Introduction

Babesiosis is a zoonotic disease, predominantly tick-borne, caused by protozoan parasites of the genus Babesia, most commonly Babesia microti. B. duncani, B. divergens, B. venatorum, and B. crassa are the other species known to infect humans. These parasites target red blood cells, leading to an infection that manifests as mild flu-like illness to severe, life-threatening conditions.¹ Understanding the epidemiological landscape and geographic distribution of babesiosis is imperative for public health leaders to concentrate efforts on mitigating the spread of this infectious disease, particularly in vulnerable populations. 

Epidemiology of babesiosis

Incidence and prevalence

Although there has been a sizable increase in babesiosis cases around the globe, certain areas have reported notable spikes. The Centers for Disease Control and Prevention (CDC) in the States has documented a rise in cases in the Northeast and upper Midwest particularly, among other parts.² Europe has also reported a spike, particularly in regions with a high tick population. In the past decade, cases have grown significantly according to the latest statistics reflecting both an actual increase in disease occurrence and improved surveillance capabilities.

The occurrence of Babesiosis differs over the world, with Europe, Asia, and Australia reported as endemic regions and rarely there have been incidents reported in South America and Africa. On a global level, the prevalence is still underreported in many regions due to inadequacy of awareness and diagnostic problems.³

Risk factors

Several factors contribute to the risk of acquiring babesiosis:^2,4

Changing geographic location

Tick habitat is quite fairly influenced by changing climate conditions, extending their range into new areas. The disease risk zone is expanding mainly due to the increased life span and activity of the vector ticks as warmer winters and longer summers make way.

Increased human-tick contact

The creation of rural settlements and changes in land usage, such as deforestation, have contributed to a higher level of engagement with tick habitats. Activities like hiking, camping, and gardening in tick-prone areas have risen, elevating exposure risks.

Pet ownership

The increased exposure to ticks via pets, particularly dogs, has been underestimated as pets can bring ticks into homes.

High-risk groups

Individuals at higher risk for severe babesiosis include:⁴

Immunocompromised individuals

Individuals with a compromised defence mechanism are often more vulnerable to severe infection and complications. HIV/AIDS, cancer, or immunosuppressive therapies are the most common conditions that weaken the immune system.²

Elderly

Older adults are more likely and easily prone to severe infections and complications.

People without a spleen

Asplenic individuals are more susceptible to chronic infections as the spleen plays a critical role in filtering parasites from the blood.

Modes of transmission

Tick-borne transmission

The nature of the tick lifecycle and behavioural pattern makes nymphal ticks more efficient in the transmission of the disease. This is further complemented by the small size and greater prevalence in peak transmission seasons.¹

Blood transfusions

Transfusion-related disease prevalence remains the second leading cause of transmission despite active efforts to implement effective blood screening processes. This mode of transmission is under more scrutiny, leading to blood banks employing rigorous testing protocols.⁵

Congenital transmission

The transmission of the parasites from an infected mother to her fetus during pregnancy is a rare occurrence but a possible mode of transmission as recent reports have highlighted. This has necessitated healthcare providers to become more vigilant, demanding further research in the area.⁶

Coinfections

Babesiosis cases frequently occur in tandem with Anaplasmosis and Lyme disease, the two other tick-borne illnesses. This coinfection scenario presents complications in clinical presentations and treatment, necessitating comprehensive diagnostic approaches. 

Diagnostic challenges

Symptoms overlap

Diagnosing babesiosis remains a challenge due to its symptoms' resemblance to those of other feverish infections. Healthcare professionals must receive enhanced training on babesiosis recognition.

Laboratory diagnosis

While advancements in diagnostic technologies, such as high-sensitive polymerase chain reaction (PCR) and improved serological tests have enhanced detection capabilities, they have also highlighted the complexities of chronic and subclinical infections.

Geographic distribution

Geographic representation of human babesiosis where main regions of transmission and prevalence are shown on the map. The text further down lists additional locations where human babesiosis has been documented but which are not depicted in the figure. Solid colours indicate human babesiosis is endemic. Transparent areas are those where there have been at least ten recorded cases of babesiosis-more like sporadic. Babesia microti (red), Babesia venatorum (purple), Babesia duncani (green), and Babesia divergens (yellow) are the etiologic agents' distinct colours.

United States

In the U.S., the Northeast and upper Midwest remain to be the regions most commonly reporting Babesiosis. States with high incidence rates include Connecticut, Massachusetts, New York, New Jersey, Rhode Island, Wisconsin, and Minnesota. Favourable tick habitats and the presence of wildlife reservoirs have resulted in specific regions like Nantucket Island, Martha's Vineyard, and Long Island, having high transmission rates.⁷

Europe

Babesiosis cases have been reported in several European countries, including Ireland, the United Kingdom, France, Spain, Portugal, Italy, Switzerland, Germany, and Sweden. Babesia divergens is the primary species causing babesiosis in Europe, which is known to affect cattle before occasionally infecting humans.⁸

Other regions³

• Asia: Cases have been reported in China, Japan, Taiwan, and Korea, with variations in Babesia species and tick vectors
• Australia: Babesia microti and Babesia duncani have been identified in this region but much has not been established
• South America and Africa: Although there have not been any reports of significance, the presence of babesiosis in these continents highlights the need for broader surveillance and awareness

Epidemiologic tools for surveillance of babesiosis

In the public health landscape, several epidemiologic tools have been employed to monitor and track the distribution and determinants of babesiosis. The role played by these tools becomes critical in that they assist policymakers and health providers make informed decisions on prevention and control measures.³

Case surveillance

Case surveillance is a cornerstone of epidemiologic monitoring, especially when we speak of the United States with such a high prevalence. Systematic data collection not only helps spot disease hotspots and monitor a particular trend over time but also makes that stretch to evaluate the effectiveness of intervention strategies. Proactive surveillance efforts, such as integrating electronic health records and real-time data monitoring, have proven to be accurate and promise improved data collection.

Serosurveys

Serosurveys is the screening of populations for antibody detection against Babesia spp. These surveys assist in quantifying disease exposure both in symptomatic and asymptomatic cases as a result of past infection. By analysing blood samples from diverse populations, public health officials can identify regions with high transmission rates and assess the effectiveness of preventive measures. Serosurveys are particularly useful in identifying subclinical infections that may not be captured through routine case surveillance.

Ecological studies

Surveillance of tick vectors and mammalian hosts is essential for understanding the risk of babesiosis transmission. Ecological studies involve sampling ticks and reservoir hosts (such as rodents) to detect the presence of Babesia spp. Using techniques like PCR, culture, and serology. These studies provide indirect estimates of human infection risk and can serve as early warning systems for emerging tick-borne diseases. For instance, monitoring tick populations in endemic areas can help predict seasonal peaks in human babesiosis cases.

Genomics

Genomic studies of Babesia spp. Enhance our understanding of the pathogen’s genetic diversity, evolution, and mechanisms of virulence. Advances in genomic sequencing have enabled researchers to map the genomes of Babesia strains, identify genetic markers associated with drug resistance, and develop molecular diagnostics. Genomic data also support the development of vaccines and targeted therapies, contributing to more effective disease control strategies.

Mathematical modeling

Mathematical models are powerful tools for simulating the dynamics of babesiosis transmission and evaluating the impact of various control measures. These models can incorporate data on tick ecology, host behaviour, climate conditions, and human demographics to predict disease spread and assess the potential outcomes of intervention strategies. For example, models can estimate the effects of tick control programs, public education campaigns, and vaccination efforts on reducing the incidence of babesiosis.

Visual representation of epidemiologic data

The following graphical representation of epidemiological data better conveys the different aspects of babesiosis in the United States and the developments between 2011 and 2020. The graphs have been generated from CDC reports that aim to provide a comprehensive overview of the condition and its demographics:

• Incidence per 100,100 (2011 vs 2019): A stacked bar graph indicating Vermont, Maine, and New Hampshire experiencing the largest per cent change in incidence between 2011 and 2019. They reported a 1,602%, 1,422% and 372% increase in incidence respectively
• Reported cases of babesiosis (2011-2020): A line graph displaying the trend in reported cases over the past decade, showing fluctuations and overall increases
• Symptoms and signs: A bar graph showing the percentage of signs/symptoms reported among case patients from whom data was available
• Race and ethnicity distribution of babesiosis cases: A pie chart detailing the racial and ethnic breakdown of reported cases, indicating the majority as white non-Hispanic

This data provides valuable insights into the epidemiology of babesiosis and reiterates the importance of continued vigilance and research to mitigate the burden of this disease.

Summary: Epidemiology and geographic distribution of babesiosis

Babesiosis is a zoonotic disease caused by protozoan parasites of the Babesia genus, often transmitted by ticks, particularly Babesia microti. It affects red blood cells, leading to symptoms ranging from mild flu-like illness to severe, life-threatening conditions. The disease is most common in the U.S., Europe, and parts of Asia, with notable increases in the Northeast and upper Midwest of the U.S. and certain European regions.

Risk factors for acquiring babesiosis include changing climates, increased human-tick contact, and pet ownership. High-risk groups include immunocompromised individuals, the elderly, and those without a spleen. Transmission occurs primarily through tick bites but can also happen via blood transfusions and, rarely, congenital transmission.

Diagnostic challenges arise due to symptom overlap with other infections and the complexity of chronic cases. Infected areas include the U.S., parts of Europe, Asia, and Australia, with underreported cases in South America and Africa.

Epidemiologic tools like case surveillance, serosurveys, ecological studies, and mathematical modelling are essential for monitoring the spread of babesiosis. These tools help guide preventive measures and provide insights into transmission patterns. Data shows a significant increase in cases in regions like Vermont, Maine, and New Hampshire, highlighting the importance of ongoing surveillance and research to manage and prevent the disease.