Introduction

Babesiosis is a less well-known parasitic disease transmitted mainly by tick bites usually the hard-bodied deer tick (Ixodes scapularis) and other less common methods of transmission include through blood transfusions or through pregnancy from mother to fetus.¹ Certain species of the Babesia parasite are responsible for causing flu-like symptoms in humans, these include Babesia divergens, Babesia venatorum, Babesia microti and there are other types of species that can affect cattle or domestic pets.² The Babesia parasite in humans is a protozoan found in red blood cells, the severity of symptoms, and the ability to destroy these red blood cells is affected by certain risk factors, which make an individual more susceptible to acquiring  Babesiosis. 

It is important to know these risk factors, even though it is a globally rare disease, in certain regions it is considered endemic and cases can be fatal.³ In this article, we’ll explore these risk factors and discuss how to recognize and prevent babesiosis. 

Risk factors 

Host-related risk factors

Age 

Although Babesiosis can be acquired in any age group, individuals over the age of 50 are at increased risk as well as in newborns. This is because these populations often have a weaker immune system and in over 50s, individuals are more likely to be immunocompromised and can have other comorbidities, which makes them more likely to have a severe and progressive course of the disease.

Although there have been reports of cases within children, the disease is often milder.⁴ Severe symptoms associated with Babesiosis in older age individuals include severe hemolytic anaemia, jaundice, low blood pressure, disseminated intravascular coagulation, and failure of organs that can lead to death.^{5, 6}

Sex

There is some data to support the theory that people assigned male at birth (AMAB) often receive a greater number of diagnoses than people assigned female at birth (AFAB). The reason behind this is that people assigned male at birth (AMAB) are more often exposed to tick-infested areas due to their jobs. For example, the primary people assigned male at birth (AMAB) populated the agriculture and farming industry.⁴

Human activities 

As mentioned, working within the agriculture and farming industry can put you at increased risk of Babesiosis, but this applies to any outdoor recreational activities within a tick habitat including camping, hiking, or gardening.⁴

Immunocompromised individuals 

Individuals with weakened immune systems are more at risk of Babesiosis; this not only includes the elderly but also individuals with Human Immunodeficiency Virus (HIV) and  Acquired Immunodeficiency Syndrome (AIDS). These are viruses which reduce the number of CD4+T cells in the immune system, making an individual more vulnerable to diseases like Babesiosis.⁷ Other forms of immunocompromised status include corticosteroid therapy, cancer, and splenectomy.⁸

Splenectomy is the procedure in which the spleen is removed. The spleen is an organ which makes white blood cells necessary to fight the parasitic infections and so without it the risk of getting Babesiosis and the symptoms are more severe. Therefore, if you do not have a spleen, prevention of tick-borne diseases is crucial.⁹

Epidemiological risk factors

Geographic distribution

Around the world, there are global hotspots where Babesiosis is most prevalent, this includes Northeastern and upper Midwest states of the United States of America, also including areas of New England, New York, and Wisconsin. Babesiosis is reported as endemic in these states and so people are at higher risk here. The reason behind these high cases is that they have a large number of hosts for the Ixodes ticks.

Hosts include deers and small vertebrates like white-footed mice, which live in these regions with susceptible climates, grassy woodlands and landscapes and due to their close proximity to humans can easily transmit the disease via the tick vector. The disease can also be found in other parts of the world like Europe and Asia, but the largest population of hosts for this disease is within the USA.¹  

In the USA and southwestern China, the most common species is Babesia microtia. In northwestern China, the main species is Babesia crassa-like pathogen and Babesia venatorum, and in Europe,e it is Babesia divergens. The habitats for ticks can be suburban or rural areas but they are all associated with woodlands and tall grassy areas.⁴ 

Seasonal patterns

As well as the location, there is an increased risk of tick transmission of Babesiosis during the late spring and summer months. With more humans spending time outside during these months, there is closer proximity to deer and other hosts and therefore increased risk of tick transmission.¹⁰

Tick-related risk factors

Although there are many types of Babesia ticks, it is the black-legged or deer tick (Ixodes scapularis) that pose the highest risk of human babesiosis. These ticks act as vectors, first feeding on the blood of either rodents or deer at various stages of their lifecycle and transmitting by biting a human.¹¹

The success of transmitting Babesia by a tick depends on: 

• The amount of parasite taken in by the feeding tick is especially significant in the last 24 hours of its life 
• If the tick develops in a parallel life cycle to the parasite 
• If a tick is at its nymphal stage as here they are small and more likely to go undetected 
• If the tick remains attached to the host for around 36 to 48 hours¹²

Blood transfusion and organ transplantation

Although not as common, Babesiosis can be transmitted through blood transfusions and organ transplantations. It has been identified as a health concern and more than 170 cases of this mode of transmission have been identified. In the USA, the primary pathogen in blood transfusion is Babesia microti and unlike tick-borne transmission, transfusion-related diseases can occur all year round.¹

Universally, blood transfusions are routinely screened for infectious diseases, but due to their unknown nature, Babesiosis is not included. It can also be transmitted through blood transfusions or transplacental transmission in neonates and, although the overall risk is low in about ⅕ of cases in the population, they have a lethal outcome.¹ 

Regulatory bodies like the FDA have issued recommendations and have identified risk factors for donors who receive blood transfusions and organ transplantations, these include: 

• Screening blood using antibody and nucleic acid amplification testing (NAT) in the global hotspots for Babesiosis e.g. Northeastern and upper Midwest states of the USA 
• There is a higher risk of acquiring babesiosis from red blood cell transfusions than plasma transfusions^3,5

In blood transfusions and organ transplantations, the risk of acquiring Babesiosis is very low but the screening process is very important to prevent it from occurring.³

Summary 

Babesiosis is a tick-borne parasitic disease transmitted mainly by hard-bodied deer ticks (Ixodes scapularis) as well as less commonly through blood transfusions or through pregnancy from mother to foetus.¹ It is associated with many risk factors which increase its prevalence in certain regions of the world which include Northeastern and upper Midwest states of the United States of America, parts of Asia, and Europe.⁴ The risk factors include increased age, male sex, outdoor activities, and an immunocompromised status.¹ 

There is also an increased risk of acquiring Babesiosis associated with an individual's environment and climate as it occurs in grassy woodlands and landscapes where hosts and tick vectors thrive.⁴ There are also tick-related risk factors as the black-legged or deer tick has the highest risk of transmission to humans. It is also dependent on its lifecycle stage and how long it attaches to a human.¹²

The other route of transmission is through blood transfusions and organ transplantations which has a low risk, but risk factors include blood transfusions of red blood cells from endemic areas.¹  

Currently, there are no vaccines for Babesiosis so prevention is necessary, especially for the immunocompromised. Public health measures for reducing cases of  Babesiosis include the use of tick repellents before being exposed to areas which have a high activity of ticks, thorough daily tick checks after exposure and an understanding of how they should be removed, wearing protective clothing, minimising the time spent outdoors or in some areas government bodies often introduce the use of pesticides to control tick habitats.⁸ Awareness of risk factors can aid in reducing the likelihood of acquiring Babesiosis and preventing fatal outcomes.¹³