Ascaris suum

Ascaris suum is a large ascarid nematode found in the small intestine of pigs.


Summary

Ascaris suum is a large ascarid nematode found in the small intestine of pigs. It is commonly found in intensively and extensively housed pigs throughout the world. Ascaris suum has a direct life cycle and a third-stage larva contained in the egg is infective. Following ingestion, the eggs hatch and the larvae undergo a complex migration in the pig. Infection is most common in animals less than six months of age but it is possible to find worms in animals of all ages. Clinical signs are rare but may include GI disturbances and ill thrift. Diagnosis is based on detecting eggs and/or adults in feces as well as post mortem observation of “milk-spots” on the livers of infected pigs.  These spots are a result or larval migration in the host. There are several products approved in Canada for treatment of A. suum. Ingestion of infective eggs of A. suum by people can result in larva migrans as well as full completion of the life cycle resulting in patent infections. Infection with A. suum in people appears to be rare in Canada.

Taxonomy

Phylum: Nematoda
Order: Ascaridida
Superfamily: Ascaridoidea
Family: Ascarididae

Among parasites of veterinary importance related to A. suum are Parascaris equorum of horses, and Toxocara species and Toxascaris leonina of dogs, cats and other carnivores. Another close relative is the human ascarid, Ascaris lumbricoides. All these ascarids live as adults in the small intestines of their hosts and all have similar life cycles.

The adults of these various ascarids are large (up to 40 cm in length), they have relatively low host specificity, and their eggs are thick-shelled, long-lived and resistant to adverse environmental conditions. Additionally, their life cycles have many similarities, including larval development to the infective stage within the egg, and similar larval migration routes in the mammalian hosts.

 

Morphology

Adult Ascaris suum are up to approximately 25 cm (males) and 40 cm (females) long, and are broad. Adults have three lips surrounding the mouth that are often visible to the naked eye.

Eggs of A. suum measure up to approximately 75 µm by 50 µm, and the rough outer surface of the shell is covered with prominent projections. Each egg contains one or two cells when freshly passed.

Host range and geographic distribution

Ascaris suum occurs in domestic and free-ranging pigs and in wild boar around the world. In general, there is a higher prevalence and intensity of infection in animals that are raised extensively than in those in confinement.

 

Life cycle - direct

Myrna. would not transfer.

Epidemiology

Adult female A. suum can produce very large numbers of eggs (perhaps up to 2 million per worm per day). The eggs are sticky and are very resistant to adverse environmental conditions and can survive in the environment for months or even years. Significant transmission of A. suum can occur both in housing and at pasture.

Pathology and clinical signs

Many pigs infected with A. suum never show any detectable clinical signs. Adult parasites in large numbers probably interfere with growth and development of pigs, especially young animals. Rarely, adults may cause intestinal obstruction.

Larval migrations in the liver cause foci of inflammation that heal as small, fibrous scars ("milk spots"), which are probably not significant unless very severe. Occasionally, large numbers of larvae migrating in the lungs of young piglets produce respiratory disease, which may be severe and sometimes fatal.

Clinically significant Ascaris suum pneumonia has also been reported in Canada in cattle in yards or pastures previously used by infected pigs.

Diagnosis

Detection of eggs in feces using a fecal flotation will detect adult infections although these can sometimes be misleading.  False positive results are common in an infected barn if there are many freshly passed eggs present in the environment.  If these are ingested by an uninfected pig they will pass unchanged in the gut.

Treatment and control

Antiparasitic drugs for pigs can be used to achieve one or more of four goals:

1) to prevent parasitic infection of the host
2) to remove parasites from the host in the absence of clinical parasitic
3) to treat parasitic disease in the host
4) to reduce contamination of the environment with parasite life cycle stages that can become infective for other hosts

Each of the wide variety of products available in Canada is approved for use to achieve one or more of these goals. The scheduling of treatment of pigs for parasites, including control programs, is very variable across western Canada.  To ensure that you are using a product appropriate for the parasite(s) of concern and for your goal, always consult the product information provided by the manufacturer.  In some instances, veterinarians may use products for purposes for which they are not approved.  The consequences of any such extra-label use are the responsibility of the veterinarian, not the manufacturer.

There are several products approved in Canada for Ascaris suum in pigs. Some are given by injection, some orally, and some in the water and/or the feed. Some of the products are effective for both adult and larval parasites.

Drug(s)

Product(s)

Dichlorvos

ATGARD

Doramectin

DECTOMAX

Fenbendazole

SAFE-GUARD

Ivermectin

BIMECTIN, IVOMEC, MEGAMECTIN, NOROMECTIN

Piperazine

VARIOUS

Pyrantel

PRO BANMINTH

Additional information is on the products mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.

The exact measures to be taken for effective control of A. suum depend on the management system, particularly whether the animals are housed, in outdoor yards or at pasture. Essential are the use of an appropriate treatment protocol, combined with efforts to minimize the levels of environmental contamination with the eggs.

Public health significance

Ascaris suum can infect people, and a wide variety of other mammals and birds, following ingestion of infective eggs. Some human infections may have resulted from the ingestion of chicken livers, or other meat or offal, containing viable larvae. In these human infections, which can affect a range of organs and tissues, the parasites remain as larvae.

In North America, clinically apparent human infections with A. suum larvae are vary rare, although asymptomatic larval infections may occur. In Canada, adult A. suum have been recovered occasionally from apparently asymptomatic human hosts exposed to pig manure. These infections with adult parasites are more common in other areas of the world.

People have their own species of ascarid, Ascaris lumbricoides, which is genetically distinct from A. suum.

 

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