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Lungworm Infection (Metastrongylosis)


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Definition

Infection with any one or more of three species of metastrongyle nematodes.

Occurrence

Swine can be infected by any of three different species of metastrongyles. All ages of swine are susceptible but heavy infections occur mostly in young pigs over six weeks of age. Infection is common in areas where pigs are raised outdoors with access to soil. Lungworms likely occur in all countries where swine and the intermediate host, the earthworm, is present.

Historical information

Lungworm infection historically was a major problem in US swine production. With the development of confinement housing and subsequent lack of access to soil (and earthworms), lungworm infection has decreased in prevalence. However, where pigs are raised in the outdoors, lungworm infection still can be a serious problem. Parasiticides effective in controlling lungworms, as well as many other parasites, have improved control. At one time, lungworms were erroneously believed to be important in the transmission of influenza virus to swine.

Etiology

Lungworms that infect swine include Metastrongylus elongatus (apri), M. pudendotectus and M. salmi. Metastrongylus elongatus infections are the most common but infection with more than one species can occur. Lungworms usually have an indirect life cycle through earthworms, but a direct cycle has been reported. Adult lungworms vary in length from 14-66 mm. They are white, threadlike and easily visible at necropsy. Lungworm ova are larvated, thick shelled, about 40 by 50 um in size, and are passed in feces.

Epidemiology

First stage lungworm larvae hatch from eggs in swine feces and survive in feces or moist soil for long periods of time. In some areas the first stage larvae can survive over winter. After being ingested by earthworms, larvae are able to develop to the third infective stage in about 10 days. They then remain quiescent in the earthworm for up to 18 months. Thousands of larvae can accumulate in a single earthworm, apparently without harm to the earthworm.

Pigs on pasture root for earthworms, especially if the soil is soft and moist. Pigs ingest the parasitized earthworms and are infected by the larvae within them. The lungworm larvae penetrate the intestinal mucosa, migrate through lymphatics and venous blood and reach the lungs. There they localize, grow to maturity and produce larvated ova about 25 days after ingestion. Infection becomes patent in about four weeks. The larvated ova are coughed into the pharynx where they are swallowed and passed out through the feces.

Survival of lungworms depends largely on the survival of infective larvae in earthworms for long periods of time; also, on the ability of first stage larvae to survive for long periods in swine feces. Breeding and other older stock can remain infected for long periods and lungworm eggs in their feces continue contaminating the environment.

Pathogenesis

Third stage larvae penetrate the small intestine mucosa and pass in lymph and blood to the liver and lungs. Some larvae migrate in the liver and cause gray to white areas of scarring similar to those caused by ascarid migration. Larvae that reach the lungs break into alveoli where they mature, extending into terminal bronchioles. The parasites preferentially localize along the caudodorsal border of the diaphragmatic lobes. Their presence results in alveolitis and bronchiolitis leading to coughing. Exudate that forms around lungworms and their ova partially or completely obstructs airways resulting in alveolar emphysema and atelectasis. Secondary bronchopneumonia is a common sequela.

Clinical signs

Common clinical signs include chronic and sometimes paroxysmal coughing that eventually leads to generalized unthriftiness. If secondary pneumonia intervenes, there may be dyspnea and perhaps an abdominal type respiration (“thumping”). Signs are similar to those caused by ascarid migration in the lungs. The two parasitisms often occur concurrently where pastures are heavily contaminated by both ascarids and lungworms.

Lesions

Pigs with many lungworms are unthrifty, have a rough hair coat, and are small for their age. Pulmonary lesions often occur on the caudodorsal border of diaphragmatic lobes. When viewed from the dorsal aspect, those lesions often have a roughly triangular shape and appear as recessed darker areas of atelectasis or raised, lightly colored, emphysematous areas. Usually both types of lesions can be present in the same pig. If the distal border of the lobe is trimmed and lesion areas squeezed, adult worms can be expelled. The bronchial tree should always be opened as lesions along the border are not always present. When this is done, inflammatory exudate and lungworms may be found in terminal airways. Scattered 1- to 2-cm areas of gray-to-white scarring may be visible in the liver.

Diagnosis

Diagnosis is made most easily by postmortem examination of a few typical, affected pigs. Also, laboratory fecal exams usually will reveal the embryonated eggs of lungworms. Postmortem exams have the advantage of permitting examination for other parasites and diseases and often require less time.

Control

Prevention of heavy infection is possible by preventing access of pigs to soil that contains larvae or earthworms. Consequently, most confinement-raised pigs are free of lungworms.

Pigs raised outdoors often can avoid heavy infection if they are farrowed and raised on clean pastures. Pasture rotation is helpful but sometimes fails to prevent infection as lungworm larvae persist in earthworms for long periods of time. Using an effective anthelmintic on sows prior to farrowing and then moving them to clean pasture is helpful. There are several effective anthelmintics. See the table, Anthelmintics and Parasiticides.