The majority of us don’t find nature disagreeable. The common consensus is that it is perfectly acceptable, even beautiful… in its right place. But its right place, we feel, is rarely in our homes, and almost never inside our bodies.
The human body is, in fact, a seething mass of other creatures, from microscopic bacteria and fungi to much larger parasitic invaders. Approximately 10 trillion cells make up the average human body, and yet there is also an additional 75 trillion cells on and in our bodies that belong not to us but foreign organisms.
One of the most famous of all internal parasites is the much-maligned tapeworm, which live as adults within the intestinal tracts of vertebrates. Long and thin, the tapeworm has a very simple body: the head, which has no eyes, mouth or even brain, attaches to the inside of the intestines by means of four suckers, while the body trails behind it through the entire digestive system. It does not have a gut of its own because it absorbs already-digested food through its paper-thin skin as it flows past.
Obtaining food if you happen to be an internal parasite, therefore, is not hard. The tricky part is ensuring your young reach a similar position within another animal of the same kind. The solution is to recruit a different kind of host – an intermediate host – that will eventually be consumed by the animal that the adult tapeworm needs to live in. Humans have had an intimate relationship with tapeworms ever since we were hominids that learned to walk upright, but it’s a relationship that most of us could probably do without.
One of the tapeworms we are most commonly infected with is the beef tapeworm. When it decides to lay its eggs (up to a million a day), they travel through our digestive system and are excreted. For the tapeworm eggs to hatch, they need to end up within the body of a cow. Since humans generally dispose of their bodily waste in a toilet, a place not usually frequented by cows, the majority of these eggs are wasted. In places with poor sanitation and sewage, however, especially in parts of the developing world, human faeces can end up in places where cows are grazing. If this happens, the tapeworm eggs can be inadvertently taken up by a cow as it feeds. The eggs hatch inside the cow and the larvae then burrow their way into the animal’s muscle tissue. This same cow may later get turned into meat and even if it is slightly undercooked, the tapeworm larvae will survive and be transferred to the human consumer. Here, they will mature and reproduce for themselves.
Such a circuit is an extremely chancy one. Only in rare places do human faeces end up in an environment where cows live, and even then, cows will not commonly eat grass that has excrement on it. If the cow is not eaten by a human but by another predator, the larvae within it will soon die. They will also die if humans do indeed eat the cow but thoroughly cook its meat beforehand. Only a miniscule percentage of the tapeworm eggs will make it back to the digestive tract of a human, but that’s fine because a tapeworm can lay as many seven thousand million eggs in its lifetime.
Tapeworms are extremely hardy creatures and can only be truly killed if their head end is destroyed, so they can live within their host quite happily for twenty-five years or more. The beef tapeworm is usually only around 10 millimetres in length, but there have been some rather worrying reports of specimens reaching 12 metres in length. I doubt there is anyone in the world who wouldn’t mind having a 12 metre long worm living inside their digestive system.