Metastrongylus species

Metastrongylus spp. are lung nematodes found in pigs. This parasite is present in many parts of the world and is only found rarely in Canada. Adult parasites live in the bronchi and bronchioles.  The life cycle is indirect and infection of pigs is by ingestion of third-stage larvae within an earthworm intermediate host. The pre-patent period is 3 to 4 weeks. Infection is much more common on pigs raised outdoors because these animals have access to the intermediate hosts in soil. Many pigs infected with small numbers of Metastrongylus show few or no clinical signs.  Clinical signs when present may include dyspnoea, coughing and weight loss.  Diagnosis is usually based on finding larvated eggs in a fecal flotation. There are a variety of approved products available in Canada to treat this infection. Although there have been occasional reports of human infection with this parasite it is not generally considered to be zoonotic.

Phylum: Nematoda
Class: Rhabditea
Subclass: Rhabditia
Order: Strongylida
Superfamily: Strongyloidea
Family: Metastrongylidae

The closest relatives of Metastrongylus species of veterinary interest are probably species of the genus Angiostrongylus, including A. cantonensis, a gastropod-transmitted zoonosis for which rats are the primary reservoir, and A. vasorum, the "French heartworm" of dogs and other carnivores.

Adult Metastrongylus species measure up to 60 mm (females) and 25 mm (males) in length. Usually, few of the other structural features are obvious, other than in mounted, stained specimens.

Eggs of Metastrongylus species are larvated when passed, each containing a first-stage larva, and measure up to approximately 60 µm by 40 µm, with a thick, rough shell. The eggs of Metastrongylus species can easily be differentiated from other parasite eggs present in the fresh feces of pigs. In feces that are not fresh, the eggs of H. rubidus and Oesophagostomum may have larvated.

Metastrongylus species occur in domestic and wild pigs, wild boar, and occasionally other hosts, in many parts of the world. They are uncommon in pigs in Canada.

Adult Metastrongylus species live in the bronchi and bronchioles. Larvated eggs pass in the feces. The eggs hatch in the environment or following ingestion by an earthworm intermediate host. In the earthworm the larvae moult twice to the infective third stage. Infection of the pig is by ingestion of an earthworm containing infective larvae, or of larvae that have been released accidentally from the intermediate host. The pre-patent period of Metastrongylus species is approximately three to four weeks.

Life Cycle: Metastrongylus species

Transmission of Metastrongylus species depends on the presence of suitable earthworm intermediate hosts. Several species of earthworm will support larval development. It is also important that the pigs are have access to the earthworms, thus infection is likely to be more common in animals that have access to the outdoors than in those that are housed.

Many pigs infected with Metastrongylus species rarely, if ever, show clinical signs, particularly if parasite burdens are small. When clinical signs do occur, a very rare occurrence in Canada, they are usually in young animals and include dyspnoea, coughing and weight loss.

Detection of the larvated eggs in feces using a fecal flotation is effective and probably has reasonable sensitivity. Because there is a slight possibility that the eggs of Metatstrongylus species have hatched in the intestine, a negative flotation might sometimes be supplemented by use of the Baermann technique for larval recovery.

are approved in Canada for Metastrongylus species in pigs.

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

Control of Metatsrongylus species in pigs depends on minimizing contact between pigs and intermediate hosts. Where this is not possible, early detection of infection and appropriate treatment may be helpful.

Metastrongylus species are not usually considered to be zoonotic, but there are reports of a very few possible cases of human infection with M. elongatus.