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Researchers reveal a link between tapeworm parasites and cancer-like infections in people with compromised immune systems.
A Museum scientist has helped pathologists at the Centers for Disease Control and Prevention (CDC) identify the cause of cancerous tumours found in a man suffering from human immunodeficiency virus (HIV). They discovered that the cancer cells came from a tapeworm infection.
The invasion of human tissue by proliferating tapeworm cells is a newly discovered disease mechanism, linking cancer and parasitic worm infection in people with compromised immune systems.
The infection is very rare. But given the millions of people worldwide suffering from immune deficiency diseases and tapeworms, the researchers say there is a chance that this condition is being frequently misdiagnosed as human cancer.
Dr Peter Olson, a tapeworm expert at the Museum and co-author of the paper, provided CDC scientists with specimens and crucial genetic data as part of the research.
'This study is an example of natural history and public health experts working together to uncover fascinating new details about the natural world,' says Dr Olson.
'It also raises fundamental questions about the conditions under which cells may become cancerous, whether they're human cells or even those of a parasitic worm.'
Hymenolepis nana, or the dwarf tapeworm, is the most common tapeworm in humans, with an estimated 75 million carriers.
Uniquely for a tapeworm, H. nana can complete its whole life cycle within the small intestine, reproducing inside the body without the need for an intermediate host.
This trait (known as autoinfection) means infestations can last for years, leading to high numbers of worms, particularly in people with immune deficiency disorders.
Experts think that chronically weak immune systems could also allow the worms to spread beyond the small intestine, such as in the case of a Columbian man whose case was brought to the attention of the CDC in 2013.
The 41-year-old, who was diagnosed with HIV in 2006, visited doctors in January 2013 with fatigue, a fever, a cough and weight loss.
The doctors found dwarf tapeworm eggs in his stools, and later performed biopsies on lung tumours and lymph nodes, yielding tissue samples that were sent to the CDC in April 2013 for diagnosis.
The case posed a 'diagnostic conundrum', according to the study's authors.
The behaviour of the tumours seemed distinctly cancer-like: the cells invaded adjacent tissue in a 'crowded and disordered growth pattern', and the patient's lymph nodes increased to five centimetres in diameter over the course of four months.
But the cells were non-human in origin. They were tiny - about 10 times smaller than most human cancer cells - and researchers noticed them fusing together, which is rare for human cancer cells.
Molecular analysis later revealed the presence of H. nana DNA in the tumours, leading the researchers to conclude that the cancer originated in the tapeworm infection.
The study's authors theorise that a continued proliferation of tapeworms inside the man allowed H. nana cell mutations to accumulate, ultimately leading to malignant growths similar in nature to human cancers.
The man died just 72 hours later. He was too sick to take the medicine that might have been effective in treating his condition.
Dr Atis Muehlenbachs, a pathologist at the CDC and lead author of the study, says: 'We were amazed when we found this new type of disease - tapeworms inside a person essentially getting cancer that spreads to the person, causing tumours.'
The risk of contracting the parasite-derived cancer is limited to people with both a compromised immune system and a severe H. nana infection.
But because both conditions are relatively widespread, especially in developing countries, the researchers say H. nana transformations could frequently be misdiagnosed as human cancer.
A lack of knowledge may also be reducing the effectiveness of common treatments for parasitic worms. Preliminary results have suggested that albendazole - the drug of choice for treating tissue-invasion stages of worm infections - may be ineffective against wider cellular proliferations of H. nana, as seen in the case of the Columbian man.
'We think this type of infection is rare. However, this tapeworm is found worldwide and millions of people globally suffer from conditions like HIV that weaken their immune system,' says Dr Muehlenbachs.
'So the potential for this to be a larger, but unrecognised, problem is there. It's definitely an area that deserves more study.'