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The roundworm, also known as Ascaris lumbricoides or the large intestinal human roundworm, is more common in areas of the world with an appropriate climate, a high population density, and where sanitation is a problem. The World Health Organization (WHO) estimates that about 25% of the world's population are infected with roundworm. The Ascaris causes an infection known as ascariasis. Living worms are pale pink in colour and have a bright red stripe of their bodies. The female roundworm typically ranges in size from 16cm to 33cm inches and can grow to the thickness of a pencil. The male roundworm is smaller than its female counterpart. Up to approximately one hundred roundworm worms can infect a single human host.

The female roundworm can lay as many as 200,000 to 300,000 eggs a day. The eggs of the roundworm have an incredibly thick shell and are very resistant to variations in temperature and humidity in the environment. They are coated with a jelly-like substance with a lumpy surface that allows the eggs to stick to almost anything. Although it takes about two to three weeks for them to become infective, roundworm eggs will remain infective in the soil for some time. In order to catch roundworm you need to be in contact with infective eggs, which have passed through contaminated faeces. This usually occurs via the faecal-oral route. Give the right conditions, polyparasitic infections involving more than one type of worm are a possibility because of the similarity of the environments favoured by hookworms, whipworms and roundworms.

Once someone ingests infective eggs, usually after contact with infected food or hands, then the eggs are carried to the lower intestines where they hatch into larvae. The larvae then migrate through the body, passing through the intestinal wall, and are carried by the blood or the lymphatic system to the lungs. Once in the lungs, the roundworm larvae break out of the pulmonary capillaries into the air sacs, ascending into the throat and descending into the small intestine again. After about two months in the small intestine the roundworms mature into adults. The roundworm adults remain in the small intestine feeding on the contents of the intestine. They do not actually feed on the human host as many of the other nematodes do.

If the roundworm infestation is only a mild one then there will be few easily identifiable symptoms. There is the possibility of intermittent abdominal pain and "flu like" symptoms. Coughing and wheezing could occur when the roundworm larvae migrates to the lung. Severe infestations can result in vomiting, restlessness, sleep disturbances and blockage of the intestine resulting in extreme abdominal pain. The large size of roundworms means that they can literally block up the gastrointestinal tract with their bodies.

A large infestation of roundworms can cause pneumonia. This would occur well before the roundworm larvae reach the intestine and so would be difficult to diagnose. The passage of the roundworm larvae through the blood vessels of the lung can cause haemorrhaging. There can also be an inflammatory response accompanied by oedema, which is the swelling caused by an excessive accumulation of serous fluid in tissue. "Ascaris pneumonia" can result, a potentially fatal caused by the accumulation of fluids in the lungs.

In rare instances roundsworms have been known to wander to places in the body outside of their normal life cycle. Certain specific circumstances, such as when a host gets a fever, takes a worming treatment or undergoes a surgical procedure requiring anaesthetic, may increase the chances of this occurring. There have been documented cases where roundworms have migrated from a patient's small intestine and out of their nose or mouth during an operation. A wandering roundworm may be passed out in a stool or, if it heads upwards through the stomach, it may be vomited out. In some very rare instances infants have been suffocated when large roundworms have crawled up behind the nasal passages and blocked their breathing. There have even been instances where roundworms have migrated sideways into a bile or a pancreatic duct, blocking them. Roundworms have also penetrated the small intestine on occasion, which can result in life-threatening peritonitis.

Diagnosis will need to be made by a doctor with the assistance of a pathology laboratory. The laboratory will be able to discover characteristic roundworm eggs in a stool sample. Adult roundworms, which can grow as big as 40cm long, may occasionally be passed in a stool or be present in vomit.

A roundworm infestation can be treated with Vermox or Combantrin-1, which contain mebendazole, or Combantrin, which has pyrantel embonate as its active ingredient.