Predicting disease: Making the leap

RNA filovirus that causes Ebola. Photo: Phil Boyer

How are diseases passed from animals to humans?

Ebola, Zika, SARS they're all diseases that have been passed from animals to humans. But how does this occur?

Han: A disease passes from an animal reservoir to a human host in a number of different ways.

Barbara Han is an infectious disease ecologist at the Cary Institute of Ecosystem Studies.

Han: A common way is for us to come into contact with fecal matter or to come into direct contact with the animal species by handling it. Other ways include our livestock might serve as a bridge. If there are wild animals sharing the pasture with livestock and seeding the ground with pathogenic particles, and the livestock pick it up, humans of course are in regular, close contact with livestock. That animal becomes a bridge for transmission.
Another very common way is for vectors to be involved, so things like ticks and mosquitoes. Zika virus is a prime example of this. It's a mosquito-borne virus which originated from primates. What happens is the mosquito vector takes the blood meal from an infected primate and then finishes the blood meal by biting a human and thereby transmitting the pathogen to the human.

Not all diseases are transmitted by the so-called disease vectors such as ticks and mosquitos.

Han: These disease causing organisms hitch a ride on mosquitoes, but in order to do that, they have various adaptations, traits that they are expressing that enable that mosquito to carry it. Not only to carry it, but to transmit it back out into another host. Those adaptations of these disease-causing organisms come from a long evolutionary history. Things like AIDS have been specially adapted to be transmitted in a particular way and they can't just, all of a sudden, be picked up by a mosquito and transmitted by a mosquito.

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