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Disease transmission

Introduction

Only a small number of tick species are vectors of important economic diseases or toxicosis in sub-Saharan Africa (see Table 1). These are Amblyomma hebraeum, Amblyomma variegatum, Hyalomma dromedarii, Hyalomma truncatum, Ixodes rubicundus, Rhipicephalus (Boophilus) microplus, Rhipicephalus (Boophilus) decoloratus, Rhipicephalus appendiculatus, Rhipicephalus evertsi evertsi, Rhipicephalus zambeziensis and argasid or soft ticks of the Ornithodoros moubata/porcinus complex. Each of these ticks occurs in specific areas suitable for survival and reproduction. In cattle they transmit diseases of economic importance such as heartwater, babesiosis, theileriosis and anaplasmosis but also non-pathogenic or mild theileriosis, spirochaetosis, benign anaplasmosis, benign babesiosis and ehrlichiosis. Diseases affecting sheep and goats are heartwater, anaplasmosis, theileriosis, spirochaetosis and Nairobi sheep disease. Diseases that affect horses, mules and donkeys are piroplasmosis, spirochaetosis and those of pigs are porcine babesiosis and African swine fever.

Several wild ruminant species are susceptible to Ehrlichia ruminantium the causal organism of heartwater or cowdriosis or can act as carriers of the organism. Some are also carriers to certain Theileria spp., while zebra (Equus spp.) are carriers 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 burneti 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.

Vector

Disease

Causative agent

Method of transmission

Animals affected

A. hebraeum

Heartwater
Benign bovine theileriosis
Foot abscesses

Ehrlichia ruminantium
Theileria mutans,T. velifera

TS, IS, TO*
TS

Cattle, goats, sheep
Cattle
Goats

A. variegatum

Heartwater
Benign bovine theileriosis
Bovine anaplasmosis
Nairobi sheep disease
Bovine dermatophilosis

Ehrlichia ruminantium
Theileria mutans,T. velifera
Anaplasma bovis
Nairobi sheep disease virus
Dermatophilus congolensis

TS
TS
TS
TS

Cattle, goats, sheep
Cattle
Cattle
Sheep
Cattle, sheep, goats, horses

H. dromedarii

Oriental theileriosis

Theileria annulata

TS

Cattle

H. truncatum

Sweating sickness
Equine piroplasmosis
Toxicosis
Crimean-Congo Haemorrhagic fever
Boutonneuse fever

Toxin
Babesia caballi

Crimean-Congo haemorrhage  fever (CCHF) virus

Rickettsia conorii

 

TO

TS

TS

Calves
Horses, donkeys, mules
Dogs
Humans

I. rubicundus

Spring lamb paralysis

Toxins

 

Lambs

R. microplus

African redwater (babesiosis)
Asian redwater (babesiosis)
Galsickness (anaplasmosis)
Borreliosis/spirochaetosis

Babesia bigemina
Babesia bovis
Anaplasma marginale
Borrelia theileri

TO
TO
TS, IS
TO

Cattle
Cattle
Cattle
Cattle, sheep, goats, horses

R. decoloratus

African redwater (babesiosis)
Galsickness (anaplasmosis)
Borreliosis/spirochaetosis

Babesia bigemina
Anaplasma marginale
Borrelia theileri

TO
TS, IS
TO

Cattle
Cattle
Cattle, sheep, goats, horses

R. appendiculatus

East Coast fever, Corridor disease
Benign bovine theileriosis
Bovine anaplasmosis
Nairobi sheep disease
Tick toxicosis

Theileria parva

Theileria taurotragi
Anaplasma bovis
Nairobi sheep disease virus
Toxins

TS

TS
TS
TS, TO

Cattle, African buffalo

Cattle, eland
Cattle
Sheep, goats
Cattle, antelope

R. evertsi evertsi

Ovine theileriosis
Equine babesiosis
Equine theileriosis
Galsickness (anaplasmosis)
Borreliosis/spirochaetosis

Tick paralysis

Theileria separata/T. ovis
Babesia caballi
Theileria equi
Anaplasma marginale
Borrelia theileri

Toxins

TS
TS
TS
IS
TS, TO

Sheep
Horses, donkeys, mules
Horses, donkeys, mules
Cattle
Cattle, sheep, goats, horses
Lambs, dogs

R. zambeziensis

East Coast fever, Corridor disease
Benign bovine theileriosis
Bovine anaplasmosis

Theileria parva

Theileria taurotragi
Anaplasma bovis

TS

TS
TS

Cattle, African buffalo

Cattle, eland
Cattle

Table 1: Tick vectors, diseases, causative agents and hosts affected.
TS = Transstadial                                               IS = Intrastadial                                                   TO = Transovarial

Some tick species readily pass their disease-causing agent to their hosts, while others are inefficient vectors or are incompetent. The most common routes of transmission are:

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