Morphological Characteristics of the Thymus in Opisthorchiasis

Opisthorchiasis is a widespread parasitic disease in the Ob and Irtysh River basins in Russia, characterized by a prolonged course in humans and carnivorous animals, which are the definitive hosts. The impact of this parasitic disease on age-related morphological changes of the thymus remains an unexplored feature of the parasite-host relationship. The parasitic Siberian liver fluke (Opisthorchis felineus Rivolta, 1884) has systemic toxic, carcinogenic, and immunosuppressive effects on the host organism. By interacting with the host immune system for camouflage, it likely leads to more pronounced morphological regression of the host thymus. It is necessary to determine the degree of involution, as well as the presence of other morphological changes in the host thymus in chronic opisthorchiasis, compared with natural involution without parasitic disease. Objective: To determine the presence and extent of age-related morphological changes of the thymus in a parasitic disease caused by the trematode O. felineus. Material and Methods. The study was conducted on 20 adult male mongrel rabbits (Oryctolagus cuniculus domesticus) aged 6 months. In the experimental group (n=10), each animal received an infectious dose of 50 metacercariae of O. felineus orally. The observation group consisted of clinically healthy animals of similar age (n=10). The animals were sacrificed 18 months after infection. Thin sections were prepared and stained with Carazzi’s hematoxylin and eosin, as well as Van Gieson’s stain to evaluate collagen fibers and vessels. The preparations were examined using light microscopy. Morphometric studies were performed on images of the preparations using Zen Blue Edition morphometric software (Carl Zeiss, Germany). The specific volume of thymic structures was estimated using the Avtandilov ocular grid. Statistical data processing was performed using Statistica v. 10, employing descriptive and nonparametric statistics for samples with a non-normal distribution at a significance level of p≤0.05. Results. Based on numerous morphological criteria, the thymus after infection with O. felineus exhibits moderate signs of age-related transformation, in contrast to the pronounced changes observed in the thymus in the course of natural aging without parasitic disease. All morphometric data showed significant differences with values below the baseline. However, the proportion of blood vessels in the medulla was significantly higher in patients with opisthorchiasis, along with the opposite trend in the cortex. The presence of Hassall’s corpuscles in the thymic cortex of infected animals can be considered a sign of acute thymic involution caused by a parasitic disease. Conclusion. Age-related thymic involution during a parasitic disease caused O. felineus is moderate vs. natural aging, which is characterized by signs of acute thymic involution.