Screening for active toxoplasmosis in patients by DNA hybridization with the ABGTg7 probe in blood samples

We report the potential use of a specific Toxoplasma gondii DNA probe (ABGTg7). We applied a dot blot hybridization assay to blood samples for the diagnosis of cerebral toxoplasmosis (CT), acute toxoplasmic lymphadenopathy (ATL), and disseminated toxoplasmosis in transplant recipients (TRs). We studied a total of 84 individuals: 38 patients and 46 controls. We found positive hybridization signals for 12 (66.7%) of 18 patients with confirmed CT, 9 (52.9%) of 17 patients with ATL, and 2 (66.7%) of 3 TRs. PCR assays were performed in parallel for patients with ATL, resulting in T. gondii DNA detection for 10 patients (58.8%). A comparative study between dot blot and PCR assays performed with the blood of mice that had been experimentally infected with tachyzoites gave similar results: 60 and 70% positive results, respectively. Finally, the sum of positive values obtained by both DNA tests (dot blot assay plus PCR) increased the rate of positivity for ATL patients to 76.4%. These results demonstrate that the T. gondii ABGTg7 repetitive DNA element is an additional useful resource for diagnosing Toxoplasma parasitemia in patients with CT and ATL and in TRs. Thus, our ABGTg7-based dot blot test may lead to an improvement in T. gondii detection methods in patients with acute toxoplasmosis.

Immunogenicity of the recombinant SAPA protein of Trypanosoma cruzi for mice

The humoral and cellular immune responses induced by the recombinant SAPA (shed acute phase antigen) of Trypanosoma cruzi were studied in mice and correlated with the immunologic control of parasitemia. The immunizing schedule used consisted of 2 weekly injections of 50 micrograms glutathione-S-transferase (GST)-SAPA in Freund's adjuvant. Specific alpha GST-SAPA antibodies were detected by enzyme-linked immunosorbent assay 1 wk after each antigen dose, the concentration of antibodies after the second injection being 30-fold higher than after the first. Immediate- (ITH) and delayed-type hypersensitivity (DTH) reactions were observed as footpad swelling after injecting 50 micrograms GST-SAPA in preimmunized mice as compared to naive controls. Adoptive transfer experiments indicated that these cutaneous reactions were mediated by lymphoid cells and not by serum. Both humoral and cellular responses were specific for the GST-SAPA antigen and did not cross-react with either the GST or the recombinant GST-1 T. cruzi antigen. Immunized mice that had developed high levels of antibody and DTH reaction to GST-SAPA were able to control the level of parasitemia after challenge with 10(3) blood trypomastigotes. The levels of parasitemia obtained were lowered to about 1/3 (P < 0.05) and mortality at day 60 was reduced from 67 to 25% (P = 0.085). Comparison of this immunizing method with other schedules involving more injections or higher antigen doses indicates that control of parasitemia can be obtained with low amounts of antigen and seems to be associated with the development of DTH.

Protective immunity induced in squirrel monkeys with a multiple antigen construct against the circumsporozoite protein of Plasmodium vivax

Saimiri boliviensis monkeys were immunized with a multiple antigen construct [(PvCS)2]2(P2)2 directed against the circumsporozoite protein of Plasmodium vivax or a combination of the multiple antigen construct with nonionic copolymer P1005, with P1005 and lipopolysaccharide, with muramyl tripeptide Mf-75, or with alum. Following intravenous challenge with 10,000 sporozoites of the Salvador I strain of P. vivax, 11 of the 26 monkeys were protected against patent parasitemia. Ten additional animals were partially protected. Following rechallenge of the 26 monkeys with 30,000 sporozoites of the homologous strain of parasite, four monkeys were totally protected and nine animals were partially protected.

Malaria in a nonimmune population after extended chloroquine or primaquine prophylaxis

Extended chemoprophylaxis against endemic malaria raises concern with regard to susceptibility after ceasing use of the drug. In this study, we measured attack rates of malaria among adult men for 28 weeks after they ended one year of prophylaxis using either weekly chloroquine (5 mg base/kg, n = 20), daily primaquine (0.5 mg base/kg, n = 30), or a placebo of primaquine (n = 41). The 28-week incidence densities, times to parasitemia, parasite densities, and symptoms of primary post-prophylaxis infections were not significantly different among the former primaquine, chloroquine, and placebo groups. However, the incidence of Plasmodium falciparum infection in the post-chloroquine group was significantly greater than in the post-primaquine group during the first (P = 0.03) and second (P = 0.02) months post-prophylaxis. Six of 10 primary P. falciparum and three of 10 P. vivax infections occurred in the former chloroquine group within one month after ending prophylaxis and the mean time to infection was 30-35 days. In contrast, only one P. falciparum and no P. vivax infections occurred during the first month after ending primaquine prophylaxis. The mean time to first parasitemia by either species of malaria parasite in this group was 72-77 days. There was no indication that daily use of primaquine for one year placed subjects at greater risk of malaria infection or to more severe clinical symptoms of malaria than subjects who had taken placebo or chloroquine, despite the potential for some degree of immunity to have been acquired in these latter two groups during the year-long prophylaxis period. The results do suggest that chloroquine suppressed P.falciparum infections until drug levels decreased, and that primaquine had effectively prevented the establishment of liver-stage parasites.

Evaluation of immunogenicity of an oral Salmonella vaccine expressing recombinant Plasmodium berghei merozoite surface protein-1

A repetitive region of Plasmodium berghei merozoite surface protein-1 (PbMSP-1) was expressed as a fusion protein with either maltose binding protein or the B subunit of heat-labile enterotoxin from Escherichia coli. Vaccination of mice with the fusion proteins indicates that this region of PbMSP-1 is antigenic as evidenced by an antibody response. The fusion proteins were also expressed in Salmonella and mice were orally immunized with the recombinant Salmonella. Some of the vaccinated mice survived a challenge with P. berghei blood-stage parasites without developing parasitemia. All control mice became patent and succumbed to the challenge infection. This partial protection was also observed with purified recombinant protein and was independent of the adjuvant used. Mice immunized with recombinant Salmonella showed either extremely low or no antibody response to PbMSP-1, suggesting that cell-mediated immunity is important for the observed protection. These studies show that it is feasible to develop a cost effective oral vaccine against the blood stage of the malarial parasite.

Plasmodium chabaudi: immunogenicity of a highly antigenic glutamate-rich protein

The immunogenicity of a 93-kDa Plasmodium chabaudi protein that contains glutamate-rich tandem repeats was investigated in this study. Immunoblotting with various monoclonal antibodies indicates that this 93-kDa protein is equivalent to a potential P. chabaudi RESA analogue. However, the sequence of the P. chabaudi protein does not exhibit any significant homology to Pf155/RESA. Antibodies against the 93-kDa protein appear early during P. chabaudi infection and reach high titers. The highest antibody titers are found when the parasitemia is descending, suggesting that this protein may play some role in immunity. Immunization of mice with the recombinant protein also results in high antibody titers, indicating that the protein is quite immunogenic. However, mice immunized with recombinant protein and challenged with P. chabaudi do not exhibit a delayed appearance of parasitemia, a reduced parasitemia, or a shortened duration of parasitemia. Glutamate-rich P. falciparum proteins such as Pf155/RESA, are being considered as vaccine candidates. The studies with P. chabaudi suggest that interpretation of serological data using glutamate-rich proteins should proceed with caution. The glutamate-rich repeats, although highly immunogenic, may not be important in host immunity against malaria. However, antibodies that appear late in the P. chabaudi infection do appear to play a role in anti-malarial immunity.

Maloprim malaria prophylaxis in children living in a holoendemic village in north-eastern Tanzania

A randomized, double-'blind', placebo-controlled trial of weekly Maloprim (dapsone-pyrimethamine, D-P) for malaria prophylaxis was conducted at Magoda village in north-eastern Tanzania. The effect of D-P on the incidence of clinical malaria, Plasmodium falciparum prevalence and density, splenomegaly, and packed cell volume (PCV) was investigated in a cohort of 249 children (126 receiving D-P and 123 receiving placebo) aged 1-9 years. The case definition of clinical malaria (malaria fever) was measured axillary temperature > or = 37.5 degrees C and/or reported fever, and P. falciparum asexual parasitaemia > or = 5000/microL. Children aged 1-4 years given D-P experienced 1.56 episodes of clinical malaria per year, whereas children on placebo experienced 2.55 episodes (relative rate [RR] = 0.61, 95% confidence interval [CI] 0.47, 0.80). Thus, D-P protective efficacy against clinical malaria, in this age group, was 39% (95% CI 20%, 53%; P = 0.0002). The annual incidence of clinical malaria among children aged 5-9 years was 0.16 episodes in the D-P group and 0.26 episodes in those receiving placebo (RR = 0.58, 95% CI 0.26, 1.28; P = 0.17). Increased malaria transmission and drug resistance, during the course of the trial, resulted in a reduction in the protective efficacy of D-P. Overall, D-P was able to reduce parasite densities and splenomegaly. D-P prophylaxis also resulted in an increase in PCV but this effect diminished towards the end of the trial. D-P exerted a suppressive effect on asexual parasitaemia throughout the trial.

Inhibition of trypomastigotes' infectivity by a monoclonal antibody directed to a glycoconjugated fraction of epimastigotes of Trypanosoma cruzi

In this work it was shown that the infectivity of trypomastigote forms of Trypanosoma cruzi was affected upon the interaction with the Monoclonal Antibody (McAb) 2E9, which was raised against a glycoconjugated fraction of membranes of epimastigotes (Tulahuen strain). Characterization of the epitope recognized by this McAb, as well as its effects on complement mediated lysis and host cell invasion are reported. Immunocytochemical analysis showed that the McAb was reactive with two macromolecules (41-58 kDa) present on Trypanosoma cruzi epimastigotes (Tulahuen and Y strain), while it recognized several trypomastigotes macromolecules, showing a more intense reactivity with a band of 80 kDa. By indirect immunofluorescence, it was found there were subpopulations of blood and tissue culture derived trypomastigotes which attach the antibody to varying degrees. Studies using chemical or enzymatically treated antigens suggested that the McAb 2E9 was directed against carbohydrate epitopes, which were identified as being--galactosyl residues. In addition, preliminary results are shown, suggesting that the epitope recognized by the McAb 2E9 is involved in adhesion/or internalization of trypomastigotes.

Depletion of CD4+ or CD8+ T-cells prevents Plasmodium berghei induced cerebral malaria in end-stage disease

The role of T-cells in development of experimental cerebral malaria was analysed in C57B1/6J and C57B1/10 mice infected with Plasmodium berghei K173 or Plasmodium berghei ANKA by treatment with anti-CD4 or anti-CD8 mAbs. Mice were protected against cerebral malaria (CM) when anti-CD4 or anti-CD8 mAbs were injected before or during infection. Even in mice in end-stage disease, i.e. with a body temperature below 35.5 degrees C, treatment with anti-CD4 or anti-CD8 antibodies or the combination protected against CM, whereas chloroquine treatment was completely ineffective in inhibiting further development of the cerebral syndrome.

Occupationally acquired infections in health care workers. Part II

BACKGROUND

Health care workers are at occupational risk for a vast array of infections that cause substantial illness and occasional deaths. Despite this, few studies have examined the incidence, prevalence, or exposure-associated rates of infection or have considered infection-specific interventions recommended to maintain worker safety.

OBJECTIVE

To characterize the type and frequency of infections, the recommended interventions, and the costs of protecting health care workers. Part II of this two-part review focuses on infections caused by bloodborne organisms, organisms spread through the oral-fecal route, and organisms spread through direct contact. It also reviews established interventions for controlling transmission.

DATA SOURCES

A MEDLINE search and examination of infectious disease and infection control journals.

DATA SELECTION

All English-language articles and meeting abstracts published from January 1983 to February 1996 related to occupationally acquired infections among health care workers were reviewed. Outbreak- and non-outbreak-associated incidence and prevalence rates were derived, as were costs to prevent, control, and treat infections in health care workers.

DATA SYNTHESIS

Occupational transmission to health care workers was identified for numerous diseases, including infections caused by bloodborne organisms (human immunodeficiency virus, hepatitis B virus, hepatitis C virus, Ebola virus), organisms spread through the oral-fecal route (salmonella, hepatitis A virus), and organisms spread through direct contact (herpes simplex virus, Sarcoptes scabiei). Most outbreak-associated attack rates range from 15% to 40%. Occupational transmission is usually associated with violation of one or more of three basic principles of infection control: handwashing, vaccination of health care workers, and prompt placement of infectious patients into appropriate isolation.

CONCLUSIONS

The risk for occupationally acquired infections is an unavoidable part of daily patient care. Occupationally acquired infections cause substantial illness and occasional death among health care workers. Further studies are needed to enhance compliance with established infection control approaches. As health care is being reformed, the risk for and costs of occupationally acquired infection must be considered.

Prophylactic treatment of experimental canine babesiosis (Babesia canis) with doxycycline

Doxycycline provided satisfactory prophylaxis against experimental infection with a highly pathogenic strain of Babesia canis. Rectal temperature, parasitaemia, packed cell volume and serology were monitored for evaluation of the prophylactic effect. Although a daily dose of 5 mg kg-1 of doxycycline did not completely prevent clinical disease, symptoms remained moderate and a full recovery was obtained within 1 week. At a dose of 20 mg kg-1 day-1 no clinical symptoms were observed, but asymptomatic infection could not be ruled out.

Infant parasite rates and immunoglobulin M seroprevalence as a measure of exposure to Plasmodium falciparum during a randomized controlled trial of insecticide-treated bed nets on the Kenyan coast

Repeated cross-sectional surveys among infants sleeping under insecticide-treated bed nets (ITBN) and contemporary control infants were used to estimate changes in Plasmodium falciparum exposure due to ITBN use on the Kenyan coast. Presence of P. falciparum parasites or total P. falciparum Immunoglobulin M (IgM) seropositivity were used independently and in combination in a constant risk catalytic conversion model to estimate the force of infection in ITBN and control communities. Such studies during infancy avoid problems of early saturation of prevalence due to high forces of infection and persistence of infection, minimize problems of self-treatment, and can be conducted among large populations covering a wide geographic area. These contrast previous parasitologic studies of ITBN among older children and the traditional entomologic studies of transmission that are logistically demanding. Our investigations demonstrated that parasite prevalence, IgM seropositivity, and the force of transmission were all significantly reduced by 50%. In addition, more infants under ITBN entered their second year of life without previous exposure to P. falciparum than control infants. These effects upon delayed acquisition of effective immunity require careful monitoring during future vector control programs using ITBN.

Comparability of treatment groups and risk factors for parasitemia at the first antenatal clinic visit in a study of malaria treatment and prevention in pregnancy in rural Malawi

The problems of Plasmodium falciparum infection in pregnant women have been described in numerous sub-Saharan African countries, but the frequency of parasitemia at the first antenatal visit and risk factors for infection have not been fully investigated. During a prospective antimalarial treatment and prophylaxis trial in pregnant women in Malawi (three groups receiving a chloroquine regimen and one group receiving a mefloquine regimen), we examined women at their first antenatal clinic visit to evaluate these issues and to verify that subjects in the study treatment/prevention arms were similar. Among 4,127 women with enrollment blood smear results, 1,836 (44.5%) were parasitemic. The highest infection rates and densities were observed in primigravidas (66% infected, geometric mean parasite density [GMPD] = 1,588 parasites/mm3 of whole blood), followed by second pregnancies (46% infected, GMPD = 615 parasites/mm3) and subsequent pregnancies (29% infected, GMPD = 238 parasites/mm3), (P < 10(-6) for both infection prevalence and density, by chi-square test for trend). Significant risk factors for parasitemia at first antenatal clinic visit in a multivariate model included low gravidity, high transmission season, no use of prophylaxis before first antenatal clinic visit, young age (< 20 years), human immunodeficiency virus (HIV) infection, low hematocrit, short stature, and second trimester (compared with third trimester). Women in the different treatment arms of the study were generally similar in many characteristics; statistically significant differences, where present, were small. Targeting malaria control efforts to women in their first or second pregnancy and during the high transmission season will be an important strategy to reach most parasitemic women and minimize resource expenditure. Women infected with HIV had a 55% increased risk of parasitemia at their first antenatal clinic visit and may represent an additional important risk group whose numbers may be increasing and who may benefit from identification and management for malaria.

Developing effective strategies for malaria prevention programs for pregnant African women

The control of malaria in pregnant African women, one of several child survival strategies applied through antenatal care, has been particularly challenging. Prevention and control recommendations for typical areas of high Plasmodium falciparum transmission have promoted the use of antimalarial chemoprophylaxis to prevent placental infection. Persistently low program coverage coupled with diminishing intervention effectiveness have forced a re-evaluation of the relative importance of malaria in pregnancy. The Mangochi Malaria Research Project (MMRP), a prospective evaluation of malaria prevention in pregnant women in rural Malawi conducted during 1987-1990, contributed to establishing new criteria for policy and program development for malaria prevention in pregnancy. The principle findings of the MMRP include: 1) populations at risk of the adverse consequences of malaria in pregnancy include women with low parity, women infected with human immunodeficiency virus, pregnancy during the high malaria transmission season, and the use of a malaria drug that is suboptimally efficacious; 2) the estimated maximum benefits of an antimalarial intervention that clears placental and umbilical cord parasitemia are a 5-12% reduction of low birth weight (LBW), an approximately 35% reduction in the risk of LBW for risks that are actually preventable once a woman has become pregnant (e.g., risks such as infectious disease or poor nutrition during gestation), and a 3-5% reduction in the rate of infant mortality; 3) the intervention must be capable of rendering the woman malaria parasite free, including clearance of parasites from the placental vascular space and umbilical cord blood; 4) other diseases adversely affect pregnancy outcome and, while the control of malaria in pregnancy may not warrant independent programming, if coupled with prevention programs to provide a range of antenatal services, the incremental costs of malaria control may prove to be highly cost-effective; and 5) the choice of a regimen must balance intervention efficacy with safety, availability, affordability, and simplicity of delivery, and several antimalarials may meet these criteria. The Malawi Ministry of Health has modified its malaria prevention in pregnancy recommendations and now faces the challenge of effective programming to improve child survival.

Mass DEC chemotherapy for control of bancroftian filariasis: comparative efficacy of four strategies two years after start of treatment

The efficacy of 4 strategies for control of bancroftian filariasis using mass diethylcarbamazine (DEC) chemotherapy was evaluated and compared in 4 endemic communities in Tanzania 2 years after the start of treatment. The strategies used were the standard 12 d treatment (strategy I), a semi-annual single dose treatment (strategy II), a monthly low dose treatment (strategy III), and DEC medicated salt (strategy IV). Treatment took place during the first study year, and no treatment was given during the second year. Among individuals who were microfilaraemic before treatment, the microfilaria (mf) clearance rates were 41.5%, 75.0%, 84.0% and 89.1%, and the pre-treatment mf geometric mean intensities (GMIs) were reduced by 97.1%, 98.9%, 99.8% and 99.8%, for strategies I, II, III and IV, respectively, 2 years after starting treatment. Statistical analysis indicated that strategies III and IV were equally effective, and superior in clearing microfilaraemias and in reducing mf GMIs compared to strategies I and II; strategy II was significantly more effective than strategy I. The rate of occurrence of new cases of microfilaraemia among individuals who were amicrofilaraemic during the pre-treatment surveys was negligible over the study period in all communities. In all the communities combined, 66.6% of males presenting hydrocele before treatment, and 61.5% of individuals presenting elephantiasis before treatment, showed improvements in these conditions 2 years after the start of treatment, either as a reduction in size or complete disappearance.

Prophylactic activity of imidocarb against experimental infection with Babesia canis

A single subcutaneous injection of imidocarb at 6 mg kg-1 gave 2 weeks protection against experimental inoculation of Babesia canis merozoites in Beagle dogs. The prophylactic effect was evaluated by daily measurements of parasitaemia, rectal temperature, haematological values and specific antibodies.

Analysis of roles of natural killer cells in defense against Plasmodium chabaudi in mice

Mice that have recovered from a primary infection with Plasmodium chabaudi have been shown to resist a secondary infection. In the present study the authors investigated how natural killer (NK) cells were involved in this resistance. Spleen cells from P. chabaudiprimed C57BL/6 mice could transfer protection against P. chabaudi infection into naive syngeneic mice, but spleen cells from unprimed mice could not. T-enriched cells purified from primed spleen cells could also transfer such protection. Transfer of NK cells from primed spleen cells failed to protect against challenge infection. However, depletion of NK cells in host mice by injection of an anti-NK1.1 monoclonal antibody resulted in higher mortality relative to controls. The possible protective roles of NK cells in P. chabaudi infection are discussed.

Effect of Mycobacterium phlei on the development of immunity to Babesia bigemina

The immunomodulatory role of Mycobacterium phlei against intracellular blood protozoan Babesia bigemina was demonstrated following experimental immunisation and challenge in bovine calves. A lysate of erythrocytes infected (6 x 10(9)) with B. bigemina was used as a source of dead antigen either with Freund's complete adjuvant (FCA) or with a trypsinised culture of M. phlei as a non-specific immunomodulation (NSI) agent with appropriate controls. Following virulent challenge with B. bigemina infected erythrocytes (1 x 10(7)), the NSI printed calves showed 100% protection, while the dead antigen alone with FCA afforded 75% protection. The protective status of the immunising regimes was studied by clinicopathological parameters and assessment of humoral and cell-mediated immune responses. The role of babesial dead antigen and the effects of M. phlei on the development of immunity to B. bigemina is discussed.

Sulfated polyanions inhibit invasion of erythrocytes by plasmodial merozoites and cytoadherence of endothelial cells to parasitized erythrocytes

Sulfated proteoglycans have been shown to be involved in the binding of sporozoites of malaria parasites to hepatocytes. In this study, we have evaluated the effect of sulfated glycosaminoglycans on the invasion of erythrocytes by Plasmodium falciparum merozoites and cytoadherence of parasitized erythrocytes (PRBC) to endothelial cells. Invasion of erythrocytes by HB3EC-6 (an HB3 line selected for high binding to endothelial cells) was inhibited by dextran sulfate 500K, dextran sulfate 5K, sulfatides, fucoidan, and heparin but not by chondroitin sulfate A. With the exception of sulfatides, the invasion-inhibitory effect was not mediated by killing of parasites. Cytoadherence of HB3EC-6 to human microvascular endothelial cells (HMEC-1) and inhibited by these sulfated glycoconjugates. The highly sulfated dextran sulfate 500K had the highest inhibitory effect on both invasion and cytoadherence, whereas the positively charged protamine sulfate promoted cytoadherence. Because preincubation of PRBC with sulfated glycosaminoglycans and treatment of target cells with heparinase had no significant inhibition on cytoadherence, it is unlikely that sulfated glycoconjugates are used directly by endothelial cells as cytoadhesion receptors. In an vivo experiment, we found that the administration of dextran sulfate 500K to CBA/Ca mice infected with Plasmodium berghei ANKA reduced parasitemia and delayed the death associated with anemia. These observations suggest that sulfated polyanions inhibit the invasion of erythrocytes by merozoites and cytoadherence of PRBC to endothelial cells by increasing negative repulsive charge and sterically interfering with the ligand-receptor interaction after binding to target cells.

The Santa Lucia strain of Plasmodium falciparum as a model for vaccine studies. II. Development of Aotus vociferans as a model for testing transmission-blocking vaccines

The Santa Lucia strain of Plasmodium falciparum and the Aotus vociferans monkey were studied as models for the testing of transmission-blocking vaccines. Virulence developed early in the passage history. Despite the use of only small quantities of chlorguanide and/or quinine to control infection coupled with the use of small inocula and delays in splenectomy, mosquito infection was markedly reduced from that seen during primary passage to this species of Aotus. It appears that the model may be most useful during its initial passage from the primary species, Aotus lemurinus griseimembra.

The Santa Lucia strain of Plasmodium falciparum as a model for vaccine studies. I. Development in Aotus lemurinus griseimembra monkeys

The Santa Lucia strain of Plasmodium falciparum and the Aotus lemurinus griseimembra monkey are proposed as models for the testing of sporozoite vaccines and transmission-blocking vaccines. Approximately 85% of splenectomized monkeys were infected when fed upon by 10 or more heavily infected Anopheles freeborni mosquitoes. Sporozoite-induced infections in monkeys with or without previous infection with P. vivax readily infected mosquitoes, thus making them candidates for testing transmission-blocking vaccines.

Observations on the biological nature of Plasmodium vivax sporozoites

The relapsing malaria parasites are characterized by the production of sporozoites with varying potential for exoerythrocytic development. Some sporozoites develop soon after introduction to produce mature schizonts and merozoites that initiate the erythrocytic stage infection. Relapsing hypnozoite forms are characteristic of some strains of Plasmodium vivax and are more apt to develop late than early with many time intervals in between. Studies in Saimiri monkeys suggest another type of sporozoite-induced infection. With the Salvador I strain of P. vivax, early developing exoerythrocytic schizonts apparently release parasites with different levels of virulence for these monkeys, ranging from those producing high-level parasitemia to a more abundant avirulent form. The induction of low-density avirulent infections requires the development of more sensitive detection methods for the evaluation of sporozoite vaccines.

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

Groups of five dogs were vaccinated against Babesia canis using soluble parasite (SPA) antigens from in vitro cultures. Although vaccination did not significantly alter peripheral parasitaemia upon challenge, protected animals had lower levels of SPA in the plasma after a challenge infection. The severity of anaemia correlated with the SPA-load during the post-challenge period in that high levels of SPA were associated with low haematocrit values. In addition, it was found that recovery was associated with the production of antibodies against SPA. The results suggest that SPA induce anaemia during B. canis infection, and that vaccination with SPA results in antibody production that can neutralize the effects of SPA after a challenge infection.

Effects of adoptive transfer of immune spleen cells on worm growth and microfilaraemia in Brugia pahangi infection in Mongolian gerbils

Protective immunity against Brugia pahangi was examined after adoptive transfer of immune spleen cells. Spleen cells obtained from gerbils at 8 weeks post-infection (p.i.) with 100 infective larvae (L3) of B. pahangi were transferred into naive recipients, and then 24 h later, they were infected with 100 L3 of B. pahangi. The recipients given normal spleen cells and infected by the same manner served as controls. Microfilarial counts in the circulation were monitored at designated times after infection and worm burden and the size of individual female worm were determined at 16 weeks p.i. to evaluate the effects of adoptive immunization. In addition, eosinophil responses and serum antibody titres were examined during the course of infection. In the control group, microfilariae first appeared in circulation at 9 weeks p.i. and continuously increased in number throughout the course examined. In contrast, microfilaraemia was almost completely suppressed in the group given immune spleen cells. Although worm burden was comparable between the two groups, the average size of female adult worms recovered from the adoptively immunized group was significantly smaller than that from the control group. Eosinophil response was hastened and enhanced by adoptive transfer of immune spleen cells in the early stage of infection. Parasite-specific antibody response was also hastened by adoptive immunization. These results suggest that immune spleen cells could confer protective immunity mainly directed against adult B. pahangi.