Column: Leeches – slimy blood-suckers or medical miracle?

Published 5:49 pm Thursday, August 25, 2005

By Staff
Walleye fishermen may consider leeches the sacred bait but to the rest of us leeches are one of the most disgusting, despicable creatures on the planet. Just the thought of them gives us the willies. If you can get past this prejudice, though, you'll find one of nature's most amazing creatures. Closely related to worms, leeches are one of the most widespread animals on earth. They are found from steamy tropic jungles to frigid arctic lakes living in freshwater and salt and even on dry land.
A leech is not just a leech, though. There are dozens of species, of which 39 can be found here in Michigan. Most are simple carnivores, eating any prey small enough to fit in their mouths. Only a few are the blood sucking type that has given leeches their infamous reputation. Most blood sucking species tend to be host specific, meaning they target only fish or turtles or crayfish or snails or birds or mammals, depending on the species of leech.
Leeches have evolved as survivors. They are highly pollution tolerant and can live in water with low oxygen content. They are hermaphroditic, an efficient means of reproduction, and they don't complicate matters with elaborate sexual rituals. One leech simply deposits a spermatophore on the outer body of another leech. The spermatozoa burrows through the skin and makes its way to the ovaries. Eggs are laid on submerged rocks or wood. As each egg is laid the leech encapsulates it in a cocoon filled with nutrient fluid. The egg hatches into a larvae which lives off this fluid. When all the fluid is consumed the larvae emerges from the cocoon as an adult leech.
Of course, it's the leech species that suck mammal blood that most concerns us. These typically lie in wait in the mud or vegetation of ponds or slow moving streams. When they sense disturbance they venture forth to investigate. If they detect blood, sweat or heat they latch on with their sucker mouth. It's this bite and subsequent treatment of the blood meal that makes leeches such a marvel to science. We're now discovering the lowly leech is virtually a high tech pharmaceutical company.
If you're going to be a host to some parasite you couldn't pick a better one than the leech. After attaching it secretes an anesthetic so the bite can't be felt. This includes a substance that makes the tissue more permeable to help the saliva spread throughout the bite area. The leech holds no ill will toward you and proves it by including a powerful antibiotic in its saliva. Leech bites rarely become infected. This antibiotic kills such hard to treat infections as staph and strep and in artificially enlarged quantities it kills the bacteria causing tuberculosis, dysentery and diphtheria. It also appears to be effective against tetanus and meningitis and is being heavily studied for potential cancer treatment.
The next step in the bite is to inject a chemical that opens the capillaries to ensure copious and prolonged bleeding. Included is an anticoagulant. This is not so much to keep the blood flowing but to prevent coagulation once the blood is inside the leech. Leeches aren't capable of digestion and rely on bacteria within their body to break the blood down so the blood must remain viable for a long time. It's in this regard that leeches are most commonly used in current medical treatment. Allowed to do their thing on skin graphs, leech bites promote blood flow to the area, increasing success and speeding healing. This chemical brew also dissolves blood clots and is being intensely scrutinized for treatment of a multitude of blood disorders.
Equally amazing to all these wondrous chemicals is that no side effects to any of them have been found. Very few, if any, artificial wonder drugs can boast of that. Perhaps that's a right Mother Nature has reserved for herself. The next time you see a leech think not of a slimy bloodsucker but rather a gift of nature leading us toward the cure of some of our most dreaded illnesses. Carpe diem.