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S. Shin1
1Chonnam National University College of Veterinary Medicine, Parasitology, Gwangju, Republic of Korea
Tick infestation and tick-borne diseases are one of
the most important factors hindering pet owners to do outdoor activities with their dogs in many countries. Ticks are basically blood-sucking arthropods that cannot survive or lay eggs without blood meal, and they are present outdoors and in the wild where blooded animals are present. Tick infestation is zoonotic and so are many tick-borne diseases. Understanding the biology of tick and their life cycle are as critical as chemotherapy in controlling ticks and tick-borne diseases in both animals and humans.
Tick biology
Although many insects consume blood meal from animals, ticks and mites are not insects. They are closer in taxonomy to spiders and scorpions than to insects. Therefore, nymph and adult stages have four pairs of legs while larval stage has three pairs of legs. Most insects do not take blood meal from host animals unless they become adult stage, while ticks have to take blood meal even from the juvenile stages. Therefore, while hard ticks can go for several months without feeding, all three stages have piercing mouth parts. Since ticks have to take blood meal even from the larval stages, they try to get close to where blooded animals wander around. Although both male and female ticks take blood meal, females have expandable abdomen to take enormous amount of blood meal from the host to lay eggs. Those engorged ticks on the dog skin, therefore, are females [1].
Most ticks that get onto humans and companion animals are hard ticks and are three-host ticks. It means all developmental stages, larvae, nymph and adult, will
fall off to the ground after blood meal from the animal host either to molt or to lay eggs. It also means that it is impossible to eradicate ticks among the wildlife animals and will keep coming up onto dogs wandering in the woods if not on preventitives. They seldom climb up onto the tree tops and fall down to animals and humans. In general, ticks that are questing on the tips of grasses and shrubs are found up to 50 cm from the ground. Unlike fleas, ticks cannot jump or fly. Completion of
the entire life cycle may take from less than a year in tropical regions to over three years in cold climates, where certain stages may enter diapause until hosts are again available. Although ticks have preferred animals
to take blood meal, they have low host specificity and will therefore get onto any blooded animals including humans [2].
Ticks spread diseases
Like mosquitoes, ticks transmit pathogens that cause disease through the process of feeding, not via the feces. When a tick finds a feeding spot after a searching period of 10 minutes to 2 hours on host, it inserts its feeding tube into the skin. After secreting a cement-
like substance that keeps them firmly attached during the blood meal, the tick secrets small amounts of saliva with anesthetic properties so that the animal cannot feel pain. The saliva also contains anticoagulants to keep the blood from hardening, and exhibits cytolytic, vasodilator, anticoagulant, anti-inflammatory, and immunosuppressive activity [3].
Ticks are major vectors of pathogens affecting both humans and animals worldwide and transmit a variety
of microorganisms including viruses, bacteria, and parasites (protozoa and helminths). Each year, more
than 30,000 cases of Lyme disease in humans are reported in the U.S., while studies suggest the actual number of people diagnosed with Lyme disease is
more likely about 300,000. Other less known, but serious tickborne diseases include Rocky Mountain spotted fever, anaplasmosis, ehrlichiosis, Powassan virus, and babesiosis. In dogs, ehrlichiosis, rickettsioses, hepatozoonosis, hemoplasmosis, tick-borne encephalitis, and tick paralysis can occur as well as anaplasmosis and babesiosis [4].
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, their association with a clinical disease in dogs is more difficult to evaluate. 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 [5].
Several of these tick-borne infections can also cause serious diseases in humans, and dogs may play an important role in the transmission of such pathogens because 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. Dogs may carry ticks of all life stages that are not
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