Key Messages

  • Babesia microti (B. microti) is a tick-borne intracellular protozoan parasite that is carried by Ixodes scapularis, the blacklegged or deer tick. 
  • The presentation of babesiosis can be nonspecific and should be suspected in cases of fever of unknown origin with a history of travel or residence in an area where Ixodes scapularis is found. 
  • Diagnosis of babesiosis is generally made via manual review of blood smears by an experienced hematopathologist. 
  • Treatment of symptomatic individuals consists of a 7 to 10 day course of oral atovaquone and azithromycin. Infectious diseases consultation may be considered.  
  • Suspected or confirmed cases of babesiosis are reportable to local public health under the Health Protection and Promotion Act.

Babesiosis is an emerging tick-borne infection caused by the protozoan parasite Babesia microti. Infection in humans occurs following a bite from the Ixodes scapularis tick (also known as the blacklegged or deer tick), which is the same type that can carry the pathogens causing Lyme disease, anaplasmosis, ehrlichiosis, and Powassan virus. Manifestations of babesiosis range from subclinical infection to fulminant disease resulting in death1.  

  • Reservoir: The mammalian hosts of B. microti primarily consists of the white-footed mouse (Peromyscus leucopus). The Ixodes scapularis tick becomes a vector of B. microti after feeding on an infected mouse. Humans serve as accidental hosts. 
  • Modes of Transmission: Human infection most often occurs following a bite from an Ixodes scapularis tick. Acquisition after transfusion of blood products harbouring B. microti has been well described particularly in endemic regions2,3. Transplacental transmission of B. microti has been described but only in a few cases4. Babesiosis generally occurs in early summer to late fall coinciding with the activity of I. scapularis ticks5.  
  • Host Susceptibility: Risk factors for severe infection include asplenia and immunocompromising conditions6 

Blacklegged ticks in various stages of feeding:

Three nymphs of the blacklegged tick are shown in different stages of feeding.

Tick nymphs

The following image shows 5 female blacklegged ticks in different stages of feeding.

5 female blacklegged ticks in different stages of feeding

For additional photos showing blacklegged ticks in various phases of engorgement from feeding visit the Tick Encounter Resource Center website.

Local Epidemiology
The geographic distribution of I. scapularis has markedly increased in the eastern United States, in particular throughout the Northeast, Upper Midwest, and mid-Atlantic7. Prior to 1998, the only known population of I. scapularis in Canada was in Long Point, Ontario8. It has since expanded considerably to include large regions of central and eastern Canada disseminated by bird migration and movement of local hosts such as white-tailed deer, small mammals, and ground dwelling birds9. Other factors such as climate change play an important role in facilitating geographic expansion by improving the suitability of environmental conditions10. Modeling studies suggest ongoing expansion within the United States and Canada with significant public health implications11,12. Ixodes scapularis ticks are present in Ottawa and surrounding regions with increasing abundance13. B. microti has been found in the Ottawa area in I.scapularis 25.
Signs and Symptoms

The incubation period of babesiosis ranges from 1 to 4 weeks in infections acquired from tick bites and 1 to 9 weeks in infections acquired from contaminated blood products1. Onset of illness is typified by gradual worsening malaise and fatigue followed by fever that may exceed 40°C15. Other common symptoms include headache, myalgias, anorexia, non-productive cough, arthralgias, nausea, vomiting, sore throat, abdominal pain conjunctival injection, photophobia, weight loss, emotional lability, depression and hyperesthesia1. Infection can be severe and life-threatening particularly in persons with asplenia, immunocompromise, or advanced age.

Physical exam is most notable for fever, but pharyngeal erythema, hepatosplenomegaly, jaundice, or retinopathy with splinter hemorrhages and retinal infarcts1

Laboratory finding include a mild-to-moderate hemolytic anemia hallmarked by low hemoglobin level, low haptoglobin and elevated lactate dehydrogenase level along with a compensatory increased reticulocyte count16

Symptomatic illness usually lasts 1 to 2 weeks without treatment, but can be followed with several months of persistent fatigue1. After treatment, asymptomatic parasitemia may persist for several months, yet without treatment, it may remain for more than a year17

Diagnosis / Laboratory testing

The diagnosis of babesiosis can be difficult due to its protean manifestations, and thus a high index of suspicion is warranted for a patient with unexplained fever accompanied with travel or residence in an endemic area within the previous two months or if a blood transfusion was received within the prior 6 months1.

Diagnosis is typically achieved via microscopical identification of B. microti on a peripheral blood film. Typical finding includes trophozoites appearing as pleomorphic ring forms. While rare, identification of tetrads of merozoites forming a “Maltese cross” pattern is pathognomonic18. Blood films should be reviewed by an experienced hematopathologist. 

Polymerase-chain-reaction is both sensitive and specific for identification of B. microti19. Testing is referred out by Public Health Ontario Laboratories (PHOL) to the National Microbiology Lab (NML) in Winnipeg with a turnaround time of up to 42 days. 

Serology is not the recommended testing method for diagnosis of acute babesiosis. If babesiosis is suspected, submit ethylenediamine tetraacetic acid (EDTA) blood and unstained slides for microscopy as per the following links:

The role of serology in babesiosis is restricted to individuals with negative microscopy and PCR results linked to a case of transfusion-transmitted infection, transplant-associated infection, or congenital infection. If serologic testing is considered in these limited instances, serologic testing requests must be pre-approved by a PHO microbiologist before submission. To do so, contact PHO’s Laboratory Customer Service Centre.

Simultaneous co-infection with other I. scapularis-borne pathogens such as Lyme disease, anaplasmosis, and Powassan virus infection is increasingly recognized21. Testing for these infections should be considered if compatible clinical findings are present. 

For further information about human diagnostic testing, the following resources are available: 

Reporting Requirements 

Suspected or confirmed cases of babesiosis are reportable to local public health under the Health Protection and Promotion Act

Monday to Friday from 8:30 am to 4:30 pm: Call 613-580-2424, extension 24224 and leave a detailed, confidential message including your contact information; or fax 613-580-9640.

After hours, on weekends, or holidays: Call 3-1-1 and ask to speak to Public Health on call. 

For more details on how to report, please visit Reporting a communicable disease.

Antimicrobial Therapy: 

Asymptomatic carriers of B. microti do not require treatment unless there are risk factors present for severe infection22. For patients with symptomatic babesiosis, treatment with oral atovaquone and oral azithromycin for 7-10 days is recommended23. Clindamycin plus quinine can be considered as an alternative, but this regimen tends to be less well tolerated24

If a patient presents with the tick attached: 

  1. Use tweezers to grasp the tick where it attaches to the skin. 
  2. Pull the tick straight out, slowly and firmly, and do not jerk or twist the tick as this can cause the tick's mouthparts to break off. Avoid squeezing the tick's abdomen. 
  3. Disinfect the feeding site after the tick is removed. 

Tick removal diagram

4. Tick identification:

a) Revised Tick Surveillance Program: (As of September 20, 2021)

  1. Public Health Ontario will continue to accept ticks for identification from members of the public through health care providers. Ottawa Public Health does not accept ticks for identification. The tick identification results will be reported to submitters. It may take up to three weeks to receive identification results back from PHO’s laboratory during peak season. The online platform ( may be an alternative option if you require a faster result.
  2. PHO will no longer send blacklegged ticks to the National Microbiology Laboratory (NML) for pathogen testing (except in rare instances by special request after consultation with NML staff). As a result of this change, submitters will no longer receive pathogen results from their submitted ticks.

b) Use Bishop’s University electronic tick identification platform ( anyone can submit a picture of a tick and receive species identification results within 48 hours, along with public health education and awareness messaging.

c) Try to identify the tick yourself using the ID guide at the University of Rhode Island’s TickEncounter Resource Center (


Prevention of tick bites is a cornerstone of babesiosis prevention. Individuals are advised to adopt the following practices: 

  • Application of a Health Canada approved insect repellent containing DEET or icaridin to exposed skin and to clothing. 
  • Wearing long pants, a long-sleeved shirt, shoes and socks to cover exposed skin. 
  • Tucking pants into socks.
  • Wearing light coloured clothing so it is easier to spot ticks. 
  • If possible, staying on the trails when hiking in the woods or walking in long grass. 
  • Performing a "full body" check (also on children, and pets if applicable) for ticks. Ticks often attach in areas such as between toes, behind knees, in the groin, armpits and scalp. 
Public Health role

Suspected and confirmed cases of babesiosis are reportable to local public health under the Health Protection and Promotion Act

For Provincial surveillance case definitions and disease-specific direction for the public health management of babesiosis, please see the Ontario Ministry of Health’s Appendix 1 - Case Definitions and Disease Specific Information. 

For more details on how to report, please visit Reporting a communicable disease.

Patient Information
Babesiosis, U.S. Centres for Disease Control (CDC)  
Physician Resources
 General Overview 

Lab Testing 

  • Refer to the Public Health Ontario Laboratory Services test information index, under Babesia 

  • Contact the Public Health Ontario Laboratories 

  • Babesiosis U.S. Centers for Disease Control and Prevention (CDC) includes parasite biology, image gallery, lab diagnosis and treatment information. 

  • Public Health Ontario will continue to accept ticks for identification from members of the public through health care providers. Ottawa Public Health does not accept ticks for identification. 

  1. Vannier E, Krause PJ. Human babesiosis. N Engl J Med. 2012;366(25):2397-2407. 
  2. Herwaldt BL, Linden JV, Bosserman E, Young C, Olkowska D, Wilson M. Transfusion-associated babesiosis in the United States: a description of cases. Ann Intern Med. 2011;155(8):509-519. 
  3. O'Brien SF, Drews SJ, Yi QL, et al. Risk of transfusion-transmitted Babesia microti in Canada. Transfusion. 2021;61(10):2958-2968.
  4. Fox LM, Wingerter S, Ahmed A, et al. Neonatal babesiosis: case report and review of the literature. Pediatr Infect Dis J. 2006;25(2):169-173. 
  5. Spielman A, Wilson ML, Levine JF, Piesman J. Ecology of Ixodes dammini-borne human babesiosis and Lyme disease. Annu Rev Entomol. 1985;30:439-460. 
  6. Mareedu N, Schotthoefer AM, Tompkins J, Hall MC, Fritsche TR, Frost HM. Risk Factors for Severe Infection, Hospitalization, and Prolonged Antimicrobial Therapy in Patients with Babesiosis. Am J Trop Med Hyg. 2017;97(4):1218-1225.
  7. Eisen RJ, Eisen L, Beard CB. County-Scale Distribution of Ixodes scapularis and Ixodes pacificus (Acari: Ixodidae) in the Continental United States. J Med Entomol. 2016;53(2):349-386. 
  8. Lindsay R, Artsob H, Barker I. Distribution of Ixodes pacificus and Ixodes scapularis re concurrent babesiosis and Lyme disease. Can Commun Dis Rep. 1998;24(15):121-122. 
  9. Clow KM, Leighton PA, Ogden NH, et al. Northward range expansion of Ixodes scapularis evident over a short timescale in Ontario, Canada. PLoS One. 2017;12(12):e0189393. 
  10. McPherson M, Garcia-Garcia A, Cuesta-Valero FJ, et al. Expansion of the Lyme Disease Vector Ixodes Scapularis in Canada Inferred from CMIP5 Climate Projections. Environ Health Perspect. 2017;125(5):057008. 
  11. Ogden NH, St-Onge L, Barker IK, et al. Risk maps for range expansion of the Lyme disease vector, Ixodes scapularis, in Canada now and with climate change. Int J Health Geogr. 2008;7:24. 
  12. Hahn MB, Jarnevich CS, Monaghan AJ, Eisen RJ. Modeling the Geographic Distribution of Ixodes scapularis and Ixodes pacificus (Acari: Ixodidae) in the Contiguous United States. J Med Entomol. 2016;53(5):1176-1191. 
  13. Burrows H, Talbot B, McKay R, et al. A multi-year assessment of blacklegged tick (Ixodes scapularis) population establishment and Lyme disease risk areas in Ottawa, Canada, 2017-2019. PLoS One. 2021;16(2):e0246484. 
  14. Nelder MP, Russell CB, Dibernardo A, et al. Monitoring the patterns of submission and presence of tick-borne pathogens in Ixodes scapularis collected from humans and companion animals in Ontario, Canada (2011-2017). Parasit Vectors. 2021;14(1):260. 
  15. White DJ, Talarico J, Chang HG, Birkhead GS, Heimberger T, Morse DL. Human babesiosis in New York State: Review of 139 hospitalized cases and analysis of prognostic factors. Arch Intern Med. 1998;158(19):2149-2154. 
  16. Hatcher JC, Greenberg PD, Antique J, Jimenez-Lucho VE. Severe babesiosis in Long Island: review of 34 cases and their complications. Clin Infect Dis. 2001;32(8):1117-1125.
  17. Krause PJ, Spielman A, Telford SR, 3rd, et al. Persistent parasitemia after acute babesiosis. N Engl J Med. 1998;339(3):160-165. 
  18. Conrad PA, Kjemtrup AM, Carreno RA, et al. Description of Babesia duncani n.sp. (Apicomplexa: Babesiidae) from humans and its differentiation from other piroplasms. Int J Parasitol. 2006;36(7):779-789. 
  19. Krause PJ, Telford S, 3rd, Spielman A, et al. Comparison of PCR with blood smear and inoculation of small animals for diagnosis of Babesia microti parasitemia. J Clin Microbiol. 1996;34(11):2791-2794. 
  20. Krause PJ, Telford SR, 3rd, Ryan R, et al. Diagnosis of babesiosis: evaluation of a serologic test for the detection of Babesia microti antibody. J Infect Dis. 1994;169(4):923-926. 
  21. Lehane A, Maes SE, Graham CB, Jones E, Delorey M, Eisen RJ. Prevalence of single and coinfections of human pathogens in Ixodes ticks from five geographical regions in the United States, 2013-2019. Ticks Tick Borne Dis. 2021;12(2):101637. 
  22. Moritz ED, Winton CS, Tonnetti L, et al. Screening for Babesia microti in the U.S. Blood Supply. N Engl J Med. 2016;375(23):2236-2245. 
  23. Wormser GP, Dattwyler RJ, Shapiro ED, et al. The clinical assessment, treatment, and prevention of lyme disease, human granulocytic anaplasmosis, and babesiosis: clinical practice guidelines by the Infectious Diseases Society of America. Clin Infect Dis. 2006;43(9):1089-1134. 
  24. Krause PJ, Lepore T, Sikand VK, et al. Atovaquone and azithromycin for the treatment of babesiosis. N Engl J Med. 2000;343(20):1454-1458. 
  25. Wilson CH, Gasmi S, Bourgeois A-C, Badcock J, Chahil N, Kulkarni MA, Lee M-K, Lindsay LR, Leighton PA, Morshed MG, Smolarchuk C, Koffi JK. Surveillance for Ixodes scapularis and Ixodes pacificus ticks and their associated pathogens in Canada, 2019. Can Commun Dis Rep 2022;48(5):208–18.


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Monday to Friday from 8:30 am to 4:30 pm: Call 613-580-2424, extension 24224, select your language of choice by pressing 1 or 2 and then leave a detailed, confidential message including your contact information.

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