The Parasite Zoo – Benjamin Boublil
The Parasite Zoo
During one of our times in London, I attended a lecture on the parasitic world at Gresham College. These types of lecture happen quite often there, with subjects varying from history to science. This one in particular appealed to me as I really like anything related to biology. The subject of parasites is not the most glamorous one, but it is interesting as parasites are everywhere, all the time. The lecturer quoted Nathan Cobb, who said that if we removed all the matter in the world except the nematodes (species of parasitic worms), we could still see the world; indeed, as parasites are on the inside and the outside every single living organism, the outline and insides of each bio-organism would still be apparent.
Parasites are small animals, defined by two things: they live inside or outside a host, living of it, and they cause harm to that host during their survival process. Of course, depending on the species, one single parasite may not cause enough harm to its host for it to be noticeable, but if we take into account the incredible number of parasites and their inclination to carry diseases, parasites can still cause a lot of harm. They are not to be confused with bacteria or viruses; bacteria are single-celled organisms, viruses have a parasitic behavior, but on an intra-cellular scale, whereas parasites are much bigger in comparison. The lecture specifically focused on worms (not the earthworm kind), and more specifically on Nematodes. These worms are very common in hot and poor countries, and tend to reproduce quite fast; in general 400 eggs are laid each year for each female. A lot of them have a very unique reproduction pattern, always a sexual reproduction, but half of the time the organisms are hermaphrodites. They tend to stick together as a couple, never leaving their partner, to ensure an optimization of the reproduction time. Nematodes specifically are round worms, and count so many different sub-species that there are more sub-species of Nematodes than there are of species of all the other insects put together.
The Ascaris is a very big nematode, the side of a big earthworm. The females can lay up to 200 000 eggs a day, which means roughly 100 eggs per minute. They live in their host’s guts, sticking to the walls of the guts with teeth-like organs, drawing a little blood from them, and then sucking it. Their lifespan is extremely interesting to follow. The eggs laid by the female will go down the digestive system to come out in the feces, which will then hopefully for them be eaten by any other organism. From there, they go through the stomach, into the guts, their destination, but they do not stop there; instead, they continue to the intestine, pass through the walls into the blood vessels, go to the lungs, are then coughed up by the host, are swallowed through the cough by the same or another organism, then go to the gut. This seemingly overcomplicated and long process allows them to grow to a respectable size, meaning they will be mature enough to defend themselves from the host’s defenses when they get to the guts the second time. From there, they’ll just live their life by feeding and reproducing.
Nematodes and other parasites usually are like that; they are very complex genetically, but are very simple organisms that live very simple lives. Alone, they are interesting to analyze, but in groups is where the really interesting stuff begins. Due to their number, there’s a high chance an individual carrying one will carry more. Actually, 80% of the worms are inside 20% of the population. This means they live in high density, but this co-infection has different effects. For instance, females in a host will lay fewer eggs per day the more numerous they are. So it would seem that their co-infection reduces their life expectancy, but it actually is the opposite; indeed, the host’s defenses, the antibodies, are far more effective when the worms are in lower numbers, as the worms, when put in groups, each hinder the host’s antibodies more and more as they grow in numbers.
There is of course much more to say on the subject, both on the analysis of individual species of parasites, and on the analysis of their impact as a group. Parasites and worms in particular are among the biggest bio-masses in the world, despite their incredibly small individual mass. When it comes to what is interesting in biology, size does not matter.