Many arthropods and particularly ticks have evolved as ectoparasites of warm-blooded animals. Only a minority of tick species, generally those with a wide host range, transmit diseases to domestic animals and humans. The biology of ticks has been extraordinarily favorable to transmit disease agents such as viruses, bacteria and protozoa (see: Tick-transmitted pathogens): Ticks attach securely to their hosts, facilitating not only effective transmission of infectious agents, but also the spread of both ticks and microorganisms to different geographical habitats via traveling hosts, esp. migrating birds and other warm-blooded animals. Pathogens ingested by ticks can be spread trans-stadially and/or trans-ovarially. Female ticks are extremely fecund (up to 5,000 eggs), which allows an effective dissemination of infectious agents. In the case of a transovarial transmission, ticks may further serve as the pathogen’s reservoir, i.e. the species with the highest abundance of pathogens.

The increasing prevalence of tick-transmitted diseases of dogs and their owners has been associated with their increased outdoor activities for mainly two reasons: Firstly, the increasing accessibility of traditional 'wilderness' environments; secondly, the growing population of wildlife species (esp. rodents and e.g. deer), the natural hosts of ticks, as a result of preservation of natural habitats.

The rapid evolution of molecular-based techniques has allowed more sensitive and accurate detection of tick-borne pathogens. For example, anaplasmosis in dogs, cats, horses and humans is now considered to be a disease of importance in Sweden, the USA, Scotland, Switzerland and Slovenia.

Spread of ticks and tick-borne diseases

Ticks and the diseases they transmit have a zoogeographical range restricted by (wild) host movement and climatic factors. However, the increased mobility of pets has resulted in rapid extension of the zoogeographical ranges for many tick species and thus tick-transmitted diseases. The range is also increasing because of the fact that tick species are finding niches in different climatic conditions. Examples for the increasing spread of ticks and tick-borne diseases are reported from different countries:

• In Germany, the Ornate Cow tick, Dermacentor reticulatus, has entered large parts of the country over the last twenty years and brought along babesiosis, a potentially life-threatening CVBD.
• In Sweden, the Castor Bean tick, Ixodes ricinus, harboring the infectious agents of anaplasmosis and Lyme borreliosis, has extended its range since the 1980s including even northern and western areas.
• In the USA, the American Dog tick, Dermacentor variabilis, a major vector of Rocky Mountain spotted fever (RMSF), has reached the North-East of the USA. Climate change has been discussed as one major reason.

Co-infection

Infection with multiple tick-transmitted pathogens, or with multiple genotypes of the same pathogenic species, can occur in an individual animal following heavy exposure to ticks. The same tick species can be a vector for several pathogens and co-infection of individual ticks can occur. Co-infection may partially explain variations in clinical presentation, pathogenicity and response to therapy.

Infection with tick-borne pathogens can also be complicated by other arthropod-borne diseases that possess overlapping distribution with the different tick species, such as leishmaniosis with sand flies as the transmitting vector. In dogs, several different co-infections of Anaplasma, Ehrlichia, Bartonella, Babesia, Hepatozoon, Leishmania and Rickettsia species occur frequently in endemic areas.

Intrastadial transmission

For most of the tick-borne disease systems, an initial attachment and a more or less prolonged feeding period is thought to be required to allow reactivation of tick-borne pathogens and subsequent transmission. However, intrastadial transmission, in which the same feeding tick acquires the infection or started the pathogen’s activation process, but moves to another host, and then transmits the infection without molting or developing to the next stage, may occur during the natural infection. This type of transmission, which would result in a much shorter feeding period needed to transmit the pathogen to the second host and thus a quicker spread of infection, is currently under investigation.

Dogs as sentinels and reservoirs for human infection

The most important tick-transmitted infectious diseases causing severe clinical illness in dogs are babesiosis, anaplasmosis, ehrlichiosis and, in the USA, RMSF and hepatozoonosis. However, although Borrelia burgdorferi and Rickettsia conorii infections commonly produce subclinical infection, as evidenced by seropositivity, their association with a clinical disease in dogs is more difficult to evaluate. Dogs also appear to be susceptible to infection with Coxiella burnetii (Q fever) and tick-borne viral encephalitis (TBE), but systematic investigations on the clinical impact are currently missing.

Several of these tick-borne infections can also cause serious diseases in humans, notably Lyme borreliosis, ehrlichiosis, RMSF, TBE and R. conorii-infection (Boutonneuse fever). Due to the following reasons, dogs may play an important role:

• If transmission of an infectious agent involves ticks with a broad host range (such as I. ricinus), dogs can act directly as sentinels for infection of humans.
• Dogs can act as a domestic reservoir for certain nidicolous ticks (such as Rhipicephalus sanguineus and I. canisuga), if they are natural hosts. Dogs significantly increase the contact between these species and humans, thereby increasing the risk of transmission.
• There is a limited risk of transmission by exposure to contents of infected ticks following damage to ticks during grooming of infested animals. This scenario has been reported for R. conorii.
• Dogs may carry ticks of all life stages that are not attached to the host or may be interrupted during feeding. These ticks occasionally leave the canine host and are able to find another host, infest and finally transmit pathogens. This is rather important for companion animals living in close contact with humans.

Epidemiological tick control and preventative measures

Strategies to reduce populations of vector ticks through area-wide application of acaricides (chemicals that will kill ticks and mites) and control of tick habitats (e.g., leaf litter and brush) have been effective in small-scale trials. Community-based, integrated, tick-management strategies may prove to be an effective public health response to reduce the incidence of tick-borne infections. This includes general preventative measures of dog owners. However, limiting exposure to ticks is currently the most effective method of prevention.

Further information

• Shaw, S.E. et al.: Tick-borne infectious diseases of dogs. Trends Parasitol., 17, 2001, 74-80 (Download)
• Little, S.E.: Changing paradigms in understanding transmission of canine vector-borne diseases: the role of interrupted feeding and intrastadial transmission. Proceedings of the 2nd Int. CVBD Symposium, 2007, 30-35 (Download)
• http://www.cdc.gov/ncidod/diseases/list_tickborne.htm