The broad-backed hippopotamus
Rests on his belly in the mud;
Although he seems so firm to us
He is merely flesh and blood.
Flesh-and-blood is weak and frail,
Susceptible to nervous shock […]
—TS Eliot, “The Hippopotamus”
When a patient once asked me whether hippopotami ever suffer from high blood pressure, I dreamed that night I was sitting in my office, trying to take the blood pressure of a visiting hippo. Decades later, learning that specific subspecies of hippos develop a disease similar to the polycystic kidney disease of humans, I decided to pursue the subject further. My first piece of information was that the collective name for a herd of hippos is a bloat, a term supposedly introduced as late as 1939 and referring to their large, bloated bellies.
Hippos’ stomachs are typically bloated from eating almost exclusively grass, which they ingest and can store in their stomachs for up to three weeks. A typical “bloat” consists of one male hippo and 10–20 females. Adult males weigh between 3,300 and 7,000 pounds, but in captivity, they have reached almost 10,000 pounds. Females are much smaller, typically weighing around 3,000 pounds.
The ancient Greeks, including Herodotus, first mentioned the hippo. It is one of the most formidable and dangerous mammals on the planet, living in rivers and lakes. Somewhere between 115,000 and 130,000 hippos are alive in the world, all in Sub-Saharan Africa, mainly in its eastern and southern parts, in Zambia (37,000), Tanzania (20,000), Mozambique (20,000), and Uganda (10,000).
Several years ago, medical scientists autopsied sixty wild hippopotami killed at random in Uganda. They found Schistosoma hepatitis as the primary disease. There were also some cases of biliary coccidiosis, fascioliasis, endocarditis, a case of congenital bronchiectasis, one case of bilateral pheochromocytomas, and a tumor-like mass of possibly viral origin in the thyroid. Arteriosclerosis, with fibromuscular medial plaques, was mild and progressive, but not causing significant morbidity.
The hippo is covered in subcutaneous fat, which helps it float. Its skin is remarkably thick but highly sensitive, requiring constant hydration. Prolonged exposure to the sun outside of the water can lead to skin cracking, dehydration, and secondary infections. To counteract its effects, hippos produce a reddish, oily secretion, often called “blood sweat,” which contains pigments with antiseptic and sunscreen-like properties. Despite this adaptation, bacterial and fungal skin infections still occur, especially in polluted or overcrowded water. Skin wounds caused by aggressive intraspecific fighting, particularly among males, can become infected, leading to abscesses or systemic bacterial infections. Poor water quality in captive settings further increases the risk of skin diseases.
Anthrax (Bacillus anthracis) is one of the most devastating diseases affecting hippos. Outbreaks occur particularly during dry seasons when animals crowd into shrinking water sources, and hippos may ingest its spores from contaminated vegetation or water. Hippos may also develop tuberculosis, leptospirosis, and infections by gastrointestinal parasites such as nematodes and trematodes, as well as by leeches and ticks, causing localized tissue damage and facilitating secondary infections. Their teeth and tusks are enormous and grow continuously. Dental fractures, malocclusion, and infections can impair feeding and lead to chronic illness.
Reduced water availability increases stress, weakens immunity, and heightens disease transmission. Digestive disorders, including colic, impaction, and fermentation imbalance, are relatively uncommon in the wild but are more common in captivity due to dietary differences. Increasingly, the most significant threat to hippo health is human activity. Habitat fragmentation, water extraction, climate change, and direct conflict expose hippos to lethal stress and trauma. At the same time, boat strikes, gunshot wounds, and snare injuries are common causes of morbidity and mortality in certain regions.
The pygmy hippopotamus, the lesser-known cousin of the common hippo, has experienced an unexpected cultural moment. In 2024, a newborn pygmy hippo named Moo Deng at a Thai zoo became an internet sensation, bringing unprecedented attention to this endangered species. With only about 3,000 pygmy hippos remaining globally, this event has given conservationists a rare opportunity to raise awareness of them, and several North American zoos throughout 2024 and 2025 have offered to establish captive breeding programs.
However, not all the news is positive. Hippos remain among the most dangerous animals in Africa. In 2024, a New Jersey woman was killed by a hippo during a Zambian safari, raising questions about safari safety protocols. A boat capsizing in the Ivory Coast resulted in eleven people missing, including children, after a hippo attack. Estimates suggest that each year, hippos kill hundreds of people across Africa. The medical implications of hippo-human interactions extend far beyond physical trauma. Research from Médecins Sans Frontières hospitals in Burundi revealed the devastating nature of hippopotamus attacks.
The adult hippopotamus possesses a bite force of 12,600 kPa, more than double that of a lion at 4,500 kPa, with razor-sharp lower canines measuring 30-50 centimeters that can bisect a human body in a single bite. Treating victims of hippo attacks presents unique challenges. Deep lacerations, crushing injuries, and near-amputations commonly result from these encounters, compounded by severe microbial contamination. A 2024 study on antibiotic resistance in hippopotamus oral flora revealed troubling findings about treatment protocols. Research has found that 24% of bacterial isolates from hippo mouths showed multidrug resistance to commonly used antibiotics, complicating treatment. The study emphasized that the hippopotamus’ mouth flora remains understudied, making appropriate antibiotic selection difficult.
Hippos have also emerged as susceptible hosts for zoonotic diseases. A twenty-year-old female hippopotamus at a zoo in Hanoi, Vietnam, died in December 2021 from SARS-CoV-2 infection, confirmed through positive PCR tests of lung, spleen, liver, and intestine samples. Likewise, two adult female hippos at a zoo in Antwerp, Belgium, became infected with SARS-CoV-2 in November 2021, though they recovered after showing only mild nasal discharge. These cases highlight the need for comprehensive monitoring systems for emerging infectious diseases in captive wildlife populations.
Hippopotami serve as important sentinels for disease in their ecosystems. A 2021 study examined an anthrax outbreak in Tanzania’s Ruaha National Park, where GPS-collared hippos provided unprecedented insights into disease transmission dynamics. Researchers found that infected hippos continued moving normally despite infection, potentially vectoring the disease up to seven kilometers per night, creating new infectious reservoirs across large distances during drought conditions when animals congregate around limited water sources.
At present, humans are the greatest threat to hippos, who face poaching and increasing commercial demand for their ivory teeth and other body parts. Thus, in 2019, Zambia reintroduced plans for a mass cull of 2,000 hippos, despite resounding protests from animal rights groups. The government reasoned that the Luangwa River’s water levels were insufficient for the nation’s 37,000 hippos. The river could only accommodate 5,000 at most, and relocation was too expensive for one of the poorest countries in the world. Although this plan would not kill off the entire species, it would place increased pressure on hippos and underscore the dire need for wildlife protection.
Habitat loss is another significant threat to hippos. Deforestation and habitat loss have impacted their ability to graze. Hippos eat nearly 88 pounds of grass nightly—about 1.5% of the animal’s body weight. As humans occupy land and water sources, fewer resources are available to hippos. Extended drought and exceedingly hot temperatures have also plagued African nations for the last few years. For instance, Kenya had no rain for over two years in the period from 2020–2023. As a result, wildlife officials regularly report on animal deaths, including those deemed threatened or endangered. Experts have attributed the drought to climate change. Without precipitation, dried-up rivers leave hippos with nowhere to go. Of course, all species need water to survive—and an animal weighing up to 7,000 pounds does drink a lot. Extended dry periods affect vegetation quality, water availability, and disease dynamics, while also concentrating animals around limited resources, increasing stress and opportunities for disease transmission.
Unfortunately, the U.S. has played a key role in a highly unregulated yet legal trade, since hippos are not yet protected under its rules. According to data from the Humane Society’s petition, the U.S. imported at least 9,000 hippo teeth, 700 pieces of skin, and 4,400 leather items from 2009 to 2018. Scientists estimate that 3,000 hippos were killed for trade during this time. The only way to stop poaching and the animal trade is by issuing global protections and regulations. A U.S. federal law enacted in 1973 aims to protect and recover imperiled species and their habitats, prohibiting harming, harassing, or killing species listed under its aegis. At present, hippos are not protected under the current U.S. Endangered Species Act (ESA). Still, conservation groups have successfully sued the US Fish and Wildlife Service, resulting in a court-ordered deadline of July 27, 2028, for the agency to decide if they should be listed.
Meanwhile, the hippo remains in scientific, political, and biological limbo, terrestrial yet aquatic, massive yet vulnerable, revered yet feared since antiquity, long impressing humans by its apparent invincibility. Beneath this formidable exterior, however, lies an animal subject to a distinctive range of diseases, shaped by anatomy, behavior, and environment. Examining the diseases of hippopotamuses offers an insight not only into animal pathology but also into ecological health, zoonotic risk, and the consequences of human disruption of natural systems.