Which cancers are known to be transmissible between individuals?

Transmissible Cancers in Humans and Animals

In humans, no cancers are naturally transmissible between individuals under normal circumstances. However, cancer transmission can occur iatrogenically through organ transplantation, and certain virus-associated cancers involve transmissible infectious agents (not the cancer cells themselves).

Human Context: Iatrogenic Cancer Transmission

Organ Transplantation

Cancer cells can be transmitted from donor to recipient during solid organ transplantation, though this represents transmission of malignant cells rather than a naturally contagious cancer. 1 This occurs when:

• Donors have occult or known malignancy at time of transplantation
• Ovarian cancer has been documented to transmit from donor to recipient 1
• Case reports exist of various cancer transmissions through transplanted organs

Key clinical pitfall: The American Society of Transplantation recommends waiting periods before transplantation for patients with prior cancer history—2 years for uterine corpus cancers and 2-5 years for invasive cervical cancers—though these guidelines may not reflect current treatment advances 1

Virus-Associated Cancers (Transmissible Agents, Not Cancer Cells)

While the viruses themselves are transmissible, the cancer cells are not:

• HPV-related cancers (cervical, vulvar, vaginal, anal, oropharyngeal): The virus transmits sexually, not the cancer 1
• HHV-8/KSHV-related Kaposi sarcoma: The virus transmits, particularly in immunosuppressed individuals 2
• Hepatitis B and C-related liver cancers: The viruses transmit, not malignant cells
• EBV-related lymphomas: The virus transmits, not the cancer

Naturally Occurring Transmissible Cancers in Animals

True transmissible cancers—where living cancer cells themselves spread between individuals as infectious agents—exist only in non-human species. 3, 4, 5

Currently Known Transmissible Cancer Lineages

Eleven distinct clonal lineages of naturally transmissible cancers have been identified: 3

Mammalian Transmissible Cancers:

1. Canine Transmissible Venereal Tumor (CTVT)

   • Transmitted sexually between dogs
   • The oldest known mammalian somatic cell line, thousands of years old 4, 5, 6
   • Spreads worldwide, not necessarily lethal
   • Evades immune detection by downregulating MHC expression 4, 5
   • Responds well to vincristine chemotherapy 7

2. Tasmanian Devil Facial Tumor Disease (DFTD)

   • Two distinct lineages exist: DFT1 and DFT2 3, 8
   • DFT1: First observed 1996, female origin
   • DFT2: Discovered 2014-2015, carries Y chromosome 8
   • Transmitted through biting and scratching
   • Highly lethal, driving species toward extinction 5, 8
   • Both lineages evade immunity through MHC downregulation 4

Marine Transmissible Cancers:

1. Bivalve Transmissible Neoplasia (BTN)

   • Eight different clonal lineages identified in bivalves 3
   • Affects soft-shell clams and cockles (Cerastoderma edule)
   • Two types in cockles: CedBTN1 and CedBTN2 9
   • Associated with high copy numbers of Steamer retrotransposon 4
   • Transmitted through seawater containing cancer cells

2. Syrian Hamster Reticulum Cell Sarcoma (HRCS)

   • Experimentally transmissible 4
   • Limited natural occurrence

Evolutionary Mechanisms of Transmissible Cancers

These cancer lineages defy evolutionary predictions about genomic decay in asexual organisms: 3

• Carry mutation loads far exceeding human cancers yet persist for centuries to millennia
• Successfully evade immune recognition across genetically diverse hosts
• CTVT appears to have reached evolutionary stalemate with hosts (stable phase) 10
• DFTD shows stepwise evolution (dynamic phase) 10
• Clam leukemia exhibits genetic instability (early dynamic phase) 10

Immune Evasion Strategies:

• MHC molecule downregulation 4, 5
• Secretion of immunosuppressive cytokines (TGF-β, IL-10) 6
• Impaired dendritic cell differentiation 6
• Immunometabolic reprogramming through lactate production 6
• Induction of neutrophil extracellular traps (NETs) 6

Clinical Implications

For human medicine, the key distinction is critical: No naturally transmissible cancers exist in humans. Cancer transmission occurs only through:

1. Iatrogenic routes (organ transplantation requiring careful donor screening)
2. Vertical transmission (rare maternal-fetal transmission during pregnancy)
3. Infectious agent transmission (viruses that cause cancer, not cancer cells themselves)

The transmissible animal cancers serve as valuable research models for understanding tumor immunology, immune evasion, and long-term cancer cell survival, but do not represent a direct human health threat 5, 10, 6