Optimal age of Trichostrongylus colubriformis larvae (L3) for the in vitro larval exsheathment inhibition test under tropical conditions

• Published in: Veterinary parasitology 2020-02, Vol.278, p.109027-109027, Article 109027
• Main Authors: Mancilla-Montelongo, Gabriela, Castañeda-Ramírez, Gloria Sarahi, Can-Celis, Alhely, Chan-Pérez, José Israel, Sandoval-Castro, Carlos Alfredo, Torres-Acosta, Juan Felipe de Jesús
• Format: Article
• Language: English
• Subjects: Anthelmintic activity; Larval exsheathment inhibition test; Optimal age; Plant extracts; Small ruminants; Trichostrongylus colubriformis
• ISSN: 0304-4017; 1873-2550
• DOI: 10.1016/j.vetpar.2020.109027

•The optimal age for larval exsheathment inhibition test (LEIT) was studied for T. colubriformis.•The optimal period for LEIT ranged from 2 to 10 weeks of age.•Motility > 85 % was recorded for L3 from weeks 1 to 7 of age.•The use of T. colubriformis L3 on the first week of age is discouraged for LEIT.•Each laboratory should find the optimal age for LEIT, considering their respective conditions. This study identified the optimal age of Trichostrongylus colubriformis larvae (L3) under tropical conditions for the in vitro evaluation of plant extracts using the larval exsheathment inhibition test (LEIT). Two T. colubriformis isolates with different anthelmintic (AH) susceptibility status were used for this study. The L3 of both isolates were maintained on refrigeration (4−5 °C) until use. For the LEIT, the isolates were tested every week during 16 weeks, using a stock solution of Acacia pennatula acetone:water extract at different dilutions (80–1200 μg/mL). Respective positive controls (levamisole 12.5 mM) and negative controls (PBS) were included. Effective concentrations 50 % (EC50), 90 % (EC90) and respective 95 % confidence intervals (95 %CI) for exsheathment inhibition were calculated at 60 min after exposure to chlorine solution. Motility of L3 (migration percentages (M%)) was recorded weekly using the larval migration test (LMT) as an indicator of L3 fitness over time. No correlation was found between L3 age or M% and the extract’s EC50 or EC90 values. However, the EC50 values for the A. pennatula extract ranged from 80 to 200 μg/mL from weeks 2–10. Beyond week 12, larval exsheathment was irregular, with higher EC50 and EC90 values and wider 95 %CI. The M% decreased below 85 % on week 7 for Paraiso isolate, and on week 10 for FMVZ isolate. A linear negative relationship was observed between the age of L3 and M% for both T. colubriformis isolates. The relationship (slope) for both isolates was similar therefore a single linear equation was estimated describing all M% data (r2 = 0.771, df = 164, P