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Risk for rabies varies around the world
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Rabies is not only an infection that is almost always fatal, but it is also a horrible disease. I had the experience many years ago of observing patients with rabies in Salvador, Brazil, virtually all from dog bites. The disease causes severe anxiety, confusion, agitation, delirium, hallucinations, insomnia, rage and the very unpleasant symptom of hydrophobia.
In the United States, rabies is a rare disease, and it is safe to say that the great majority of US physicians, including those specializing in infectious diseases, have never seen a patient with rabies. There has been an average of two to three cases of human rabies in the United States each year during the past 25 years.
Donald Kaye
Unfortunately, this is not the case in many other parts of the world. According to WHO, rabies occurs in more than 150 countries and territories, and more than 55,000 people die of rabies every year. More than 95% of human deaths occur in Asia and Africa.
As an example of the widespread exposure to suspect or proven rabid animals, more than 15 million people worldwide receive a postexposure vaccination to prevent the disease every year, according to WHO statistics.
Rabies in travelers
Human rabies also occurs in certain tourist venues such as Bali, Indonesia, and parts of India and the Philippines. Among travelers to Southeast Asia, India and Africa, exposure to nonhuman primates is an important potential risk factor because they are the animals most likely to produce injury.
Rabies in travelers has resulted in two to four recognized cases per year worldwide in the past 25 years, with most cases occurring in European travelers. Most of the exposures occurred in Asia (mainly India), followed by the Philippines, the countries of Central America (including Mexico and the Caribbean) and Africa. Many of the patients were immigrants or descendants of immigrants visiting their country of origin or immigrants who were infected before moving to their new country. There have been close to 20 cases in US travelers during the past 25 years.
In most of the world, dog bites are the main source of human rabies cases and account for about 50,000 deaths. However, in the Americas, bats are the most frequent cause. Insectivorous bats can presumably transmit rabies even without any evidence of a bite. Other rare causes of rabies include exposure to cats, foxes, raccoons, skunks, jackals, mongooses and other wild carnivore host species. Horses, cows and other farmyard animals may develop rabies but are generally regarded to be dead-end victims, rarely causes of exposure to people.
This micrograph, prepared using a hematoxylin-eosin (H&E) stain, depicts the histopathologic changes associated with rabies encephalitis.
Image: CDC
Countries such as Great Britain, some other European countries, Japan, Australia and New Zealand are free of rabies in wild life other than bats.
There have been two episodes of rabies transmission to organ transplant recipients whose organs were from donors with unrecognized rabies. One event in the United States resulted in rabies in four donor recipients, and one in Germany resulted in transmission of rabies to three recipients.
It must be stressed that numbers of cases of rabies are underestimated because diagnoses of rabies are missed and sometimes just labeled as encephalitis.
Immunization best prevention
Rabies is a preventable disease. Many countries have virtually eliminated rabies in dogs by mandatory immunization. Immunization of wildlife with bait containing rabies vaccine can reduce the prevalence of rabies in wild animals. However, since it is difficult if not impossible to eliminate rabies in bats and wildlife, the key to prevention is to give vaccine to those most likely to be exposed: veterinarians, animal handlers, including dog, cat, and wildlife rescue volunteers, rabies laboratory workers, spelunkers and rabies biologics production workers. If someone is bitten, cleaning the wound promptly and giving vaccine after possible exposure in others are also effective prevention measures. The vaccine is given pre-exposure in three doses to high-risk candidates at 0, 7 and 21 to 28 days and in four doses after exposure at 0, 3, 7 and 14 days. Rabies immune globulin is given at the same time as the first dose for postexposure prophylaxis.
Natural immunity through exposure
One interesting and controversial aspect of rabies is the possible development of immunity in certain populations that may have been exposed to small doses of virus on multiple occasions. A study was performed in the Amazon basin of Peru, where rabies exposure is common and usually from vampire bats feeding on the sleeping individual. It demonstrated antibody titers against rabies virus in 11% of 63 people tested. There have been similar reports among various small populations of animal hunters and trappers.
In addition, a study conducted in Nigeria, where dog meat is often consumed, detected rabies virus in the saliva and brain tissue of 5% of healthy-appearing slaughtered dogs. Butchers take few precautions around symptom-free dogs before slaughter, and they often use their bare hands to muzzle animals without using adequate protection against bites, which potentially places butchers at high risk for contracting rabies. However, no rabies was reported in butchers. Almost all of those questioned had been bitten in the past, but none had been vaccinated, which led to speculation that the butchers had become immune during repeated minor exposures. Unfortunately, no serological tests were reported.
There also is a report of a girl in the United States who developed encephalitis after bat contact in a cave, but she recovered and never required intensive care. She had high antibody titers against rabies virus, which suggests abortive rabies. Although these observations are of interest from an intellectual point of view, no one who is likely to be, or has been, exposed to rabies should forgo prophylaxis.
References:
CDC. MMWR. 2010;59:185-190.
Gilbert AT. Am J Trop Med Hyg. 2012;87:206-215.
Donald Kaye, MD, is a professor of medicine at Drexel University College of Medicine, associate editor of the International Society for Infectious Diseases’ ProMED-mail, section editor of news for Clinical Infectious Diseases and is an Infectious Disease News Editorial Board member.
Disclosure: Kaye reports no relevant financial disclosures.