Trichinella spiralis in Newborn Mice: Course of Infection and Effect on Resistance to Challenge

Abstract

Newborn mice were infected with Trichinella spiralis by transabdominal intragastric injection or stomach intubation of 200 mature larvae per mouse. Newborns were found to be 5 to 10 times more resistant than adult mice to this infection as measured by mean adult and larval worm burdens at necropsies during a 45-day postinfection period. Mice infected at birth were found to be more resistant to subsequent single or multiple challenge infection as compared with littermate controls. No significant differences were found in the worm burdens of male and female mice in these experiments or between the weight gain of mice infected at birth and as adults and mice infected only as adults. Previous attempts to alter the resistance of mice to Trichinella spiralis by exposing young mice to various trichinella antigens have been reported (Ewert and Olson, 1960; Olson and Ewert, 1961). These workers injected newborn or fetal mice with metabolites or extracts from larvae; resistance to subsequent challenge infection was not significantly altered in these mice as compared to suitable controls. This report presents our results from experiments designed (1) to compare the susceptibility of newborn and adult mice to this infection, and (2) to test the effect of infection at birth on the subsequent resistance of mice to homologous challenge. A portion of these results has been reported in abstract (Bass and Olson, 1963a, b). METHODS AND MATERIALS White mice (Fairfield-Webster) were purchased as young adult males for organ distribution studies and as pregnant females (14 to 16 days gestation) to obtain newborn mice for tolerance experiments. The stock infection of trichinella was maintained by passage in adult male mice. Larvae for stock and experimental infections were recovered from mice during a 6to 12-week period after primary infection (Ewert and Olson, 1960). Adult worms were recovered from mice by NaOH treatment of gut segments (Larsh and Kent, 1949). Pepsin digestion of organs for recovery of larvae was done by the methods of Ewert and Olson (1960). Intragastric, subcutaneous, and intraperitoneal injections of newborn mice were performed with a 30-gauge needle attached to a tuberculin syringe. The stomach, visible through the skin of a newborn, was injected with viable larvae suspended in Received for publication 30 September 1964. * This investigation supported by National Science Foundation Research Grant G-20994 and U. S. Public Health Service Training Grant 2-G-185-C. nutrient broth gelatin (Olson, 1962); 0.05 ml of inoculum was found to be the safe maximum for newborn stomachs and was used as the standard volume in these studies. Stomach intubation of newborns was done with a tuberculin syringe equipped with a 27-gauge needle to which was attached a 1-inch length of polyethylene tubing (Clay Adams, Intramedic, PE10). Newborns were held under light ether anesthesia during intubation. Each lot of larvae recovered for experimental use was examined microscopically to establish that the larvae were vigorously moving and viable. In addition larvae from each lot were used to infect several adult male mice (200 larvae/ mouse) which were necropsied 5 or 7 days later for the recovery of adult worms. Statistical analyses utilized Student's t-test, 2tail, with no significance given to differences with a probability greater than 0.05 (Snedecor, 1959). EXPERIMENTS AND RESULTS Course of T. spiralis following infection of adult and newborn mice A preliminary experiment was planned to determine whether or not the newborn mouse is susceptible to this infection. One hundred newborn mice were inoculated within 12 hr postpartum with 200 larvae per mouse; animals were divided into four groups and inoculated as follows: intraperitoneal and subcutaneous routes; stomach intubation; and intragastric (IG) injection. One-half of each group was necropsied during the first 10 days after inoculation; the intestines, blood, and carcasses were examined for adult worms and for migrating or encysted larvae. The remaining animals in each group were necropsied at 30 days postinfection for digestion of carcasses and larval recovery. Animals were graded at necropsy as positive or negative for adult or larval worms. This experiment revealed that newborn mice were infected when infective larvae were ad-