Can cetaceans transmit diseases to humans?

Disease transmission from cetaceans to humans is real, scientifically documented, but remains rare. This kind of interspecies transmission has a technical name: zoonosis. Cases remain confined to very specific groups, field scientists, marine veterinarians, keepers in dolphinaria, and trained volunteers of the Réseau National Échouage, who regularly handle live or dead animals. For the general public the risk remains very low, provided basic precautions are observed.

Who is exposed?

Exposure to cetacean pathogens is largely proportional to the frequency and the closeness of contact. Those most concerned are marine biology researchers who take biopsies or samples from live animals, the veterinarians who carry out necropsies on stranded cetaceans, the keepers who work daily in rescue centres and dolphinaria, and the trained RNE volunteers who respond on beaches.

For the general public, the risk becomes relevant in two situations: finding a stranded cetacean (alive or dead, the risk often being higher with decomposing carcasses), and swimming or diving with sociable solitary dolphins, which prolongs physical contact. With the rise in strandings linked to climate change and the growth of whale watching tourism, these situations are becoming more frequent, hence the growing interest in the subject.

The main bacteria involved

Several bacteria are involved in documented cetacean zoonoses. Erysipelothrix rhusiopathiae, the agent of erysipelas, is particularly notable. In cetaceans it can cause fatal septicaemia. In a human who is injured while handling an infected animal, it produces a typical, painful skin lesion, usually on the fingers or the hand. This lesion, known as Rosenbach’s erysipeloid, heals on its own but is not trivial and sometimes calls for antibiotic treatment.

Marine brucellosis (Brucella ceti) is an emerging health issue identified on the French coasts. Cetaceans can carry it without apparent symptoms, but human infection, harder to diagnose than the terrestrial brucelloses, can cause chronic fever and persistent joint and bone pain. Human cases remain very rare but documented, in particular among researchers who had handled porpoise carcasses in the North Sea.

Rarer still, certain bacteria of the Mycobacterium family can cause lung or skin conditions close to tuberculosis. Leptospirosis, present in many cetaceans, is rare but possible in humans who swim in contaminated water.

Viruses and parasites

Several morbilliviruses circulate in cetaceans and periodically cause large epizootics (mass mortality events), in particular among Mediterranean dolphins. Although they are related to the human measles virus, no case of cetacean → human morbillivirus transmission has been documented to date, in all likelihood because of a fairly strong species specificity.

On the parasite side, several cetacean worms and protozoa can in theory infect humans. The best known is Toxoplasma gondii, a parasite that cetaceans contract through water contaminated by cat faeces, a striking example of the connection between land wastewater and marine fauna. Marine toxoplasmosis does not produce in humans a picture any different from ordinary toxoplasmosis, but it illustrates how pathogens move across ecological interfaces.

The right precautions

The precautions are simple and effective. Faced with a stranded cetacean, alive or dead, never touch it with bare hands. If you have to intervene (for example while waiting for the RNE to arrive), wear waterproof gloves. Cover any open wound before acting. Wash your hands thoroughly with soap after any contact. Do not go near the blowhole, the droplets of the blow can carry pathogens. Avoid skin to skin contact, particularly with the cetacean’s mucous membranes.

If you are injured after contact, watch over the following weeks for the appearance of redness, pain, swelling, fever or joint pain. Always mention the contact with a cetacean during a medical consultation, many of these zoonoses are poorly known to general practitioners, and the clinical information is essential in guiding the diagnosis.

For the general public at the seaside, the message is clear: watch respectfully, do not touch, alert the RNE (national number: 05 46 44 99 10) as soon as a stranded animal is found. This basic discipline protects both human health and that of the cetaceans.

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