MONOGENEA TREMATODE PARASITES SYSTEMATICS AND MODE OF INFECTIONS.

Morphology

Monogenes are small parasitic flatworms. Two types of monogeneans may be identified based upon opisthaptor morphology. In the more common Monopisthocotylea, there is a single attachment organ with several large, centrally located, sclerotized anchors, and small marginal hooklets. 

In the Polyopisthocotylea, the opisthaptor consists of a group of small, muscular, adhesive suckers or clamps that are supported by cuticular sclerite. Two of the most common families of monopisthocotylid monogeneans infesting fishes are the Gyrodactylidae and Dactylogyridae. Gyrodactylids are small (1 mm or less), and are found on the skin and gills of freshwater and bottom to develop. With hatching, eggs release free-swimming, infective, oncomiracidia, which seek out and attach to a new host (Paperna, 1975). In contrast, gyrodactylids are viviparous. The young are retained in the uterus until they develop into functional subadults, and usually remain with the host on which they developed. Transmission of both families is greatly enhanced by overcrowding.

 Pathologic Effects

 Heavy gill infestations cause epithelial hyperplasia, destruction of gill epithelium, clubbing of gill filaments, and hypersecretion of mucus, resulting in death by asphyxiation. Heavy skin infestations can cause body and fin necrosis.

Clinical Disease

 Monogeneans feed on mucus, epithelium, and blood. Heavy infestations usually indicate poor sanitation, deteriorating water quality, or inadequate water flows. Clinical signs of dactylogyrid infestations include rapid respiratory movements, clamped fins, and flashing.  The opisthaptor is membranous with one pair of anchors, two transverse bars, and 16 marginal hooks. Dactylogyrids are also small (up to 2mm) and are found on the gill filaments. The opisthaptor is membranous with one to two pairs of anchors, zero to three transverse bars, and 14 to 16 marginal hooks. The prohaptor has four prominent eyespots.

Hosts

Monogenes are common parasites of both marine and freshwater fish. Gyrodactylus causes disease in a variety of captive fish species, including catfish, and others. Dactylogyrus occurs on the gills of goldfish, carp, gulf killifish, sheepshead minnows, and other species (Cone and Odense, 1984). There are numerous species of Gyrodactylus and Dactylogyrus, and most are host-specific

 Life Cycle

 Members of both families have direct life cycles. Dactylogyrids are oviparous. Eggs settle to the lethargic, swim near the surface, and refuse food.

Diagnosis

 Diagnosis of monogenean infestation is based on clinical signs, and is confirmed by identification of parasites in wet mounts or histological sections of skin or gills.

Treatment

Treatment with the organophosphate trichlorfon, Praziquantel and Formalin at as a bath is very effective.

Prevention

Monogenean infestations are best prevented by not introducing wildcaught or hatchery-rearedfishes or invertebrates into laboratory systems.