Carcinogenic parasites are parasitic organisms that depend on other organisms (called hosts) for their survival, and cause cancer in such hosts. Three species of flukes ( trematodes) are medically-proven carcinogenic parasites, namely the urinary blood fluke ('' Schistosoma haematobium''), the Southeast Asian liver fluke ('' Opisthorchis viverrini'') and the Chinese liver fluke (''

Clonorchis sinensis ''Clonorchis sinensis'', the Chinese liver fluke, is a liver fluke belonging to the class trematode, Trematoda, phylum Platyhelminthes. It infects fish-eating mammals, including humans. In humans, it infects the common bile duct and gall bladder, ...

''). ''S. haematobium'' is prevalent in Africa and the Middle East, and is the leading cause of

bladder cancer Bladder cancer is the abnormal growth of cells in the bladder. These cells can grow to form a tumor, which eventually spreads, damaging the bladder and other organs. Most people with bladder cancer are diagnosed after noticing blood in thei ...

(only next to tobacco smoking). ''O. viverrini'' and ''C. sinensis'' are both found in eastern and southeastern Asia, and are responsible for cholangiocarcinoma (cancer of the bile ducts). The

International Agency for Research on Cancer The International Agency for Research on Cancer (IARC; ) is an intergovernmental agency forming part of the World Health Organization of the United Nations. Its role is to conduct and coordinate research into the causes of cancer. It also cance ...

declared them in 2009 as a Group 1 biological carcinogens in humans. Other parasites are also linked to various cancers. Among protozoan parasites, ''

Toxoplasma gondii ''Toxoplasma gondii'' () is a species of parasitic alveolate that causes toxoplasmosis. Found worldwide, ''T. gondii'' is capable of infecting virtually all warm-blooded animals, but members of the cat family (felidae) are the only known d ...

'', '' Cryptosporidium parvum'', ''

Trichomonas vaginalis ''Trichomonas vaginalis'' is an Anaerobic organism, anaerobic, flagellated protozoan parasite and the causative agent of a Sexually transmitted infection, sexually transmitted disease called trichomoniasis. It is the most common pathogenic protoz ...

'' and '' Theileria'' are associated with specific cancer cells. ''

Plasmodium falciparum ''Plasmodium falciparum'' is a Unicellular organism, unicellular protozoan parasite of humans and is the deadliest species of ''Plasmodium'' that causes malaria in humans. The parasite is transmitted through the bite of a female ''Anopheles'' mos ...

'' can also be an indirect cause of cancer. Tapeworms such as ''

Echinococcus granulosus ''Echinococcus granulosus'', also called the hydatid worm or dog tapeworm, is a cyclophyllid cestode that dwells in the small intestine of canids as an adult, but which has important intermediate hosts such as livestock and humans, where it caus ...

'' and ''

Taenia solium ''Taenia solium'', the pork tapeworm, belongs to the cyclophyllid cestode family Taeniidae. It is found throughout the world and is most common in countries where pork is eaten. It is a tapeworm that uses humans (''Homo sapiens'') as its definit ...

'' may directly or indirectly cause cancer. Liver flukes such as ''Opisthorchis viverrini'' and ''Platynosomum fastosum'' can cause cancer in domesticated animals. Roundworms such as '' Strongyloides stercoralis'', '' Heterakis gallinarum'', and '' Trichuris muris'' are known to cause cancer in animals.

History

A rat roundworm ''Gongylonema neoplasticum'' was the first parasite discovered—allegedly—to cause cancer. A Danish physician Johannes Fibiger discovered it in 1907, and experimentally showed that he could induce stomach cancer in rats using the roundworm infection in 1913. In 1914, he gave the name ''Spiroptera'' (''Gongylonema'') ''neoplastica'', but later changed it to ''Spiroptera carcinoma''. Fibiger won the 1926 Nobel Prize in Physiology or Medicine "for his discovery of the Spiroptera carcinoma". However, his interpretation was later found to be false, and that the roundworm was not carcinogenic on its own. Fibiger's Nobel Prize was described as "one of the biggest blunders made by the Karolinska Institute." The first true carcinogenic parasite discovered was ''Schistosoma haematobium''. Theodor Maximillian Bilharz, a German physician at the Kasr el-Aini Hospital in Cairo recovered the adult fluke from a dead soldier in 1851. He named it ''Distomum haematobium''. The disease is often called bilharzia in honour of the discoverer. The infectivity and life cycle was discovered by Scottish physician Robert Thomson Leiper in 1915. A British Surgeon Reginald Harrison, at the Liverpool Royal Infirmary, was the first to note its role in cancer. In 1889, he found that four people out of five cancer victims had bilharzia. A German physician Carl Goebel confirmed in 1903 that bladder tumour occurred in most bilharzia patients. By 1905, he was convinced that

carcinoma Carcinoma is a malignancy that develops from epithelial cells. Specifically, a carcinoma is a cancer that begins in a tissue that lines the inner or outer surfaces of the body, and that arises from cells originating in the endodermal, mesoder ...

of bladder was due to bilharzia.

Group 1 carcinogens in human

Three flukes, urinary blood fluke (''Schistosoma haematobium''), Southeast Asian liver fluke (''Opisthorchis viverrini'') and Chinese liver fluke (''Clonorchis sinensis'') are classified as Group 1 carcinogens, i.e. they are substantiated and directly cancer-causing agents.

''Schistosoma haematobium''

''S. haematobium'' is a digenetic

trematode Trematoda is a Class (biology), class of flatworms known as trematodes, and commonly as flukes. They are obligate parasite, obligate Endoparasites, internal parasites with a complex biological life cycle, life cycle requiring at least two Host ( ...

found in Africa and the Middle East. It is the major agent of

schistosomiasis Schistosomiasis, also known as snail fever, bilharzia, and Katayama fever is a neglected tropical helminthiasis, disease caused by parasitism, parasitic Schistosoma, flatworms called schistosomes. It affects both humans and animals. It affects ...

, the most prevalent parasitic infection in humans. It is the only blood fluke that infects the urinary tract, causing urinary schistosomiasis, and is the leading cause of

(only next to tobacco smoking). Its life cycle is transmission between humans and freshwater snail, species of ''Bulinus''. The larvae live in water bodies from where they infect the hosts by penetrating the skin. Adults are found in the venous plexuses around the

urinary bladder The bladder () is a hollow organ in humans and other vertebrates that stores urine from the Kidney (vertebrates), kidneys. In placental mammals, urine enters the bladder via the ureters and exits via the urethra during urination. In humans, the ...

and the released eggs travels to the wall of the urine bladder causing

haematuria Hematuria or haematuria is defined as the presence of blood or red blood cells in the urine. "Gross hematuria" occurs when urine appears red, brown, or tea-colored due to the presence of blood. Hematuria may also be subtle and only detectable with ...

and fibrosis of the bladder. The bladder becomes calcified, and there is increased pressure on ureters and kidneys (

hydronephrosis Hydronephrosis is the hydrostatic dilation of the renal pelvis and Renal calyx, calyces as a result of obstruction to urine flow downstream. Alternatively, hydroureter describes the dilation of the ureter, and hydronephroureter describes the dila ...

). Inflammation of the genitals due to ''S. haematobium'' may contribute to the propagation of HIV. Antigens produced by the eggs induce granuloma formation. Granulomata in turn coalesce to form tubercles, nodules or masses that often ulcerate. This creates the pathological lesions found in the bladder wall, ureter and renal; and also tumour, both

benign Malignancy () is the tendency of a medical condition to become progressively worse; the term is most familiar as a characterization of cancer. A ''malignant'' tumor contrasts with a non-cancerous benign tumor, ''benign'' tumor in that a malig ...

and

malignant Malignancy () is the tendency of a medical condition to become progressively worse; the term is most familiar as a characterization of cancer. A ''malignant'' tumor contrasts with a non-cancerous benign tumor, ''benign'' tumor in that a malig ...

''Opisthorchis viverrini''

''O. viverrini'' is a food-borne liver fluke that mainly attacks the area of the

bile duct A bile duct is any of a number of long tube-like structures that carry bile, and is present in most vertebrates. The bile duct is separated into three main parts: the fundus (superior), the body (middle), and the neck (inferior). Bile is requ ...

. Infection with the parasite, called opisthorchiasis is the major cause of cholangiocarcinoma, a cancer of the

s, in northern Thailand, the Lao People's Democratic Republic, Vietnam and Cambodia. ''O. viverrini'' has three successive host for its life cycle – the first intermediate hosts are freshwater snails of the genus ''Bithynia'', the second intermediate hosts are different cyprinid fish, and humans are the definitive hosts. Generally opisthorchiasis due to ''O. viverrini'' is harmless without any clinical symptoms, but in rare cases, cholangitis,

cholecystitis Cholecystitis is inflammation of the gallbladder. Symptoms include Right upper quadrant (abdomen), right upper abdominal pain, pain in the right shoulder, nausea, vomiting, and occasionally fever. Often gallbladder attacks (biliary colic) precede ...

, and cholangiocarcinoma can develop. ''O. viverrini'' invades the bile ducts and, rarely, the gall bladder and pancreatic duct. Heavy infection can produce problems such as fibrosis in the liver, gall bladder and bile ducts. Pathological effects on the bile ducts including inflammation, epithelial desquamation, goblet cell metaplasia, epithelial and adenomatous

hyperplasia Hyperplasia (from ancient Greek ὑπέρ ''huper'' 'over' + πλάσις ''plasis'' 'formation'), or hypergenesis, is an enlargement of an organ or tissue caused by an increase in the amount of Tissue (biology), organic tissue that results from ...

and periductal fibrosis collectively promote cholangiocarcinoma. Though it is not immediately life-threatening, cancer develops after 30–40 years, and the ensuing death is rapid—within 3–6 months of diagnosis.

''Clonorchis sinensis''

''C. sinensis'' is also a food-borne liver fluke. It is the most prevalent human trematode in Asia, and is found in Korea, China, Vietnam and also Russia. 85% of the cases are found in China. It is transmitted similarly to ''O. viverrini'', but the species of snails are varied, of which '' Parafossarulus manchouricus'' is the most common. The cyprinid fish hosts are also different. General clonorchiasis is indicated with fatigue, abdominal discomfort, anorexia, weight loss, diarrhea,

liver cirrhosis Cirrhosis, also known as liver cirrhosis or hepatic cirrhosis, chronic liver failure or chronic hepatic failure and end-stage liver disease, is a chronic condition of the liver in which the normal functioning tissue, or parenchyma, is replaced ...

and jaundice. The most severe infections cause cholangiocarcinoma and hepatic carcinoma.

Indirect or putative carcinogens

Infection with malarial parasite ''Plasmodium falciparum'' is classified by IARC as probable (Group 2A) carcinogen. ''Schistosoma japonicum'' is a possible (Group 2B) carcinogen. There is a close association between the cat liver fluke ''Opisthorchis felineus'' and bile duct cancer among people in Russia. ''Toxoplasma gondii'' and eye cancer (intraocular lymphoma) was detected by PCR from two human cases. ''Strongyloides stercoralis'' eggs and adult worms may be linked with gastric adenocarcinoma and colon adenocarcinoma in Korea. ''Cryptosporidium parvum'' infection is associated with colorectal carcinoma.

Carcinogens in animals

The roundworm ''Trichuris muris'' infection can increase the number of tumours in mice. Heavy infection with the trematode ''Platynosomum fastosum'' is associated with cholangiocarcinoma in cats. ''Cryptosporidium parvum'' infection can be the cause of carcinoma in the gut of mice.

References

{{Reflist, 30em Parasitic diseases Infectious causes of cancer Helminthiases Helminthology