Ticks are blood sucking ectoparasites of mammals, birds, and reptiles. These arthropods have a six legged larval stage, one or more eight legged nymphal stages, and an eight legged adult stage, all of which take blood meals. Females of all species and the males of most species feed on blood. In addition to annoyance and discomfort, ticks are efficient vectors of a number of serious diseases of humans and domestic animals. A pathogen is an organism which causes disease in the animal it infects. Pathogens occur in several forms. Typical viral diseases transmitted by ticks include Colorado tick fever, Powassan encephalitis, and tick borne encephalitis. Pathogenic bacteria transmitted by ticks include Lyme disease, relapsing fever, and tularemia organisms. There is at least one protozoan pathogen carried by ticks-human babesiosis. Finally, tick borne rickettsia cause human diseases such as Rocky Mountain spotted fever, ehrlichiosis and endemic typhus. Pathogen acquisition and behavior inside the vector is very important in disease transmission. Two factors contribute greatly to the successful spread of a pathogen in ticks. A pathogen can be passed through the stages of he vector’s development, a process known as trans-stadial transmission. With trans-ovarial transmission, the pathogen is passed from the mother to her offspring through her eggs. These two processes are found in both ticks and mites. For example, newly hatched tick larvae are capable of transmitting the Rocky Mountain spotted fever organisms to animals and humans if the mother has passed the pathogen on to her eggs.
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Ticks occur in two basic forms and are thus divided into two families: Ixodidae, the hard ticks and Argasidae, the soft ticks. Hard and soft, when describing ticks, refers to presence or absence of a hard plate on their back, called the scutum.
Hard ticks are characterized as follows: only one nymphal stage; mouthparts projecting forward and visible from above; respiratory openings or stigmata located near the fourth pair of legs; a scutum, or shield like plate, present on the back of the body and covering the entire back of the male but only the front part of the back of the female; the coxa, or basal segment, of each leg usually with a spur; festoons present along the posterior margin of the body on all North American hard ticks except the genus Ixodes; palpi containing four segments, the fourth segment very small. Hard ticks feed on blood once for each of the stages.
Soft ticks have more than one nymphal stage, some having up to eight. Their mouthparts are ventrally located and covered by the front margin of the body, making them not visible from above. Stigmata occur between legs three and four, the bodies lack a scutum, and the integument appears wrinkled and leathery. The coxae lack spurs, and festoons are absent. Palpal segments are equal in size. Soft ticks feed periodically during all life stages.
Some hard ticks have a one host life cycle, wherein engorged larvae and nymphs remain on the host after feeding. Adults mate on that same host, and only the engorged female drops to the ground to lay eggs. While some hard ticks complete their development on only one or two hosts, most commonly encountered ixodids have a three host life cycle, meaning they feed, drop off, and then get on another animal each time.
Most soft ticks are “multi host ticks.” The six legged larva and the three or four nymphal stages each feed once, while the adult female will feed several times, with each engorgement usually followed by oviposition. Five molts are typical, which means that five different host animals must be located for the entire life cycle to be completed.