Can acaricide treatment of sheep control ticks and tick-borne disease in the environment?

Published on 10 January 2011 in Sustainability and Communities , Food, health and wellbeing

Introduction

The tick Ixodes ricinus is the most important pathogen-carrying vector in Europe, including the UK. This species of tick is a generalist parasite, feeding off most types of terrestrial vertebrate, including sheep, deer, horses, dogs and humans. Because these ticks can feed off so many types of host, it is a particular challenge to control their numbers in order to reduce risk of disease. This also makes it a problem to empirically test the effectiveness of potential tick control methods, due to the difficulty in setting up controlled experiments in the natural environment. Therefore, in this project, we use mathematical models to predict how effective tick control methods might be at controlling ticks and disease. As a case study we use the example of louping ill virus. This tick-borne pathogen can kill livestock and red grouse, and the disease therefore has serious implications for rural economies and livelihoods, as well as animal health and welfare.

Key Points

Ticks and incidence of tick-borne diseases, such as Lyme borreliosis and louping ill, are currently increasing rapidly in the UK.

There are various management strategies used in an attempt to control tick-borne diseases. Culling hosts, such as deer and mountain hares, that carry ticks and carry the pathogens is one method that is sometimes used.

However, large-scale culls of keystone species can have wide-ranging implications. For example, they can have knock-on effects on habitat quality and biodiversity, including on predators of high conservation priority.

Therefore, in this study, we use mathematical models to predict the effectiveness of a more benign tick control method – applying acaricides to sheep. The idea is that sheep are treated frequently with acaricides, e.g. 6 times per year during the tick activity season, and vaccinated against louping ill virus. It is hoped that the acaricide on the sheep’s skin and wool will kill the ticks that try to attach, such that the sheep will act like “tick mops” and lessen the tick population in the environment. This is hoped, in turn, to reduce the risk of tick-borne diseases such as louping ill and Lyme borreliosis to susceptible animals and humans.

Research Undertaken

The mathematical model was built by Ros Porter in collaboration with Rachel Norman, both at Stirling University’s Department of Computing and Mathematics. The model, termed an “SIR” (Susceptible, Infected, Recovered) model, was a series of differential equations describing the rate of change of numbers of red grouse and ticks that are susceptible, infected and recovered from louping ill virus. Values for the model parameters were derived from empirical data (e.g. counts of ticks on sheep, deer and grouse – Fig. 1) and from the published scientific literature.

Inspecting a sheep for ticks.

Figure 1: Inspecting a sheep for ticks.

The model was run under scenarios of difference acaricide efficacies, sheep numbers in the flock and wild deer densities. The model was used to test the predicted effect of these factors on the effectiveness of sheep “tick mops” in controlling ticks and louping ill virus. It was used to answer the following specific questions:

How many treated sheep are needed for effective tick control?
What efficacy of acaricide is required?
What density of wild deer can there be before the methods becomes ineffective?

The model predicted that, with very low deer densities, using sheep as tick mops can theoretically reduce the tick populations and virus prevalence under certain conditions. However, above a certain density of deer, treatment was ineffective at controlling ticks and disease (Fig. 2). The model also predicted that high efficacy levels of acaricide are essential, and must be maintained long-term, for effective tick control in the environment.

The effect of different wild deer densities on the speed of the effect of adding a flock of 50 treated sheep (with 100% acaricide efficacy) on tick abundance

Figure 2. The effect of different wild deer densities on the speed of the effect of adding a flock of 50 treated sheep (with 100% acaricide efficacy) on tick abundance.

This research was co-funded by the Scottish Government, the Natural Environment Research Council and the Macaulay Development Trust. These results are currently published online in the journal “Theoretical Ecology”:

Porter, RB, Gilbert, L & Norman, RA. (2010) Controlling tick borne diseases through domestic animal management: a theoretical approach. Theoretical Ecology. (published online 20 May 2010, DOI: 10.1007/s12080-010-0080-2)

Policy Implications

These findings have clear implications for policy on sustainable tick control strategies, sheep acaricide treatment strategies, and deer population management. These are as follows:

Benignly managing one host species to protect another species from a vector and pathogen can, theoretically, be effective, albeit under certain strict conditions.
The importance of understanding the ecological complexity of a system, in order to target control methods under only specific circumstances for maximum effectiveness. In other words, it identifies conditions under which attempting this control method is unlikely to work.
It also identifies the strict conditions that are needed for this treatment method to be effective, which can hopefully help land managers make informed decisions about whether or not the method is worth is trying.

Author

Dr. Lucy Gilbert (MLURI), Ros Porter and Rachel Norman (Stirling University) l.gilbert@macaulay.ac.uk

Topics

Sustainability and Communities , Food, health and wellbeing