Microsporidia are spore forming, obligate eukaryotic intracellular protozoan parasites which have been previously recognised in a variety of animals especially invertebrates and lower vertebrates. They have recently come to medical attention as opportunistic pathogens in humans with Acquired Immune Deficiency Syndrome (AIDS) and have been implicated in conditions ranging from enteritis to keratoconjunctivitis.

Morphology and Life cycle

Microsporidia are primitive organisms.  They possess no mitochondria and have prokaryotic like ribosomes.  Classification is based on the ultrastructural features which include the number of coils in the polar tubes,  the configuration of nuclei and the spore size.  The spores are the infective stage of the organism. Infection occurs when the infective sporoplasm within the organism is injected into the host cell through the polar tube.  Microsporidia multiply rapidly within the cytoplasm of the cell to produce sporoplasts (merogony), followed by sporogony which results in the production of infective, thick walled spores which are released into the intestinal lumen in the case of Enterocytozoon species and Encephalitozoon species.

Clinical disease

The most common microsporidia found in patients with AIDS are Enterocytozoon bieneusi, Encephalitozoon intestinalis and Encephalitozoon hellem . These patients tend to be severely immuno-deficient with a CD4 count less than 100 x10⁶/L.

Enterocytozoon bieneusi

Infections with E. bieneusi are restricted to the enterocytes of the small intestine, resulting in villous atrophy and malabsorption.  Clinical symptoms include chronic watery, non-bloody diarrhoea, malaise and weight loss.

Encephalitozoon intestinalis

Infection with Encephalitozoon intestinalis occurs in the enterocytes of the small intestine but is more widely disseminated than E. bieneusi and has been found in the colon, liver and kidney.

Encephalitozoon hellem and Encephalitozoon cuniculi

These organisms have also been found in disseminated microsporidiosis.  Clinical symptoms may include sinusitis, nephritis, hepatitis, keratoconjunctivitis and peritonitis.

Nosema corneum

This organism has been detected in AIDS patients with keratoconjunctivitis.

Microsporidia	Size	Associated disease
Enterocytozoon bieneusi	1m by 1.5m	gastrointestinal and biliary tract infections
Encephalitozoon intestinalis	1.5m by 2.5m	gastrointestinal tract and systemic infections
Encephalitozoon hellem	1.5m by 1m	keratopathy, respiratory tract infection
Encephalitozoon cuniculi	1.5m by 1m	central nervous system disease
Nosema connori	2m by 4m	systemic infections
Nosema corneum	2m by 4m	keratopathy
Pleistophora species	2.8m by 3.4m	myositis

Laboratory Diagnosis

Initially, the diagnosis of intestinal microsporidiosis depended on tissue biopsies which were stained with Gram's stain and examined by light microscopy.  However, in order that ill patients were not subjected to unnecessary invasive procedures, non-invasive diagnostic procedures were developed. The modified Trichrome stain and the Fungiqual fluorescent stain are the stains of choice.

Microsporidia stained with modified Trichrome stain

Microsporidia stained with Fungiqual

Microsporidia stained with modified Trichrome stain