Microsporidia

Definition

A diverse phylum of obligate intracellular parasites, historically classified as protozoa but now recognized as fungi or fungal relatives. Microsporidia produce distinctive spores containing a coiled polar tube that everts to inject the sporoplasm into host cells. They infect all major animal groups, with most described species parasitizing insects, though crustaceans, fish, and vertebrates (including humans) are also affected. In entomology, they are significant as pathogens of beneficial and pest insects, with some species developed as biological control agents.

Etymology

New Latin, plural of microsporidium, from Greek mikros (small) + sporos (seed, spore)

Example

Nosema apis, a microsporidian parasite of honey bees, causes nosemosis by infecting the midgut epithelium and disrupting nutrient absorption, contributing to colony losses worldwide.

Synonyms

• Microsporida (variant spelling)
• microsporidians (common name)

Related Terms

• polar tube
• sporoplasm
• obligate parasite
• intracellular parasite
• Nosema
• Biological control
• Microsporidiosis
• Pathogen
• host specificity

Usage Notes

Taxonomic placement remains debated: treated as a phylum within Fungi in many modern classifications, but alternatively placed in Protozoa or as a sister group to true fungi. The singular form Microsporidium refers to an individual spore or the organism in its spore stage, not a taxonomic unit. Host specificity is generally high; most species infect single host species or closely related taxa. In insects, microsporidia often cause chronic, debilitating infections rather than rapid mortality, reducing fecundity and longevity.