What Diseases Are Carried by Ticks in Washington State?

Ticks in Washington State carry a wider range of organisms than many people realize. While Lyme disease often dominates public conversation about ticks, the Pacific Northwest is home to multiple tick species that can transmit bacteria, viruses, and protozoa causing illnesses with very different symptoms and severities. Because tick species, host animals, and local environments vary across the state — from damp coastal woodlands to dry inland sagebrush and high-elevation cabins — the patterns of risk and the suite of diseases you might encounter are also varied.

The western blacklegged tick (Ixodes pacificus) is the primary human-biting Ixodes species in Washington and is the main vector here for Borrelia burgdorferi (Lyme disease). That same species can also transmit Anaplasma phagocytophilum (anaplasmosis) and is associated, although less commonly in this region, with Babesia species (babesiosis) and Borrelia miyamotoi (a relapsing-fever–type illness caused by a hard-tick Borrelia). Soft ticks of the Ornithodoros genus can transmit Borrelia hermsii, the agent of tick-borne relapsing fever, which is classically linked to rodent-infested mountain cabins. Dermacentor species (wood and dog ticks) are present in parts of the state and are implicated in transmission of spotted-fever group rickettsiae (including rare cases of Rocky Mountain spotted fever and other rickettsial infections), Colorado tick fever virus in high-elevation areas, and can also be associated with tularemia or tick paralysis in occasional cases.

Clinically, these infections can present very differently — from the classic erythema migrans rash of early Lyme disease to high fevers, severe headache, rash, or relapsing fevers with waxing and waning symptoms — and some require rapid antibiotic treatment to avoid serious complications. Diagnosis can be challenging because symptoms overlap, laboratory tests have limits (especially early in disease), and awareness of locally relevant tick-borne pathogens varies. Prevention remains the most effective strategy: avoiding tick habitat, using repellents and protective clothing, conducting thorough tick checks after outdoor activity, promptly and properly removing attached ticks, and protecting pets that can bring ticks into the home.

This article will map the major tick species found in Washington to the specific pathogens they can carry, summarize the typical signs and diagnostic approaches for each disease, outline current treatment recommendations, and give practical guidance on reducing exposure. Understanding which ticks and infections are present where you live or recreate helps you take targeted precautions and recognize symptoms early if exposure occurs.

 

Lyme disease (Borrelia burgdorferi)

Lyme disease is an infection caused by the spirochete Borrelia burgdorferi and is transmitted to people by the bite of infected blacklegged ticks. In the Pacific Northwest the primary vector is the western blacklegged tick (Ixodes pacificus). Early infection commonly produces a characteristic expanding skin lesion (erythema migrans) at the bite site, often accompanied by fever, fatigue, headache and muscle aches. If untreated, the organism can disseminate and cause neurologic symptoms (e.g., facial nerve palsy, meningitis), cardiac involvement (e.g., heart block), or intermittent large-joint arthritis.

Diagnosis and management combine clinical judgment with laboratory testing. The classic erythema migrans rash alone is sufficient for clinical diagnosis in many cases because serologic tests can be negative early in infection; when used, serologic testing typically follows a two-step algorithm (sensitive screening test followed by a confirmatory test) or newer validated single-step assays. Early localized disease is usually treated with oral antibiotics and has a good prognosis when therapy begins promptly; more advanced or neurologic disease may require different antibiotic choices or intravenous treatment. If you are bitten by a tick, develop a rash, fever, new neurologic symptoms, or joint swelling, seek medical evaluation rather than trying to self-diagnose.

Ticks in Washington State can carry several different pathogens besides B. burgdorferi. The western blacklegged tick can transmit anaplasmosis (Anaplasma phagocytophilum) and has been associated with Borrelia miyamotoi (a relapsing-fever–group spirochete) and, less commonly in this region, Babesia species (a protozoal infection that causes babesiosis). Tick-borne relapsing fever caused by Borrelia hermsii is typically transmitted by soft ticks (Ornithodoros species) that live in rodent-infested cabins at higher elevations. Tularemia (Francisella tularensis) has also been reported in Washington and can be transmitted by tick bites or contact with infected animals. The risk of infection varies by tick species, local pathogen prevalence, and how long an infected tick is attached; standard prevention—personal protective measures, prompt tick removal, pet protection and environmental precautions—reduces risk, and anyone with concerning symptoms after a tick bite should get medical evaluation.

 

Anaplasmosis (Anaplasma phagocytophilum)

Anaplasmosis is an infection caused by the bacterium Anaplasma phagocytophilum, transmitted to humans primarily by bites of infected Ixodes ticks (the same group that transmits Lyme disease). Typical symptoms begin within about 1–2 weeks after a tick bite and include fever, chills, headache, muscle aches, and sometimes gastrointestinal symptoms. Laboratory findings that raise suspicion include low white blood cell count (leukopenia), low platelets (thrombocytopenia), and elevated liver enzymes. Early diagnosis is based on clinical suspicion plus tests such as PCR (more sensitive early in illness) and serology (antibody tests that may take time to become positive). Prompt antibiotic treatment—doxycycline is the recommended therapy for suspected or confirmed anaplasmosis—greatly reduces the risk of complications.

In Washington State, the primary vector for Anaplasma phagocytophilum is Ixodes pacificus (the western blacklegged tick), which is found along the coast, in many forested and brushy areas, and in some inland regions. Nymphal Ixodes ticks, which are small and harder to detect, are most active in late spring and early summer and are often responsible for human infections; adult ticks are more active in cooler months. Anaplasmosis is less commonly reported in Washington than some other tick-borne illnesses overall, but clinicians in affected areas should consider it when patients present with the characteristic symptoms after possible tick exposure. Co-infection with other tick-borne pathogens (for example Borrelia burgdorferi or Babesia species) can occur because the same tick species can carry multiple agents, and co-infection can complicate presentation and management.

Beyond anaplasmosis, ticks in Washington can carry several other human pathogens. Those include Lyme disease (Borrelia burgdorferi), babesiosis (Babesia species), relapsing-fever group Borrelia (Borrelia hermsii transmitted by soft Ornithodoros ticks and Borrelia miyamotoi transmitted by Ixodes ticks), and tularemia (Francisella tularensis) which can be transmitted by some tick species or by handling infected animals. Prevention is the best approach: avoid known tick habitats when possible, use EPA-registered repellents (or permethrin-treated clothing), wear long sleeves and tuck pants into socks, perform full-body tick checks after being outdoors, and remove attached ticks promptly and properly. If you develop fever, new rash, or other concerning symptoms after a tick bite or outdoor exposure, seek medical evaluation and tell your clinician about the tick exposure so appropriate testing and timely treatment can be initiated.

 

Babesiosis (Babesia species)

Babesiosis is a malaria-like parasitic infection of red blood cells caused by microscopic Babesia species (in North America most commonly Babesia microti and a Pacific Coast–associated species often called Babesia duncani or WA1-type). Ticks of the Ixodes genus (in the western United States, primarily Ixodes pacificus, the western blacklegged tick) are the usual blood-feeding vectors that transmit Babesia to people; transmission can also occur through blood transfusion or, rarely, from mother to baby. Clinical presentation ranges from asymptomatic infection to a febrile illness with chills, sweats, fatigue, myalgias, headache and hemolytic anemia; severe disease — with high fever, pronounced hemolysis, jaundice, respiratory failure, renal failure or disseminated intravascular coagulation — occurs most often in people who are elderly, asplenic, or immunocompromised.

Diagnosis relies on demonstration of the parasite in blood (thin blood smear showing intraerythrocytic parasites, classically the “Maltese cross” tetrad when present), plus nucleic acid testing (PCR) and serology to support the diagnosis. Laboratory findings commonly include hemolytic anemia, low haptoglobin, elevated lactate dehydrogenase and thrombocytopenia. Treatment choices depend on severity: milder or moderate babesiosis is usually treated with atovaquone plus azithromycin, whereas severe cases are treated with clindamycin plus quinine and, for very high parasitemia or life-threatening hemolysis, red cell exchange transfusion may be necessary. Because prevention is easier than treatment, recommended measures include avoiding tick habitat, using EPA‑registered repellents on skin and permethrin on clothing, performing thorough daily tick checks after outdoor exposure, and removing attached ticks promptly and correctly.

In Washington State ticks are known to carry several human pathogens in addition to Babesia. The western blacklegged tick (Ixodes pacificus) is the principal vector locally for Lyme disease (Borrelia burgdorferi), anaplasmosis (Anaplasma phagocytophilum), and can transmit Babesia species and Borrelia miyamotoi (a relapsing-fever–group organism transmitted by hard ticks). Soft ticks of the Ornithodoros genus are the classic vectors for tick-borne relapsing fever spirochetes such as Borrelia hermsii, which is associated with rodents and rustic cabins. Dermacentor species (e.g., Dermacentor andersoni and related Dermacentor ticks) are present in parts of the state and can be involved with tularemia (Francisella tularensis), Colorado tick fever virus, and tick paralysis; spotted‑fever group rickettsial infections are uncommon but possible. Because risk varies by specific region, season and the behavior of local ticks, take preventive measures, be alert for symptoms after a tick bite, and seek prompt medical evaluation for fever or unexplained systemic illness following tick exposure.

 

Relapsing fever–group Borrelia (Borrelia hermsii, Borrelia miyamotoi)

Relapsing fever–group Borrelia includes two different clinical/ecologic syndromes relevant to Washington State. Borrelia hermsii is the classic agent of tick-borne relapsing fever in the western U.S.; it is transmitted by soft ticks (Ornithodoros hermsi) that live in rodent-infested cabins, woodpiles, and similar sheltered environments. Infections typically produce abrupt high fever, headache, myalgias and rigors followed by spontaneous defervescence and one or more relapses as spirochetemia waxes and wanes. By contrast, Borrelia miyamotoi is carried by hard-bodied Ixodes ticks (the same vector group that transmits Lyme disease) and causes a relapsing-fever–type illness that can range from a self-limited febrile syndrome to more severe disease (including meningoencephalitis) in immunocompromised persons.

Diagnosis and management differ somewhat between the two agents. During febrile episodes B. hermsii is often detectable by direct microscopy of blood (many organisms circulate), whereas B. miyamotoi is more reliably identified by PCR and specific serologic testing; serologic cross-reactivity with Borrelia burgdorferi (Lyme) can complicate interpretation. Treatment is typically with a tetracycline-class antibiotic (doxycycline is commonly used) for uncomplicated illness, and severe or neurologic infections are often treated with parenteral agents such as ceftriaxone or penicillin-based regimens under clinical guidance. Clinicians and patients should be aware of the risk of a Jarisch–Herxheimer reaction (an acute inflammatory response) shortly after initiating therapy. Prevention focuses on avoiding exposure to vector habitats: rodent control and cabin sealing to reduce encounters with Ornithodoros ticks, use of repellents and treated clothing for Ixodes exposure, and prompt evaluation for febrile illness after possible tick contact.

More broadly, ticks in Washington State can carry multiple pathogens. Well-recognized agents include Borrelia burgdorferi (Lyme disease), Anaplasma phagocytophilum (anaplasmosis), Babesia species (babesiosis), the relapsing-fever Borrelia described above (B. hermsii and B. miyamotoi), and Francisella tularensis (tularemia), among others. Different tick species are associated with different pathogens: Ixodes pacificus transmits Lyme, anaplasma, babesial species and B. miyamotoi in parts of the state; Ornithodoros hermsi transmits B. hermsii in rustic/cabin settings; Dermacentor species can be involved in transmission of tularemia and other rickettsial agents. The geographic distribution and incidence vary by habitat, season, and local ecology, so preventive measures (personal protection, habitat modification, and prompt medical evaluation for fever or signs of infection after tick exposure) remain the cornerstone of reducing risk.

 

Tularemia (Francisella tularensis)

Tularemia is an infectious disease caused by the bacterium Francisella tularensis. It can present in several clinical forms depending on the route of exposure: ulceroglandular (skin ulcer with swollen local lymph nodes) is most common after a tick bite or direct handling of infected animals; glandular disease (lymphadenopathy without an obvious skin ulcer); pneumonic or typhoidal forms (more severe and systemic, caused by inhalation or bloodstream spread); and oculoglandular or oropharyngeal forms after eye or mouth exposure. Symptoms typically begin within a few days of exposure and can include fever, chills, malaise, localized pain or swelling, and, in pulmonary cases, cough and difficulty breathing. Without prompt appropriate antibiotic therapy, tularemia can be severe, but it responds well to specific antimicrobials.

In the western U.S., including Washington State, ticks are one of several important vectors for tularemia; transmission also occurs from contact with infected mammals (especially rabbits and rodents), biting flies, and by inhaling contaminated dust. Various hard-bodied ticks (Dermacentor species and related hard ticks) are the most commonly implicated tick vectors, and in some circumstances Ixodes species can also be involved. Diagnosis is made by clinical suspicion supported by laboratory testing (culture is hazardous and specialized; serology and PCR are commonly used), so clinicians should be informed about possible exposures. First-line treatment for severe tularemia historically includes aminoglycosides (e.g., streptomycin or gentamicin); doxycycline or fluoroquinolones (e.g., ciprofloxacin) are commonly used for less severe cases or oral therapy. Early recognition and treatment reduce complications and speed recovery.

Ticks in Washington State can carry several pathogens of human concern. The primary tick-borne illnesses reported or known to occur in Washington include Lyme disease (Borrelia burgdorferi) transmitted mainly by the western blacklegged tick (Ixodes pacificus); anaplasmosis (Anaplasma phagocytophilum) and babesiosis (Babesia species), both of which can also be transmitted by Ixodes pacificus; relapsing fever group Borrelia, where pathogens such as Borrelia hermsii are usually transmitted by soft ticks (Ornithodoros species) and Borrelia miyamotoi by hard Ixodes ticks; and tularemia (Francisella tularensis), which can be transmitted by several tick species as noted above. Other tick-associated pathogens (for example, spotted fever group rickettsiae) are less commonly reported in Washington but can occur. Preventive measures—regular tick checks after outdoor activity, prompt removal of attached ticks with fine-tipped tweezers, use of EPA-registered repellents on skin, permethrin-treated clothing, and reducing rodent and rabbit habitat around homes—are effective at lowering risk; seek medical evaluation if fever, rash, swollen lymph nodes, or unexplained illness develop after a tick bite or likely exposure.