Health Alert: Human Vector-Borne Diseases

Summary and Action Items

West Nile virus (WNV), Lyme disease, and Rocky Mountain spotted fever (RMSF), are the most commonly reported human vector-borne diseases (VBDs) in Illinois. As summer approaches and more people are inclined to engage in outdoor activities, tick and mosquito vectors are also becoming more active, increasing the risk of disease transmission to humans. IDPH sends this health alert to implore health care providers (HCPs) to consider and test for VBDs in their differential diagnoses. This alert addresses the following items:

  1. Examine common exposures and provide additional details on the regional distribution
    of common VBDs.
  2. Consider reporting requirements, such as the significance of exposure and travel
  3. Update on the new laboratory tests recommended for Lyme disease diagnosis.

As the temperature rises, mosquitoes and ticks become more active, increasing risk for human exposure and infection. Provisional case counts for the most commonly reported VBDs in Illinois during 2021 are as follows: WNV (61), Lyme disease (526), and RMSF (18). Reported cases of Lyme disease have been on a steady incline for several years with a notable jump from 276 reported cases in 2018 to 395 in 2019. There was a dip in reported cases in 2020 (295) from 2019 (395), possibly because many people stayed home during the height of the COVID-19 pandemic and did not seek non-emergency medical attention. A jump from 276 reported cases in 2018 to 526 provisionally reported cases in 2021 accounts for a 90.6% increase in reported Lyme disease cases in the span of 3 years. In Illinois, rapid increases have not been observed with other reported VBD cases during this time span.

Potential Exposures

Ticks and mosquitoes have distinct habitats, although both vectors are most active during the warmer months until the temperatures drop below freezing. Below are links to websites that outline the geographic distributions of common VBDs.


In many cases the symptoms of VBDs are similar and include fever, rash, headache, arthralgia, myalgia, and neurologic symptoms. VBDs may have highly distinct symptoms that are not seen in other illnesses. The following is a list of all the VBDs that must be reported to the local health department (LHD), along with CDC links to the symptoms page:
Tickborne Diseases: Anaplasmosis, Babesiosis, Ehrlichiosis, Lyme disease, RMSF, other spotted fever group rickettsioses, Powassan virus, Heartland virus, and Bourbon virus.

Mosquito-borne Diseases: California serogroup virus diseases (includes California encephalitis, Jamestown Canyon, Keystone, La Crosse, Snowshoe hare, and Trivittatus), Chikungunya, Dengue, Eastern Equine Encephalitis, Malaria, St. Louis encephalitis, West Nile virus, and Yellow Fever.


The bite of an infected mosquito or tick transmits VBDs to humans. Blood transfusion, organ or tissue transplant, and possibly laboratory exposure are other possible modes of transmission. Zika can be passed from a pregnant woman to her baby during or shortly after birth.


IDPH Labs do not perform testing of VBD specimens, with the exception of Malaria, primarily when the specimen has not been speciated. Healthcare providers must contact their LHDs to request authorization prior to Malaria specimen submission to the Springfield IDPH Laboratory.
Testing for other VBD pathogens should be done via commercial labs. In select cases where the infecting pathogen cannot be determined, e.g., due to high cross-reactivity, testing of acute and convalescent-phase VBD specimens can be performed at CDC. Physicians should consider testing for Heartland and Bourbon viruses at CDC if patients who have been treated for tick-borne illness show no clinical response. Testing of tick-borne pathogens at CDC requires IDPH to obtain prior approval from CDC, so LHDs should contact IDPH CD VBD program staff to facilitate testing approval and specimen submission.

LHDs should visit the 2022 Lyme Disease Case Definition Page to review updated criteria for reporting cases with symptom onset dates beginning January 1, 2022. It should be noted that a physician diagnosis is no longer accepted in the case definition and Borrelia mayonii (B. mayonii), a new pathogen that causes Lyme disease, has been added to two of the 2022 lab testing criteria detailed below.

  • Western Blot (immune blot) test positive for Borrelia burgdorferi (B. burgdorferi)-specific IgM or IgG;
  • Antibody to B. burgdorferi detected by EIA or IFA;
  • Detection of B. burgdorferi or B. mayonii in a clinical specimen by a B. burgdorferi group-specific NAAT assay – this is a PCR test;
  • Detection of B. burgdorferi group-specific antigens by immunohistochemical assay on biopsy or autopsy tissues;
  • Isolation of B. burgdorferi or B. mayonii in culture.


Preventing tick and mosquito bites is the greatest approach to avoid contracting VBDs. For more info, please visit the CDC Division of Vector-Borne Diseases website.

IDPH and LHD Response

Healthcare professionals suspecting a VBD in their patients should contact their LHD to report cases using the steps highlighted in the Communicable Disease Code, Section 690.200, and to discuss consideration of confirmation testing at IDPH or CDC. In addition to full patient demographic information and clinical information, travel history (with departure and return dates) as well as exposure history should be reported as described below to respective LHDs.

Important exposure history information includes the following:
Was the patient in a potential or known tick habitat (wooded, brushy, grassy area)?
Where was the tick habitat? (provide address, including city and state)
What type of tick habitat was the patient in? (park, campground, forest preserve, etc.)
Collect all dates up to 6 months prior to illness onset when patient was in the tick habitat. Note: some symptoms may take up to 6 months to appear.


For additional information regarding human vector-borne disease surveillance, local health departments should contact Debbie Freeman ( or Jonathan Popovitch ( at IDPH Communicable Disease Control Section.

Additional Resources

For additional information on vector-borne diseases, visit the following links:
CDC Tickborne Diseases of the United States, A Reference Manual for Healthcare Providers
CDC Division of Vector-Borne Diseases (DVBD) A-Z Topics Index

Target Audience

Local Health Departments, Infectious Disease Physicians, Family Practice and Internal Medicine Physicians, Pediatricians, Geriatric Physicians, Nurse Practitioners, Physician Assistants, Hospital Emergency Departments, Infection Control Preventionists, and Laboratories

Date Issued

May 2, 2022


Communicable Disease Control Section

Adult Health Topics