Ticks in dogs: Significance as a disease vector
Ticks in dogs can act as vectors of various diseases, some of which have zoonotic potential.
Introduction
Ticks are one of the ectoparasites isolated most frequently in dogs. Taxonomically, they are arthropods that belong to the subclass Acari. Due to their haematophagous nature, severe tick infestations can cause anaemia in dogs.
However, from a clinical perspective, the most important role of ticks is their ability to act as a disease vector, which can affect several animal species and humans.After mosquitoes, ticks are thought to be the second-most important transmitter of diseases to dogs and humans. Evidence indicates that the global incidence of vector-borne diseases is increasing due to a greater degree of interaction between pathogens, vectors and hosts as a consequence of climate change and population movements.1-3
Geographical distribution
Ticks that parasitise dogs belong to two families: Ixodidae, or hard ticks, and Argasidae, or soft ticks. In Europe, tick infestations in dogs are caused by ticks from the family Ixodidae, genera Ixodes, Rhipicephalus, Dermacentor, Haemaphysalis and Hyalomma. The distribution of different tick species found on dogs varies according to geographical location:
- Ixodes spp. predominate throughout central and northern Europe and the British Isles.
- Whereas R. sanguineus is much more common in southern Europe . Similarly, most isolates of Hyalomma spp. are found in south-eastern Europe.2
- With respect to Spain, a study into the prevalence of different types of ticks and the pathogens they carry showed that the most common species was R. sanguineus (53%), followed by D. reticulatus (9%), I. ricinus (9%) and I. hexagonus (4%). On rare occasions, patients may be infested by more than one species at a time. Each species adheres to a well-defined geographical distribution.I. ricinus predominates in the more humid north of Spain. Similarly, I. hexagonus and D. reticulatus are mainly isolated in the north, while R. sanguineus is prevalent throughout the country.4
- In the United States, the most common dog ticks are Amblyomma americanum, A. maculatum, D. variabilis, Haemaphysalis longicornis, I. pacificus, I. scapularis and Rhipicephalus spp. There are two genera of soft ticks (Otobius spp. and Ornithodoros spp.) that also infest dogs.2,5
Seasonality
Tick infestations are usually seasonal.
- For example, in Britain and Central Europe there are two typical periods: one from March to June and another from August to November.
- In more southerly climates, species such as R. sanguineus and others are more prevalent during spring and summer but may feed throughout the year.
- In northern European countries, R. sanguineus cannot normally survive outdoors, but can complete its life cycle in kennels or inside homes.
- However, current patterns of seasonality may be shifting due to climate change.2
- Spain: In Spain, ticks can be encountered throughout the year, but the periods of peak activity vary according to the species.
- R. sanguineus peaks between March and July.
- D. reticulatus is mainly active in autumn/winter.
- And no clear seasonality has been reported for I. ricinus or I. hexagonus.4
Tick-borne diseases in dogs
In Europe
Tick-borne diseases include: babesiosis, hepatozoonosis, ehrlichiosis, anaplasmosis, rickettsiosis, borreliosis or Lyme disease, flavivirus, filariasis (A. dracunculoides), coxiellosis (Q fever) and European tick-borne encephalitis.2
Not all ticks transmit all of these diseases, but it is possible for a tick to harbour more than one pathogen, so coinfections should be considered.2
- Ixodes ricinus is a vector for A. phagocytophilum and B. burgdorferi, causative agents of granulocytic anaplasmosis and Lyme disease, respectively. In addition, lxodes spp. are probably involved in the transmission of B. vulpes (previously known as B. microti-like and Theileria annae). D. reticulatus can transmit B. canis, while R. sanguineus is a vehicle for various pathogens, such as A. platys, Rickettsia spp., B. vogeli, Hepatozoon spp. and Ehrlichia spp..3
In the USA
- Ixodes scapularis and I. pacificus are vectors of Lyme disease, anaplasmosis and babesiosis, and I. cookie has been associated with Powassan encephalitis.D. variabilis is the main vector of R. rickettsii, the causative agent of Rocky Mountain spotted fever, which can also be acquired, although less frequently, through contact with R. sanguineus.These ticks are also transmitters of ehrlichiosis and tularaemia, as is D. occidentalis, which has been linked to Pacific Coast tick-borne fever in humans due to Rickettsia philippi.A. americanum is the vector of the disease known as STARI, in addition to carrying Ehrlichia spp. and acting as a bridge in the transmission of tularaemia to humans. Genetic material from Hepatozoon americanum, A. platys, E. chaffeensis, and E. ewingii has been isolated by PCR from examples of A. maculatum. Finally, Haemaphysalis longicornis can transmit Theileria orientalis.5,6
Conclusions
Clinicians need to be familiar with the various diseases transmitted by different tick species. If a vector-borne disease is suspected, the differential diagnosis can be quickly narrowed if the vet has sound knowledge of the tick species present in the area where the animal has been. On the other hand, it is important to advise owners to follow appropriate prophylactic treatment throughout the year. Finally, in areas with zoonotic vector-borne diseases, owners must be made aware that if their dog has ticks, then they may also be exposed to possible infection.
