Since the early part of the last century, the number of rabies cases in humans has declined dramatically in the United States. An average of only one or two human deaths from rabies are now reported each year. In the majority of cases over the past 45 years, no documented exposure to a rabid animal could be established. Messenger and associates presented circumstantial evidence implicating bats as the likely vector in most reported cryptic rabies cases.

Prophylaxis for possible rabies exposure is expensive and difficult to access, because of the limited supply of human rabies immunoglobulin. In the past two decades, the median number of persons who required prophylaxis because of contact with a human rabies case was 54. The authors noted that earlier identification of cryptic cases could decrease this burden.

Molecular typing has revealed that rabies viruses native to insectivorous bat species accounted for 32 of the last 35 cases of human rabies, and for 26 of the last 28 cryptic cases in which no bite could be documented. Surprisingly, common house bat species were not the usual vector. Two reclusive species not usually seen near humans, the silver-haired bat and the eastern pipistrelle, were responsible for 19 of the 26 cryptic cases.

Surveillance studies of bats sampled randomly from natural populations have shown that fewer than 1 percent are infected with rabies. The majority of infected bats are less than one year old. Most exposures to rabid bats occur in the late summer months; this finding correlates with an increase in human rabies during the fall, because of the usual one- or two-month incubation period between rabies exposure and the onset of illness.

Studies of human rabies cases have identified two factors that are significantly associated with a premortem diagnosis of rabies: a definite or probable exposure to a known rabid animal, and the presence of aerophobia or hydrophobia symptoms in the human victim. The authors suggested the importance of maintaining an increased index of suspicion for possible rabies in any patient who has an acute, rapidly progressing encephalitis. Paresthesias at the site of the bite wound are frequently reported in rabies cases. Agitation or paralysis also may be early signs. In almost all cases, patients with rabies are hospitalized less than one week after the appearance of clinical symptoms; coma usually ensues less than a week after the appearance of signs of encephalopathy.

Different methods are available to establish the diagnosis of rabies. State health departments can provide instructions for submitting samples for diagnosis of the disease.

Preventive measures suggested by the authors include avoiding exposure to wildlife, particularly bats, removing bat roosts in homes, vaccinating pets against rabies, testing for rabies in domestic animals that die rapidly after the onset of illness, and testing for rabies in bats found in the home when exposure cannot be ruled out.