The ascarid or roundworm of pigs (Ascaris suum) is the most common and widespread parasite of swine. Larval migration through the liver results in focal hepatitis (milk spots) and condemnation of the liver at slaughter but no clinical signs. Migration through the lungs causes pulmonary lesions that can lead to pneumonia, particularly in animals coinfected with other viral or bacterial pathogens. Small intestinal stages compete with the host for nutrition, resulting in poor feed conversion and slower weight gain. Infections usually stimulate immunity, which often results in older animals having fewer nematodes and excreting fewer eggs than younger ones. Infections can be detected via fecal flotation during the patent period; however, postmortem examination or ELISA is required for detection of prepatent infections. Numerous anthelmintics are available for use against the intestinal stages; however, control programs must incorporate strict sanitation as well.
Adults of the ascarid or roundworm of pigs (Ascaris suum) live in the small intestine. Adult A suum are large white worms up to 40 cm (15.7 inches) long. Females produce thousands of eggs per gram of feces daily, which can develop to the infective stage (eggs containing third-stage [L3] larvae) in 3–4 weeks under optimal conditions.
Eggs cannot develop at low temperatures (< 10–15°C [50–59°F]) but resume development when temperatures rise in spring and summer (1). The eggs are thick shelled and highly resistant to environmental changes and chemical agents; however, heat, low humidity, or direct sunlight decrease their survivaltime (2). Under optimal conditions, eggs in the environment can remain infective for 6–7 years (3).
When the eggs are ingested, the larvae hatch in the stomach and small intestine, penetrate the wall of the cecum and colon, and enter the portal circulation. After several days in the liver, they are carried by the circulation to the lungs, where they pass through the capillaries into the alveolar spaces. The larvae then migrate to the lungs, pass up the bronchial tree, and are coughed up and swallowed. They return to the small intestine, where they mature into adult worms. The first eggs are passed approximately 6–8 weeks after infection (4). Lifespan is approximately 6–9 months. Earthworms and dung beetles can serve as paratenic hosts (in which parasite development does not occur).
Distribution and Host Range of Ascaris suum in Pigs
A suum is found in pigs worldwide, including wild boars and feral hogs. In addition, infections can also rarely occur in cattle, manifesting as acute atypical interstitial pneumonia (5).
Some evidence indicates that the ascarid nematode found in humans, Ascaris lumbricoides, and A suum of pigs are genetically independent populations. However, other evidence suggests they should be synonymized (6). Ascarid infections in multiple reported human cases where A lumbricoides is considered nonendemic were mainly related to pig-derived Ascaris (7). In areas where both parasites occur, cross-infections have been documented (8). Thus, the pig ascarid is considered zoonotic. Visceral larva migrans in humans caused by migrating larvae of A suum has been described (9).
Clinical Findings of Ascaris suum in Pigs
Infections of A suum do not always cause clinical disease. Adult roundworms can appreciably decrease the growth rate of young pigs; in rare cases, the worms can cause mechanical obstruction of the intestine. Adult worms can also occlude bile ducts, causing icterus (10).
Migration of larvae through the liver causes hemorrhage, fibrosis, and accumulation of lymphocytes, observed as white spots or "milk spots" under the capsule, leading to condemnation of the liver at slaughter. These lesions become visible 7–10 days after infection and will regress within 4 weeks if reinfection does not occur; therefore, their presence indicates recent infection or reinfection. If reinfection occurs, milk spots can enlarge and remain visible in the liver (10).
If only a few larvae reach the liver, the number of white spots might be low despite continual reinfection. Therefore, the number of white spots and the liver condemnation rate are both poor measures of the severity of herd infection.
In acute roundworm infections with migrating larvae in the lungs, coughing can be frequent, and the larval infection can cause pulmonary edema and consolidation as well as exacerbate other pathogenic respiratory infections. Pigs can develop interstitial pneumonia (10) and abdominal breathing, commonly referred to as "thumps."
In addition to the respiratory clinical signs, marked unthriftiness and weight loss can occur with roundworm infections. Infection generally induces development of acquired resistance to reinfection, and prevalence is highest in young growing pigs. If the treatment rate is very low and the level of herd immunity is also low, prevalence can be highest in breeding animals.
Diagnosis of Ascaris suum in Pigs
Standard fecal flotation methods (patent infections)
Postmortem examination and ELISA (prepatent infections)
During the patent period, diagnosis of A suum in pigs can be made by demonstrating typical ascarid eggs (see ), which are golden brown with a thick, rough shell (50–76 × 68-84 mcm), on fecal analysis (3) or by observation of large worms in feces. False-negative fecal results can occur in single-sex infections.
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Ascaris suum eggs recovered by fecal flotation, 63 × 53 mcm. 400X total magnification.
Courtesy of Dr. Heather Walden.
Eggs are most often found in pigs between 5 and 6 months old, with one study also showing that eggs were more often recovered from fecal samples rectally collected rather than from the floor (11), which can lead to false-negative results (3). Pigs are coprophagic; thus, low egg counts (< 200 eggs/g) can indicate coprophagy rather than actual infection; however, it does indicate environmental contamination with A suum eggs and that the parasite might be infecting other pigs.
A presumptive diagnosis of A suum infection can be made at postmortem examination based on demonstration of the typical milk spots in the liver, and worms can be recovered from the lungs (smaller larval stages) and the small intestine (large immature or adult stages). An indirect-antigen ELISA has been developed for the detection of A suum in animals where an accurate diagnosis is difficult, such as on farms with few infected animals or a low infection rate or during the prepatent period (12, 13).
Treatment of Ascaris suum in Pigs
Anthelmintics (routine and prefarrowing)
Supportive treatment as needed
Herd management and husbandry
Many anthelmintics have been used to remove adult ascarids from pigs. Piperazine preparations have low toxicity. The benzimidazoles and probenzimidazoles, levamisole, dichlorvos, ivermectin, doramectin, and pyrantel are effective and have a broader spectrum of activity than piperazine. Hygromycin B is active against ascarids when administered as a low-level additive to the feed.
Less information is available concerning the control of migratory stages of ascarids; pyrantel and fenbendazole show some activity. Supportive treatment, including treatment for secondary bacterial infections, might be necessary during the acute respiratory phase of infection.
Prevention of roundworms is more effective than treatment; however, it can be difficult to achieve. Good ascariasis control in confinement systems can include deworming sows approximately 1 week before farrowing, washing sows to remove any adherent ascarid eggs before moving to sanitized farrowing crates, early weaning (2–4 weeks old, before eggs become infective), and using all-in/all-out production systems, which has been shown to substantially lower ascarid incidence on farms (14), with thorough cleaning between groups.
Pigs that are free from ascarids are at risk of developing severe pneumonia if placed into contaminated grower/finisher facilities. Feeding a continuous dewormer for the first 30 days is an option (15), and pigs in these situations should be dewormed at no greater than 8-week intervals to prevent additional contamination of the facility.
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In pasture or open-lot housing systems, deworming sows before moving to a pasture that has not been grazed in several years (ie, a clean pasture) is often recommended. Pasture rotation combined with anthelmintic use can also help decrease swine contact with ascarid eggs.
Key Points
Ascariasis, the infection of pigs with the swine ascarid or roundworm A suum, occurs worldwide, wherever swine are found.
Clinical signs, including chronic coughing, unthriftiness, and failure to gain weight, tend to be more prevalent in young, growing pigs.
Severe, sometimes fatal, respiratory disease can occur, particularly in naive animals placed in heavily contaminated facilities.
Patent infections (> 2 months) can be detected using fecal flotation to find eggs; diagnosis of prepatent (< 2 months) infections requires postmortem examination or ELISA.
Control should not depend solely on anthelmintics; however, it must also incorporate appropriate sanitation and herd management.
For More Information
Bowman DD. Helminths. In Bowman, DD. Georgis' Parasitology for Veterinarians. 11th ed. WB Saunders; 2021:135-260.
References
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