Susceptibility and reactivity of sheep to Trichinella spiralis infection.

Susceptibility and reactive manifestation to Trichinella spiralis infection were studied in atypical hosts (sheep) for the period of 247 days. Sheep produced anti-Trichinella antibodies as early on Day 11 (low titer 1:200), with maximum reached at Day 35 (titer 1:800). From Day 42 the antibody level was declining with a negative result of examination on Day 70. Mice exhibited anti-Trichinella antibodies only on Day 32 (titer 1:200). This level was rising, reaching high titer (1:1600) on Day 56. This antibody level persisted until Day 156. In the following period, a rapid decrease in the titer was observed (Graph). On Day 32, T. spiralis larvae in sheep were present in all groups of the muscles examined. The highest larval counts during the entire experiment were detected in the masseter. The initially high counts in the diaphragm and tongue were reduced to only 1/4 or 1/10 at the end of the experiment. In mice, the larvae occurred evenly throughout the entire experiment (Tab. I). The first appearance of a capsule around the T. spiralis larva in muscles was observed on Day 32 p. i. No cell response was detected around the capsule (Fig. 1). Neither was any response observed around necrotizing larvae, even though the surrounding myofibrils were caused to die off (Fig. 2). Certain differences in the degree of myofibril degradation by larvae were evident as early as on Day 32. The least damaged myofibrils were those in the masseter, tongue and diaphragm. This finding correlates with the histological recovery of a different number of necrotized larvae from the individual muscle groups examined. Fresh blood extravassations around larvae were observed on Day 59 (Fig. 3). They could be caused by the migration of larvae to a parasitation site. Live uncapsulated larvae were also found on Day 115 p. i. (Fig. 4). An increased cellular presence around some larvae was observed on Day 84. The larvae surrounded by lymphocytes consequently died off, those without lymphocytic responses formed capsules and survived (Fig. 5). The necrotizing larvae were subject to a powerful phagocytic process, presented by histiocytes, forming multinuclear symplasms (Fig. 6). On Day 11 p. i., larvae inside a capsule were dying off as well. The initial stage of larval necrosis in a capsule is also accompanied by an increased lymphocytic response (Fig. 7). The condition of larvae in capsules and the cellular unresponsiveness as late as on Day 247 indicate the long-lasting viability of the larvae. The capsules surrounding T. spiralis larvae in mice were distinctly seen as early as on Day 32 p. i. Lymphocytic aggregations around the capsule were observed throughout the entire experiment (247 days)-Fig. 8.