Because the same disease can be transmitted by more than one species of tick and because some tick species are vectors of more than one disease, the simplest way to outline the disorders and vectors involved is to list them separately.
The Centers for Disease Control and Prevention (CDC) has identified 17 tick-borne diseases in the United States as of 2018. Those that are notifiable (required to be reported to public health authorities) are identified with (N).
The most important species of ticks identified as vectors of disease in North America are as follows:
Risk factors for tick-borne diseases include:
Being immunocompromised, lacking a spleen, or having a history of alcohol abuse are risk factors for severe infection. As far as was known in 2018, race or ethnicity are not risk factors for tick-borne diseases.
In general, humans develop a tick-borne disease only if the tick is infected in the first place and only if it is allowed to feed for a period of time (about 3 hours for RMSF; 36–48 hours for lyme disease; as little as 15 minutes for Powassan virus). While between 30% and 50% of Ixodes ticks carry the agent that causes lyme disease, other viruses and bacteria are much less common. The best protection against developing a tick-borne infection is to check one's body carefully after being outdoors in a wooded area and removing any ticks as soon as possible.
Tick-borne diseases are common worldwide, as ticks are responsible for more types of disease in humans than any other vector except for mosquitoes. In the United States, ticks are the single most common vector of vector-borne disease. There are about 30,000 cases of Lyme disease—the most prevalent tick-borne disease in North America—reported to the CDC each year. The figures for other tick-borne diseases range between 3,600 cases of anaplasmosis, 3,000 cases of RMSF, and 1,200 cases of ehrlichiosis in an average year to 45 cases of TBRF, 10 cases of Powassan virus, and 6 cases of Heartland virus.
The immediate cause of a tick-borne disease is the specific infectious organism (bacterium, virus, or protozoan) injected into the human or animal host through the tick's bite. Ticks have four life stages (egg, larva, six-legged nymph, and eight-legged adult), and, after hatching from the egg, they must obtain a blood meal at each stage in order to survive. Ticks can feed on birds, reptiles, and amphibians as well as mammals, and many ticks prefer a different host animal at each stage. It can take a tick as long as three years to complete its life cycle.
The symptoms of tick-borne infections vary somewhat according to the type of infectious organism transmitted and whether the tick transmitted more than one pathogen (co-infection) in its bite. The following may serve as a general summary of the so-called classic symptoms of tick-borne diseases:
Diagnosis of tick-borne diseases is often difficult or delayed because the symptoms of these infections overlap with those of many other bacterial and viral diseases. In addition, the patient may be infected by more than one tick-borne pathogen. Still another factor complicating diagnosis is the different geographic ranges of various tick species and the pathogens they carry; many tick-borne diseases were limited to certain regions of the United States as of 2018.
The first step in diagnosis is a patient history. The CDC recommends that doctors ask patients about the following: 1) a known tick bite or exposure to ticks; 2) recreational or occupational exposure to tick-infested areas; 3) recent travel to areas where tick-borne diseases are endemic; and 4) diagnosis of similar illnesses in family members, workplace colleagues, or pet dogs.
The second step is a close look at the patient's clinical symptoms. If the doctor suspects a tick-borne disease, even if the patient does not recall being bitten, the doctor will usually prescribe antibiotics without waiting for test results. This approach to treatment is known as empiric therapy.
Diagnostic tests that can be used to identify diseases caused by Rickettsia bacteria include polymerase chain reaction (PCR) testing, tissue biopsy with indirect immunofluorescence assay (IFA), and blood tests. Ehrlichiosis and anaplasmosis can similarly be diagnosed by PCR testing of blood samples. Lyme disease is most often diagnosed by Western blot or ELISA testing of blood samples, although the patient's cerebrospinal fluid (CSF) may also be tested for antibodies to the infectious organism. TBRF is diagnosed by a blood smear. Babesiosis can be diagnosed by IFA, fluorescent in-situ hybridization (FISH), PCR, or blood smears. PCR can also be used to diagnose Heartland virus and CTF; there was no diagnostic test for Bourbon virus as of 2018. Powassan virus infection can be diagnosed by testing blood serum or CSF for antibodies to the virus.
Patients suspected of having a bacterial tick-borne disease are often given empiric antibiotic therapy before diagnostic test results are available.
More specific treatments are as follows:
Surveillance of tick-borne diseases in the United States requires doctors to report suspected cases to state or local departments of public health. These authorities can assist the physician in obtaining laboratory testing to confirm the diagnosis. The Council of State and Territorial Epidemiologists and the CDC National Notifiable Disease Surveillance System (NNDSS) work together to collect case reports and data that include the diagnosis, date of onset, basic demographics, and geographic data related to each confirmed case. Analysis of the data enables the CDC and state public health agencies to identify emerging trends in the incidence and spread of tick-borne diseases. This information is critical because of the rising frequency of tick-borne infections in the United States and Canada since 2005. Reasons for the increase in tick-borne diseases include the spread of tick species previously limited to one geographic region to other regions by travelers and campers; the growing popularity of hiking or camping with pet dogs, which are favorite hosts of ticks; the expansion of human populations into formerly wilderness areas; and the spread of tick species as a result of increasing average surface temperatures and climate change.
Growing recognition of the number of wildlife as well as domestic animal species that can serve as reservoirs of tick-borne diseases has led to active participation on the part of veterinarians in surveillance as well as diagnosis of these diseases in animals.
Public education includes a number of online resources on tick identification and the prevention of tick-borne diseases: the websites of the CDC and several state health departments and the website of the Lyme Disease Association. Two university-related websites, the TickEncounter Resource Center (TERC) of the University of Rhode Island, and the Laboratory of Medical Zoology (LMZ) of the University of Massachusetts, provide information about tick identification, removal, and disease prevention, and also tick identification services that allow the public to participate in surveillance and research. People can send photos of ticks they have spotted to TERC or the actual tick to the LMZ for disease testing. This type of participation on the part of the general public is known as passive surveillance; it is an important complement to the active surveillance conducted by epidemiologists and infectious disease specialists.
The American Veterinary Medical Association (AVMA) has a number of resources on its website for hunters and hikers for protecting themselves and companion animals against tick-borne diseases.
The rising rate of tick-borne diseases in North America, along with the identification of several new diseases in this category since 2013, has intensified the search for newer and more effective antibiotics for the bacterial tick-borne diseases; effective treatment for the viral diseases; and faster and more rapid diagnostic techniques. One promising innovation is the development of a multiplex assay called the TBD-Serochip to test blood serum for the presence of antibodies to eight different tick-borne diseases, including anaplasmosis, babesiosis, Lyme disease, ehrlichiosis, Powassan fever, RMSF, Borrelia miyamotoi infection, and Heartland virus. The TBD-Serochip is a joint project of Columbia University's Mailman School of Public Health; the CDC Division of VectorBorne Diseases; and the National Institute of Allergy and Infectious Diseases (NIAID).
The prognosis of tick-borne diseases varies considerably depending on the specific pathogen(s) involved, the presence of co-infection, and the patient's age and general health status. Children younger than 10 years have a higher rate of mortality than adults. Babesiosis can be fatal in the elderly and in patients with no spleen. Heartland virus has a mortality rate of 20%, and Powassan virus has a mortality rate of 20% with 50% of survivors having permanent neurological damage. CTF is rarely fatal but about 50% of patients develop neurologic complications.
Bacterial tick-borne infections also have varying prognoses. Lyme disease is rarely fatal, but if untreated or inadequately treated, can develop into disseminated (distributed throughout the body) disease with symptoms ranging from arthritis and difficulty with movement to neurologic symptoms that include memory difficulties, a general sense of feeling unwell, and personality changes. RMSF has the highest case fatality rate (10%) of the tick-borne diseases caused by bacteria; it may develop into pneumonia, tissue death requiring amputation, and circulatory failure if treatment is delayed. TBRF if untreated follows a pattern of several days of fever followed by a week without fever followed by another episode of fever, but death is rare.
There were no human vaccines for tick-borne diseases as of 2018. As ticks are too numerous and widespread for environmental control measures to be effective, prevention depends primarily on tick avoidance and control on the part of individuals.
See also Lyme disease ; Zoonoses .
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Rebecca J. Frey, Ph.D.