Parasitic causes of cancer

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Estimates place the worldwide risk of cancers from infectious causes at 16.1%. Viral infections are risk factors for cervical cancer, 80% of liver cancers, and 15–20% of the other cancers.

Helicobacter pylori

Estimates place the worldwide risk of cancers from infectious causes at 16.1%. Viral infections are risk factors for cervical cancer, 80% of liver cancers, and 15–20% of the other cancers. Viruses are the usual infectious agents that cause cancer but Mycobacterium, some other bacteria and parasites also have an effect.

A virus that can cause cancer is called an oncovirus. These include human papillomavirus (cervical carcinoma), Epstein-Barr virus (B-cell lymphoproliferative disease and nasopharyngeal carcinoma), Kaposi's sarcoma herpesvirus (Kaposi's Sarcoma and primary effusion lymphomas), hepatitis B and hepatitis Cviruses (hepatocellular carcinoma), and Human T-cell leukemia virus-1 (T-cell leukemias). Bacterial infection may also increase the risk of cancer, as seen in Helicobacter pylori-induced gastric carcinoma. Parasitic infections strongly associated with cancer include Schistosoma haematobium (squamous cell carcinoma of the bladder) and the liver flukes, Opisthorchis viverrini and Clonorchis sinensis (cholangiocarcinoma).

Common oncogenic viruses

In Western developed countries, human papillomavirus (HPV), hepatitis B virus (HBV) and hepatitis C virus (HCV) are the most frequently encountered oncogenic DNA viruses.

Human papillomavirus

Worldwide, HPV causes the second largest fraction of infection-associated cancers or 5.2% of the global cancer burden.

In the United States, HPV causes most cervical cancers, as well as some cancers of the vagina, vulva, penis, anus, rectum, and oropharynx (cancers of the back of the throat, including the base of the tongue and tonsils). Each year in the United States, about 39,800 new cases of cancer are found in parts of the body where HPV is often found. HPV causes about 31,500 of these cancers.

Hepatitis B and hepatitis C viruses

Hepatitis virus-associated hepatocarcinogenesis is a serious health concern. Liver cancer in the United States is primarily due to three main factors: hepatitis C virus (HCV) (22%), hepatitis B virus (HBV) (12%) and alcohol use (47%).In 2017 there will be about 40,710 new cases of liver cancer in the United States. World-wide, liver cancer mortality is more often due to hepatitis B virus (HBV) (33%), less often due to hepatitis C virus (HCV) (21%), and still frequently due to alcohol use (30%).

H. pylori, a common oncogenic bacterium

H. pylori causes over 63% of all stomach cancers, which corresponds to more than 5.5% of all cancers in the world. As reviewed by Chang and Parsonnet, chronic H. pylori infection in the human stomach is characterized by chronic inflammation. This is accompanied by epithelial cell release of reactive oxygen species (ROS) and reactive nitrogen species (RNOS), followed by the assembly of activated macrophages at the stomach site of infection. The macrophages also release ROS and RNOS. Levels of 8-oxo-2'-deoxyguanosine (8-OHdG), one of the predominant forms of free radical-induced oxidative DNA damages, are increased more than 8-fold in DNA after infection by H. pylori, especially if the H. pylori are cagA positive.[24] The increase in 8-OHdG likely increases mutation. In addition, oxidative stress, with high levels of 8-OHdG in DNA, also affects genome stability by altering chromatin status. Such alterations can lead to abnormal methylation of promoters of tumor suppressor genes.
Tuberculosis is a risk factor for lung cancer.

Viruses

Viruses are one of the most important risks factor for cancer development in humans.

Infection by some hepatitis viruses, especially hepatitis B and hepatitis C, can induce a chronic viral infection that leads to liver cancer in about 1 in 200 of people infected with hepatitis B each year (more in Asia, fewer in North America), and in about 1 in 45 of people infected with hepatitis C each year. People with chronic hepatitis B infection are more than 200 times more likely to develop liver cancer than uninfected people. Liver cirrhosis, whether from chronic viral hepatitis infection or alcohol abuse or some other cause, is independently associated with the development of liver cancer, and the combination of cirrhosis and viral hepatitis presents the highest risk of liver cancer development.

Human papillomaviruses (HPV) are another particularly common cancer-causing virus. HPV is well known for causing genital warts and essentially all cases of cervical cancer, but it can also infect and cause cancer in several other parts of the body, including the larynx, lining of the mouth, nose, and throat, anus, and esophagus. The Papanicolaou smear ("Pap" smear) is a widely used cancer screening test for cervical cancer. DNA-based tests to identify the virus are also available.

Herpesviruses are a third group of common cancer-causing viruses. Two types of herpesviruses have been associated with cancer: the Epstein–Barr virus (EBV) and human herpesvirus 8 (HHV-8).

EBV appears to cause all nonkeratinizing nasopharyngeal carcinomas and some cases of lymphoma, including Burkitt's lymphoma—the association is especially strong in Africa—and Hodgkin's disease.

EBV has also been found in a variety of other types of cancer cells, although its role in causing these other cancers is not well established. KSHV/HHV-8[31] causes all cases of Kaposi's sarcoma, and has been found in some cases of a cancer-related condition called Castleman's disease.

Studies involving other kinds of cancer, particularly prostate cancer, have been inconsistent.[30] Both of these herpesviruses are commonly found in cancerous cells of primary effusion lymphoma.

Herpesviruses also cause cancer in animals, especially leukemias and lymphomas.

Human T cell lymphotropic virus (HTLV-1) was the first human retrovirus discovered by Robert Gallo and colleagues at NIH.

The virus causes Adult T-cell leukemia, a disease first described by Takatsuki and colleagues in Japan and other neurological diseases.
Merkel cell polyomavirus is the most recently discovered human cancer virus, isolated from Merkel cell carcinoma tissues in 2008, by the same group that discovered KSHV/HHV-8 in 1994, using a new technology called digital transcriptome subtraction. About 80% of Merkel cell carcinomas are caused by Merkel cell polyomavirus; the remaining tumors have an unknown etiology and possibly a separate histogenesis. This is the only member of this group of viruses known to cause human cancer but other polyomaviruses are suspects for being additional cancer viruses.

HIV does not directly cause cancer, but it is associated with a number of malignancies, especially Kaposi's sarcoma, non-Hodgkin's lymphoma, anal cancer and cervical cancer. Kaposi's sarcoma is caused by human herpesvirus 8. AIDS-related cases of anal cancer and cervical cancer are commonly caused by human papillomavirus. After HIV destroys the immune system, the body is no longer able to control these viruses, and the infections manifest as cancer.Certain other immune deficiency states (e.g. common variable immunodeficiency and IgA deficiency) are also associated with increased risk of malignancy.

Bacteria

In addition to viruses, certain kinds of bacteria can cause some cancers. The most prominent example is the link between chronic infection of the wall of the stomach with Helicobacter pylori and gastric cancer. Although only a minority of those infected with Helicobacter go on to develop cancer, since this bacterial infection is quite common, it may be responsible for most of these cancers. The mechanism by which H. pylori causes cancer may involve chronic inflammation, or the direct action of some of its virulence factors, for example, CagA has been implicated in carcinogenesis.

Parasites

The parasites that cause schistosomiasis (bilharzia), especially S. haematobium, can cause bladder cancer and cancer at other sites. Inflammation triggered by the worm's eggs appears to be the mechanism by which squamous cell carcinoma of the bladder is caused. In Asia, infection by S. japonicum is associated with colorectal cancer.

Distomiasis, caused by parasitic liver flukes, is associated with cholangiocarcinoma (cancer of the bile duct) in East Asia.
Malaria is associated with Burkitt's lymphoma in Africa, especially when present in combination with Epstein-Barr virus, although it is unclear whether it is causative.

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