Introduction to emerging diseases and zoonoses, part 1

So, I'm back from Atlanta. While there, I attended two back-to-back conferences. First, ICEID: International Conference on Emerging Infectious Diseases; second, ISEZ: International Symposium on Emerging Zoonoses. I thought I'd do a multi-part series this week discussing some of the highlights.

To make sure everyone is on the same page, first I'll discuss just what is meant by an "emerging infectious disease" and a "zoonosis" or "zoonotic disease."

An "emerging disease" is generally defined as "any of a group of diseases, of various cause, that have newly appeared or are rapidly expanding their range in the human species." These can include viruses, bacteria, fungi, or other organisms that are completely new to scientists--organisms such as the Marburg and Ebola viruses discovered in the 1960s and 1970s, or HIV in the 1980s. They can also include organisms that were known, but typically not thought to infect humans (such as the group B streptococcus prior to the 1970s); or organisms that have in the past been fairly benign, and suddenly become a problem to human health due to the acquisition of virulence factors or antibiotic resistance genes. Pathogens that are new to a particular geographic area may also be labeled as "emerging" diseases. The West Nile virus is a recent example of this phenomenon: until the late 1990s, it had never been seen in the United States, and now has spread throughout the country. Emerging diseases are such a problem that the Centers for Disease Control and Prevention (the CDC) has dedicated an entire journal to research in that area: Emerging Infectious Diseases.

What, then, is a zoonotic disease? This means a disease that is transmitted between animal species, or emerged from an animal reservoir. These diseases include viruses such as rabies, Ebola, and influenza H5N1, as well as many strains of E. coli. Something like HIV, which originated in non-human primates but has become established in the human population and no longer requires contact with the animal reservoir can also be considered a zoonotic disease.

Why were these conferences held back-to-back, and why are they of interest to so many people? Zoonotic diseases and emerging diseases are highly correlated. Indeed, it's estimated that ~75 percent of newly emerging infectious diseases come from an animal reservoir--that is, they're zoonotic. Just in the last few years, we've seen outbreaks of SARS, Hendra and Nipah viruses, Streptococcus suis, and of course, H5N1, among others. Many of these require ongoing close contact with animals in order to contract the disease (such as Hendra, Nipah, and S. suis, which are not transmissible human-to-human). Others, such as SARS, were able to be spread among people--once they jumped into our species, ongoing contact with the reservoir was no longer necessary. Some, such as Ebola and Marburg, seem to be transmissible by close direct contact, but their spread (especially in the case of Ebola) has been facilitated by conditions in the country of outbreak, and by other routes introduced by the re-use of needles and cultural burial practices, for example. So, while zoonotic diseases and emerging infectious diseases are distinct groups, there is a lot of overlap between the two.

Often the impression is that these zoonotic diseases are a problem of developing countries, where they have much closer contact with many animals, living alongside goats, chickens, cows, ducks. And sure, that's often the case, but it's not limited to the third world. Think of how many Americans sleep with (or kiss!) their dogs or cats, for instance. And of course, we can transmit pathogens between them and us. For example, measles virus is closely related to the canine distemper virus, suggesting either a long-ago transmission event or a common ancestral virus that was spread to both species. Pregnant women and those with compromised immune systems are advised to avoid changing kitty litter, as they may become infected with toxoplasma. Organisms that are capable of infecting both of us, such as Staphylococcus aueus, for example, can also serve to transmit antibiotic resistance genes between species, spreading resistance from them to us or vice-versa (more on this later in the week).

The rest of this week, I'll discuss some new research, summarize some old that was presented, talk a bit more about preparedness, and touch on some other topics I've discussed before (such as avian flu, obviously a hot topic at the meetings) and others I've not spent very much time on previously.

[Edited to add: here is a nice little overview of many of the issues, from Bloomberg.com]