Clinical Risk Assessment and Treatment of a Man Attacked by a Rabid North American Beaver ( Castor canadensis ) in the Connecticut River: Case Report

Abstract

A 46-y-old male was swimming in the Connecticut River near Hatfield, Massachusetts, when he suffered an unprovoked attack from a North American beaver (Castor canadensis). The beaver attacked the man 3 separate times, inflicting multiple bite wounds to his arms and torso, ultimately attaching itself to his arm without releasing. The patient was brought to our emergency department, where the wounds were treated, and rabies postexposure prophylaxis was begun. The beaver tested positive for rabies shortly thereafter. Here we review the case of an unprovoked attack by a beaver and clinical risk assessment for rabies, along with management including postexposure prophylaxis, tetanus immunization, and wound care.

Introduction

Beavers are the largest rodent in the United States, weighing on average 13 to 32 kg. They are also excellent swimmers, nocturnal, and normally docile. ¹ Reported attacks within the news media and the medical literature are extremely rare and usually associated with rabid beavers. Only 40 confirmed cases of positive rabies-infected beavers were reported nationally from 2002 to 2021.^2–5 Additionally, there has been no reported transmission of rabies from beavers to humans, and only 2 case reports exist within the medical literature of rabies-infected beavers attacking humans.^6,7

Rabies is an acute progressive encephalomyelitis that is almost always fatal, caused by an infection with the RNA virus, family Rhabdoviridae, genus Lyssavirus. Symptoms include pain or paresthesias around the wound site, fever, paralysis, delirium, convulsions, and/or hydrophobia. Once a patient is exposed, an incubation period before clinical symptoms arise begins and can vary between 1 and 3 mo, but incubation periods of up to more than a year have been documented previously. The most frequent mechanism of transmission is through a bite from a rabid animal introducing rabies-laden saliva into the wound, and although rare, exposure to neural tissue or cerebrospinal fluid may transmit rabies through a penetrating wound or mucous membrane exposure. The Centers for Disease Control and Prevention Annual Surveillance Report of all reported animal rabies cases during 2021 showed that wildlife rabies represented 91.5% of all reported animal rabies cases. ³ Here we discuss the case of a patient exposed to rabies from a beaver while swimming in the Connecticut River in western Massachusetts and review the public health clinical guidance for treatment.

Case Report

A 46-y-old male with a past medical history significant for atrial fibrillation presented to the emergency department after he was swimming in the Connecticut River and suddenly felt something brush against him and then viciously attack him. The patient felt something biting his chest and left and right arms. He then noticed that the beaver was clamped down on his left arm. The patient reported that the beaver was attempting to drag him under the water, but he was able to swim back to shore with the beaver still attached to his arm. He successfully got out of the water and fatally removed the beaver by applying blunt force to the head with a log. Animal control then was contacted and submitted the body of the animal for rabies testing. On arrival at the emergency department, the patient had 2 separate lacerations on his left forearm that were ∼3 cm in length across, one linear and the other stellate in shape. He also had a crescent-shaped 2-cm-long laceration on his right forearm and a small crescent-shaped wound measuring 1 cm on his left anterior chest. The wounds were debrided and flushed with copious amounts of normal saline, and x-rays of the affected areas were obtained to rule out foreign bodies or fractures. Computed tomography with angiography of the left upper extremity was obtained to evaluate for vascular compromise after x-rays were negative. The study was negative. Due to the complexity and contamination of the wounds, they were partially closed and allowed to heal by secondary intention. The patient was given a tetanus toxoid and prophylactically treated for rabies with both human rabies immunoglobulin (HRIG) at 20 IU·kg^–1 infiltrated around the wounds with the remaining volume in the deltoid and an intramuscular dose of rabies vaccine in the contralateral deltoid, awaiting further testing of the collected beaver. The decision to initiate rabies postexposure prophylaxis (PEP) was made based on the abnormally aggressive behavior of the animal described by the patient, specifically considering that it was an unprovoked attack. Neither the local nor the state health department was contacted. The decision was made clinically to begin rabies PEP, a 4-dose course of rabies vaccines and HRIG, considering that the patient had not been vaccinated previously. As for the wounds, the patient was prescribed a course of amoxicillin/clavulanate 875 mg/125 mg 2 times a day for 1 week.

The patient did return for all repeat rabies vaccinations on 3 different visits, totaling the 4-vaccine course. By the first follow-up visit on the third day following initial injury, the patient had been contacted by animal control and was informed that the beaver was in fact positive for rabies. Wounds were noted to be healing appropriately with the exception of the larger of the 2 left upper extremity wounds, which was more erythematous and swollen. Levofloxacin was prescribed given the patient’s exposure to an aquatic environment for additional gram-negative coverage. During the third visit, the wounds were still healing appropriately with the exception of the aforementioned lesion (Figures 1–3). The infectious disease service was consulted, and it was recommended that an additional course of amoxicillin/clavulanate be administered in addition to the previously prescribed therapy for more longitudinal coverage. Finally, for the fourth and final visit on day 14 after initial exposure, the patient visited a different emergency department, where all his wounds were noted to be healing well without further complications, completing his 4-dose vaccine course. The patient was contacted after a month for follow-up and stated that his wounds were still healing appropriately and that he never suffered any constitutional symptoms such as fever.

Figure 1.

Left anterior chest wall wound.

Figure 2.

Left forearm wounds.

Figure 3.

Right forearm wound.

Discussion

Large rodents such as beavers and groundhogs, although not rabies reservoir species, are susceptible to rabies infection through interactions with raccoons in an eastern raccoon rabies virus variant geographic area such as Massachusetts. Rabies is rare in rodents and lagomorphs. There are no documented human rabies deaths from interactions with rodents or lagomorphs in the United States, but the risk cannot be ruled out, especially with exposures to large rodents. ⁸ There are only 2 reported human attacks by rabid beavers in the literature, including one that was exhibiting aggressive behaviors by charging kayakers in Florida in 2001 as well as a report of a male from upstate New York in 2014 bitten various times while in his kayak. In both cases, testing of the beavers in question revealed raccoon variant rabies, as in this case.^6,7 This appears to be an established trend because lab testing of rabid rodents, such as groundhogs and beavers, has revealed infection with the raccoon variant since the 1980s. There has been a positive correlation between the reported number of rabid raccoons and rodents, such as groundhogs, infected with the raccoon variant of rabies virus. ⁹ Regardless of the species of exposing mammal that may be a carrier for rabies, the most important patient-management actions are prompt assessment and recognition of a possible rabies exposure, appropriate initiation of rabies PEP, and management of presenting comorbidities and wounds. Importantly, in cases involving rodents, the state or local health department can be consulted to help determine whether PEP is indicated because rodents are not a natural reservoir for the rabies virus. ¹⁰

Any individual exposed to rabies should first have a thorough cleansing of all wounds, ideally with copious irrigation using water or dilute providone-iodine solution to decrease the likelihood of bacterial and viral infection. Devitalized tissues also should be excised by dissection, foreign bodies should be removed, and most wounds should be left open or packed to heal by secondary intention. If circumstances dictate a dirty wound with aquatic exposure, as in our case, then broader coverage with a fluoroquinolone to cover freshwater Aeromonas and Shewanella species would be necessary.^11,12 Tetanus prophylaxis is also warranted on a case-by-case basis depending on the patient's immunization history.

The importance of timely rabies PEP after a potential exposure must be emphasized because a prompt response can prevent death if initiated early after exposure. Ideally, PEP should begin on the day of exposure, if possible, but if the exposure is recognized later or there is a delay, PEP may be initiated up to months after exposure if signs and symptoms of rabies infection have not appeared. To provide passive immunity to persons not previously vaccinated with a complete pre-exposure prophylaxis or PEP regimen, HRIG should only be administered once at 20 IU·kg^–1 on the first day of the PEP regimen (day 0), but HRIG may be given through day 7 of the PEP regimen if not administered on day 0. Administer HRIG around the wound sites, with any remaining dose administered intramuscularly at an anatomic site distant from the site of rabies vaccine administration, such as the contralateral deltoid. This should be complemented by the rabies vaccine to develop active immunity. Previously unvaccinated individuals should receive a 1-mL dose of rabies vaccines in the deltoid muscle on the following schedule: days 0 (date of initial administration), 3, 7, and 14. Serum antibody titer testing is not recommended for healthy patients as opposed to immunosuppressed individuals, for whom antibody titer testing is recommended. Additionally, immunosuppressed patients should receive HRIG and a 5-dose regimen of the vaccine following the schedule of days 0 (date of initial administration), 3, 7, 14, and 28. Postvaccination serologic testing is recommended for immunosuppressed individuals 1 to 2 weeks after the last dose of vaccine, and state public health officials should be contacted for guidance. In the setting of previously vaccinated patients who have documented appropriate antibody titers, only 2 administrations of the vaccines should be performed, on days 0 and 3, without any HRIG. The recommended sites for vaccine placement include the deltoid for adults, with the anterolateral thigh also being appropriate for children. Gluteal administration should be avoided due to decreased immunologic response from administration there.^13,14

Conclusion

Prompt recognition and appropriate treatment of patients potentially exposed to rabies are indispensable. Administration of rabies PEP is essential to induce both passive and active immunity when it comes to rabies prevention, but other comorbidities need to be addressed as well during the initial encounter and during subsequent visits, including wound care and tetanus prophylaxis. Local or state public health authorities can help provide information on the epidemiology of rabies, assess animal species rabies risk, and advise on the need for PEP. Although North American beavers are not rabies reservoirs and human attacks are rare, rabid beavers have been documented in the literature with an increasing incidence since the early 1960s. A patient attacked by a beaver is an alert to the clinician to begin rabies PEP.