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After ingestion, these infective ova are acted upon by the gastric juices which stimulate hatching of the operculated ova in the small bowel, freeing tiny larvae which penetrate the epithelium of the intestinal mucosa and enter either the venules of the portal system on their way to the liver or the intestinal lymphatics en route to the thoracic duct. By either route they eventually reach the lungs after passing through the right heart and pulmonary capillaries, and, after a few days, they perforate into the alveoli (Fig. 10.3).

Respiratory symptoms occur from 5 to 26 days after ingestion of viable eggs and at this stage the larvae may cause migratory, transient, localized intra-alveolar inflammatory reactions ("Ascaris pneumonia"). The alveoli may he filled with red and white blood cells, including many eosinophils and desquamated alveolar lining cells. Small areas of necrosis and minute intrapulmonary hemorrhages may be seen. The bronchi and bronchioles dilate and there is peribronchial infiltration with eosinophils and histiocytes. There may be a variable degree of peripheral eosinophilia at this time. Larval ascariasis is the most common cause of Löffler's syndrome, the classic findings of which were described in 1956 as consisting of fever, cough, sputum, asthma, eosinophilia, and infiltrates on the chest radiograph.

If there is a severe larval infection as a result of a large initial inoculum or repeated ingestion of infective ova, the bronchial mucosa may show ulceration and replacement by granulation tissue. Third and fourth stage larvae surrounded by edema fluid and inflammatory cells may fill the lumina of small bronchi and give rise to a more severe disseminated lobular pneumonitis, characterized radiologically by diffuse pulmonary infiltrates. This inflammatory process usually subsides within a week. A solitary pulmonary nodule can occur if the larva dies during this stage. Similar granulomatous lesions may be found in the peritoneum and mesentery following the death of larvae migrating through those tissues.

Fig. 10.3 Coiled larva of A. lumbricoides in bronchiole of experimentally infected guinea pig. Two molts occur in the lungs (L2>L3>L4 over a ten day period).

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