Cancers that pass between animals
A few animal cancers persist as clonal lineages that can move from one host to another. The oldest known example is a dog cancer likely thousands — not eleven thousand — years old. This is not a naturally occurring human cancer.
Written retrospectively and published on August 3, 2026.
Background reading, not clinical guidance. This describes published research; it is not a recommendation for any individual case, and the cited studies may have been superseded.
Most cancers begin and end in one body. A few exceptions are living cancer-cell lineages that move between hosts: canine transmissible venereal tumour in dogs, two facial-tumour lineages in Tasmanian devils, and several transmissible neoplasias in marine bivalves.
Their routes differ. Dog tumour cells are transferred during mating; devil facial tumours during biting; bivalve cancer cells can be shed into seawater and taken up by filter-feeding animals. These are genuine cancer-cell transfers, not viruses that merely raise cancer risk.
The famous dog lineage is younger than the usual headline
An early two-genome estimate led to the widely repeated figure of roughly 11,000 years for canine transmissible venereal tumour. A later time-resolved analysis of 546 tumour exomes estimated its origin at about 6,200 years ago, with a broad uncertainty interval. The age is a molecular-clock estimate, not a date on a calendar.
Counts also change as wildlife is sampled: a 2024 review counted eleven known lineages, and reports in 2025 raised the total to at least thirteen. A lineage count is not the same as a count of affected species.
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