Treatment of psoroptic mange with reference to epidemiology and history

Abstract

Treatment methods in the last century involved the use of substances such as sulphur, mercury, hellebore, arsenic, nicotine and others, applied in various ways. The advent of dips in 1843 signalled an advance. However, the biology of the mite, Psoroptes ovis, the epidemiology of sheep scab and the lack of persistence of the acaricides necessitated repeated laborious treatments to ensure success. In 1947 this changed with the use of organo chlorines (OCs) which had up to 3 months residual activity. The use of OCs led to the eradication of psoroptic mange of sheep in USA. Organo phosphates (OPs) were introduced in the late fifties and synthetic pyrethroids (SPs) in the early seventies. In 1985, due to sheep tissue residues, lindane (OC) was withdrawn from the market and this greatly reduced the capacity for effective sheep scab control. Before the arrival of the endectocidal avermectin, ivermectin in 1978 and its successful use as an acaricide in 1992, control of psoroptic mange was limited to plunge dipping. In 1994 moxidectin, a milbemycin, was found to be effective and to have the added benefit of at least 4 weeks persistent activity. Another avermectin doramectin was shown, in 1995, to successfully treat scab and recently an ivermectin bolus has been introduced which has a 100 days’ activity and is fully therapeutic and prophylactic. Strict attention to detail in the use of injectable products is essential in order to achieve satisfactory results. Work is progressing on the use insect growth regulators (IGRs) and also on naturally occurring substances such as linalool, neem and lavender oil. At the CVRL Dublin, promising results have been achieved with neem and some IGRs. Other areas of interest are allemones, synergists, microclimate manipulation, sheep breed resistance and vaccines.

Successful control depends on epidemiological knowledge, accurate diagnostic techniques, intimate knowledge of the mite’s life cycle, its behaviour on and off the host, its macro and molecular biology, the nature of the pathogenesis of the disease, sheep husbandry practices, nutritional and environmental factors; also farmer awareness and attitudes. The variable responses of sheep to the mite, the unpredictable incubation period, course, manifestations and outcome make this an intriguing and perplexing disease. Ways to overcome these problems and to achieve possible eradication are discussed.

Introduction

In the past, treatment and control of psoroptic mange was often reactive or indiscriminate, although sometimes it was successful, even to the extent of achieving eradication. These successes were in great part due to favourable circumstances that no longer exist. The lack of success of the past three decades has been mainly due to the failure to recognise this. While control programmes were in place in the British Isles the disease was kept at bay and few scientists showed any interest in the condition, even though it causes severe suffering and significant economic losses. ( Kirkwood, 1980, Sargison et al., 1995).

However, the new awareness and interest in animal welfare issues and the spread of the disease following the abolition of the compulsory dipping schemes, caused a surge in scientific activity. This must now be directed and used in the right way to ensure final conquest of this dreaded disease. It is also important to allow the results of history and epidemiological experience be guides in the decisions taken on future strategies and control measures.