Cryptosporidium parvum - Introduction, Classification, History, Habitat, Morphology, Culture

Introduction of Cryptosporidium parvum

Cryptosporidium spp is a coccidian parasite which causes infection of the small intestine resulting in diarrhoea. Numerous species of Cryptosporidium are known to affect amphibians, fish, birds, and mammals while Cryptosporidium parvum is infectious to human beings.

Classification of Cryptosporidium parvum

Phenotypically, Cryptosporidium parvum can be classified on

Kingdom: Chromista

Superphylum: Alveolata

Phylum: Apicomplexa

Class: Conoidasida

Order: Eucoccidiorida

Suborder: Eimeriorina

Family: Cryptosporidiidae

Genus: Cryptosporidium

Species: parvum

History of Cryptosporidium parvum

Tyzzer, in 1907, was the first to describe the parasite obtained from the peptic glands of laboratory mice and named it Cryptosporidium. These coccidians were first thought to be non-pathogenic as only 15 reports of Cryptosporidium infections were reported in animals before 1975. However, in 1976, Cryptosporidium infection was reported in a three-year-old healthy girl in the USA.

Since then the Cryptosporidium infection has been diagnosed in individuals with AIDS or people undergoing immunosuppressive therapy.

Habitat of Cryptosporidium parvum

Cryptosporidium parvum inhabits the small intestine’s surface epithelial cells of villi or crypts of the small intestine.

The parasite may also inhabit the stomach, appendix, colon, rectum, and pulmonary tree.

Morphology of Cryptosporidium parvum

Cryptosporidium parvum exists in six different morphological forms:

• Oocyst

• sporozoite

• trophozoite

• meront

• microgamont

• macrogamont

Oocyst

• diagnostic form excreted in human faeces

• colourless, spherical to oval

• Cryptosporidium parvum oocyst measures 4.5 μm to 6 μm in diameter

• the cyst is surrounded by a 50 nm thin cyst wall which in turn consists of an electroluminescent middle zone surrounded by two electron-dense layers

• does not stain with iodine and is acid-fast

• each oocyst consists of up to four sporozoites

• the sporozoites are slender, bow-shaped, and fusiform which remain parallel to each other

• these sporozoites are released after partial digestion of oocyst

• micropile polar granules are absent in Cryptosporidium parvum

• oocyst, which sporulates inside the host, are excreted in small numbers in faeces

• the severity of the infection and the number of oocysts excreted has no co-relation

• oocysts are either thick-walled or thin-walled

• thick-walled Cryptosporidium parvum oocysts are infectious to susceptible humans while thin-walled oocysts cause autoinfection in the same host

Sporozoite

• Cryptosporidium parvum sporozoite is slender, crescent-shaped

• measures 1.5 μm to 1.75 μm in diameter

• pointed anterior end

• the posterior end contains a rounded prominent nucleus

Trophozoite

• Cryptosporidium parvum trophozoite is an intracellular transitional form of the parasite

• round or oval

• 2μm to 2.5μm in diameter

• each trophozoite consists of a large nucleus with/without a conspicuous nucleolus

Meronts

• Cryptosporidium parvum meronts are crescent-shaped

• measures 1μm to 5μm in diameter

• rounded anterior and posterior ends

Culture of Cryptosporidium parvum

Cryptosporidium parvum can be cultured in the media as well as the laboratory animals.

In media

• Cryptosporidium parvum can be cultured in-vitro in tissue culture using a variety of cell lines such as monolayered human foetal lung, primary chick kidney, porcine kidney etc as well as in chick embryos

• in cell lines, development takes place through type I meronts, type II meronts, sexual forms (microgamont, macrogamont), and oocysts

Laboratory animals

Cryptosporidium parvum is infectious to sucking mice and can be cultured in them.