Egg sizes and differential egg counts in relation to sheep nematodes.

Worm egg counts have been used for the assessment of nematode infections of sheep for many years. Different methods have been used, but recently the Stoll method (Stoll, 1930) and the McMaster Slide (Gordon & Whitlock, 1939) technique have almost entirely superseded the others. The McMaster technique is the best method, although it suffers from the slight disadvantage that eggs of Trichuris ovis are difficult to detect because they do not float in the salt solution used. In most methods partial differentiation of the eggs into species can be made because the eggs of Trichuris, Nematodirus and Strongyloides can be easily recognized. This recognition is based upon the fact that these eggs differ considerably in size and/or shape from each other, and also from the main body of trichostrongyle eggs found in sheep faeces. For many purposes, however, the proportion of eggs of Trichuris, Nematodirus and Strongyloides is relatively unimportant, and knowledge of the numbers of the more pathogenic species is required. In such cases, faeces-charcoal cultures are prepared and the resulting larvae identified. This latter method has numerous faults, and most workers who have employed it extensively will have been conscious of difficulties in standardizing the technique. The recovery of different species of larvae from cultures seems to be dependent upon the amount of moisture in the culture jars and the temperature and time of incubation. Thus, apart from the method being slow and laborious, dif(6) Interpretation of class distributions within a mixed sample of eggs . 283 (c) Method of application of the derived relationships to differential egg counts 285 VI. The classifying scale and its construction 285 VII. Discussion of method of egg differentiation 285 VIII. Summary 286 References 286