I’d like to continue the overview of emerging infectious diseases (part one is here) by discussing some reasons why diseases “emerge.” Obviously, this will be somewhat of a simplification; many diseases may emerge due to a combination of the topics mentioned below, or may have factors involved that I don’t mention, so these should be considered broad categories rather than an all-inclusive list.

So, one reason: climate change. An obvious example of this are diseases borne by arthropods, which live in a fairly narrow range of temperatures or environments. Global warming or cooling may extend or decrease the range of such vectors–and as such, the range of the diseases they transmit. (A recent example is the recent study described here suggesting an increase in temperature has extended the range of mosquitoes in the African highlands, though it should be noted that other researchers hadn’t seen this connection). Drought or flooding can also lead to a mixing of populations (both animal and humans), facilitating spread of disease into new populations.

Two: urban expansion. Many diseases, particularly those which are transmitted from human to human (or similarly within an animal population), thrive under crowded conditions. Close contact allows for more efficient exposure, and a higher initial innoculum of the infectious agent makes disease development (and in many cases, subsequent infection of others) more likely. Overcrowding can also lead to unsanitary conditions, resulting in the accumulation of garbage, sewage, and other agents that facilitate disease transmission.

Three: marketing and commerce. Intense production of goods and their rapid transport means that a microbe can be around the world (and potentially into a naïve population) in a matter of days, and price differentials of some goods across country borders lead to importation and/or frequent border crossing–potentially spreading disease along the way.

Four: somewhat related, tourism and global trade. Clearly, tourism moves millions of people around the globe on a daily basis, and can cause diseases to be brought along with them. For instance, here in the United States we’ve had an imported case of Lassa fever (a hemorrhagic fever with symptoms similar to Ebola), and tourism in Toronto took a heavy hit after the importation of SARS in 2003. It doesn’t even have to be global travel–consider the 1976 conference in Philadelphia that led to the recognition of Legionella pneumophila, the causative agent of so-called Legionnaire’s disease. Additionally, exotic souvenirs, including wild animals, may be brought back illegally, and their microbes along with them. Animals, of course, are also moved legally around the world, and such trade has led to the importation of Ebola into the United States on more than one occasion, as well as the outbreak of monkeypox in 2003–more on this in a later post in the series.

Five: wealth and accessibility. Again, this goes hand-in-hand with the previous two. As more people are able to travel, to make purchases abroad and bring them back with them, to introduce themselves into areas with microbes that are novel to them, the better the chance for them to bring an unintentional stowaway back home with them.

Six: food in dwellings/food storage in general. Though obviously not a new concept, having food inside our homes brings in other uninvited guests, which may also carry harmful microbes. One type of animal that takes advantage of our stored food are rodents, such as mice and rats. These carry their own microbes, such as the deadly hantavirus, which humans can inhale upon aerosolization of dried rodent urine. Grain storage can also lead to mold infestation, leading to poisoning by aflatoxin, for example.

Seven: civil disturbances and refugee camps. War often causes large population displacements. Millions worldwide live in refugee camps today, which often are overcrowded with poor sanitation. The mixing of people from different areas into a crowded, dirty camp is a perfect situation for the emergence or re-emergence of diseases. Add to this the fact that many families may be traveling with animals either for food or to assist in carrying their possessions, and these camps act as a “perfect microbial storm,” mixing animal and human pathogens in a population of malnourished, immunocompromised hosts.

Eight: increased surveillance and detection, or acquisition of new genetic material. Many “new” diseases have, in all likelihood, affected us for many years, decades, or even millennia, but they were unknown to us until recently. For example, several studies have been carried out examining the genetic diversity of Helicobacter pylori, and have used this information to track human migrations throughout the millennia. Others, such as methicillin-resistant Staphylococcus aureus, are well-known pathogens that have become increasingly deadly due to the acquisition of genes increasing their resistance to a host of antibiotics. In my own field, a particular serotype of Streptococcus agalactiae (the group B streptococcus, GBS) has emerged just in the last decade that is frequently resistant to erythromycin and clindamycin, and causes disease more often in adults than other serotypes of GBS have in the past.

Luckily, many microbes seem to not have the ability to cause overt disease, due to an intrinsic lack of “virulence factors,” or because they are held in check by our immune systems (or a combination of the two). All of us are colonized with hundreds of different of species of bacteria, on our skin, in our gastrointestinal tract, in our mouth. Very few of these harm us. It is similar with microbes in our environment: our soil, our lakes, our rivers and oceans are teeming with microbial life. And indeed, our domestic and wild animals harbor their own microbial ecology, which rarely do us harm. Considering all of the contact we have with microbes, the emergence of a new disease-causing microbe is a fairly rare event; and the emergence of one that is transmissible between humans is even rarer. This is, however, a major concern, and of course, we are in the midst of the emergence of a new zoonotic disease with the potential for dramatic effects on the human population: influenza H5N1. More on this in later posts.

Comments

  1. #1 Dave S.
    March 28, 2006

    I’m curious to know…are there examples of diseases that go the other way? That is, are there natural examples of micro-organisms that once caused disease in humans but now do not? I ask because I’ve been reading about mimiviruses (very intriguing beasties!) and it’s been suggested they once caused a type of pneumonia in humans.

  2. #2 Tara C. Smith
    March 28, 2006

    Hmm. I’m sure there are some, but none that I can think of right off the top of my head, at least as far as “no longer causing disease in humans.” There is some argument, however, that the 1918 influenza virus moved from humans into swine rather than the other way around, and more recent reports of MRSA found in horses suggest it was transferred to them from their human caretakers, so there definitely are examples where the disease moved from humans –> animals rather than the other way around.

  3. #3 Dave S.
    March 28, 2006

    I’m sure medical science is more interested in stuff that causes diseases rather than the things that don’t! :)

    Interesting to know about the human —> animal disease vector. It’s to be expected when you think about it.

  4. #4 AlanW
    March 28, 2006

    Following up on Dave S, I’ve heard that there are diseases that become less harmful, or act slower, after their initial epidemics. IIRC, the examples included syphilis and maybe leprosy. The hypothesis was that there was an evolutionary advantage for the strains that did not kill the host so quickly. Is this just an urban legend or is there evidence?

  5. #5 Tara C. Smith
    March 29, 2006

    There’s a whole field studying the evolution of virulence (which is what you’re referring to–species that kill at high rates initially, and then become more endemic in the population and generally less deadly). It’s still sort of conventional wisdom that most species eventually “evolve to benignity,” with the rhinoviruses and the common cold given as an example. That doesn’t always happen, however, and people such as Paul Ewald (and many others) have discussed why. (You can find one of Ewald’s articles here. Maybe I’ll write a future post on this. I don’t always agree with Ewald’s conclusions, but it’s a very interesting area of study.

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