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Mesocestoidiasis

[Mesocestoides spp.]

Causal Agents

Cestodes (tapeworms) in the genus Mesocestoides are common in carnivores but only very rarely infect humans. Although Mesocestoides spp. are classified as Cyclophyllidea, they share biological characteristics with Pseudophyllidea. Some life cycle features remain unknown. Species implicated in zoonotic infections include M. lineatus and M. variabilis, though the relationships between species in this genus is a status of ongoing research.

Life Cycle

Most evidence suggests three hosts (including an arthropod intermediate host, a vertebrate intermediate host, and a carnivore definitive host) are required for completion of the life cycle of Mesocestoides spp. Gravid, motile proglottids are shed in feces of the definitive host image . Proglottids contain many eggs within the parauterine organ, a structure unique to Mesocestoides image . Eggs lack a true shell, possessing only a thin embryophore enveloping a six-hooked oncosphere. The first intermediate host (presumed to be an arthropod) image becomes infected after eating proglottids or oncospheres. In the first intermediate host, it is believed the oncosphere develops into an undescribed larval stage probably similar to a cysticercoid or procercoid. When the first intermediate host is eaten by the vertebrate second intermediate host image , the larva develops into an infective larva (tetrathyridium) in the peritoneal cavity or in viscera. Asexual proliferation of tetrathyridia within the intermediate host has been observed occasionally in some Mesocestoides spp., although it is not known if this represents an abnormal state. The carnivorous definitive host ultimately becomes infected after eating tissue infected with tetrathyridia image . Upon ingestion, the scolex of the tetrathyridium evaginates and becomes anchored to the small intestinal mucosa, where it matures into an adult cestode. Gravid proglottids are passed typically 2-3 weeks post infection in the natural host. Humans are believed to become infected after eating undercooked meat or viscera containing tetrathyridia image .

Hosts

The normal definitive hosts are terrestrial carnivores, including canids, felids, procyonids, mustelids, and opossums. The actual host specificity of Mesocestoides spp. is poorly defined due to extensive morphological overlap among species.

Tetrathyridia have been recovered from hundreds of intermediate host species of mammals (particularly rodents), birds, amphibians, and reptiles. Domestic canids and occasionally felids, which typically serve as definitive hosts, may also become infected with tetrathyridia as intermediate hosts.

The identity of the arthropod intermediate host of Mesocestoides spp is unknown. Several arthropods have been proposed as first intermediate hosts, including ants and oribatid mites. However, none of these species have been definitively demonstrated to serve as intermediate hosts. Experimental attempts to infect arthropods have not been successful.

Geographic Distribution

Mesocestoides spp. occur broadly in wild and domestic animals and have been found on every inhabited continent except for Australia. M. lineatus is an Old World species reported from Asia, Europe, and Africa. M. variabilis is endemic to North America.

Zoonotic infections have been identified in Japan and Korea (M. lineatus) following ingestion of raw snake and turtle liver. Cases of M. variabilis have been reported from the United States.

Clinical Presentation

Infections with Mesocestoides spp. reported in humans have typically involve low numbers of worms. Non-specific, recurrent gastrointestinal signs/symptoms have been reported.

To date, no larval-stage infections (tetrathyridia) have been identified in humans.

Mesocestoides sp. proglottids and scoleces

 

Adults are 40-150 cm long and the strobila can contain more than 400 proglottids. Immature proglottids are about three times broader than long, and mature but non-gravid proglottids are nearly 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 Mesocestoides sp., collected from the stool of a dog.
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
Figure C: Proglottid of Mesocestoides sp. stained with carmine (lateral view). Shown in this specimen are the uterus (UT), which is a simple tube parallel to the body axis, and excretory ducts (ED).
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).
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.
Mesocestoides sp. tetrathyridia.

 

Numerous vertebrates can serve as the second intermediate host, in which the infective third-stage larva (tetrathyridium) develops. The definitive host becomes infected upon ingestion of tetrathyridia, which are found within body cavities or organs of second intermediate hosts. Tetrathyridia possess an invaginated scolex with four suckers and are often surrounded by a thin cyst wall.

Figure A: Tetrathyridium of Mesocestoides sp. in the liver of a laboratory-infected mouse, Giemsa stain. 100x magnification.
Figure B
Figure B: Closer view of the suckers from the specimen in Figure A. 400x magnification.
Figure C
Figure C: Another view of a tetrathyridium from the same specimen. Features typical of cestodes are visible, such as the sucker (arrow) and microtriches (darts), which are fine hair-like projections of the cuticle. Calcareous corpuscles may be seen in some specimens. 200x magnification.
Figure D: Multiple tetrathyridia visible in a liver section from an experimentally-infected mouse. 40x magnification.

Laboratory Diagnosis

Diagnosis of Mesocestoides sp. is based on the morphological identification of proglottids in the stool. While gravid proglottids have a characteristic morphology, immature proglottids may be difficult to differentiate from those of Diphyllobothriidae. Accurate identification of species is complicated and requires complete specimens.

Laboratory Safety

Standard protocols for the processing of stool samples apply.

Suggested Reading

Fuentes, M.V., Galan-Puchades, M.T. and Malone, J.B., 2003. A new case report of human Mesocestoides infection in the United States. The American journal of tropical medicine and hygiene, 68(5), pp.566-567.

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