Wouldn't it be great to live in a place where it was nice and comfortable, where there was unlimited food and drink, no need to go to work, no stress. All you have to do is eat and have sex all the time?
Welcome to the world of the parasitic worm.
These creatures which live inside our bodies are viewed by many people with horror and disgust, but in a biologist's eyes they are exquisite examples of the evolution of highly specialised life forms, finely tuned to the habits of the host animals (including human beings) in which they live.
The biggest problem facing a parasitic worm is how to find a suitable home for its descendants. That is, how to get its offspring from inside this host to inside another one. This is the problem of "transmission".
If you are a worm living in the gut of an animal, then the obvious method is to send your children out through the host's anus and ensure that they get into another host through its mouth.
The direct method
Some worms use unsubtle methods. That is, they produce as many eggs as possible and rely on the host's dirty habits and a lot of luck to transmit their offspring to new hosts. An example of this approach is that taken by Ascaris lumbricoides, the large nematode worm which infects man. Females of this species can reach 30cm in length and be 6mm wide (which is great going, when you realise that most of the 100,000 or so other nematode species are microscopic).
Male and female worms live in the small intestine and produce lots of tiny eggs (one female can lay over 200,000 per day). These eggs are passed out in the host's faeces. They mature in about two weeks and can live for years in damp shady soil (the type of place where people like to go and defecate).
The egg shells are sticky, and when people (especially children) touch contaminated soil, they get eggs on their fingers. If they don't wash their hands, they get infected through the mouth. Once inside the host, the Ascaris eggs hatch out, but before settling down, the young worms take a scenic tour of their new home. They burrow through the gut wall and enter the bloodstream for a trip via the heart to the lungs. Here they burrow out and make their way up to the throat (sometimes causing pneumonia) and finally down into the intestine again. During this trip they grow in size about ten times.
Why do they have this complicated tour before they settle down? The answer lies in the basic life cycle of all nematodes. Like insects, nematodes have a tough outer skin (cuticle) and must molt and pass through several stages in their development. A butterfly passes from egg, caterpillar and pupa to adult butterfly. Nematodes pass through four larval and two adult stages. The third larval stage is the infective one. So, basically the scenic tour of Ascaris is a "time out" for the worm to grow up and develop to a stage ready for living in the new host's intestine.
How did Man and Ascaris evolve to come together in this way? Fossil human faeces show that Ascaris has been a companion of ours since at least the Stone Age. We have also been close companions of pigs, which carry their own related worm, Ascaris suum. Since man and pigs are so similar physiologically (pig heart valves are used to repair human hearts) it is no wonder that the pig worm became adapted to humans. How pigs got parasitised in the first place is another (and more complex) problem. Nematodes are found everywhere in our environment, in the soil, sea, freshwater, ice, deserts, and plants, and DNA studies suggest that different groups took on the parasitic life style independently at different times.
The Indirect method - via dinner
Tapeworms are more subtle in their methods of transmission. They use an "intermediate" host, and exploit the main (definitive) host's feeding habits to complete their life-cycle.
One example is the "Beef Tapeworm" of man, Taenia saginata, the adult of which lives in human intestines. The worm can be up to 25 metres long and live for over 20 years. The body form is highly adapted to its life as a parasite. It has a simple, tiny head (1.5 - 2 mm) called a scolex, which consists of four suckers to hold on to the wall of the host's intestine.
Unlike a nematode which feeds using its mouth, a tapeworm does not have a gut and absorbs nutrients through the surface of its body. The entire animal is specialised as an egg-making machine. It is hermaphrodite, meaning it has sets of both male and female sex organs and these are repeated again and again in the long strip of segments, which makes up its tape-like body.
This grows continuously with young segments forming at the scolex end, while the most mature segments at the other end are nothing more than sacks full of eggs. These drop off and can be seen moving about on the surface of freshly passed faeces. They are very active, crawling about like white caterpillars and often they creep out of the anus. Strange as it may seem, a well fed, healthy host can live with his tapeworm without ill effects for many years.
To get to the next (intermediate) host, the Taenia eggs depend upon the less than hygienic grazing habits of cattle. Coprophagy (the eating of stool) is common in herbivores, providing the animals with important nutrients. In India, cattle frequently follow human beings to their defecation sites in anticipation of a fecal meal.
While the cattle snack, they ingest tapeworm eggs which, instead of developing into adult tapeworms, hatch into tiny hooked larvae which burrow their way out of the intestine. They are then carried by the blood stream to the muscles where they hide in small 10mm cysts and wait.
They must wait for the lifetime of the cow. If the animal is slaughtered and becomes someone's dinner, and if that person likes their meat lightly barbequed in large chunks, then the parasite is lucky because the cysts can survive this gentle cooking and once inside a human gut, they turn into adult worms.
There is a similar, smaller tapeworm of man called Taenia solium. This normally infests the meat of pigs. Unfortunately, unlike the beef tapeworm, it is not so choosy about the intermediate host and man can become infected both by the adult worm, and more dangerously by the cysts. A few cysts in a muscle will not do much harm, but can cause problems and even death if they develop in the eye, heart or brain.
It is thought that tapeworms first evolved millions of years ago from free-living marine flat-worms which became parasites of crustaceans. They then became parasites of fish which ate crustaceans, and then parasites of fish which ate fish which ate crustaceans....When fish evolved into amphibians, reptiles, birds, and mammals, their tapeworms evolved and diversified too.
Taenia tapeworms have been living with us since even before the evolution of Man. That is, they were parasites of our early ancestors, the ape-like creatures in Africa, which shared the kill of carnivores and became infected through eating raw meat. Today, religious dietary habits mean that the pork tapeworm is rare in Jewish and Muslim communities, and the beef tapeworm is rare in Hindu ones. No such dietary restrictions affect Christians, who are host to both species.
A very indirect method - via a magical mystery tour
But the most elegant method of transmission belongs to the Trematode worms. Those which infect man are not typical of the group (nor so interesting from a biological point of view) so here we shall look at one called Diplostomum, which lives as an adult in the intestine of a sea-gull, and whose young take a magical mystery tour to reach a new bird host.
The adult worms are small, only 2mm in length. Like tapeworms they are flat in shape, hermaphrodite and have suckers to attach themselves to the intestine of the host. However they are not segmented, instead large numbers of them constantly produce eggs which are passed out in the faeces of the sea gull.
If the bird defecates over the sea or the land, then it is bad luck for the parasite because the eggs must land in fresh water (a river or lake) in order to continue the life cycle. Once in water they develop over a few days and then hatch, releasing tiny torpedo-shaped creatures called miracidia.
These are covered with hair-like cilia which enable them to swim swiftly in search of the next host, which is certain species of freshwater snail. They must complete their search within a few hours before they use up their energy reserves and die.
If a miracidium is lucky, and the right species of snail is nearby, it burrows inside and settles down in the snail's liver (the part which is coiled up inside the shell). Here it transforms itself into a long sack which acts as brood chamber for the next generation of larval form called cercariae. These are just visible to the naked eye.
Large numbers escape from the snail like puffs of smoke every morning. They are shaped like tiny "Y"s and wriggle up to the surface of the water, then drift down slowly to wriggle up again. The Y shape of their tails acts like a parachute to slow their descent and save energy, and thus they wait.
Like miracidia, they only live for a few hours during which time they must encounter the next host or die. Unlike miracidia which actively seek slow-moving snails, cercariae wait in ambush to bump into fast-moving freshwater fish. If they make contact, they burrow through the skin of the fish and migrate through the body to the lens of the fish's eye. Here they wait. In heavy infections the fish goes blind. Blind fish cannot feed properly and they swim weakly near the surface of the water, becoming easy prey to a passing sea-gull.
An incredible amount of luck is required for Diplostomum to complete its life-cycle, and yet it survives. At each stage it takes advantage of the hosts' behaviour in order to transmit itself via this complicated path to the intestine of a different sea-gull host.
How could such an intricate life cycle have evolved? The one thing which is common to all trematode life-cycles is the presence of a snail intermediate host. It is believed that, like tapeworms, these parasites evolved from free-living primitive flat worms. However in this case, the worms first became parasites of snails, later adding other hosts so that they could spread from one place to another (snails do not move very fast nor very far!)
A parallel evolution
While worm infections still have a considerable impact on public health in developing countries, they are becoming rarer in modern western society. This is not because of advanced medicines, but simply because of improved hygiene. Eventually, perhaps, man will succeed in making our long term companion, Ascaris lumbricoides, extinct. This product of millions of years of evolution will become a victim, not of volcanoes, climate change or meteors, but the invention of latrines and soap.
Christine Betterton Jones - BSc. (Zoology), PhD (Parasitology)
