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Tumours consisting of tapeworm cancer cells found in a human

Unique case raises questions about misdiagnosis and treatment.

Scientists at the Centers for Disease Control and Prevention (CDC) in the USA and the Natural History Museum in London have discovered that a common tapeworm can cause cancer-like infections in people with compromised immune systems.

In the 5 November edition of the New England Journal of Medicine researchers report on a case of a person becoming ill from cancer cells that arose from the human dwarf tapeworm, Hymenolepis nana, rather than human cells. The case raises concern that other similar cases, if they occur, may be misdiagnosed as human cancer – especially in underdeveloped countries where this tapeworm and immune system suppressing illnesses like HIV are widespread.

In 2013, Colombian doctors asked CDC to help diagnose bizarre-looking biopsies from lung tumours and lymph nodes of 41-year-old HIV-positive man. The tumours looked like cancer, but lab studies showed the cells were not human. That revelation prompted a nearly three-year hunt to unravel how the man became sick and what caused his illness.

The growth pattern of the tumour was decidedly cancer-like, with many undifferentiated cells crowded into small spaces and quickly multiplying. But the cells themselves were tiny – about 10 times smaller than a normal human cancer cell. Researchers also noticed cells fusing together, which is rare for human cells. Eventually, tests on DNA from the man's tumour revealed that these were tapeworm cells. Unfortunately, the man died 72 hours later, too sick to take medicine that might have treated the infection.  

CDC investigators sought out Dr Peter D Olson, a researcher and tapeworm expert at the Natural History Museum in London, who provided specimens and unpublished genetic data critical to the study. He said, 'This study is an example of natural history and public health experts working together to uncover fascinating new details about the natural world. 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.' The Natural History Museum research team includes a number of parasitologists who are working to control and eradicate parasites and the diseases they cause.

Dr Atis Muehlenbachs, lead author of the study and a staff pathologist at CDC's Infectious Disease Pathology Branch, said, '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.'

'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. So the potential for this to be a larger, but unrecognized, problem is there. It's definitely an area that deserves more study.'

H. nana infects up to 75 million people at any given time, more than any other known tapeworm. People may accidentally ingest tapeworm eggs in the environment or larvae that develop in beetles, so children are most often affected.

Most people show no symptoms and eventually excrete the tapeworms. However, in people whose immune systems are compromised – such as HIV-positive people – the tapeworm thrives, causing large numbers of worms in the gut.

Of about 9,000 known tapeworms, H. nana is the only one that can complete its entire lifecycle from egg to adult in an individual's small intestine. This can lead to large numbers of H. nana in the intestine, especially in people with compromised immune systems whose bodies have a harder time clearing the infection.

Researchers theorise that this lack of normal immune function in the human host contributed to the spread of the disease in the body; such infections would be normally restricted to the host’s intestine.

Establishing how to treat such infections presents a further challenge for doctors as therapies to treat adult tapeworms may not be effective against their individual cells. It is also not clear whether normal cancer treatments for humans would be effective for this type of infection, though researchers think that similarities between tapeworm development and the growth of the tumours could point to a common method of treatment.

Doctors and clinicians in developing nations should be aware of the possibility of similar illnesses, especially if they have patients with weakened immune systems who have tumours.

Notes for editors

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Caption: A six-centimetre-long Hymenolepis tapeworm.
Credit: © Dr Peter Olson

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