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Mesocestoidiasis

[Mesocestoides spp.]

Causal Agents

Cestodes in the genus Mesocestoides; M. lineatus and M. variabilis have been reported to infect humans.

Life Cycle

lifecycle

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 The number 1. Within the proglottids, hundreds of oncospheres are contained within the parauterine organ The number 2. The first intermediate host is presumed to be an arthropod The number 3, 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 number 4, 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 The number 5. 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 The number 6.

Geographic Distribution

North America, Europe, Asia and Africa; M. lineatus is an Old World species; M. variabilis is North American.

Clinical Presentation

Mesocestoides species usually parasitize humans in low numbers, causing mild gastrointestinal symptoms: nausea, diarrhea, abdominal discomfort, vomiting.

Mesocestoides spp. proglottids and scoleces

 

Definitive hosts for Mesocestoides spp. are usually carnivores, including canids, felids and mustelids. Human infections are rare, but humans may serve as incidental definitive hosts for Mesocestoides spp. Adults are 40-150 cm long and the stroblia can contain more than 400 proglottids. Immature proglottids are about three times broader than long. Mature proglottids are also broader than long, but closer to being square. Gravid proglottids are longer than wide and contain a parauterine organ that encloses a mass of eggs. The scolex has four suckers but no rostellum or hooklets.
Figure A: Proglottids of <em>Mesocestoides</em> sp., collected from the stool of a dog.
Figure A: Proglottids of Mesocestoides sp., collected from the stool of a dog.
Figure E: Scolex of <em>Mesocestoides</em> sp. stained with carmine. In this field, two of the suckers are clearly visible. Note that lack of rostellar hooklets.
Figure E: Scolex of Mesocestoides sp. stained with carmine. In this field, two of the suckers are clearly visible. Note that lack of rostellar hooklets.
Figure B: Higher magnification of the proglottids in Figure A, showing the uterus (red arrow), ovary (blue arrow) and parauterine organ (green arrow).
Figure B: Higher magnification of the proglottids in Figure A, showing the uterus (red arrow), ovary (blue arrow) and parauterine organ (green arrow).
Figure C: Gravid proglottid of <em>Mesocestoides</em> sp. stained with carmine. Shown in this specimen are the uterus (UT) and excretory ducts (ED).
Figure C: Gravid proglottid of Mesocestoides sp. stained with carmine. Shown in this specimen are the uterus (UT) and excretory ducts (ED).
Figure D: Mature proglottids of <em>Mesocestoides</em> sp. stained with carmine. Shown in this specimen are the vagina (VA), cirrus sac (CS), bilobed ovary (OV) and numerous testes (TE).
Figure D: Mature proglottids of Mesocestoides sp. stained with carmine. Shown in this specimen are the vagina (VA), cirrus sac (CS), bilobed ovary (OV) and numerous testes (TE).
Mesocestoides spp. tetrathyridia.

 

Mesocestoides spp. require two intermediate hosts for completion of their life cycle. The first is an arthropod, usually a soil mite or coprophagous insect. The second intermediate hosts are vertebrates, including reptiles, amphibians and small mammals. In the second intermediate host, the second-stage larva develops into the infective third-stage larva, also known as a tetrathyridium. The definitive host becomes infected upon ingestion of tetrathyridia.
Figure A: Tetrathyridium of <em>Mesocestoides</em> sp. in the liver of a laboratory-infected mouse.
Figure A: Tetrathyridium of Mesocestoides sp. in the liver of a laboratory-infected mouse.

Laboratory Diagnosis

The diagnosis is based on the microscopic identification of proglottids and eggs in the stool. Mature proglottids need to be differentiated from Dipylidium caninum; immature proglottids may be difficult to differentiate from Diphyllobothrium spp.

Treatment Information

For information about treatment please contact CDC-INFO.

DPDx is an education resource designed for health professionals and laboratory scientists. For an overview including prevention and control visit www.cdc.gov/parasites/.

Page last reviewed: December 29, 2017