Tick-Borne Diseases: Are You at Risk? New Insights into Ehrlichia ewingii Infections
"A recent study reveals a higher prevalence and wider geographic distribution of Ehrlichia ewingii in the United States, raising concerns about underreported cases of tick-borne illnesses."
Ehrlichiosis is a bacterial infection transmitted to humans through the bite of infected ticks. In the United States, Ehrlichia chaffeensis has long been recognized as the primary culprit. However, a recent study has uncovered a concerning trend: a higher rate of infection from another species, Ehrlichia ewingii, than previously reported.
The study, which retrospectively analyzed samples from an 18-month investigation of ehrlichiosis cases across the United States, found that Ehrlichia ewingii was present in 9.2% of the cases, which is 10 out of 109 individuals. What’s more, two of these patients resided in states where cases of Ehrlichia ewingii had not previously been documented: New Jersey and Indiana.
These findings suggest that Ehrlichia ewingii may be more prevalent and have a wider geographic distribution in the U.S. than previously understood. All patients with available case histories recovered, so there is no need to panic. But, this does underscore the importance of understanding the risks associated with tick bites and knowing how to protect yourself.
Ehrlichiosis Data and Impact
CDC case data from 2000–2022 show seasonality in ehrlichiosis and identify at-risk states and populations. The CDC also provides data on ticks, tick bites, and tick-borne diseases. A 2021 NNDSS table displays provisional cases of Ehrlichia ewingii infection and other notifiable diseases in the United States, U.S. territories, and among non-U.S. residents; the CDC noted that data for some jurisdictions could be incomplete for week 7 because of processing issues.
From Rickettsia to Ehrlichia
Ehrlichial organisms are transmitted during tick feeding through tick saliva. ScienceDirect describes Ehrlichia ewingii as targeting neutrophils and occasionally eosinophils, whereas several other Ehrlichia species show tropism for monocytes and macrophages. Ehrlichial disease was first reported in South Africa in the 19th century, recognized as tick-borne in the early 1900s, and the organism was initially classified as Rickettsia ruminantium before later being placed in the genus Ehrlichia.
Ehrlichia Ewingii: Understanding the Risks and Symptoms
Ehrlichiosis, in general, is a zoonotic disease, meaning it spreads from animals to humans. In the United States, three Ehrlichia species are known to cause the disease: Ehrlichia chaffeensis, Ehrlichia ewingii, and a less common E. muris-like pathogen. A related organism, Anaplasma phagocytophilum, causes Anaplasmosis, which has a similar clinical presentation to ehrlichiosis.
- Fever: A high temperature is one of the most common symptoms.
- Chills: Shivering and feeling cold, even when it's not cold.
- Headache: Persistent or severe headaches.
- Myalgia: Muscle aches and pains.
- Malaise: A general feeling of discomfort, illness, or lack of well-being.
- Thrombocytopenia: A decrease in the number of platelets in the blood, which can lead to bleeding problems.
- Leukopenia: A decrease in the number of white blood cells in the blood, which can increase the risk of infection.
- Elevated Liver Enzymes: Increased levels of liver enzymes, indicating liver damage.
Expanding Recognition of E. ewingii
A CDC Emerging Infectious Diseases study found that all 10 E. ewingii/E. canis-positive cases were subsequently identified as E. ewingii, representing 10 of 109 ehrlichiosis cases during the study period, or 9.2%. A later national surveillance report notes that the ehrlichiosis case definition was revised in 2024 to require molecular methods for identifying and classifying specific Ehrlichia species. Recent review material also characterizes Ehrlichia and Anaplasma as clinically and economically significant tick-borne obligate intracellular bacteria affecting humans and animals.
Delayed Recognition and Severe Disease
Ehrlichiosis and anaplasmosis were identified as important human health threats in a 2021 Tick-Borne Disease Working Group subcommittee report, but the infections had not received focused discussion in the group's first report. A 2024 study states that ehrlichiosis can progress to central nervous system involvement, or neuro-ehrlichiosis, particularly when diagnosis and treatment are delayed. Another analysis reports that U.S. ehrlichiosis cases increased more than eightfold since reporting began in 2000 and identifies Ehrlichia chaffeensis, E. ewingii, and E. muris eauclairensis as closely related causes of human infection.
Timing and Clinical Contrast
The CDC reports that fever, chills, and potentially severe ehrlichiosis generally begin 1–2 weeks after a tick bite. A comparative clinical resource describes ehrlichiosis as primarily associated with Ehrlichia chaffeensis and the Lone Star tick, while noting that it often resembles influenza and rarely causes an early rash compared with Lyme disease. That source also reports that ehrlichiosis is highly treatable with doxycycline.
Protecting Yourself from Tick-Borne Diseases
The rise of Ehrlichia ewingii infections highlights the importance of taking preventive measures against tick bites. Here are some practical steps you can take:
Shared Risks Across Species
A serological survey reports that canine vector-borne diseases are prevalent in the United States, Canada, and the Caribbean. The survey lists Ehrlichia ewingii alongside Anaplasma phagocytophilum, Anaplasma platys, Ehrlichia canis, Ehrlichia chaffeensis, and Borrelia burgdorferi among pathogens that infect dogs and humans. It also notes that changing tick distributions affect this broader disease landscape.
Emerging Tick-Borne Threats
A 2026 Frontiers in Microbiology review describes ticks as important vector arthropods capable of carrying and transmitting diverse pathogenic microorganisms. It frames these organisms as a serious threat to global public health. The review covers established and emerging pathogens, including Lyme disease-related Borrelia, Rickettsia, Babesia, severe fever with thrombocytopenia syndrome virus, and tick-borne encephalitis virus.
Co-Infection and Evidence Gaps
A review of tick-borne co-infections in Lyme disease synthesizes evidence on epidemiology, clinical impact, diagnostic limitations, and treatment approaches. It emphasizes that co-infections can complicate clinical understanding and management. The review also identifies critical evidence gaps and calls for further research to improve patient outcomes.
Ehrlichiosis in Human History
An ASCLS case study reports that several tick-borne diseases, including babesiosis, Lyme disease, and ehrlichiosis, were identified across the United States near the end of the 20th century. It states that ehrlichiosis was initially recognized in 1935. The disease is caused by bacterial species in the genus Ehrlichia.