Human-Animal Infections: A Primer for Clinicians

Tara C. Smith, PhD


December 16, 2013

In This Article

Emerging and Reemerging Microbes

Microbes are also constantly evolving, which leads to an ever-expanding reservoir of pathogens. Bacteria are frequently acquiring novel genes from other isolates or species, including genes that encode resistance to antibiotics. In recent decades, we have seen the emergence and spread of novel strains of methicillin-resistant Staphylococcus aureus, some of which are zoonotic.[10]

Although many of these examples may seem to be rare events, or to occur internationally but not domestically, zoonotic disease can occur anywhere. Recent studies have shown that many urinary tract infections (approximately 8 million per year)[11] and cases of bacterial sepsis caused by E coli[12] are, in fact, caused by zoonotic E coli strains that evolved in poultry and pigs.

Viruses, such as influenza, are also constantly "reemerging" through the accumulation of mutations and reassortment between different strains of virus. A recent multistate outbreak of influenza was caused by a novel swine influenza virus (called H3N2v) that was transmitted between pigs and people attending agricultural fairs.[13]

Other viruses can stealthily invade through healthy intermediates. The importation of Gambian giant rats and dormice (originating in Ghana) led to a multistate outbreak of monkeypox in the midwestern United States in 2003. The African rodents, already infected subclinically with the virus, were housed in close proximity to prairie dogs, which contracted the virus and were subsequently sold as pets at flea markets or from other distributors. At least 71 human cases of monkeypox resulted from this chain of events.[14]

Other pets, including reptiles and small turtles, can harbor Salmonella strains that young children in particular can pick up and then become ill. Salmonella has also caused infections in both humans and animals as a result of handling or consumption of contaminated pet food or treats.

Adorable baby chicks from a mail-order hatchery have caused outbreaks of Salmonella at Easter, particularly among children.[15] An act as common as sharing a bed with your pet can spread disease between species.[16]


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