1980 marked a milestone in infectious disease epidemiology: the World Health Organization declared the smallpox virus eradicated in the wild. However, while smallpox currently exists only in frozen stocks, poxviruses as a class certainly haven’t disappeared. A related virus, monkeypox, regularly causes illness in Africa, and even spread half a world away in the American midwest.
Additionally, Africa isn’t the only area with endemic poxvirus infections. Brazil has been dealing with their own poxvirus outbreak, and poxviruses have popped up in Europe as well. More on both of those after the jump…
I mentioned yesterday that we need to move beyond just responding to outbreaks, but instead, anticipating them. Emergent poxviruses are a good example of this philosophy. Since 1999, in several different regions in Brazil, the poxvirus they’ve been dealing with is vaccinia virus. The history of the virus in this country is detailed in this 2007 publication. Briefly, vaccinia virus was initially brought in on the arms of slaves returning to the country from Portugal, and was propagated arm-to-arm for the better part of a century. By mid-century, the virus was found to be present in mice and cattle on dairy farms, and has recently caused several outbreaks in the country.
Human patients identified thus far have largely been young (75% under the age of 25) and unvaccinated, with a seasonal peak from July through September (winter in Brazil, and the dry season). Cows are highly affected, with attack rates ranging from 80-100%. This has a huge economic impact: infected cows experience a 30-50% drop in milk production. Most of the human cases have worked on dairy farms, and specifically, have manually milked the cows there.
Is this a case of a vaccine strain gone “feral”–that is, moving from a vaccinated human into an animal reservoir, where it’s continued to evolve and be transmitted back to other animals (including humans?) Molecular analyses suggest this isn’t the case, but a reservoir hasn’t yet been identified.
In Europe, the related cowpox virus is somewhat better understood, with a rodent reservoir and occasional infections of cattle, humans, and other species. A 2006 paper documents transmission of cowpox virus between rats and monkeys at an animal sanctuary in the Netherlands. In the April 2008 Emerging Infectious Diseases, an even more complicated outbreak is described, with the virus moving from rat to elephant to the elephant’s keeper in Germany. Zoo and circus animals seem to be particularly susceptible to the virus, and while rats have been identified as a reservoir species, the prevalence of the virus in the rodent population in Europe isn’t known.
Will human poxvirus infections increase in the coming years? It seems likely. Any residual immunity to vaccinia that’s come via vaccination is not only waning in those who were vaccinated, but that proportion of the population that received the smallpox vaccine is decreasing every day, leaving us unprotected in the face of an emergent poxvirus.
Trindade, G.S. (2007). Brazilian Vaccinia Viruses and Their Origins. Emerging Infectious Diseases, 13(7), 965-972.
Martina, B. (2006). Cowpox Virus Transmission from Rats to Monkeys, the Netherlands. Emerging Infectious Diseases, 12(6), 1005-1007.