January 10th, 2012

First Rabies Case in State in Over 75 Years Raises Questions (Again) About Preventive Strategies

The recent case of bat-related rabies in a Barnstable man has prompted my colleague Larry Madoff, director of the Division of Epidemiology and Immunization at the Mass Department of Public Health, to write this fine commentary in the Atlantic.

I particularly like these passages:

Rabies is perhaps the archetypal zoonotic disease, one spread between animals and humans. It has an extremely broad host range, with the ability to infect all mammals. The ancients understood that when a mad dog bit another dog, it too became mad … While rabies is transmissible between any species, most transmission occurs within the species — bat to bat, raccoon to raccoon — and the virus adapts slightly to its host. That means each virus carries a signature in its genetic sequence indicating the species and geographic location of the “donor.”

He also states, “Keeping bats out of our homes, particularly our sleeping quarters, is a key public health measure in reducing human rabies.”

But as I’ve written before, I’m not so sure the numbers back this up as an effective preventive measure — or at least one we can quantify. Sure, we should try to keep bats out of our bedrooms, and I’d be quick to get immunized if one bit me.

But will these individual efforts — or the recommended practice of giving rabies immune globulin plus the vaccine series for “bats in the bedroom” exposures — actually reduce the number of rabies cases? Maybe not.

To recap the logic in this Canadian study:

  • Bats in houses — incredibly common.
  • People seeking preventive immunization with bats in the house — very rare (though some summer evenings I bet emergency room docs feel otherwise).
  • Cases of bat-related rabies — EXTRAORDINARILY rare, often zero cases in a calendar year. The man from Barnstable was the fourth case in 2011 in the entire United States, the first rabies case in Massachusetts since 1935 — and first-ever bat-related case in the State.

Since most people with bats in the house aren’t seeking immunization, and since the “number needed to treat” to prevent a single case of rabies is estimated to be 2.7 million (!), the Canadians scrapped the recommendation that people who awaken to find a bat flying around their room get immunized.

And according to Larry, the World Health Organization now advocates a middle ground strategy of vaccine alone for minimal risk exposures. (US guidelines recommend rabies immune globulin as well.)

Time will tell whether these less aggressive approaches to rabies prevention will be the right move.

But I have a sneaking suspicion that sporadic cases will continue to occur — fortunately rarely — no matter what we do.

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HIV Information: Author Paul Sax, M.D.

Paul E. Sax, MD

Contributing Editor

NEJM Journal Watch
Infectious Diseases

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