Hermaphroditic hook up
Larval and adult nematodes lodge, migrate or encyst within tissues resulting in obstruction, inflammation, oedema, anaemia, lesions and granuloma formation.Infections by adult cestodes are generally benign as they are not invasive, but the larval stages penetrate and encyst within tissues leading to inflammation, space-occupying lesions and organ malfunction.Worms grow, moult, mature and then produce offspring which are voided from the host to infect new hosts.Worm burdens in individual hosts (and often the severity of infection) are therefore dependent on intake (number of infective stages taken up).These stages mature to cercariae which are released from the snails and either actively infect new definitive hosts or form encysted metacercariae on aquatic vegetation which is eaten by definitive hosts.Helminth eggs have tough resistant walls to protect the embryo while it develops.
Many helminths are free-living organisms in aquatic and terrestrial environments whereas others occur as parasites in most animals and some plants.
Trematodes have more complex life-cycles where ‘larval’ stages undergo asexual amplification in snail intermediate hosts.
Eggs hatch to release free-swimming miracidia which actively infect snails and multiply in sac-like sporocysts to produce numerous rediae.
Other helminths cause serious clinical diseases characterized by high morbidity and mortality.
Clinical signs of infection vary considerably depending on the site and duration of infection.