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Should we fear being licked by our dogs?

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Anyone who has ever interacted with a dog knows about the species’ proclivity to lick. Licks are thought of, at most, as sloppy so recent headlines reporting the story of a Wisconsin man who contracted a fulminant limb-destroying infection from a dog’s lick are likely puzzling to the general public.

How did this rare infection occur? The 48-year-old man hospitalized in this case developed an infection with a bacterial species called Capnocytophaga canimorsus . This bacterial species is highly prevalent in the mouths of both dogs and cats and isn’t harmful to them, or most humans. However, certain individuals, when exposed to the bacterium through a lick, or more frequently a bite, can develop aggressive infections that are often life-threatening. For a lick to cause infection, the bacteria has to find its way into the body through an opening such as a wound or abrasion. A few hundred cases of Capnocytophaga occur annually in the United States (while countless numbers of dog licks occur per hour).

It is important to emphasize that damaging infections like this are a complex interplay between a microbe and the specific immune system of the individual involved. Immune responses that are either underwhelming or over exuberant both can result in severe symptoms. In the case of Capnocytophaga it is almost always the case that an immune impairment explains why the infection occurred. The chief risk factor is lacking a spleen or having a dysfunctional spleen. The spleen is a key organ involved in immune system function. Other risk factors include age over 40, alcoholism, and the use of medications that suppress the immune system (e.g. steroids). However, cases have occurred in those without the traditional risk factors and the man involved apparently has no known risk factors other than his age.

Once the Capnocytophaga infection begins, it can disseminate widely and lead to sepsis and septic shock, a condition characterized by a dysregulated response to infection that can lead to severely low blood pressure in which vital tissues can be deprived of sufficient oxygen-rich blood flow to such a degree that they become compromised. In this case, it appears that this happened with his limbs and was severe enough to require multiple amputations.

Capnocytophaga is readily treated with standard antibiotics but, in many cases, the disease process has progressed far by the time antibiotics are administered and some consequences are not reverisible. The mortality rate from Capnocytophaga cases in which septic shock ensues can reach 80 percent. Because of the well-characterized risk of Capnocytophaga infections in certain patients, such as those lacking a spleen, some physicians immediately begin antibiotics after a dog bite in such patients to forestall infection.

While the scary headlines describing this case are a reminder of the speed and ferocity of certain infectious diseases, it is crucial to remember that this man’s infection was a rare event that happened with a nearly ubiquitous bacterial species that resides in the mouths of dogs.

Dr. Adalja is a Senior Scholar at the Johns Hopkins Center for Health Security and a Pittsburgh-based infectious disease physician. Follow him on Twitter at @AmeshAA and read his blog at www.trackingzebra.com .

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