Immunopathological Changes Caused by Oesophagostomum radiatum in Calves: Insights into Host–Parasite Interactions

The intensity and prevalence of different gastrointestinal nematode species vary across regions worldwide. <i>Oesophagostomum radiatum</i> commonly shows a high occurrence in young cattle. <i>O. radiatum</i> causes anaemia, hypoproteinaemia, and immunopathological changes in the large intestine wall, impairing calves' body weight gain. This study aimed to assess the impact of natural <i>O. radiatum</i> infection on haematological parameters and immune responses in 23 Nellore calves, considering sex-based differences. Assessments included <i>Oesophagostomum</i> egg count (EPG), worm count, packed cell volume (PCV), total plasma protein, histopathological and immunohistochemistry analyses. A large number of parasites attached to the colon mucosa were observed, along with massive nodule formation and haemorrhagic lesions, mainly within a 20-30 cm-long segment adjacent to the nodules. The maximum mean egg shedding was approximately 165 EPG for males and 173 EPG for female calves; however, males presented a significantly higher worm count (969 ± 200.5) than females (460 ± 99.5). There were significant positive correlations between the total <i>O. radiatum</i> worm count and <i>O. radiatum</i> EPG for both female and male calves. Significant negative correlations were observed between the total <i>O. radiatum</i> worm count and PCV in female calves. Our results demonstrated that natural <i>O. radiatum</i> infection in Nellore calves induced marked immunopathological alterations, including chronic inflammatory responses that impaired intestinal function. Sex-related differences suggested that female calves may develop more effective tissue responses. These findings emphasise the economic impact of subclinical infections and reinforce the importance of control strategies to minimise productivity losses in cattle.