Microsporidia are unicellular and obligate intracellular eukaryotic parasites related to fungi and characterized by a unique mechanism of infecting host cells. They are widely considered underdiagnosed agents of infection (in both HIV-positive and HIV-negative individuals) due to impediments in their detection, various clinical presentations, and subsequent dissemination to the environment. The disease they cause is known as microsporidiosis.

Since the advent of the acquired immunodeficiency syndrome (AIDS), research endeavors on these agents have increased substantially, resulting in the identification of novel species and reclassification of older ones. Both symptomatic and asymptomatic infections have been observed, but it is highly influenced by the host immune status.

Infections in immunologically competent hosts

In immunologically competent individuals, microsporidial protists usually cause asymptomatic infection, self-limited diarrheal disease or (less frequently) chronic diarrhea. In apparent infections, dominant clinical sign is watery and non-bloody diarrhea, often accompanied by fever, nausea and diffuse abdominal pain.

However, the correlation between observation of microsporidia in stool specimens and gastrointestinal symptoms can be transient; infection with Enterocytozoon bieneusi, Encephalitozoon species and other microsporidial agents may result in clinical manifestations in the early stages of the disease that tend to resolve – regardless of the persistent evidence of microsporidia in diagnostic assessment.

Ocular infections presenting with superficial or stromal keratitis (in association with eye pain) have been described in immunocompetent individuals. Species that are most commonly associated with such presentations include Nosema ocularum, Vittaforma corneum and Microsporidium africanum. This condition can be pretty serious and sometimes can even lead to ocular enucleation.

Cerebral infections caused by Encephalitozoon cuniculi can be seen in immunocompetent patients, albeit rarely. Dominant symptoms include headache, seizures and vomiting. A case of pacemaker endocarditis caused by the same putative microorganism has been described, while some other species may cause myositis (i.e. inflammation of the muscles).

Infections in patients with HIV/AIDS

Microsporidiosis and its accompanying manifestations are much more common in immunocompromised hosts, most notably those with AIDS. It is known that infection of the intestinal epithelium with Encephalitozoon intestinalis or Enterocytozoon bieneusi is the most frequent manifestation of microsporidiosis in AIDS patients, while unexplained diarrhea is seen in up to 30% patients infected with human immunodeficiency virus (HIV).

Clinical manifestations of gastrointestinal disease include chronic diarrhea, wasting and damage to bile ducts (cholangiopathy) – often compared to cryptosporidiosis and cytoisosporiasis in AIDS patients. Diarrheal stools are watery without blood, continuous or intermittent, and there is often malabsorption of carbohydrates and fat.

In patients with compromised immune system, a plethora of microsporidial species may disseminate and damage muscles, kidneys, liver, eye and/or urinary tract. Although specific for the eye, Vittaforma corneae has also been found in urine and sinonasal aspirates of patients with AIDS.

Encephalitozoon cuniculi has been detected in AIDS patient with central nervous system (CNS) infection, who presented with multifocal hypodense lesions in the brain. The diagnosis in this case was established when microsporidial spores were detected in cerebrospinal fluid, stool, urine and sputum. And this species is not the lone case of CNS infection – some representatives of Trachipleistophora genus have also been associated with encephalitis (brain inflammation) and subsequent death.