Parastic worms or ‘helminths’ are worms that have a lifecycle that involves living in a human host
Nothing turns the stomach more of the traveller to distant lands than the thought of enormous worms living for years in your intestine without you knowing it. Yet this is the case for billions of people around the world including many of us in hyper-sanitary North America and Europe.
Worldwide, an estimated one to one and a half billion people are infected with an average of seven 15–60 centimetre long Ascaris lumbricoides roundworms while another 600–800 million people have hookworms. Perhaps two billion people have the tiny pinworm living in their bowel and millions of others have various beef and fish tapeworms in their intestines, some of which can reach 10–20 metres in length.
Unlike their microbial parasitic counterparts, helminthes (parasitic worms) are only microscopic in the egg or larvae stage, with most oval-shaped eggs measuring in the 25 micron by 30 micron to 50 micron by 150 micron range.
North America and northern Europe are relatively free of most types of parasitic worm species with a few notable exceptions. The knowledge that parasitic worm eggs are not naturally found in North American and northern European waterway may add to your peace of mind. Nevertheless, raw sewage and waterways polluted with sewage often contain enormous numbers of eggs from infected individuals, particularly in areas frequented by tourists.
Fortunately, only a few types of eggs can infect humans without intermediary hosts that are not indigenous to North America and Europe. This means that water in wilderness areas, and to a lesser extent in more frequented recreational areas, can be considered generally free of parasitic worm eggs and larvae.
Even if eggs or larvae are present, they are easily filtered or destroyed by heat. The greatest threat in North America and Europe is from worms that produce eggs that can infect humans immediately after being passed in the faeces, without needing an intermediary host or specific soil conditions.
There are also several worm parasites that infect animals, such as dogs, which can live in humans but die without maturing or reproducing. Since parasitic worms are rarely acquired from drinking water in North America and Europe and realtively easily dealt with while travelling outside of these areas this section contains only be a brief summary of waterborne parasitic worms.
Biologists divide helminthes into the two main categories of platyhelminths (flatworms), which includes the subdivisions of cestodes (tapeworms) and trematodes (flukes), and nematodes (roundworms).
Humans get worm infections in two ways. The first is via oral ingestion of eggs in undercooked meat or fish, or in food and water, or from hands and objects contaminated with eggs.
The second method involves certain types of microscopic larvae burrowing through the skin in body parts exposed to contaminated soil or water—conditions not found in North America and Europe.
As with microbial parasites, the faeces of people infected with intestinal worm parasites can contain an enormous number of microscopic eggs. For example, each Ascaris lumbricoides worm produces over 200,000 eggs per day, which are shed in the host’s faeces. Eggs can remain viable for long periods of time. Eschinoccus granulosus (dog tapeworm) eggs are infectious for up to one year and Ascaris lumbricoides eggs can remain infectious for up to nine years.
Infectious Dose and Reservoirs
In theory, infection can start after ingesting one egg. Human parasitic worms are generally restricted to humans with occasional crossovers from other mammals. Many types of helminthes live in people, in North America, but only a small number can reproduce effectively because of unfavourable environmental conditions. For example, all trematodes—intestinal, liver, lung and blood flukes—require specific species of freshwater snails to complete their lifecycles before they can infect humans. These snails do not live in North America and Europe.
Taenia (Beef and Pork Tapeworms)
Two versions of Taenia infect humans. The larvae of T. solium are found in pigs, and T. saginata live in cattle. Humans get infected by eating raw or undercooked flesh from animals that have infective cysticerci (larvae) imbedded in their flesh.
While Taenia can exceed 20 metres in length in the human intestine they have only a minimal effect on infected people. There is also the possibility that the round, 35–40 micron diameter T. solium eggs might hatch into cysticerci in humans. Because cysticerci cannot develop in the flesh of humans they die in bodily tissues, including the brain, and are calcified by the body within a year. Known as cysticercosis, this syndrome can be acquired by ingesting eggs passed in human faeces.
Humans get infected with Echinococcus granulosus only by accident. Normally the worm’s lifecycle involves a larvae stage in sheep, goats, cattle or horses and an adult phase in dogs. Generally only humans who work closely with sheep and dogs, such as sheep ranchers, get infected. Echinococcus granulosus eggs are roundish in shape and 35–40 microns in diameter.
Hymenolepis nana is the most common tapeworm in humans, particularly in Asia. Mature worms measure only 1.5–4 centimetres long and Hymenolepis nana are the only tapeworms that do not require intermediary hosts before infecting humans.
Most eggs released by the worms in the ileum of the intestine are capable of hatching and reinfecting the already infected host. Oval-shaped 30 micron by 45 micron eggs are passed in the faeces and can survive up to two weeks. Given the concentration of this parasite in Asia and the short survival period of eggs, North American and European water outside of urban or recreational areas frequented by Asia tourists is probably egg free. Water within Asia has a considerably higher probabilty of containing the eggs.
Diphyllobothrium latum is a fish tapeworm common in Scandinavia and northern North America. Humans contract it by eating infected raw or undercooked fresh water fish.
Adult worms are several metres long and release oval-shaped 45 micron by 65 micron eggs in the host’s faeces. Eggs hatch in fresh water and infect fresh water copepods, which are then eaten by fish. The larvae develop in the fish, and if infected fish are eaten raw or in an undercooked state, the larvae can develop into mature egg-producing adults, in the human intestine.
These flukes are common in East Asia in humans and pigs, and produce eggs in the 80 micron by 140 micron range.
The eggs of the most common intestinal fluke, Fasciolopsis buski, are passed in the faeces and hatch into larvae in fresh water where they infect particular snails and develop into cercariae. The cercariae then escape and become encysted as metacercariae on aquatic vegetation such as water caltrop or water chestnut, which are eaten by humans.
The metacercariae encyst in the human intestine and eventually develop into seven centimetre long flukes that attach themselves to the intestinal wall. Similar intestinal flukes include Heterophyes and Metagonimus.
Fasciola hepatica are parasitic liver flukes of the bile ducts that are found in sheep, cattle, and often humans. They produce eggs in the 80 micron by 140 micron range, which eventually hatch into larvae that are ingested on contaminated watercress. Once they reach the intestine, they encyst before hatching into tiny flukes that penetrate the intestinal wall, reach the liver, and finally take up residence in the bile ducts. Eggs are passed via the faeces and the cycle continues. Other liver flukes with similar lifecycles include Clonorchis sinensis, Opisthorchis felineus and Opisthorchis viverrini.
Paragonimus westermani are lung flukes usually isolated in East Asia, West and Central Africa, and Central and South America.
Humans acquire the fluke by eating larvae encysted in fresh-water crabs and crayfish that are raw or undercooked. After ingestion, the larvae encyst in the small intestine and penetrate the gut wall before migrating to the lungs. Adults grow in the lungs within fibrous cysts to about one and a half centimetres long. Oval-shaped 35 micron by 85 micron eggs are coughed up in sputum, and are spit out or are swallowed to eventually pass in the faeces.
In fresh water, the eggs hatch to release miracidia that enter a particular snail where they develop into cercariae. The cercariae then escape into the water and penetrate the flesh of fresh water crabs or crayfish, where they await ingestion by humans.
Schistosomes (Blood Flukes)
An estimated 200–300 million people, primarily in Africa, South America and East Asia, are infected with this fluke which causes up to 1.5 million deaths annually. Three species of schistosomes infect humans. The fluke is pervasive in areas such as the Nile River delta and other common traveller destinations in Africa and Asia.
The adults exist as thin threadlike worms in the abdominal veins of humans and a few domestic animals. Eggs are laid in the capillaries that drain the intestine and most eggs pass into the intestine or bladder and are excreted in the faeces or urine with a small amount of blood. With severe infection, blood loss can be extreme and is easily visible in the urine and faeces.
Some eggs are carried in the blood to the liver or lungs, depending on the species, and become encased in a fibrous granuloma. Eggs passed from the body hatch in fresh water and infect particular snails before being released as 80 micron by 200 micron cercariae.
The cercariae eventually infect humans by burrowing through the skin of people standing in or handling infected water and the cycle continues. In North America and Europe, duck schistosomes commonly penetrate and die in the skin of people, causing the well-known ailment ‘swimmer’s itch.’
Ascaris lumbricoides live in 20% of the world’s population with the average infected human carrying seven worms. Infection is usually limited to tropical areas but can occur anywhere an infected person deposits untreated faeces so infection is common. The worm’s oval-shaped, 30 micron by 60 micron eggs are tough and can survive months or years in the environment while they await ingestion by a human host.
Once eaten, Ascaris lumbricoides eggs hatch in the intestine. Larvae breach the intestinal wall and are carried to the lungs in the blood stream. In the lungs, they enter the alveolar spaces and begin to mature before being coughed up and swallowed. Once in the intestine for the second time, the worms mature into adults that resemble 15–60 centimetre long earthworms. Adult worms live for one to two years.
Trichuris trichiura simply hatch in the intestine and penetrate the colon wall where they mature. The worms then re-enter the large intestine, with their tails remaining in the lumen, where they lay 20 micron by 50 micron eggs that pass out in the faeces. The worms live for several years.
Ancylostoma duodenale, Necator americanus (Hookworm) and Strongyloides stercoralis
These roundworms live in humans in warm tropical areas and produce eggs in the 30 micron by 60 micron size.
Eggs in the case of Ancylostoma duodenale and Necator americanus and larvae in the case of Strongyloides stercoralis, are passed in the faeces and develop into infective larvae in warm moist soil. The larvae then burrow through any human flesh that comes into contact with infected soil including bare feet.
The larvae enter the bloodstream and are carried to the lungs. In the lungs, they break into the alveolar spaces and are coughed up and swallowed.
Back in the intestine, the larvae grow into egg-producing adults. Strongyloides stercoralis larvae often migrate from the intestine, through body tissues, causing inflamation and often reinfecting the host. In these cases, infection can continue for a decade or more and, in immunocompromised people, the end result can be complete tissue infestation and death by septicaemia.
Enterobius vermicularis (Pinworm)
Enterobius vermicularis are known popularly as pinworms or threadworms. These tiny one to two centimetre worms are common worldwide, especially in children.
Adult worms live in the lumen and squirm out of the anus when the infected person is resting, usually at night, and lay thousands of 25 micron by 50 micron eggs on the skin surrounding the anus.
The light eggs are easily ingested from bedding lint from under the fingernails of the infected person who scratches the irritated area or from door handles, furniture, recreational equipment or other objects handled by the infected person. Once eggs are swallowed they quickly develop into adults that continue the cycle.
Trichinella spiralis infect some rodents, hyenas, polar bears and occasionally humans who have eaten raw or poorly cooked meat including pork.
These roundworms live for six to eight weeks in the intestine where the females pass large numbers of 6 micron by 100 micron larvae that burrow through the intestinal wall and enter the bloodstream. Once in the blood, they are carried to voluntary muscles where they encyst and die as they are calcified by the body. Larvae that escape in faeces can be reingested and mature in animal and human hosts.
Among these parasitic worms are members of the genera Wuchereria, Brugia, Onchocera, Loa and Dracunculus (Guinea worm).
All of these parasites are restricted by environmental considerations to areas in Central and South America, Southeast Asia, Central Africa, parts of the Middle East, India, the West Indies and South Pacific islands.
People become infected when 7 micron by 200 micron microfilariae get into the bloodstream via the bite of mosquitoes or blackflies. Only Dracunuculus, which are found in West Africa, parts of the Middle East and India, are transmitted by drinking water that contains small fresh water crustacea that act as an intermediary host.
Nematodes with Non-Human Hosts
Several round worms that infect non-human mammals can infect humans and die without reproducing.
Most importantly, the larvae of Ancylostoma braziliensis (dog hookworms) can penetrate the human skin and migrate for several months beneath the skin, causing allergic reactions, before dying.
The eggs of Toxocara canis, which are similar to those produced by its close relative Ascaris, are common in dogs and cats and sometimes hatch in humans. The larvae can migrate around the body before dying in an immature state.
Soil in parks, playgrounds and beaches that is contaminated with dog faeces is the most common source of this canine parasite.