DISCUSSION
To our knowledge, this is the first case series reporting on the clinical presentation and outcome of the largest cohort of cases (11 cases) of hippopotamus bites in Africa. To date, there are only three published reports of hippopotamus bites in the medical literature discussing a total of six cases [4,5,10].
Our data showed that during 2017-2018, animal bites represented 0.6% of the overall emergency department visits, of which 8% was caused by hippopotami.
The incidence of deep wound infection in our series was 36.4%. Animal bite wound infections are reported to be between 2 and 64% in the literature [11]. The relatively high infection rate reported in our series could be explained by crushing trauma rather than the commonly described bite wounds of scratches, punctures and avulsions [11]. Crushing can devitalize tissues far beyond clinical identification during surgical debridement of wounds. Hippopotamus saliva could also play strong role in wound infection. The mammals are known to have powerful salivary digestive enzymes (lysozyme and peroxidase) that can chemically damage human flesh. Furthermore, oral bacterial flora inoculated during biting could be particularly virulent in human tissues [12]. Due to the limited resources in our practice, we were unable to isolate the causative organisms. Delayed presentation could be an added factor.
Amputation was required for four patients; three had open fractures. This high rate of post-traumatic ischemia may reflect the extent of soft tissue damage caused by these animals. Lin et al. [4] reported acute traumatic ischemia in one patient who required vascular repair by specialized team. Their patient received 34 units of packed red blood cells and 16 units of fresh frozen plasma during this limb-saving procedure. Drake et al. [5], on the other hand, decided to perform an amputation for their reported case. Our hospital set-up is neither equipped with vascular imaging machines nor staffed with specialized surgeons to do sophisticated vascular repairs, so amputation was the safest option for our patients.
We reported three cases of limb fractures in our series, all of which were open. Pickles reported the management of four cases; two of which presented with open fractures [10]. Lin et al. [4] and Drake et al. [5] each reported one case of open fracture. Our data confirm that limb fractures resulting from Hippopotamus attacks will most likely be open.
The human mortality rate from hippopotamus attacks is unknown but it is estimated to range from 500 to 3000 per year [1]. This estimate comes from a few non-medical published papers; we did not find data in the medical literature regarding fatality rates. Treves et al. [2], in their retrospective study analyzing the data of wildlife-caused casualties over 58 years, found that hippopotamus attacks produced the highest percentage of fatalities (86.7%) compared to lion and leopard attacks (75.0% and 32.5%, respectively).
Hippopotami are one of the main tourist attractions in Africa, and attacks on tourists tend to get a lot of media attention. For instance, the Australian Broadcasting Corporation reported the death of 13 people while on a boat trip due to a single hippopotamus attack [13]. A total of 18 people were aboard this boat, translating into a death rate of 72%.
The probability of been killed by a hippopotamus attack (case fatality rate) is thought to be in the range of 29 to 87% [2,6]. This compares to a death rate following a grizzly bear attack of 4.8%, shark attack at 22.7% and crocodile attack at 25%, all of which indicate that a hippopotamus attack is far more dangerous encounter than the public knows and media publicize [14-16]. In our study, we received 11 patients over 2 years who had survived hippopotamus attacks and were medically stable enough to reach our hospital from distant villages using public and private transportation. We were unable to know how many deaths occurred on the spot or before reaching our facility, but it may be extrapolated from the published ratios; we calculated that between 5 and 74 people may have died before reaching the hospital.
Our results highlight several important operational lessons. First, the high incidence of hippopotamus bite wound infection should raise a red flag to all treating surgeons to be more aggressive in the wound debridement considering them as crushing injuries rather than penetrating wounds. This has a significant impact on patient outcomes where patients are at risk of chronic osteomyelitis and permanent disability. Second, hippopotamus injury to the limbs carries a high risk of amputation so a meticulous assessment of peripheral circulation on arrival and close observation for several hours afterwards should be part of the treatment strategy. Third, hippopotamus bites are serious injuries as many patients required blood transfusions and complex orthopedic interventions, which may not be available in low-resource settings. Fourth, almost half of our patients had a permanent disability at discharge, which has not been reported in the literature to date.
A future study should be designed to identify the causative organisms involved in these wound infections to better inform appropriate antibiotic choices. In particular, hippopotamus mouth flora has not been studied and would provide useful information. In view of the high morality and case-fatality rates ensuing from hippopotamus bites and other dangerous mammals in sub-Saharan Africa, we call for increased surveillance of this public health risk and greater sensitization of local populations and tourists to danger zones and situations.
