Infectious cancers

ALSO KNOWN AS: Viral cancer, bacterial cancer, parasitic cancer

RELATED CONDITIONS: Oncogenic infections (bacterial, viral, parasitic), hepatitis, herpes, infection with human immunodeficiency virus (HIV), (AIDS), mononucleosis, schistosomiasis

DEFINITION: Infectious cancers are malignant tumors that result from disruptions and diseases caused by the invasion and multiplication of microorganisms and parasites in body tissues and cells, which could be transmitted from person to person.

Risk factors: Some human infections of ribonucleic acid (RNA) and deoxyribonucleic (DNA) viruses, bacteria, and parasitic worms cause diseases, inflammation, and suppression of the immune system, leading to a higher risk of mutations and cancer. Long-term infection with Helicobacter pylori (H. pylori) bacteria increases the risk of developing stomach cancer and lymphoma, which is also influenced by diet and smoking.

Two RNA viruses and the linked cancers are HIV, which induces sarcoma, and the human T-cell lymphotropic virus (HTLV), which causes leukemia. HIV infection promotes viral cancers such as Kaposi sarcoma, cancers linked to human herpesvirus 8 (HHV-8) and human papillomavirus (HPV), invasive cervical cancer, lymphomas, invasive anal cancer, Hodgkin disease, lung cancer, cancer of the mouth and throat, cancer of the testicles, and skin cancers.

DNA viruses cause cancers of the nasopharynx and cervix (HPV), lymphoma (Epstein-Barr virus, EBV), and liver (hepatitis B virus, HBV).

Certain tropical parasitic worm infections increase the risk of developing cancer. Opisthorchis viverrini is a flatworm or flukeworm linked to bile duct cancer of the liver, called cholangiocarcinoma, if not promptly treated with antiparasitic drugs. These worms are primarily found in Southeast Asia and infect humans who eat uncooked fish. Another parasitic flatworm, Schistosoma hematobium, is found in contaminated freshwater in Africa and the Middle East and can lead to bladder cancer if untreated.

Etiology and the disease process: Infectious cancer is induced by three main mechanisms—agents indirectly interfering with the immune system (HIV), infectious agents creating tissue destruction and chronic inflammation (H. pylori), and agents that directly trigger the proliferation of infected cells (viruses and some bacteria). Many oncogenic viruses remain latent in cells until activation by toxic chemicals, hormones, radiation, or other viruses. People infected with human T-cell lymphotropic virus type 1 (HTLV-1) develop adult T-cell leukemia/lymphoma (ATL) after a latent period of twenty years or more.

Hepatitis C virus triggers long-term chronic infections, 5 percent of which result in liver cancer after several decades. HIV infection stimulates Kaposi about two years after the virus is first detected. HIV infects and destroys white blood cells (helper T cells) and weakens the body’s immune system, allowing viruses such as HPV to attack and trigger cancer development. H. pylori bacterial infections secrete toxins that cause chronic inflammation followed by abnormal cells that become cancerous, especially in the lower part of the stomach.

EBV causes infectious mononucleosis, cancers of the throat (nasopharyngeal carcinoma) and stomach, Hodgkin disease, and lymphomas (cancers in the lymphatic system including the spleen, tonsils, and thymus), particularly in people with organ transplants. The hepatitis B virus (HBV) and hepatitis C virus (HCV) can cause chronic infections that lead to most liver cancers. Some human papillomaviruses (HPV) initiate cervical cancer, promoted by hormones in oral contraceptives and Chlamydia trachomatis infection of the reproductive system. HPVs also cause some cancers of the penis, anus, vagina, and vulva, and are possibly linked with mouth, throat, head, and neck cancers.

Blood flukes create chronic inflammation in the blood vessels of the intestine or bladder, which occasionally causes bladder and bile duct cancers.

Incidence: Roughly 15 percent of all cancers worldwide are linked to viral (11 percent), bacterial (4 percent), or parasitic infections (0.1 percent). In the United States and other developed countries, fewer than 10 percent of cancers are linked to infectious agents, but they account for 20 percent of cancers in developing countries.

Cancers from EBV infections are few but are more common in Africa and parts of Southeast Asia. HBV infections (causing liver cancer) are prevalent in China, Southeast Asia, northern Canada, Africa, Alaska, and Amazonia. Up to 30 percent of liver cancers are related to hepatitis B or C virus infections.

Kaposi sarcoma is a common cancer among individuals with HIV infection. HPV causes 70 percent of cervical cancers and is one of the most common cancers among women worldwide. HTLV-1 infections, mostly in southern Japan, the Caribbean, Central Africa, and parts of South America, create up to a 5 percent chance of developing adult T-cell leukemia/lymphoma.

Symptoms: Patients with infectious cancers exhibit symptoms of the viral, bacterial, or parasitic worm diseases associated with the particular infectious cancer, plus other general cancer symptoms such as swelling (indicating a tumor), ongoing pain, fever, constipation, weakness, fatigue, loss of appetite, anemia, nausea, vomiting, bone fractures, weakness or numbness in legs, weight loss, repeated infections, and urination problems.

For example, symptoms of infectious mononucleosis (caused by EBV) are fever, sore throat, swollen lymph glands, and occasionally a swollen spleen or liver. Kaposi sarcoma (associated with HIV infection) appears as reddish-purple or blue-brown tumors just underneath the skin. Patients infected with the hepatitis B virus have symptoms such as a flu-like illness and a yellowing of the eyes and skin (jaundice of the liver). The hepatitis C virus typically has no symptoms but causes chronic infections that lead to liver damage.

Screening and diagnosis: Screening of all donated blood in the United States has greatly reduced the chance of infection through transfusion, which has helped to control the spread of HIV, HTLV-1, and many other forms of infection. Epidemiological studies, combined with molecular analysis, enhance the diagnosis, screening, and analysis of stages in and development of infectious cancers. Such studies have shown, for example, that infections of H. pylori are present before the appearance of stomach cancer.

Modern technology allows viral infections to be monitored by polymerase chain reaction methods. Biochemical markers are useful for determining carcinogen exposure, such as urine biochemical studies confirming a synergism between aflatoxin fungal and hepatitis B viral infections.

Treatment and therapy: Antibiotics, antiparasitics, and other medicines effectively destroy carcinogenic infectious microorganisms. Immunotherapy is a modern method of treating infectious cancers. Antigens expressed by virus-induced cancers generate host antibodies that kill cancer cells. HPV vaccines protect against HPV types 16 and 18, which cause more than 70 percent of HPV-induced cancers, and HPV-6 and HPV-11, which are associated with genital warts. These vaccines include the 9-valent HPV vaccine (Gardasil 9, 9vHPV), the quadrivalent HPV vaccine (Gardasil, 4vHPV), and the bivalent HPV vaccine (Cervarix, 2vHPV). Medical professionals recommend vaccinations for young women at age eleven or twelve, but individuals aged nine to forty-five may receive the vaccine.

Few drugs can treat hepatitis B or C, but a vaccine is available to prevent hepatitis B virus infection. It is recommended for all children and adults, particularly healthcare workers and intravenous drug users. The Hepatitis C virus has no preventive vaccine at present. Anti-HIV drugs reduce the risk of Kaposi sarcoma and cervical cancer in those infected with HIV.

Prognosis, prevention, and outcomes: Epidemiological studies, as well as descriptive and clinical studies, provide information on carcinogenic microorganisms and the effectiveness of cancer prevention and treatment. Antimicrobial drugs that kill or suppress the growth of infectious cancer-causing agents can be administered as a preventive measure against infectious cancers or after surgery that removes suspected abnormal cells. Stomach cancers from H. pylori infection, for example, have been effectively eliminated with antibiotics after the removal of superficial cancerous areas.

Exposure to carcinogenic infectious agents can be avoided or reduced through lifestyle changes, including diet management and healthy sexual practices (avoiding multiple partners and regulating contraception). Condoms reduce the risk of viral infection but do not protect exposed areas. Pap tests detect precancerous cervical cells for treatment and prevention of HPV-induced cancer. Screening of donated blood greatly reduces the chance of infection through transfusion. The Kaposi sarcoma rate has dropped in the United States because of better treatment of HIV infections.

Investigations of carcinogenic infections continue, alongside the exploration of novel cancer treatments with minimum side effects, such as gene therapy, immunotherapy, and proton radiotherapy.

Bibliography

Bunz, Fred. Principles of Cancer Genetics. 3rd ed., Springer, 2023.

"Infectious Agents." National Cancer Institute, 4 Mar. 2019, www.cancer.gov/about-cancer/causes-prevention/risk/infectious-agents. Accessed 20 June 2024.

"Is Cancer Contagious?" American Cancer Society, 4 Mar. 2021, www.cancer.org/cancer/risk-prevention/understanding-cancer-risk/is-cancer-contagious.html. Accessed 20 June 2024.

"Infections that Can Lead to Cancer." American Cancer Society, www.cancer.org/cancer/risk-prevention/infections/infections-that-can-lead-to-cancer.html. Accessed 20 June 2024.

Kleinsmith, Lewis J. Principles of Cancer Biology. Pearson, Benjamin Cummings, 2014.

Tissot, Sophie, et al. “Transmissible Cancer Evolution: The Under-Estimated Role of Environmental Factors in the "Perfect Storm" Theory.” Pathogens (Basel, Switzerland) vol. 11, no. 2, 12 Feb. 2022, doi:10.3390/pathogens11020241.