When we think about cancer, we usually imagine a disease that begins inside the body of a person or animal due to alterations that accumulate in their own cells. In nearly every case, this is correct: a cell acquires mutations, begins to grow uncontrollably, and forms a tumor that genetically belongs to the affected individual.
However, extraordinary exceptions exist in which cancer cells can leave their organism of origin, reach another individual, and continue growing there. In these cases, cancer behaves in a way similar to a parasite: the tumor cells themselves become the agent that is transmitted and “spreads” the disease.
A study published in *Nature* presents evidence of a new example of this phenomenon in the brown bullhead (*Ameiurus nebulosus*), which would make it the first contagious cancer documented in fish and the first identified in a freshwater ecosystem.
An unusual emergence of black tumors
Lake Memphremagog extends between Vermont in the United States and Quebec in Canada. Beginning in 2012, fishermen and biologists started observing a strikingly high number of brown bullheads with raised, black lesions on different parts of their bodies.
Histological analyses confirmed that many of these lesions were malignant melanomas—tumors originating from melanocytes, the cells responsible for producing pigment. Between 2014 and 2017, these lesions were detected in approximately 23–37% of the fish examined, a frequency much higher than what is normally observed in other bodies of water.
The situation raised concern for several reasons. The brown bullhead is considered an indicator species of environmental quality, and the lake serves as a source of drinking water for a large population. Initially, one suspicion was that a contaminant, perhaps associated with runoff or declining water quality, might be promoting the development of cancer.
The possible presence of a virus, bacterium, or another infectious agent capable of inducing tumors was also considered. However, the first genomic analyses revealed something even more unusual: the DNA of the tumors was considerably different from the normal DNA of the fish carrying them.
What exactly is a transmissible cancer?
A transmissible cancer begins as a conventional tumor in a single founding organism. Some of its cells acquire the ability to leave that individual, reach another organism, evade its immune system, and establish a new tumor, much like a pathogen.
In conventional metastasis, cancer cells move to another part of the same body. In a transmissible cancer, something far more extreme occurs: those cells move between different organisms.
The new tumor does not develop from the recipient’s healthy cells. Instead, it is composed of descendants of malignant cells that originally emerged in another individual. For this reason, these tumor lineages can be considered cellular clones that survive for years, centuries, or even millennia by passing from one host to another.
Before this study, naturally occurring transmissible cancers had been confirmed primarily in dogs, Tasmanian devils, and several species of mollusks, including clams. Canine transmissible venereal tumor is transmitted mainly during mating. Tasmanian devils spread facial tumors through biting, while certain mollusk neoplasms can travel through seawater.
The genetic fingerprint that revealed the transmission
To determine whether the bullhead melanomas were independent tumors or belonged to the same transmissible lineage, researchers compared the genetic material of the tumors with that of healthy tissues from their respective hosts.
The logic was straightforward. In a conventional cancer, the tumor should retain much of the DNA of the individual in which it originated. Even if it contains numerous mutations, it should still be genetically more similar to its own host than to tumors found in other animals.
By contrast, if multiple lesions belong to a transmissible cancer lineage, their genomes should closely resemble one another, even when they are growing in genetically distinct fish. The tumors would share a large number of variants inherited from an ancestral cancer cell.
The researchers performed whole-genome sequencing on tumor samples and non-tumor tissues. They analyzed affected and unaffected fish collected over several years, as well as reference populations from other locations.
Mitochondria told the first part of the story
Mitochondria are cellular structures primarily responsible for producing energy, and they possess a small genome of their own. Because they are abundant within each cell, mitochondrial DNA can be especially useful for studying genetic relationships.
In all the tumors examined, the researchers detected mitochondrial variants that did not match the healthy tissue of the fish carrying the tumor. In addition, the mitochondrial genomes from the different lesions formed a single evolutionary group.
This means that tumor mitochondria from different fish were more closely related to one another than to the mitochondria of the organisms in which they were growing. The pattern remained consistent in samples collected during different years, indicating that the tumor lineage already existed before 2015.
On its own, this evidence was already highly suggestive. However, the researchers also analyzed nuclear DNA, which contains most of the cell’s genetic information.
Figure 1. Tumor cells can spread between individual fish, while shared genetic mutations reveal that the lesions belong to the same transmissible cancer lineage.
Hundreds of thousands of shared mutations
The analysis of the nuclear genome considerably strengthened the conclusion. The tumors formed a distinct genetic group separate from the healthy tissues of their hosts.
The researchers identified hundreds of thousands of variants that were present in the tumors but absent from the healthy brain tissue of the fish. Most importantly, a large proportion of these variants appeared repeatedly in tumors collected from many different individuals.
This pattern would be extremely unlikely if the melanomas had developed independently in each fish.
How might the tumor cells be transmitted?
The study demonstrates the existence of a transmissible lineage, but it does not yet determine precisely how the cells pass from one fish to another.
One possibility is physical contact during reproduction. Brown bullheads gather in relatively confined spaces during spawning, which may facilitate rubbing, injuries, and the direct exchange of cells.
An environmental route may also exist. As occurs in certain mollusks, tumor cells might survive temporarily in the water or sediment and enter a new host through the skin, mouth, or gills.
Brown bullheads lack scales and live near the bottom, so contact with sediments containing malignant cells could be relevant. In addition, hormonal changes associated with reproduction or exposure to contaminants could temporarily weaken immune defenses and facilitate the engraftment of foreign cells. These explanations remain hypotheses and will need to be tested experimentally.
The challenge of evading the immune system
For a transmissible cancer to survive, it must overcome two major barriers. First, its cells must physically pass from one organism to another. Second, they must avoid being rejected by the new host’s immune system.
Under normal circumstances, cells originating from another individual are recognized as foreign and eliminated, similar to what can occur during transplant rejection.
The cells of a transmissible cancer must possess mechanisms that allow them to conceal their identity, reduce antigen presentation, or alter the recipient’s immune response. Understanding how this melanoma accomplishes this could provide important information about immune evasion, tumor evolution, and the ability of malignant cells to adapt to new environments.
After considering all of the above, it is natural to ask the question…
Does it pose a risk to humans?
The study presents no evidence that this cancer can be transmitted to humans or other species. It is a disease identified in brown bullheads and adapted to the biology of that particular host.
Known transmissible cancers generally encounter major barriers when moving between species because genetic and immunological differences make it difficult for cells from one animal to survive inside another. The relevance of this finding is primarily ecological, veterinary, and evolutionary. It does not indicate that the lake water can transmit cancer to humans.
Nevertheless, it will be important to determine whether the tumor lineage exists in other lakes or bullhead populations and to prevent its accidental spread into new bodies of water while its behavior is better understood.
A new way of understanding cancer
The discovery expands the boundaries of what was previously considered possible for a cancer cell. A tumor does not always disappear with the death of its host. Under exceptional circumstances, it can become an independent cell lineage that survives by passing from one organism to another.
Brown bullhead melanoma represents the fourth major type of naturally occurring transmissible cancer recognized in animals, following those found in dogs, Tasmanian devils, and clams. Its presence in freshwater fish suggests that this phenomenon may be more common and diverse than previously believed.
Fundamental questions remain: In which fish did it originally emerge? How long has it been circulating? How does it evade immune defenses? How exactly is it transmitted? What impact will it have on wild populations?
Answering these questions will not only help protect these fish more effectively. It could also reveal new principles concerning cancer evolution, immunity, disease transmission, and cellular survival in natural environments.
Main Reference
Kanemaru, H., Luong, S., Yamamoto, Y., Mizukami, Y., & Ito, F. (2026). Thymulin restrains age-associated myeloid inflammation and enhances cancer immunotherapy. *Nature Communications, 17*(1). https://doi.org/10.1038/s41467-026-75383-0
Other References
Blazer, V. S., Shaw, C. H., Smith, C. R., Emerson, P., & Jones, T. (2020). Malignant melanoma of brown bullhead (*Ameiurus nebulosus*) in Lake Memphremagog, Vermont/Quebec. *Journal of Fish Diseases, 43*(1), 91–100. https://doi.org/10.1111/jfd.13112
Metzger, M. J., & Goff, S. P. (2016). A sixth modality of infectious disease: Contagious cancer from devils to clams and beyond. *PLoS Pathogens, 12*(10), e1005904. https://doi.org/10.1371/journal.ppat.1005904
