Innate resistance to Babesia infection is influenced by genetic background and gender

An adjuvant formulation containing Toll-like Receptor 7 agonist stimulates protection against morbidity and mortality due to Anaplasma marginale in a highly endemic region of west Africa

Efficient cattle production and provision of animal-sourced foods in much of Africa is constrained by vector-borne bacterial and protozoal diseases. Effective vaccines are not currently available for most of these infections resulting in a continuous disease burden that limits genetic improvement. We tested whether stimulation of innate immunity using the Toll-like Receptor (TLR) 7 agonist imiquimod, formulated with saponin and water-in-oil emulsion, would protect against morbidity and mortality due to Anaplasma marginale, a tick-borne pathogen of cattle highly endemic in west Africa. In Trial 1, haplotype matched Friesian x Sanga (F1) A. marginale negative calves were allocated to either the experimental group (n = 10) and injected with the synthetic TLR 7 agonist/saponin formulation or to an untreated control group (n = 10). TLR7 agonist/saponin injected calves responded with significantly elevated rectal temperature, enlarged regional lymph nodes, and elevated levels of IL-6 post-injection as compared to control group calves. All calves were then allowed to graze in pasture for natural exposure to tick transmission. All calves in both groups acquired A. marginale, consistent with the high transmission rate in the endemic region. The need for antibiotic treatment, using pre-existing criteria, was significantly lower in the experimental group (odds ratio for not requiring treatment was 9.3, p = 0.03) as compared to the control group. Despite treatment, 6/10 calves in the control group died, reflecting treatment failures that are typical of anaplasmosis in the acute phase, while mortality in the experimental group was 1/10 (odds ratio for survival was 13.5, p = 0.03). The trial was then repeated using 45 Friesian x Sanga calves per group. In Trial 2, the odds ratios for preventing the need for treatment and for mortality in the TLR7 agonist/saponin experimental group versus the control group were 5.6 (p = 0.0002) and 7.0 (p = 0.004), respectively, reproducing the findings of the initial trial. Together these findings demonstrate that innate immune stimulation using a TLR7 agonist formulated with saponin and water-in-oil emulsion provides significant protection against disease caused by tick borne A. marginale in highly susceptible cross-bred cattle, critically important for their potential to increase productivity for smallholder farmers in Africa.

Babesia duncani, a Model Organism for Investigating Intraerythrocytic Parasitism and Novel Antiparasitic Therapeutic Strategies

Pathogens such as Plasmodium, Babesia, and Theileria invade and multiply within host red blood cells, leading to the pathological consequences of malaria, babesiosis, and theileriosis. Establishing continuous in vitro culture systems and suitable animal models is crucial for studying these pathogens. This review spotlights the Babesia duncani in culture-in mouse (ICIM) model as a promising resource for advancing research on the biology, pathogenicity, and virulence of intraerythrocytic parasites. The model offers practical benefits, encompassing well-defined culture conditions, ease of manipulation, and a well-annotated genome. Moreover, B. duncani serves as a surrogate system for drug discovery, facilitating the evaluation of new antiparasitic drugs in vitro and in animals, elucidating their modes of action, and uncovering potential resistance mechanisms. The B. duncani ICIM model thus emerges as a multifaceted tool with profound implications, promising advancements in our understanding of parasitic biology and shaping the development of future therapies.

Immune Mediators Important for a Protective Secondary Response to Babesia microti

Babesia microti (B. microti) is a tick-transmitted protozoan parasite that invades red blood cells. It is the primary cause of human babesiosis in the US. The severity of babesiosis caused by B. microti infection can range from asymptomatic to fatal. Risk factors for severe disease include general immune suppression, advanced age (>50) and lack of a spleen. However, severe disease can occur in the absence of any known risk factors. The degree to which tick-transmitted B. microti infection confers protection from subsequent exposure is largely unexplored. This is an important question as both the prevalence and geographic range of tick-transmitted B. microti infection continues to increase and individuals in endemic regions may have multiple exposures over their lifetime. In the current study we used a mouse model to evaluate the degree to which primary infection with B. microti protected against secondary challenge with the same parasite strain. We show that CD4 T cells, and to a lesser extent B cells, contribute to protection. However, mice exhibited significant protection from secondary parasite challenge even in the absence of either CD4 T cells or B cells. The protection mediated by CD4 T cells did not depend on their production of IFN-γ as mice with a targeted gene deletion for the IFN-γ receptor remained fully protected against secondary challenge. Other factors including inducible nitric oxide synthase (iNOS) and the adaptor protein MyD88, important for toll-like receptors, IL-18 and IL-1 signaling, were not important for protection against primary or secondary challenge with B. microti. Thus, our study shows that resolution of primary infection with B. microti results in robust protection against secondary challenge with parasites, at least in the short term. Further studies are needed to evaluate the length of protection and the degree to which protection is impacted by parasite heterogeneity. Although we show an important role for CD4 T cells in protection against secondary challenge, our results suggest that no single aspect of the immune system is solely responsible for adequate protection against secondary challenge with B. microti.

Molecular detection of Babesia ovis and blood parameters' investigation reveal hematological and biochemical alterations in babesiosis-infected Lohi sheep in Multan, Pakistan

Background

Babesia infections in sheep can cause a wide range of clinical and laboratory presentations. Changes in blood parameters are a meaningful manifestation of physiological and pathological changes in an organism.

Aim

Therefore, the present study was conducted to analyze and compare hematological and biochemical parameters between blood profiles of Lohi sheep naturally infected and uninfected with Babesia ovis, the main causative agent of ovine babesiosis.

Methods

Initially, blood and serum samples from 67 Lohi sheep were collected, DNA was extracted and babesial infection was detected through polymerase chain reaction. The overall infection rate of B. ovis was 37% (25/67). Sixteen infected (experiment group) and 16 uninfected (control group) sheep that were apparently healthy with no history of previous treatment for babesiosis, were selected for hemato-biochemical analysis. Blood samples were analyzed through an automatic CBC analyzer, while serum collected from gel vacutainers was analyzed for blood urea, blood urea nitrogen (BUN), creatinine, and total bilirubin. Each parameter was compared between infected and uninfected animals using a paired t-test in Minitab Express™ software for statistical analyses.

Results

Erythron comparison showed a highly significant ( p < 0.0001) decrease in RBC, hemoglobin, and Hct. A nonsignificant increase in mean corpuscular volume (MCV), red cell distribution width (RDW), and RDW-standard deviation (RDW-SD), while a nonsignificant decrease in mean corpuscular haemoglobin (MCH) and MCH concentration (MCHC) values was recorded in infected sheep. Leukon comparison showed a significantly low level of total leukocyte (p < 0.001) in infected sheep. Platelet (Plt) along with platelet crit (Pct) and platelet distribution width (PDW) were nonsignificantly higher, whereas a nonsignificant decrease in mean Plt volume was recorded in infected sheep as compared to uninfected animals. Among biochemical parameters, blood urea, BUN, and total bilirubin showed significant differences (p < 0.05), while creatinine showed a nonsignificant difference.

Conclusion

To the best of our knowledge, this is the first report on hemato-biochemical changes associated with babesiosis in the Lohi breed. Consistent with hemolytic anemia, these data would justify physical examination and, together with the medical history, would provide an excellent basis for the diagnosis of babesiosis.

Animal models of the immunology and pathogenesis of human babesiosis

Animal models of human babesiosis have provided a basic understanding of the immunological mechanisms that clear, or occasionally exacerbate, Babesia infection and those pathological processes that cause disease complications. Human Babesia infection can cause asymptomatic infection, mild to moderate disease, or severe disease resulting in organ dysfunction and death. More than 100 Babesia species infect a wide array of wild and domestic animals, and many of the immunologic and pathologic responses to Babesia infection are similar in animals and humans. In this review, we summarize the knowledge gained from animal studies, their limitations, and how animal models or alternative approaches can be further leveraged to improve our understanding of human babesiosis.

Babesia duncani in Culture and in Mouse (ICIM) Model for the Advancement of Babesia Biology, Pathogenesis, and Therapy

Babesiosis is a tick-borne disease caused by pathogens belonging to the genus Babesia. In humans, the disease presents as a malaria-like illness and can be fatal in immunocompromised and elderly people. In the past few years, human babesiosis has been a rising concern worldwide. The disease is transmitted through tick bite, blood transfusion, and transplacentally in rare cases, with several species of Babesia causing human infection. Babesia microti, Babesia duncani, and Babesia divergens are of particular interest because of their important health impact and amenability to research inquiries. B. microti, the most commonly reported Babesia pathogen infecting humans, can be propagated in immunocompetent and immunocompromised mice but so far has not been successfully continuously propagated in vitro in human red blood cells (hRBCs). Conversely, B. divergens can be propagated in vitro in human red blood cells but lacks a mouse model to study its virulence. Recent studies have highlighted the uniqueness of B. duncani as an ideal model organism to study intraerythrocytic parasitism in vitro and in vivo. An optimized B. duncani in culture and in mouse (ICIM) model has recently been described, combining long-term continuous in vitro culture of the parasite in hRBCs with an animal model of parasitemia (P) and lethal infection in C3H/HeJ mice. Here, we provide a detailed protocol for the use of the B. duncani ICIM model in research. This model provides a unique and sound foundation to gain further insights into the biology, pathogenesis, and virulence of Babesia and other intraerythrocytic parasites, and has been validated as an efficient system to evaluate novel strategies for the treatment of human babesiosis and possibly other parasitic diseases. This protocol was validated in: J Infect Dis (2022), DOI: 10.1093/infdis/jiac181 Graphical abstract ICIM model [Adapted and modified from Pal et al. (2022)].

Babesia duncani as a Model Organism to Study the Development, Virulence, and Drug Susceptibility of Intraerythrocytic Parasites In Vitro and In Vivo

Human babesiosis is a malaria-like illness caused by tick-borne intraerythrocytic Babesia parasites of the Apicomplexa phylum. Whereas several species of Babesia can cause severe disease in humans, the ability to propagate Babesia duncani both in vitro in human erythrocytes and in mice makes it a unique pathogen to study Babesia biology and pathogenesis. Here we report an optimized B. duncani in culture-in mouse (ICIM) model that combines continuous in vitro culture of the parasite with a precise model of lethal infection in mice. We demonstrate that B. duncani-infected erythrocytes as well as free merozoites can cause lethal infection in C3H/HeJ mice. Highly reproducible parasitemia and survival outcomes could be established using specific parasite loads in different mouse genetic backgrounds. Using the ICIM model, we discovered 2 new endochin-like quinolone prodrugs (ELQ-331 and ELQ-468) that alone or in combination with atovaquone are highly efficacious against B. duncani and Babesia microti.

Babesia microti: Pathogen Genomics, Genetic Variability, Immunodominant Antigens, and Pathogenesis

More than 100 Babesia spp. tick-borne parasites are known to infect mammalian and avian hosts. Babesia belong to Order Piroplasmid ranked in the Phylum Apicomplexa. Recent phylogenetic studies have revealed that of the three genera that constitute Piroplasmida, Babesia and Theileria are polyphyletic while Cytauxzoon is nested within a clade of Theileria. Several Babesia spp. and sub-types have been found to cause human disease. Babesia microti, the most common species that infects humans, is endemic in the Northeastern and upper Midwestern United States and is sporadically reported elsewhere in the world. Most infections are transmitted by Ixodid (hard-bodied) ticks, although they occasionally can be spread through blood transfusion and rarely via perinatal transmission and organ transplantation. Babesiosis most often presents as a mild to moderate disease, however infection severity ranges from asymptomatic to lethal. Diagnosis is usually confirmed by blood smear or polymerase chain reaction (PCR). Treatment consists of atovaquone and azithromycin or clindamycin and quinine and usually is effective but may be problematic in immunocompromised hosts. There is no human Babesia vaccine. B. microti genomics studies have only recently been initiated, however they already have yielded important new insights regarding the pathogen, population structure, and pathogenesis. Continued genomic research holds great promise for improving the diagnosis, management, and prevention of human babesiosis, and in particular, the identification of lineage-specific families of cell-surface proteins with potential roles in cytoadherence, immune evasion and pathogenesis.

Treatment of Human Babesiosis: Then and Now

Babesiosis is an emerging tick-borne disease caused by apicomplexan parasites of the genus Babesia. With its increasing incidence worldwide and the risk of human-to-human transmission through blood transfusion, babesiosis is becoming a rising public health concern. The current arsenal for the treatment of human babesiosis is limited and consists of combinations of atovaquone and azithromycin or clindamycin and quinine. These combination therapies were not designed based on biological criteria unique to Babesia parasites, but were rather repurposed based on their well-established efficacy against other apicomplexan parasites. However, these compounds are associated with mild or severe adverse events and a rapid emergence of drug resistance, thus highlighting the need for new therapeutic strategies that are specifically tailored to Babesia parasites. Herein, we review ongoing babesiosis therapeutic and management strategies and their limitations, and further review current efforts to develop new, effective, and safer therapies for the treatment of this disease.

Babesia microti Confers Macrophage-Based Cross-Protective Immunity Against Murine Malaria

Malaria and babesiosis, the two primary intraerythrocytic protozoan diseases of humans, have been reported in multiple cases of co-infection in endemic regions. As the geographic range and incidence of arthropod-borne infectious diseases is being affected by climate change, co-infection cases with Plasmodium and Babesia are likely to increase. The two parasites have been used in experimental settings, where prior infection with Babesia microti has been shown to protect against fatal malarial infections in mice and primates. However, the immunological mechanisms behind such phenomena of cross-protection remain unknown. Here, we investigated the effect of a primary B. microti infection on the outcome of a lethal P. chabaudi challenge infection using a murine model. Simultaneous infection with both pathogens led to high mortality rates in immunocompetent BALB/c mice, similar to control mice infected with P. chabaudi alone. On the other hand, mice with various stages of B. microti primary infection were thoroughly immune to a subsequent P. chabaudi challenge. Protected mice exhibited decreased levels of serum antibodies and pro-inflammatory cytokines during early stages of challenge infection. Mice repeatedly immunized with dead B. microti quickly succumbed to P. chabaudi infection, despite induction of high antibody responses. Notably, cross-protection was observed in mice lacking functional B and T lymphocytes. When the role of other innate immune effector cells was examined, NK cell-depleted mice with chronic B. microti infection were also found to be protected against P. chabaudi. Conversely, in vivo macrophage depletion rendered the mice vulnerable to P. chabaudi. The above results show that the mechanism of cross-protection conferred by B. microti against P. chabaudi is innate immunity-based, and suggest that it relies predominantly upon the function of macrophages. Further research is needed for elucidating the malaria-suppressing effects of babesiosis, with a vision toward development of novel tools to control malaria.

Rat volatiles as an attractant source for the Asian tiger mosquito, Aedes albopictus

Aedes albopictus is a vector of dengue, chikungunya, and dirofilariasis. Volatile compounds are crucial for mosquitoes to locate their hosts. This knowledge has allowed the identification of attractants derived from human odours for highly anthropophilic mosquito species. In this study, we used rats as a experimental model to identify potential attractants for host-seeking Ae. albopictus females. Porapak Q extracts from immature female rats were more attractive to Ae. albopictus females than those from mature and pregnant females, and males. Phenol, 4-methylphenol, 4-ethylphenol, and indole were identified compounds in male, immature, mature, and pregnant female extracts. There were quantitative differences in these compounds among the extracts that likely explain the discrepancy in their attractiveness. Ae. albopictus females were not attracted to the single compounds when was compared with the four-component blend. However, the binary blend of 4-methylphenol  + 4-ethylphenol and the tertiary blend of 4-methylphenol + 4-ethylphenol + indole were as attractive as the four-component blend. In the field trials, BGS traps baited with the tertiary or quaternary blends caught more Ae. albopictus females and males than BGS traps without lures. This is the first laboratory and field study to identify compounds that mediate the attraction of Ae. albopictus to one of its hosts.

Protozoan Parasite Babesia microti Subverts Adaptive Immunity and Enhances Lyme Disease Severity

Lyme disease is the most prominent tick-borne disease in the United States. Co-infections with the tick-transmitted pathogens Babesia microti and Borrelia burgdorferi sensu stricto are becoming a serious health problem. B. burgdorferi is an extracellular spirochete that causes Lyme disease while B. microti is a protozoan that infects erythrocytes and causes babesiosis. Testing of donated blood for Babesia species is not currently mandatory due to unavailability of an FDA approved test. Transmission of this protozoan by blood transfusion often results in high morbidity and mortality in recipients. Infection of C3H/HeJ mice with B. burgdorferi and B. microti individually results in inflammatory Lyme disease and display of human babesiosis-like symptoms, respectively. Here we use this mouse model to provide a detailed investigation of the reciprocal influence of the two pathogens on each other during co-infection. We show that B. burgdorferi infection attenuates parasitemia in mice while B. microti subverts the splenic immune response, such that a marked decrease in splenic B and T cells, reduction in antibody levels and diminished functional humoral immunity, as determined by spirochete opsonophagocytosis, are observed in co-infected mice compared to only B. burgdorferi infected mice. Furthermore, immunosuppression by B. microti in co-infected mice showed an association with enhanced Lyme disease manifestations. This study demonstrates the effect of only simultaneous infection by B. burgdorferi and B. microti on each pathogen, immune response and on disease manifestations with respect to infection by the spirochete and the parasite. In our future studies, we will examine the overall effects of sequential infection by these pathogens on host immune responses and disease outcomes.

Short- and long-term effects of orally administered azithromycin on Trypanosoma brucei brucei-infected mice

Human African trypanosomosis (HAT) and animal African trypanosomosis (AAT) are diseases of economic importance in humans and animals that affect more than 36 African countries. The currently available trypanocidal drugs are associated with side effects, and the parasites are continually developing resistance. Thus, effective and safe drugs are needed for the treatment of HAT and AAT. This study aimed to evaluate the effects of azithromycin (AZM) on Trypanosoma brucei brucei-infected mice. Mice were randomly divided into 7 groups consisting of a vehicle control group, 5 test groups and a diminazene aceturate (DA)-treated group. Mice were treated orally for 7 and 28 days, as short-term and long-term treatments, respectively. Short-term AZM treatment cured 23% (16 of 70) of the overall treated mice whereas long-term treatment resulted in the survival of 70% of the mice in the groups that received AZM at doses of 300 and 400 mg/kg. Trypanosomes treated in vitro with 25 μg/mL of AZM were subjected to transmission electron microscopy, which revealed the presence of increased numbers of glycosomes and acidocalcisomes in comparison to the vehicle group. The current study showed the trypanocidal effect of AZM on T. b. brucei in vivo. The demonstrated efficacy increased with an increase in treatment period and an increased concentration of AZM.

Innate Lymphoid Cells in Protection, Pathology, and Adaptive Immunity During Apicomplexan Infection

Apicomplexans are a diverse and complex group of protozoan pathogens including Toxoplasma gondii, Plasmodium spp., Cryptosporidium spp., Eimeria spp., and Babesia spp. They infect a wide variety of hosts and are a major health threat to humans and other animals. Innate immunity provides early control and also regulates the development of adaptive immune responses important for controlling these pathogens. Innate immune responses also contribute to immunopathology associated with these infections. Natural killer (NK) cells have been for a long time known to be potent first line effector cells in helping control protozoan infection. They provide control by producing IL-12 dependent IFNγ and killing infected cells and parasites via their cytotoxic response. Results from more recent studies indicate that NK cells could provide additional effector functions such as IL-10 and IL-17 and might have diverse roles in immunity to these pathogens. These early studies based their conclusions on the identification of NK cells to be CD3–, CD49b+, NK1.1+, and/or NKp46+ and the common accepted paradigm at that time that NK cells were one of the only lymphoid derived innate immune cells present. New discoveries have lead to major advances in understanding that NK cells are only one of several populations of innate immune cells of lymphoid origin. Common lymphoid progenitor derived innate immune cells are now known as innate lymphoid cells (ILC) and comprise three different groups, group 1, group 2, and group 3 ILC. They are a functionally heterogeneous and plastic cell population and are important effector cells in disease and tissue homeostasis. Very little is known about each of these different types of ILCs in parasitic infection. Therefore, we will review what is known about NK cells in innate immune responses during different protozoan infections. We will discuss what immune responses attributed to NK cells might be reconsidered as ILC1, 2, or 3 population responses. We will then discuss how different ILCs may impact immunopathology and adaptive immune responses to these parasites.

Robust adaptive immune response against Babesia microti infection marked by low parasitemia in a murine model of sickle cell disease

The intraerythrocytic parasite Babesia microti is the number 1 cause of transfusion-transmitted infection and can induce serious, often life-threatening complications in immunocompromised individuals including transfusion-dependent patients with sickle cell disease (SCD). Despite the existence of strong long-lasting immunological protection against a second infection in mouse models, little is known about the cell types or the kinetics of protective adaptive immunity mounted following Babesia infection, especially in infection-prone SCD that are thought to have an impaired immune system. Here, we show, using a mouse B microti infection model, that infected wild-type (WT) mice mount a very strong adaptive immune response, characterized by (1) coordinated induction of a robust germinal center (GC) reaction; (2) development of follicular helper T (TFH) cells that comprise ∼30% of splenic CD4+ T cells at peak expansion by 10 days postinfection; and (3) high levels of effector T-cell cytokines, including interleukin 21 and interferon γ, with an increase in the secretion of antigen (Ag)-specific antibodies (Abs). Strikingly, the Townes SCD mouse model had significantly lower levels of parasitemia. Despite a highly disorganized splenic architecture before infection, these mice elicited a surprisingly robust adaptive immune response (including comparable levels of GC B cells, TFH cells, and effector cytokines as control and sickle trait mice), but higher immunoglobulin G responses against 2 Babesia-specific proteins, which may contain potential immunogenic epitopes. Together, these studies establish the robust emergence of adaptive immunity to Babesia even in immunologically compromised SCD mice. Identification of potentially immunogenic epitopes has implications to identify long-term carriers, and aid Ag-specific vaccine development.

Age-Related Differential Stimulation of Immune Response by Babesia microti and Borrelia burgdorferi During Acute Phase of Infection Affects Disease Severity

Lyme disease is the most prominent tick-borne disease with 300,000 cases estimated by CDC every year while ~2,000 cases of babesiosis occur per year in the United States. Simultaneous infection with Babesia microti and Borrelia burgdorferi are now the most common tick-transmitted coinfections in the U.S.A., and they are a serious health problem because coinfected patients show more intense and persisting disease symptoms. B. burgdorferi is an extracellular spirochete responsible for systemic Lyme disease while B. microti is a protozoan that infects erythrocytes and causes babesiosis. Immune status and spleen health are important for resolution of babesiosis, which is more severe and even fatal in the elderly and splenectomized patients. Therefore, we investigated the effect of each pathogen on host immune response and consequently on severity of disease manifestations in both young, and 30 weeks old C3H mice. At the acute stage of infection, Th1 polarization in young mice spleen was associated with increased IFN-γ and TNF-α producing T cells and a high Tregs/Th17 ratio. Together, these changes could help in the resolution of both infections in young mice and also prevent fatality by B. microti infection as observed with WA-1 strain of Babesia. In older mature mice, Th2 polarization at acute phase of B. burgdorferi infection could play a more effective role in preventing Lyme disease symptoms. As a result, enhanced B. burgdorferi survival and increased tissue colonization results in severe Lyme arthritis only in young coinfected mice. At 3 weeks post-infection, diminished pathogen-specific antibody production in coinfected young, but not older mice, as compared to mice infected with each pathogen individually may also contribute to increased inflammation observed due to B. burgdorferi infection, thus causing persistent Lyme disease observed in coinfected mice and reported in patients. Thus, higher combined proinflammatory response to B. burgdorferi due to Th1 and Th17 cells likely reduced B. microti parasitemia significantly only in young mice later in infection, while the presence of B. microti reduced humoral immunity later in infection and enhanced tissue colonization by Lyme spirochetes in these mice even at the acute stage, thereby increasing inflammatory arthritis.

Investigating disease severity in an animal model of concurrent babesiosis and Lyme disease

The incidence of babesiosis, Lyme disease and other tick-borne diseases has increased steadily in Europe and North America during the last five decades. Babesia microti is transmitted by species of Ixodes, the same ticks that transmit the Lyme disease-causing spirochete, Borrelia burgdorferi. B. microti can also be transmitted through transfusion of blood products and is the most common transfusion-transmitted infection in the U.S.A. Ixodes ticks are commonly infected with both B. microti and B. burgdorferi, and are competent vectors for transmitting them together into hosts. Few studies have examined the effects of coinfections on humans and they had somewhat contradictory results. One study linked coinfection with B. microti to a greater number of symptoms of overall disease in patients, while another report indicated that B. burgdorferi infection either did not affect babesiosis symptoms or decreased its severity. Mouse models of infection that manifest pathological effects similar to those observed in human babesiosis and Lyme disease offer a unique opportunity to thoroughly investigate the effects of coinfection on the host. Lyme disease has been studied using the susceptible C3H mouse infection model, which can also be used to examine B. microti infection to understand pathological mechanisms of human diseases, both during a single infection and during coinfections. We observed that high B. microti parasitaemia leads to low haemoglobin levels in infected mice, reflecting the anaemia observed in human babesiosis. Similar to humans, B. microti coinfection appears to enhance the severity of Lyme disease-like symptoms in mice. Coinfected mice have lower peak B. microti parasitaemia compared to mice infected with B. microti alone, which may reflect attenuation of babesiosis symptoms reported in some human coinfections. These findings suggest that B. burgdorferi coinfection attenuates parasite growth while B. microti presence exacerbates Lyme disease-like symptoms in mice.

Babesia microti Infection Changes Host Spleen Architecture and Is Cleared by a Th1 Immune Response

Babesia microti is a malaria-like parasite, which infects ∼2000 people annually, such that babesiosis is now a notifiable disease in the United States. Immunocompetent individuals often remain asymptomatic and are tested only after they feel ill. Susceptible C3H/HeJ mice show several human-like disease manifestations and are ideal to study pathogenesis of Babesia species. In this study, we examined parasitemia of B. microti at different time points and assessed its impact on hemoglobin levels in blood, on spleen pathology and overall immune response in C3H/HeJ mice. Peak parasitemia of 42.5% was immediately followed by diminished hemoglobin level. Parasitemia at 21 days of infection was barely detectable by microscopy presented 5.7 × 10⁸ to 5.9 × 10⁹B. microti DNA copies confirming the sensitivity of our qPCR. We hypothesize that qPCR detects DNA released from recently lysed parasites or from extracellular B. microti in blood, which are not easily detected in blood smears and might result in under-diagnosis of babesiosis in patients. Splenectomized patients have been reported to show increased babesiosis severity and result in high morbidity and mortality. These results emphasize the importance of splenic immunity in resolution of B. microti infection. Splenomegaly in infected mice associated with destruction of marginal zone with lysed erythrocytes and released B. microti life forms in our experiments support this premise. At conclusion of the experiment at 21 days post-infection, significant splenic B and T cells depletion and increase in macrophages levels were observed in B. microti infected mice suggesting a role of macrophage in disease resolution. Infected mice also showed significantly higher plasmatic concentration of CD4 Th1 cells secreted cytokines such as IL-2 and IFN-γ while cytokines such as IL-4, IL-5, and IL-13 secreted by Th2 cells increase was not always significant. Thus, Th1 cells-mediated immunity appears to be important in clearance of this intracellular pathogen. Significant increase in IL-6 that promotes differentiation of Th17 cells was observed but it resulted in only moderate change in IL-17A, IL-17F, IL-21, and IL-22, all secreted by Th17 cells. A similar immune response to Trypanosoma infection has been reported to influence the clearance of this protozoan, and co-infecting pathogen(s).

Allocation, not male resistance, increases male frequency during epidemics: a case study in facultatively sexual hosts

Why do natural populations vary in the frequency of sexual reproduction? Virulent parasites may help explain why sex is favored during disease epidemics. To illustrate, we show a higher frequency of males and sexually produced offspring in natural populations of a facultative parthenogenetic host during fungal epidemics. In a multi-year survey of 32 lakes, the frequency of males (an index of sex) was higher in populations of zooplankton hosts with larger epidemics. A lake mesocosm experiment established causality: experimental epidemics produced a higher frequency of males relative to disease-free controls. One common explanation for such a pattern involves Red Queen (RQ) dynamics. However, this particular system lacks key genetic specificity mechanisms required for the RQ, so we evaluated two other hypotheses. First, individual females, when stressed by infection, could increase production of male offspring vs. female offspring (a tenant of the "Abandon Ship" theory). Data from a life table experiment supports this mechanism. Second, higher male frequency during epidemics could reflect a purely demographic process (illustrated with a demographic model): males could resist infection more than females (via size-based differences in resistance and mortality). However, we found no support for this resistance mechanism. A size-based model of resistance, parameterized with data, revealed why: higher male susceptibility negated the lower exposure (a size-based advantage) of males. These results suggest that parasite-mediated increases in allocation to sex by individual females, rather than male resistance, increased the frequency of sex during larger disease epidemics.

Elimination of Babesia microti Is Dependent on Intraerythrocytic Killing and CD4+ T Cells

Babesiosis is a tick-borne zoonosis caused by protozoans of the genus Babesia, apicomplexan parasites that replicate within erythrocytes. However, unlike related Plasmodium species, the pathogenesis of Babesia infection remains poorly understood. The primary etiological agent of babesiosis in the United States is B. microti. In healthy individuals, tick-transmitted infection with Babesia causes no specific clinical manifestations, with many having no symptoms at all. However, even in asymptomatic people, a Babesia carriage state can be established that can last up to a year or more. Current blood bank screening methods do not identify infected donors, and Babesia parasites survive blood-banking procedures and storage. Thus, Babesia can also be transmitted by infected blood, and it is currently the number one cause of reportable transfusion-transmitted infection in the United States. Despite a significant impact on human health, B. microti remains understudied. In this study, we evaluated the course of Babesia infection in three strains of mice, C57BL/6J, BALB/cJ, and C3H-HeJ, and examined the contribution of multiple immune parameters, including TLRs, B cells, CD4⁺ cells, IFN-γ, and NO, on the level of parasitemia and parasite clearance during acute babesiosis. We found that B. microti reaches high parasitemia levels during the first week of infection in all three mice strains before resolving spontaneously. Our results indicate that resolution of babesiosis requires CD4 T cells and a novel mechanism of parasite killing within infected erythrocytes.

Babesia in a Nonsplenectomized Patient Requiring Exchange Transfusion

Babesiosis is a tick born zoonosis caused by red blood cell parasites of the genus Babesia. It is caused predominantly by B. microti and B. divergens, microti being more common in the US. The parasites are transmitted by Ixodes tick to their host but infection can also spread by blood transfusion and perinatally. Clinical manifestations vary from subclinical infection to fulminating disease depending upon the immune status of the patient. About half of patients, hospitalized with babesiosis, develop complication with fatality rates of 6 to 9% which increase up to 21% among those with immunosuppression. A case of 58-year-old previously healthy man, infected by B. microti, was reported on 2000 who presented with severe disease characterized by severe anemia, DIC, and renal and respiratory failure. First case of overwhelming septic shock without respiratory involvement due to babesiosis in a healthy patient with an intact spleen was published in a case report on 2011. Since our patient here is an immunocompetent healthy male with intact spleen presenting with severe babesiosis requiring exchange transfusion, this presentation of Babesia is rare and warrants further study.

Changes in selected subpopulations of lymphocytes in dogs infected with Babesia canis treated with imidocarb

OBJECTIVE

The purpose of this study was to track changes in selected subpopulations of lymphocytes in the blood of dogs infected with Babesia (B.) canis and treated with imidocarb.

MATERIAL AND METHODS

The study included 16 dogs divided into two groups. The first group (n = 6) consisted of healthy control animals. Dogs of the se- cond group (n = 10) were infected with B. canis and after establishment of the diagnosis each animal received a single dose of imido- carb (5 mg/kg). Flow cytometry was used to enumerate several immune cell phenotypes.

RESULTS

It was concluded that the invasion of B. canis contributes to the decreased percentage of CD3+, CD4+, CD8+, CD21+ lymphocytes in the blood of infected animals. The decreased level of tested subpopulations of lymphocytes in group 2 persisted for the entire 12-day period of the test. After the administration of imidocarb, each tested lymphocyte fraction in the blood of the dogs with babesiosis increased, but did not reach physiological values.

CONCLUSION

The presented results indicate that the resolution of clinical signs associated with babesiosis may be related to the stimulation and intensity of cellular immunity, dependent on the CD4+ T cells profile. After administration of imidocarb, the parasitemia is cleared which allows the recovery of the lymphocyte populations.

Parasite evolution in response to sex-based host heterogeneity in resistance and tolerance

Heterogenity between sexes in terms of both the level and the type of immune response to infection is documented in many species, but its role on parasite evolution is only beginning to be explored. We adopt an evolutionary epidemiology approach to study how the ability of a host to respond to infection through active immunity (resistance) or through minimizing deleterious effects of a given parasite load (tolerance) affects the evolution of parasite virulence. Consistently with earlier models, we find that increases in host resistance and tolerance both favour more virulent parasite strains. However, we show that qualitatively different results can be obtained if dimorphism between the sexes occurs through resistance or through tolerance depending on the contact pattern between the sexes. Finally, we find that variations in host sex ratio can amplify the consequences of heterogeneity for parasite evolution. These results are analysed in the light of several examples from the literature to illustrate the prevalence of sexually dimorphic immune responses and the potential for further study of the role of sexual dimorphism on parasite evolution. Such studies are likely to be highly relevant for improving treatment of chronic infections and control of infectious diseases, and understanding the role of sex in immune function.

Macrophages are the determinant of resistance to and outcome of nonlethal Babesia microti infection in mice

In the present study, we examined the contributions of macrophages to the outcome of infection with Babesia microti, the etiological agent of human and rodent babesiosis, in BALB/c mice. Mice were treated with clodronate liposome at different times during the course of B. microti infection in order to deplete the macrophages. Notably, a depletion of host macrophages at the early and acute phases of infection caused a significant elevation of parasitemia associated with remarkable mortality in the mice. The depletion of macrophages at the resolving and latent phases of infection resulted in an immediate and temporal exacerbation of parasitemia coupled with mortality in mice. Reconstituting clodronate liposome-treated mice at the acute phase of infection with macrophages from naive mice resulted in a slight reduction in parasitemia with improved survival compared to that of mice that received the drug alone. These results indicate that macrophages play a crucial role in the control of and resistance to B. microti infection in mice. Moreover, analyses of host immune responses revealed that macrophage-depleted mice diminished their production of Th1 cell cytokines, including gamma interferon (IFN-γ) and tumor necrosis factor alpha (TNF-α). Furthermore, depletion of macrophages at different times exaggerated the pathogenesis of the infection in deficient IFN-γ(-/-) and severe combined immunodeficiency (SCID) mice. Collectively, our data provide important clues about the role of macrophages in the resistance and control of B. microti and imply that the severity of the infection in immunocompromised patients might be due to impairment of macrophage function.

Looking for prognosticators in ovine anaplasmosis: discriminant analysis of clinical and haematological parameters in lambs belonging to differently susceptible breeds experimentally infected with Anaplasma ovis

BACKGROUND

A study was carried out to evaluate the response of different native sheep breeds to experimental infection with Anaplasma ovis, the most prevalent sheep tick-borne pathogen in Apulia (Southern Italy). Thirty-four lambs belonging to a Northern European breed (Suffolk) and two Southern Italian breeds (Comisana and Altamurana) were infected. Eleven clinical as well as haematological parameters were monitored at different temporal resolutions on the same subjects before and after the infection, resulting in a data set of 435 observations. The present work, aiming to further the research, presents the results of a multivariate analysis carried out to identify which parameters out of the eleven considered are the most reliable parameters to be considered as markers of the disease phenotype as well as prognosticators of practical clinical importance.

RESULTS

Data were analysed by discriminant analysis. Out of the eleven considered variables (red blood cells, packed cell volume, mean corpuscular volume, mean corpuscular haemoglobin, mean corpuscular haemoglobin content, haemoglobin concentration, white blood cells, neutrophils, leukocytes, platelets, rectal temperature), only seven were included in the step-wise model since significantly increasing the Mahlanobis distance between the two closest groups. Both discriminant functions resulted to be highly significant (P < 0.0001) and the percentage of variation accounted for by the first discriminant function was 63.6% of the variance in the grouping variable.

CONCLUSIONS

Taken together, the observed results stress the marked differentiation among the three breeds in terms of physio-pathological phenotypes indicating packed cell volume and red blood cell count as the most informative parameters in the routine clinical practice for A. ovis infection in sheep.

Effect of sex steroids on Babesia microti infection in mice

Sex-based-differences are known to affect susceptibility to protozoan infections, but their effects on parasitemia and clinical symptoms in Babesia infections remain unclear. We examined the sex-based susceptibility of various mouse strains to Babesia microti Munich strain infection. In all strains, male mice exhibited significantly higher peak parasitemia and more severe anemia than female mice. Testosterone and estradiol-17β treatment caused an increase in parasitemia and aggravation of anemia. Orchidectomized male mice receiving testosterone exhibited smaller splenic macrophage populations three days after infection, smaller B cell populations 10 days after infection, and reduced splenic tumor necrosis factor-α and interferon-γ mRNA expression than mice that did not receive testosterone. Mice receiving estradiol-17β did not exhibit immunosuppressive effects. Thus, a weakened and delayed innate immunity response may lead to acquired immunity failure. The results suggested that testosterone directly affects T or B cells, leading to delayed acquired immunity, dramatically increased parasitemia, and severe anemia.

Macrophages are critical for cross-protective immunity conferred by Babesia microti against Babesia rodhaini infection in mice

Although primary infection of mice with Babesia microti has been shown to protect mice against subsequent lethal infection by Babesia rodhaini, the mechanism behind the cross-protection is unknown. To unravel this mechanism, we investigated the influence of primary infection of mice with nonlethal B. microti using different time courses on the outcome of subsequent lethal B. rodhaini infection. Simultaneous infections of mice with these parasites resulted in rapid increases in parasitemia, with 100% mortality in BALB/c mice, as observed with control mice infected with B. rodhaini alone. In contrast, mice with acute, resolving, and chronic-phase B. microti infections were completely protected against B. rodhaini, resulting in low parasitemia and no mortalities. Mice immunized with dead B. microti were not protected from B. rodhaini infection, although high antibody responses were induced. Interestingly, the protected mice had significantly decreased levels of antibody response, cytokines (including gamma interferon [IFN-γ], interleukin-2 [IL-2], IL-8, IL-10, and IL-12), and nitric oxide levels after infection with B. rodhaini. SCID mice and IFN-γ-deficient mice with chronic B. microti infections demonstrated protective responses comparable to those of immunocompetent mice. Likewise, in vivo NK cell depletion did not significantly impair the protective responses. Conversely, macrophage depletion resulted in increased susceptibility to B. rodhaini infection associated with changes in their antibody and cytokines profiles, indicating that macrophages contribute to the protection against this challenge infection. We conclude that future development of vaccines against Babesia should include a strategy that enhances the appropriate activation of macrophages.

Course of infection by Babesia sp. BQ1 (Lintan) and B. divergens in sheep depends on the production of IFNgamma and IL10

Ovine babesiosis is an important disease in China and responsible for economic losses. Several Babesia strains are involved, but Babesia sp. BQ1 (Lintan) and Babesia sp. BQ1 (Ningxian) are particularly prevalent in the Guansu region. Babesia divergens, in contrast, can experimentally infect spleen-intact sheep, but does not induce clinical signs. The immune response of spleen-intact sheep to Babesia sp. BQ1 (Lintan) and to B. divergens was therefore compared to identify the immune mechanisms involved in pathogenicity. The greater pathogenicity of Babesia sp. BQ1 (Lintan) than that of B. divergens was confirmed: sheep infected with Babesia sp. BQ1 (Lintan), but not with B. divergens, developed hyperthermia and showed patent parasitaemia in Giemsa-stained blood smears from the ear vein. Furthermore, more parasites were also detected in the blood from the jugular vein of Babesia sp. BQ1 (Lintan)-infected sheep. Pathogenicity of Babesia spp. involved cellular responses, but not humoral responses. Interferon-gamma was produced only by specifically activated PBMC from B. divergens-infected sheep and interleukin-10 only by specifically activated PBMC from Babesia sp. BQ1 (Lintan)-infected sheep. The role of these cytokines in the course of infection by Babesia spp. is discussed.

Prospects for recombinant vaccines against Babesia bovis and related parasites

Babesial parasites infect cattle in tropical and temperate regions of the world and cause significant morbidity and mortality. Discovery of protective antigens that could be used in a killed vaccine has been slow and to date there are few promising vaccine candidates for cattle Babesia. This review describes mechanisms of protective innate and adaptive immune responses to babesial parasites and different strategies to identify potentially protective protein antigens of B. bovis, B. bigemina, and B. divergens. Successful parasites often cause persistent infection, and this paper also discusses how B. bovis evades and regulates the immune response to promote survival of parasite and host. Development of successful non-living recombinant vaccines will depend on increased understanding of protective immune mechanisms and availability of parasite genomes.

Immune control of Babesia bovis infection

Babesia bovis causes an acute and often fatal infection in adult cattle, which if resolved, leads to a state of persistent infection in otherwise clinically healthy cattle. Persistently infected cattle are generally resistant to reinfection with related parasite strains, and this resistance in the face of infection is termed concomitant immunity. Young animals are generally more resistant than adults to B. bovis infection, which is dependent on the spleen. Despite the discovery of B. bovis over a century ago, there are still no safe and effective vaccines that protect cattle against this most virulent of babesial pathogens. Immunodominant antigens identified by serological reactivity and dominant T-cell antigens have failed to protect cattle against challenge. This review describes the innate and acquired immune mechanisms that define resistance in young calves and correlate with the development of concomitant immunity in older cattle following recovery from clinical disease. The first sections will discuss the innate immune responses by peripheral blood- and spleen-derived macrophages in cattle induced by B. bovis merozoites and their products that limit parasite replication, and comparison of natural killer cell responses in the spleens of young (resistant) and adult (susceptible) cattle. Later sections will describe a proteomic approach to discover novel antigens, especially those recognized by immune CD4+ T lymphocytes. Because immunodominant antigens have failed to stimulate protective immunity, identification of subdominant antigens may prove to be important for effective vaccines. Identification of CD4+ T-cell immunogenic proteins and their epitopes, together with the MHC class II restricting elements, now makes possible the development of MHC class II tetramers and application of this technology to both quantify antigen-specific lymphocytes during infection and discover novel antigenic epitopes. Finally, with the imminent completion of the B. bovis genome-sequencing project, strategies using combined genomic and proteomic approaches to identify novel vaccine candidates will be reviewed. The availability of an annotated B. bovis genome will, for the first time, enable identification of non-immunodominant proteins that may stimulate protective immunity.

The role of sex steroids in the complex physiology of the host-parasite relationship: the case of the larval cestode of Taenia crassiceps

Sex steroids play a significant role in regulating the parasite load in experimental intraperitoneal Taenia crassiceps cysticercosis of male and female Balbc/anN mice. Briefly, oestrogens increase parasite loads and androgens decrease them (1) by acting directly on the parasite, favouring or hindering its reproduction, respectively, and (2) by biasing the hosts' immune response towards a parasite-permissive Th2 or a parasite-restrictive Th1 response. The infected male host also undergoes drastic endocrinological and behavioural changes that may impinge upon the course of infection, and the host's mating behaviour and its exposure to predators. In addition, at different times of infection, significant changes occur in the expression of c-fos in the host's hyppocampus, hypothalamus and preoptic area. Thus, the host's brain seems to sense and/ or react to intraperitoneal infection. The physiological domains of the network affected by the infection, which classically included the hypothalamus-pituitary-axis and the immune system, must now incorporate the host's sexual hormones and other areas of the brain. The network's complex circuitry and functions may help understand some basic questions of parasitology (i.e. the hosts' sexual dimorphism in parasite infections, host-parasite specificity, heterogeneity in the course and outcome of infections at different stages of parasite and host development). The plurality of elements and the complexity of the network that regulates the host-parasite relationship also point to additional strategies for the treatment and control of infections.

Sex-dependent susceptibility to Listeria monocytogenes infection is mediated by differential interleukin-10 production

It is well documented that sex-dependent factors affect susceptibility to infection, with most mouse models demonstrating higher resistance in females. We made the unexpected observation that infection with the intracellular bacterium Listeria monocytogenes showed an opposite pattern in several commonly used inbred mouse strains: female C57BL/6J, BALB/c, C3H/HeN, and CBA/J mice were significantly more susceptible to Listeria infection. The pronounced sensitivity of females to Listeria, which was revealed by significantly higher lethality rates, correlated also with increased bacterial numbers in organ tissues (spleen and liver) and several immunological changes in peripheral blood samples. Surprisingly, increased severity of infection in females was associated with elevated interleukin-10 (IL-10) levels in plasma. Experiments using Il10 knockout mice, for which no differences between the susceptibilities of males and females to Listeria infection could be detected, confirmed the important role of this immunosuppressive cytokine for the outcome of disease. Our findings are likely to have clinical relevance, since similar sex differences with regard to infection with Listeria monocytogenes and other intracellular pathogens have been reported for humans.

The treatment of mice with Lactobacillus casei induces protection against Babesia microti infection

In this study, we report that administration of Lactobacillus casei confers protection to mice against the intracellular protozoan Babesia microti. Mice treated with L. casei orally or intraperitoneally were inoculated 7 days later with an infectious dose of B. microti. Mice treated with lactobacilli showed significant reduction in the percentage of parasitized erythrocytes (PPE) compared to untreated mice. When mice were inoculated intraperitoneally with L. casei 3 or 0 days before challenge with B. microti, the PPE was significantly lower compared to untreated mice and there were no differences between treated mice and mice immune to B. microti infection. When mice treated with live or dead L. casei were compared to mice inoculated with Freund Complete Adjuvant before a B. microti infection, a significant reduction of PPE was observed. These results show the protective effect of L. casei administered to mice against a B. microti infection and suggest that it might act by stimulating the innate immune system.

The pathogenicity of Entamoeba histolytica is related to the capacity of evading innate immunity

The host and parasite factors that influence susceptibility to Entamoeba histolytica infection and disease are not well understood. Entamoeba histolytica pathogenicity has been considered by focusing principally on parasite rather than host factors. Thus, research has concentrated on explaining the molecular differences between pathogenic E. histolytica and non-pathogenic E. dispar. However, the amoeba molecules considered most important for host tissue destruction (amoebapore, galactose/N-acetyl galactosamine inhibitable lectin, and cysteine proteinases) are present in both pathogenic E. histolytica and non-pathogenic E. dispar. In addition, the genetic differences in pathogenicity among E. histolytica isolates are unlikely to completely explain the different outcomes of infection. Considering that the principal difference between pathogenic and non-pathogenic amoebas lies in their surface coats, we propose that pathogenicity of the amoebas is related to the composition and properties of the surface coat components (or pathogen-associated molecular patterns, PAMPs), and the ability of innate immune response to recognize these components and eliminate the parasite. According to this hypothesis, a key feature that may distinguish pathogenic (E. histolytica) from non-pathogenic (E. dispar) strains is whether or not they can overcome innate immune defences. A corollary of this hypothesis is that in susceptible individuals the PAMPs are either not recognized or they are recognized by a set of Toll-like receptors (TLRs) that leads to an inflammatory response. In both cases, the result is tissue damage. On the contrary, in resistant individuals the innate/inflammatory response, induced through the activation of a different set of TLRs, eliminates the parasite.

Hormonal and immunological mechanisms mediating sex differences in parasite infection

The prevalence and intensity of infections caused by protozoa, nematodes, trematodes, cestodes, and arthropods is higher in males than females. The primary thesis of this review is that immunological differences exist between the sexes that may underlie increased parasitism in males compared to females. Several field and laboratory studies link sex differences in immune function with circulating steroid hormones; thus, the roles of sex steroids, including testosterone, oestradiol, and progesterone, as well as glucocorticoids will be discussed. Not only can host hormones affect responses to infection, but parasites can both produce and alter hormone concentrations in their hosts. The extent to which changes in endocrine-immune interactions following infection are mediated by the host or the parasite will be considered. Although males are more susceptible than females to many parasites, there are parasites for which males are more resistant than females and endocrine-immune interactions may underlie this sex reversal. Finally, although immunological differences exist between the sexes, genetic and behavioural differences may explain some variability in response to infection and will be explored as alternative hypotheses for how differences between the sexes contribute to dimorphic responses to parasites.

Resistance to acute babesiosis is associated with interleukin-12- and gamma interferon-mediated responses and requires macrophages and natural killer cells

We examined the role of the cytokines gamma interferon (IFN-gamma) and interleukin-12 (IL-12) in the model of acute babesiosis with the WA1 Babesia. Mice genetically deficient in IFN-gamma-mediated responses (IFNGR2KO mice) and IL-12-mediated responses (Stat4KO mice) were infected with the WA1 Babesia, and observations were made on the course of infection and cytokine responses. Levels of IFN-gamma and IL-12 in serum increased 24 h after parasite inoculation. The augmented susceptibility observed in IFNGR2KO and Stat-4KO mice suggests that the early IL-12- and IFN-gamma-mediated responses are involved in protection against acute babesiosis. Resistance appears to correlate with an increase in nitric oxide (NO) production. In order to assess the contribution of different cell subsets to resistance against the parasite, we also studied mice lacking B cells, CD4+ T cells, NK cells, and macrophages. Mice genetically deficient in B lymphocytes or CD4+ T lymphocytes were able to mount protective responses comparable to those of immunosufficient mice. In contrast, in vivo depletion of macrophages or NK cells resulted in elevated susceptibility to the infection. Our observations suggest that a crucial part of the response that protects from the pathogenic Babesia WA1 is mediated by macrophages and NK cells, probably through early production of IL-12 and IFN-gamma, and induction of macrophage-derived effector molecules like NO.

Cellular immunity, but not gamma interferon, is essential for resolution of Babesia microti infection in BALB/c mice

A new strain of Babesia microti (KR-1) was isolated from a Connecticut resident with babesiosis by hamster inoculation and adapted to C3H/HeJ and BALB/c mice. To examine the relative importance of humoral and cellular immunity for the control of B. microti infection, we compared the course of disease in wild-type BALB/c mice with that in BALB/c SCID mice, JHD-null (B-cell-deficient) mice, and T-cell receptor alphabeta (TCRbeta(-/-)) or gamma interferon (IFN-gamma) (IFN-gamma(-/-)) knockout mice following inoculation with the KR-1-strain. SCID mice and TCRalphabeta knockouts sustained a severe but nonlethal parasitemia averaging 35 to 45% infected erythrocytes. IFN-gamma-deficient mice developed a less severe parasitemia but were able to clear the infection. In contrast, in six of eight JHD-null mice, the levels of parasitemia were indistinguishable from those in the wild-type animals. These data indicate that cellular immunity is critical for the clearance of B. microti in BALB/c mice but that disease resolution can occur even in the absence of IFN-gamma.