Effect of Hypothyroidism on Resistance of Mice to Infection with Trichinella spiralis

Abstract

Propylthiouracil (PTU) was employed to induce hypothyroidism in albino mice which were subsequently given 6,400 or 10,000 Trichinella spiralis larvae. During the first 8 days of infection there was no difference in the mean number of hypothyroid and euthyroid (normal) animals that died. But differences in survival times between the respective hypothyroid and euthyroid groups were statistically significant. The mean survival times of mice receiving 6,400 larvae were 79 and 82 hr for the two groups fed PTU, and 110 hr for the control group. When 10,000 larvae were used, the survival times of mice in hours were 64 and 59 for the PTU-treated animals and 106 for the controls fed a PTU-free diet. When survival time is used as a criterion, the results suggest that the hypothyroid state lowers the resistance of mice to heavy T. spiralis infections. In another experiment second generation larvae were recovered from hypothyroid and euthyroid mice 6 weeks after each received 300 larvae. Although more larvae were recovered from hypothyroid animals (37,000 per mouse) than from controls (27,000 per mouse), the difference was not significant. It is unlikely that larviposition is affected by the thyroid status of the mouse host. Studies of the relation between host thyroid activity and parasitic infections have been few, the results variable. Thus, Todd (1949) working with chickens made hypothyroid with thiouracil or hyperthyroid with iodocasein reported that Heterakis gallinae developed to a greater size in hypothyroid hosts, but Ascaridia galli grew larger in hyperthyroid birds. However, the number of nematodes recovered from chickens with either thyroid abnormality was not significantly different from those in controls. According to Dobson (1964), surgical thyroidectomy increased the number of Oesophagostomum columbianum surviving in lambs, particularly female lambs. Thyroidectomy had no effect, however, on the number of nodules found in hosts of either sex. Using the number of cysticercoids observed 93 hr after infection as the criterion of resistance, Larsh (1947, 1950) reported that the hyperthyroid condition favored the development of Hymenolepis nana in mice. No effect was noted in thiouraciltreated hypothyroid hosts. On the other hand, Landt and Goodchild (1962) recovered heavier Received for publication 11 July 1966. * This investigation was supported by NIH Research Grant NBO 1716 awarded to Dr. M. Hamburgh. t This project was carried out with the assistance of Mr. F. Weil, Rutgers University, and Miss D. Gelfand, Brooklyn College, who participated under the terms of an NSF Summer Institute Grant awarded to Yeshiva University. specimens of H. diminuta from thyroidectomized female, but not male, rats. Palva (1963) detected no relation between hypothyroidism and impaired absorption of vitamin B12 in patients infected with the broad tapeworm, Diphyllobothrium latum. Wheeler et al. (1948) found that hypothyroidism did not alter the natural resistance of thiouracil-fed chicks to Eimeria tenella. Differences in technique and biology of the parasites may account for the diversity of these results. The need for more information on the relation between thyroid activity and parasitism stimulated the present work. The experiments reported herein were undertaken to determine whether hypothyroidism in mice would affect (1) the survival of mice infected with massive doses of Trichinella spiralis, and (2) the number of second generation larvae recovered from host tissues when nonlethal infections were given. MATERIALS AND METHODS The mice used in these experiments were male Swiss albinos from the Charles River Breeding Laboratories. The animals ranged between 5 and 6 weeks of age at the start of each experiment. They were tagged and housed in individual metal cages on wood shavings. The cages were cleaned and new wood shavings added every week. During the experiments all animals were maintained on powdered Rockland mouse diet. The food was issued in porcelain bowls and changed every day. The animals were divided into three groups. Group A consisted of mice that were offered a