Parascaris univalens Intestinal Roundworm of Horses; Equine Ascarid		Contents		Rev 10/25/2025
		Classification		Biology and Ecology
		Morphology and Anatomy		Life Cycle
Return to Parascaris menu				Ecosystem Functions and Services
		Distribution		Management
Return to Ascarididae Menu		Feeding		References
		Go to Nemaplex Main Menu		Go to Dictionary of Terminology

Classification:

Class:    Chromadorea

Subclass: Chromadoria

Order:   Rhabditida 

Suborder: Spirurina

Superfamily: Ascaridoidea

Family:  Ascarididae

 

            Parascaris univalens van Beneden, 1883

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Morphology and Anatomy:

• Very large, heavy-bodied, whitish nematodes
• Adult parascarid worms have three lips anteriorly with the inner surface provided with a transparent membrane bordered by dentigerous ridges

Male:

• Length 15-28 cm, 3-6 mm wide
• Spicules equal size, 2-2.5 mm long
• Posterior end rounded or bluntly conical with small caudal alae

Female:

• Length 18-50,  2-8 mm wide
• Posterior end rounded.
•  Eggs brownish, subspherical with a finely-punctated, thick shell. Eggs 90-100 um diam, containing a single cell when passed in the feces. Eggs have an outer sticky, protein coating allowing them to adhere to the horse’s haircoat, teats, and udder as well as to vertical housing surfaces in addition to contaminating the soil

Ref. Cruthers et al., 2020

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Distribution:

Worldwide:  inhabit the small intestine of horses and other equids throughout the world.

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Economic Importance:

Two main species that parasitize horses throughout the world: P. univalens and P. equorum. The species are morphologically indistinguishable, and it is presumed that their life cycles, host susceptibility, and other biological features are esimilar, if not identical. The only difference between the species is that P. univalens has only one pair of chromosomes, whereas P.equorum has two chromosome pairs have been reported. Recent karyotyping of ascarid eggs recovered from horses on multiple continents revealed that P. univalens is the more prevalent species in managed horse populations today (Cruthers et al., 2020).

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Hosts:

Horses and other equids

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Feeding:

Parasites of the mucosa of the small intestine.

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Life Cycle

Life cycle direct, no intermediate host.

• Females deposit eggs in the host intestine which pass out in the feces of the horse.
• Eggs in the soil develop to the L1 and then to the infective L2 in 10-14 days. Eggs are highly resistant to weather conditions and can remain infective for up to 10 years.
• Horses become infected by ingesting the eggs directly from the environment.
• The infective L2 is released from the egg in the stomach and small intestine and penetrates the wall of the small intestine. It passes to the liver via the lymphatics and venules draining the small  intestine within 2-7 days
•  L2 migrate about the liver parenchyma causing mechanical damage with inflammation which gives rise to white, fibrotic lesions (milk spots).
• Larvae leave the liver and travel to the lungs, where they develop into L3 in about 2 weeks post infection
•  L3 enter alveoli, rupture out into the airways and lungs, are coughed up and swallowed to reach the small intestine within 4 weekspost infection.
• The L3 molt to the L4 and then to the L5 and become mature adults.
• Prepatent period is 75-90 days (Cruthers et al., 2020).

Ecophysiological Parameters:

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Management:

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References:

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