Case
A 65-year-old woman presented to the ED with a chief complaint of an animal bite, in Southwest Ohio. The patient reported picking up what she thought to be an injured bird, which in fact turned out to be a chipmunk. When she handled the creature, it jumped up and bit her on the right arm, latching on through the sleeve of her sweater. Her husband struck the chipmunk with a shovel, thus terminating the threat. The patient was brought to the ED by her husband for evaluation, along with the lifeless animal which had been placed in a plastic bag. On examination, the patient had a small superficial abrasion to her right forearm with shallow skin puncture. This was treated with local wound care, including copious irrigation with normal saline and chlorhexidine. Given that she sustained an animal bite wound, she was treated empirically with amoxicillin/clavulanic acid (Augmentin) orally. Her tetanus vaccine was up to date. The critically important question regarding management of the patient was whether she should be treated with post-exposure prophylaxis for rabies after being bitten by the rodent.
Discussion
According to the Centers for Disease Control and Prevention (CDC) website regarding the indications for rabies post-exposure prophylaxis, it was found that small rodents, including chipmunks, have not been known to transmit rabies to humans. While these small animals are rarely infected with rabies, there have been reports of squirrels infected with rabies in the United States.1,2 Therefore, it is recommended that in all cases of rodent bites, the state or local health department should be contacted prior to making a decision regarding post-exposure prophylaxis.1 The state health department was contacted and agreed that post-exposure prophylaxis was not indicated; however, they requested the animal be sent to them for testing, which was arranged. An in-depth literature review found a case report in India of a 7-year-old boy who contracted rabies after being bitten by a squirrel. Per this research, small rodents are rarely infected with rabies, with woodchucks being indicated as local vectors. This particular patient received a tetanus vaccine and wound care initially after the squirrel bite, but presented 2 months later with difficulty with oral feeding, fever, and cough. He succumbed to his illness just 4 hours after admission, and rabies was confirmed through a corneal impression smear.3 Birhane et al4 reported three human deaths in the United States from rabies during 2015. The deaths included one due to a rabid dog bite while abroad, one due to contact with a bat, and one from a mongoose bite. None of these patients had been treated with post-exposure prophylaxis.4
Rabies is an RNA virus in the genus Lyssavirus. Once contracted, the rabies virus initially binds to the nicotinic acetylcholine receptor in muscle, replicates, and ascends along the nerves until it reaches the central nervous system (CNS), then propagates outward via the peripheral nerves. Replication in the CNS occurs in the Negri bodies, which are highly specific for rabies. There are two forms of rabies, a paralytic form and an encephalitic form, with the encephalitic form occurring far more commonly. The encephalitic form presents with hallucinations, and disorientation intermixed with lucid intervals. The paralytic form presents with weakness in the affected limb(s), progressing to quadriparesis and facial weakness, eventually leading to organ failure. The classic hydrophobia that is thought of in connection with rabies is present in only 50% of cases.5Transmission occurs from an infected host via bite most commonly, but can occur through exposure to mucous membranes, aerosol transmission, or exposure in a laboratory setting. The rabies virus can be “shed in the saliva concomitantly with, before or after the development of clinical signs.”6 Lyssaviruses, such as the rabies virus, do not persist in the environment. Once outside the host, the virus is rapidly inactivated, therefore, fomites do not play a role in transmission.6 Rabies hosts vary significantly, and the virus has been found in almost all mammalian orders and on all continents except for Antarctica. The primary reservoir for rabies is the bat, followed by dogs; however, cats, foxes, coyotes, jackals, wolves, mongoose, and raccoons are all vectors of rabies. Animals infected with rabies typically show signs of CNS disturbance. According to Rupprecht,2 “the most reliable signs, regardless of species, are acute behavioral changes and unexplained progressive paralysis.” Notably, wild animals infected with rabies “may lose their fear of people, and nocturnal species may be seen wandering about during the daytime.”2According to Rupprecht et al,6 “Rodents and lagomorphs, although used heavily as laboratory models, are not important in the epidemiology of the disease [rabies], except in the public-health resources devoted to consultation or prophylaxis after routine contact with these ubiquitous small mammals.”