Heard anything about the flu virus? As the mass media gets you ready for bedlam and hysteria, here’s a quick primer on what you need to know.
There are literally thousands of different virus (some you’ve probably heard of before such as herpes, Norwalk, HIV, measles, rhinovirus, ebola), and they come in various shapes and sizes. Viruses infect different tissues (respiratory tract, gastrointestinal tract, skin cells), and have many different properties. I’ll try to cover some general virus background and then focus on the Influenza virus A and B, which fall under the classification of orthomyxoviruses.
First, to put it into perspective, a virus can be about 100 times smaller than a single bacterial cell, so we’re talking tinier than tiny. Viruses are actually submicroscopic.
Next, viruses are unlike any other living things because they are not living (at least not by commonly accepted standards). Viruses are simply strings of proteins that are designed to hijack living cells—the ONLY way viruses can be active is if they invade other living cells (be that of a plant, an animal or bacteria). Viruses contain either DNA or RNA, (yep, genetic material), a protein covering, and some are also enclosed in a layer of lipoproteins called an envelope. The envelope is somewhat similar to the lipid bilayer membrane that encloses human cells. In fact, it’s that similarity that allows an enveloped virus to fuse to a human cell to initiate the hijacking process. Influenza A and B are both enveloped RNA viruses, which does mean they do have some ease in infecting human cells, but that envelope is also fairly fragile—good cleaning practices should be able to “kill” the virus.
The envelope also helps in the identification of the different influenza strains as they have special “spikes” (surface glycoproteins) called hemagglutin and neuraminidase. Hemagglutin binds to the surface of human cells to start the infection process, while neuraminidase acts as an enzyme to break down the protective layer of mucus we have in our respiratory tract (decreasing our defenses), and it breaks open the surface of the infected cell after the virus has replicated to help it spread to other cells. When you hear flu strains described as H1N1 or H5N1, that’s classifying the virus on properties of the hemagglutin and neuraminidase.
Properties of the RNA allow for classifying the virus as Influenza A or B. Influenza A has different strains that infect other animals including birds, swine and horses. Influenza B is primarily a human virus. The ability of a virus to infect certain species is based on its hemagglutin and the blueprint to produce different types of hemagglutin is carried in the RNA.
Every time a flu virus replicates, it has the opportunity to mutate. Some of these mutations and changes are small—called antigenic drifts, and some are major—called antigenic shifts.
The scary mutations—one that we may be seeing now—are when RNA segments of different species’ strains combine to form a new virus that human cells may have never faced before because the virus previously lacked the ability to infiltrate human cells.
Antibodies (the body’s good guys) against hemagglutin, prevent infection (remember, thats what helps the virus envelope bind to human cells) and antibodies against neuraminidase help prevent the spread of the infection from cell to cell (remember, neuraminidase helps the virus break out of the infected cell after it has replicated).
Flu vaccines are designed on what’s predicted as the upcoming strain, based on epidemiologic studies of worldwide infections. The keyword here is “predicted.” Vaccine makers don’t know exactly what strain will be prevalent, and mutations can (and do) occur faster than vaccines can be formulated, produced in mass quantities, distributed world-wide and administered to billions of people.
Transmission is typically by inhaling respiratory droplets from an infected person who has coughed or sneezed. If infected with a regular Influenza A strain, symptoms usually appears within 24-48 hours and include sore throat, cough, headache, fevers, and significant muscle pain. Gastrointestinal symptoms are rare for Influenza A and B infections. A flu infection may be hard to differentiate from other types of infections (bacterial pneumonia for example), but pneumonia is diagnosed with specific findings on chest X-rays, and the flu symptoms typically hit hard and fast, especially the fever and muscle aches. Viral swabs are available for diagnostic testing however.
As with most viral infections, treatment usually is supportive, meaning targeted at the symptoms (treating pain and coughing, etc), because there is no way to CURE the flu.
Zanamavir (Relenza) and oseltamivir (Tamiflu) are in a class of medications called neuraminidase inhibitors. They don’t work on the viruses themselves, but they prevent further spread of the virus from cell to cell (only within the infected person). These neuraminidase inhibitors are reportedly effective against both influenza A and B, and while not technically curative, they reduce the duration of symptoms for a couple of days but only if treatment is started within 48hrs of onset of symptoms.
Wikipedia has a great, up to date, page on the current situation with the emerging swine flu: http://en.wikipedia.org/wiki/2009_swine_flu_outbreak
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The bottom line is that infection is preventable with simple, basic hygeine. Wash your hands and avoid close contact with sick people. I can do the first thing... but not the second!