Historically, new viruses were discovered when they popped up and did something nasty of humans, their animals, or occasionally, to plants. But for the last few decades, most new viruses have been discovered as a result of people going looking for them. Awareness of emerging human pathogens as public health threats has grown rapidly since HIV was identified in the 1980s. Annual references to “emerging disease” in the medical literature have risen a hundred-fold, from 11 in 1993 to 313 in 2012; a similar search of popular print media uncovered two references in 1993 and 241 in 2012. Does this really mean there are lots more emerging viruses these days, or is there another explanation? A new paper in PNAS looks a some of the factors behind this increase.
By one estimate, 335 “emergent events” occurred during 1940–2004. If the definition is limited to only newly recognized causes of human disease, viruses now predominate. Of the six classes of agents causing disease (prions, viruses, bacteria, fungi, protozoa, and helminths), viruses comprised 67% of the 87 pathogens discovered during 1980–2005, a period that saw the first descriptions of HIV, severe acute respiratory syndrome, hepatitis C, and Nipah viruses. Nearly 85% of these emergent viruses had single-stranded RNA (ssRNA) genomes. In an analysis of 188 viruses first found to cause human disease between 1900 and 2005, the rate at which new viruses were identified in humans appeared to quicken between 1950 and 1970 but then to slow. To examine if changes in approach or methods might account for the change in pace, the authors compiled a list of the viruses found to cause human illness from 1897, when the first virus infecting vertebrates (foot-and-mouth disease virus) was discovered, through 2010 – take a look at the list, your favourite virus is likely to be on there! Not surprisingly, when people go looking for viruses, they find them.
There are some interesting gems in this data. West Nile virus, first discovered in Uganda in 1937, rapidly spread across the United States after its introduction in 1999; in 2012 it produced severe neuroinvasive disease in about 3,000 Americans, which has been estimated to represent only about 1% of infections. The unprecedented 2004–2009 pandemic of chikungunya virus, also first described in East Africa in 1957, infected at least 2 million people in the Indian Ocean region. This analysis implies that not only has the emergence of arboviruses in the tropics been underestimated over the last 40 years but that the discovery of nonarboviruses has lagged as well because of the lack of systematic searching. The development of faster, more sensitive methods for virus identification will continue to be important in discovering new pathogens, but strategy, support, and commitment for looking in the right places in the right ways will be critical to their most effective use.
Search strategy has influenced the discovery rate of human viruses (2013) PNAS USA 05 August doi: 10.1073/pnas.1307243110
A widely held concern is that the pace of infectious disease emergence has been increasing. We have analyzed the rate of discovery of pathogenic viruses, the preeminent source of newly discovered causes of human disease, from 1897 through 2010. The rate was highest during 1950-1969, after which it moderated. This general picture masks two distinct trends: for arthropod-borne viruses, which comprised 39% of pathogenic viruses, the discovery rate peaked at three per year during 1960-1969, but subsequently fell nearly to zero by 1980; however, the rate of discovery of nonarboviruses remained stable at about two per year from 1950 through 2010. The period of highest arbovirus discovery coincided with a comprehensive program supported by The Rockefeller Foundation of isolating viruses from humans, animals, and arthropod vectors at field stations in Latin America, Africa, and India. The productivity of this strategy illustrates the importance of location, approach, long-term commitment, and sponsorship in the discovery of emerging pathogens.