The risk for specific tickborne diseases varies based upon the tick vector being present in an area and capable of transmitting a pathogen. Other tickborne diseases occur infrequently with just a handful of cases reported each year. Humans: Lyme disease is the most common tickborne disease that affects humans in the U.S., with approximately 400,000 cases reported each year, though this number underestimates the true burden of the disease. Complex life cycles that involve the interplay of humans, wildlife, domestic animals, habitat, climate, and socioeconomic factors make tick-borne diseases a true One Health issue. Ticks exist throughout the United States, although the type of tick, the pathogens they carry, and the prevalence varies geographically. “Soft ticks” (lacking a scutum) can transmit the Borrelia bacteria that cause Tick-Borne Relapsing Fever. Toxins of ticks (most commonly the Rocky Mountain wood tick and the American dog tick in the U.S.) can also cause a rare condition in animals and humans known as tick paralysis, which usually resolves following removal of the tick. Alpha-gal allergy, also known as the "meat allergy," can be triggered by a tick bite, with growing evidence pointing to the bite of the lone star stick. Hard ticks can transmit multiple pathogens responsible for diseases such as anaplasmosis, babesiosis, ehrlichiosis, Lyme disease, Rocky Mountain Spotted Fever, tularemia, and Powassan virus disease. “Hard ticks” have a visible dorsal shield (scutum) and include the black-legged tick (also called the deer tick) Ixodes scapularis, the Lone Star tick ( Amblyomma americanum), and Dermacentor ticks, including the American Dog Tick and Rocky Mountain wood tick. In most cases, a tick must be attached for >36 hours for the Lyme disease bacterium to be transmitted, but other pathogens can be transmitted in shorter periods of time. The amount of time a tick needs to be attached to transmit the pathogen. Infected ticks can transmit pathogens (bacteria, viruses, and protozoa) to humans or animals when they bite and attach. Larval and nymphal stage ticks are even smaller. Adult ticks are approximately 3 to 5 millimeters in length (roughly the size of a sesame seed to an apple seed) depending on age, sex, species, and feeding status. The full life cycle can take up to 3 years. A tick must take in a blood meal to develop to the next life stage (e.g., larva to nymph, nymph to adult). Most ticks go through four life stages: egg, six-legged larva, eight-legged nymph, and adult. Ticks are external parasites that feed on the blood of vertebrate species (mammals, birds, reptiles and amphibians) and can cause a range of illnesses in both humans and animals.
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