Monthly Archives: September 2012

Epidemics

An epidemic of fear

7 Comments

How does fear of epidemic disease influence the spread of disease?  In the movie “Contagion” we see many examples of how individuals and governments respond to an epidemic.  There is panic, people fleeing cities, people isolating themselves in their homes, governments closing borders and imposing quarantines.   The tagline of the movie, “nothing spreads like fear” may actually be quite accurate.

Using mathematical models to understand the spread of disease is a common tool in epidimiology.  At its most basic, we can think about the spread of a disease through a population by considering that an individual can be susceptible to the disease, infected, or  removed.  Someone who is susceptible can become infected.  Someone who is infected can spread the disease to others who are susceptible.  Infected people can be “removed” from the population by either recovering and becoming immune, or dying.  Either way, they are no longer in the susceptible or infected categories.   These S-I-R models can become quite complicated when you begin to consider all the possible variations:  how easily the disease is transmitted, how long someone is contagious, whether transmission is dependent on direct contact, etc.  These models don’t have to be limited to study the spread of infectious disease: indeed, one could consider fear to be contagious.

So what happens to the spread of disease during an epidemic if we add normal human behavior into the SIR model? If we consider fear to be contagious as well, during an epidemic there there would be two things spreading through the population: fear and disease.  And when we fear disease we change our behaviour.  In a recent paper, the simultaneous spread of fear and disease were modeled using an SIR model.  It turns out that, as the tagline for “Contagion” suggests, fear spreads more quickly than disease.  In their model, someone susceptible to fear can become “infected” with fear either through contact with someone with the disease, someone who has fear but no disease, or someone who has both fear and disease.  Since you can only get infected with the disease by contact with someone else who has the disease, there are more ways to catch fear than disease, and it spreads faster.

So once someone is infected with fear, what do they do?  What would you do? My instinct would be to hide at home and remain isolated; others I have spoken to would flee to a small town in the middle of nowhere.   The researchers considered that too.  How will the spread of the disease change if some people run and some hide and others don’t do anything?  It turns out it is best to hide.  Hiding has the greatest impact on halting the spread of the disease: not only do fewer people become infected, but the disease doesn’t spread as far geographically.  Fleeing reduces the number of people infected (compared to not doing anything) but spreads the disease further.

The model demonstrated another important point.  Over time, our fear decays.  For whatever reasons, after hiding for a while, we may feel more confident about wandering out into the now hopefully less plague ridden world.  Bad idea.  As long as there are still some people infected with the actual disease, re-entering the world adds fuel to the epidemic by introducing more susceptibles.  This causes a second wave of the epidemic.  One could speculate that this could have been the cause of the two waves of influenza in 1918 – people came out of quarantine too early.

As the number of infected people increase (red), more susceptible people (blue) will hide (black). That causes less people to become infected, people become less afraid and leave quarantine re-entering the population and causing a second wave of disease. From Figure 3 of Epstein et la 2008.

We had a terrific discussion about this paper in class and we identified several reasons why we though that fear would actually spread even faster than the paper suggested, and that the effects of fear (fleeing or hiding) on spread of the disease might be greater.  Obviously, fear can spread without direct contact.  You can get infected with fear pretty easily by reading things on the internet or watching the news.  Fear of a disease could leap across the globe quickly, well ahead of the disease itself.  Also, people don’t behave independently.   So if one member of a family becomes afraid and wants to flee or hide, the rest of the family may do so as well.  As people flee from cities, its likely that choke-points and crowds would further fuel the epidemic.

So in the event of a massive, lethal epidemic, would the politicians and army have the willpower to impose a long enough population-wide quarantine and prevent people from fleeing an infected city?  Would President Obama/Romney say “No really, everyone must stay at home, see there’s this paper in PLoS ONE by Epstein et al…”

Reference:

Epstein, Parker, Cummings and Hammond. (2008). Coupled Contagion Dynamics of Fear and Disease: Mathematical and Computational Explorations.  PLos ONE, 3(12): e3955

Share
Evolution, Immunology, Pathogenesis

Fever!

6 Comments

Most viral infections start out with the same general symptoms: fever, malaise, aches. Those are usually the sign of your immune system starting to fight back. Fever is one of our defense mechanisms, and while it can be quite uncomfortable it is very rarely dangerous. One of the common questions I get asked in my classes is whether it is good or bad to take fever reducing medicines when you are sick. Well, Im not that kind of doctor so I can’t really answer that question, but I can tell you what research has been done on the benefits of fever.

The adaptive value of Saturday Night Fever, caused by listening to Disco, remains unknown

Presumably, by raising your body temperature, you can do a better job of fighting off an infection. The higher temperature may make it more difficult for the pathogen to grow because it grows optimally only at normal body temperature, or it may help the immune system work faster (or both). Some studies in animals have shown that reducing fever results in increased growth of bacteria or virus in the infected animal, and other studies have found increased proliferation, migration and activity of immune cells.

Whatever the mechanism, it is clear that fever must provide some advantage. There are many studies that demonstrate that fever is beneficial in overcoming infection. None of these studies alone is definitive, however taken together, they do seem to support a role for fever in fighting infection. For one, the febrile response is highly conserved in vertebrates (even “cold blooded” animals) and many invertebrates. Some lizards, for example, will seek warmer spots to rest and as a result, raise their body temperature when infected. Fever is also energetically very expensive, requiring 20% more energy to maintain a temperature even a few degrees above normal. It would be unlikely for such an expensive mechanism to be maintained by natural selection if it didn’t have some benefit.

In addition to the evolutionary perspective, several studies in animals show that a fever of a few degrees correlates with better survival rate from infection. Being correlations, we must be cautious in over-interpreting this. Another good way to test if something like fever is useful is to get rid of it. Infected animals can be treated with anti-pyretic (fever reducing) drugs to see what happens to their recovery in the absence of fever. These studies typically show that animals treated with anti-pyretic drugs take longer to recover, and in some cases even to increased mortality. There are some problems with anti-pyretic studies however, and one of the major problems is that many anti-pyretic drugs don’t just reduce fevers. They can have other effects on the body, not all of which are known, and so we can’t always be certain that fever reduction is the reason for the changes in morbidity and mortality.

Of course, I also like to look at the pathogens themselves for hints of what our immune system is doing. They are pretty good at defending themselves against our immune response, so if we look at their defenses we can learn more about how we attack them. The poxvirus Vaccinia encodes a protein that blocks fever. This protein interferes with the function of interleukin-1B, a component of our own immune system that regulates fever. Animals infected with Vaccinia lacking that protein develop fever, showing that when the viral protein is present, the virus can prevent fever. However, interleukin-1B does many other things too, not just regulation of fever, so it is possible that the virus is blocking interleukin-1B for a different reason.

So it is highly likely that fever is good for fighting off infections, but this is not to be taken as medical advice to avoid fever reducing medicines. In the case of naturally occurring infections in humans, we need much more research into fevers resulting from specific infections to decide when a fever is beneficial and should be left alone, whether the fever is dangerous, or if the fever is helpful but the risks of taking an anti-pyretic are worth alleviating the uncomfortable symptoms.

References:
Kluger, M.J. (1996) The adaptive value of fever. Infectious disease clinics of north america. 1(10):1-20.
Alcami, A. and Smith, G.L. (1996) A mechanism for the inhibition of fever by a virus. Proc Natl Acad Sci. 93:11029-11034.

Share