What Are the Most Common Tick-Borne Diseases in the Pacific Northwest?
As outdoor recreation and suburban development bring more people into contact with wooded, brushy and grassy areas, awareness of tick-borne illness in the Pacific Northwest has grown. The region’s tick fauna and the diseases they carry differ from the better-known patterns in the Northeast and Midwest, so it’s important to know which infections are most relevant locally, how they usually present, and where and when risk is highest.
The most commonly discussed tick-borne infection in the Pacific Northwest is Lyme disease, transmitted locally by the western blacklegged tick (Ixodes pacificus). Although overall Lyme incidence in the region is lower than in the northeastern U.S., cases do occur and can cause the classic erythema migrans rash, fever, fatigue and joint or neurologic symptoms if untreated. Ixodes pacificus is also a vector for anaplasmosis (Anaplasma phagocytophilum), and less frequently for babesiosis (Babesia spp.); these illnesses present with fever, headache, muscle aches and, in more severe cases, blood abnormalities or organ dysfunction. Uniquely relevant to the mountainous PNW are relapsing fevers caused by Borrelia hermsii and related species transmitted by soft ticks (Ornithodoros spp.), often associated with rodent-infested cabins and characterized by repeated episodes of high fever.
Less common but clinically important conditions include Rocky Mountain spotted fever and tularemia — both possible in the region through Dermacentor ticks or other routes — and tick paralysis, a toxin-mediated weakness that may occur after attachment of certain hard ticks. Timing matters: nymphal Ixodes activity and most human infections peak in late spring and early summer, while adult ticks can be active in cooler months; soft tick exposure is often nocturnal and linked to specific lodging. Because early laboratory tests can be insensitive and symptoms overlap, prompt recognition, careful tick removal and appropriate clinical evaluation are essential.
This article will examine each of these diseases in more detail — how they’re transmitted, typical symptoms, diagnostic challenges, treatment options and practical prevention strategies tailored to Pacific Northwest habitats and seasons — so you can reduce your risk and respond quickly if exposure occurs.
Lyme disease (Borrelia spp.)
Lyme disease is an infection caused primarily by Borrelia burgdorferi (and related Borrelia species) transmitted by infected blacklegged ticks. Early infection commonly produces a characteristic expanding skin lesion (erythema migrans) and nonspecific flu‑like symptoms such as fever, headache, fatigue, and muscle aches. If untreated, infection can progress over weeks to months to involve the joints (intermittent or persistent arthritis), the nervous system (meningitis, cranial nerve palsies, radiculoneuritis), or the heart (atrioventricular block). Laboratory testing is supportive but has limitations: serologic tests are most useful a few weeks after symptom onset, and clinical judgment is important for early localized disease when the rash is present.
In the Pacific Northwest the principal vector is the western blacklegged tick (Ixodes pacificus). Lyme disease does occur in the region but at lower overall incidence than in the northeastern and upper-midwestern United States; risk is highest in coastal and forested habitats where these ticks are common and during spring to early summer when nymphs are active. Other tick-borne pathogens carried by the same tick in the region include Anaplasma phagocytophilum (anaplasmosis) and Borrelia miyamotoi (a relapsing-fever Borrelia), and there are occasional cases of babesiosis, spotted fever group rickettsioses, and tularemia transmitted by other local tick species. Because multiple pathogens can be transmitted by local ticks, clinicians in the Pacific Northwest consider a range of tick‑borne infections when evaluating compatible symptoms after a tick exposure.
Prevention centers on reducing tick exposure and prompt tick removal: wear long clothing and treat clothing with permethrin, use EPA‑registered repellents on skin, avoid walking through tall grass or leaf litter, perform full‑body tick checks after outdoor activities, and remove attached ticks as soon as they are found. The probability of Borrelia transmission increases with the duration of tick attachment (risk rises substantially after roughly 36–48 hours), so early removal reduces risk. If a person develops erythema migrans or systemic symptoms after a tick bite, they should seek medical evaluation promptly; early antibiotic therapy is generally effective and reduces the risk of late complications, while diagnostic testing and treatment choices depend on timing, clinical presentation, age, pregnancy status, and local clinical guidelines.
Anaplasmosis (Anaplasma phagocytophilum)
Anaplasmosis is a bacterial infection transmitted to humans by infected Ixodes ticks (the same group that transmits Lyme disease). After a tick bite there is typically an incubation period of about 1–2 weeks before symptoms develop. Typical presentation includes fever, headache, malaise, myalgias, sometimes cough or gastrointestinal complaints; patients often have laboratory abnormalities such as leukopenia, thrombocytopenia, and elevated liver enzymes. Diagnosis is most reliable with PCR testing on acute blood samples or by demonstrating a fourfold rise in specific antibodies; peripheral blood smears can occasionally show morulae (intracellular bacterial clusters) within granulocytes but this finding is insensitive.
Doxycycline is the treatment of choice and should be started promptly when anaplasmosis is suspected, because early therapy reduces risk of severe complications. Typical treatment courses last about 10–14 days and are continued until the patient has been afebrile for at least 48–72 hours and is clinically improving. Severe disease can occur, particularly in older or immunocompromised patients, and may progress to respiratory failure, neurologic involvement, or secondary complications; hospitalized care and supportive measures are sometimes required. Preventive measures include avoiding tick habitat, using protective clothing and EPA-registered repellents, performing thorough tick checks after outdoor activity, and removing attached ticks promptly and correctly.
In the Pacific Northwest the most commonly encountered tick-borne infections include Lyme disease (caused by Borrelia burgdorferi and transmitted by Ixodes pacificus) and anaplasmosis; both share the same vector and can occur in overlapping areas. Tick-borne relapsing fever (caused by Borrelia hermsii and transmitted by soft Ornithodoros ticks) is also regionally important and characterized by recurring febrile episodes. Babesiosis is reported in the region but is less common than on the U.S. Northeast, while Rocky Mountain spotted fever and tularemia occur sporadically; overall local risk can vary by county, season, and habitat, so local public health guidance and prompt clinical evaluation for fever after a tick bite remain important.
Babesiosis (Babesia spp.)
Babesiosis is a malaria-like parasitic infection caused by intraerythrocytic protozoa of the genus Babesia. In humans the disease most often produces nonspecific flu-like symptoms (fever, chills, sweats, myalgia, fatigue) and signs of hemolytic anemia (jaundice, dark urine, low hemoglobin) when red blood cells are extensively infected. Diagnosis is made by identifying parasites on a thin blood smear (sometimes showing the characteristic “Maltese cross” tetrads), by PCR, or by serology; laboratory evidence of hemolysis (elevated LDH, low haptoglobin, reticulocytosis) supports the diagnosis. Treatment depends on severity: mild-to-moderate illness is commonly treated with atovaquone plus azithromycin, whereas severe infections (high parasitemia, severe hemolysis, respiratory or renal compromise) are treated with clindamycin plus quinine and may require red cell exchange transfusion. People who are asplenic, elderly, or immunocompromised are at higher risk of severe or prolonged infection.
In the Pacific Northwest the epidemiology differs from the more familiar pattern in the northeastern and midwestern United States. While Babesia microti is the dominant species in the Northeast and upper Midwest, the Pacific coast has documented cases of a distinct species often referred to as Babesia duncani (WA1-type). These Pacific cases are less numerous overall but are clinically important; transmission in the region is linked to tick exposure and there have also been documented cases associated with blood transfusion and, rarely, congenital transmission. Local tick ecology differs from the East (Ixodes pacificus is the principal blacklegged tick in the PNW rather than Ixodes scapularis), so risk patterns, seasonal activity, and co-infections with other Ixodes-borne pathogens can vary; clinicians in the region should consider babesiosis in patients with compatible symptoms, especially if hemolysis is present or if a patient has risk factors for severe disease.
When people ask, “What are the most common tick-borne diseases in the Pacific Northwest?” the short answer is that a handful of infections account for most of the recognized tick-borne illness in the region: Lyme disease (Borrelia burgdorferi and related species), anaplasmosis (Anaplasma phagocytophilum), and babesiosis (including Babesia duncani) are among the more frequently encountered; tick-borne relapsing fever (Borrelia hermsii and related species transmitted by soft Ornithodoros ticks) is also well recognized in the PNW. Rocky Mountain spotted fever and tularemia occur less commonly but do appear regionally; other pathogens common elsewhere (for example certain Ehrlichia species or Powassan virus) are rare or geographically restricted. Prevention—wearing protective clothing, using EPA-registered repellents on skin or permethrin on clothing, doing thorough tick checks after outdoor activity, and promptly removing attached ticks—reduces risk of most of these infections, and early medical evaluation is advised for fever after a tick bite or after potential exposure.
Rocky Mountain spotted fever (Rickettsia rickettsii)
Rocky Mountain spotted fever (RMSF) is an acute, potentially severe tick-borne infection caused by the bacterium Rickettsia rickettsii. It is transmitted primarily by infected Dermacentor ticks in much of North America; after an incubation period of about 2–14 days typical early symptoms include fever, headache, malaise, myalgia, and gastrointestinal complaints. A characteristic rash — often starting on the wrists and ankles and potentially spreading to the trunk, palms, and soles — may appear a few days after fever onset, but it can be absent or delayed in some patients. Without prompt treatment, RMSF can cause vasculitis with complications such as organ dysfunction, neurologic involvement, and shock; mortality can be substantial in untreated cases.
Diagnosis of RMSF is clinical and empirical treatment decisions are time-sensitive: because early laboratory tests may be insensitive, suspected cases are treated promptly on clinical grounds. Doxycycline is the recommended first-line therapy for suspected RMSF in adults and children; treatment should not be delayed while awaiting confirmatory tests. Laboratory methods such as serology (paired acute and convalescent sera), PCR, or immunohistochemical staining of biopsy material can support the diagnosis retrospectively. Prevention focuses on avoiding tick exposure (protective clothing, repellents, checking for and promptly removing ticks) and public-health measures to reduce tick habitat; early recognition and treatment markedly reduce the risk of severe outcomes.
In the Pacific Northwest, the most common tick-borne diseases differ somewhat from other U.S. regions because of the local tick species and their infection rates. Lyme disease (Borrelia burgdorferi sensu lato), anaplasmosis (Anaplasma phagocytophilum), and babesiosis (Babesia spp.) are among the more frequently reported illnesses and are primarily transmitted by the western blacklegged tick (Ixodes pacificus). RMSF occurs in the region but is less common than in some central and southeastern areas of the country because Dermacentor tick activity and R. rickettsii prevalence are lower; nevertheless, clinicians and outdoor workers should remain aware of it because of its rapid progression and need for immediate treatment. General prevention strategies across the region are the same: minimize tick habitat, use personal protective measures (repellents, permethrin-treated clothing), perform regular tick checks after outdoor activity, and remove attached ticks promptly to reduce the chance of pathogen transmission.
Tularemia (Francisella tularensis)
Tularemia is an infectious disease caused by the bacterium Francisella tularensis. It can present in several clinical forms depending on route of exposure; the most common is ulceroglandular tularemia, characterized by a fever, an ulcer at the site of inoculation and nearby swollen lymph nodes. Other presentations include glandular (lymphadenopathy without an obvious skin lesion), oculoglandular (eye involvement), oropharyngeal (sore throat, abdominal pain), and pneumonic forms (lung involvement) that can be severe. Incubation is typically a few days (commonly 3–5 days), and diagnosis relies on clinical suspicion plus confirmatory testing such as serology, PCR, or culture (culture requires high-containment laboratory handling). Because F. tularensis is highly infectious in small numbers, laboratory handling follows strict biosafety procedures.
Transmission occurs through multiple routes: tick and deer-fly bites, direct handling of infected animals (especially rabbits and rodents), ingestion of contaminated food or water, and inhalation of contaminated aerosols. Ticks are one of several natural vectors and can transfer the organism from wildlife reservoirs to humans. Treatment requires antibiotics active against intracellular bacteria; historically streptomycin was the treatment of choice, but gentamicin is commonly used as a parenteral alternative, while doxycycline or ciprofloxacin may be used for less severe disease or oral therapy. Early recognition and appropriate antibiotic therapy significantly reduce complications, so anyone with febrile illness plus a compatible exposure (tick bite, contact with wild mammals, or exposure to vectors in endemic areas) should seek prompt medical evaluation.
In the Pacific Northwest, tick-borne disease risk is driven largely by the local tick species and the pathogens they carry. The western black‑legged tick (Ixodes pacificus) is the primary vector for Borrelia burgdorferi (Lyme disease) and can also transmit Anaplasma phagocytophilum (anaplasmosis) and Borrelia miyamotoi (a relapsing-fever–type infection). Lyme disease and anaplasmosis are the most commonly reported tick-borne infections in the region; babesiosis and other infections occur less frequently but have been documented. Tularemia occurs sporadically in the Pacific Northwest and can be transmitted by ticks there as well as by other vectors and animal contacts. Preventive measures that reduce risk include avoiding tick habitat, using EPA‑registered repellents on skin and permethrin on clothing/gear, performing thorough tick checks after outdoor activity and removing ticks promptly with fine-tipped forceps, and seeking medical advice for fever or localized infection after a tick bite — clinicians can advise about diagnostic testing and, when appropriate, a single-dose doxycycline prophylaxis for high‑risk Ixodes bites.