The phrase “emerging disease”, to describe an infectious disease that is new to humans or which is suddenly increasing its geographical range or number of cases, dates back to the 1960s. But it was the realisation in the late 1970s and early 1980s that the world was in the throes of previously unrecognised pandemics of genital herpes and AIDS, that really propelled the term into the mainstream.
The causative agent of genital herpes was type 2 herpes simplex virus (HSV-2), a pathogen that was reasonably well-known at the time, but whose capacity for explosive spread had been underestimated. AIDS, on the other hand, was a completely new infectious agent – one which we now know had been spreading unrecognised since the early 20th century.
Since then, emerging diseases have been appearing at an accelerating rate. Part of the explanation for this may simply be that we are much better at detecting them now. On the other hand, population pressure, climate change and ecological degradation may be contributing to a situation where zoonosis – the movement of a disease from a vertebrate animal to a human host – is more common.
Whatever the explanation, hepatitis C (1989), West Nile virus (1999), SARS (2003), Chikungunya (2005), swine flu (2009), MERS (2012), Ebola (2014) and Zika (2015) have all since had their time in the media spotlight. A further 33 diseases have featured in the World Health Organisation’s Disease Outbreak News since its inception in 1996. Of the “big eight” listed above, six are known zoonotic diseases – and the remaining two (hepatitis C and Chikungunya) are assumed to be so, although the animal reservoir remains undiscovered.
So what other new infectious diseases are on the horizon? These are the ones to watch for in 2017.
Leishmaniasis:. Historically known as “Aleppo boil”, this parasitic infection has recently, as the name suggests, become a problem among Syrian refugees. Producing disfiguring skin ulcerations, and occasionally spreading to internal organs with fatal consequences, the increase of cases turning up in Europe among migrants has made it the subject of considerable media interest. Leishmaniasis is spread by the bite of the sandfly, however, which means it has a northern limit to its range.
Rift Valley Fever:. This virus is spread by a variety of biting insects but fortunately does not transmit from person to person. Humans appear only to be infected by mosquitoes that have previously bitten livestock. Nevertheless, RVF has been expanding its range in Africa, most recently pushing north-west into the Sahel region.