Pest Library: Ticks

Ticks are small arachnids, part of the order Parasitiformes. Along with mites, they constitute the subclass Acari. Ticks are ectoparasites (external parasites), living by feeding on the blood of mammals, birds, and sometimes reptiles and amphibians. Ticks had evolved by the Cretaceous period, the most common form of fossilisation being immersed in amber. Ticks are widely distributed around the world, especially in warm, humid climates.

Almost all ticks belong to one of two major families, the Ixodidae or hard ticks, which are difficult to crush, and the Argasidae or soft ticks. Adults have ovoid or pear-shaped bodies which become engorged with blood when they feed, and eight legs. As well as having a hard shield on their dorsal surfaces, hard ticks have a beak-like structure at the front containing the mouthparts whereas soft ticks have their mouthparts on the underside of the body. Both families locate a potential host by odour or from changes in the environment.

Ticks have four stages to their lifecycle, namely egg, larva, nymph, and adult. Ixodid ticks have three hosts, taking at least a year to complete their lifecycle. Argasid ticks have up to seven nymphal stages (instars), each one requiring a blood meal. Because of their habit of ingesting blood, ticks are vectors of at least twelve diseases that affect humans and other animals.

Ticks are implicated in the transmission of a number of infections caused by pathogens such as bacteria, viruses and protozoa. Sometimes the tick harbours more than one type of pathogen, making diagnosis of the infection more difficult. Species of the bacterium Rickettsia are responsible for typhus, rickettsialpox, Boutonneuse fever, African tick bite fever, Rocky Mountain spotted fever, Flinders Island spotted fever and Queensland tick typhus (Australian tick typhus). Other tick-borne diseases include Lyme disease and Q fever, Colorado tick fever, Crimean Congo hemorrhagic fever, tularemia, tick-borne relapsing fever, babesiosis, ehrlichiosis, Bourbon virus and tick-borne meningoencephalitis, as well as bovine anaplasmosis and probably the Heartland virus.

Some species, notably the Australian paralysis tick, are also intrinsically venomous and can cause tick paralysis. Eggs can become infected with pathogens inside a female tick’s ovaries, in which case the larval ticks are infectious immediately at hatching, before feeding on their first host. Tropical bont ticks transmit the rickettsial disease heartwater which can be particularly devastating in cattle. The ticks carried by migratory birds may act as reservoirs and vectors of infectious diseases.

Not all ticks in an infective area are infected with pathogens, and both attachment of the tick and a long-feeding session seem to be necessary for transmission of these diseases to take place. Thus tick bites often do not lead to infection, especially if the ticks are removed within 36 hours. Adult ticks can be removed with fine-tipped tweezers or proprietary tick removal tools, disinfecting the wound. It is also possible to freeze them off with a medical wart remover. If the tick’s head and mouthparts break off during removal, they can be removed with tweezers like a splinter.