Pathologic changes and pathogenesis of Parascaris equorum infection in parasite-free pony foals.

Parasite-free pony foals (n = 10) were infected orally with 1000,000 Parascaris equorum embryonated eggs. One pony foal each was euthanatized on days 1, 3, 5, 7, 11, 16, 23, 27, 42, or 80 after infection. Foals infected for more than 7 days showed signs of coughing, anorexia, rough coat, and weight loss. Cellular changes in the blood were mild anemia, marked eosinophilia, and leukopenia. Gross postmortem lesions included hemorrhage, edema, and white-to-yellow necrotic foci (0.5 to 1.00 mm) in lungs, liver, and bronchial and hepatic lymph nodes. Microscopically, the liver was hemorrhagic and had focal necrosis and eosinophilic granulomas. Eosinophilic lymphadenitis with edema was in the hepatic and bronchial lymph nodes. The lungs had focal areas of necrosis with hemorrhage and interstitial pneumonia with hyalinization of the alveolar walls. Larvae were first seen in the liver at postinfection hour 24 and in the lungs on postinfection day (PID) 11. Average length of larvae in the liver was 1 mm and 2.5 mm in the lungs. Larvae were not found in the liver after PID 11 or in the lungs after PID 42. The migratory pathway of P equorum was similar to that of Ascaris lumbricoides because the larvae migrated via lymph and blood vessels to the liver and then to the lungs. After migrating through the lungs, the larvae were coughed up and swallowed and then developed in the small intestine.