Aquatic Parasite Observatory

Mesocestoides spp. (Vaillant, 1863)

    • Species Name: Mesocestoides spp. (Vaillant, 1863)
    • Synonyms: Monodoridium (Walter, 1866); Ptychophysa (Hamann, 1885)
    • Taxonomy: (Vaillant, 1863) Animalia, Platyhelminthes, Cestoda, Eucestoda, Mesocestoididae, Mesocestoides spp.
    • Description: "The head is 600 to 900 microns in diameter, massive and somewhat flattened anteriorly. The rostellum is replaced by a slight central depression. The suckers are oval and display a widely opened longitudinal aperture. The neck is comparatively short and thick. The strobila measures from 30 cm to 2.5 m, and shows a feebly reddish color along the median line. The first segments become square, with slightly convew borders and with slightly prominent posterior angles. Segments become mature in the middle third of the strobila. The terminal segments measure 4 to 6 mm long and 2 to 3 mm wide, are shaped like melon seeds and are swollen in the median line by the ovoid uterine capsule filled with eggs. The male genitalia develop first.

      Male genitalia: Testes large and numerous and scattered through the segment both median and lateral of the longitudunal excretory canals. Vas deferens describes numerous loops near the mid-dorsal line to the anterior extremity of the segment, where it turns abruptly and enters the cirrus pouch. The cirrus pouch is in the anterior portion of the segment, is well developed and prominent, piriform, and with its posterior aperture opening alternately, usually regularly but at times irregularly, a little to the right and to the left of the median line. The cirrus from 50 microns to 1mm long, swollen at its proximal extremity and commonly found protruding in mature segments.

      Female genitalia: The ovaries are located in the posterior fourth of the segment, and are irregularly spherical to oval. There are two vitellaria which are partly posterior of the ovaries and partly underneath the posterior portion of the ovaries. The vagina extends anteriorly and then returns in a sinuous curve posteriorly from the genital pore and on the side of the median line opposite to the cirrus pouch; it is without a receptaculum seminis. The uterus forms as an elongate sac in the median line, and presents anteriorly a curve to one side, the cirrus pouch always lying in the concavity of this curve, and the curve being alternately, regularly or irregularly, to the right or to the left. The posterior dilation of the uterus persisting as attached cord-like structures, a short on posteriorly and a longer, sinuous one anteriorly. The eggs are ovoid, 40 to 60 microns long by 35 to 43 microns wide, and have two very thin shells" (Hall, 1915).

    • Life Cycle: Mesocestoides spp. require a three-host life cycle to complete their development. The definitive hosts are primarily carnivores, including canids, felids and mustelids. Gravid, motile proglottids are shed in feces. Within the proglottids, hundreds of oncospheres are contained within the parauterine organ. The first intermediate host is presumed to be ants, and becomes infected after eating proglottids or oncospheres. Several arthropods have been looked at as potential first intermediate hosts for Mesocestoides spp., including ants and oribatid mites; however, none of these species have been demonstrated in nature to serve as an intermediate host. In the first intermediate host, it is believed the oncosphere develops into a second-stage larva (cysticercoid or procercoid). When the first-intermediate host is eaten by a second intermediate host, including small mammals, birds, reptiles and amphibians, the second-stage larva develops into an infective, third-stage larva (tetrathyridium). Domestic and wild canids, which usually serve as the definitive host, may also serve as dead-end intermediate hosts upon ingestion of infected first intermediate hosts. The definitive host ultimately becomes infected after eating meat contaminated with tetrathyridia. Upon ingestion, the cestode settles in the small intestine where it matures. Gravid proglottids can be seen in the stool as early as two weeks. Humans are not usual definitive hosts, but can serve as such after eating undercooked meat containing tetrathyridia” (CDC, 2013).
    • Sources: Hall, M.C. 1915. The adult taenioid cestodes of dogs and cats and of related carnivores in North America. Proceedings of the United States National Museum, Vol.55, p. 60.
    • CDC. 2013. http://www.cdc.gov/dpdx/mesocestoidiasis/index.html accessed January 21, 2016.
    • APO Parasite Records: (by Life Cycle)

  • University of Colorado Boulder