Blastocystis

# Overview
Blastocystis is a highly prevalent single-celled parasite that infects the gastrointestinal tract of humans and animals.  Many different types of Blastocystis exist, and they can infect humans, farm animals, birds, rodents, amphibians, reptiles, fish, and even cockroaches.

# Blastocystosis
Infection with Blastocystis can produce the disease Blastocystosis.  The most frequently described symptoms of Blastocystosis are abdominal pain, constipation, diarrhea.

# Genetic classification
Blastocystis has presented a challenge to the medical and scientific community due to the diversity of hosts the organism can infect, the diversity of Blastocystis species which exist, and the fact that most species of Blastocystis found in mammals and birds are able to cause infection in humans.  The organism has been called controversial, cryptic, and enigmatic.  Even its classification has proved challenging.  Blastocystis was originally classified as a yeast, then as a protozoan.  An analysis of gene sequences was finally performed in 1996, which placed it into the Stramenopile kingdom.
For many years, scientists believed one species of Blastocystis infected humans, while different species of Blastocystis infected other animals.  So they called Blastocystis from humans Blastocystis hominis and gave different species names to Blastocystis from other animals, for example Blastocystis ratti from rats.  Various genetic analysis showed Blastocystis hominis as a unique entity does not really exist -- there is no single species of Blastocystis that infects humans.  In fact, nine distinct 'species' of Blastocystis (as defined by genetic differences) can infect humans, including those previously called Blastocystis ratti.  Because of this, in 2007 scientists proposed discontinuing the use of the term Blastocystis hominis.  Their proposal is to refer to Blastocystis from humans and animals as Blastocystis sp. subtype nn where nn is a number from 1 to 9 assigned to each species group.  

# Microbiology
The appropriate classification of Blastocystis has only recently been resolved. The original description of Blastocystis was as a yeast due to its yeast-like glistening appearance in fresh wet mounts and the absence of pseudopodia and locomotion.  This was then contradicted by Zierdt who reclassified it under subphylum Sporozoa based on some distinctive protistan features that the Blastocystis cell has, such as the presence of nuclei, smooth and rough endoplasmic reticulum, Golgi complex, and mitochondrion-like organelles. Its sensitivity to antiprotozoal drugs and its inability to grow on fungal media further indicated that it was a protozoan. However, major revisions were made to its classification more recently based on modern molecular approaches to classification, and these studies have shown that Blastocystis is neither yeast nor a protozoan.  It is placed in a new Kingdom known as the Stramenopiles.  Other Stramenopiles include brown algae, mildew, diatoms, the organism that caused the Irish potato famine, and the organism responsible for Sudden oak death disease. 
The great diversity of morphological forms in which Blastocystis exists in poses identification and diagnostic problems. Four commonly described forms are the vacuolar (otherwise known as central body), granular, amoeboid, and cyst forms. The appearance of the organism is largely dependent upon environmental conditions as it is extremely sensitive to oxygen. Whether all of these forms exist in the host intestine is unclear.
- Vacuolar form
- Granular form
- Amoeboid form
- Cyst form
The proposed life cycle begins with ingestion of the cyst form.  After ingestion, the cyst develops into other forms which may in turn re-develop into cyst forms. Through human feces, the cyst forms enter the external environment and are transmitted to humans and other animals via the fecal-oral route, repeating the entire cycle.
Obtaining and culturing Blastocystis
The ATCC maintains a collection of Blastocystis isolates.  Some records show whether the isolates were obtained from symptomatic or asymptomatic carriers.  As yet, no publication has identified the subtypes of most of the ATCC isolates, which are mostly axenic.  Researchers have reported that patients with Irritable bowel syndrome may provide a reliable source for xenic Blastocystis isolates.  Some researchers have reported being able to culture Blastocystis from 46% of IBS patients.   Researchers have described different culture mechanisms for growing Blastocystis.  Colony growth on solid medium colonies on solid culture medium using a synthetic medium with added supplements have both been described.  However, most cultivation is performed in liquid media of various types.

# Treatment

## Antimicrobial regimen
- Blastocystis
- Preferred regimen (1): Metronidazole 750 mg PO tid or 1.5 g qd for 10 days
- Preferred regimen (2): Trimethoprim-sulfamethoxazole 1 DS PO bid or 2 DS PO qd for 7 days
- Preferred regimen (3): Iodoquinol 650 mg PO tid for 20 days
- Preferred regimen (4): Nitazoxanide 500 mg PO bid for 3 days
- Preferred regimen (5): Paromomycin 25-35 mg/kg/day PO tid for 7 days
- Note (1): Treatment of asymptomatic infections is unnecessary
- Note (2): One double strength tablet contains 160 mg trimethoprim/800 mg sulfamethoxazole