Regular readers know that I like to touch back to the published research when we talk about a topic. Obviously swine flu is brand new, and it would be pretty darn fast to get good research out on the subject. Luckily there is at least one very recent article that is very relevant.
The CDC releases a weekly morbidity and mortality report online, they can be found here. For some reason the researchblogging.org citation isn’t quite working, so the article we’re discussing is found here, and an update to it found here.
So what do the articles say? Sadly they don’t say a lot. The initial article is a pair of case studies in southern California, the update includes 6 more cases – four from California and two from Texas, and mentions the outbreak in Mexico. In the two initial cases in the report, the same virus appears to have been isolated – it is an H1N1 strain of influenza. The CDC has further described this as a swine flu virus. The new cases in the update were still in the process of typing as of the publication if this article, but preliminarily it looks like the same virus.
In all cases, the patients have not been exposed to pigs. It looks more like the disease spread from person to person. As I said previously, you don’t get swine flu from eating pork. You get it from being exposed to a person with swine flu. If you work closely with pigs, it’s entirely possible you could get swine flu from a sick, life pig.
The viruses they found were resistant to amantadine and rimantadine – two common antiviral medications. However the viruses were sensitive to zanamavir and oseltamivir – better known as Relenza, and Tamiflu respectively. This is great news. The US has pretty large stockpiles of zanamavir and oseltamivir, and the government is already deploying these for use. So if this does end up being a pretty large breakout, or even a pandemic, we are in good shape treatment-wise.
That’s about all that’s covered in the article, I’d like to use that as a jumping off point for some relevant education on influenza in general, and this outbreak in particular. It’ll be in multiple parts, but today we’ll just discuss some basics – including why public health officials are so trigger happy about influenza outbreaks.
We talked about H1N1 earlier, what does that mean? Well there are two types of influenza: influenza A and influenza B. We won’t be discussing influenza B today, the virus we’re worrying about with this outbreak is a subtype of influenza A virus. Influenza A viruses are divided up into subtypes based on two surface proteins – hemagglutinin, and neuraminidase. These proteins are numbered, and different subtypes can infect sets of different species. The species that most commonly infect people are H1N1, and H3N2. The much feared Avian flu is a variety of H5N1.
Now that doesn’t mean that one H1N1 is the same as every other H1N1. Influenza can be further broken down into different strains. The influenza vaccine this year contains a H1N1 strain, but not the same strain that is in this outbreak.
This is where it gets complicated. The influenza virus evolves over time, to evade our immune systems. It can do this in two big ways. One is called antigenic drift, and the other is called antigenic shift.
Antigenic drift is what we normally think of when we think of evolution. Over time, as the virus reproduces, mutations occur and as they occur, those proteins – hemagglutinin and neuraminidase – change too. Eventually, the right mutations occur and the proteins are different enough to look “new” to our immune system, to not be the same virus to our immune system. The thing is, since they just mutated from a virus we’re familiar with, they’re still somewhat familiar to us, not completely different.
Antigenic shift is far scarier. Each gene in influenza’s genome is in it’s own separate chunk of DNA. If two or more different influenza viruses infect the same cell, their DNA chunks can mix together, and the virus can pops out can be completely new. For example it could have some primarily human infecting DNA, and some primarily pig infecting DNA. A virus like that could be completely new to our immune system, and that can be scary.
The balance of evidence says that this swine flu outbreak results from an antigenic shift event. That’s why people are freaking out. That’s one of the reasons that the CDC was pretty quick to declare this a public health emergency. We don’t know yet if this is or even if it could be a pandemic, but we’re prepping in case it is.
It’s worth noting that as far as we can tell so far, the American cases are very different from the Mexican cases. The American cases appear to be relatively self limited –people don’t seem to be getting so sick that they die. Many of the Mexican cases appear to be resulting in hospitalizations and death. This is despite the fact that they seem to be the same virus, genetically we are seeing different things. We do not why that is. If I see credible information on the reason, I’ll be sure to post it here in followup.
So, is this swine flu outbreak the new 1918 flu? Or is it a mere hiccup, not even worth worrying about in the grand scheme of things? The unsatisfying real answer is that it’s too early to tell. It would be like watching a college basketball game, and after 5 minutes saying “aha, this is the next Magic vs Bird in NCAA finals!”
In the mean time, do your part: wash your hands, and if you or your child is sick, take the day off. If you’d like more posts like this, or like my initial swine flu post, check back here, I’ll be trying to post at least once a day.
Centers for Disease Control and Prevention (CDC). (2009). Swine Influenza A (H1N1) infection in two children–Southern California, March-April 2009. MMWR Morb Mortal Wkly Rep., 58 (15), 400-402 DOI: 19390508