Lyme disease dominates the public conversation about ticks, but it is one threat among many. In the eastern United States alone, three major tick species collectively transmit at least 14 human pathogens Journal of Medical Entomology / PMC, 2021. Some of these diseases kill faster than Lyme. Some have no treatment. Some can be transmitted in minutes. The breadth of the tick-borne disease landscape is itself a public health problem — because a medical system organized around Lyme disease will miss the others.

"Tickborne rickettsial diseases continue to cause severe illness and death in otherwise healthy adults and children, despite the availability of low-cost, effective antibacterial therapy." Centers for Disease Control and Prevention, 2016

Of the nearly 50,000 cases of tickborne disease reported in the US in 2018, nearly 8,000 were caused by pathogens associated with non-Ixodes ticks Centers for Disease Control and Prevention, 2020. CDC data for 2019-2022 reported 46,115 cases total, with the five most prevalent being Lyme disease, anaplasmosis, spotted fever rickettsiosis, ehrlichiosis, and babesiosis ScienceDirect / Elsevier, 2025. And these are only the reported cases.

Anaplasmosis: The 24-hour pathogen

Anaplasmosis, caused by Anaplasma phagocytophilum (anaplasmosis bacterium), is transmitted by Ixodes scapularis (blacklegged tick / deer tick) — the same tick that carries Lyme disease. What makes it distinctive is speed. Transmission of A. phagocytophilum from tick to human can occur within 24 hours of attachment — significantly shorter than the 36-to-72-hour window for Borrelia burgdorferi American Society for Microbiology, 2019. Rapid tick removal, the cornerstone of Lyme prevention, may not be fast enough to prevent anaplasmosis.

Incidence increased from 1.4 to 6.1 cases per million people between 2000 and 2010 American Society for Microbiology, 2019. The average annual incidence reached 6.3 cases per million during 2008-2012, concentrated in northeastern and upper Midwestern states, with the geographic range expanding Centers for Disease Control and Prevention, 2016. The case fatality rate is 0.3%, with higher rates among persons 70 and older and those with immunosuppression Centers for Disease Control and Prevention, 2016.

Doxycycline is the treatment of choice for all suspected tickborne rickettsial diseases, including anaplasmosis, in adults and children of all ages Centers for Disease Control and Prevention, 2016.

Ehrlichiosis: Similar symptoms, ten times the fatality rate

Ehrlichiosis, caused primarily by Ehrlichia chaffeensis (ehrlichiosis bacterium), is transmitted by Amblyomma americanum (lone star tick). It presents with symptoms nearly identical to anaplasmosis — fever, headache, malaise, myalgia — but the case fatality rate of E. chaffeensis (3%) is ten times that of A. phagocytophilum (0.3%), making accurate differentiation clinically consequential American Society for Microbiology, 2019. Mortality is approximately 3% of affected individuals ScienceDirect / Elsevier, 2025.

E. chaffeensis was reported in 35 states by 2015, up from 29 states in 2008 American Society for Microbiology, 2019. Age-specific case-fatality rates are highest in children under 10 and adults 70 and older Centers for Disease Control and Prevention, 2016.

A related species, Ehrlichia ewingii, is likely significantly underreported because morulae found in neutrophils are routinely attributed to A. phagocytophilum rather than E. ewingii, and serological cross-reactivity between the two species makes differentiation difficult with standard assays American Society for Microbiology, 2019.

Three additional tickborne rickettsial agents were newly recognized as causes of human disease since 2004: Rickettsia parkeri, the EML agent, and Rickettsia species 364D Centers for Disease Control and Prevention, 2016.

Babesiosis: A malaria-like parasite in the blood supply

Babesiosis, caused by Babesia microti (Babesia parasite), is a parasitic infection — not bacterial — transmitted by the blacklegged tick. It attacks red blood cells in a manner similar to malaria, and it carries a distinctive and underappreciated public health risk: blood supply contamination.

"Indeed, B. microti is the infectious agent with the highest number of reports of transfusion transmission and is associated with the most deaths due to pathogen contamination of blood products." American Society for Microbiology, 2019

There were 159 transfusion-transmitted cases in the US from 1979 to 2009, and 55 cases in New York State alone from 2004 to 2015 American Society for Microbiology, 2019. Babesiosis became nationally notifiable in 2011, with 1,124 confirmed or probable cases that year, rising to 1,910 cases in 2016 American Society for Microbiology, 2019.

The fatality rate is 2-9% of hospitalized B. microti infections and up to 20% for transfusion-transmitted cases American Society for Microbiology, 2019. In Europe, B. divergens infections in splenectomized patients carry a fatality rate of 40-60% American Society for Microbiology, 2019. Immunocompromised hosts face the additional danger of relapsing babesiosis that develops drug resistance during prolonged therapy — mutations in cytochrome b and ribosomal protein L4 genes have been identified, paralleling resistance mechanisms in Plasmodium falciparum malaria American Society for Microbiology, 2019.

Treatment is atovaquone plus azithromycin, which has a 15% side effect rate, compared to 72% (33% severe) for the older regimen of quinine plus clindamycin American Society for Microbiology, 2019.

Coinfection with B. microti or A. phagocytophilum should be considered in patients with initial symptoms more severe than commonly observed with Lyme disease alone, especially with high-grade fever persisting more than 48 hours despite appropriate antibiotic therapy Centers for Disease Control and Prevention, 2022.

Powassan virus: No grace period for tick removal

Powassan virus represents a fundamentally different category of tick-borne threat. Unlike bacterial and parasitic pathogens that require hours of tick feeding to transmit, Powassan virus (deer tick virus lineage) can be transmitted from tick to host in as little as 15 minutes of attachment American Society for Microbiology, 2019. There is no grace period for tick removal to prevent Powassan infection.

"Transmission of DTV to naive P. leucopus by infected nymphal I. scapularis ticks in ≤15 min was demonstrated in a more recent study." American Society for Microbiology, 2019

Through the data available in the ASM review, 98 total US cases had been reported, 88 neuroinvasive, with 11 deaths — a fatality rate of approximately 10% American Society for Microbiology, 2019. Survivors frequently suffer long-term neurological damage. There is no specific antiviral treatment and no vaccine MDPI (Vaccines), 2024. A lipid nanoparticle-encapsulated modified mRNA vaccine carrying Powassan prM and E genes has shown protection in mice against both lineage 1 and 2 strains and induced cross-neutralizing antibodies against other tick-borne flaviviruses, but this remains in research stages MDPI (Vaccines), 2024.

Tick-borne encephalitis: A major burden outside the US

Tick-borne encephalitis (TBE), caused by TBE virus, is the major tick-borne viral disease globally, with 10,000 to 20,000 clinical cases reported per year in Europe and Asia American Society for Microbiology, 2019, MDPI (Vaccines), 2024. The disease is transmitted by Ixodes ticks and exists in three subtypes with dramatically different severity: the Far Eastern subtype has a case fatality rate of up to 30%, compared to approximately 2% for the European subtype MDPI (Vaccines), 2024.

TBE virus prevalence in ticks in China increased from 4.8% (2000-2010) to 6.3% (2011-2023), concentrated in Jilin and Inner Mongolia ScienceDirect / Elsevier, 2025. Like Powassan virus, TBE virus can be transmitted within 15 minutes of tick attachment during blood feeding ScienceDirect / Elsevier, 2025. Uniquely among the diseases covered here, TBE can also be transmitted to humans through consumption of raw milk and milk products from infected ruminants ScienceDirect / Elsevier, 2025.

TBE vaccines are widely available in Europe but remain underused MDPI (Vaccines), 2024. No TBE vaccine is marketed in the United States, relevant for travelers to endemic areas.

Heartland and Bourbon viruses: Newly emerged, poorly understood

Heartland virus and Bourbon virus are recently emerged tick-borne viruses in the United States, both associated with Amblyomma americanum (lone star tick).

Heartland virus has caused more than 35 cases in the US since its identification in 2012, with 3 fatalities American Society for Microbiology, 2019. Bourbon virus is even rarer, with approximately 5 known human cases American Society for Microbiology, 2019. Both viruses lack specific treatments or vaccines, and their true incidence is unknown because diagnostic testing is not widely available Centers for Disease Control and Prevention, 2022.

These viruses illustrate an ongoing pattern: over the past decade, approximately 30% of emerging zoonotic diseases have been linked to arthropod vectors such as ticks, and several recently identified pathogens — including Neoehrlichia mikurensis, Heartland virus, SFTSV, Alongshan virus, and Yezo virus — demonstrate that new tick-borne threats continue to be discovered ScienceDirect / Elsevier, 2025.

Tularemia: A tick-borne disease with bioterrorism potential

Tularemia, caused by Francisella tularensis (tularemia bacterium), can be transmitted by several tick species including Dermacentor variabilis (American dog tick) and Amblyomma americanum (lone star tick) Connecticut Agricultural Experiment Station / CDC, 2007, Centers for Disease Control and Prevention, 2022. Unlike most tick-borne diseases, tularemia has multiple transmission routes beyond tick bites — including handling infected animals, contaminated water, and inhalation — which is why F. tularensis is classified as a potential bioterrorism agent Connecticut Agricultural Experiment Station / CDC, 2007.

STARI: The rash without a known cause

Southern tick-associated rash illness (STARI) produces an expanding rash resembling the erythema migrans of Lyme disease, but it occurs in the range of the lone star tick, and its causative agent remains unknown Connecticut Agricultural Experiment Station / CDC, 2007, Centers for Disease Control and Prevention, 2022. The IDSA/AAN/ACR clinical practice guidelines note that antibiotic treatment may be appropriate because STARI cannot be clinically distinguished from early Lyme disease IDSA / AAN / ACR, 2020. The inability to identify its pathogen means there is no specific diagnostic test, making STARI a persistent clinical problem in the southeastern US.

Tick paralysis: A neurotoxin, not an infection

Tick paralysis is mechanistically distinct from every other disease on this list. It is not caused by an infectious pathogen but by a neurotoxin secreted in the saliva of certain tick species during prolonged feeding Connecticut Agricultural Experiment Station / CDC, 2007. The paralysis is ascending — beginning in the legs and progressing upward — and can mimic Guillain-Barre syndrome. The critical difference is that tick paralysis is rapidly reversible once the tick is removed. The danger lies in failure to find the tick: if the feeding tick is not discovered and removed, paralysis can progress to respiratory failure.

Coinfections: When one tick carries multiple pathogens

Because major tick-borne pathogens share the same vectors and reservoir hosts, a single tick bite can transmit more than one pathogen simultaneously. The white-footed mouse is the principal reservoir for B. burgdorferi, Babesia microti, and A. phagocytophilum Connecticut Agricultural Experiment Station / CDC, 2007, making coinfection a predictable consequence of the ecology.

"Coinfection with Babesia microti or Anaplasma phagocytophilum should be considered in patients who present with initial symptoms that are more severe than are commonly observed with Lyme disease alone." Centers for Disease Control and Prevention, 2022

Coinfection complicates diagnosis, treatment, and prognosis. Patients coinfected with Lyme disease and babesiosis, for example, require treatment for both a bacterial and a parasitic infection — regimens that do not overlap. The clinical significance is that any patient with a blacklegged tick bite in an endemic area who presents with unusually severe symptoms should be evaluated for multiple infections, not just the most common one.

Crimean-Congo hemorrhagic fever: A global threat not yet in the US

While not present in the United States, Crimean-Congo hemorrhagic fever virus (CCHFV) is the most widespread tick-borne viral infection of humans globally, reported in more than 30 countries with a case fatality rate of 5-30% American Society for Microbiology, 2019. An estimated 88% of infections are subclinical American Society for Microbiology, 2019.

"CCHFV must also be considered a potential agent for bioterrorism because of the ease of amplifying the virus to high titers in large volumes, infectiousness for humans, the severity of the disease, the ability to be transmitted by aerosol, and the lack of measures available for its control." American Society for Microbiology, 2019

The pattern: a growing list with no single solution

"Ticks notably surpass all other arthropods in their capability to transmit a diverse array of infectious agents, including viruses, bacteria, and parasites." MDPI (Vaccines), 2024

The tick-borne disease landscape is not static. New pathogens continue to be identified, existing diseases expand their geographic range as tick populations spread, and the medical system remains organized around the assumption that Lyme disease is the primary tick-borne concern. Antibiotic prophylaxis after a tick bite has been shown to reduce the risk of only one of these diseases — Lyme — and is explicitly not recommended for anaplasmosis, babesiosis, ehrlichiosis, or RMSF Centers for Disease Control and Prevention, unknown. There is no single pill, no single vaccine, and no single strategy that addresses the full range of threats a tick bite can deliver.

Sources

Compiler Notes

  • The Cao 2026 Nature Reviews Microbiology source on tick-borne viruses is unavailable (paywall) and could not be used for claims. Its full text would likely strengthen the Powassan, TBE, and Heartland/Bourbon sections.
  • The Piesman 2008 Annual Review of Entomology source is unavailable (abstract only) and could not be used for direct claims.
  • Tularemia coverage is thin — the vault has clinical reference entries but no dedicated tularemia source. A dedicated CDC tularemia factsheet or clinical review would strengthen that section.
  • STARI pathogen identification remains an open research question. The vault contains no sources specifically investigating STARI etiology.
  • Colorado tick fever is mentioned in the CDC reference manual's table of contents but has minimal indexed content in the vault. It is not covered here.
  • Tick paralysis coverage relies on a single source (Stafford 2007). A clinical case series or review article would add depth, particularly on differential diagnosis with Guillain-Barre syndrome.