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

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 an arthropod , 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 .

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

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

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.

Diagnostic findings

Microscopy

Treatment:Praziquantel and niclosamide have been used with success in human infections.  For additional information on the general treatment of cestode infections, see the recommendations in The Medical Letter (Drugs for Parasitic Infections).

Microscopy

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.

A B

A: Proglottids of Mesocestoides sp., collected from the stool of a dog.B: Higher magnification of the proglottids in Figure A, showing the uterus (red arrow), ovary (blue arrow) and parauterine organ (green arrow).

C D

C: Gravid proglottid of Mesocestoides sp. stained with carmine.  Shown in this specimen are the uterus (UT) and excretory ducts (ED).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).