Ticks and infectious disease
Ticks are small arachnids that play a significant role in the transmission of various infectious diseases to humans. They are categorized into two main families associated with human disease: Argasidae (soft ticks) and Ixodidae (hard ticks). Ticks undergo a four-stage life cycle—egg, larvae, nymph, and adult—requiring blood meals at each stage for development and survival. While soft ticks tend to inhabit warmer, humid environments and live for many years, hard ticks are more prolific in disease transmission, particularly in the United States.
Among the diseases caused by ticks, Lyme disease is the most prevalent, transmitted primarily by hard ticks such as the deer tick. Other notable tick-borne diseases include anaplasmosis, babesiosis, and various viral infections like Powassan disease. The incidence of tick-borne diseases has reportedly increased, with environmental factors such as climate change contributing to expanding tick habitats. Preventive measures against tick bites include wearing protective clothing, thorough tick checks, and using insect repellents. Proper tick removal and post-bite care are crucial to minimize infection risks. Understanding the ecology and health implications of ticks is essential for public health and safety.
Ticks and infectious disease
Definition
Ticks are members of the Arthropod phylum of the animal kingdom, and they serve as the vectors for a wide variety of human pathogens. Ticks are not insects; they are arachnids like scorpions, spiders, and mites. Ticks are divided into four families, but only two families, the Argasidae and the Ixodidae, are associated with human disease. Of the over 800 known species, about 60 have been reported to feed on people; however, only 49 species commonly do so.
Anatomy
The body of the tick comprises two fused parts, the capitulum (a “false” head) and the idiosome (the main body) to which the legs are attached. Larvae have six legs, and the nymphs and adults have eight legs. The body of the ixodid, or hard tick, bears a dorsal shield called the scutum. In males, the scutum covers the entire body, but in females, the hard cuticle is smaller and makes up only the anterior portion of the body. Unfed hard ticks are flattened and seed-like in appearance. In contrast, the argasid, or soft tick, has a nearly absent hard cuticle and is shaped like a raisin. There is little difference in the appearance of males and females.
Life Cycle and Feeding
Ticks have four life stages: egg, larvae, nymph, and adult. They are ectoparasites that require a blood meal to survive and to develop or move to the next stage and reproduce. Ticks feed upon a variety of vertebrate hosts, with mammals and birds predominating. After the larvae hatch from the egg, the first meal is taken, which enables movement to the next stage of development, the nymph. Nymphs may progress through several stages (instars) before advancing to the final adult stages. Blood meals are taken between stages. Adults, both male and female, take blood meals for survival and reproductive development.
Soft ticks are found mostly in tropical and subtropical regions, and they tend to live in narrow ranges of temperature and humidity. They are nest-dwelling and live up to twenty years. Nymphs may have as many as seven instars, each preceded by a blood meal.
Hard ticks are much more significant in disease transmission in the United States. Hard ticks, which can live from one to six years, may have from one to three hosts during their life cycles. Female hard ticks die after laying one batch of eggs; the male dies after mating. Hard-tick habitats vary according to species, but they live in environments, such as open fields and wooded areas, that are less sheltered than those of soft ticks.
Ticks employ a variety of methods to find their hosts. Soft ticks leave their nests and crawl to their hosts for a quick meal before returning to their nests. Hard ticks quest by crawling to the tips of vegetation and waiting for a passing host, which the tick detects through the host’s vibration, heat, shadow, odor, or carbon dioxide output. When the tick detects a host, it releases from the vegetative perch and quickly clings to the host.
A host can carry a tick to an entirely new place. In the case of a migrating bird host, the tick may even be carried to a new continent. The body weight of a soft tick may increase five to twelve times following a blood meal, but the female hard tick may weigh 125 to 150 times more after feeding. Hard ticks feed slowly and for several days before dropping from their hosts.
Ticks spend only a small portion of their lives on hosts taking blood meals. Ticks spend most of their time struggling to stay alive in a fasting state and often in a less-than-ideal environment. An ideal environment for a tick is one with 85 percent or greater humidity and a temperature between 43 and 45 degrees Fahrenheit (6 and 7 degrees Celsius). Humidity is paramount, and rain is welcomed by ticks. Ticks can prevent desiccation while fasting, and they respond to overhydration during feeding.
Tick-Borne Diseases
Ticks transmit several bacterial infections, but humans are only rarely infected; for humans, the usual method of acquisition is by inhaling spores or bacteria associated with infected animals, such as sheep and goats. Spirochetal infections caused by ticks fall into three categories: Lyme disease, southern tick-associated rash illness, and tick-borne relapsing fevers. Ticks can also carry and transmit certain flaviviruses to humans. Tick-borne flaviviruses include Powassan disease, tick-borne encephalitis, Kyasanur forest disease, Alkhurma disease, and Omsk hemorrhagic fever.
In 2024, Lyme disease was the most common arthropod-borne infectious disease in the United States and in Europe. This disease is caused by a spirochete, Borrelia burgdorferi, and is transmitted by several species of hard ticks. A relative of that bacterium, B. miyamotoi, causes Borrelia miyamotoi disease (BMD), which was first documented in North America in 2013. It is carried by the deer tick and the western black-legged tick. A less common bacterial infection is tularemia, which is caused by the gram-negative coccobacillus Francisella tularensis. The disease may be transmitted by the bite of the deer fly or hard tick or by direct contact with an infected animal, such as a rabbit or muskrat. The American dog tick, Dermacentor variabilis, is the usual vector. The newest bacteria to be added to the list are the Ehrlichia and Anaplasma. Human ehrlichiosis and anaplasmosis are transmitted by hard ticks and are now endemic to many regions of the United States. Q fever is caused by a gram-negative, intracellular bacterium, Coxiella burnetti, that is related to Legionella pneumophila.
The tick-borne rickettsial diseases are in the spotted fever group. In the United States, the most significant of these fevers is Rocky Mountain spotted fever, caused by Rickettsia rickettsii and transmitted by hard ticks. The illness, which was first described in Idaho's Snake River Valley, is now more commonly found in the American South.
Protozoa also can be transmitted to humans by ticks. Texas cattle fever, the first infectious disease attributed to tick bites, was described in 1893 by Theobald Smith. This illness is caused by Babesia bigemina and does not affect humans. Two other species, B. divergens and B. microti, cause disease in humans. The most common form of babesiosis (which is similar to malaria) in the United States is caused by B. microti and is transmitted by the same hard tick that carries the pathogen for Lyme disease.
The viral pathogens transmitted by ticks fall into three groups: encephalitide, hemorrhagic fever, and coltivirus. Powassan and West Nile are the two encephalitis viruses seen in the United States. Colorado tick fever and Salmon River viruses are coltiviruses that occasionally cause disease in Western states. Powassan virus disease is a rare but serious tick-borne viral illness, with approximately 322 cases being reported in the United States between 2010 and 2023. Most cases of Powassan virus disease occurred in the northeastern United States and the Great Lakes region. Many cases of Powassan virus disease are asymptomatic, but the disease can cause encephalitis and meningitis, with symptoms including fever, headache, vomiting, weakness, confusion, and loss of coordination.
The Centers for Disease Control and Prevention (CDC) published the results of a study that showed the recorded instances of mosquito-, tick-, and flea-borne illness tripled in the United States between 2004 and 2016. The study focused on sixteen diseases, including six tick-borne diseases—Lyme disease, anaplasmosis/ehrlichiosis, spotted fever, babesiosis, tularemia, and Powassan virus. Tick-borne diseases accounted for more than 75 percent of the cases of insect-borne diseases during this time period and showed the most consistent increase over those years, while flea-borne illnesses remained uncommon and mosquito-borne illnesses showed a pattern of intermittent, virulent but brief epidemics. Anaplasmosis/ehrlichiosis showed the largest increase, with more than six times as many cases in 2016 as in 2004. In addition, while most cases of mosquito-borne diseases occurred in US territories, primarily in Puerto Rico, the tick-borne diseases occurred almost exclusively in the continental United States.
Tick paralysis is a rare disease associated with the bite of a hard tick. Acute ataxia and ascending paralysis occur four to seven days after the bite of a female tick. The exact mechanism of disease is unknown, but the disease is cured by removal of the tick. No infectious agent is involved.
Prevention and Treatment
Tick bites can be prevented by avoiding infested areas and wearing protective, light-colored clothing. Conduct a thorough, full-body tick check after spending time in areas where ticks live, and carefully examine clothing, gear, and pets for ticks. The US Centers for Disease Control and Prevention recommend bathing or showering within two hours after being outdoors to wash off and find any ticks that may be on the body before the ticks have a chance to bite and embed. A chemical that can be used on clothes to kill and repel ticks is permethrin. For skin protection, one can use an insect repellant containing NN-diethyl-metatoluamide (DEET). Ticks should be carefully removed from the skin using tweezers or forceps to avoid handling or crushing the ticks, as the infected blood of an engorged tick can cause infection if it comes into contact with the eyes, with other mucous membranes, or with broken skin. After carefully removing the entire tick from the body, clean the bite site with alcohol or soap and apply bacitracin. Specific antibiotic treatment is available for all of the infectious diseases transmitted by ticks, except for the viral diseases. Severe cases of the viral diseases may be treated with hospitalization, respiratory support, and intravenous fluids.
Impact
Tick-borne diseases are varied and many. The exact number of cases occurring annually worldwide is unknown, but it certainly numbers in the hundreds of thousands. Many of these diseases, such as Lyme disease and tick-borne encephalitis, appear to be increasing. The climate changes associated with global warming have resulted in large areas of ideal tick habitat, providing both warmth and moisture. A direct connection between climate change, tick populations, and tick-borne diseases has been scientifically documented.
Though the COVID-19 pandemic caused a decrease in tick-borne diseases, this is most likely due to patients not actively seeking medical care due to lockdown restrictions. In 2021, the CDC released a Tick Bite Data Tracker to track the number of patients seeking medical treatments for tick bites. The CDC reported in 2024 that tick-borne illnesses remained on the rise. Changes in land use patterns, suburban development, and healthcare worker education are important factors in increasing instances of tick-borne diseases and their treatment.
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