The epidemiology and control of nematode parasites of ruminants in Texa

A series of ten studies were done focusing on Ostertagia ostertagi and Haemonchus contortus although other nematodes were also considered. The object of the studies was to determine seasons and weather conditions favoring or hindering transmission of these organisms in Texas and to develop systems to control them.;Ostertagia ostertagi is primarily transmitted during the winter and can be controlled with treatments in the fall that preclude pasture infestation. Other nematodes of cattle such as Haemonchus placei are transmitted primarily during the warmer months, especially in areas of higher rainfall. They can be controlled by spring treatments, but stocking density must be kept relatively low, and the animals should be moved to clean pastures after treatment.;In the more arid regions of Texas, grazing systems may affect transmission, but stocking density has more effect on nematode acquisition than the grazing system does. Differences between goats and sheep are mainly related to their foraging strategies. Availability of browse reduces the infection rate in goats.;Haemonchus contortus is found in most areas of Texas including arid areas. It is transmitted during the summer if rainfall is sufficient to allow development of the freeliving forms. It can be controlled by use of an effective anthelmintic early in the spring at the time of parturition since it usually does not survive the winter on pasture. Reinfection later in the summer may require retreatment followed by a move to clean pasture.;Because of frequent treatments with the available anthelmintics, H contortus has developed resistance to all the drugs available. The level and type of resistance depends on the treatment history, and a producer needs to check the efficacy of his anthelmintic on the parasites affecting his animals. An egg count reduction test is feasible and accurate enough to be used by lay-men.