fatal, infections in humans. This highly pathogenic strain can kill over 90 percent of exposed poultry populations within 48 hours. Since 1997, more than 200 million domestic birds have died or been destroyed in Asia, Europe and Africa due to avian influenza. In China, approximately 60 percent of the country’s 13.2 billion chickens are raised in close proximity to humans and other domestic animals. Pigs are a concern because they can become infected by both human and avian influenza viruses and could serve as a perfect vessel to introduce the two virus types where they could readily exchange genetic information and become more easily transmitted to a human host.
In a four-year prospective surveillance of poultry in southern China, over 20 percent were found to be affected by H5N1 in the winter. The close proximity of humans to backyard chickens in Asia increases the chance for the virus to emerge in a human host and greatly complicates control efforts.1 Although infected migratory birds are capable of spreading the disease to domestic birds, poultry in the U.S. are often raised in confined facilities, protected from exposure to wild bird populations.
The human host
Nearly 200 cases have been positively identified in humans and reported to World Health Organization (WHO) since 2003, resulting in 109 deaths. As of April 2006, Egypt had reported 12 cases of H5N1 infections in humans. As with most cases of human avian influenza, all 12 victims were known to have had direct contact with diseased birds. Unlike commonly circulating strains of human influenza A, death from avian influenza A is not limited to the young or elderly. The outcome of disease is largely correlated to the nature of the infecting virus strain, with H5N1 being the most virulent (capable of causing disease). Two cases of avian influenza A have been documented in the U.S. since 2003 but involved a less-virulent strain of the virus (H7N2).
Health experts often say that it is a matter of when, not if, we will be dealing with widespread infections of avian influenza A in humans. Each additional human infection represents a potential for adaptation that may ultimately lead to person-to-person transmission. Historically, flu pandemics occur about once every 33 years (H1N1 in 1918 and 1977; H3N2 in 1957; and H2N2 in 1968). The 1918 influenza pandemic (Spanish Flu) is estimated to have killed up to 40 million people of all age groups, worldwide. WHO predicts that a pandemic today could result in up to 7.4 million deaths globally.
A waterborne route
Potential transmission routes of avian influenza include air, aerosol droplets and person-to-person spread including fomite (inanimate surface) contamination. Unlike human influenza, which primarily replicates in the respiratory tract, avian influenza replicates in the lung and the intestine.2 The fecal-water-oral route of transmission is considered significant among birds and thus, conceivable among humans.3 Symptoms of diarrhea have been reported in human infections and the virus has been isolated in feces.4 Two fatal infections in children presented with diarrhea but no obvious respiratory symptoms. Therefore, the disease in humans may be respiratory, enteric or both.
Wild waterfowl (i.e., ducks, geese) are considered significant contributors of infectious viruses to water reservoirs. Avian influenza has been isolated from unconcentrated lake water in Canada, the U.S. and Hong Kong, suggesting that waters subject to wild waterfowl or to waste run-off from duck and chicken farms are contaminated at relatively high levels. One duck has been shown to excrete up to 109 infectious viruses per gram of feces.5 Studies on avian influenza subtype H3N6 re-suspended in Mississippi River water survived for over a month at 4oC and for four days at 22oC. Survey of an Alaskan lake with nesting ducks showed that 23 percent of the water samples were positive for a strain of influenza A. Following migration of the ducks, 14 percent of the samples were still positive indicating long-term persistence of the virus.6
More difficult to control would be the potential exposure to avian influenza via recreational waters. Although recreational water transmission has not been proven, it has been suggested in two cases in Vietnam following bathing in contaminated waters.7 Possible entry routes include the eyes, mouth or nose.
Controlling a pandemic
With no prior exposures in the human population and thus no immunity, health experts around the world are preparing for the possibility of an avian influenza A pandemic. The first line of defense in an avian flu outbreak is to avoid exposure to the virus. Once an outbreak in humans is detected, a vaccine could be developed but experts estimate that it would require a minimum of six months. Administering vaccinations presents an additional challenge and many people are expected to be exposed before effective distribution. Unfortunately, the H5N1 viruses isolated from humans and poultry in Asia have already shown resistance to two of the four drugs effective for treating influenza, prompting the CDC to recommend they not be used in the U.S. during last season’s outbreak (January 14, 2006 CDC Health Alert Notice).
Like other strains of influenza, avian influenza A is expected to be easily inactivated by conventional methods of disinfection, including chlorine, ozone and ultraviolet light. Point of use treatment utilizing one of these technologies, or even just boiling the water, is also expected to be effective. Conventional drinking water treatment methods, as outlined by the WHO,8 should be adequate to treat contaminated source waters and thus quantitative risk assessment calculations estimate the risk of infection from contaminated source waters to be below health-based standards.9
Previous influenza pandemics spread globally in six to nine months. With the frequency and volume of international travel today, if adapted to a human host, the avian influenza virus is expected to spread more rapidly, reaching every continent in less than three months.10 With no immunity to the novel virus, widespread illness is considered likely. The potential for a severe outcome related to human avian influenza A infections warrants the application of every reasonable control measure that could help to reduce risks of exposure. Experts recommend that precautions be exercised to minimize exposure from all potential routes as long as resources allow.1 If drinking water is identified as a significant transmission route, proven technologies of municipal and household treatment are available for the control of these infectious viruses.
References and recommended reading
- Yuen, KY and Wong, SSY (2005) Human infection by avian influenza A H5N1. Hong Kong Med. J. 11: 189-99.
- Uiprasertkul M, Puthavathana P, Sangsiriwut K, Pooruk P, Srisook K, Peiris M, et al. (2005) Influenza A H5N1 replication sites in humans. Emerging Infectious Diseases. 11(7): 1313.
- Markwell DD and Shortridge KF (1982) Possible waterborne transmission and maintenance of influenza viruses in domestic ducks. Applied and Environmental Microbiology, 43(1):110–116.
- De Jong MD, Van Cam B, Qui PT, Hien VM, Thanh TT, Hue NB, Beld M, et al. (2005) Fatal avian influenza A (H5N1) in a child presenting with diarrhea followed by coma. The New England Journal of Medicine. 352:686–691.
- WHO (2006) Review of latest available evidence on risks to human health through potential transmission of avian influenza (H5N1) through water and sewage. Available online at: http://www.who.int/water_sanitation_ health_ emerging/h5n1background.pdf. Last updated: 24/03/2006.
- Ito T, Okazaki K, Kawaoka Y, Takada A, Webster RG and Kida H (1995) Perpetuation of influenza A viruses in Alaskan waterfowl reservoirs. Archives of Virology, 140:1163–1172
- Cooperative Research Centre (CRC) (2005) Avian influenza: Is there a risk to water supplies? Health Stream, Issue 40, December 2005. Available online at http://www. water quality.crc.org.au/hsarch/hs40m.htm.
- World Health Organization. (2004). Avian influenza A (H5N1)–update 31: Situation (poultry) in Asia: need for a long-term response, comparison with previous outbreaks. http://www. who.int/csr/don/2004_03_02/en/index.html.
- Schijven JF, Teunis PFM and de Roda Husman AM (2005) Quantitative risk assessment of avian influenza virus infection via water. Report prepared for the Environmental Inspectorate for the Netherlands (RIVM report 703719012).
- WHO Guidelines for drinking-water quality (2004a) Available at: http://www.who. int/water_sanitation_health/dwq/gdwq3/en/index.html.
About the author
Dr. Kelly A. Reynolds is a research scientist at the University of Arizona with a focus on development of rapid methods for detecting human pathogenic viruses in drinking water. She holds a master of science degree in public health (MSPH) from the University of South Florida and doctorate in microbiology from the University of Arizona. Reynolds has been a member of the WC&P technical review committee since 1997. She can be reached via email, [email protected]