Dictionary of Tropical Medicine

Ornithorhynchus anatinus

See Platypus.

Ocellus

The "eye", present midway between the corners of cubozoan ("box") jellyfish. It is capable of distinguishing light and dark, and is probably responsible for evasive action by the jellyfish. Term ocellus also refers to the simple eyes of insects and spiders as opposed to their compund eyes.

Oesophagus

Structure attached to the oral cavity/mouth of an organism which connects to the rest of the digestive system. The oesophagus can be classified according to the shape and structure.

Onchocerciasis

(Syn. "river blindness") A. disease caused by the parasitic filarial nematode Onchocerca volvulus. (See also filariasis).

Oncogene

A gene in a cancer cell that causes its growth to be uncontrolled. A regulatory gene that has changed and is responsible for uncontrolled growth.

Oncologist

Doctor who specialises in treating cancer.

Oncology

The study of cancer.

Oncosphere

A hexacanth embryo of cestodes.

Oocyst

Fertilised female cell (zygote) after the ookinete penetrates and encysts in the wall of the mosquito stomach. This cell undergoes division to produce sporozoites.

Ookinete

Motile (mobile) stage of the malaria parasite resulting from fertilisation of the macrogametocyte by microgametocyte(s) in the mosquito gut. After passing through the gut wall, it becomes an oocyst.

Ootype

The fertilising chamber where the ovum is fertilised by the spermatozoon.

Operculum

A lid-like structure covering certain cestode and most trematode eggs.

Opisthorchiasis

A disease caused by liver flukes from the Opisthorchis species, e.g. O. viverrini seen widely in southeast Asia. See also cholangiocarcinoma.

Opportunistic infections

A variety of diseases which occur in some individuals who do not have healthy immune systems. These are microorganisms which do not usually cause diseases in a healthy individual. They are seen in AIDS patients and include Pneumocystis carinii pneumonia, massive or overwhelming herpes infections, atypical mycobacteria, toxoplasmosis or chronic or overwhelming candidiasis.

Oral

Relating to the mouth. The area with the mouth opening.

Order

The taxonomic term for a group, above Genus, but below Class. See Taxonomy.

Outbreak of malaria

A sudden increase in the number of people sick with malaria in a particular area (village, town, district).