Can viruses of animal origin cause cancer in humans? What is known and what is not
Zoonotic describes a virus that passes from animals to people. Oncovirus describes a virus associated with cancer. The question now being debated in oncovirology is whether any virus of animal origin, besides those already known, can contribute to cancer in humans. A recent review by oncovirologist Francisco Aguayo (Universidad de Tarapacá, Chile), published in 2025, organizes what is known and what remains to be proven. This article summarizes its main ideas and compares them with other sources.
The starting point: viruses that do cause cancer
About 13% of all human cancers are estimated to be attributable to viral infections, with differences between regions. According to Aguayo's review, the International Agency for Research on Cancer (IARC, part of WHO) recognizes seven viruses that naturally infect people as definitive carcinogens: high-risk human papillomavirus (HPV), Epstein-Barr virus (EBV), Kaposi sarcoma-associated herpesvirus (KSHV), hepatitis B and C viruses, Merkel cell polyomavirus, and human T-cell lymphotropic virus type 1 (HTLV-1).
IARC Monograph 100B (2012) also includes HIV-1. The exact count depends on how it is done.
What a virus needs to cause cancer
The review points to two traits shared by cancer-causing viruses: they persist in the body for a long time (years, decades or a lifetime) before a tumor appears, and they maintain a latent or abortive infection, in which the virus does not destroy the cell and its products alter the control of cell growth.
According to the author, most known zoonotic viruses lack that persistence, which is why their role in cancer is unclear.
The hypothesis: susceptible but non-permissive cells
For a virus of animal origin to contribute to cancer in humans, a human cell would have to be susceptible (letting the virus in) but non-permissive (not letting it complete its cycle and destroy the cell). In that scenario the virus would stay latent and could alter the cell without killing it. This idea was put forward by Harald zur Hausen, Nobel laureate for his work on HPV and cervical cancer.
There is laboratory evidence that some animal viruses can enter human cells: feline leukemia virus (limited replication), bovine leukemia virus (several human cell lines) and mouse mammary tumor virus (enters and integrates its genetic material into human breast cells). All of this has been seen in cultured cells, not in people.
“Hit and run”: why it is so hard to prove
An abortive infection can go unnoticed. The virus could act as an initial “hit” that transforms the cell and then disappear, or stay at levels so low that it is not detected in the tumor (the hit-and-run model). This mechanism has even been proposed for known human viruses, such as some beta-HPVs linked to skin cancer. That is why proving a link between an initial exposure and a tumor years later is very difficult.
Three debated cases
| Virus (animal) | What has been observed | What is missing |
|---|---|---|
| SV40, simian virus 40 (rhesus macaque) | Reached human populations through contaminated polio vaccines between 1955 and 1963. Causes tumors in rodents and transforms human cells in the laboratory | Its relationship with human mesothelioma and brain tumors remains controversial |
| MMTV, mouse mammary tumor virus (mouse) | MMTV-like sequences have been detected more often in breast cancer tissue than in normal tissue | Evidence is inconclusive and proposed transmission routes (for example, contact with rodents) are speculative |
| BLV, bovine leukemia virus (cattle) | Its DNA has been found in human breast tissue and blood, and correlations with breast cancer incidence have been described; undercooked meat and unpasteurized milk have been proposed as routes | No evidence of active replication or a clear mechanism in humans; a causal relationship has not been confirmed |
SV40, simian virus 40 (rhesus macaque)
MMTV, mouse mammary tumor virus (mouse)
BLV, bovine leukemia virus (cattle)
In all three cases the link with human cancer remains unproven.
Source: Aguayo F. Infectious Agents and Cancer. 2025;20:72.
Another example is bovine papillomavirus: human infections are rare and anecdotal, suggesting strong species barriers, though not impenetrable ones.
Human oncoviruses that come from animals
- HIV-1: arose from several cross-species transmissions of simian immunodeficiency viruses (SIV) from African primates. One of them, from chimpanzee SIV in southeastern Cameroon, gave rise to group M, the cause of the pandemic.
- HTLV-1: it is hypothesized that it arose from multiple transmissions of STLV-1 from non-human primates. It is estimated to affect 10 to 20 million people, and 2% to 5% develop adult T-cell leukemia/lymphoma.
Respiratory viruses and cancer
The review mentions a mouse study in which influenza and SARS-CoV-2 “awakened” dormant breast cancer cells in the lung through an inflammatory response. Observational data from the UK Biobank and Flatiron Health suggest that SARS-CoV-2 infection increases the risk of lung metastasis and mortality in cancer survivors. Precision matters here: this refers to the progression of an existing cancer, not to the virus causing it, and most of what is known about the mechanisms comes from laboratory studies that need clinical validation.
When the cancer appears in the animal
A useful example is jaagsiekte sheep retrovirus (JSRV), which causes lung adenocarcinoma in sheep and has also been found in goats. Its envelope protein acts as an oncogene, which is unusual. The authors consider it a good model for studying human lung cancer; a 2007 review noted that a viral cause had not yet been identified for the similar human cancer.
What researchers propose
- Broad sequencing of tumors of unknown cause or that appear in clusters.
- Serological studies in people with heavy animal exposure, such as agricultural workers, veterinarians and market vendors.
- More sensitive tests to detect very low viral traces.
- A One Health approach that integrates surveillance of animals, people and the environment.
The author also argues that it is plausible that some sporadic cancers of unknown cause stem from undetected zoonotic infections, although definitive proof for most candidate viruses does not yet exist.
What this means for you
- Today there is no basis for avoiding contact with animals or animal products for this reason. The sources reviewed give no such recommendations.
- This is an active research field with many open questions.
- If you have concerns about your health or an infection, consult your doctor.
This article provides general educational information and does not replace an individual medical assessment.
Sources
- Aguayo F. Viral zoonosis and human cancer: a perspective. Infectious Agents and Cancer. 2025;20:72. doi:10.1186/s13027-025-00701-y
- IARC. A Review of Human Carcinogens: Biological Agents. IARC Monographs, vol. 100B. Lyon, 2012. publications.iarc.who.int/119
- Origins of HIV and the AIDS Pandemic. Cold Spring Harbor Perspectives in Medicine. 2011. doi:10.1101/cshperspect.a006841
- Folgosi VÂ, Carneiro LA, Sanabani SS, et al. Investigating STLV-1 infection in African green monkeys: a model for understanding HTLV-1 pathogenesis. Frontiers in Medicine. 2025;12:1616406. doi:10.3389/fmed.2025.1616406
- Leroux C, Girard N, Cottin V, Greenland T, Mornex J-F, Archer F. Jaagsiekte Sheep Retrovirus (JSRV): from virus to lung cancer in sheep. Veterinary Research. 2007;38(2):211-228.
- Fan H, Hofacre A. Jaagsiekte Sheep Retrovirus Biology and Oncogenesis. Viruses. 2010;2(12):2618-2648. doi:10.3390/v2122618
- Buehring GC, Sans HM. Breast Cancer Gone Viral? Review of Possible Role of Bovine Leukemia Virus in Breast Cancer, and Related Opportunities for Cancer Prevention. Int J Environ Res Public Health. 2019;17(1):209.