Not peripheral parasitaemia but the level of soluble parasite antigen in plasma correlates with vaccine efficacy against Babesia canis

Immune Response of Mice Against Babesia canis Antigens is Enhanced When Antigen is Coupled to Gold Nanoparticles

PURPOSE

The aim of this study was to isolate Babesia canis soluble antigens and to investigate the effect of their conjugates with gold nanoparticles on the immunogenicity in laboratory animals.

METHODS

A procedure was developed for isolating and purifying B. canis antigens. The isolated culture antigen of B. canis 495 was coupled to gold nanoparticles, and the conjugate was used to immunize laboratory mice.

RESULTS

Western blotting showed that the resultant antiserum specifically recognized the proteins of the B. canis strains isolated from naturally infected dogs. The antibody titer, the respiratory activity of peritoneal macrophages, the proliferative activity of splenocytes, and the production of cytokines were maximal when the animals were immunized with the antigen-nanoparticle conjugate emulsified in complete Freund's adjuvant. Without adjuvant, the babesial antigen was weakly immunogenic.

CONCLUSION

Therefore, the use of gold nanoparticles as an antigen carrier induced a broad immune response involving both cellular and humoral responses. The antibodies raised by the proposed procedure are potentially effective at immunodetection of Babesia canis infections in dogs.

Mechanisms Involved in the Persistence of Babesia canis Infection in Dogs

Dogs that are infected with Babesia canis parasites usually show severe clinical signs, yet often very few parasites are detectable in the blood circulation. The results showed that large numbers of B. canis-infected red blood cells accumulate in the microvasculature of infected subjects. The initial process leading to the attachment of infected erythrocytes to the endothelial cells of small capillaries (sequestration) appears to involve the interaction of parasite molecules at the erythrocyte surface with ligands on the endothelial cells. Since parasites continue to develop in the sequestered erythrocyte, it would be expected that the infected erythrocyte is destroyed when the mature parasites escape the host cell, which would make it hard to explain accumulation of infected erythrocytes at the initial site of attachment. Apparently, additional processes are triggered that lead to consolidation of parasite sequestration. One possible explanation is that after initial attachment of an infected erythrocyte to the wall of a blood capillary, the coagulation system is involved in the trapping of infected and uninfected erythrocytes. The data further suggest that newly formed parasites subsequently infect normal red blood cells that are also trapped in the capillary, which finally leads to capillaries that appear to be loaded with infected erythrocytes.

Canine Babesiosis: Where Do We Stand?

Canine babesiosis is a tick-borne disease caused by protozoal haemoparasites of different Babesia species. Babesiosis is one of the most important globally extended and quickly spreading tick-borne infections of dogs. This comprehensive review gives an in-depth overview of Babesia species currently identified in dogs together with relevant vector tick species and their geographical distribution, life cycle and transmission of parasite. The main mechanisms in the pathogenesis of babesiosis are described and elucidated by recent literature overview. As Babesia infection causes a disease with very variable clinical manifestations, special attention is given to clinical signs, laboratory features and clinicopathological findings. The diagnosis of canine babesiosis by microscopy, serological and molecular methods is reviewed, together with recent advances in mass spectrometry based assays. Accurate detection and species recognition are important for the selection of the appropriate therapy, monitoring and prediction of the outcome of the disease. Finally, guidelines for the treatment and prevention of canine babesiosis are given.

First molecular detection of Babesia canis in dogs from Bosnia and Herzegovina

Babesia spp. are tick-transmitted protozoan haemoparasites of great economic, veterinary and medical impact worldwide. Herein we reported the very high prevalence of autochthonous babesiosis in symptomatic dogs from Bosnia and Herzegovina in the period from 2014 to 2016. Eighty dogs that did not leave the country were examined using parasitological and molecular analyses and babesiosis was diagnosed in 82.5% and 85.0% of them, respectively (p < 0.001). One species, Babesia canis was identified using molecular methodology (PCR and sequence analysis). Statistical analyses showed that epizootiological characteristics have no influence on the possibility of infection. Agglomerative hierarchical clustering (AHC) analyses used for comparing the symptoms and clinical signs of infection in dogs pointed out that a high degree of anemia, followed by thrombocytopenia (89%), lethargy (100%), loss of appetite (95%), fever (66%) and icterus (61%) was dominant. In addition, results of the statistical analysis performed showed that more dogs with no data of tick prophylaxis (70%) were found Babesia infected. Those results point to further intensified epizootic surveys in the territory of Bosnia and Herzegovina.

Discovery of a recombinant Babesia canis supernatant antigen that protects dogs against virulent challenge infection

Soluble parasite antigens (SPA) in supernatants of in vitro cultures of Babesia canis can be used to vaccinate dogs against virulent B. canis infection. The moment that immunity becomes apparent coincides with the appearance of antibodies against SPA in the serum of the vaccinated animals. This so-called vaccination-challenge serum (VC-serum) was used to precipitate antigens from B. canis culture supernatants in agarose gels. This antigen preparation was then used to analyse the reactivity of sera from vaccinated dogs on western blots.

RESULTS

showed that the first appearance of antibody reactivity against a protein that migrated at the 39kDa position in SDS-PAGE gels was associated with the moment vaccinated dogs started to recover from a virulent challenge infection. In addition, pulse-chase experiments revealed that a 39-40kDa doublet was released into the supernatant of B. canis cultures starting 15min after the chase. This doublet was specifically precipitated by VC-serum, thus corroborating that the 39-40kDa doublet in SPA preparations was of parasite origin. Partial amino acid sequencing allowed the discovery of the gene that encoded the 39-40kDa doublet (canine Babesia antigen; CBA). The full-length gene was cloned and expressed in E. coli. The recombinant CBA protein (rCBA) was recognized by VC-serum, and antibodies against rCBA precipitated the 39kDa antigen of SPA preparations and of merozoites of B. canis. In addition, anti-rCBA serum reacted with the surface of B. canis merozoites (but not with B. rossi merozoites) in immunofluorescence. Vaccination of dogs with rCBA induced antibodies against rCBA, which recognized B. canis merozoites. Vaccinated dogs were protected against virulent challenge infection by limiting parasite proliferation. As a result, the development of clinical signs was prevented and the animals self-cured. In contrast, six out of seven non-vaccinated control dogs developed relatively high parasitaemia and serious clinical signs associated with poor tissue perfusion. This antigen can be used to replace the SPA antigen in the conventional B. canis vaccines, which eliminates the need for dog blood and serum for vaccine production.

An ELISA for the early diagnosis of acute canine babesiosis detecting circulating antigen of large Babesia spp

Babesia canis is the predominant Babesia species in dogs in Europe and is responsible for a severe and fatal disease. An increase in global pet tourism and a widening of the geographic distribution of the tick vector has led to the emergence of infections in areas where previously only imported cases have been reported. Due to the potential for rapid and serious disease progression, direct parasite detection by stained blood smears and light microscopy or DNA-based methods have traditionally been used for the diagnosis of acute infections. This study describes the production of a murine monoclonal antibody ('mAb BcFIII 7/1/2') that reacts to a 65kDa corpuscular epitope present in B. canis-infected erythrocytes and can be used in an ELISA to detect circulating Babesia antigen during acute infections. The sensitivity of the ELISA was 100% (95%CI: 84.5-100) as determined using blood lysate samples from 27 dogs with acute B. canis infections. Sensitivity was reduced to 53.8% in 13 patent Babesia vogeli infections (95%CI: 26.1-79.6) based on the current test design using convalescent serum from a B. canis-infected dog. The specificity was determined to be 86.4% (95%CI: 64-96.4) using 22 samples from healthy canine blood donors. In the course of acute B. canis infections, the ELISA showed a positive result at the same time as a positive PCR result was recorded. This was 24-48h before parasites could be detected by light microscopy. Convalescent samples collected from 6 B. canis-infected dogs at least 14days post treatment resulted in negative ELISA reactions. The hyper-acute to acute phase of a B. canis infection represents an emergency situation with high mortality. To increase the chances of survival, a fast and accurate diagnosis and immediate treatment is required. The current study demonstrates the opportunity of an early and specific detection of acute infections by an AgELISA that is potentially translatable to a rapid diagnostic test design.

Protective efficacy of Babesia gibsoni culture-derived exoantigens against the challenge infection in dogs

The aim of this study is to determine the efficacy of exoantigens derived from Babesia gibsoni cultures to induce protective immunity against challenge exposure of virulent organisms. An attenuated B. gibsoni Oita strain was maintained in vitro by the microaerophilus stationary phase (MASP) method, and exoantigens-containing supernatant fluids were collected for preparation of the immunization. Two dogs received three subcutaneous immunizations with a 20-day interval of B. gibsoni exoantigens plus 0.5 mg saponin (Quil A). On day 68 after the prime immunization, the immunized dogs and control dogs were challenged intravenously with 2 × 10(8) virulent parasites of a homologous B. gibsoni strain. The results showed that exoantigens could induce a high degree of protection against virulent homologous challenge exposure. Two dogs immunized with exoantigens showed a lower parasitemia, accompanied by a slight decrease in the PCV that returned to normal values. Control dogs developed typical acute clinical signs, including severe anemia and hyperthermia. The immunization elicited humoral immune responses. In dogs immunized with exoantigens, the maximum antibody titer was 2,560 and 5,120 by indirect fluorescent antibody test (IFAT), respectively. Preliminary Western blot analysis of the immunogen revealed five dominant proteins of molecular weights of 18, 37, 43, 50, and 57 kDa. These results suggested that the culture-derived exoantigens were candidates for non-viable vaccine.

Babesiosis in dogs and cats--expanding parasitological and clinical spectra

Canine babesiosis caused by different Babesia species is a protozoal tick-borne disease with worldwide distribution and global significance. Historically, Babesia infection in dogs was identified based on the morphologic appearance of the parasite in the erythrocyte. All large forms of Babesia were designated Babesia canis, whereas all small forms of Babesia were considered to be Babesia gibsoni. However, the development of molecular methods has demonstrated that other Babesia species such as Babesia conradae, Babesia microti like piroplasm, Theileria spp. and a yet unnamed large form Babesia spp. infect dogs and cause distinct diseases. Babesia rossi, B. canis and Babesia vogeli previously considered as subspecies are identical morphologically but differ in the severity of clinical manifestations which they induce, their tick vectors, genetic characteristics, and geographic distributions, and are therefore currently considered separate species. The geographic distribution of the causative agent and thus the occurrence of babesiosis are largely dependent on the habitat of relevant tick vector species, with the exception of B. gibsoni where evidence for dog to dog transmission indicates that infection can be transmitted among fighting dog breeds independently of the limitations of vector tick infestation. Knowledge of the prevalence and clinicopathological aspects of Babesia species infecting dogs around the world is of epidemiologic and medical interest. Babesiosis in domestic cats is less common and has mostly been reported from South Africa where infection is mainly due to Babesia felis, a small Babesia that causes anemia and icterus. In addition, Babesia cati was reported from India and sporadic cases of B. canis infection in domestic cats have been reported in Europe, B. canis presentii in Israel and B. vogeli in Thailand. Babesiosis caused by large Babesia spp. is commonly treated with imidocarb dipropionate with good clinical response while small Babesia spp. are more resistant to anti-babesial therapy. Clinical and parasitological cure are often not achieved in the treatment of small Babesia species infections and clinical relapses are frequent. The spectrum of Babesia pathogens that infect dogs and cats is gradually being elucidated with the aid of molecular techniques and meticulous clinical investigation. Accurate detection and species recognition are important for the selection of the correct therapy and prediction of the course of disease.

Immunity against Babesia rossi infection in dogs vaccinated with antigens from culture supernatants

Soluble parasite antigens (SPA) from different Babesia species have been shown earlier to induce protective immunity when used as vaccine. However, initial attempts to produce such vaccine against Babesia rossi infection using SPA from B. rossi culture supernatants were not or only partially successful. Here we show that when dogs were vaccinated with a vaccine comprising SPA from B. rossi combined with SPA from Babesia canis protective immunity against experimental challenge infection was induced. Immunity was reflected in reduced clinical signs that resolved spontaneously, and reduction of parasitaemia and SPA in the blood. Not a single infected erythrocyte could be found in blood smears of dogs that had been repeatedly boosted (three vaccinations in total). In contrast, three out of four control dogs required chemotherapeutic treatment to prevent death. The fourth control dog showed a transient parasitaemia that resolved spontaneously. Vaccination did not prevent the development of a transient anaemia. It is concluded that a vaccine containing a mixture of SPA obtained from in vitro culture supernatants of B. rossi and B. canis induces protection in dogs against heterologous challenge infection with B. canis (as shown before) or B. rossi.

Capillary and venous Babesia canis rossi parasitaemias and their association with outcome of infection and circulatory compromise

This observational study of 100 dogs naturally infected with Babesia canis rossi determined whether severity of parasitaemia was associated with outcome of infection and documented the relative distribution of parasitised red blood cells (pRBC) in capillary and venous circulation. The association between increased parasitaemias and outcome with a clinically compromised circulation was also investigated. Outcome was defined as either hospitalisation with death, or hospitalisation with eventual recovery or treatment as an outpatient. Dogs were enrolled if large babesias were found on stained thin capillary blood smears made from an ear prick. Thin venous smears were prepared from jugular or cephalic blood. Parasitaemias were manually counted and expressed as the percent pRBC. Ten dogs died, 50 recovered after hospitalisation and 40 were treated as outpatients. Venous sampling site did not affect venous parasitaemia (P=0.6). Both capillary and venous parasitaemias of dogs that died were significantly higher than those of dogs that recovered after hospitalisation (P=0.002) and dogs that were treated as outpatients (P<0.0001). When assessing the whole group, capillary parasitaemia (median 0.61%, range <0.05-71.6%, interquartile range (IQR) 0.22-3.75%) was significantly higher than venous parasitaemia (median 0.14%, range 0-30.6%, IQR 0.046-0.52%) with P<0.0001. The 21 dogs with a clinically compromised circulation were more likely to die (P<0.0001) and had significantly higher capillary (median 5.98%, range 0.09-71.6%, IQR 2.44-19.41%) and venous (median 2.81%, range <0.05-30.6%, IQR 0.17-9.03%) parasitaemias than the 79 dogs with a clinically normal circulation (capillary median parasitaemia 0.38%, range <0.05-12.87%, IQR 0.16-1.42%; venous median parasitaemia 0.096%, range 0-6.13%, IQR <0.05-0.33%; P<0.0001). This study shows that high parasitaemia is significantly associated with death in B c rossi infected dogs. The previous clinical suspicion that capillary parasitaemias are usually higher than venous parasitaemias is confirmed. Thus capillary samples are the most appropriate diagnostic samples. Prior observations that a clinically compromised circulation is associated with death are confirmed. Despite the highly significant association between compromised circulation and higher parasitaemia, it is thought unlikely that parasite burden is the sole trigger for circulatory collapse.

Onset and duration of immunity against Babesia canis infection in dogs vaccinated with antigens from culture supernatants

It has previously been shown that dogs can be vaccinated against heterologous Babesia canis infection using a vaccine containing soluble parasite antigens (SPA) from in vitro cultures of B. canis and B. rossi that are adjuvanted with saponin. In the present study the onset and duration of immunity of vaccinated dogs were studied. Results showed that 3-26 weeks after initial vaccination, dogs effectively limit the level of SPA in plasma upon challenge infection, which was reflected in limited duration and extent of clinical manifestations. There was no statistically significant effect of vaccination on the parasite load in the circulation, which was determined from blood smears. It was further shown that the level of immunity of primary vaccinated dogs (priming and booster vaccination with a 6-week interval) and that of repeatedly vaccinated dogs (a single additional vaccination 6 months after primary vaccination) is comparable. From this study it is concluded that vaccination with this preparation induces protective immunity against clinical babesiosis from 3 weeks after booster vaccination onwards, and remains effective for a period of at least another 6 months. A single booster vaccination is sufficient to maintain immunity for at least another 6 months.

Vaccination against canine babesiosis

It has been known for several decades that the soluble parasite antigen (SPA) of several Babesia species can be used as a vaccine against the clinical manifestations of babesiosis. Originally observed in the plasma of infected animals, SPA can also be recovered from the supernatants of in vitro cultures of these parasites. Variable success has been reported for vaccines against the bovine and canine Babesia parasites, which seems to be related to antigenic diversity within Babesia species. In this article, an overview is presented of the development of such vaccines for dogs, and additional research that has led to improvement of an SPA-based vaccine against Babesia canis in dogs.

Vaccination of dogs against heterologous Babesia canis infection using antigens from culture supernatants

Soluble parasite antigens (SPA) from European Babesia canis can be used to protect dogs against a homologous but not heterologous challenge infection. In this study it is shown that when dogs are vaccinated with a mixture of SPA from both, a European B. canis isolate and a South African Babesia rossi isolate, protective immunity against heterologous B. canis infection is induced. Three groups of five beagle dogs each were vaccinated twice with graded doses of SPA derived from in vitro cultures of B. canis and B. rossi, with a 3-week interval. Saponin was used as adjuvant. Three weeks after booster vaccination immunological responsiveness against heterologous B. canis antigen was measured by seroconversion against infected erythrocytes and lymphocyte transformation using SPA. Upon vaccination dogs produced antibodies against infected erythrocytes and lymphoblastogenic responses against SPA in a dose-dependent manner. Dogs were then challenged with heterologous B. canis parasites. Dogs appeared to be protected against challenge infection, which was reflected in less severe decrease of packed cell volume (PCV) and reduced clinical signs. The level of protection to clinical signs (but not excessive PCV drop) was related to the level of SPA in plasma and spleen size, and not related to peripheral parasitaemia. The results suggest that vaccination with this bivalent vaccine primes T-helper cells that recognise common epitopes on SPA from an antigenically distinct B. canis isolate. These cells provide the essential Th signal to mount an effective and timely antibody response against SPA and parasites or parasitised erythrocytes, which prevents the further development of clinical babesiosis.

Vaccination of dogs against Babesia canis infection

This paper describes the clinico-pathological parameters measured in dogs that were vaccinated against Babesia canis using soluble parasite antigens (SPA) and then challenged. The packed cell volume (PCV) and the plasma creatinine value decreased immediately after challenge. Actual PCV values could be predicted in the first 5-6 days of the infection, assuming that creatinine values were modulated by increase of plasma volume. This association no longer existed after that time, and observations indicated splenic involvement in reduction of numbers of circulating erythrocytes. The anaemia due to B. canis infection appears to be the result of a multifactorial process including plasma volume increase, erythrocyte retention in the spleen and erythrocyte destruction, partly due to parasite proliferation. Vaccination limited the reduction of PCV values, and the development of splenomegaly. Differences in protection between vaccinated and control animals became apparent 6 days after infection, when a memory immune response becomes operative, and the onset of recovery of vaccinated animals correlated with the onset of antibody production against SPA.