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Parascaris equorum, the horse roundworm is a species of parasitic roundworms that belongs to the family of Ascarids and infects horses and other equids (e.g. donkeys, mules, zebras, etc.).

Parascaris equorum in gut of a weanling. Image from Tetiana Kuzmina taken from https://download.e-bookshelf.deIt is found worldwide, and is highly significant for young foals, particularly in stud farms. 

These worms do not affect cattle, sheep, goats, pigs, dogs or cats.

Related Ascarid species are Toxocara vitulorum that infects cattle, Ascaris suum that infects swine, Toxocara canis, that infects dogs and Toxocara cati that infects cats.

The disease caused by Parascaris equorum is called parascaridosis or parascariasis.

Are horses infected with Parascaris equorum contagious for humans?

  • NO: There is no evidence that Parascaris equorum can infect humans.

You can find additional information in this site on the general biology of parasitic worms and/or roundworms.


Final location of Parascaris equorum

Predilection site of adult Parascaris equorum is the small intestine. Migrating larvae can be found in several organs: particularly in the lungs, trachea, bronchi and the liver.


Anatomy of Parascaris equorum

Egg of Parascaris equorum. Image from http://www.proprofs.com

Adult Toxocara vitulorum are rather large worms. Males can reach up to 28 cm length, females up to 50 cm. They have a whitish color and a translucent aspect, and look very much like cooked spaghetti. As in other roundworms, the body of these worms is covered with a cuticle, which is flexible but rather tough. It forms two characteristic wing-like projections (cervical alae) in the anterior end. The worms have a tubular digestive system with two openings, the mouth and the anus. The mouth has three rather large lips. They also have a nervous system but no excretory organs and no circulatory system, i.e. neither a heart nor blood vessels. The female ovaries are large and the uteri end in an opening called the vulva. Males have a copulatory bursa with a single spicule for attaching to the female during copulation.

The eggs are almost spherical, about 90x100 micrometers, and of a brownish color. They have a thick and pitted membrane and contain mostly a single cell (zygote). They are extremely resistant to dessication, high temperatures and sunlight, and can survive up to 10 years in the environment. They are very sticky and adhere to any surface they come in contact with, including the vegetation and the skin or hair of other horses, which makes it very easy to be ingested by other horses.


Life cycle of Parascaris equorum

Parascaris equorum has a direct life cycle, i.e. there are no intermediate hosts involved. Adult females lay eggs in the small intestine of the host that are shed with the feces. A single female worm can shed more than 150'000 eggs daily, and up to 60 millions yearly. Once in the environment L2-larvae develop inside the eggs that become infective in about 20 to 40 days after shedding, depending mainly on temperature. These eggs can survive for more than 12 weeks indoors and are capable of overwintering outdoors in regions with cold climate.

Foals become infected after ingesting embryonated eggs containing L2 larvae. Larvae emerge from the eggs in the gut, penetrate the gut's wall and are carried to the lungs through the bloodstream or the lymphatic system. There they cross the alveolar wall and get to the trachea where they molt to L3 larvae and migrate further or are coughed to the mouth, to be swallowed. This migration may takes 2 to 3 weeks. Once swallowed they reach the small intestine where they molt twice to complete development to adult worms, mate and the females start producing eggs. The life span of adult worms in the intestine is about 12 months.

The prepatent period (time between infection and first eggs shed) is 6 to 12 weeks in foals.


Harm caused by Parascaris equorum, symptoms and diagnosis

Parascaris equorum can be very harmful to foals, even fatal. Foals are particularly susceptible to acute infections during the first 6 months after birth but the disease can become manifest in foals of up to 2 years of age.

In adult horses infections are not or only mildly pathogenic because they become immune after exposure. Adult horses may develop a chronic infection with mild or no clinical signs, but remain infected and a source of eggs that will contaminate their environment.

Migration through the lungs can cause, cough, fever, difficult breathing (dispnea), loss of appetite and disturbed development. A typical symptom is chloroform smelling of the exhaled air. Secondary infections may also occur. In the gut severe infections can cause enteritis characterized by diarrhea, colic and loss of appetite. Obstruction of the bile duct and even of the intestine is possible. Gut perforation and peritonitis with fatal outcome may also occur.

Diagnosis is based on detection of typical eggs in the feces. However, this works only once adult worms have started shedding eggs in the small intestine, not if the worms have reached only the lungs.


Prevention and control of Parascaris equorum infections

Non-chemical prevention

Foals become infected mainly through ingestion of eggs shed by foals of the previous year in pastures, paddocks or stalls, which remain infective for up to 10 years! Hygienic measures indoors such as removing manure and potentially contaminated bedding and/or food are highly recommended in stud farms or wherever numerous foals are hold together, particular if they have a history of Parascaris infections. In such cases steam vapor disinfection of stables and premises may be considered. However, these measures are unlikely to completely eliminate the infections.

Alternate grazing with livestock (cattle, sheep) that are not susceptible to Parascaris equorum infection may be considered as well, but livestock can carry other parasites that affect horses as well. Or the location of paddocks for young foals may be changed every year, ensuring that they had not been visited by horses during the last year. Such measures are likely to reduce the infection rates, but may not be practicable everywhere.

Other preventative measures for gastrointestinal roundworms are explained in a specific article in this site (click here). 

So far no true vaccine is available against Parascaris equorum. To learn more about vaccines against parasites of livestock and pets click here.

Biological control of Parascaris equorum (i.e. using its natural enemies) is so far not feasible. Learn more about biological control of worms.

You may be interested in an article in this site on medicinal plants against external and internal parasites.

Chemical control

Numerous broad-spectrum anthelmintics are effective against adult worms and larvae in the gut, e.g. several benzimidazoles (febantel, fenbendazole, mebendazole, oxibendazole, etc), as well as macrocyclic lactones (e.g. ivermectin, moxidectin). But not all of them are effective against migrating larvae in the tissues. Read the product label carefully to find it out. Most of them control other roundworms that may affect horses together with Parascaris equorum.

A few other narrow-spectrum anthelmintics such as tetrahydropyrimidines (e.g. morantel, pyrantel) and piperazine derivatives are effective against adult worms but may not control larvae and other roundworm species that often infect livestock simultaneously with Parascaris equorum worms. 

Several commercial products contain mixtures of two or even more active ingredients of different chemical classes. This is sometimes done to extend the spectrum of activity (e.g. to ensure efficacy against roundworms and tapeworms) or to increase the chance that at least one active ingredient is effective against gastrointestinal worms that have become resistant, or to delay resistance development by those worms that are still susceptible.

Depending on the country most of these anthelmintics are available for oral administration as pastes, gels or other formulations for oral administration (e.g. tablets, drenches, feed additives). Very few are available as injectables. Many horse owners complain about the high prices of ivermectin formulations  (mostly pastes or gels) for horses when compared with injectables for cattle and other livestock. The reason why ivermectin injectables are usually not used on horses is apparently that shortly after introduction, it was noticed that horses were more prone to develop severe clostridial infections at the injection site (due to contamination of the needles) and other undesired side effects than cattle or sheep. In addition, the pharmacokinetic behavior of ivermectin on horses is different than in ruminants. For these reasons oral pastes were developed for horses that do not show such side effects.

Most wormers containing benzimidazoles (e.g. febantel, fenbendazole, oxibendazole, etc), tetrahydropyrimidines (e.g. morantel, pyrantel) and other classic anthelmintics kill the worms shortly after treatment and are quickly metabolized and/or excreted within a few hours or days. This means that they have a short residual effect, or no residual effect at all. As a consequence treated animals are cured from worms but do not remain protected against new infections. To ensure that they remain worm-free the animals have to be dewormed periodically, depending on the local epidemiological, ecological and climatic conditions. Exceptions to this are macrocyclic lactones (e.g. ivermectin, moxidectin) that are stored in the body fat and progressively released. They offer several weeks protection against re-infestation, depending on the dosage, the delivery form and the specific parasite. As a general rule, moxidectin achieves longer protection periods (e.g. up to ~12 weeks for suppressing egg production by the worms) than most ivermectin formulations (up to ~8 weeks).


Resistance of Parascaris equorum to anthelmintics

Resistance of Parascaris equorum to most commonly used anthelmintics is increasing. Resistance to macrocyclic lactones (e.g. ivermectin, moxidectin) has been reported in the USA, UK and Australia. Tolerance (i.e. reduced efficacy) to benzimidazoles (e.g. fenbendazole, oxibendazole, etc) has also been reported in the USA, and resistance to pyrantel is also known to occur in several countries (e.g. Australia, USA, Brazil, Japan). There are also reports on cases of multi-resistance to macrocyclic lactones and tetrahydropyrimidines, e.g. in the USA.

All this strongly suggests that resistance is well established in many regions. And most likely its prevalence will increase and it will also spread. Product rotation, i.e. alternating anthelmintics with different modes of action may delay this process.

This means that if an anthelmintic fails to achieve the expected efficacy against Parascaris equorum, chance is significant that it is due to resistance and not to incorrect use, which is the most frequent cause of product failure.

Learn more about parasite resistance and how it develops.

Ask your veterinary doctor! If available, follow more specific national or regional recommendations for Parascaris equorum control.

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