Clicky

Authors: Prof Maxime Madder, Prof Ivan Horak, Dr Hein Stoltsz

Introduction

Ticks are one of the best-known groups of parasites and evoke different reactions in different people. Some are revolted, others are afraid, particularly if they have recently been bitten, and others are fascinated, while farmers, whose livelihood may be threatened by tick-borne diseases, think of control. We hope to address some of these aspects in this module and its accompanying modules devoted to disease transmission and control. To many of us ticks are either those little red ones that bite us when we walk in the veld or those big, baggy, blue ones clinging to the sides of cattle or protruding through the hair of our pet dogs. Both the little red ones and the big blue ones are part of the life cycle of every species of tick and we will explain how the two are connected. Many people are surprised to hear that the little red ones may belong to an entirely different species from the big blue ones. Few people, even those who realise that there are more than one species of tick, are able to identify even the most common species of tick infesting our household pets or domestic livestock, and even fewer can identify those species infesting wildlife as small as mice or as large as rhinoceroses and elephants. On completion of this module you should at least be able to identify the most common and important species of ticks as well as some of the less common species.

Ticks belong to an ancient lineage, with specimens found in Burmese amber dating back to about 100 million years ago. They are a very successful group of arthropods and have adapted to feeding on practically every terrestrial mammal, bird and reptile, and have attacked humans and infested the animals that they have domesticated for thousands of years. During the past 150 years they have become an important concern of veterinary and medical research, not only because of the direct effects they have on their hosts, such as injury at their points of attachment, blood loss and paralysis caused by toxins in their saliva, but also because they are efficient vectors of a wide variety of micro-organisms.

Globally a total of close to 900 tick species have been described, of which slightly more than 700 species belong to the Ixodidae or hard ticks and the remainder to the Argasidae or soft ticks. Ticks are particularly abundant in the Afrotropical region with its rich animal fauna and climatic zones ranging from arid to tropical. Approximately 200 ixodid tick species (hard or shield ticks) and 40 argasid tick species (soft ticks or tampans) are present in the Afrotropical region, but only a small number are of veterinary and medical importance. Many of the ticks and tick-borne diseases occur usually in specific geographical areas but with globalisation and climate change their range may expand and may even spread intercontinentally. Although it is common to consider domestic animals as being the preferred host of ticks, most species occur on wildlife, and several would not be able to complete their life cycles without the presence of small wild mammals or birds as hosts for their immature stages. Most importantly, at the livestock/wildlife interface transfer of tick-borne pathogens frequently occurs and poses a risk to livestock farming and development.

Failure to control ticks and tick-borne diseases effectively is a major factor limiting livestock production. The worldwide economic loss due to tick infestation and the additional burden of protecting livestock against ticks and tick-borne diseases is estimated to be in the billions of dollars annually.

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