Andrew Wilner, MD, Neurology, 10:46PM Sep 1, 2011
Deadly Bat Bite
A flurry of fictional vampires has recently invaded American television, movie screens, and bookstores. While the Centers for Disease Control and Prevention (CDC) offers no statistics on vampire attacks, the CDC did report the first case of fatal rabies in the US from a vampire bat (August 12, 2011).
Fruit Bat just hanging around...(photo AW)
Clinical History
According to the patient's mother, the 19 year old boy had been bitten by a bat on the left foot while sleeping in Michoacan, Mexico, before coming to Louisiana to work as a laborer on a sugar cane plantation. After one week of work, he developed generalized fatigue, left shoulder pain, and left hand numbness. Initially, his symptoms were attributed to overexertion. He then experienced hyperesthesia of his left shoulder, left hand weakness, generalized areflexia, and drooping of the left upper eyelid. A presumptive diagnosis of the Miller-Fisher variant of acute inflammatory demyelinating polyneuropathy (Guillain Barre) syndrome was made. He became febrile, had respiratory distress, and lapsed into a coma. A lumbar puncture revealed 87 WBCs (97% lymphocytes) and a protein of 233 mg/dL. Rabies virus specific immunoglobulin G and immunoglobulin M were present in the CSF. Rabies virus antigen was detected in postmortem brain tissue, and antigenic typing isolated the vampire bat rabies variant.
I chatted with Brett Petersen, MD, MPH, Medical Officer, Poxvirus and Rabies Branch, CDC, who was kind enough to answer my many questions. He told me that this case is unusual because vampire bats are only found in Latin America, not in the US. Dr. Petersen explained that patients may develop hypersalivation and hydrophobia due to painful laryngeal spasms. "Even the sight of water can create pain," he stated.
According to Dr. Petersen, bat rabies is uniformly fatal, even for infected bats. However, the long incubation period of the virus allows it to be transmitted from bat to bat. In human cases, the median incubation period is 85 days. In general, for a person to be infected with bat rabies, the virus must be inoculated under the skin from the bat's saliva. This requires a bite or a scratch (Hooper et al. 2011), although infection by aerosolized virus has been proposed.
Rabies is caused by a Lyssavirus and has the highest case fatality of any infectious disease (Blanton et al. 2010). With rare exceptions, every patient dies.
Vampire Bats
Characterized by big ears and razor sharp teeth, vampire bats feed at night, quietly landing or jumping onto their prey. However, because of the bat's padded feet and wrists, the victim may be unaware of the bat's presence. Heat sensors in the bat's nose detect accessible blood vessels close to the skin's surface. The bat has an anticoagulant in its saliva that allows it to lap up blood with its tongue. After feeding for approximately 30 minutes, the bat may have ingested so much blood that it is barely able to fly. Victims may not realize they have been bitten. Bat teeth are very fine and may leave only pinpoint puncture marks <1mm that may be nearly undetectable (De Serres et al. 2008). In the past 20 years, most of the people infected with bat rabies did not report a bat bite (De Serres et al. 2008).
The number of rabies cases in the US has decreased dramatically due to the elimination of canine rabies by vaccination programs for dogs. Rabies now comes from wildlife such as raccoons, bats, skunks and foxes (Blanton et al. 2010). This is in contrast to the global situation, where rabies kills approximately 55,000 people per year, mostly due to rabid dogs (De Serres et al. 2008). Humans are not natural reservoirs for rabies virus (Hooper et al. 2011).
Since the elimination of dogs as a rabies reservoir in the US, bat rabies has become the most common cause of human rabies. In 2009, only 4 cases of rabies were identified in the US. Of these, 3 were due to bats. A fourth case of rabies was in a physician who had been bitten by a rabid dog while traveling in India.
Vampire bats are the leading cause of human rabies in Latin America. One concern about global warming is that it could possibly affect the range of vampire bats, introducing them into the Southern USA, resulting in an increase in bat rabies.
Post-Exposure Prophylaxis
Over 20,000 people receive rabies post-exposure prophylaxis in the US each year, and there are no reported failures (Hooper et al. 2011). The purpose of post-exposure prophylaxis is to prevent the virus from reaching the central nervous system. While the neurotropic virus travels through peripheral nerve axons to the central nervous system, there is no clinical evidence of infection. Post-exposure prophylaxis is 100% effective if administered before symptoms develop. However, once the rabies virus has entered the central nervous system and caused symptoms, the outcome is nearly always fatal.
The current recommendations for postexposure prophylaxis are 4 doses of rabies vaccine and 1 dose of rabies immunoglobulin. The wound should be vigorously cleaned and infiltrated with rabies immunoglobulin. The immunoglobulin provides immediate protection while the vaccination induces endogenous antibodies. While the older rabies vaccine was made from nervous tissue and was painful, the current vaccine is made from human diploid cell culture or purified chick embryo cells and is no more painful than other vaccines. In 2008, the CDC revised its vaccination guidelines from 5 shots down to 4, administered on days 0 (right away), 3, 7, and 14. Allergic reactions are infrequent (1/1000), but patients should be closely supervised (De Serres et al. 2009). Persons with altered immunocompetence should receive the older 5 dose regimen. If work or travel predispose individuals to rabies exposure, they can be vaccinated prophylactically. There is also research on an intranasal vaccine (Cruz et al. 2008).
Conclusions:
For those who have received excessive exposure to vampires from TV, cinema and other media, their suffering may continue, as a vaccine is still unavailable. However, if one is bitten by a bat, the CDC recommends the following:
1. If the bat is available, test it for rabies. If the test is negative, no anti-rabies prophylaxis is needed.
2. If the bat flies away, assume it was rabid and administer post-exposure prophylaxis according to the CDC guidelines.
3. Treat as soon as possible after the bite.
Rabies, although rare, should be considered in the differential diagnosis of unexplained acute, progressive, encephalomyelitis. Because of the relatively long incubation period of the rabies virus, a travel history should be obtained from the patient because of the possibilty of infection outside the US. Prompt post-exposure treatment is critical-once a patient has developed symptoms, there is no established therapy (Jackson 2011).
More information on bat rabies can be found on the CDC rabies web page.
References
Blanton JD, Palmer D, Rupprecht CE. Rabies surveillance in the United States during 2009. JAVMA 2010;237(6):646-657.
Cruz ET, Romero IAF, Mendoza JGL et al. Efficient post-exposure prophylaxis against rabies by applying a four-dose DNA vacine intranasally. Vaccine 2008;36:6936-6944.
De Serres G, Skowronski DM, Mimault P et al. Bats in the bedroom, bats in the belfry: Reanalysis of the rationale for rabies postexposure prophylaxis. Clinical Infectious Diseases 2009;48:1493-9.
De Serres G, Dallaire F, Cote M, Skowronski DM. Bat rabies in the United States and Canada from 1950 through 2007: Human cases with and without bat contact. Clinical Infectious Diseases 2008:46:1329-37.
Hooper DC, Roy A, Barkhouse DA et al. Rabies virus clearance from the central nervous system. Chapter 4 Advances in Virus Research 2011;79:55-71.
Jackson AC. Therapy of human rabies. Chapter 17 Advances in Virus Research;79:365-372.
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