Schistosoma is a genus of parasitic flatworms, or blood flukes, responsible for the disease schistosomiasis. This condition affects millions of people globally, particularly in regions with limited access to sanitation and clean water. Because the parasite lives inside the blood vessels of an infected person, it cannot be seen directly. Therefore, microscopic examination of a patient’s urine or stool for the parasite’s eggs is the primary method for diagnosing an active infection.
Adult Worm Morphology
Adult Schistosoma worms exhibit sexual dimorphism, meaning the males and females have distinctly different body forms. The male worm is shorter and wider, measuring approximately 10–18 mm in length. Its primary feature is a groove that runs along its ventral surface, known as the gynaecophoric canal, which is formed by the sides of the male’s flat body curling inward.
The female worm is much longer and more slender, appearing almost thread-like, and can reach up to 20 mm in length. She resides permanently within the male’s gynaecophoric canal, a state described as “in copula,” or paired. This pairing is necessary for reproduction, after which the female lays the eggs that cause disease symptoms and are used for diagnosis.
Identifying Schistosoma Eggs
The definitive diagnosis of schistosomiasis relies on the microscopic identification of eggs in a patient’s excretions. The eggs of the three most common human-infecting species have unique shapes and sizes, particularly regarding the placement and appearance of a spine on the eggshell. An experienced microscopist can also determine if an egg is viable by observing the living larva, or miracidium, inside.
Schistosoma haematobium eggs are typically found in urine and are a leading cause of bladder cancer. These are large, oval-shaped eggs measuring between 110–170 micrometers (µm) long and 40–70 µm wide. Their most prominent feature is a sharp, terminal spine, meaning it protrudes from one end of the egg.
Infections with Schistosoma mansoni are diagnosed by finding eggs in fecal samples. These eggs are also large, ranging from 114 to 180 µm in length, and possess a very distinct and prominent lateral spine that juts out from the side of the eggshell.
The third major species, Schistosoma japonicum, also passes its eggs in feces. These eggs are noticeably different, appearing more rounded or broadly oval compared to the others. They are slightly smaller and possess a very small, inconspicuous spine or knob that can be difficult to see.
Other Microscopic Life Stages
Beyond the adult worms and eggs, the Schistosoma life cycle includes other microscopic stages, though these are not typically seen in clinical patient samples. When schistosome eggs are released into freshwater, they hatch and release a larval stage called the miracidium. Miracidia are oval-shaped and covered in cilia that they use to swim in search of a specific freshwater snail, which serves as an intermediate host.
After developing within the snail, the parasite emerges as a new, infectious larva known as a cercaria. This stage is characterized by a body and a distinctive forked tail, which it uses to swim through the water. It is the cercaria that penetrates the skin of a human host, shedding its tail in the process to begin the infection. Observing these free-swimming stages usually occurs in a laboratory or environmental assessment context.
Preparing Samples for Viewing
To visualize Schistosoma eggs, which may be present in low numbers, samples must be properly prepared to concentrate them for microscopic viewing. The method used depends on the species suspected, as this determines whether urine or feces is the correct sample to analyze. Both techniques aim to make the eggs easier to find.
For infections with S. haematobium, a urine sample is collected. The eggs are then concentrated using either filtration or a process of centrifugation and sedimentation. In filtration, a specific volume of urine is passed through a fine filter that traps the eggs, and this filter is then placed on a microscope slide. Sedimentation involves spinning the urine in a centrifuge, which causes the heavier eggs to collect at the bottom of the tube for examination.
Diagnosing infections from S. mansoni or S. japonicum requires a stool sample. The most common method for preparing fecal samples is the Kato-Katz technique. This procedure uses a template to measure a standardized amount of fecal material, which is then pressed onto a slide and covered with a glycerin-soaked cellophane strip. The glycerin helps to clear the fecal debris, making the spined eggs visible.