Human-Animal Infections: A Primer for Clinicians

Tara C. Smith, PhD


December 16, 2013

In This Article

What Causes Zoonotic Emergence?

Climate change may also play a role in disease emergence, particularly in the establishment of recognized pathogens in new areas. Increasing temperatures have allowed mosquitoes to move into areas that were previously inhospitable to these insects, bringing with them their associated microbes: viruses, including dengue, yellow fever, and West Nile, or parasites, such as malaria. Ticks and their pathogens also are moving into new territories, potentially exposing expanding human populations to disease.

The globalization and intensification of our food production may also contribute to disease emergence. For any given meal, the products may come not only from several different states, but even different countries. This may make tracing the origins of foodborne disease difficult, because the origin of the ingredients or whole products implicated in an outbreak may be unknown. Farming intensification, particularly with livestock, can also amplify the exposure of animals (and their meat products) to emergent microbes, including antibiotic-resistant bacteria.

Approximately 80% of antibiotics used in the United States are given to animals[6]; as a result, bacteria that are resistant to several classes of drugs have arisen in the farming setting and can be transmitted to humans. Other types of organisms can also be generated in an agriculture setting, including reassortant influenza viruses (which can result from swine and poultry species being raised in close proximity), or animal-origin pathogens with novel virulence genes, such as Escherichia coli O157:H7.

Globalization also increases human travel and the movement of products. It is possible to travel to any location worldwide in approximately 24 hours -- well within the incubation period of many infectious diseases. As such, even an asymptomatic individual could transport a pathogen around the world in just 1 day. Indeed, international transportation certainly facilitated the spread of SARS from China to Europe, Australia, and Canada and throughout Asia.

In addition to humans, live animals and animal products can also be quickly transported between countries. Between 2000 and 2006, approximately 1.5 billion live animals were imported into the United States,[7] and 25 million kilograms of meat from wildlife enter through various ports. A recent study examined several shipments of imported bush meat (hunted from wild African mammals) and found several zoonotic viruses in the meat products.[8] Imported products, including live animals, are rarely inspected upon arrival, allowing for the potential introduction of zoonotic pathogens into the destination country.

Novel technology can also lead to the introduction or spread of new pathogens. In the early days of the HIV epidemic, the virus was spread through blood transfusions, and it continues to be spread by the sharing of needles (especially by persons using intravenous drugs). Early outbreaks of Ebola were likewise exacerbated by needle reuse in impoverished hospitals.

In other cases, novel technologies are in place that can detect organisms that were previously undiagnosed.[9] Researchers suggest that there may be as many as 320,000 undiscovered mammalian viruses.


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