COMMENTARY

CDC Commentary: Rabies Transmission, Identification, Diagnosis, and Safety

Brett W. Petersen, MD, MPH

Disclosures

March 31, 2010

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Hello, I'm Dr. Brett Petersen from CDC's rabies program. I am here today to speak with you as part of the CDC Expert commentary series on Medscape.

Rabies is a rare but deadly disease in humans that can generate significant fear and apprehension. I'd like to talk about how rabies virus is transmitted, identifying and diagnosing the disease, and safety precautions to be aware of when rabies is confirmed or suspected.

Rabies virus is transmitted most commonly by a bite from an infected mammal. Worldwide, the vast majority of cases are caused by rabid dogs. However, in the United States dogs are rarely infected due to successful animal control and vaccination programs. Nevertheless, every year approximately 39,000 people in the U.S. receive postexposure prophylaxis due to rabies exposure.

Death from rabies usually occurs due to exposures to indigenous rabid bats, skunks, or raccoons, or to rabid dogs while traveling overseas. For this reason, it is important that rabies be considered in all cases of unexplained encephalitis. Rabies is nearly always fatal once symptoms appear. Fortunately, it can be prevented with almost 100% efficacy when postexposure prophylaxis, including rabies vaccine and immunoglobulin, is administered soon after a rabies exposure occurs.

Several factors need to be evaluated when considering rabies as a diagnosis. A history of a recent animal bite greatly increases the likelihood of rabies. However, its absence should not preclude suspicion, since patients often fail to recognize or report a history of animal contact. The incubation period of rabies can be several months or more and patients may not remember an exposure or may not realize its significance. Nearly a third of human rabies cases in the United States are acquired in rabies-endemic countries, so a recent history of overseas travel should also raise suspicion.

Almost all cases of rabies begin with a nonspecific prodrome of fever and malaise prior to the onset of neurologic signs. Pain and/or pruritus at the bite wound that are unrelated to the injury are often early symptoms as well. In the acute neurological phase, the most common signs and symptoms include altered mental status, hypersalivation, limb pain, paresthesias, and dysphagia. Hydrophobia and aerophobia are the most specific signs and can be elicited respectively by offering a cup of water or fanning air at the face. The clinical course is usually rapid and progressive and patients show signs of encephalitis or myelitis within days of symptom onset. Rabies is an unlikely diagnosis if a patient has unchanging or improving neurologic status or the illness has lasted greater than 2 to 3 weeks.

The combination of acute onset fever, a lymphocytic pleocytosis, and hydrophobia/aerophobia, all occurring 1-3 months after a suspicious animal bite should place rabies high on the differential list. When appropriate, diagnostic testing is available and can be coordinated by local public health personnel and the CDC. Samples required for diagnosis include serum, saliva, CSF, and a skin biopsy from the nape of the neck. A rabies diagnosis can also be made post-mortem through examination of tissue from the medulla, cerebellum, and hippocampus.

In treating patients with rabies, palliative therapy is the standard of care though experimental, aggressive treatment protocols can be considered in select cases. Whenever a diagnosis of rabies is confirmed or suspected, safety is a concern.

Hundreds of thousands of human rabies cases have been treated and human-to-human transmission has not been proven except in cases of organ or tissue transplantation. Nonetheless, the theoretical risk of transmission remains when a patient's saliva or other potentially infectious material comes in direct contact with broken skin or mucous membranes.

This risk can be minimized by following routine safety precautions such as wearing gowns, goggles, masks, and gloves whenever contact with the patient occurs, and particularly during intubation and suctioning. Contacts with blood, urine, or stool are not considered indications for rabies postexposure prophylaxis.

For further information about rabies, please visit www.cdc.gov/rabies, which includes contact information for local or state health officials, or call 1-800-CDC-INFO.

Thank you.

Web Resources

Centers for Disease Control and Prevention. Rabies. Page last updated: March 22, 2010. Available at: www.cdc.gov/rabies Accessed March 24, 2010.

Brett Petersen, MD, MPH , is a Lieutenant Commander in the United States Public Health Service. He currently serves as an Epidemic Intelligence Service Officer in the rabies program of CDC. Prior to joining the CDC, Dr. Petersen completed his MD at the University of Michigan Medical School, his residency in internal medicine at the UCSD Medical Center, and his Masters of Public Health from the Johns Hopkins Bloomberg School of Public Health. He is also a graduate of UCSD with a bachelor's degree in microbiology.

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