Faecal Egg Count Variability in Foals and Parasite Control Practices in Australia

Gastrointestinal helminths are recognised as important causes of clinical disease in adult horses and foals. Resistance to anthelmintics is a global problem involving parasites of horses, including species with considerable pathogenic potential. As the ongoing development of anthelmintic resistance (AR) is likely to continue, there is increased risk of greater challenges to parasite control with potential consequences to the health and welfare of horses. A number of strategies have been developed and promoted within the equine and other livestock industries in an attempt to slow the development of AR and extend the efficacy of the currently available anthelmintics. Surveillance-based treatment regimens have been recommended and these regimens rely on the use of faecal egg counts (FEC) to identify horses that require treatment, based on egg shedding potential. It is important that the limitations of FECs are considered and improved understanding of egg distributions in faeces and FECs is developed to best apply this important diagnostic method.
The aims of the research presented in this thesis were to: (i) examine the density of Parascaris eggs in solution and faeces and to describe the use of hypothesis testing for treatment decision-making; (ii) determine the potential impact of FEC variability on anthelmintic treatment decisions for foals in a selective treatment regimen scenario and the reliability of a single FEC result of zero; (iii) use an online questionnaire to investigate parasite control practices used on Thoroughbred and Standardbred studs in Australia and determine factors that may be associated with increased risk of development of AR; (iv) investigate the efficacy of a morantel–abamectin combination for the treatment of ML-resistant Parascaris infections in foals.
In this study, it was determined that the density of nematode eggs in faeces is inhomogenous and egg counts conform to a negative binomial distribution. Methods for calculation of the negative binomial confidence interval for the estimation of the egg distribution mean and application of these intervals for statistical inference for decision-making were presented. It was confirmed that point estimates do not account for the sample variance and provide a poor estimate of the egg distribution mean. The outcomes of this research can inform improved techniques for faecal sample collection and analysis of FEC data through statistical inference and this is applicable in both a research and clinical settings.
The results of the questionnaire study of parasite control practices currently employed on stud farms in Australia confirmed that there is considerable over-reliance on the use of anthelmintics with variable knowledge or understanding of the importance of other management strategies, including pasture hygiene.
The results of the field study investigating the efficacy of a morantel-abamectin combination for treatment of resistant Parascaris indicate that the use of combination anthelmintics has the potential for effective control of parasites for which there is documented evidence of AR. However, the use of these drugs needs to be carefully instigated within an appropriately designed targeted treatment regimen.
This thesis provides valuable contributions to the understanding of nematode egg density in faeces and the influence this has on FEC results and the application of FECs in decision-making in a clinical setting. The research generated important information on parasite control practices on Australian stud farms which can be used to inform changes necessary for more sustainable programmes. Evidence was also provided that combination anthelmintics may be useful as a component of these programmes for the management of existing resistance and to delay the development of resistance.