Update on Bird Flu [Avian Influenza A(H5N1)] February 2007

The avian influenza A virus is back in the headlines and television news shows with the recent outbreak affecting domestic turkey farms in the UK. During the first week of February 2007, g overnment veterinary officials were called to a farm near Lowestoft in eastern England after the death of 2500 birds. The UK government is enforcing European Union-agreed controls to contain the outbreak, which involved destroying the more than 150,000 birds on the farm and setting up a protection zone with a radius of 2 miles and a surveillance zone of 6 miles around the infected farm. It is the second confirmed case of H5N1 in the 27-country European Union in 2007, following one in Hungary.

The H5N1 virus was first isolated from wild birds (terns) in South Africa in 1961. Like all bird flu viruses it is very contagious among birds and circulates around the world through migrating wild bird populations. These infected wild birds (primarily waterfowl) may develop few signs of illness but carry the virus over long distances during migrations. At the same time it is highly infectious to and causes severe respiratory illness in domestic poultry (chickens, turkeys, ducks) and therein lies the problem and concern with the H5N1 virus. Over time the virus spread to Asia and in 1997 the first case of disease transmission from poultry to human was reported during a bird flu outbreak in Hong Kong. The virus caused severe respiratory disease in 18 humans, 6 of whom died. During late 2003 and 2004 outbreaks of influenza H5N1 occurred among poultry in eight countries in Asia (Cambodia, China, Indonesia, Japan, Laos, South Korea, Thailand and Vietnam) and human cases of this H5N1 infection were reported in Cambodia, Thailand and Vietnam. During the 2005-2006 wild bird migratory season the H5N1 virus spread to parts of Africa and the European Union, and to date the H5N1 virus is known to have infected 270 people and killed at least 164 worldwide since 2003, most of them in Asia, and over 200 million birds have died from it or have been killed to prevent its spread. However, the H5N1 virus has still not made the critical mutational leap from bird to man that would allow an altered form of the virus to be contagious and spread from one infected person to another. The global pandemic that was predicted some 18 months ago has not become reality…..yet.

There are many different subtypes of type A flu viruses that can affect many different animals including pigs, whales, horses and seals. The various subtypes are identified by certain viral surface proteins: the H stands for Hemagluttinin protein and the N for Neuraminidase protein. Many different combinations of H and N proteins are possible and each combination is a different subtype. All subtypes of the influenza A virus can be found in birds and most do not infect humans. Wild birds are considered the natural reservoir because more Influenza A subtypes circulate among wild birds than any other species. So when we talk about “bird flu” viruses we are talking about flu A subtypes that occur mainly in birds and usually do not infect humans. Overall, at least 16 distinct antigenic subtypes of H (H1 to H16) and nine subtypes of N (N1 to N9) have been identified in wild aquatic birds. Currently there are only 3 known subtypes of “human flu viruses” (H1N1, H1N2, H3N2) and it is believed that some genetic components of these human flu viruses originally came from birds. This information becomes important as we discuss how the avian flu infects humans and how we can prevent infection.

So why do some avian flu viruses infect people and why do so many flu viruses originate in Southeast Asia? Infected birds shed virus in feces, saliva and nasal secretions and humans are exposed via direct contact or contaminated drinking water. Most cases of the H5N1 virus occurred in people in close association with infected poultry; they worked on the farms as caretakers or in slaughterhouses where they came in contact with infected bird blood or other body fluids. As well, many humans in SE Asia live in close association with birds where it is not uncommon to raise backyard poultry as a source of food or to shop for dinner at a live bird mart. In these living arrangements the ‘backyard’ may be the room directly behind the kitchen! This environment of close human-poultry contact in situations of crowding, stress, high heat and humidity and less than ideal sanitary conditions creates a situation ripe for viral spread. Of additional importance is the fact that China alone produces 67 % of the world’s ducks and 90% of the geese. Raised outdoors, these domestic poultry are exposed to wild and potentially infectious birds. The H5N1 virus is highly infectious to poultry that become sick and then shed the virus in body excrement and fluids to new populations of migratory waterfowl that continue to spread the virus elsewhere around the globe.

The concern for H5N1 viral infection in humans stems from the fact that viruses are masters at mutating. Due to their microscopic size and simple genetic structure it is easy for viruses to mutate and spread within a population anew or cross over to a different species. It is how they survive and thrive. So far with the H5 N1 there have been very few reports of human-human spread. The fear is that the H5N1 will mutate and change genetically to a highly contagious virus where human-to-human transmission is possible and a susceptible world population will result in a pandemic (worldwide epidemic). Influenza is not a trivial disease with typical U.S. seasonal epidemics resulting in approximately 36,000 deaths, 226,000 hospitalizations and $1-3 billion annually in costs for medical care. Influenza pandemics typically occur 3-4 times a century with the 1918 Spanish flu pandemic resulting from mutation of and spread of H1N1 Influenza A virus and the 1968 Hong Kong flu pandemic associated with H3N2 Influenza A virus. These two pandemics alone resulted in over 180 million illnesses and over 2 million deaths.

An alarming fact with the H5N1 virus is its resistant to amantadine and rimantadine, two antiviral drugs commonly used to lessen the severity of and shorten the duration of symptoms associated with human influenza. Two other antiviral drugs, Tamiflu and Relenza, may work to reduce symptoms associated with avian flu and as a result the U.S. Government has stepped up the effort to encourage the manufacture and supply of these pharmaceuticals. Another concern is that there is no vaccine currently available to protect against the H5N1 subtype of bird flu. The human flu vaccine used for the 2005-2006 flu season protected against three influenza viruses—one A (H3N2) virus, one A (H1N1) virus, and one B virus. The viruses in the vaccine change each year based on international surveillance and scientists’ estimations about which types and strains of viruses will circulate in a given year and at this time development of a H5N1 vaccine is only in the research stages. The National Institutes of Health and MedImmune Corporation of Gaithersburg, MD are working in conjunction to produce and test vaccines against potential pandemic flu strains including the H5N1 avian influenza virus. Human influenza viruses that have circulated in the last century contain one of three possible hemagglutinin proteins (H1, H2, H3). Not having any prior exposure to the hemagglutinin in H5N1 bird flu, the human immune system is unprepared to fight it. For all these reasons the concern for a pandemic situation is very real and thus is getting a lot of attention by international governments and the media.

What about our pet birds—are they susceptible to this avian influenza virus? There has been one report of an imported parrot dieing of H5N1 avian flu at a quarantine station in the United Kingdom. Importation of psittacine birds is still allowed in the UK and Europe and this particular parrot (species not identified) was from Suriname, South America. As H5N1 bird flu virus has not been identified in South America it has been theorized that this deceased bird acquired the disease from another bird in quarantine that harbored the virus. Now that we know this bird flu can infect our pet psittacines is it a concern? First of all the disease does not appear to be highly infectious to psittacine or passerine birds and additionally has not been reported in the Western Hemisphere. However due to its spread along the routes of principally migratory waterfowl it is felt that it is only a matter of time before it hits our shores. If that happens then one needs to use common sense in preventing exposure of pet birds. Keep birds indoors and off of screened porches or outdoor perches where they could get exposed to wild bird populations. If traveling to a park or taking a walk where waterfowl collect, then leave boots at the door and change clothes prior to handling pet birds. If the virus does mutate and humans start to get infected, there is a good chance that this new virus subtype will not result in human to bird transmission.

Also of concern is how it will affect the poultry industry if it makes its way to the United States. Zoological parks with collections of rare bird species and waterfowl will also be on the alert. Fortunately with the U.S. poultry industry most birds are raised indoors which would help limit exposure to wild bird populations carrying the virus. With zoological collections guidelines have been set forth by the American Zoo and Aquarium Association (AZA) which include; isolation and decontamination control measures, cleaning, disinfecting and personnel protection protocols, employee education and guidelines if infected birds are identified, and facility closure and isolation during known regional virus identification.

As a side note, the Kowloon Zoological Park in Hong Kong vaccinated some of their avian collection with an experimental H5N3 (note the different N [Neuraminidase] protein is this flu subtype) avian influenza virus in 2002 following a severe outbreak of H5N1 avian flu in their collection. The H5N3 strain of avian influenza is nonpathogenic (does not cause disease) and it was hoped that the H5 component would result in protection against the more virulent strain of H5N1. Of those birds vaccinated nearly 100% of the waterfowl and 90% of the psittacines tested developed protective titers against the H5N3 virus subtype at 56 days post vaccination. Vaccine doses and route of administration are factors not yet properly studied in most avian species and whether these birds will be protected against the H5N1 avian influenza subtype is yet to be proven.

If and when the bird flu virus will mutate and become infectious among humans or make its way to North America is unknown. Only time will tell on how this interesting and concerning scenario will pan out.