Resumen

MORALES, Gustavo; PINO, Luz A; SANDOVAL, Espartaco  y  RONDON, Zoraida. Evaluation of two utility criteria in a program of control of the infection for gastrointestinal nematodes in sheep by means of selective antihelmintic treatment. Zootecnia Trop. [online]. 2008, vol.26, n.2, pp.141-150. ISSN 0798-7269.

A lot of sheep selected for breeding and naturally infected with gastrointestinal nematodes were submitted during nine months to hematological and quantitative coprological exams. The packed cell volume or haematocrit (Ht) determination was done by means of the microhaematocrit centrifugation method and determined the numbers of strongyle eggs per gram of faces (epg) by the McMaster counting technique. The animals were classified by using the epg counts as wormy animals or parasite accumulators (≥ 800 epg, drench) or not wormy animals (< 800 epg, no drench). When the haematocrit values were used, two groups were established: Ht≤22%, drench and Ht≥23%, no drench. The combination of both criteria plus hemoglobin values, first manually and then by mean of discriminant analysis allowed to classify the sheep in the following categories: resistant (no drench), resilient (no drench), and sensible or wormy animals (drench). The haematocrit values and weight were similar between resilient and resistant sheep, but higher than the sensible. However, for the epg counts there were found no statistical differences between wormy animals and resilient, but in both cases higher than resistant. The goal of these criteria is the identification of animals within the flock that require drug treatment to reduce drug usage. No statistical differences for the numbers of sheep to be drenched were found when the criteria for selecting animals were the haematocrit (11.5%) values or the epg count (10.8%), but in both cases this fraction was higher when the sheep condition (2,93%) was incorporated. These results showed a considerable decrease in number of sheep to be drenched when the classification of an animal as resilient, resistant, or wormy animals was used. The different criteria for selecting sheep for drug treatment in a program of control against gastrointestinal nematode infestation to reduce drug usage are discussed.

Palabras clave : nematodes; gastrointestinal nematodes; digestive strongyle; hematology; resistant; resilient; sen.