Cross-species transmission

Cross-species transmission, (CST) or spillover, is the ability for a foreign virus, once introduced into an individual of a new host species, to infect that individual and spread throughout a new host population. Steps involved in the transfer of viruses to new hosts include contact between the virus and the host, infection of an initial individual leading to amplification and an outbreak, and the generation within the original or new host of viral variants that have the ability to spread efficiently between individuals in populations of the new host Often seen in emerging viruses where one species transfers to another, which in turn transfers to humans. Examples include HIV-AIDS, SARS, ebola, swine flu, rabies, and avian influenza.Bacterial pathogens can also be associated with CST.

The exact mechanism that facilitates transfer is unknown, however, it is believed that viruses with a rapid mutation rate are able to overcome host-specific immunological defenses. This can occur between species that have high contact rates. It can also occur between species with low contact rates but usually through an intermediary species. Bats, for example, are mammals and can directly transfer rabies to humans through bite and also through aerosolization of bat saliva and urine which are then absorbed by human mucous membranes in the nose, mouth and eyes. A host shifting event is defined as a strain that was previously zoonotic and now circulates exclusively among humans.

Similarity between species, for example, transfer between mammals, is believed to be facilitated by similar immunological defenses. Other factors include geographic area, intraspecies behaviours, and phylogenetic relatedness. Virus emergence relies on two factors: initial infection and sustained transmission.

Cross-species transmission is the most significant cause of disease emergence in humans and other species. Wildlife zoonotic diseases of microbial origin are also the most common group of human emerging diseases, and CST between wildlife and has appreciable economic impacts in agriculture by reducing livestock productivity and imposing export restrictions. This makes CST of major concern for public health, agriculture, and wildlife management.

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