A Clear and Present Danger: Tick-borne Diseases in Europe

Paul Heyman; Christel Cochez; Agnetha Hofhuis; Joke van der Giessen; Hein Sprong; Sarah Rebecca Porter; Bertrand Losson; Claude Saegerman; Oliver Donoso-Mantke; Matthias Niedrig; Anna Papa


Expert Rev Anti Infect Ther. 2010;8(1):33-50. 

In This Article


Initially described by Babes in 1888 in Romanian cattle,[123,124] babesiosis is a worldwide tick-borne hemolytic disease that is caused by intraerythrocytic apicomplexan protozoa of the genus Babesia (Apicomplexa, Sporozoea, Piroplasmida and Babesiidae).[123,125–127] The clinical expression of babesiosis varies from asymptomatic to acute and/or fatal infections depending on host factors, and on the species and virulence of the parasite involved.[127–129] The different species belonging to the genus Babesia are known to infect a large variety of animals[124,126] and some of them are zoonotic.[125,130] Furthermore, Babesia infection in cattle can induce animal mortality, ill-thrift, abortions, loss of milk/meat production, loss of draft power, extra expenses of control measures, a negative impact on international cattle trade and, sometimes, neurological symptoms.[131] Thus, babesiosis is not only recognized as a dangerous zoonosis but also as a significant economical hazard.[124,132,133] The rise in cattle international movement and the entry into the EU of several countries where the disease is prevalent could also worsen the existing problem.[133] The present review focuses on the zoonotic aspects of Babesia divergens, Babesia microti and Babesia venatorum (also called Babesia EU1).

Human infections by Babesia spp. remain infrequent.[133] Nevertheless, human babesiosis appears to be increasing in prevalence[134] and is considered an emerging tick-borne disease.[132] In Europe, most clinical cases of human babesiosis are caused by Babesia divergens.[124,127,133,135] The infected humans can present a fulminant, life-threatening disease.[129] Within 1–3 weeks postinfection, the individual presents fever (up to 40°C), weakness, fatigue, anorexia, gastrointestinal symptoms, myalgia, arthralgia, respiratory symptoms and headaches.[129,123,127] With progression of the destruction of the erythrocytes by the parasite, hemoglobinuria and/or jaundice develop.[129,123,127] Neurological symptoms can be observed.[129,123] Signs of CNS involvement include headache, photophobia, neck and back stiffness, altered sensorium and emotional lability.[123] Severe cases can result in renal failure and myocardial infarction. Congestive heart failure, disseminated intravascular coagulation and acute respiratory syndrome are frequently reported in complicated cases of babesiosis.[123,129] The lethality rate is estimated to be up to 42%.[129,136,137] Most human cases reported in Europe have occurred in asplenic patients and, after an acute febrile haemolytic disease, have led to death.[123,129,133,134,136] Advanced age, previous splenectomy and immunodefiency states are known to worsen the prognosis of infection.[123] The individuals considered at risk are farmers, foresters and other individuals who visit cattle raising areas, cattle being an important reservoir for B. divergens.[134]

The known vector of B. divergens is I. ricinus.[123,124,127,129,133] Therefore, the distribution of B. divergens is dependent upon the distribution of its tick vector.[133] Each lifecycle stage appears to be a competent vector for the transmission of B. divergens.[123,133,138] Larvae, present in large numbers and most active in the trough between the spring and autumn babesiosis cases, mainly feed on micromammals and birds.[139] Due to these feeding preferences, larvae are most probably of limited importance in the epidemiology of bovine and human babesiosis.[133] Nymphs generally feed on small-sized mammals but can also feed on larger mammals. Adult ticks preferentially feed on large ruminants such as cattle and deer.[138] With the promotion of extensive agricultural systems and climate change (prolonging the period of tick activity), infected tick populations are likely to increase in number. This increase in tick population raises the pressure of infection leading to an increase in the prevalence of clinical babesiosis.[133,140] Tick activity peaks in spring and autumn when the climatic conditions are the most favorable for tick reproduction.[133,135,138,141] This causes a similar bimodal seasonal distribution for disease occurrence.[141] Indeed, the first cases of babesiosis are usually diagnosed approximately 2 weeks after the peak of tick activity.[133] The 2-week lag phase represents, first, the period of time necessary for the tick to find an ideal host, to attach itself to the host and transmit the disease and, second, the incubation period of the disease in the host.[133]B. divergens and certain other Babesia spp. are maintained in the tick vector throughout the molts (transtadial transmission) and are transmitted from the engorged female to her numerous eggs (transovarial transmission). Moreover, B. divergens is capable of persisting in the tick population for as long as 4 years even in the absence of bovine hosts. A single tick bite is sufficient to transmit the disease to a susceptible host.[133]B. divergens is maintained in the environment by cattle, which have recovered from previous infections or carry mild subclinical infections, by cervids, caprids, other bovids and by rodents.[133] The main reservoir, however, is thought to be I. ricinus.

Babesia divergens is not a unique protozoon capable of infecting humans.[140] Indeed, several other Babesia spp. are known to cause human disease or are considered potential zoonotic agents.[142] Furthermore, I. ricinus ticks are not exclusively vectors for B. divergens. Indeed Babesia capreoli, B. venatorum and Babesia microti have been isolated in I. ricinus and could be transmitted to the host during the tick's blood meal.

B. capreoli infections seem limited to the roe deer and are not considered a zoonotic threat at present.[140,143] Reports on B. venatorum (EU1) infections in humans are rare but it has been isolated in splenectomized humans in Europe and must be included in the differential diagnosis of hemolytic anaemia.[140] Its zoonotic potential remains poorly documented. Babesia major has been isolated in cattle in Asia and in Europe. Clinically, this protozoon is not considered a threat to cattle or to humans. However, in a Chinese study by Yin et al.,[144] experimental infections using ticks infected by B. major were performed and lead to clinical illness in cattle.[140] To our knowledge, no human cases of B. major infection have been reported in Europe.[140]

Babesia microti, essentially a rodent protozoon, causes a less acute illness than B. divergens in Europe.[142] Elderly and HIV-positive patients are the main population at risk of developing an acute and potentially fatal infection.[134] However, this protozoon has also been isolated in normal hosts.[134,145] In Europe, infections by B. microti are considered less frequent than could be expected. Indeed, the prevalence of B. microti infection in rodents is high.[134] This could be due to the vector of B. microti, Ixodes trianguliceps, a nest-dwelling tick that does not frequently bite humans.[134] However B. microti has also been isolated in I. ricinus ticks collected from vegetation.[127] In Europe, despite serological evidence of human infections, several authors remain skeptical concerning the ability of B. microti to cause human disease.[127] In the USA, B. microti is recognized as the main cause of babesiosis in humans and has been associated with asymptomatic to severe and fatal clinical cases.[134]B. bovis, B. canis and Babesia of unknown species have been isolated in humans in Europe[134] but their respective importance is unknown.

Babesiosis is an important emerging disease for veterinary, commercial and medical reasons. Its potential for future geographical expansion and development in previously exempt areas around the world renders babesiosis an international threat. Increased awareness of the disease and of its differential diagnosis must be promoted in the veterinary and medical professions but also in the agricultural and general population. Early diagnosis of babesiosis infections would allow the implementation of specific therapy to prevent the potentially disastrous consequences of this tick-borne disease.