Chemotherapy against babesiosis

Babesiosis is caused by a haemotropic protozoal parasite of the genus Babesia, member of the phylum Apicomplexa and transmitted by the bite of an infected tick. There are many Babesia species affecting livestock, dogs, horses and rodents which are of economic significance. Infections can occur without producing symptoms, but babesiosis may also be severe and sometimes fatal caused by the intraerythrocytic parasite development. The disease can cause fever, fatigue and haemolytic anemia lasting from several days to several months. There are a number of effective babesiacides, but imidocarb dipropionate (which consistently clears the parasitaemia; often the only available drug on the market) and diminazene aceturate are the most widely used. Some Babesia spp. can infect humans, particularly Babesia microti and Babesia divergens, and human babesiosis is a significant emerging tick-borne zoonotic disease. Clinical manifestations differ markedly between European and North American diseases. In clinical cases, a combination of clindamycin and quinine is administered as the standard treatment, but also administration of atovaquone-azithromycin is successful. Supportive therapy such as intravenous fluids and blood transfusions are employed when necessary. More specific fast-acting new treatments for babesiosis have now to be developed. This should be facilitated by the knowledge of the Babesia spp. genome and increased interest for this malaria-like parasite.

First confirmed autochthonous case of human Babesia microti infection in Europe

A 42-year-old female patient with acute myeloid leukemia presented with fever and heavy chest pain after her first cycle of specific chemotherapy. Acute myocardial infarction was excluded, but surprisingly, parasitic inclusions in erythrocytes became obvious in Pappenheim and Giemsa-stained peripheral blood smears. The patient did not remember a tick bite but acknowledged having received several blood transfusions in her recent medical history. Suspicion of malaria was ruled out by use of a dip-stick test. The diagnosis of Babesia microti infection was finally established by specific polymerase chain reaction (PCR). Six weeks after initiation of specific treatment, PCR turned negative and a positive immunoflourescence assay (IFA) with an IgG titer of 1:128 indicated seroconversion. Subsequent screening of donors involved in the transfusion of blood products to the patient demonstrated borderline reactivity for Babesia microti (IgG-titer 1:32) in 1 out of 44 individuals. Neither the patient nor the positively tested blood donor had travelled to North America or Asia. Therefore, this is the first confirmed autochthonous human infection in Europe.

Pathogen transmission in relation to duration of attachment by Ixodes scapularis ticks

The blacklegged tick, Ixodes scapularis, is the primary vector to humans in the eastern United States of the deer tick virus lineage of Powassan virus (Powassan virus disease); the protozoan parasite Babesia microti (babesiosis); and multiple bacterial disease agents including Anaplasma phagocytophilum (anaplasmosis), Borrelia burgdorferi and Borrelia mayonii (Lyme disease), Borrelia miyamotoi (relapsing fever-like illness, named Borrelia miyamotoi disease), and Ehrlichia muris eauclairensis (a minor causative agent of ehrlichiosis). With the notable exception of Powassan virus, which can be transmitted within minutes after attachment by an infected tick, there is no doubt that the risk of transmission of other I. scapularis-borne pathogens, including Lyme disease spirochetes, increases with the length of time (number of days) infected ticks are allowed to remain attached. This review summarizes data from experimental transmission studies to reinforce the important disease-prevention message that regular (at least daily) tick checks and prompt tick removal has strong potential to reduce the risk of transmission of I. scapularis-borne bacterial and parasitic pathogens from infected attached ticks. The most likely scenario for human exposure to an I. scapularis-borne pathogen is the bite by a single infected tick. However, recent reviews have failed to make a clear distinction between data based on transmission studies where experimental hosts were fed upon by a single versus multiple infected ticks. A summary of data from experimental studies on transmission of Lyme disease spirochetes (Bo. burgdorferi and Bo. mayonii) by I. scapularis nymphs indicates that the probability of transmission resulting in host infection, at time points from 24 to 72 h after nymphal attachment, is higher when multiple infected ticks feed together as compared to feeding by a single infected tick. In the specific context of risk for human infection, the most relevant experimental studies therefore are those where the probability of pathogen transmission at a given point in time after attachment was determined using a single infected tick. The minimum duration of attachment by single infected I. scapularis nymphs required for transmission to result in host infection is poorly defined for most pathogens, but experimental studies have shown that Powassan virus can be transmitted within 15 min of tick attachment and both A. phagocytophilum and Bo. miyamotoi within the first 24 h of attachment. There is no experimental evidence for transmission of Lyme disease spirochetes by single infected I. scapularis nymphs to result in host infection when ticks are attached for only 24 h (despite exposure of nearly 90 experimental rodent hosts across multiple studies) but the probability of transmission resulting in host infection appears to increase to approximately 10% by 48 h and reach 70% by 72 h for Bo. burgdorferi. Caveats to the results from experimental transmission studies, including specific circumstances (such as re-attachment of previously partially fed infected ticks) that may lead to more rapid transmission are discussed.

Life history and demographic drivers of reservoir competence for three tick-borne zoonotic pathogens

Animal and plant species differ dramatically in their quality as hosts for multi-host pathogens, but the causes of this variation are poorly understood. A group of small mammals, including small rodents and shrews, are among the most competent natural reservoirs for three tick-borne zoonotic pathogens, Borrelia burgdorferi, Babesia microti, and Anaplasma phagocytophilum, in eastern North America. For a group of nine commonly-infected mammals spanning >2 orders of magnitude in body mass, we asked whether life history features or surrogates for (unknown) encounter rates with ticks, predicted reservoir competence for each pathogen. Life history features associated with a fast pace of life generally were positively correlated with reservoir competence. However, a model comparison approach revealed that host population density, as a proxy for encounter rates between hosts and pathogens, generally received more support than did life history features. The specific life history features and the importance of host population density differed somewhat between the different pathogens. We interpret these results as supporting two alternative but non-exclusive hypotheses for why ecologically widespread, synanthropic species are often the most competent reservoirs for multi-host pathogens. First, multi-host pathogens might adapt to those hosts they are most likely to experience, which are likely to be the most abundant and/or frequently bitten by tick vectors. Second, species with fast life histories might allocate less to certain immune defenses, which could increase their reservoir competence. Results suggest that of the host species that might potentially be exposed, those with comparatively high population densities, small bodies, and fast pace of life will often be keystone reservoirs that should be targeted for surveillance or management.

Molecular survey of Babesia microti, Ehrlichia species and Candidatus neoehrlichia mikurensis in wild rodents from Shimane Prefecture, Japan

A significant number of patients are diagnosed with "fevers of unknown origin" (FUO) in Shimane Prefecture in Japan where tick-borne diseases are endemic. We conducted molecular surveys for Babesia microti, Ehrlichia species, and Candidatus Neoehrlichia mikurensis in 62 FUO cases and 62 wild rodents from Shimane Prefecture, Japan. PCR using primers specific for the Babesia 18S small-subunit rRNA (rDNA) gene and Anaplasmataceae groESL amplified products from 45% (28/62) and 25.8% (16/62) of captured mice, respectively. Of the 28 18S rDNA PCR positives, 23 and five samples were positive for Hobetsu- and Kobe-type B. microti, respectively. In contrast, of the 16 groESL PCR positives, eight, one and seven samples were positive for Ehrlichia muris, Ehrlichia sp. HF565 and Candidatus N. mikurensis, respectively. Inoculation of selected blood samples into Golden Syrian hamsters indicated the presence of Hobetsu- and Kobe-type B. microti in four and one sample, respectively. Isolation of the latter strain was considered important as previous studies suggested that the distribution of this type was so far confined to Awaji Island in Hyogo Prefecture, where the first case of transfusion-associated human babesiosis originated. DNA samples from 62 FUO human cases tested negative for B. microti 18S rDNA gene, Anaplasmataceae groESL gene, Rickettsia japonica 17K genus-common antigen gene and Orientia tsutsugamushi 56K antigen gene by PCRs. We also conducted seroepidemiological surveys on 62 human sera collected in Shimane Prefecture from the FUO patients who were suspected of carrying tick-borne diseases. However, indirect immunofluorescent antibody tests using B. microti- and E. muris-infected cells detected IgG against E. muris in only a single positive sample. This study demonstrates the presence of several potentially important tick-borne pathogens in Shimane Prefecture and suggests the need for further study on the causative agents of FUOs.

Genome-wide diversity and gene expression profiling of Babesia microti isolates identify polymorphic genes that mediate host-pathogen interactions

Babesia microti, a tick-transmitted, intraerythrocytic protozoan parasite circulating mainly among small mammals, is the primary cause of human babesiosis. While most cases are transmitted by Ixodes ticks, the disease may also be transmitted through blood transfusion and perinatally. A comprehensive analysis of genome composition, genetic diversity, and gene expression profiling of seven B. microti isolates revealed that genetic variation in isolates from the Northeast United States is almost exclusively associated with genes encoding the surface proteome and secretome of the parasite. Furthermore, we found that polymorphism is restricted to a small number of genes, which are highly expressed during infection. In order to identify pathogen-encoded factors involved in host-parasite interactions, we screened a proteome array comprised of 174 B. microti proteins, including several predicted members of the parasite secretome. Using this immuno-proteomic approach we identified several novel antigens that trigger strong host immune responses during the onset of infection. The genomic and immunological data presented herein provide the first insights into the determinants of B. microti interaction with its mammalian hosts and their relevance for understanding the selective pressures acting on parasite evolution.

Age‐Associated Decline in Resistance toBabesia microtiIs Genetically Determined

BACKGROUND

Although infection by the protozoan Babesia microti is rarely symptomatic in immunocompetent young people, healthy individuals aged >50 years may experience life-threatening disease. To determine the basis for this age relationship, we developed a mouse model of babesiosis using a novel clinical isolate of B. microti.

METHODS

Mice were infected at 2, 6, 12, or 18 months. Parasitemia was monitored on Giemsa-stained blood smears or by flow cytometry.

RESULTS

In DBA/2 mice, early and persistent parasitemias increased with age at infection. BALB/c and C57BL/6 mice were resistant, regardless of age, which indicates that allelic variation determines resistance to B. microti. Unlike immunocompetent mice, SCID mice, which retain an innate immune system but lack the lymphocytes needed for adaptive immunity, developed high and persistent levels of parasitemia that were markedly reduced by transfer of naive BALB/c or DBA/2 splenocytes. BALB/c cells reduced the persistent parasitemia to a greater extent than did age-matched DBA/2 cells. Of importance, there was an age-associated loss of protection by cells of both strains.

CONCLUSION

The resistance to B. microti infection conferred by the adaptive immune system is genetically determined and associated with age. We postulate that there are age-related differences in the expression of alleles critical for adaptive immunity to B. microti.

Cost-effectiveness of blood donor screening for Babesia microti in endemic regions of the United States

BACKGROUND

Babesia microti is the leading reported cause of red blood cell (RBC) transfusion-transmitted infection in the United States. Donor screening assays are in development.

STUDY DESIGN AND METHODS

A decision analytic model estimated the cost-effectiveness of screening strategies for preventing transfusion-transmitted babesiosis (TTB) in a hypothetical cohort of transfusion recipients in Babesia-endemic areas of the United States. Strategies included: 1) no screening; 2) Uniform Donor Health History Questionnaire (UDHQ), "status quo"; 3) recipient risk targeting using donor antibody and polymerase chain reaction (PCR) screening; 4) universal endemic donor antibody screening; and 5) universal endemic donor antibody and PCR screening. Outcome measures were TTB cases averted, costs, quality-adjusted life-years (QALYs), and incremental cost-effectiveness ratios (ICERs; $/QALY). We assumed a societal willingness to pay of $1 million/QALY based on screening for other transfusion-transmitted infections.

RESULTS

Compared to no screening, the UDHQ avoids 0.02 TTB cases per 100,000 RBC transfusions at an ICER of $160,000/QALY whereas recipient risk-targeted strategy using antibody/PCR avoids 1.62 TTB cases per 100,000 RBC transfusions at an ICER of $713,000/QALY compared to the UDHQ. Universal endemic antibody screening avoids 3.39 cases at an ICER of $760,000/QALY compared to the recipient risk-targeted strategy. Universal endemic antibody/PCR screening avoids 3.60 cases and has an ICER of $8.8 million/QALY compared to universal endemic antibody screening. Results are sensitive to blood donor Babesia prevalence, TTB transmission probability, screening test costs, risk and severity of TTB complications, and impact of babesiosis diagnosis on donor quality of life.

CONCLUSION

Antibody screening for Babesia in endemic regions is appropriate from an economic perspective based on the societal willingness to pay for preventing infectious threats to blood safety.

Babesia microti primarily invades mature erythrocytes in mice

Babesia microti is a tick-borne red blood cell parasite that causes babesiosis in people. Its most common vertebrate reservoir is the white-footed mouse. To determine whether B. microti invades reticulocytes, as does the canine pathogen B. gibsoni, we infected the susceptible inbred mouse strains C.B-17.scid and DBA/2 with a clinical isolate of B. microti. Staining of fixed permeabilized red blood cells with 4',6'-diamidino-2-phenylindole or YOYO-1, a sensitive nucleic acid stain, revealed parasite nuclei as large bright dots. Flow cytometric analysis indicated that parasite DNA is primarily found in mature erythrocytes that expressed Babesia antigens but not the transferrin receptor CD71. In contrast, CD71-positive reticulocytes rarely contained Babesia nuclei and failed to express Babesia antigens. Accordingly, the frequency of YOYO-1-positive, CD71-negative cells strongly correlated with parasitemia, defined as the frequency of infected red blood cells assessed on Giemsa-stained blood smears. Importantly, the absolute numbers generated by the two techniques were similar. Parasitemia was modest and transient in DBA/2 mice but intense and sustained in C.B-17.scid mice. In both strains, parasitemia preceded reticulocytosis, but reticulocytes remained refractory to B. microti. In immunocompetent C.B-17 mice, reticulocytosis developed early, despite a marginal and short-lived parasitemia. Likewise, an early reticulocytosis developed in resistant BALB/cBy and B10.D2 mice. These studies establish that B. microti has a tropism for mature erythrocytes. Although reticulocytes are rarely infected, the delayed reticulocytosis in susceptible strains may result from parasite or host activities to limit renewal of the mature erythrocyte pool, thereby preventing an overwhelming parasitemia.

Misdiagnosis of Babesiosis as Malaria, Equatorial Guinea, 2014

We report a case of babesiosis, caused by Babesia microti, in a missionary who worked in Equatorial Guinea but also visited rural Spain. The initial diagnosis, based on clinical features and microscopy, was malaria. The patient’s recovery was delayed until she received appropriate treatment for babesiosis.

Host distribution and pathogen infection of fleas (Siphonaptera) recovered from small mammals in Pennsylvania

The number of recognized flea-borne pathogens has increased over the past decade. However, the true number of infections related to all flea-borne pathogens remains unknown. To better understand the enzootic cycle of flea-borne pathogens, fleas were sampled from small mammals trapped in central Pennsylvania. A total of 541 small mammals were trapped, with white-footed mice (Peromyscus leucopus) and southern red-backed voles (Myodes gapperi) accounting for over 94% of the captures. Only P. leucopus were positive for examined blood-borne pathogens, with 47 (18.1%) and ten (4.8%) positive for Anaplasma phagocytophilum and Babesia microti, respectively. In addition, 61 fleas were collected from small mammals and tested for pathogens. Orchopeas leucopus was the most common flea and Bartonella vinsonii subspecies arupensis, B. microti, and a Rickettsia felis-like bacterium were detected in various flea samples. To the best of our knowledge, this is the first report of B. microti DNA detected from a flea and the first report of a R. felis-like bacterium from rodent fleas in eastern North America. This study provides evidence of emerging pathogens found in fleas, but further investigation is required to resolve the ecology of flea-borne disease transmission cycles.

Case report: spontaneous splenic rupture during acute parasitemia of Babesia microti

Babesia is a malaria-like protozoan parasite spread by Ixodes ticks primarily from the white-footed deer mouse to humans. Typically it causes subclinical disease, but occasionally causes acute febrile disease with hepatosplenomegaly. We report a case of spontaneous splenic rupture of a 56-yr-old man with acute Babesia microti infection.

Presence of potentially pathogenic Babesia sp. for human in Ixodes ricinus in Switzerland

We have designed and performed a new PCR method based on the 18S rRNA in order to individuate the presence and the identity of Babesia parasites. Out of 1159 Ixodes ricinus (Acari: Ixodidae) ticks collected in four areas of Switzerland, nine were found to contain Babesia DNA. Sequencing of the short amplicon obtained (411-452 bp) allowed the identification of three human pathogenic species: Babesia microti, B. divergens, for the first time in Switzerland, Babesia sp. EU1. We also report coinfections with B. sp. EU1-Borrelia burgdorferi sensu stricto and Babesia sp. EU1-B. afzelii.

A Novel Thioredoxin-Like Protein of Babesia microti Involved in Parasite Pathogenicity

Babesiosis poses a serious threat to immunocompromised individuals and the major etiological species of Babesia for human babesiosis is Babesia microti. Merozoites are a critical stage in the life cycle of Babesia microti. Several merozoite proteins have been demonstrated to play important roles in this process; however, most of the merozoite proteins of B. microti remain unknown. In the present study, we identified a novel merozoite protein of B. microti with similar structure to the thioredoxin (Trx)-like domain of the Trx family, which was named as B. microti Trx-like protein (BmTLP). Western blot assays demonstrated that this protein was expressed by B. microti during the erythrocytic infection process, and its expression peaked on day 7 post-infection in vivo. Immunofluorescence assay further showed that this protein is mainly expressed in B. microti merozoites. BmTLP hold both heparin- and erythrocyte-binding properties, which are critical functions of invasion-related proteins. Immunization with recombinant BmTLP imparted significant protection against B. microti infection in mice. Taken together, these results suggest that the novel merozoite protein, BmTLP, is an important pathogenic molecule of B. microti and may be a possible target for the design of babesiosis control strategy.

Tick HRF-dependent ferroptosis pathway to promote tick acquisition of Babesia microti

B. microti is a tick-transmitted zoonotic erythrocytic intracellular parasite. Ferroptosis is an iron-dependent form of programmed cell death that affects pathogen replication in the host. Currently, there is limited research concerning the effect of tick ferroptosis on Babesia infection and the underlying mechanism of action. The present study used a B. microti -mouse- Haemaphysalis longicornis infection model in which nymphs fed on the blood of B. microti-infected mice. The midgut divalent iron (p<0.01) and reactive oxygen species (ROS) (p<0.05) levels were significantly elevated in infected ticks, and transmission electron microscopy (TEM) showed that mitochondrial ridges were absent or decreased in size. Downregulation of ferritin 1 and glutathione peroxidase 4 (GPX4) in ticks infected with B. microti suggests that these changes promote ferroptosis. In vivo studies demonstrated that the ferroptosis promoter Erastin increased B. microti load (p<0.05), while the inhibitor Ferrostatin-1 effectively decreased load (p<0.01). Tick histamine-releasing factor (HRF), a protein related to the antioxidant system, was downregulated in infected nymphs compared with uninfected nymphs (p<0.05), and interference with HRF promoted tick acquisition of B. microti (p<0.001). Transcriptomic analyses showed that HRF interference promotes tick ferroptosis by downregulating ferritin 1 and GPX4. Meanwhile, interference with tick HRF molecules showed increased divalent iron and ROS and decreased mitochondrial ridges compared with controls. These findings highlight the critical role of tick HRF molecules in regulating ferroptosis and acquisition of B. microti, thereby providing important insights for a deeper understanding of the tick-Babesia interaction.