The essentials

Spotted fever is a group of infections caused by Rickettsia bacteria and passed on by tick bites. The most serious form, Rocky Mountain spotted fever in the United States, also called Brazilian spotted fever in Brazil, starts with fever and headache and can become life-threatening within days. Doctors give doxycycline as soon as they suspect it, without waiting for tests, because blood tests are often negative in the first week.

What is spotted fever, at a glance?

  • Cause: Rickettsia rickettsii, a bacterium spread by the bite of an infected tick. It does not pass from person to person[1].
  • Other names: Rocky Mountain spotted fever (RMSF) in the United States, Brazilian spotted fever in Brazil[5][7].
  • Where: throughout the United States, mostly from April to September[1], and in Mexico, Central and South America, including Brazil[5].
  • How serious: about 5% to 10% of people with RMSF die today, more when treatment starts late[5].
  • Key tests: blood count and metabolic panel for warning signs, then antibodies and sometimes PCR to confirm. None should delay treatment[3].
  • Not Lyme disease: Lyme disease is another tick infection, with a different bacterium, course and tests (see below).

What are the symptoms of Rocky Mountain spotted fever?

Symptoms begin 3 to 12 days after the bite of an infected tick[2]. The first days look like many other infections, which is why the disease is easy to miss[1].

  • Common Sudden fever and headache, often with nausea, vomiting, loss of appetite, muscle pain and stomach pain[1][2]. Children report headache less often[2].
  • Common A rash, usually 2 to 4 days after the fever: small, flat, pink spots on the wrists, forearms and ankles, spreading to the trunk and sometimes the palms and soles[2]. About 90% of people eventually get it, but fewer than half in the first 3 days; pinpoint red-purple spots usually come around day 5 or 6[2].
  • Possible Swelling around the eyes and on the back of the hands in the first days[2].
  • Rare From day 5 without treatment: confusion, breathing problems, kidney failure, skin damage on fingers and toes, coma[2].

Only 55% to 60% of patients remember a tick bite[5]. No bite and no rash do not rule it out: a late or absent rash is linked to delayed diagnosis and more deaths[5].

When should I see a doctor after a tick bite?

When to see a doctor

  • Emergency: call 911 for fever with confusion or unusual sleepiness, trouble breathing, very little urine, or dark, cold fingers or toes: signs of severe disease[2].
  • Within 24 to 48 hours, ideally the same day for fever, a strong headache or a rash within two weeks of a tick bite or of time in woods, tall grass or brush[1][4]. Mention the tick. Treatment works best in the first 5 days of illness[4].
  • At your next appointment if you removed a tick and still feel well. Antibiotics are not recommended just because of a bite: watch for fever or rash for two weeks[4].

What causes spotted fever, and who is at risk?

In the United States, three ticks carry R. rickettsii: the American dog tick, the Rocky Mountain wood tick, and the brown dog tick, found in parts of the southwestern states and Mexico[1]. Only about 1 tick in 1,000 carries the bacterium[6].

  • Common Outdoor exposure in warm months, in woods, grass or brush[1].
  • Possible Severe illness is more likely with late treatment, age under 10 and G6PD deficiency (an inherited red cell enzyme deficiency)[2]. Children are less than 6% of cases but 22% of deaths[2].
  • Possible High-risk areas: in Mexico and Brazil, outbreaks also occur in cities, with fatality rates that can exceed 50%, often due to late treatment; in Brazil, capybaras keep infected ticks close to people[8].

Related species cause milder spotted fevers. In Brazil, the severe form (R. rickettsii) is reported in northern Paraná and the Southeast, while R. parkeri, in Atlantic Forest areas, is milder[7]. Mediterranean spotted fever (boutonneuse fever), caused by R. conorii and spread by the brown dog tick, usually shows a black scab at the bite, the "tache noire"[9], which is rare in RMSF[5].

How is it different from Lyme disease? Both follow a tick bite, but RMSF can turn severe within days and is treated on suspicion[4][5]. Lyme disease, caused by another bacterium, develops more slowly, often starts with a "bullseye" rash and uses a different test strategy: see our Lyme disease guide.

Which blood tests are done for spotted fever, and how do I read them?

Routine tests do not prove spotted fever, but they show its typical pattern: low platelets, slightly raised liver enzymes, sometimes low sodium as the disease advances, and a normal or slightly raised white cell count[5]. Early on, they can still be normal[2]. Here is a fictitious report.

Fictitious example of a lab report (United States format)
TestResultReference rangeFlag
Platelet count92 x10³/µL150-400 x10³/µLL
Sodium131 mEq/L135-145 mEq/LL
AST (aspartate aminotransferase)78 U/L8-33 U/LH
Rickettsia rickettsii IgG antibodies (IFA)Negative (below 1:64)Negative
  1. Platelets, flagged L: below 150,000 per microliter is called thrombocytopenia[10], a typical finding in spotted fever[5][7].
  2. Sodium, flagged L: a mild drop fits an advancing infection[5], but vomiting, diarrhea or some medicines can also lower sodium[11].
  3. AST, flagged H: a slight rise of AST and ALT fits the picture[5] but is not specific: hepatitis, muscle injury or medicines can do the same[12].
  4. Antibodies, negative: expected in the first week; it does not rule out the infection[3]. The cutoff shown is an example.
  5. Ranges: examples from MedlinePlus[10][11][12]. Your laboratory's range applies.

Low platelets, low sodium and raised liver enzymes on the same report? BloodSense reads every line of your complete report, shows which results fit together and helps you prepare your questions for your doctor.

Get my full report explained

This is a pattern, not a diagnosis. The Brazilian Ministry of Health lists dengue fever, leptospirosis, hepatitis, malaria and meningitis among the look-alikes[7], and severe infections such as sepsis also lower platelets. See our pages on the platelet count, blood sodium and high liver enzymes. In hospital, creatinine kidney test, the prothrombin time and a urinalysis may be added[6]. Lab values must not decide early treatment[5].

How is spotted fever confirmed: antibodies, fourfold rise and PCR?

The reference test is an IgG antibody test by indirect immunofluorescence (IFA)[3]. Three traps:

  • Too early: antibodies are often absent in the first week; a negative first test never justifies stopping treatment[3].
  • One sample is not enough: confirmation needs two samples 2 to 10 weeks apart (ideally 2 to 4) showing at least a fourfold rise[3][5]. A single high result never confirms a recent infection[5].
  • Old antibodies and look-alikes: antibodies can stay high for up to four years, 10% or more of healthy people in some areas have raised levels, and most laboratories cannot tell spotted fever species apart[3].

An IgM antibody test is less specific than IgG antibodies and should not be used alone[3][5]. PCR finds the bacterium's DNA: on blood it is less sensitive than on a skin biopsy of the rash, a negative result does not rule out the disease, and doxycycline lowers its sensitivity[3][5]. Confirmation often comes after you are better; it still allows the case to be reported to health authorities[3].

Which tests are repeated during and after treatment?

With doxycycline, fever usually goes down within 24 to 48 hours[4]. In hospital, the blood count, sodium, kidney and liver tests may be repeated to check recovery. The second antibody sample, weeks later, confirms the diagnosis[3].

Most people without early blood vessel damage recover fully, and R. rickettsii does not cause a chronic infection[2]. After severe illness, some keep nerve damage, hearing loss or organ damage, or need an amputation[1][2], and need longer follow-up.

How is Rocky Mountain spotted fever treated?

No doses are given here: your doctor chooses the treatment.

  • Doxycycline, started on suspicion, is the treatment of choice in adults and children of all ages[4], never delayed for test results[3][5]. Patients treated after day 5 are more likely to die; treated on days 8 or 9, 40% to 50% died[5].
  • Children: short courses did not stain teeth in a study of 89 children[5].
  • Duration: at least 3 days after the fever ends, 5 to 7 days minimum[4]. In Brazil, the public health system (SUS) provides the antibiotic[7].
  • Pregnancy and allergy: doxycycline is unlikely to harm the fetus, but risks and benefits are discussed; an allergy may be managed by supervised desensitization[4].
  • Avoid sulfa antibiotics: they may worsen the disease[4]. Severe cases need hospital care[7].

How can I prevent spotted fever?

There is no vaccine[1]. Prevention means avoiding tick bites on yourself, your pets and in your yard[1].

  • In tick areas, wear long sleeves and long pants and use an insect repellent.
  • Check your body, your children and your dogs for ticks after time outdoors; protect dogs with a tick product advised by your veterinarian[1].
  • Remove an attached tick quickly with fine tweezers, pulling steadily[6]. Quick removal lowers the risk of infection[7].
  • Note the date and place of the bite, and watch for fever or rash for two weeks[4].

Questions to ask your doctor

  1. Could my fever be spotted fever, and should I start doxycycline now?
  2. Which results on my blood count and metabolic panel fit this infection?
  3. When should the second antibody sample be taken?
  4. Which warning signs should make me go to the emergency room?
  5. Do my platelets, sodium or liver enzymes need to be rechecked, and when?
  6. Should the people I was outdoors with, or my dog, be checked for ticks?

Frequently asked questions

Is Rocky Mountain spotted fever contagious?

No. It is spread only by tick bites, not from person to person[1].

Can you fully recover from Rocky Mountain spotted fever?

Yes, most people do, especially when doxycycline starts early. Lasting damage mainly affects people who had severe illness with blood vessel damage[2].

Should I take antibiotics right after a tick bite?

No. Antibiotics after a bite are not recommended to prevent RMSF. Watch for fever or rash in the next two weeks and see a doctor if they appear[4].

Is "doença do carrapato" in dogs the same disease?

Not always. The name is used in Brazil for several tick infections of dogs. Dogs can also get R. rickettsii, and dogs and their ticks can bring infected ticks close to people[8]. In people, the serious tick disease in Brazil is spotted fever[7].

  1. Centers for Disease Control and Prevention. About Rocky Mountain spotted fever. May 15, 2024. cdc.gov (accessed November 5, 2026)
  2. Centers for Disease Control and Prevention. Clinical signs and symptoms of Rocky Mountain spotted fever. May 15, 2024. cdc.gov (accessed November 5, 2026)
  3. Centers for Disease Control and Prevention. Clinical testing and diagnosis for Rocky Mountain spotted fever. May 15, 2024. cdc.gov (accessed November 5, 2026)
  4. Centers for Disease Control and Prevention. Clinical care of Rocky Mountain spotted fever. March 6, 2025. cdc.gov (accessed November 5, 2026)
  5. Biggs HM, Behravesh CB, Bradley KK, et al. Diagnosis and management of tickborne rickettsial diseases: Rocky Mountain spotted fever and other spotted fever group rickettsioses, ehrlichioses, and anaplasmosis — United States. MMWR Recomm Rep. 2016;65(2):1-44. doi:10.15585/mmwr.rr6502a1 (accessed November 5, 2026)
  6. MedlinePlus Medical Encyclopedia. Rocky Mountain spotted fever. Reviewed May 12, 2025. US National Library of Medicine. medlineplus.gov (accessed November 5, 2026)
  7. Ministério da Saúde (Brazil). Febre maculosa. gov.br (accessed November 5, 2026)
  8. Foley J, López-Pérez AM, Álvarez-Hernández G, et al. A wolf at the door: the ecology, epidemiology, and emergence of community- and urban-level Rocky Mountain spotted fever in the Americas. Am J Vet Res. 2025;86(3). doi:10.2460/ajvr.24.11.0368 (accessed November 5, 2026)
  9. Spernovasilis N, Markaki I, Papadakis M, et al. Mediterranean spotted fever: current knowledge and recent advances. Trop Med Infect Dis. 2021;6(4):172. doi:10.3390/tropicalmed6040172 (accessed November 5, 2026)
  10. MedlinePlus Medical Encyclopedia. Platelet count. Reviewed February 3, 2025. US National Library of Medicine. medlineplus.gov (accessed November 5, 2026)
  11. MedlinePlus Medical Encyclopedia. Sodium blood test. Reviewed November 6, 2025. US National Library of Medicine. medlineplus.gov (accessed November 5, 2026)
  12. MedlinePlus Medical Encyclopedia. Aspartate aminotransferase (AST) blood test. Reviewed January 1, 2025. US National Library of Medicine. medlineplus.gov (accessed November 5, 2026)

This page is general information. It does not replace the advice of a doctor, who interprets your results with your symptoms, tick exposure and examination.