ISSN 2398-2942      

Ixodes pacificus


Synonym(s): Western black legged tick




  • Class: Arachnida.
  • Family: Ixodidae.
  • Genus: Ixodes.

Active Forms

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Clinical Effects



On the host
  • On the larger domestic hosts I. pacificus attaches primarily to the head, ears, axilla and inguinal regions although they may be found anywhere on the body.
Off the host
  • The tick spends most of its lifecycle off the host.
  • Occurs in forests, coastal scrub, high brush and open grassland.
  • Nymphs are often found on moss-covered tree trunks.
  • May be present at any time of year.


  • Eggs.
  • Larva.
  • Nymph.
  • Adult.
  • Immature stages prefer to feed on small vertebrates in the environment, including rodents, birds, and lizards. Nymphs will feed on medium sized mammals.
  • Adults prefer to feed on medium-sized, eg deer, sheep, cattle, man, dog, etc.


  • The ticks move up onto the vegetation and 'quest' for a host. Size is important. Various tactile and olfactory sensory organs detect shadow, movement, CO2, temperature, odor, etc of a preferred host.

Pathological effects

  • A basophil/eosinophil hypersensitivity response occurs around the mouthparts of engorging ticks.
  • On first infection there is a decrease in Langerhans cells in the dermis surrounding the mouthparts as they migrate to, and present antigen in, the draining lymph node. This is followed, 4-5 days after infection, with infiltration into the dermis and degranulation of particularly basophils and eosinophils. By this time the tick may be detaching but the response continues to the injected materials. The response occurs within a few hours on reinfection.
  • This hypersensitivity response has been associated with some, although not marked, protective immunity resulting in reduced weights of fed ticks, fewer eggs produced, etc.
  • A basophilic eosinophilic granulomatous nodule forms around the mouthparts of the feeding tick.
  • The inflamed nodule may persist for some time after detachment of the tick.
  • The pruritus and itching induced may result in scab formation or secondary infection with pustule formation.

Other Host Effects

  • Ixodes pacificus is the main host in North America of the spirochete, Borrelia burgdorferi, that causes Lyme disease.
  • Also a host for Anaplasma phagocytophilum, a cause of anaplasmosis.
  • Larval I. pacificus become infected with B. burgdorferi in the spring or autumn when feeding on rodents which have been infected by nymphs feeding on them earlier in the spring or on rodents in which the infection has overwintered.
  • The infection is then transmitted stage to stage in the tick, ie the larva acquires the infection and transmits it to humans as a nymph feeding. Or a nymph will acquire the infection and transmit it to a dog as an adult. There is limited amount of transovarian transmission of the spirochete from the female through the egg to the larva but this seems much less important epidemiologically than is infection overwintering in the nymphs and rodents.
  • As the main reservoir species for B. burgdorferi are rodents (although many other host species including man and dog, sheep, deer, lagomorphs, etc are infected) the highest proportion of infected ticks are larvae transmitting the infection as nymphs. The nymph to adult cycle is less important but as it is the adults that prefer to feed in the larger hosts they must contribute significantly to infection in man and domestic animals.


Control via animal

  • Ixodes pacificus are usually are found on dogs singly or in small numbers.
  • The long mouthparts of I. pacificus penetrate well into the dermis and so make the tick difficult to remove.
  • Remove the tick(s) manually, ideally using a tick hook. Care must be taken to remove the mouthparts by grasping the tick very close to the mouthparts with a tick hook or forceps and extracting with a 'twist and pull' action.
  • Topical acaricides, paraffin wax, burning and freezing have been used to aid removal but are not required and will increase the risk of disease transmission by regurgitation of tick stomach contents.
  • The tick should be examined microscopically to ensure removal of the mouthparts.
  • The skin should be examined carefully to detect any small nymphs or larvae.
  • Use of acaricide washes or sprays may be required if a large number of ticks are present.

Control via chemotherapies

For treatment and/or prophylaxis
  • Chemoprophylaxis. Amitraz Amitraz , fipronil Fipronil , pyrethroids Pyrethrum and isoxazolines are approved for treatment and prophylaxis in dogs.

Control via environment

  • Avoid known infected areas or use persistent acaricide collars, etc.

Other countermeasures

  • Check dog after walking and remove ticks with tick hook.
  • Important to remove tick as soon as it is found to reduce risk of disease transmission.
  • If the tick is removed but the mouthparts remain this will increase the risk of local soft tissue reaction and disease transmission.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Piesman J & Eisen L (2008) Prevention of tick-borne diseases. Ann Review Entomol 53, 323-343 PubMed.
  • Dryden M W & Payne P A (2004) Biology and control of ticks infesting dogs and cats in North America. Vet Therap (2), 139-154 PubMed.

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