02/27/2006

Before Worrying Over Avian Flu, Know Your Viruses

Writer:

Mauricio Espinoza
espinoza.15@osu.edu
(330) 202-3550

Source:

Mo Saif, Food Animal Health Research Program
saif.1@osu.edu
(330) 263-3743

Mike Lilburn, Animal Sciences
lilburn.1@osu.edu
(330) 263-3992

WOOSTER, Ohio — As avian influenza spills outside eastern Asia into Europe, Africa, the Middle East and India, the viral disease many fear will cause the world’s next pandemic is becoming a recurrent topic in news shows and family conversations. It has even struck an entrepreneurial cord, as some companies are now selling “bird flu prevention kits” over the Internet.

But how much should Ohioans worry? Mo Saif, an expert on poultry diseases and chair of the Food Animal Health Research Program (FAHRP) on the Ohio Agricultural Research and Development Center’s (OARDC) Wooster campus, said the first thing people should do is understand what the media or public officials exactly mean when they say “bird flu,” “avian influenza,” “H5N1” or any of various terms that are being tossed around in the evening news or presidential speeches.

“Avian influenza is a very complex disease, involving many virus strains that can cause different levels of illness in animals or people,” said Saif, also a poultry-disease specialist with Ohio State University Extension. “That’s why it’s very important to know how avian influenza viruses work and which virus strains are involved in a particular outbreak.”

So what is avian flu anyway?

Avian flu is an infection in birds caused by avian influenza viruses. First identified in Italy more than 100 years ago, this infection is common among wild birds and poultry and occurs worldwide. Wild birds, especially waterfowl, act as hosts for avian flu by carrying the virus in their intestines and shedding it. Even when infected, some birds do not show any disease symptoms.

The viruses responsible for avian flu are classified as type A influenza viruses; other influenza virus types are B and C. Influenza A viruses are no strangers to humans — the flu epidemics that take place almost every winter in the United States and kill an average of 36,000 people, for example, are caused mostly by type A viruses, with type B viruses sometimes being the culprit. Influenza type C infections cause only mild respiratory illness in people.

So far, so good. But things get a little more complicated, Saif said. That’s because there are 144 subtypes of avian influenza — and even though all of them can infect poultry, they are not created equal. These subtypes result from the combination of two proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase (NA). There are 16 known HA subtypes and nine known NA subtypes. An “H7N2 virus,” for example, designates an influenza A subtype that has an HA 7 protein and an NA 2 protein.

“HA is a molecule that allows the virus to attach to cells of the respiratory tract, while NA is an enzyme that enables the virus to leave infected cells in search of new cells to attach to,” Saif explained. “Only H5 and H7 subtypes can cause severe disease in birds, wild or domestic. But even within H5 and H7 viruses there are variations in how much they can sicken a bird.”

Which is why there’s a third way we can classify avian influenza viruses: according to their disease-causing abilities — low pathogenic or highly pathogenic. Most avian flu viruses are low pathogenic, typically associated with mild disease in poultry. On the other hand, highly pathogenic strains of the virus (H5 and H7) can cause severe illness and high mortality in domestic birds. But since change is the word when it comes to influenza viruses, low-pathogenic H5 or H7 viruses have been known to mutate into highly pathogenic forms.

Outbreaks of both low-pathogenic and highly pathogenic H5 and H7 avian flu viruses have been reported and controlled in the United States in the past few years, both in poultry and in humans. In 2003, an individual infected with an H7N2 virus and showing serious respiratory symptoms was admitted to a New York hospital. In 2004, a highly pathogenic H5N2 virus caused an avian flu outbreak in poultry in Texas, and a low-pathogenic H5N2 virus was found in poultry in Delaware, New Jersey and Maryland.

But what about the H5N1 virus?

First identified in Hong Kong in 1997, the H5N1 virus is a highly pathogenic strain of avian influenza that has proven deadly to wild birds and poultry. For reasons that still remain a riddle to scientists, this virus managed to jump into humans, thus far infecting and/or killing dozens of people in southeast Asia, China, Turkey and Iraq — all of whom were in close contact with infected chickens.

But H5N1 is not the only avian flu virus that has claimed human lives in recent years. In 2003, in The Netherlands, an H7N7 virus infected 83 people, one of whom died.

However, the H5N1 virus is the one making headlines because of its aggresiveness, the number of birds and humans it has infected, and the fear that it could mutate into a form that spreads from human to human — as deadly as the Asian strain and as contagious as the U.S. winter flu viruses. Such combination could result in a pandemic.

“Viruses are constantly replicating, and every time they replicate there’s a change,” Saif said. “Any species, humans included, could harbor influenza viruses that came from other species. If a person is harboring a highly contagious human influenza virus and the H5N1 virus at the same time, there’s a possibility that those two viruses could recombine into a new virus that possesses the characteristics of both original viruses. In order for that to happen, both viruses would have to infect the same cell — and the mixing of the viruses would have to occur in that very cell. I can’t tell you how often that happens. But it could happen.”

Avian flu in Ohio?

Yes, there have been cases of avian flu in Ohio. But not H5N1. Not even a highly pathogenic subtype. Recent cases include a mild H3N2 virus that infected turkeys on a western Ohio farm in 2004, resulting in the slaughter of some 12,000 gobblers to contain the spread. Avian flu is a constant threat to the state’s $3.3 billion poultry industry, but fears of a pandemic are not likely to impact the already strict biosecurity measures producers take to keep infectious diseases away, said Mike Lilburn, an OARDC scientist and OSU Extension poultry-nutrition specialist.

“Most commercial operations today are taking blood samples on a regular basis for avian influenza testing conducted by the Ohio Department of Agriculture,” said Lilburn, a professor in the Department of Animal Sciences. “The level of biosecurity is the same today as it has always been, which is pretty tight. The distinction between virus types will dictate the response to a viral confirmation rather than anything on the prevention side.”

For Lilburn, continuous monitoring — including migratory birds, backyard flocks and birds sold state-to-state in live markets — is our best shot at keeping highly pathogenic forms of avian influenza out of the country and Ohio.

OARDC and OSU Extension are part of Ohio State’s College of Food, Agricultural, and Environmental Sciences.

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