Scientists in Vermont have just discovered something that shouldn’t really be possible: a skin cancer that’s spreading from catfish to catfish. It’s only the fourth transmissible cancer ever found in nature.

Catfish now join Tasmanian devils, dogs and shellfish as the only animals known to carry naturally occurring cancers that can pass between individuals. In each case, cancer cells break free from one body, travel to another and carry on living and dividing as cancer cells in their new host. No virus, bacteria or parasite is involved – it’s the cancer cells themselves doing the spreading, which is what makes them so unusual.

Tasmanian devils bite each other’s faces when mating or fighting, letting tumour cells pass directly from one animal’s wounds into another’s. Something similar happens in dogs, where canine transmissible venereal tumour cells spread between mating partners via damage to their genitals.

Shellfish do it differently again, passing cancer cells to........

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