Things To Know About The Parasitic Trematode: Alaria americana

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Alaria americana is a trematode worm that belongs to Family Diplostomidae and to Superfamily Strigeioidea. Similar species under the genus Alaria have already been identified by parasitologists. All of these species infect carnivorous mammals by living in their small intestines as mature worms. A. americana has been isolated to the different North American mammals belonging to Family Canidae such as foxes, wolves, coyotes and the like. The worm measures from about 2.5 mm to 4.5 mm long and its body is divided into two distinct parts: the longer forebody and the shorter hindbody. Each side of the worm’s oral sucker has pointed process which is used in attaching to its host’s intestinal mucosa. The worm has also a tribocytic organ located behind its acetabulum; this organ secretes special proteolytic enzymes capable of digesting the host mucosa. The organ is considerably large in A. americana and has a distinct ventral depression in the center. (Mehlhorn 2001)

Life Cycle of Alaria Americana

Before A. Americana reaches the mature stage and completes its life cycle, it requires 4 hosts. The worm lays her eggs unembryonated and these eggs hatch for about two weeks. After the eggs were hatched, actively swimming miracidia emerge and begin to attack and penetrate any of the numerous species of planorbid snails (Pearson 1956). When inside the snails’s body, the miracidia develop to mother sporocysts in the snail’s renal vein and they produce daughter sporocyts within 2 weeks. The daughter sporocyts then travel from the renal vein towards the digestive gland of the snail where they grow there for about a year. After a year, they are already mature and begin producing cercariae (singular: cercaria). The cercariae leave the snail (probably through the anus) during day time and swim in the water until they reach their next hosts, the swimming tadpoles. A cercaria enters a tadpole’s body by first dropping its skin, then penetrating the tadpole’s skin, and start moving inside the amphibian’s body. As the tadpole undergoes metamorphosis, the cercaria develops into a mesocercaria, an unencysted form between the cercaria stage and the metacercaria stage. Mesocercaria stage is already infective to definitive hosts: dogs, foxes, wolves, etc. These hosts acquire the mesocercariae by eating the tadpoles or adult frogs. Upon reaching the small intestine of the hosts, they penetrate the coelom and move to the diaphragm and lungs. They stay in the lungs for 5 weeks and after this period they already transformed to diplostomulum metacercariae. These newly emerged metacercariae begin to travel towards the trachea an eventually to the intestine, where they mature within a month. (Mehlhorn 2001)

Alaria Infection

Not only canids are susceptible of cercariae infection but also human who eats undercooked frog flesh and canid meat. Mature Alaria causes severe enteritis (infection of the intestine) to infected animals and human; death is the result of extreme infection associated to enteritis. When accumulated in large number, mesocercaria of Alaria Americana could lead to deadly infection.

Transmission of the A. americana among Mammals

In an experiment conducted by Shoop and Corkum (1987), it was demonstrated that another Alaria species, the A. marcianae can be transmitted to the young mammals by drinking their mothers infected milk. The breast tissue of the mothers contains diplostomula metacercariae that would eventually mix into the milk. It was also demonstrated experimentally that an infected cat has transmitted the trematode successfully to all her kittens via her milk. Like cats, primates also transfer the trematode through their milk (Shoop, Font, & Malatesta, 1990).


Shoop, W.L., W.F. Font, and P.F. Malatesta. 1990. Transmammary transmission of mesocercariae of Alaria marcianae (Trematoda) and the concept of amphiparatenesis. J. Parasitology. 73:110-113.

Shoop, W.L., and K.C. Corkum. 1987. Maternal transmission by Alaria marcianae (Trematoda) and the concept of amphiparatenesis. J. Parasitology. 73: 110-113

Pearson, J.C. 1956. Observations on the morphology and life cycle of Strigea elegans Chandler and Rausch, 1947 (Trematoda: Strigidae). J. Parasitology. 45:155-174.

Mehlhorn , Heinz, P M Armstrong. 2001. Encyclopedic Reference of Parasitology. Published by Springer.