Babesia argentina: the infectivity and immunogenicity of irradiated blood parasites for splenectomized calves

Variable and Variant Protein Multigene Families in Babesia bovis Persistence

Cattle infected with Babesia bovis face a bifurcated fate: Either die of the severe acute infection, or survive and carry for many years a highly persistent but generally asymptomatic infection. In this review, the author describes known and potential contributions of three variable or highly variant multigene-encoded families of proteins to persistence in the bovine host, and the mechanisms by which variability arises among these families. Ramifications arising from this variability are discussed.

Babesia bovis merozoite surface antigen 2 proteins are expressed on the merozoite and sporozoite surface, and specific antibodies inhibit attachment and invasion of erythrocytes

The Babesia bovis merozoite surface antigen 2 (MSA-2) locus encodes four proteins, MSA-2a(1), -2a(2), -2b, and -2c. With the use of specific antibodies, each MSA-2 protein was shown to be expressed on the surface of live extracellular merozoites and coexpression on single merozoites was confirmed. Individual antisera against MSA-2a, MSA-2b, and MSA-2c significantly inhibited merozoite invasion of bovine erythrocytes. As tick-derived sporozoites also directly invade erythrocytes, expression of each MSA-2 protein on the sporozoite surface was examined and verified. Finally, statistically significant inhibition of sporozoite binding to the erythrocytes was demonstrated by using antisera specific for MSA-2a, MSA-2b, and MSA-2c. These results indicate an important role for MSA-2 proteins in the initial binding and invasion of host erythrocytes and support the hypothesis that sporozoites and merozoites use common surface molecules in erythrocyte invasion.

Tick-borne diseases in transfusion medicine

Ticks are effective vectors of viral, bacterial, rickettsial and parasitic diseases. Many of the tick-borne diseases (TBDs) are of significance to transfusion medicine, either because of the risks they pose to the blood supply or the necessity for blood products required in their treatment. The transmission of tick-borne pathogens via blood transfusion is of global concern. However, among transfusion medicine practitioners, experience with most of these microorganisms is limited. Transfusion transmission of TBDs has been documented largely by means of single case reports. A better understanding of the epidemiology, biology and management of this group of diseases is necessary in order to assess the risks they pose to the blood supply and to help guide effective prevention strategies to reduce this risk. Unique methods are required to focus on donor selection, predonation questioning, mass screening and inactivation or eradication procedures. The role of the transfusion medicine service in their treatment also needs to be better defined. This article reviews the growing body of literature pertaining to this emerging field of transfusion medicine and offers some recommendations for transfusionists in dealing with TBDs.

Babesia bovis merozoite surface antigen 1 and rhoptry-associated protein 1 are expressed in sporozoites, and specific antibodies inhibit sporozoite attachment to erythrocytes

We examined Babesia bovis sporozoites for the expression of two molecules, merozoite surface antigen 1 (MSA-1) and rhoptry-associated protein 1 (RAP-1), that are postulated to be involved in the invasion of host erythrocytes. Both MSA-1 and RAP-1 were transcribed and expressed in infectious sporozoites. Importantly, monospecific MSA-1 and RAP-1 antisera each inhibited sporozoite invasion of erythrocytes in vitro. This is the first identification of antigens expressed in Babesia sp. sporozoites and establishes that, at least in part, sporozoites and merozoites share common targets of antibody mediated inhibition of erythrocyte invasion.

The effect of neutrophils, tumor necrosis factor, and granulocyte macrophage/colony stimulating factor on Babesia bovis and Babesia bigemina in culture

Bovine neutrophils, human recombinant tumor necrosis factor-alpha (TNF), and bovine recombinant granulocyte macrophage/colony stimulating factor (GM/CSF) were added to microaerophilic cultures of Babesia bovis and Babesia bigemina to determine if those substances could inhibit growth. Incorporation of [3H]hypoxanthine by the Babesia spp. was utilized as an indirect measure of parasite growth. When neutrophils were added to cultures of B. bovis and B. bigemina, the highest percentage inhibition of growth was attained. There was no significant enhancement of neutrophil killing when TNF or GM/CSF or both were added to either Babesia spp. Addition of TNF or GM/CSF or both substances (without neutrophils) resulted in an increase in growth of B. bovis and B. bigemina. For B. bovis, the group that contained neutrophils only and the group that contained neutrophils and TNF resulted in significantly higher growth inhibitions than the treatment group which contained neutrophils and GM/CSF or the group that contained neutrophils, TNF, and GM/CSF. No significant differences in inhibition were observed for the same treatment groups between B. bovis and B. bigemina.

Detection of Babesia bovis carrier cattle by using polymerase chain reaction amplification of parasite DNA

Carrier cattle infected with Babesia bovis are difficult to detect because of the low numbers of parasites that occur in peripheral blood. However, diagnosis of low-level infections with the parasite is important for evaluating the efficacies of vaccines and in transmission and epidemiological studies. We used the polymerase chain reaction (PCR) to amplify a portion of the apocytochrome b gene from the parasite and tested the ability of this method to detect carrier cattle. The target sequence is associated with a 7.4-kb DNA element in undigested B. bovis genomic DNA (as shown previously), and the amplified product was detected by Southern and dot blot hybridization. The assay was specific for B. bovis, since no amplification was detected with Babesia bigemina, Trypanosoma brucei, Anaplasma marginale, or leukocyte DNA. The target sequence was amplified in DNA from B. bovis Mexico, Texas, and Australia S and L strains, demonstrating the applicability of the method to strains from different geographic regions. The sensitivity of the method ranged from 1 to 10 infected erythrocytes extracted from 0.5 ml of blood. This sensitivity was about 1,000 times greater than that from the use of unamplified parasite DNA. By the PCR method, six B. bovis carrier cattle were detected 86% of the time (range, 66 to 100%) when they were tested 11 times, while with microscopic examination of thick blood smears, the same carrier cattle were detected only 36% of the time (range, 17 to 66%). The method provides a useful diagnostic tool for detecting B. bovis carrier cattle, and the sensitivity is significantly improved over that of current methods. The results also suggest that characteristics of the apocytchrome b gene may make this a valuable target DNA for PCR-based detection of other hemoparasites.

Investigations of breakdowns in protection provided by living Babesia bovis vaccine

Field investigations of protection afforded by live Babesia bovis vaccine in Australia revealed that a ninefold increase in vaccine failures occurred in the period from 1985 to 1990. Laboratory trials using 189 experimental cattle were conducted to evaluate the protection afforded by the Babesia bovis strain used in the commercial vaccine during this time. Four isolates from clinical cases of babesiosis in vaccinated cattle were assessed. The results showed that the strain used in the vaccine during the 5 year period was poorly protective against three isolates while a recently isolated and prepared vaccine strain was strongly protective. Circumstantial evidence is provided that indicates the vaccine failures were due to change in the field populations of Babesia bovis, rather than change in the strain used in the vaccine. Implications of the results for the future of Babesia bovis vaccines are discussed.

Comparison between Sri Lankan and Australian strains of Babesia bovis in the vaccination of imported cattle in Sri Lanka

A Sri Lankan strain of Babesia bovis (designated A strain) was isolated from larval ticks and prepared for use as vaccine by syringe-passage in 20 splenectomised calves followed by irradiation. The A strain and a vaccine strain of Babesia bovis (designated K strain) brought in frozen form from Australia were used to vaccinate 37 susceptible bulls imported from southern Australia. Rectal temperatures, packed cell volumes, parasitaemias and overt clinical signs were monitored for three weeks following vaccination. The results indicated that the A strain was slightly more virulent than the K strain but suitable for the vaccination of well-supervised cattle.

Immunological control of ticks and tick-borne parasitic diseases of livestock

Parasitic diseases inflict major losses on livestock production throughout the world. Currently, control of the diseases relies largely on prophylactic or therapeutic application of anti-parasitic drugs. In many instances, these measures are only partially effective. Moreover, they must be applied frequently, are therefore costly and time-consuming, and lead to the selection of drug resistance within the parasite populations. Thus, it has been recognized for several decades that effective methods of vaccination against parasitic diseases would have a major impact on livestock production. However, despite considerable efforts over the last 30 years, only a few parasite vaccines are currently in use and all of these involve the administration of live organisms.

Induction of protective immunity to Babesia divergens in Mongolian gerbils, Meriones unguiculatus, using culture-derived immunogens

Immunogens derived from microaerophilous stationary phase (MASP) cultures of Babesia divergens grown in bovine erythrocytes were used to inoculate the laboratory host of B. divergens, the Mongolian gerbil, Meriones unguiculatus. Animals inoculated subcutaneously twice with preparations of freeze-thawed merozoites in complete Freund's adjuvant were fully protected against homologous challenge, as were gerbils immunised with a non-viable preparation of parasite-enriched lysed infected bovine erythrocytes. Animals which had been infected with small numbers of parasitised erythrocytes from cultures cooled to 4 degrees C, allowed to recover, then challenged, also survived. All three groups had high antibody titres which dropped immediately after challenge and then rose again. Gerbils given culture supernatants containing soluble merozoite protein coat antigens were partially protected only after receiving a third inoculation. Non-immunised animals all died 4 days after challenge.

Attenuation of Babesia bovis by in vitro cultivation

A virulent strain of Babesia bovis was adapted to grow in erythrocyte culture in the presence of equine serum and in lieu of bovine serum. Four splenectomized calves inoculated with the adapted strain, 429, developed hematologic signs of infection and a low grade fever, but remained free of central nervous system (CNS) signs and recovered. All of six control animals inoculated with a virulent strain reacted severely and five showed CNS signs and died. The calves injected with the attenuated strain were solidly immune when challenged with the virulent strain at 44 or 78 days after vaccination.

cDNA clone encoding a high molecular weight antigen of Babesia bovis

An expression library was constructed by inserting cDNA copied from mRNA of the blood stages of Babesia bovis isolate KA into bacteriophage lambda gt11-amp3. An antigen-positive cDNA clone detected by screening the library with antibodies from cattle vaccinated with the KA isolate was shown to encode part of a high-molecular weight polypeptide antigen of B. bovis. This molecule was a dominant immunogen and was found by immunofluorescence to be within the parasite in infected erythrocytes.

Protective vaccination against virulent Babesia bovis with a low-molecular-weight antigen

A Babesia bovis low-molecular-weight antigen was purified from crude material by using affinity adsorption techniques first with a mouse monoclonal antibody and then by adsorption with normal bovine sera. The antigen was then further purified by gradient gel electrophoresis. Analysis by Western transfer revealed only one antigen band, with an apparent molecular mass of 29 kilodaltons. A band (12 by 0.3 by 0.6 cm) corresponding to this antigen was excised from the acrylamide gel and injected twice 4 weeks apart, together with 2.5 ml of Freund complete adjuvant, into nine nonsplenectomized adult cattle. The vaccinated cattle and five susceptible control animals were challenged with a virulent homologous strain 4 weeks after the second vaccination. None of the vaccinated animals was clinically affected, whereas three of five controls were severely affected. Control animals had significantly greater declines in packed cell volume and greater rises in temperature and parasitaemias than vaccinated animals.

Suppression of Babesia microti infection in mice concurrently infected with Fasciola hepatica

Blood cell parasitaemia of Babesia microti and the associated haematological changes were examined in mice harbouring patent Fasciola hepatica infections and in fluke-free control mice. B. microti parasitaemia was markedly suppressed in mice harbouring primary 7-week F. hepatica infections, as reflected in a reduction in the percentage of erythrocytes parasitised and in the incidence of multiple B. microti infections in the red cells. This suppression was accompanied by an annulment of B. microti induced reductions in haemoglobin and haematocrit levels in F. hepatica infected mice. In naive recipient mice, inoculated with blood from mice concurrently infected with F. hepatica and B. microti, the course of B. microti infection was characterised by a prolonged pre-parasitaemia period, a reduced peak parasitaemia and a delayed fall in haematocrit levels as compared to those inoculated with blood from mice infected with B. microti only. This feature may presumably be dose-related. The present study does not reveal the actual mechanism(s) involved in the suppression of the blood protozoan by F. hepatica. However, since B. microti has a preference for mature erythrocytes, the suppression may be a result of the altered erythrocyte kinetic state induced by the removal of erythrocytes by the blood-sucking fluke resulting in high levels of reticulocytes.

The irradiation of babesia bovis. II. The immunogenicity of irradiated blood parasites for intact cattle and splenectomised calves

Intraerythrocytic forms of B. bovis were exposed to 350 Grays (Gy) gamma irradiation and were then injected intravenously into intact two and three year old Hereford steers. One of 15 steers died on initial infection and subsequently six steers were given a virulent heterologous challenge three weeks after recovery; all six animals were highly immune. The remaining eight animals were kept under quarantine conditions for 10 months and were then challenged with a different virulent heterologous strain of B. bovis. Seven of eight were highly immune, but one animal died. Subsequently a further 12 steers were injected intravenously with 1 X 10(8) irradiated organisms. All showed only mild transient clinical signs. After 12 months quarantine in a tick-free area these animals were then challenged with a virulent heterologous strain and all 12 were shown to be highly immune. Irradiation reduced the infective dose from 1 X 10(8) to 2.5 X 10(3) parasites. These parasites multiplied at the same rate, and achieved the same maximum parasitaemia as the parent non-irradiated strain, but the disease produced by them was not severe. A dose of 2.5 X 10(3) non-irradiated parasites was lethal to all of the four animals which received it. It was concluded that irradiation had produced a predominantly avirulent parasite population.

The irradiation of Babesia bovis. 1. The difference in pathogenicity between irradiated and non-irradiated populations

Babesia bovis parasites attenuated by 35 krads gamma irradiation and parasites not exposed to irradiation, were injected into intact 2-year-old Hereford steers. All five animals receiving non-irradiated blood died but the five animals which received irradiated blood were only mildly affected. Highly significant differences were observed in changes to plasma fibrinogen, serum fibrinogen-like proteins, packed cell volume, partial thromboplastin time, prothrombin time, blood kinins, and plasma kininogen levels in the control animals but non-significant changes in these parameters occurred in the group receiving iradiated blood. Significant changes in the antiplasmin alpha 2M, and the antithrombin levels occurred in control cattle but not in the group receiving irradiated blood. Parasite multiplications rates and maximum parasitaemias were similar in both groups. Irradiation reduced the dose of living parasites from 1 x 10(8) to 2.5 x 10(3), but this was not the reason for the mild reactions. It was concluded that irradiation had selected an avirulent parasite population.

The invasion and growth of Babesia bovis in tick tissue culture

Erythrocytic forms of Babesia bovis inoculated into cell cultures of the tick Boophilus microplus invaded the tick cells and showed multiplication for up to 48 h after inoculation.

The identification of Babesia equi in Australia

A Babesia parasite, isolated from the blood of a horse at Bowral, New South Wales, was identified on the basis of its morphological features, host specificity and serological reactions, as Babesia equi (Laveran 1901). The case was originally reported by Churchill and Best (1976, Aust. vet. J. 52: 487) and is the first record of equine babesiosis in Australia. In preliminary studies, the organism produced only a mild disease in an intact horse, but caused the typical clinical syndrome of acute babesiosis in a splenectomised horse, which died 19 days after the intravenous inoculation of the parasites.

Acute Babesia bigemina infection: changes in coagulation and kallikrein parameters

Plasma from splenectomized calves infected with B. bigemina were assayed for disturbances in the coagulation system. Changes were observed in the WBCT, PTT, PT, TT and RT as well as in fibrinogen, calcium and thrombocyte levels. These changes were most marked during days 6--7 when parasite numbers were greatest. Massive intramuscular haemolysis coincided with these changes. The plasma kallikrein level fell progressively from days 6--11 and the level of activated kallikrein rose slightly. Parasite doubling time was calculated at 8.13 h. The similarities and differences between this disease and B. argentina infections of cattle are discussed.

Effect of trasylol on packed cell volume and plasma kallikrein activation in acute Babesia argentina infection of splenectomised calves

Three groups of four splenectomised calves, three months old, were infected with Babesia argentina parasites. One group was inoculated intravenously with 2000 kallikrein inhibitor units (kiu)/kg body weight of the broad-spectrum proteinase inhibitor, Trasylol, three times a day for 5 days commencing on the 3rd day after infection (early treatment). The same dose of Trasylol was given to a second group for 3 days commencing 8 days after infection (late treatment). The third group was untreated. Parasite multiplication rates were similar in the three groups. In the early treated group levels of activated kallikrein in plasma were significantly lower than those in the other two groups. The early treatment group also showed significantly higher levels of plasma kallikrein inhibitor. No significant differences in total plasma kallikrein levels were seen among the three groups. Packed cell volumes in the treated groups remained significantly higher than those in the controls. The implications of these findings are discussed.