It is the season for colds and flu in Cusco. But why are colds ‘seasonal?’ What makes the flu, and colds, spread more frequently when the outside temperature is cold? Or is that simply an old-wives’ tale?
Looking for scientific reasons to better understand the factors that lead to viral spread among human populations yields surprising results.
First, at cooler temperatures, ‘viral shedding’ takes place in greater abundance and the viability of the virus increases. Viral shedding is the number of viruses that are given off by the host through breathing, sneezing or coughing. This, however, is only one factor.
In low humidity, the virus tends to stay aloft for a longer period of time – and while it is airborne it has a greater chance of being breathed in by a new host. It is also less stable, and more likely to infect hosts. At 80% relative humidity, the virus spreads at a steady rate – and mostly by actual contact.
Thirdly, the host itself may play an important part in ‘catching’ the virus during the cold season, with the mucosa becoming ‘stickier’ and more likely to attach to the virus. Also, because the virus is more ‘unstable’ or ‘alive,’ it is able to grow quickly in the upper respiratory tract of the host.
Additionally, in the winter season people are more likely to stay indoors, rebreathing air from others, thus further increasing the possibility of ‘catching’ a cold. Many seasonal celebrations bring people together where they can share their viral load.
So, colder temperatures and relative humidity have a great impact on the spread of the flu virus. Why can’t we get a single influenza vaccine and become immune, just like getting a shot for measles prevents a person from contracting the disease? Why are there new flu shots each year?
The trouble with influenza is that the virus mutates between seasons, often using different hosts. Birds can catch the flu, and wild birds have been known to transmit the flu to flocks of domestic turkeys or chickens, often killing many of them. Bird migrations are known to spread the flu around the world. (reference)
The same flu carried by birds can be contracted by humans, and avian flu is one of the deadliest. WebMD claims H5N1 (Avian flu). has killed nearly 60% of the people who have been infected. (reference)
Pigs are also susceptible to flu strains (e.g., Swine flu). But these are not the only animals that can contract the flu, and spread it between themselves and humans. Guinea pigs and ferrets are also capable of catching and transmitting and transmuting the virus to each other and to human hosts.
Guinea pigs are used in the lab for studying the effects of the flu. They are also able to contract human virus strains. When a virus is shared between different hosts, it mutates, and creats a new strain of flu virus. Influenza is tracked by the different hemagglutinin and neuraminidase proteins that it carries in its genetic material. There are 16 types of hemagglutinin and 9 types of neuraminidase. The flu is identified by the shorthand version H5N1 designating what the components are. There are a possible 144 different combinations of the flu virus.
The reassortment of the Hemagglutinin and neuraminidase takes place in the swapping of the virus between hosts, particularly between different mammalian and avian hosts. Where people and birds or animals that are capable of carrying the virus are in close proximity, there are more chances for not only reassortments, but also mutations of the genes to take place. Some mutations are more deadly than others, which is why some flu epidemics are more likely to cause death.
In 2009, the CDC (Center for Disease Control) reported that “More than 100 people have died in Mexico as a result of an outbreak of swine flu, a strain of the influenza virus that normally targets pigs but has occasionally mutated enough to infect and spread in humans. This ability to mutate from one host to the next, or one species to the next, is one of the traits that has given the influenza virus a long life and made it both nearly impossible to eradicate and potentially dangerous to animals and humans alike.” (reference)
The CDC tracks the flu internationally, and warns when a particular strain seems most virulent and thus capable of causing widespread death. The pandemic of 1918, which was most deadly for people between 20 and 40 years old, killed 20% of the known population of the world. (reference)