Vectors: Ticks

Previous authors: R A I NORVAL^† AND I G HORAK

Current authors:
F JONGEJAN - Director, Utrecht Centre for Tick-borne Diseases, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 1, Utrecht, The Netherlands / Extraordinary Professor, Department of Veterinary Tropical Diseases, Faculty of Veterinary Science, University of Pretoria, Private Bag X04, Onderstepoort, Pretoria, South Africa
G UILENBERG - Retired, PhD, A Surgente, Route du Port, Cargese, Corse, 20130, France

Introduction

Ticks and tick-borne diseases are ranking high in terms of their impact on animal and human health worldwide. They are efficient vectors of a variety of pathogenic protozoa, rickettsiae, spirochaetes and viruses, which are causing major diseases affecting livestock, humans and companion animals. In livestock, diseases transmitted by ticks constitute a major constraint to animal production in particular in tropical and sub-tropical areas of the world.⁸⁹ Moreover, there is a great impact of ticks on public health primarily in the northern hemisphere due to Lyme borreliosis and zoonotic tick-borne illnesses of viral origin. This mounting array of tick-borne zoonotic diseases in the temperate regions of the world poses an ever increasing public health risk.³⁸ Also, companion animals are at risk, in particular dogs pay a heavy toll to protozoan and rickettsial tick-borne diseases.¹³

Globally, there are 896 species recognized in three families. ⁵⁷ The Nuttalliellidae is monotypic, containing a single species Nuttalliella namaqua. The Argasidae, the soft ticks, consist of 193 species, whereas the hard ticks,  the Ixodidae, count 702 species in 14 genera according to a list of valid tick species names published in 2010 and based on an earlier list published in 2002.^{57, 69}

The focus of this chapter is on ticks of African livestock. Since previous editions of this chapter, some changes in tick taxonomy and nomenclature have occurred or have become more generally accepted. Where relevant these changes have been incorporated. At least 200 hard tick species occur in the Afrotropical region, whereas around 40 soft tick species occur here. Interesting, the single representative of the Nuttalliellidae, N. Namaqua, has recently been rediscovered in South Africa and, due to its intermediate characteristics between hard and soft ticks, is considered a “living fossil”.^{99, 110, 111} As far as African livestock is concerned, the relevance of the Ixodidae is far greater than the Argasidae, although the latter contains important disease vectors in the genera  Ornithodoros and Argas.  

Hard ticks are the main contributors as vectors of diseases of domestic livestock and of commercially farmed wildlife in the Afrotropical region. The Ixodidae are divided into 12 different genera, including Amblyomma, Hyalomma and Rhipicephalus (among which are the five species from the former genus Boophilus, which is still considered valid by some authors since they differ quite distinctly from the other Rhipicephalus species). Ticks of the genera Ixodes, Dermacentor, Margaropus, Cosmiomma, Haemaphysalis and Rhipicentor all occur in the Afrotropical region, but most are either geographically limited or prefer other hosts over livestock. The remaining three genera, Nosomma, Anomalohimalaya, Bothriocroton are not found on the African continent and also not on domestic animals.⁵⁷

The only species of the genus Dermacentor that infests domestic livestock in Africa is D. marginatus, but it is not of major economic importance in Africa, where it only occurs in vert limited regions of Morocco and Tunisia.¹⁸⁶ Its vector capabilities do not appear to have been thoroughly investigated.

There are no Haemaphysalis spp. in Africa of major importance to domestic livestock. H. punctata is one of the few species in Africa known to feed on livestock and is found only in the more humid parts of North African countries (Morocco, Algeria, Tunisia and Libya). This 3-host tick is a vector of a Babesia and a non-pathogenic Theileria of small ruminants (B. motasi, causing small ruminant babesiosis, but the Theileria has so far not received a valid name) and of a usually non-pathogenic species of Theileria of cattle, T. buffeli, which may cause confusion when studying bovine tropical theileriosis (T. annulata). It has also been found infected with Rickettsia spp. Its range extends from western and southern Europe, through northern Africa to western and central Asia. H. punctata does not occur south of the Sahara, and theileriosis in cattle by T. buffeli, must be transmitted by other ticks. A second tick of this genus found on livestock in Africa is H. sulcata. It has approximately the same geographical distribution as H. punctata: northern Africa (Morocco, Algeria, Tunisia, Libya), southern Europe, and much of Asia. The adults of this 3-host tick feed on large animals, the immatures are mainly parasites of reptiles.¹⁸⁶ Ticks of the H. leachii group occur all over Africa, but parasitise mainly carnivores, including dogs, and transmit a particularly virulent form of canine babesiosis (Babesia rossi). Those in South Africa have been distinguished from H. leachii under the old name of Haemaphysalis elliptica by,⁷ and this means that the results of transmission experiments carried out in South Africa are no longer valid for H. leachii. ⁷

In this chapter, descriptions of the most important ticks infesting livestock in Africa are provided...