Female worms are prolific egg layers and as a result infected foals may pass
millions of eggs daily in their feces.
These thick-shelled eggs are resistant to freezing and drying and can therefore
survive for long periods in the environment. The outer layer is sticky and ensures
that eggs will be present almost everywhere in a foal's environment.
Optimal temperatures for development of eggs to the infective stage(25'c to 35'C)
ensure that transmission of Parascaris will be maximum during the summer months in most
temperate areas of the world.
Infection rates in foals begin to decline significantly by the time they are
six months old. This is due to a combination of an age-dependent resistance and a potent
immune response. By the time foals are 12 months old, patent infections are
relatively uncommon and and when they are found, worm numbers and egg counts are usually
low. This state of solid immunity, combined with infrequent patent infections usually
lasts throughout the adult lives of equines.
Experimental studies have shown that the immune response acts in the liver and
lungs since infections of mature horses with large numbers of eggs produce significant
lesions in livers and lungs but few larvae reach the small intestine.
P. equorum eggs may hatch in rodents and undergo a limited migration but
the nematode seems not to be a cause of visceral larva migrans in humans.
|A coughing foal with a nasal discharge is a likely
indication of a prepatent infection with P. equorum. Otherwise a diagnosis is
dependent on finding the distinctive thick-shelled egg in the feces of horses with patent
Parascaris equorum egg
on fecal floation