Parascaris equorum


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 infections. Asc21ath.jpg (13858 bytes)

Parascaris equorum egg
on fecal floation

rbtn_pre.gif (1751 bytes) rbtn_cnt.gif (1897 bytes) rbtn_top.gif (1435 bytes)


Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on January 24, 2000