Swine flu – why is a mild pig disease dangerous to humans?
The numbers are rising with every news broadcast, but as I write this, over a hundred people have died in Mexico from Swine Flu. Cases have been confirmed in five states across the USA, in Canada, and in Europe, the first one has been found in Spain. A WHO emergency committee will meet tomorrow, examining the spread of the virus to decide whether to increase the alert for a possible pandemic, or global epidemic, after the UN agency declared the outbreaks in Mexico and the U.S. a "public health emergency of international concern" on Saturday. Countries across the world are increasing border security in an attempt to stem the spread of the virus.
Articles are springing up on the Internet explaining more about this virus, but I'm focusing on the aspect that intrigues me: the fact that a common, mild, pig disease is striking global fear into the human race.
The whole problem with the flu virus lies in its capacity to change. The basic virus blueprint has been around for thousands of years, and although it started off as a bird virus, it has gradually developed into forms that are specific for every species of warm blooded animal on the planet, as well as a few cold blooded ones (even snakes have their own flu virus). The established human flu viruses don't cause too much of a problem to most of us. Many humans have immunity due to previous exposure, and vaccines are available to protect the most vulnerable sectors of society such as elderly people.
Animal flu viruses don't usually present a threat to humans. Most often, the flu virus remains in its host species, and if it does occasionally adapt so that it can infect a human, it isn't able to be passed on from that person to anyone else. A single human infection is the end of the line for that virus. Changes in the flu virus don't happen often enough or fast enough for a virus from one species to cause an epidemic in a different species.
So what's happened that's allowed this pig flu virus to spread so fast amongst humans? It's presumed that it all started with somebody in Mexico who was working closely with pigs. One of the pigs must have had a mild dose of swine flu; coincidentally, the pig worker happened to have a mild human flu at the same time. The pig was then infected with the human flu virus, and the swine flu and the human flu viruses then co-existed in the pig for a short time. The flu viruses did their usual trick – so-called "reassortment"- which is the exchange of genetic material between two different viruses. Usually this just happens between two flu viruses of the same type, making them a little more tricky for the host immune system to tackle. But when two entirely different viruses reassort in one animal, the result is a new hybrid virus with properties of both swine flu and human flu.
In many cases, this "new" virus would just stop there, dwindling away to nothing. In this instance, the new hybrid virus must have been passed back to the pig worker. Again, in other similar scenarios, it could have stopped there, with just one human casualty. But it seems that this particular swine flu virus has the capacity not just to infect one human, but to spread rapidly from human to human. And because it's a new virus, made up of a unique new combination of genetic material, no humans have immunity to it. The health authorities haven't yet had time to prepare a vaccine, so the virus is spreading very rapidly amongst the human population.
The global spread of the virus is happening because we've become such a mobile species – it's reckoned that there are something like 500,000 people airborne around planet Earth at any one time. This means the potential for the virus to spread widely and quickly is very high.
The virus isn't as fearsome as some viruses – like rabies, which kills every human who develop signs of illness. Swine flu has a high morbidity (i.e. if infects a high proportion of the people in an area) but a low mortality (less than 5%). It may only kill one in twenty people who are infected, but as you look around your colleagues and friends, you'll realise that it's a significant mortality rate when the victims are part of your own circle.
Viruses are traditionally much more of a challenge to doctors than bacteria, being unaffected by antibiotics, but in recent years, powerful anti-viral drugs have been developed. The ever-adaptable flu viruses do have the ability to develop resistance to some of these new drugs, but the word is that this latest virus is susceptible to oseltamivir ("Tamiflu") as well as to zanamivir ("Relenza"). These anti-viral drugs act on metabolic pathways to prevent new viral particles being released by infected human cells, so stopping the spread of the virus in its tracks, and preventing it from spreading to other people.
Just three months ago, the UK government put in orders with Roche and Glaxo-Smith-Kline, doubling their stockpile of these antiviral drugs, and it's reported that the UK is amongst the best prepared countries in the world if this does become the pandemic that people fear.
The Swine Flu virus, in its harmless, pig-only form, is seen in all pig producing countries, including the UK. The current episode happened to start in Mexico, but it could have happened in any place where a human with flu was in contact with a pig with flu. This is a good example of the importance of efficient state agencies in a range of areas, from disease monitoring in the animal population (as well as in humans), to the dishing out of anti-viral drugs when they're needed. Without this type of coordinated country-wide (and planet-wide) action, we'd all be in a much worse place.
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