Ticks

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Ticks

  • obligate, blood-feeding ectoparasites of vertebrates 
  • bites may be directly damaging - cause irritation, inflammation, or hypersensitivity, if large numbers - anaemia and production losses
  • important in pathogen transmission
  • two families of ticks - Ixodidae and Argasidae 
  • Ixodidae - most important - hard ticks, due to rigid chitinous scutum
  • Argasidae - soft ticks, lack a scutum - includes bird ticks
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Ixodidae

  • relatively large ticks (2-20mm), flattened dorsoventrally 
  • enlarged fused coxae of palps - basis capituli 
  • lower wall extended anteriorly - hypostome (anchors tick when feeding), lies below chelicerae 
  • palps and chelicerae are anterior and visible from dorsal surface 
  • have scutum - whole surface male, small area larva/nymph/female
  • may have row of notches - festoons on posterior border of body
  • gonopore ventral behind gnathostoma
  • anal groove posterior to genital grooves and 4th pair of legs
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Ixodidae - Life Cycle

  • temporary parasites, single hexapod larval stage, single octopod nymphal stage, reproductive adult stage 
  • number of hosts variable 
    • one-host ticks - entire parasitic development on one host 
    • two-host ticks - larvae and nymphs on one host and adults on another
    • three-host ticks - each stage of development on different hosts 
  • important vectors of protozoal, bacterial, viral and rickettsial diseases 
  • trans-stadial transmission - pathogen remains in vector throughout cycle 
  • trans-ovarical - pathogen transmitted to offspring via ovaries 
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Ixodes spp.

  • largest ixodidae genus - ~250 species 
  • small inornate ticks, no eyes or festoons, long mouthparts
  • anterior position of anal groove - important in identification
  • ixodes ricinus - example species 
  • adult female light grey, 1cm in length and bean shaped, when engorged legs not visible from above, males 2-3mm long, legs visibile 
  • hosts - sheep, cattle, goat mainly 
  • not in North America 
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Ixodes ricinus - Life Cycle

  • three-host tick, 3-year life cycle 
  • tick only feeds for a few days a year (26-28 total) - in each life stage
  • mating takes place on the host 
  • once fertilised female feeds for about 14 days, drops to ground lays several thousand eggs in soil for 30 days then dies 
  • eggs hatch and produce larvae 
  • once host located - larvae feed for 3-5 days, drop back to vegetation digest and moult to nymphs 
  • nymphs find host next year feed 3-5 days, drop and moult to adult 
  • host size increases with each moult 
  • unfed larvae can survive 13-19 months, nymphs 24 months, adults 21-31 months temp/humidity dependent 
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Ixodes ricinus - Pathogenesis

  • sometimes heavy infections can cause anaemia 
  • may damage host at site of attachment causing local injury - predisposing to bacterial infections 
  • transmits pathogens 
    • Babesia divergens - Western Europe in cattle - red water fever
    • Anaplasma marginale - anaplasmosis
    • sheep and cattle - louping-ill, and rickettsia causing tick-borne fever 
    • Borrelia burgdorferi  - Lyme disease in humans 
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Amblyomma spp.

  • large, often highly ornate ticks, with long often based legs
  • eyes and festoons present, males lack ventral plates
  • long mouthparts - deep, painful bite  - secondary infection
  • 100 species, central and eastern North America, tropical/subtropical Africa 
  • three-host tick life cycle 
  • pathogen transmission 
    • A. americanum -  Rickettsia rickettsi   - Rocky Mountain Spotted Fever; Francisella tularensis (tularaemia); Lyme disease, Q fever, canine ehrilichiosis, human monocytic ehrlichiosis 
    • A. variegatum  - Ehrlichia ruminatum - heartwater in cattle; Coxiella burnetii - Q fever 
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Boophilus spp.

  • 'blue ticks' 
  • important vectors of Babesia spp. and Anaplasma marginale - in cattle, tropical and subtropical countries 
  • unfed adults 2-3mm long up to 12mm engorged 
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Dermacentor spp.

  • medium to large ticks, with ornate patterning 
  • festoons and eyes present 
  • most species, three-host ticks, some one-host ticks 
  • small genus of ~30 species mostly in New World 
  • several species associated with, Rocky Mountain Spotted Fever, Q Fever, tularaemia, Colarado tick fever, some saliva may produce tick paralysis 
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Hyalomma spp.

  • medium or large ticks, eyes and long mouthparts 
  • two-host ticks (some three-host) 
  • most commonly found on legs, udder, tail or perianal region 
  • ~20 species Asia, Southern Europe, North Africa 
  • can survive exceptionally cold and dry conditions 
  • usually inornate with banded legs, eyes present, sometimes festoons
  • pathogens 
    • H. anatolicum - transmits Theileria annulata, T. equi, Babesia aballi, Anaplasma marginale, Trypanosoma theileri and at least 5 arboviruses
    • H. aegyptium - found mainly on tortoises
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Rhipicephalus spp.

  • genus of ~60 species, originally endemic to Old World, for most part - Sub-Saharan Africa 
  • three-host ticks (some are two-host) 
  • pathogen transmission
    • R. appendiculatus - vector of east coast fever (Theileria parva); T. lawrencei, Nairobi sheep disease virus; Ehrlichia bovis, Hepatozoon canis, Rickettsia conorii 
    • R. sanguineus - primarily on dogs - Babesia canis and Ehrlichia canis - can cause tick paralysis; east coast fever in cattle, Rocky Mountain Spotted fever in USA/Mexico
    • other infections in animals/humans - Theileria equi, Babesia caballi, Anaplasma marginale, Hepatozoon canis, Coxiella burnetii, Rickettsia conorii, R. canis, R. rickettsi, Pasteurella tularensis, Borrelia hispanica, Nairobi sheep disease viruses 
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Argasidae

  • soft ticks, have leathery and unsclerotised body with textured surface 
  • integument is inornate
  • palps appear leg-like 3rd/4th segment equal in size 
  • gnathostoma located ventrally not visible from dorsal view in nymphs and adults 
  • if present - eyes are in lateral folds above legs 
  • stigmata are small and anterior to coxae of fourth pair of legs - legs similar to those of hard ticks 

hosts - mainly birds in warmer climates but occasionally in humans

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Argasidae - Life Cycle

  • ticks are drought resistant and capable for living for several years 
  • multi-host development cycle 
  • larva feeds once before moulting to first stage nymph
  • between 2-7 nymph stages, each feeds and leaves host before moulting
  • adults mate away from the host and feed several times 
  • adult female lays batches of 400-500 eggs after each feed 
  • argasids only feed for a few minutes 
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Argas spp.

  • dorsoventrally flattened ticks with definite margins 
  • mostly nocturnal, - birds, bats, reptiles or small insectivorus mammals, seldom humans 
  • A. persicus - breeds in shelters in cracks and crevices in structure of poultry houses 
  • heavy infestations can take enough blood to bring about death of host 
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Ornithodoros spp

  • ~90 species, tropical/subtropical habitats in Old and New Worlds
  • nocturnal, mouthparts well developed - mainly dens, caves, nests and burrows 
  • only nymphs and adults are parasitic - considerable irritation 
  • heavy infestations - mortality of stock from blood loss 
  • several species inflict painful bites - major vectors of relapsing fever and African swine fever 
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Otobius spp.

  • small fenus contains only 2 species - Otobius megnini and O. lagophilus 
  • commonly infests wild and domestic animals - sheep, cattle, dogs, horses, and occasionally humans 
  • larvae and nymphs feed in external ear canal of host - severe inflammation and a waxy exudate in ear canals - secondary bacterial infection can occur, may extend up ear canal
  • infested hosts may scratch and shake their heads - scratching can cause local skin trauma possibly perforating ear drum - infection, ulceration, and in some cases meningitis 
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Principles of tick control

  • largely based on chemical acaricides - total immersion in dipping bath or spray, shower, spot-on or slow-release ear tags 
  • organophosphates (malathion, chlorpyrifos, fenthion, dichlorvos, cythoate, diazinon, propetamphos, phosmet) and pyrethroids (permethrin, deltamethrin) - sprays, dips, spot-on, showers
  • macrolytic lactones or closantel - parenteral route 
  • topical acaricidal compounds in companion animals (fipronil, imidacloprid, selamectin, amitraz, and organophosphates and carbamates) - kill ticks on host 
  • pyrethroids (permethrin, deltamethrin) - not in cats 
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