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Showing posts with label parasites. Show all posts
Showing posts with label parasites. Show all posts

Saturday, February 11, 2017

Unionids Along the Missouri

Figure illustrating the river system of Nebraska. At top right a single position is indicated by concentric red circles.

Green is where Unionids were found in research.
Red is where I collected Unionid shells.
[Figure derived from those at link.]
Several years ago, my brother and I went on an overnight road-trip to Nebraska. Why? ...well, mostly because it was there and it was close enough to make an overnight trip. Among other memories, one of the highlights was hanging out on the banks of the Missouri River at Decatur.

As we wandered around the river edge, we found numerous large mussel shells. I collected a few, with intentions of identifying the species that made them at some later time.


Fast forward a few years and I'm digging through some boxes in the basement. I'm not sure what I was looking for, but the shells grabbed my attention. It was time to figure out what they were.

I had collected two pairs of shells. One pair was thinner and the other was thicker. One of the thinner shells broke while in storage. The remaining shell is 13.8 cm long, 8.0 cm tall, and 2.6 cm deep. After some looking around at various documents, I realized I could identify these thinner shells as a specimen of a Unionid species called the Great Floater (Pyganodon grandis). The species seems to get this name because of the penchant for their shells to float away when one has died and has begun to rot.

Four different views of a large freshwater mussel shell. The shell is very thick, showing signs of damage and healing over its long life.
Thick-shelled Unionid. Lower-right
is a closeup of growth ridges.
The second pair of shells was much thicker. They're similar in size to the previous shell (12.6 cm x 7.7 cm x 3.5 cm). The shells both look scarred and aged. The best way to determine the age of Unionid shells involves destructive dissection of the shells. Instead, I used the less accurate method of counting the yearly growth ridges. I estimated the shell at ~140 years old (which is well within the age range known for these animals), but I still haven't had any luck with an ID.

Through the process of trying to identify these shells, I accidentally identified a shell I had found in central Texas when I was in highschool. This shell is from another Unionid that is called the Threeridge (Amblema plicata). I had long ago given up on finding the name of this shell, so this was a cool bonus.


Unionids have all sorts of interesting biology. Like most bivalves, they make their living filter feeding water as they hide buried in the sediment. The live in freshwater river systems worldwide, with the most diversity present in North America. Adult Unionids can only travel very slowly by shifting their foot, so you would think they'd have a difficult time traveling up rivers. The Unionids have developed a very special trick to get around this limitation. They use fish to transport their babies.

Unionid larvae (called glochidia) spend some time as a parasite in the gills of fish. The fish can travel upstream or downstream, much further than the adult could ever crawl. After some interval, the glochidia drop from their fishy host and start living the traditional life of a c.

Five images showing Unionid mussels with various features they use to attract fish which are hosts to the larvae of the mussel. Most show fleshy extensions that look like worms or small fish. At bottom-right is a shell edge which has sharp inward pointing teeth growing along it.
Images from unionid.missouristate.edu
How the glochidia get into a fish is kinda awesome. The general strategy is to convince a fish (of an appropriate species) to inhale a bunch of the babies. How they do this varies all over the place. The females of some Unionid species develop large flanges/flaps of tissue that are shaped and colored to mimic small fish or other aquatic creatures. These organs have musculature and so can even move in a realistic manner, which all aids to draw the interest of fish. When the fish come close to try and get a meal, they instead get a mouthful of larval bivalves. Other Unionid species release their larvae in sacs (ovisacs) that attach to rocks or the parent shell and are buffeted around by the current. These sacs look like little fish, again drawing the interest of fish looking for a meal. One group of Unionids (the Epioblasma) even bites onto the heads of fish (using tooth-lined shell edges), so they can shove their babies directly into the fishes' mouths.


These species are long-lived, but sensitive to environmental disruption. They can't survive in a river that dries up and are they're unable to get out of the way when water quality is impaired by human activities. Because of this sensitivity, there are legal restrictions on their harvest (in every state I've checked).

All the shells I have were collected on dry land, which sidesteps the legal restrictions designed to protect the live animals from harm. Frankly, I couldn't imagine collecting living animals to get their shells. You have to respect your elders, even if they happen to be living on the bottom of a river.


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Saturday, November 26, 2016

Future of the Guinea Worm

Guinea worm (Dracunculus medinensis) is one of those parasites that nightmares are made of. Juvenile worms infect freshwater copepods, which invariably end up getting ingested [by humans] when drinking contaminated water. The adult female grows up to a few feet long. It migrates to the skin (usually in the lower leg) and induces an extremely painful blister about a year after infection. The blister is described like being set on fire. The pain is alleviated best by standing in water, which is exactly what the worm wants. When the blister is in water, the female worm releases hundreds or thousands of babies into the water.

Former US President Jimmy Carter has been leading an organization working to make the worm go extinct. As a disease organism, few people are going to lament its extinction. When I first learned about this organism, it was invariably described as infecting humans only. This would make the process of wiping it out so much simpler. Unfortunately, the story isn't quite so simple. The worm has other plans.

Hind leg of a dog with a parasitic worm hanging off the side.
Figure 1 from paper.
Increasingly, dogs in Chad are being found with lower-leg lesions that have worms hanging out of them. Genetic analysis has shown it is the same species as the Guinea worm which infects people. Even if we prevent all human infections for long enough to interrupt the parasite's life cycle, it can still persist in other animals. It looks like it would take continuing diligence to keep it from erupting into an active human disease again.

Figure showing increased incidence of parasite in each of four years.
Adapted from page.
Over the last several years, the number of infections observed in dogs has been going up and up, while human infections have been minimal. This pattern of yearly increases suggests the worms have been adapting to their new hosts.

The researchers did find evidence for human behavior that helped give the parasite the opportunity to make this transition. At the end of the dry season, the locals do a mass harvest of fish. The fish are processed and dried/smoked for later use. The guts and other undesirable bits are discarded for the dogs, chickens, etc. to deal with. The dogs are then getting infected by eating the fish guts. It also appears that uncooked/undercooked fish are responsible for the human cases of infection.

Figure showing life cycle of parasite through copepods to fish/humans/dogs and back to living in the water where they infect new copepods.
Adapted from Figure 9 of paper.
Historically, most human infections by this parasite were due to ingestion of water contaminated by infected copepods (an host to an earlier stage of the worm). With increasing knowledge about this mode of transmission, it became dramatically less useful of a pathway for the parasite. At the same time, any alternate pathway for the parasite to get into its main host would have been positively selected. Essentially, we've just seen a parasite go from a life-cycle with one intermediate host to a life-cycle with two intermediate hosts.

Many parasites are known to have complicated life-cycles passing through several intermediate hosts, but this is the first case I've come across that helps to illustrate how those complex life-cycles could have evolved.


Better control of the fish discards will help minimize the infection pathway through dogs, but it won't necessarily get rid of the problem. While adapting to their new hosts, the worms have had to evolve to better escape notice by the canine immune system. A consequence of this is that they will be better prepared to infect dogs later by other pathways, even if fish discards aren't available. Maybe dogs will start getting infected by ingesting infected copepods like humans used to. Maybe dogs will start getting infected by eating scavenged fish that died in the dry season. I can't predict what will happen exactly, but I understand the power of natural selection and very much believe the worm will find another way to survive even if we completely prevent transmission to humans in the near future.


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