


Jul 01, 2006

Avian influenza continues its flight path around the world. The H5N1 strain has been the culprit to 200 human infections and counting.

With such notoriety, are you prepared to answer questions from clients regarding this disease du jour?

This article is intended to offer a kind of primer about avian influenza — the ABC's of the bird flu. I've thrown in a few other letters to help address the questions you will likely receive from clients.

A = Influenza A

The influenza viruses are divided into three large categories — A, B and C, based on two proteins found in their inner cores. Influenza B and C viruses infect humans only. The influenza A viruses, which can be found in numerous species, are further divided according to the surface characteristics, in particular two different molecules, called hemagglutinin (16 different kinds of this molecule exist) and neuraminidase (nine different kinds of this molecule exist). As a result, every influenza A virus has a designation H#N#. The bad human influenza viruses are all of the H1, H2 and H3 varieties. To date, all serious avian influenza viruses are H5 or H7. Influenza A viruses are supposed to be restricted to their own species, so, in 1997, when a boy in Hong Kong died of influenza A and the authorities discovered that it was an H5 virus, public health communities around the world sat up and took notice.

B = Birds

As the name indicates, avian influenza (AI) is a disease of birds! Avian influenza viruses are widely distributed in nature. Wild waterfowl, in particular, harbor many AI viruses because the intestinal epithelium of wild waterfowl has receptors that welcome AI viruses and help them proliferate. Even though the AI viruses are replicating quite happily in the guts, the water birds don't usually get sick. The infected birds continue to move around and as they fly overhead, spread plenty of virus their manner of indiscrete defecation. When avian influenza viruses infect poultry, they can cause disease and if the disease is severe, it is referred to as "highly pathogenic avian influenza" (HPAI).

C = Crossing species boundary

In the last nine years, there have been numerous, but sporadic, instances of HPAI viruses causing sickness and sometimes death in humans. The H5N1 strain has proven to be particularly worrisome, with more than 200 human infections, each of which has been associated with contact with poultry. Why is it that bird flu spreads readily bird to bird and human flu spreads readily human to human, but bird flu does not spread readily to humans?

The receptor that allows the avian virus entry into people is present only in the deepest recesses of the human's pulmonary alveoli. This explains why the number of humans infected to date with H5N1 is relatively low, as successful infection requires that virus make its way deep into the respiratory tract to the alveolar structures. It also explains why the virus is not spread readily from human to human. It could happen, but it would require a massive paroxysmal sneeze or productive coughing from a heavily infected person followed by deep inspirations from nearby susceptible recipients.

We have seen some clusters of human infection, each with attendant concerns that the virus has mutated to allow easier person-to-person spread, but in each instance, the infections have been due to very close contact without any pandemic-presaging mutations occurring.

In addition to the relatively small number of people infected with the avian influenza H5N1 virus, there have also been documented infections in large cats, domestic cats, a dog and a stone marten. All have contracted the disease from infected birds.

D is for disease

In poultry, morbidity and mortality with HPAI are both very high. Once within the body, the virus replicates in endothelium in many organs, often spilling over to affect parenchymal cells as well. Death occurs due to multi-organ failure within two to three days of infection. Post-mortem lesions include edema in the subcutaneous tissues anywhere, but especially neck, and foci of hemorrhage in many locations, most prominently trachea and shanks.