Predicting the Locations of Zoonotic Disease Outbreaks: Global Distribution of Mammal Hosts

Where are zoonotic disease outbreaks likely to occur? One clue could be the geographic distribution of host species. Ecologists from the Cary Institute of Ecosystem Studies, New York, and the University of Georgia have compiled maps comparing mammal species distribution and global hotspots of zoonotic disease. Their work was recently published in Trends in Parasitology.

The authors included data from all 27 orders of terrestrial mammals, the source of most emerging human zoonotic diseases. The greatest diversity of mammal host species is in the tropics (Central and South America, Central East Africa, and Southeast Asia), with an additional European species hotspot that is probably due to the large number of rodent and insectivore species. However, locations of zoonotic disease outbreaks do not necessarily correspond to the areas of highest host species diversity.

“I was rather surprised to see that hotspots of zoonotic diseases didn't match hotspots of biodiversity more closely,” said lead author Barbara Han in a press release. “For example, there is high species diversity in the tropics, so I expected to see a similar pattern of more zoonotic parasites and pathogens in the tropics as well. We do find more zoonotic hosts in the tropics, but we find more zoonotic diseases in temperate regions, possibly because these diseases can occur in multiple host species.”

Reporting bias could influence disease hotspot mapping, write the authors. Because more resources are available for disease research in northern temperate regions, identification of infectious organisms and host species could be higher there. However, this would not explain the relative paucity of disease hotspots in the United States and Canada. The authors suggest that human population density could be a factor: increased contact between humans and host species (or between humans and domesticated animals that come in contact with wild hosts) increases the risk of disease transmission. Factors intrinsic to hosts or pathogens could also explain the fact that more species in temperate regions than in tropical regions carry zoonotic diseases.

Observations about specific mammalian orders include the following:

• Rodents carry 85 zoonotic diseases. The order of rodents includes the most zoonotic host species (244 of 2220 species, over 10%) of all mammals.
• Nearly half of all carnivore species, the highest percentage among the mammalian orders, carry zoonotic disease (139 of 285 species, 49%). Carnivores carry 83 zoonoses, some of which might have originated in prey species.
• Ungulates are important zoonotic hosts because of high human contact through hunting and domestication (livestock). The number of zoonotic pathogens carried by a domesticated ungulate species increases with the time since domestication. Wild ungulates carry 68 zoonotic diseases.
• High phylogenetic relatedness between human and nonhuman primates (which carry 63 zoonoses) may increase the risk of disease transmission.

The review also includes global maps of zoonotic disease by type of pathogen: bacteria, viruses, helminths, and protozoa. Bacteria are the most common cause of zoonotic disease. Carnivores host the most bacterial and viral diseases, rodents carry the most zoonotic helminths, and ungulates carry the most zoonotic protozoa.

The study was funded by the National Institutes of Health.