"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

January 10, 2011

Alaria marcianae

This horned little devil is the mesocercaria of the trematode, Alaria marcianae, which has a very unusual life cycle. When the metacercariae of this species are ingested by a lactating carnivore such as a Florida panther, they migrate to the tissues instead of developing in their normal site, the small intestine, and develop to this stage. They can then be passed in the milk to the kittens, where they develop normally in the intestine to the adult stage. Females can continue to transmit mesocercariae to future litters until exhausted of their infections.

Contributed by Mike Kinsella.


  1. They're also a risk (minimal but there nonetheless) to students dissecting frogs. An errant metacercaria on a finger scratching an eye can go invasive.

  2. That picture is not a mesocercaria. It looks like the metacercaria from the lungs. Also, the previous comment has a mistake. The stage from the frog that can get in the eye is a mesocercaria not a metacercaria.

  3. The reader is correct. This photo came from a study of Florida panthers which had both metacercariae and mesocercariae in the lungs. The metacercariae have lappets or "horns" and the mesocercariae do not. Sorry for the error.

  4. does this mean that it can no longer infect if it's passed to a male panther?

    because i am looking into a parasite which its survival is absolutely dependent to a host.