What’s up with the flu? Novel H1N1? SWINE FLU??? Mexican flu? swine-origin influenza A? A(H1N1)? S-OIV? North American flu? California flu? Schweingrippe ? la epidemia?

“Human social ecology” includes population density, where we live, the way we travel, and the way we produce our food. Changes in any of these factors can alter interactions among human populations, domestic animal populations, and wild animal populations. These interactions, in turn, affect the way pathogens evolve, get transmitted, and make us sick.

Influenza A virus (electron micrograph) Influenza A virus (electron micrograph)

Surface Proteins

Hemagglutinin Binds to receptors on host cell surfaces, “tricking” those cells into taking the virus inside. Hemagglutinin Binds to receptors on host cell surfaces, “tricking” those cells into taking the virus inside.

Neuraminidase helps newly-formed virus particles leave the host cell. Neuraminidase helps newly-formed virus particles leave the host cell.

Viral genome: 10 genes carried on 8 separate strands of RNA.

How do flu viruses make us sick?

These forms of H (16 known)and N(9 known) are assigned numbers (H1-H16; N1-N9) used in naming flu strains. “Bird flu” = H5N “swine flu” = H1N1 flu strains evolve different forms of Hemagglutinin & Neuraminidase.

Now the BIG question: Why are some flu strains basically just nuisances, while others turn out to be deadly? There are two main factors: 1. How easily a flu strain is transmitted from person to person. (more on this shortly) 2. How rapidly our immune system can recognize and respond to a particular viral strain.

The immune system recognizes pathogens by recognizing and responding to antigens.

Usually, evolutionary changes in viral antigens are small. The illness these “antigenic drift” mutants produce is annoying, but not usually life-threatening. But sometimes, a major evolutionary change in antigens occurs. These “antigenic shifts” can produce lethal, global epidemics. The most serious occurred during World War I; It killed 675,00 Americans and between 20 and 50 million people worldwide. That’s more than all the wars of the 20th century put together.

What happened in 1918? Can it happen again? The first part of the answer relates to the fact that influenza A can infect a range different animals species.

ZOONOSIS: A disease which is naturally transmissible from animals to humans. ZOONOSIS: A disease which is naturally transmissible from animals to humans. Can you think of any zoonotic diseases in the news in recent years? (Mad Cow, Influenza, West Nile Virus, Lyme Disease, SARS, Ebola)

Why is this zoonosis so important? Because although all influenza strains belong to the same viral “species,” some populations are reproductively isolated from each other in different hosts. This isolation means that not all strains have the same kind of genetic variation. And that variation can spell trouble for the human immune system.

Genetic variation that evolves in other host species is important to humans because of a phenomenon we could call... “viral sex.”

Viral strain 1 Viral strain 2 Host Cell Two viral strains infect the same host cell at the same time

Viral strain 1 Viral strain 2 Host Cell RNA from both strains is injected into the host cell and replicated

As new viral particles are assembled, RNA strands from the two strains can sort independently, forming new combinations of genes

Some newly-formed viruses that leave the host cell carry new combinations of genes from both parent strains

As the disease established itself and spread across Asia, the CDC and WHO stepped up efforts to understand how it was transmitted - among birds and from birds to people.

Akl

Strains of Avian H5N1 we know about so far have: High virulence -they cause serious illness and can be lethal)... but... Low human-to-human transmissibility - they do not spread easily from person to person.

Strains of Swine H1N1 that we know about so far have: Low virulence - they rarely cause serious illness and are rarely lethal, but... High human-to-human transmissibility - they spread easily from person to person).

The H1 variant carried by this new flu is different from “garden variety” H1 found in widely circulating human strains... so the immune systems of most people don’t recognize it quickly. That fact, combined with high transmissibility, explains why so many people are becoming infected.

So... what are public health officials so worried about?

Certain evolving strains of H5N1 avian flu strains have accumulated 5 of an estimated 10 mutations related to human- human infectivity. High virulence, and high human-human transmissibility appear to be dependent on a small number of changes in the flu genome.

Combine “viral sex” with viral strains that have evolved in different parts of the zoonosis... and we could be in serious trouble.

The best way to deal with this mix of knowns and unknowns is by: Distinguishing between scientific reports, news reports, and sensationalistic reporting. Remembering that that science rarely “proves” anything; scientists aim for the best approximation of reality that our methods can reveal. Uncertainty is part of the process. Suggesting that, despite uncertainty, science can make prudent recommendations

Teach about infectious disease as if all our lives depend on it … because they do! So let your enthusiasm and imagination take wing!