Aetiology

For the final post of the series, I want to discuss yet another outbreak, this one a bit closer to home: that of monkeypox in the United States in 2003.

First, I should note that "monkeypox" is a bit of a misnomer. Though the virus--a relative of smallpox and cowpox--can infect monkeys (and humans), the reservoir host is likely a rodent. Previously, monkeypox had been found mainly in sub-Saharan Africa, in forested regions. 2 clades of the virus had been identified. One was common in West Africa, and generally didn't cause severe disease when humans became infected with it. The other was more common in Central Africa, and viruses in this clade were more likely to cause larger outbreaks in humans, and to be more easily spread between humans. Symptoms in humans are similar to those caused by smallpox, though less severe and the disease is less frequently fatal. Fever, rash, and blistering begin after an incubation period of roughly 7-12 days; symptoms generally subside after 3-7 days. The fatality rate for the disease ranges from around 1-10%.

Monkeypox identified in 1958, but was only found to infect humans in 1970 during an outbreak in the Democratic Republic of Congo (then Zaire). Fearing the re-introduction of smallpox into the country, an investigation was conducted, and monkeypox identified as the causative agent of disease. Another large outbreak of disease occurred in the DRC in 1996-7, resulting in over 500 cases. (Bringing in this post on war and EIDs and this one on bushmeat, it's likely that one reason the outbreak was so large was because of the increased hunting of wild animals during the ongoing civil war).

So, how did this African virus end up infecting people in the United States? It's a story of rodents and dogs--prairie dogs, that is.

I mentioned in here that global trade was one factor that could influence the emergence of a disease. Monkeypox is an excellent example of this. As was noted at the conference, "it's not a good idea to capture rats, stick them on a plane, fly them to the United States, and put them in the bedroom of a 12-year-old girl within a week." Yet this is pretty much what happened to cause the outbreak of monkeypox here in the United States. Trappers in Ghana captured a variety of "exotic" rodents--Gambian rats, rope squirrels, and African doormice, among others. The Gambian rat is known to be susceptible to monkeypox, and an animal distributor who had a sick Gambian rat sold prairie dogs housed along side the rat to other dealers in Illinois, Wisconsin, Indiana, and potentially 12 other states. (Dissemination of the prairie dogs was difficult to trace, as some of them were sold at animal "swap meets" and not tracked). Human cases then were contracted via close contact with the infected prairie dogs, or in one case, with a rabbit who'd been housed alongside an ill prairie dog.

As many as 80 probable cases of monkeypox were identified during the outbreak--all of them survived. Most of them were fairly mild, and analysis of the virus showed that it belonged to the West African clade--the one that's less virulent and infrequently transmitted human-to-human. This was fortunate--had it been from the other, Central African virus clade, things could have been worse. Many of those infected were children, and the virus is even more deadly in that age group.

The outbreak did have a few positive results. An embargo was placed on African rodents, and as some collateral damage, toughened restrictions were placed on the sale of wild prairie dogs (as well as the movement of captive-bred prairie dogs between states). It also tested bioterrorism preparedness, and was either a success (it was recognized and controlled within a month!) or a failure (it took a month to recognize and control it!), depending on who you ask.

Since then, we've not had any additional outbreaks of monkeypox, but imported species remain a problem. The very Gambian rats I mentioned, for instance, have become a headache in the Florida Keys. Though more of a problem because of the threat they pose to native species rather than because of any particular disease they carry, as a recent New York Times stories notes, these invasions can have unintended consequences, often involving disease transmission. In the linked story, an attempt to control a plant pest (gallweed) by introducing an enemy (the gall fly) has led to an increased population of deer mice (who eat the fly larvae)--and as a result, more mouse-carried hantavirus. Though this wasn't an introduced disease, per se, it still confers the opportunity for a disease to emerge in a new area, due to a disruption of the native ecosystem. Indeed, that was a concern I voiced last year when the suggestion was made to bring elephants and other African megafauna to the Great Plains--what about their diseases?

We were pretty lucky with monkeypox. We just don't know when that luck will run out.

And with that comes the end of the series. Thank you for reading!

(Image from http://www.afip.org/Departments/infectious/mp/images/72main.jpg)