Records are meant to be broken, and rabies incubation period is no exception.

I’m often asked what the incubation period of rabies is in people. My general answer is "a long time, and we don’t really know how long it can be."

A report in the Annals of Neurology (Boland et al 2014) highlights this fact. It describes a case of rabies in a person who emigrated from Brazil to the US eight years before dying of rabies virus infection.

But, you might say, how do we know the incubation period was 8 years, since rabies is endemic in the US?  Good question, and this is where molecular epidemiology comes in handy:

• The rabies virus isolated from the person was determined to be a Latin American dog rabies virus strain.
• This strain isn’t present in the US. Furthermore, the man had not returned to Brazil (or even left Massachusetts) in the previous 8 years, nor had he had any contact with animals from outside the country.
• It was also reported that the man had contact with a dog that was acting strangely prior to leaving Brazil. He killed the dog with a piece of wood and handled the body without gloves.

It’s a pretty convincing story and tops earlier well-documented lengthy incubation reports.

How and why rabies does this is unclear. It’s unusual for such a virus to lay low in the body for many years, and then cause rapidly fatal disease.

A major disadvantage to long incubation periods (for rabies or any other pathogen) is you can’t say “Well, that exposure occurred a few months/years ago, so there’s nothing to worry about." Avoiding exposure in the first place is always best.

On the up side, it’s generally believed that if someone gets post-exposure treatment at any time before signs of rabies develop, it can be effective. So, if somehow the potential exposure of this person had been identified, even years after the event but prior to the development of disease, and he’d been treated, he probably wouldn’t have gotten rabies. From a practical standpoint, though, would post-exposure treatment be prescribed, particularly given its cost?

In some ways it would make sense to query past animal exposure in people, especially those who have been in areas where canine rabies is highly endemic, and to treat anyone reporting a potential exposure. Yet, given the low incidence of imported rabies in people and the high cost of post-exposure treatment, it’s unlikely to be done.