Longitudinal Follow-Up of Clinical Superficial Ovine Caseous Lymphadenitis

Caseous lymphadenitis is an infectious disease that has a significant economic impact on sheep breeding. The objectives of this study were to evaluate the effect of season, animals’ age, sex, body score and shearing on the clinical incidence of caseous lymphadenitis, relapses and abscess location in sheep from Settat province, Morocco. In this longitudinal study, 274 clinically healthy sheep were recruited in six flocks among 1451 inspected sheep and followed during 12 months to cover four seasons. The animals involved in this study were assigned to different age categories at continuous recruitment from February to July 2021 (lambs younger than 6 months, young sheep between 6 and 24 months and old sheep more than 24 months) and to sex categories. Selected animals were clinically examined every 2 months to detect superficial abscesses by palpation of superficial lymph nodes. Information about the number of abscesses and their locations was recorded and analyzed in multivariable statistical models. The results showed that 185/274 (67.5%) of the monitored animals developed superficial CL abscesses. The risk ratio (RR) of superficial CL was significantly higher between April and September compared to between January and March (RR~4.4; <i>p</i> < 0.0001). No significant difference was found between October and December compared to between January and March (RR = 1.2; <i>p</i> = 0.64). Regarding the effect of age, the results revealed that the RR was significantly lower in old sheep compared to lambs (RR = 0.45, <i>p</i> < 0.0001). No significant difference was detected between lambs and young sheep (RR = 0.7, <i>p</i> = 0.07). The prevalence of caudally located abscesses (prefemoral and popliteal lymph nodes, as well as in testicles, scrotum and mammary glands) was significantly higher in old sheep than in lambs (20% versus 3%; odds ratio = 7.8, <i>p</i> = 0.02). The sex, body score and shearing since the last examination did not show any significant effect on CL incidence (<i>p</i> > 0.1). Abscess relapse was significantly lower in old sheep than in lambs (IRR = 0.4, <i>p</i> = 0.003). The highest clinical CL incidence was observed in young animals between April and September and was likely due to indoor intensive rearing management. To conclude, both season and age had significant effects on superficial CL incidence. Farmers, veterinarians and technicians should focus attention and preventive measures against CL on young animals during spring and summer.