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: