Leeches in the Southwest

by Jay W. Sharp

As you would probably expect, you stand a far higher chance of coming face to face with a leech if you hike a tropical rain forest or tow Humphrey Bogart's African Queen than if you explore our Southwest desert basins and mountain ranges. You may be surprised to discover, however, that you can find a respectable community of leeches, including blood sucking species, in locations such as our quiet river basin wetlands and high country ponds. 

For example, in Montezuma Well, a pond in a geologically distinctive setting in the Sonoran Desert north of Phoenix, "Blood sucking leeches are…abundant in the aquatic vegetation," according to the National Park Service in its Internet site. Other species are even more abundant.

Meet the Leech

Worldwide, somewhere between 700 and 1000 species populate freshwater, saltwater, estuarine and moist land ecosystems, said leading authorities Fredric R. Govedich and Bonnie A. Bain in the All About Leeches Internet site. About 10 percent of the species occur in the United States.

Related to earthworms, leeches tend to have a colorful, tear-drop-shaped, flattened and segmented body, ranging in length from less than an inch to several inches in length. They have suckers on the underside of each end of their body—an oral sucker at the forward (narrower) end and a second sucker at the rear end. They have, on the head and body, sensory organs that enable them to detect light, odors, vibrations and temperature variations. 

In water, leeches swim with an undulating motion, something like eels. On solid surfaces, they move "by alternately attaching and detaching [their] suckers, crawling about in a looping inchworm like motion," said Govedich and Bain. 

The leeches, all with their mouth parts centered in their forward suckers, comprise three kinds of carnivores. One kind – the "engulfer," which has no teeth or jaws – feeds on small invertebrates, swallowing prey whole. A second kind – equipped with a hollow needle-like proboscis – spears the flesh of worms and snails, secretes an anticoagulant into the tissue to assure the free flow of blood, and sucks out the juices of prey. The third kind – the parasitical blood sucker, which has jaws and razor-sharp teeth – bites into prey, including amphibians, reptiles, fish, waterfowl, mammals and – given the opportunity – human beings. Using both the forward and rear suckers to remain attached to its host, the toothed blood sucker excretes a mucous that, along with suction, fixes it in place while it feeds. It releases an anesthetic to numb the wound site. It secretes an anticoagulant into the wound. If left undisturbed, it will ingest several times its weight in blood. Engorged, it releases its hold and drops off, seeking out a quiet dark refuge to digest its meal, a process that may last for several months. While all leeches are predators, they also serve as prey for fish, waterfowl, reptiles, large aquatic insects and even other leeches. 

Historically, possibly since the Stone Age, leeches have played medicinal roles. According to the BBC Internet site, leeches served Egyptians, Persians, Greeks, Chinese and other early civilizations in bloodletting ceremonies with purposes such as sucking out evil spirits from one's body.  

After falling out of favor for most of the 20th century, leeches have returned to the medical scene, providing anticoagulants and anesthetics for micro- and reconstructive surgery and, possibly, in the future, heart attack and stroke treatments. 

Biology of the Leech

Any two leeches of the same species can mate because all have both male and female sex organs. That is, they are "hermaphrodites," like earthworms. In mating, two leeches entwine their bodies and deliver sperm into each other. "The sperm," said Govedich and Bain, "are then transported to the eggs where fertilization occurs." 

The leech deposits its eggs in a gelatinous cocoon. While there is considerable variability among species, a typical leech may attach the cocoon to submerged plant material or to a stone surface. The cocoon is so tough that it will survive passage through the digestive track of a waterfowl. After several weeks or months, the eggs hatch, and the young emerge, looking like diminutive adult leeches. 

Surprisingly, many leech species make excellent parents, according to Govedich and Bain. A leech may care for its young "in a manner that resembles the care shown by birds or even mammals." It may build a nest for its brood. It may carry its brood attached to its under side or quartered in an internal, marsupial-like pouch. It may even capture and kill prey for its babies until they can provide for themselves. The leech dies after one or two reproduction cycles. 

Attack of the Blood Suckers

According to the Field Guide to Venomous and Medically Important Invertebrates Affecting Military Operations, "When a person enters leech-infested habitats, the leeches quickly swim toward the source of the water disturbance." A leech will quickly attach to the skin with one of its suckers.  It explores the body in its inchworm style until it finds its choice site for a meal, for instance, at the toes or along the shin. It quickly perforates the skin and begins to feed. After it releases its hold, engorged, the wound may continue to bleed, a result of the anticoagulant, for an hour or more. 

According to a horror story on the BBC Internet site, leeches in unfiltered and untreated water drunk by Napoleons' soldiers operating in the Syrian Desert in 1799 attached themselves to the men's noses, mouths and throats. As they filled with blood, they caused some soldiers to die of suffocation and others of blood loss.

Fortunately, leeches raise comparatively small risk in the Southwest. (After all, we filter and treat our water.) While repulsive, the bites tend to be small and inconsequential, especially if you remove the leech and treat the wound properly. (See Removing Leeches and Treating Leech Bites for more information.) 


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