Schistosoma mansoni adults in the vertebrate definitive host

Schistosomiasis (human infection with schistosome worms) is a debilitating disease, rarely killing but often disabling infected individuals. Today we will see the destructiveness of schistosomes firsthand. The US National Institutes of Health maintains the life cycle of Schistosoma mansoni in their laboratories near Washington, DC, using Biomphalaria glabrata snails as intermediate hosts and laboratory mice as definitive hosts. They provide infected mice to research and teaching laboratories around the country. We have access to these infected animals for our teaching laboratory. These mice gave their lives for science, so please treat them with respect.

Mice are great models for human schistosomiasis. Much of the schistosome-related pathology suffered by the mice is closely mirrored in human (and other vertebrate) patients. A few weeks ago (24 August), the mice you see before you were exposed to water with active Schistosoma mansoni cercariae. They were then shipped to UW, and we held them for six weeks, while their S. mansoni parasites developed. Some of these mice may be uninfected, but most will carry extremely heavy infections.

Begin by performing an external examination of your mouse. Do you see anything unusual or notable? Measure your mouse’s length, from snout to rear and from snout to tail tip.
Lay your mouse on its back and begin by making an incision from the anus to the throat using the small scissors. Keep your incision shallow to avoid damaging internal organs.
Pin out the flaps of skin on either side of the mouse, exposing the body cavity.
Observe and draw the internal organs. Note anything unusual.
Compare the liver and intestines of the infected mouse to pictures of healthy mice (see below). What pathology do you observe in the infected mouse? The white spots on the liver of the infected animal are granulomas. Encapsulation of schistosome eggs by the host causes granulomas. What is the consequence to the host? How did the eggs get into the liver? Where should they be?
Now use your dissecting scope to explore the internal organs more closely. The adult worms are visible in the portal veins of your mouse.
Once you have finished drawing your host, remove its parasites. Tease apart the organs to access the portal veins. Breach the portal vein to pull out the worm, or pull out the entire vein.
Place adult worms into Ringers’ solution. Can you differentiate between the male and female worms? Describe any movement or behavior that you see in these living specimens. Draw both the adults and eggs.
Many of these worms will have ripe eggs. Any student who is able to obtain a miracidium will be awarded +3 points on the next exam. Hint: what kind of fluid will trigger hatching in a schistosome egg?
The prominent lateral spine is diagnostic for this species. The eggs of Schistosoma japonicum have a less prominent spine and those of haematobium are terminal. How are spined eggs an adaptation for these worms? Most helminth parasite infections in humans are diagnosed by the presence of characteristic eggs in the feces or urine. You should be able to identify schistosome eggs on the exam.

healthy mouse schistosome-infected mouse

Cercaria batillariae larvae in the snail intermediate host

The Japanese mud snail Batillaria attramentaria is native to Japan but was introduced to the US west coast in the early 1900s, probably in shipments of Asian oysters. In California, it has driven out a native mud snail, Cerithidea californica, in part because Batillaria has only one parasite (Cercaria batillariae), whereas Cerithidea can have up to 20! Greg Jensen collected these snails in mid-September from the intertidal zone in Silverdale, WA.

Please familiarize yourself with gastropod internal anatomy before you begin the dissection:

snail anatomy

Place three snails in a beaker of seawater. Add just enough water to cover the tops of their shells. Give them a few minutes, then remove the snails into a watchglass. Check the water for tiny, swimming cercariae with the naked eye. Place the water from the beaker into a watch glass and check closely with a stereomicroscope for cercariae.

Place one snail on the benchtop. Take a total length measurement (the longest dimension of the shell) and record this in your lab notebook. Crack the spire with a hammer (gently) to reveal the snail. Put the snail in a watchglass and moisten the animal with seawater. Carefully remove the shell pieces for better viewing and examine the animal under a dissecting scope. Infection may or may not be immediately apparent. The visceral hump (the spiral organ that contains the digestive gland and gonads) is the narrow, “corkscrew” extremity of the snail. If the visceral hump is whitish, tan, or mottled, the snail is probably infected with trematodes. If it is dark green or gold, the snail may be uninfected.

Examine the gonad for sporocysts and/or rediae. If you don’t see any at first, tear the tissue with forceps to examine the deeper tissues.

• Draw any cercariae, rediae, or sporcysts that you find and record your observations. Have Chelsea check your snail for infection before you dispose of it! Record your snail data on the board (remember that zeros are data, too).

Note: If you do not succeed in finding an infection in your snails, please dissect another one or observe another student’s infected snail.