Worm burdens, egg elimination and retention, and schistosome pigment accumulated in the liver were studied in 111 white mice infected with Puerto Rican strain of S. mansoni, in infections lasting from 40 to 330 days. Ninety-four of these animals had been exposed to both sexes of worm, of which eight were given chemotherapy starting on the 57th day. Seventeen mice were exposed only to male worms.
The purpose of the investigation was to provide an explanation for the sudden fall in fecal elimination of eggs after the first maximum is reached, which has been observed in different species of animal.
In the laboratory mouse, egg elimination reached an early peak of 104 eggs per pair a day on the 60th day of infection, a rapid decline then taking place. This phenomenon cannot be attributed either to a decrease in the rate of oviposition or to a decreased worm burden at this early period, and the author postulates that immunologic factors may be involved.
However, after the 141st day, dead flukes were seen frequently in liver-press preparations. By employing quantitative data for egg load and schistosome pigment, it was possible to prove that a spontaneous decline in the original worm burden had taken place in 45% of the mice killed after the 141st day. Only two of these animals were entirely rid of the infection.
For the interval 43 to 65 days of infection the daily oviposition was found to be 257 per pair of flukes. Only 12% of the eggs produced from onset of oviposition to the 59th day after infection were passed in the feces.
The total egg load retained in the tissues per pair of parasites rose until the 220th day, then remained essentially constant. Schistosome pigment extracted from the liver, however, increased continuously throughout the period investigated and was found to be a more useful quantitative index of infection.
No “crowding effect” was apparent, either in egg or in pigment production.
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