Metastrongyles of Pigs

Only one genus occurs in pigs, Metastrongylus, and it is exceptional in having earthworms, rather than mol­luscs, as intermediate hosts.

Identification

• Slender white worms, up to 6.0 cm in length
• the host, site and long slender form are sufficient for generic identification.
• The eggs have rough, thick shells, and are larvated when laid (Ovo-viviparous).

Epidemiology

• Metastrongylosis shows a characteristic age distribu­tion, being most prevalent in pigs of 4-6 months old.
• The parasite is common in most the countries although outbreaks of disease do not often occur, probably due to the fact that most systems of pig husbandry do not allow ready access to earthworms by pigs.
• Though it is often suggested that Metastrongylus may transmit some of the porcine viruses, and may enhance the effect of pathogens already present in the lungs, the role of the worm is not conclusively proven

Life Cycle

Eggs hatch almost immediately, the intermediate host ingesting the L₁

 

In the earthworm, development to L₃ takes about ten days at optimal temperatures of 22-26 °C.

The pig is infected by ingestion of earthworms

the L₃ released by digestion, travel to the mesenteric lymph nodes and moult to L₄

Then, L₄ reach to the lungs by the lymphatic-vascular route

the final moult occurred after arrival in the air passages.

Female lay eggs which are swallowed and passed in the feces

PPP: 4wks

Note:

• In cold temperatures the eggs are very resistant and can survive for over a year in soil
• The longevity of the L₃ in the earthworm is similar to that of the intermediate host itself, and may be up to seven years.

Pathogenesis

• During the prepatent period areas of pulmonary con­solidation, bronchial muscular hypertrophy, and peribronchial lymphoid hyperplasia develop often accompanied by areas of overinflation.

• When the worms are mature, and eggs are aspirated into the smaller air passages and parenchyma, consoli­dation increases and emphysema is more marked
•   About six weeks after infection, chronic bronchitis and emphysema are established and small greyish nodules may be found in the posterior part of the diaphragmatic lobes; these may aggregate to form larger areas and are slow to resolve.
• Purulent staphylococcal infection in the lungs has been noted in many cases of metastrongylosis

Clinical Signs

Most infections are light and asymptomatic. However in heavy infections

• coughing accom­panied by dyspnoea and nasal discharge.
• Secondary bacterial infection may complicate the signs.

Diagnosis

• Clinical History
• Clinical signs
• Colonoscopy:
• Flotation is recommended along with the use of magnesium sulphate because of the heavy density of the eggs.
• The small rough-shelled larvated eggs are characteristic, but it should be recollected that Metastrongylus is often present in nor­mal pigs.

Treatment

·         Many anthelmintics including the modern benzi-midazoles, levamisole and the avermectins/milbe-mycins are highly effective.

Control

·         When pig husbandry is based on pasture, control is extremely difficult because of the ubiquity and longev­ity of the earthworm (intermediate host).

·         On farms where severe outbreaks have occurred pigs should be housed, dosed, and the infected pasture should be cultivated or grazed with other stock.