Sunday, April 26, 2009

Of Swine, Birds, and Humans.

During the past two days, I have read many confused statements regarding the recent swine flu outbreak, and decided to write an educational post rather than my typical emotional rant. In this post, I will attempt to shed some clarity on the subject of influenza, from the perspective of a physician.

The swine flu is a respiratory infection in swine which can occasionally infect humans. Similarly, the bird flu is a respiratory infection commonly seen in birds, which can also occasionally infect other animals, including humans.

How can one catch the flu?

The flu is caused by a number of genetically related viruses, and they gain access to our bodies through special cells located in the back of the nose. These viruses have specific proteins on their surface that allow them to attach to these cells and once inside, they take over the cellular machinery to manufacture many more viruses. These proteins need to be specific enough to be able to attach to these cells, otherwise the attachment is not strong enough to allow entry, and consequently no infection can occur. You can think of these viral surface proteins as keys and these nasal cells have doorlocks that these keys can fit. This specificity explains why we are not susceptible to most viruses that infect other species. To reiterate, the only entrance for these flu viruses is through the cells located in the back of the nose. That means you can only catch the flu if someone coughs on you or if you touch a contaminated object, like a door knob or a hand, and then rub your nose. You cannot get swine flu from eating pork.

Flu symptoms.

Once your body's cells are infected, they start producing these offensive foreigners and spreading them throughout the rest of your body. These troublemaking cells will eventually become recognized by your immune system, and consequently destroyed by your own white blood cells. As this battle within your body rages, you develop inflammation at the affected site, and accumulate lots of fluid there. For this reason, the most common symptoms of the flu are a stuffy nose and a mucus discharge from the nose. The best place for this mucus to go, is to be expelled outside of your body. Otherwise, it can cause a lot more trouble as it reaches other body organs and systems.

Headache results from stuffed sinuses, but it can also be due to other effects of the flu virus on the body. Whenever there is significant inflammation or activation of the immune machinery, fever may occur and some develop a headache directly due to this fever. Rarely, influenza inspired immune response may result in inflammation of the brain meninges. These are covering tissues of our brain which lie near blood vessels and sensory nerves. Inflammation of these specific structures is known to cause severe headaches, and is given the diagnosis of meningitis.

Other common symptoms of the flu include cough, which usually results from this inflammatory mucus draining down the back of your throat. The contents of this mucus include viruses, inflammatory cells, and dead respiratory cell debris. This fluid irritates any tissue it comes in contact with, and consequently we cough and our throat feels sore. There are specialized cells in our respiratory passages that sweep such junk upwards and prevent all this nasty fluid from reaching our lungs. Cough aids this cleaning process and it is a normal mechanism of keeping our lungs free of unwanted fluid. The lungs are technically air passages that branch into smaller and smaller tubes, which eventually lead to small sacks where we exchange oxygen for carbon dioxide. It is very difficult to accomplish this task of gas exchange, in lungs filled with fluid. If this fluid is accumulating in your lungs, you might feel shortness of breath. People who have damaged lungs from many years of smoking, have a very difficult time with this aspect of flu and they may need to be hospitalized.

Our mouth and nasal passages meet posteriorly in the same cavity, in the back of our throat, called the pharynx. From there, two different pathways arise. One leads further into the respiratory system and the lungs. The other, leads into our digestive tract. As this inflammatory exudate, from infected nasal cells, enters our stomach and intestines, symptoms such as nausea, abdominal discomfort, and even diarrhea arise. Inability to keep your food and fluid down, causes dehydration and this is the most common reason for needing to be hospitalized.

As your body is battling the flu virus, you will often feel tired, and many people develop joint aches. The reason for joint aches has to do with your immunity system activity, but to explain that adequately would require a very long discussion.

This above scheme is somewhat simplified as these viruses can also infect cells in many other parts of the body, once they establish the beach-head in your nose. Respiratory cells may become infected, and some people even develop a viral pneumonia. Intestinal cells are also susceptible to penetration by these virii, which may explain why some have primarily gastrointestinal symptoms. Rarely the liver can become infected causing symptoms of hepatitis. However, for a general understanding of how the flu virus causes its damage and gives rise to common flu symptoms, the above system is very useful.

Swine flu outbreak.

Now back to the current swine flu outbreak. This virus does seem to have the ability to infect humans but it also seems to be better at infecting swine. That is likely because its attachment surface proteins are better fitted for swine nasal cells, than for human ones. Also, it may not be produced in large enough numbers within a human host, to successfully infect other humans. There will be rare exceptions as everyone has slightly different susceptibilities to these foreign invaders, and that may account for the few documented human infections. To appreciate this comparison, you need to contrast the 20 US documented cases of this swine flu, with a number that accounts for 5-15% of the US population. That is how many Americans become infected with a different, more human prone, flu virus each year. These other outbreaks infect millions, cause an estimated 200,000 hospitalizations, and 40,000 deaths each year within the US. That said, there is a very real fear that this swine flu virus may mutate its proteins enough to gain the ability of direct human-human transmission, and then we will see widespread human infection. So far the CDC has not indicated that this is actually taking place.

Is this swine flu more deadly that other seasonal flu viruses? Too little is known about this outbreak at this time, to make such an assessment. The archaeologist who was in contact with visiting Obama, is reported of dying from pneumonia complications. Although, it is not clear what exactly caused the recently reported deaths in Mexico, if they were flu-related, I suspect that a secondary bacterial pneumonia infection was the real culprit in most cases.

The 1918 Spanish Flu Pandemic.

Speaking of pneumonia, I want to reflect on the infamous 1918 Spanish Flu pandemic. This virus was also a swine flu type virus, and it led to widespread human-human transmission. It is estimated to have killed 20-100 million world-wide and 500-675 thousand in the US. It typically killed about 5% of those infected, while this current outbreak in Mexico may have a kill rate of 2.5%. Yearly flu epidemics in America might lead to death 0.1% of the time. So far, this flu outbreak within the US, has a kill rate of 0%.

Why was the Spanish flu of 1918 so deadly? One should consider that the world had just emerged from a world war, many were starving and in poor health even prior to contracting the flu. When pathologists examined lung samples of those 1918 flu victims, most deaths were attributed to secondary bacterial pneumonia. That particular flu strain might have been very effective at destroying the respiratory epithelium, and interfering with the self cleaning function of our airways I described above. This way, the flu victims became very susceptible to bacterial infections. Although such pneumonia is now easily treated with antibiotics, these drugs were not available at that time. Penicillin was only discovered in 1928.

Why is it so much milder in the US than in Mexico?

First of all, no one really knows what the Mexicans in news reports actually died from. There is a great deal of confusion regarding this data. If you assume that in Mexico people actually died from this swine flu, while in the US swine flu patients seem to have very mild symptoms, you should consider the difference in the standard of living and the medical care between these two countries. Yearly flu vaccines in the US, may be playing a role here. There are contradicting reports about whether this year's flu vaccinations offer any protection for this particular strain of swine flu, but these vaccinations may be partly responsible for this difference in symptom severity. Furthermore, experts report that even when you don't obtain a diligent yearly flu shot, you may still have some protection from the prior year's vaccination. Another possible explanation is that this flu virus may have developed an improved ability to cause human-human transmission. However, it had to mutate its genetic code to accomplish this transformation. Often when this change happens, the viral ability to cause severe symptoms diminishes. It is sort of like giving up strength in one area, to become stronger in another.


For those who are feeling tired, have a stuffed nose, headache, fever, and an upset stomach, there is still some comforting news. If you really believe that you contracted the dreaded swine flu, you can take an effective medicine. Tamiflu and Relenza should decrease the severity and duration of your symptoms, and the treatment course is only 5 days. If you live with someone who has a flu and you want to minimize your chances of catching it, these medications are also effective at prevention. Aside from taking drugs, you should also get plenty of rest and drink lots of liquids. Obviously if you are having trouble keeping down liquid or feel short of breath, see a doctor. Nevertheless, you have an excellent chance of recovery regardless of taking or not taking any drugs.

Concluding comments.

In summary, even if this flu does become an epidemic in this country, there is little indication that it will be any worse than the other flues that we have suffered this year. Don't despair, and don't forget to get your flu shot next year.

UPDATE (4/29/2009)
I initially miscalculated the mortality rate for the yearly flu, and have since corrected my numbers.  The method I used at arriving at the earlier erroneous numbers was by dividing the number of flu related deaths by the estimated number of people who developed flu symptoms.  I should have then multiplied that number by 100 to obtain the percent mortality. Sorry for the confusion.  Regarding the mortality of this swine flu among the people in Mexico, there is simply no way to accurately assess the situation.   Nevertheless, my earlier statement that there is no indication that this flu will be any worse than other recent flues remains true.  Please see my latest post for details.


GW said...

Thanks for all that Vinny. That puts a lot of this in perspective.

Carolynn said...

Thanks for this! I've been wondering about how the Mexican victims actually died (pneumonia, dehydration?) None of the news outlets seem interested in that distinction.

At least in this post you acknowledge that the pathophysiology of this virus matters...and how much we don't know about Mexican's who've succumbed to this disease.

Anonymous said...

What are the recent news about the swine flu?

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