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PubMed This is a summary of 13 peer-reviewed journal articles Updated
Infectious Disease

What States Have the Most RMSF Cases? | Inciteful Med

At a Glance

Most cases of Rocky Mountain spotted fever occur in North Carolina, Tennessee, Oklahoma, Arkansas, and Missouri. The southwestern U.S., particularly Arizona, is a major emerging hotspot due to the brown dog tick, which can infest homes and spread the disease year-round.

In this answer

3 sections

01

Historical High-Risk States

02

Emerging Hotspots in the Southwest

03

Why Geography Matters for Your Care

Although its name suggests otherwise, Rocky Mountain spotted fever (RMSF) is no longer most common in the Rocky Mountains. When the disease was first discovered, it was linked to the Rocky Mountain wood tick (Dermacentor andersoni), but today the vast majority of cases occur in the south-central and southeastern United States, with new hotspots emerging in the Southwest [1][2][3].

Historical High-Risk States

The geographic distribution of RMSF has shifted significantly over the past century [4]. For several decades, the disease has been most heavily concentrated in five states that consistently report the highest number of RMSF cases [1][5]:

  • North Carolina
  • Tennessee
  • Oklahoma
  • Arkansas
  • Missouri

In these south-central and southeastern states, the disease is primarily spread by the American dog tick (Dermacentor variabilis). The risk in these areas is highest during the spring and summer months when adult ticks are most active and people spend more time outdoors.

Emerging Hotspots in the Southwest

While the Southeast and South-Central states account for the bulk of historical cases, RMSF has recently become a major public health concern in the southwestern United States—particularly in Arizona—and in northern Mexico (such as the state of Sonora) [3][6].

In this region, the disease is spread by a different tick: the brown dog tick (Rhipicephalus sanguineus) [7]. Unlike other ticks that live in wooded or brushy areas, the brown dog tick can survive and thrive indoors and around human homes, often infesting free-roaming domestic dogs [8][7].

This creates a unique exposure risk:

  • Peridomestic transmission: This means the disease is spread in and around the home environment, rather than in the woods [6].
  • Year-round risk: Because brown dog ticks can live indoors, the risk of RMSF in these southwestern areas is not strictly limited to the spring and summer [7].
  • Community clusters: High tick burdens on neighborhood dogs can lead to clusters of disease within communities [9].

Actionable Tip: If you live in or visit these areas, regularly check your pets for ticks to help prevent indoor infestations and reduce your exposure risk.

Why Geography Matters for Your Care

Knowing where RMSF is most common helps you and your doctor accurately assess your risk [10]. If you develop a fever and headache after traveling to or living in one of these high-risk areas, it is critical to inform your healthcare provider about your geographic exposure [11].

Do not wait for a rash to appear before seeking care; the characteristic RMSF rash often does not show up until several days into the illness, and sometimes not at all. While RMSF can be very dangerous if left untreated, it is highly curable when caught early. Doctors are advised to prescribe the antibiotic doxycycline immediately based on clinical suspicion and your geographic risk, without waiting for blood test results to confirm the diagnosis [12][13].

Common questions in this guide

Which states have the highest number of Rocky Mountain spotted fever cases?
Historically, the disease is most heavily concentrated in North Carolina, Tennessee, Oklahoma, Arkansas, and Missouri. However, the southwestern United States, particularly Arizona, has recently become a major emerging hotspot.
Do you only get Rocky Mountain spotted fever in the woods?
No. While the American dog tick, which spreads the disease in the Southeast, is found in wooded areas, the brown dog tick is common in the Southwest and can survive indoors. This means the disease can spread in and around human homes.
Is Rocky Mountain spotted fever only a risk in the summer?
In southeastern and south-central states, risk is highest during spring and summer when ticks are most active outdoors. However, in the Southwest, the brown dog tick can live indoors and pose a risk of infection year-round.
What should I do if I get sick after traveling to an RMSF hotspot?
If you develop a sudden fever and headache after visiting a high-risk area, see a doctor immediately. Be sure to inform them of your travel history so they can evaluate you for tick-borne diseases and consider early treatment with antibiotics.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.I recently traveled to a high-risk state and may have been exposed to ticks; what symptoms should I monitor for over the next two weeks?
  2. 2.Given that I live in an area where the brown dog tick is common, what specific precautions should I take around my home and pets?
  3. 3.If I develop a sudden fever and headache after visiting an area known for RMSF, how quickly can I start doxycycline?
  4. 4.Should we begin antibiotic treatment based on my recent travel and symptoms, even if my blood tests haven't confirmed a diagnosis yet?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

Related questions

Are There Alternatives to Doxycycline for RMSF?Can You Have Rocky Mountain Spotted Fever Without a Rash?Does Doxycycline Stain Teeth in Children Under 8?What Are the Long-Term Effects of Severe RMSF?What Does a Rocky Mountain Spotted Fever Rash Look Like?Why Are Early Blood Tests for RMSF Often Negative?Why Does RMSF Cause Severe Nausea and Vomiting?

References

References (13)
  1. 1

    Intravenous Immunoglobulin Therapy for Cerebral Vasculitis Associated with Rocky Mountain Spotted Fever.

    Allen HC, Welliver RC, Fogarty MW, et al.

    Journal of pediatric intensive care 2017; (6(2)):142-144 doi:10.1055/s-0036-1587327.

    PMID: 31073439
  2. 2

    From Tick to Test: A Comprehensive Review of Tick-Borne Disease Diagnostics and Surveillance Methods in the United States.

    Rowan S, Mohseni N, Chang M, et al.

    Life (Basel, Switzerland) 2023; (13(10)) doi:10.3390/life13102048.

    PMID: 37895430
  3. 3

    Rocky Mountain Spotted Fever Mimicking Multisystem Inflammatory Syndrome in Hospitalized Children, Sonora, Mexico.

    Álvarez-Hernández G, Rivera-Rosas CN, Calleja-López JRT, et al.

    Emerging infectious diseases 2024; (30(7)):1463-1466 doi:10.3201/eid3007.240033.

    PMID: 38861505
  4. 4

    Rocky Mountain spotted fever in Mexico: past, present, and future.

    Álvarez-Hernández G, Roldán JFG, Milan NSH, et al.

    The Lancet. Infectious diseases 2017; (17(6)):e189-e196 doi:10.1016/S1473-3099(17)30173-1.

    PMID: 28365226
  5. 5

    Increasing Incidence of Spotted Fever Group Rickettsioses in the United States, 2010-2018.

    Bishop A, Borski J, Wang HH, et al.

    Vector borne and zoonotic diseases (Larchmont, N.Y.) 2022; (22(9)):491-497 doi:10.1089/vbz.2022.0021.

    PMID: 36037000
  6. 6

    Rickettsial Pathogens in Dogs and Ticks During an Epidemic of Rocky Mountain Spotted Fever in Ensenada, Baja California, México.

    Backus L, Rubino F, López-Pérez AM, et al.

    The American journal of tropical medicine and hygiene 2025; (112(5)):1096-1112 doi:10.4269/ajtmh.24-0497.

    PMID: 39965213
  7. 7

    The emergence of Rocky Mountain spotted fever in the southwestern United States and northern Mexico requires a binational One Health approach.

    Foley J, Álvarez-Hernández G, Backus LH, et al.

    Journal of the American Veterinary Medical Association 2024; (262(5)):698-704 doi:10.2460/javma.23.07.0377.

    PMID: 38417252
  8. 8

    POTENTIAL SHARED DISEASE RISK AMONG DOGS AND COYOTES (CANIS LATRANS) EXEMPLIFIED BY THE ECOLOGY OF RICKETTSIOSIS IN A ROCKY MOUNTAIN SPOTTED FEVER-EPIDEMIC REGION IN NORTHERN MEXICO.

    Marcek J, Foley J, Backus L, et al.

    Journal of wildlife diseases 2023; (59(4)):722-733 doi:10.7589/JWD-D-22-00179.

    PMID: 37846906
  9. 9

    Epidemiology and Clinical Features of Rocky Mountain Spotted Fever from Enhanced Surveillance, Sonora, Mexico: 2015-2018.

    Álvarez-López DI, Ochoa-Mora E, Nichols Heitman K, et al.

    The American journal of tropical medicine and hygiene 2021; (104(1)):190-197.

    PMID: 33146112
  10. 10

    Associations between rocky mountain spotted fever and veterinary care access, climatic factors and landscape in the State of Arizona, USA.

    Lin Y, Hridoy AEE, Li M, et al.

    Geospatial health 2025; (20(2)) doi:10.4081/gh.2025.1390.

    PMID: 40679413
  11. 11

    A forty-year review of Rocky Mountain spotted fever cases in California shows clinical and epidemiologic changes.

    Kjemtrup AM, Padgett K, Paddock CD, et al.

    PLoS neglected tropical diseases 2022; (16(9)):e0010738 doi:10.1371/journal.pntd.0010738.

    PMID: 36108065
  12. 12

    [Rocky Mountain spotted fever in Mexican children: Clinical and mortality factors].

    Álvarez-Hernández G, Candia-Plata Mdel C, Delgado-de la Mora J, et al.

    Salud publica de Mexico 2016; (58(3)):385-92.

    PMID: 27598937
  13. 13

    [Complications and cause of death in mexican children with rocky mountain spotted fever].

    Martínez-Medina MÁ, Rascón-Alcantar A

    Gaceta medica de Mexico 2016; (152(6)):789-795.

    PMID: 27861477

This page provides information on the geographic risks of Rocky Mountain spotted fever for educational purposes. Always consult a healthcare professional if you develop a sudden fever after suspected tick exposure or travel to high-risk areas.

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