What Are Microfilariae? Worm Larvae on Lab Reports

What it means

Microfilariae (singular: microfilaria) are microscopic larvae released by adult filarial worms living in the body. Depending on the worm, they move through the skin (as in onchocerciasis) or circulate in the blood (as in lymphatic filariasis and loiasis). Biting insects pick them up and spread the infection to other people. Doctors can find them in skin snips or blood samples, which are sometimes taken at specific times of day because some types appear in the blood mainly at night.

The word appears on ivermectin, diethylcarbamazine and albendazole labels because many of these medicines mainly kill microfilariae rather than adult worms. When large numbers die quickly, the body can react with itching, rash, fever and swelling (the Mazzotti reaction). In people with a very high number of Loa loa microfilariae in the blood, treatment can cause serious reactions, so labels advise checking first in areas where that worm is common.

Microfilariae are not adult worms. Killing microfilariae reduces symptoms and stops spread, but adult worms may survive, which is why repeated treatment is often needed.

Where you may see it

• The 'Clinical Pharmacology' and 'Warnings' sections of ivermectin and antifilarial labels
• Blood film and skin snip lab reports
• Tropical medicine clinic letters