Vectors: Ticks

Vectors: Ticks

R A I NORVAL AND I G HORAK

Introduction

Ticks are of great economic importance as vectors of several diseases of domestic livestock and of commercially farmed wildlife in sub-Saharan Africa. Globally, 690 species and subspecies of ixodid ticks are recognized, and slightly more than 200 of these are present in the Afrotropical region.27 In addition, 37 of the 179 species or subspecies of argasid ticks occur here.27

Only a small number of ticks are major vectors of diseases or causes of toxicoses in this region. These are Amblyomma hebraeum, Amblyomma variegatum, Boophilus decoloratus, Boophilus microplus, Hyalomma dromedarii, Hyalomma truncatum, Ixodes rubicundus, Rhipicephalus appendiculatus, Rhipicephalus evertsi evertsi, Rhipicephalus zambeziensis and argasid ticks of the Ornithodoros moubata/porcinus complex. However, several other species also serve as vectors, but to a lesser degree or in a more geographically localized context.

It is customary to consider domestic animals as the preferred hosts of those tick species that transmit diseases to them. However, the only ticks that nearly exclusively parasitize these animals are the introduced Asiatic blue tick, Boophilus microplus, of cattle; the shiny Hyalomma, Hyalomma detritum, of North Africa; and the cosmopolitan kennel tick, Rhipicephalus sanguineus, of domestic dogs. The vast majority of indigenous ticks in the sub-Saharan region are parasites of wildlife, and indeed a large number of species would be unable to complete their life cycles if there were no wild hosts available. Many of the tick species deemed to be parasites of domestic cattle, sheep, goats, horses and pigsare frequently more abundant or prevalent on equivalently sized or even smaller wild animals.

Thus giraffe (Giraffa camelopardalis), African buffalo (Syncerus caffer) and eland (Taurotragus oryx) are excellent hosts of all stages of development of A. hebraeum and A. variegatum; impala (Aepyceros melampus), eland, bushbuck (Tragelaphus scriptus), greater kudu (Tragelaphus strepsiceros) and sable antelope (Hippotragus niger) of B. decoloratus; giraffe, African buffalo and eland of adult Hyalomma marginatum rufipes and H. truncatum; caracal (Caracal caracal) and mountain reedbuck (Redunca fulvorufula) of adult I. rubicundus; African buffalo, eland, male nyala (Tragelaphus angasii), greater kudu and sable antelope of all stages of development of R. appendiculatus; zebra (Equus spp.) and eland of all developmental stages of R. evertsi evertsi; zebra, black rhinoceros (Diceros bicornis), eland and gemsbok (Oryx gazella) of Rhipicephalus pulchellus; large carnivores, zebra and wild suids of adult Rhipicephalus simus; impala and greater kudu of all stages of R. zambeziensis; and warthog (Phacochoerus africanus) of ticks of the O. moubata/porcinus complex. Gerbilline rodents are hosts of the immature stages of H. truncatum and murid rodents the preferred hosts of those of R. simus. Scrub hare (Lepus saxatilis) are preferred hosts of all parasitic stages of Rhipicephalus warburtoni, as well as the immature stages of H. marginatum rufipes and H. truncatum. They are also good hosts of the immature stages of A. hebraeum, R. appendiculatus, R. evertsi evertsi, R. pulchellus and R. zambeziensis. Rock elephant shrews (Elephantulus myurus) are excellent hosts of the immature stages of I. rubicundus and of R. warburtoni. Helmeted guinea fowl (Numida meleagris) and leopard tortoise (Geochelone pardalis) are good hosts of the immature stages of A. hebraeum, and ground-frequenting birds of those of H. marginatum rufipes.

The diseases of major economic importance affecting cattle and transmitted by these and other ticks are heartwater, babesiosis, anaplasmosis, the theilerioses caused by various strains of Theileria parva, and tropical theileriosis. Of lesser importance in cattle are the generally non-pathogenic mild theilerioses, spirochaetosis, benign anaplasmosis, benign babesiosis and bovine ehrlichiosis. Sheep and goats are susceptible to the organisms causing heartwater, anaplasmosis, theileriosis, spirochaetosis, and to the virus causing Nairobi sheep disease.

Horses, mules and donkeys are affected by equine piroplasmosis as well as by spirochaetosis, and pigs by porcine babesiosis and African swine fever. In addition to transmitting infectious diseases to livestock, some tick species are also associated with toxicoses such as sweating sickness, paralysis (e.g. Karoo paralysis, spring lamb paralysis) and brown ear-tick toxicosis, or with bovine dermatophilosis. Several wild ruminant species are susceptible to Ehrlichia ruminantium, the causal organism of heartwater or cowdriosis, or can act as carriers of this organism (see Heartwater). Some are also susceptible to certain Theileria spp., while zebra are susceptible to Babesia caballi and Theileria equi, the cause of equine piroplasmosis, and wild suids to Babesia trautmanni, the cause of porcine babesiosis, and to infection with the virus of African swine fever.

Ixodid ticks are also important vectors of several organisms causing disease in humans in sub-Saharan Africa. These are Rickettsia conori, the cause of tick-bite fever or tick typhus; Coxiella burnetii, the cause of Q-fever; and the virus causing Crimean-Congo haemorrhagic fever. In addition, argasid ticks of the Ornithodoros moubata complex can transmit Borrelia duttoni, the cause of African relapsing fever, to humans.

The known...

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