Notwithstanding notable progress in medicine and biomedical science, parasitic diseases still represent a burden and a threat to human health. Among a myriad of parasitic diseases, those transmitted by vectors (mainly arthropods) play a significant role. Furthermore, such diseases are very prevalent in the poorest countries of the world, affecting an extensive portion of the human population, as well as posing a health hazard in developed countries.
One of these parasitoses is Babesiosis, an infectious diseases caused by the protozoan parasites of the genus Babesia. Already recognized for some time as pathogens that impose a substantial health burden on domesticated animals, Babesia parasites have been progressively ascertained over the last fifty years to be a cause of human morbidity around the globe.
In short, Babesia parasites infect vertebrate animals and humans, causing the lysis of red blood cells of the host. The disease is generally considered a zoonosis, since it is acquired by a tick bite when individuals unintentionally interact with the natural life cycle of the parasite. Nonetheless, Babesiosis transmitted via blood-transfusion represents a significant problem in highly endemic areas.
Life cycle of Babesia microti
The Hallmarks of Babesiosis
Infection with Babesia parasites in a human host occurs when the vector (i.e. an infected tick) takes the blood meal from an infected rodent. The parasite can enter the human’s body together with the tick’s saliva as it feeds on the human host. Once in the body, Babesias penetrate the erythrocytes or red blood cells, proliferate and then cause their lysis – resulting in a hemolytic anemia.
Even though the first case was characterized by a fatal outcome, Babesia infections range in clinical presentation from asymptomatic to severe forms of the disease. In any case, the severity of infection largely depends on the Babesia species and the underlying immune status of the affected host. Therefore many (otherwise healthy) people infected with Babesia parasites remain asymptomatic and do not exhibit any symptoms.
On the other hand, some individuals develop non-specific flu-like symptoms (such as fever, sweats, chills, body aches, headache, nausea, loss of appetite or unexplained fatigue). A life-threatening disease may emerge in people who do not have a spleen, have a weak immune system, have other serious health conditions, or in the elderly.
Most human cases of Babesiosis are due to Babesia microti species complex or to Babesia divergens, although other species (some of them newly described) are now emerging. The joint occurrence of Babesiosis and Lyme disease results in a more severe and protracted illness when compared with either of those infections alone.
In the diagnosis of Babesiosis, standard diagnostic techniques (investigation of Giemsa-stained thin blood smears and serologic tests) have been supplemented with modern molecular techniques, such as polymerase chain reaction (PCR). Current treatment recommendations for Babesiosis are focused on clindamycin and quinine as the drugs of choice, but certain novel drugs have shown some promise.
Epidemiology of the Disease
The constantly changing ecology has contributed significantly to the expansion of human Babesiosis in the United States (US), and also around the world. It is often considered an increasing problem due to the spread of tick habitats and the intensified mobility of animals, promoting in turn the dissemination of parasites to new geographical regions.
Since 1957, approximately forty cases of human infection with bovine Babesias (usually Babesia divergens) have been identified. A total of 84% of those European cases were patients that previously underwent splenectomy. Nevertheless, infection with Babesia divergens confirmed with PCR was also found in an immunocompetent adult patient from France, emphasizing the fact that this parasite may also cause illness in previously healthy people.
The highest number of human Babesiosis cases is found in the US, with Babesia microti being the predominant species (endemic in the Northeast and upper Midwest). In Washington State and California there were cases caused by Babesia duncani, and infections with parasites considered to be Babesia divergens were reported from Kentucky, Missouri and Washington State.
Human Babesiosis has also been found in Australia, Taiwan, Japan, Korea and India, and there were reports from Africa and South America. The disease also appears to be endemic on the China-Myanmar border. Serosurveys were also introduced as a useful technique to survey a population for babesial infection, which may result in finding this infection in other parts of the world as well.