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Zoonoses

The city is not a concrete jungle, it is a human zoo - Desmond Morris

Zoonoses

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NEW human pathogens arise in two ways. They may evolve from old ones, or they may jump to humanity from other species. The second is the more common route. Infections that jump in this way are called zoonotic diseases, or zoonoses. This seems to have been the route taken by Ebola fever, the latest outbreak of which has claimed more than 10,000 lives.

Ebola is suspected of being bat-borne, though that has yet to be proved beyond doubt. Bats also look like the origin of MERS, a viral illness that appeared in 2012 in the Middle East, and SARS, another virus, which burst upon the world from southern China at the end of 2002. HIV, meanwhile, came from other primates. The pandemic version, HIV1, was once a chimpanzee virus. HIV2, largely restricted to west Africa, came from the sooty mangabey, a monkey.

Some older human diseases, too, are constantly replenished from animals. Influenza is an infection of pigs and poultry that subsequently spreads to people. Not every crossover is successful from the virus’s point of view. HIV1, which has been researched intensively, is known to have spread many times to people and then petered out, before one strain of it got lucky. But it only takes one strain to make the leap successfully for trouble to start.

Zoonoses are particularly likely to develop when people and animals live in close proximity to each other. One reason southern China often spawns them (SARS was not unique; a lot of influenza begins there, too) is that the region has a plethora of small farms, in which many species of animal live in close quarters with each other and with human beings. The constant crossing of pathogens between the species involved makes it more likely that one will emerge that can thrive in people.

Agriculture is not the only sort of proximity that can foster zoonotic disease. HIV1 is suspected to have started with a hunter who killed a chimpanzee in the forest. In this context, the extensive clearance of forests, at present a serious environmental issue in many poor countries, brings people into habitats they might previously not have visited. That, in turn, is suspected by some to be increasing the amount of zoonotic disease.

All this suggests that disease-surveillance, which currently concentrates on people, needs to be expanded to look at animals as well. That is beginning to happen. In particular, a group called Global Viral, based in San Francisco, is developing a network of investigators in tropical countries who are watching for signs of crossover by monitoring both animals and people. As the example of HIV shows, not every transfer of a pathogen from an animal to a human being results in an epidemic. But, as in all battles, forewarned is forearmed.

Source: GC, What zoonotic diseases are, and how to stop them, The Economist, March 24, 2015.

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Last Updated : Thursday, May 10, 2018