March and April, 1997
Robert M. Corwin, DVM, PhD
College of Veterinary Medicine, University of Missouri, Columbia, Missouri 65211; email firstname.lastname@example.org
Diagnostic notes are not refereed.
Parasites in swine have an impact on performance, with effects ranging from impaired growth and wasteful feed consumption to clinical disease, debilitation, and perhaps even death. It is particularly important to diagnose subclinical parasitism, which can have serious economic consequences and which should be treated with ongoing preventive measures.
Internal parasitism is caused by nematode roundworms and coccidia in the gastrointestinal tract, lungworms in the respiratory tract, and by ectoparasites. The most commonly encountered gastrointestinal parasites are the large roundworm Ascaris suum, the threadworm Strongyloides ransomi, the whipworm Trichuris suis, the nodular worm Oesophagostomum dentatum, and the coccidia, especially lsospora suis and Cryptosporidium parvum in neonates and Eimeria spp at weaning.
Diagnosis of internal parasites is best accomplished by fecal examination using a flotation technique and/or by necropsy. Although there are several flotation techniques, the net result is the microscopic appearance of nematode eggs and of coccidian oocysts. Fortunately, identification is relatively easy because pig parasite eggs and oocysts have such distinctive appearances. However, you should also consider size, so that you don't incorrectly identify artifacts--e.g., pine pollen--as eggs. Use an ocular micrometer for measurements in microns ((micro)m).
You should also take the age of the pig into account when identifying parasites. Some parasites occur most often or in greatest numbers in young pigs, e.g., Strongyloides, Ascaris, and Isospora, whereas Oesophagostomum is often found in adult pigs because they have little immune response to it. The type of management and husbandry, e.g., pasture versus confinement, may determine the species of parasite present as well. Ascarids are ubiquitous and are found in confinement-reared pigs as well as in pigs reared on dirt. Others, such as nodular worms, lung worms, and, in southern climates, kidney worm (Stephanurus dentatum), are associated with dirt lots and pastures.
Prepatent period--the time it takes for the parasite to develop to reproductive age and produce eggs or oocysts --is another important identification factor. These periods are fairly specific and have a bearing on how early eggs could be expected to be found in pigs by fecal examination. The infection is then dependent on the initial time of exposure and the persistence or life span of the parasite.
Postmortem examination should reveal adult worms in their principal sites of infection, e.g., ascarids in the small intestine. Lesions associated with larval infection, such as nodules of Oesophagostomum in the colon may not have larvae present or apparent. Lungworms also have rather specific sites at least in young worm populations, viz., the bronchioles of the diaphragmatic lobes of the lungs.
All of these parasites are directly transmissible from the environment with ingestion of eggs or larvae. Strongyloides may also be passed in the colostrum or penetrate skin, and transmission of the lung worm and the kidney worm may involve earthworms.
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Ascaris suum --"large roundworm"
- (Figure 1A)77K JPEG
- egg: 45-60 (micro)m, yellowish brown, spherical, mammillated (Figure 1B58K JPEG)
- egg development: 2-5 weeks with survivability up to 10-12 years
- prepatent period: 6-10 weeks
- site of infection: small intestine (Figure 1C76K JPEG) but may migrate into bile ducts, eosinophilic tracts in liver and lungs due to larval migration (Figure 1D85K JPEG)
- adult size: 10-15 inches (25-40 cm)
- life span of adult: 8-12 weeks
- affected pigs: nursing to growing
Strongyloides ransomi--"intestinal threadworm"
- egg: 50-55 (micro)m, thin-shelled, ovoid, embryonated or larvated
- egg development: 2-3 days but larvae may be passed in colostrum
- prepatent period: 3-5 days, thus may be found in neonates with scouring
- site of infection: small intestine with adult in tissue, adult seen in histosection
- life span: 5-10 days
- affected pigs: all ages
- (Figure 2A140K JPEG)
- egg: 45 (micro)m, yellowish brown, ovoid, bipolar plugs (Figure 2B54K JPEG)
- egg development: 3 weeks, long survivability in environment
- prepatent period: 6-7 weeks
- site of infection: cecum, proximal colon
- adult size: 2-3 inches (5-8 cm), long, whiplike anterior end imbedded in mucosa
- life span: 2-3 months
- affected pigs: > 2 months
Oesophagostomum dentatum--"nodular worm"
- (Figure 3A58K JPEG)
- egg: 90 * 40 (micro)m, thin-shelled, morulated (Figure 3B48K JPEG)
- egg development: egg hatches in 1 week; infective larvae may survive up to 1 year on pasture
- prepatent period: 4-6 weeks
- site of infection: colon
- adult size: 0.5 inches (1.25 cm), larva-induced nodules
- life span: 4-7 weeks
- affected pigs: finishing and breeding adults
Metastrongylus spp--"lung worm"
- egg: 40-50 (micro)m, shell often covered with mucus, developing larva within (Figure 4A51K JPEG)
- egg development: egg hatches in earthworm or in soil
- prepatent period: 4 weeks
- site of infection: bronchioles of diaphragmatic lobes of lungs (Figures 4B66K JPEG and 4C100K JPEG)
- adult: 1-2 inches (2.5-5 cm)
- life span: several weeks dependent on immune response
- affected pigs: adult and finishing hogs
Stephanurus dentatus--"kidney worm"
- (Figure 5A46K JPEG)
- egg: 120 * 70 (micro)m, thin-shelled, morulated; passed in urine (Figure 5B40K JPEG)
- egg development: egg hatches in 1-2 days in soil or in earthworm
- prepatent period: 9 months-1 year
- site of infection of adult: perirenal fat; larvae in liver with abscessation likely (Figures 5C44K JPEG and 5D90K JPEG). Many ectopic sites such as lungs, spleen, back muscles
- adult size: up to 1 inch (2.5 cm), black and white mottling
- life span: based on egg shedding up to 3 years
- affected pigs: breeding stock
Isospora suis --"neonatal coccidiosis"
- oocyst: 25 (micro)m, smooth shelled without cap, two sporoblasts in older specimens
- development: infective following sporulation; transmission probably from sow
- site of infection: small intestine
- affected pigs: neonates and early nursing pigs
Cryptosporidium parvum --"cryptosporidiosis"
- oocyst: 5 (micro)m diameter, contains four sporozoites when passed
- development: immediately infective; note cross-transmissibility from other host species
- site of infection: jejunum, ileum, cecum, colon; in epithelial tips
- affected pigs: 1- to 12-week-old piglets, but found in feces of 30-week-old pigs
Eimeria spp--"coccidiosis" of older pigs
- oocysts: 20-30 (micro)m, depending on species, may be translucent to brownish, with or without micropylar cap depending on species
- development: infective following sporulation 6-9 days
- affected pigs: weaning and older
An excellent reference for identification of nematode eggs and of coccidian oocysts is: Sloss, Kemp, and Zajac. Veterinary Clinical Parasitology, 6th edition. Iowa State University Press:Ames Iowa.