Blastocystis hominis is a microscopic parasite that causes an intestinal infection known as blastocystosis. It is primarily transmitted through the ingestion of contaminated food or water. The parasite is commonly found in tropical and subtropical regions, but cases have been reported worldwide due to international travel and imported food. Blastocystis hominis infection can lead to symptoms such as watery diarrhea, abdominal cramping, and fatigue, which can last for several weeks if left untreated. It is important to practice good hygiene and food safety measures to prevent the spread of this parasite. Some preventive measures include washing fruits and vegetables thoroughly before consumption, avoiding raw or undercooked food, and drinking only bottled or boiled water in areas with poor sanitation. Additionally, individuals should be cautious when consuming food from street vendors or unfamiliar establishments, as these may have a higher risk of contamination.

Through this article, you’ll be able to answer the following questions:

• What does Blastocystis hominis cause?
• What is the best treatment for Blastocystis hominis?
• What type of protozoa is Blastocystis hominis?
• What are the 4 forms of Blastocystis?
• What is another name for Blastocystis hominis?

Blastocystis was originally classified as a yeast form due to its appearance in fresh wet mounts as a round, yeast-like glistening image. Initially, no amoeboid characteristics, such as pseudopodia for locomotion, were observed. Later research by (Zierdt CH., 1988 Jan;) led to reclassification under the subphylum Sporozoa based on some similarities between Blastocystis cells and Protista cells. The presence of an organized nucleus, smooth and rough endoplasmic reticulum, Golgi complex, mitochondrionlike organelles, and pseudopodia influenced this decision. Its sensitivity to antiprotozoal drugs and inability to grow on fungal media further indicated that it was a protozoan rather than a yeast (Mehlhorn H. Brumpt, 1988). Further revisions were eventually made, and more recently, the classification of Blastocystis was based on modern molecular approaches to classification, which revealed that Blastocystis is neither a yeast nor a protozoan. It is now part of a new kingdom called the stramenopiles. Brown algae, mildew, diatoms, the organism responsible for the Irish potato famine, and the organism responsible for sudden oak death disease are all stramenopiles.

Classification

• Domain: Eukaryota
• Kingdom: Protista
• Phylum: Bigyra
• Class: Blastocystea
• Order: Blastocystidida
• Family: Blastocystidae
• Genus: Blastocystis
• Species: B. hominis

Morphology

The morphological forms of Blastocystis exhibit significant diversity, making identification of the organism as the causative organism for diarrhea difficult. The vacuolar (also known as central body), granular, amoeboid, and cyst forms are the most commonly described, and they appear to be mostly dependent on environmental conditions due to an extreme sensitivity to oxygen levels. It has not been proven whether all four of these forms exist concurrently in the host intestine.

• Although the vacuolated form of B. hominis can grow to be 5 to 32 m long, the average form is only 7 to 10 m long.
• This morphologic form is distinguished by a large, central, fluid-filled vacuole that consumes nearly 90% of the cell’s volume.
• The remaining 10% constitutes the organism’s periphery, consisting of a ring of cytoplasm containing two to four nuclei.

Figure 1 Trichrome stain, 1000×. Typical Blastocystis hominis vacuolated form.

Life Cycle of Blastocystis hominis

B. hominis reproduces through either sporulation or binary fission. During these processes, the organism takes on a variety of morphologic forms. B. hominis reproduces both sexually and asexually and exhibits pseudopod extension and retraction. A detailed discussion of the life cycle of B. hominis has not been widely presented.

Clinical Disease (Signs, symptoms, causes)

Blastocystosis diagnosis is debatable because it has not been proven that symptoms associated with the presence of the organism are valid or are a result of the infection. Many people found to have Blastocystis are thought to have other infectious organisms causing their symptoms. Blastocystis hominis organisms are mostly found in asymptomatic people, with only a few people diagnosed with the infection experiencing any symptoms. Common complaints from patients infected with B. hominis include:

• Watery diarrhea
• abdominal pain or cramps
• Perianal pruritis (itch)
• Excessive flatulence

Disease Transmission

A number of researchers believe B. hominis is at least a potentially pathogenic organism. The fecal-oral route of transmission is assumed to be responsible for infections, but this belief is not supported by any specific characteristics or body of scientific knowledge.

Diagnosis

• The cyst form is the more recognizable and recent discovery that has aided in the advancement of understanding of how the infection spreads.
• For determining the presence of this organism, a permanent stained smear using a stain such as the trichrome stain is preferred. Iodine smears, which are used to identify B. hominis organisms, are also used to screen for a variety of other parasites.
• The cyst is generally smaller in size than the other three forms, with a thicker cyst wall made up of several layers.
• It lacks a large central vacuole and has only a few nuclei, though it may have small multiple vacuoles and food storage deposits.
• Because of its thick cyst wall, the cyst form of this parasite is the most resistant and can survive in environmental conditions that would be unsuitable for other organisms due to temperature extremes and drying out.
• The organisms appear to be able to survive the high levels of acidic hydrogen chloride in stomach acid and other gastric juices such as bile when ingested.
• When cysts are placed in distilled water, they are not lysed (destroyed), whereas normal cells may rupture in the low-density water.
• The cysts have been observed to survive at room temperature for up to 19 days, indicating the form’s high resistance (Zaman V et al, 1995).
• In another experiment, the cyst form was able to survive in a liquid culture medium containing antiprotozoal drugs commonly used to cultivate bacterial and yeast cultures.
• This ability to survive in such a confined environment lends credence to the theory that the cyst form of Blastocystis is the most resistant of the four forms.
• The most likely life cycle begins with cyst ingestion. Following ingestion, the cyst transforms into three other forms, which can then redevelop into cyst forms.
• The cyst forms enter the external environment via human feces and are transmitted to humans and other animals via the fecal-oral route, repeating the entire cycle.

Treatment and Prevention

• Metronidazole
• Iodoquinol

Prevention and Control

To stop the spread of Blastocystis, proper fecal material treatment, thorough hand washing, and subsequent proper food and water handling are critical.

Conclusion

Blastocystis hominis is a complex and multifaceted parasite that has been linked to various gastrointestinal symptoms and disorders. Its presence in the human gut has been associated with conditions such as irritable bowel syndrome, chronic diarrhea, and abdominal pain. However, the exact role of Blastocystis hominis in causing these symptoms is still unclear and further research is needed to fully understand its pathogenicity. Despite its potential to cause harm, it should be noted that not all individuals with Blastocystis hominis infection experience symptoms, indicating that other factors may contribute to the development of gastrointestinal disorders. In conclusion, while Blastocystis hominis is a parasite that is often found in individuals with gastrointestinal symptoms, more studies are required to determine its true impact on human health.

References

• Mehlhorn H. Brumpt. (1988). Blastocystis hominis, are there different stages or species? Parasitolology Research, 74(4):393-5. doi: 10.1007/BF00539464. PMID: 3387411.
• Zaman V et al. (1995). Ultrastructure of Blastocystis hominis cysts. Parasitology Research, 81(6):465-9. doi: 10.1007/BF00931787. PMID:7567903.
• Zierdt CH. (1988 Jan;). Blastocystis hominis, a long-misunderstood intestinal parasite. . Parasitolology Today, 4(1):15-7. doi: 10.1016/0169-4758(88)90049-x. PMID: 15462991.
• Parasitology for Medical and Clinical Laboratory Professionals by Ridley, John W. Ridley (z-lib.org). Page 59
• Clinical Parasitology A PRACTICAL APPROACH Elizabeth A. Gockel-Blessing (formerly Zeibig), PhD, MLS(ASCP)CM page 173