Risk factors for human disease emergence by M. E. J. Woolhouse, C. Dye, Louise H. Taylor, Sophia M. Latham, Mark E.J. woolhouse

Summary

Woolhouse and colleagues examine some databases regarding zoonotic pathogens. They find that many pathogens that spill out into people are zoonotic and can cause human impacts.

Main Points

We typically under-sample environmentally transmitted pathogens. This includes fungi and other environmentally transmitted pathogens (e.g., bacteria and parasites). Helminths have a high likelihood of spilling over driven likely by exposure.

Abstract

A comprehensive literature review identifies 1415 species of infectious organism known to be pathogenic to humans, including 217 viruses and prions, 538 bacteria and rickettsia, 307 fungi, 66 protozoa and 287 helminths. Out of these, 868 (61%) are zoonotic, that is, they can be transmitted between humans and animals, and 175 pathogenic species are associated with diseases considered to be ‘emerging’. We test the hypothesis that zoonotic pathogens are more likely to be associated with emerging diseases than non–emerging ones. Out of the emerging pathogens, 132 (75%) are zoonotic, and overall, zoonotic pathogens are twice as likely to be associated with emerging diseases than non–zoonotic pathogens. However, the result varies among taxa, with protozoa and viruses particularly likely to emerge, and helminths particularly unlikely to do so, irrespective of their zoonotic status. No association between transmission route and emergence was found. This study represents the first quantitative analysis identifying risk factors for human disease emergence.

Notes

“Out of the emerging pathogens, 132 (75%) are zoonotic, and overall, zoonotic pathogens are twice as likely to be associated with emerging diseases than non-zoonotic pathogens.” (Woolhouse et al., 2001, p. 983)

“protozoa and viruses particularly likely to emerge, and helminths particularly unlikely to do so, irrespective of their zoonotic status” (Woolhouse et al., 2001, p. 983)

“We carry out such a test using the published literature to compile a list of organisms known to be pathogenic to humans, together with available information on whether they are zoonotic, whether they are regarded as emerging, and on their transmission routes and epidemiologies.” (Woolhouse et al., 2001, p. 983)

“Although the de¢nition of species is di¤cult for some infectious organisms, this is the most appropriate level of classi¢cation for the vast majority of pathogens and avoids biases that would otherwise be introduced by organisms that exhibit a large amount of subspeci¢c variation (e.g. some species of Salmonella and Listeria).” (Woolhouse et al., 2001, p. 984)

“To appear in the database a species name must ¢rst have appeared in an up-to-date source text (published within the last ten years), and second appeared in an upto-date nomenclatural reference source, where available (see above), or appeared in a second up-to-date source text, or appeared in an ISI Web of Science Citation Index search of the last 10 years.” (Woolhouse et al., 2001, p. 984)

“A genus was considered to be zoonotic, and/or emerging, and/or transmissible by a particular route if at least one species in it had that characteristic.” (Woolhouse et al., 2001, p. 984)

“Analyses were performed comparing emerging and non-emerging species by taxonomic division, transmission route and zoonotic status and by combinations of these characteristics.” (Woolhouse et al., 2001, p. 984)

“Overall, 19% are viruses or prions, 31% are bacteria or rickettsia, 13% are fungi, 5% are protozoa, and 32% are helminths. Thirty-¢ve per cent of zoonotic pathogens can be transmitted by direct contact,” (Woolhouse et al., 2001, p. 985)

“61% by indirect contact, 22% by vectors, and for 6% the transmission route is not known.” (Woolhouse et al., 2001, p. 985)

“The clearest patterns are that helminths are overrepresented among zoonoses and that fungi are underrepresented.” (Woolhouse et al., 2001, p. 985)

“zoonoses are more likely to be transmitted by indirect contact or by vectors, and are less likely to be transmitted by direct contact when compared with all pathogens” (Woolhouse et al., 2001, p. 985) have to consider some level of detection bias here. if fungi are underrepresented then we would expect environmental transmission to be underestimated in the literature

“This is substantially more than expected if zoonotic and nonzoonotic species were equally likely to emerge, and corresponds to a relative risk of 1.93.” (Woolhouse et al., 2001, p. 986)

“The majority of pathogen species causing disease in humans are zoonotic (868 species, i.e. 61% of the total; electronic Appendix A).” (Woolhouse et al., 2001, p. 986)

“In addition, zoonoses are relatively likely to be transmitted indirectly (including transmission by intermediate hosts) or by vectors, suggesting that these transmission routes may be associated with lower host speci¢city (Woolhouse et al. 2001).” (Woolhouse et al., 2001, p. 987)

“The observation that the route of transmission of over 200 human pathogens (both zoonotic and non-zoonotic) remains unknown emphasizes the need for improved understanding of the biology of infectious agents in general.” (Woolhouse et al., 2001, p. 987)

“Human-to-human transmissibility is a risk factor for emergence across all pathogens, with a relative risk of 2.60.” (Woolhouse et al., 2001, p. 987)

“Nonetheless, we anticipate that pathogen biology also contributes to the likelihood of emergence, including such factors as genetic diversity, generation time and existence of a reservoir (whether zoonotic or environmental).” (Woolhouse et al., 2001, p. 987)

“Nonetheless, animal and human diseases can be closely associated; recent examples include Rift Valley fever in Kenya and Somalia (WHO Press Release 1998), Nipah virus in Malaysia and Singapore (Chua et al. 2000), West Nile virus in the United States (Lanciotti et al. 1999) and Hendra virus in Australia (Westbury 2000).” (Woolhouse et al., 2001, p. 987) .

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Implications for Infectious Diseases

Imported: 2024-05-04 2:16 pm

Woolhouse MEJ, Dye C, Taylor LH, Latham SM, woolhouse MEJ. Risk factors for human disease emergence. Philosophical Transactions of the Royal Society of London Series B: Biological Sciences 2001;356:983–9. https://doi.org/10.1098/rstb.2001.0888.

Related:: taylorRiskFactorsHuman2001

epidemiology zoonoses public health risk factors emerging diseases