Rabies: When to worry

July 2008

With the summer season upon us, we are wont to wander to places that are more likely to put us in contact with all kinds of natural wonders, including rabid species. The Advisory Committee on Immunization Practices recently issued new recommendations for the prevention of rabies (MMWR. 2008;57:1-26,28). Almost simultaneously, we were told from the CDC about a shortage of rabies vaccine (www.cdc.gov/RABIES/news/2008-05-20_PreEVax.html).

The latter recommends limiting preexposure vaccination, which is not likely to be an issue for most of us except for protection of some occupational workers, spelunkers and those traveling to areas where rabies may be a threat.

We take it for granted that dogs generally are free of rabies, based on our experience in the United States. This is not true in many areas of the world. It is particularly important when traveling with children who feel comfortable befriending dogs that might not be safe to pet in some foreign countries. One might consider preexposure vaccination against rabies amongst the other vaccines that are to be given to these children. Although I love dogs, I tend to avoid them in these places. I remember being out jogging and being followed by a group of dogs. They stopped following me, at which point I felt relieved, only to find that I jogged into a cul-de-sac. These were hairy moments.

Several years ago when I was discussing a case of unknown encephalitis in Thailand, I went into what I thought was a very scholarly discussion of differential diagnosis, only to be reminded that the most common cause of sporadic encephalitis in that area was rabies.

We assume that dogs in the United States are usually rabies free and therefore make decisions on this basis. However, I was once called about a child who had been bitten by a dog while I was at Bellevue Hospital in New York City. I felt comfortable telling the family that we had not had rabies in a dog in New York in several decades. I knew about the last case as my pet, which I loved, had wandered off and was confiscated and destroyed because of the fear of rabies in New York about 60 years ago. After reassuring the family that dogs in our area were not likely to be a problem, they told me they saw the dog jump out of a car with Texas license plates. I had to quickly backtrack. The only case of human rabies I have seen in the United States was in a child bitten by a dog who was believed to have come over from Mexico from a border town. One probably should continue to be cautious about dog bites in areas close to the Mexican border and in areas where dogs may come in contact with wildlife that might be infected with rabies. All dogs, cats and ferrets that are kept as pets should be vaccinated against rabies.

Stray cats are more likely to be infected with rabies than dogs. Many are not vaccinated or restrained and wander nocturnally where they may be bitten by rabid wild animals. A recent exposure was reported from a kitten that had been found at a baseball tournament with attendees from four states. The kitten had bitten two people, and several others had been licked or scratched. The animal died and on examination of its brain tissue was found to be infected with a virus of raccoon origin (MMWR. 2008;56:1337-1340).

With new housing developments encroaching on natural animal habitats, animals that we domesticate become increasingly at risk for coming in contact with rabid wild animals. Cats, dogs or ferrets that have been bitten should be confined and observed for 10 days. Signs of illness during this period would mandate examination by a veterinarian, a report to the health department and initiation of prophylaxis in the victim.

In the Morbidity and Mortality Weekly Report statement, human exposure is defined as “the bite of a rabid animal or when the virus is introduced into fresh open cuts in the skin or onto mucous membranes such as the eyes, mouth or nose.”

In addition to raccoons, skunks and foxes are wildlife that are most likely to be infected with rabies virus. Bites by these animals should be considered a possible exposure. When euthanizing these animals, one should try not to damage their heads so their brain tissue can be tested.

Bats and rabies

The most common cause of human rabies in this country is bats. Between 1990 and 2007, there were 34 bat-associated human cases of rabies in the United States. According to the CDC report, a bite was reported in six cases. Contact with a bat and a probable bite were reported in two cases. Physical contact, such as the removal of a bat from the home or workplace or the presence of a bat in the room where a person had been asleep occurred in 15 cases. However, no actual bite was documented in these cases. In 11 cases, no encounter with a bat was reported. “In these cases, an unreported or undetected bat bite remains the most plausible hypothesis because the genetic sequences of the human rabies viruses closely matched those of bats,” the CDC researchers wrote. Bat bites may appear to be insignificant and may have occurred months previously. Thus, the diagnosis of encephalitis for which the etiology is unclear should include the possibility of rabies.

Recently, a bat was discovered in the gym bag hanging in the closet in the room in which one of my colleague’s sons was present; the incident caused near panic. Although the recommendations call for prophylaxis when a bat is found in the room of a sleeping person and the others in the household may be considered safe, it is unlikely in practice that these recommendations will be followed. Similarly, people attending a patient with rabies and their contacts need not be considered at risk unless their skin or mucous membranes have been exposed to the victims’ secretions. Other than transplantation of organs, human-to-human transmission has not been documented.

According to the CDC report, “Attending an event where a rabid animal was present, petting a rabid animal or contact with the blood, urine or feces of a rabid animal does not constitute a risk for transmission.” Also, the ACIP indicated that rodents “are not reservoirs of rabies virus. Small rodents and lagomorphs (including rabbits and hares) are rarely infected with rabies and have not been known to transmit rabies to humans.” Woodchucks accounted for 93% of the infected rodents.

The management of a suspect wound should always include thorough cleansing. Tetanus vaccination should be considered. For those who have not had preexposure prophylaxis, rabies immune globulin should be administered at the same time as the first dose of the five-dose series of rabies vaccine. As much as possible of the rabies immune globulin should be infiltrated in the wound area and the rest injected elsewhere.

RabAvert is available at www.corporatecitizenship.novartis.com/patients/drug-pricing/assistance-programs.shtml.

There are about 16,000 to 39,000 possible exposures to rabid animals annually with only rare cases of rabies. It is essential to make considered decisions to initiate this painful and very expensive regimen. If it is found the suspected animal is not rabid, the course of injections need not be continued.

For more information:

• Philip A. Brunell, MD, is a special volunteer at the National Institute for Allergy and Infectious Disease and professor emeritus at the University of California, Los Angeles School of Medicine.