Effect of CGP 20376 on Brugia malayi and parasite antigenemia in jirds

This study was designed to investigate the activity of CGP 20376, a benzothiazole derivative, against Brugia malayi in jirds and to illustrate the utility of parasite antigen detection as a means of monitoring drug efficacy in filariasis. Drug treatment was 100% effective in jirds treated 3 or 24 days after infection. Microfilaria and adult worm counts were reduced (relative to counts in sham-treated control animals) by 96% and 95%, respectively, in animals treated 153 days after infection. Four of 6 animals in this treatment group cleared their microfilaremias and were free of adult worms 5 mo after treatment. Thus, CGP 20376 was effective against all life cycle stages of B. malayi in jirds. Parasite antigen levels in jird sera were consistent with parasitological results in all treatment groups, but antigen clearance was incomplete in some cases after apparently successful treatment of mature and immature infections.     

Brugia malayi: detection of parasite antigen in sera from infected jirds

Sera from Brugia malayi-infected jirds were demonstrated to contain a heat-stable, 95- to 105-kDa parasite antigen by immunoblot with rabbit antibody to the parasite and with a monoclonal antibody that binds to phosphorylcholine. This antigen is a major component of B. malayi adult worm excretory/secretory antigen, and it is present in lavage fluid obtained from ip-infected animals. The antigen was detected by enzyme immunoassay in all sera collected from jirds 9-54 weeks after sc injection with 100 or 300 infective larvae (L3). Parasite antigen titers were higher in animals infected with the higher L3 dose. Antiphosphorylcholine antibodies were present in jird sera for the first 12 weeks after larval injection, but thereafter, antibody titers decreased to undetectable levels. Parasite antigen was not detected by immunoblot or enzyme immunoassay in sera from 21 human subjects with B. malayi microfilaremia. Antigen may be cleared from human sera by antiphosphorylcholine antibodies, which were present in all sera tested. The practical significance of B. malayi antigen detection in the jird is that it provides a sensitive means of noninvasively monitoring the status of infection in this important experimental filariasis model.     

Circulating parasite antigen in Brugia pahangi-infected jirds

The Mongolian jird is used widely in filariasis research for studies of protective immunity, pathogenesis, and therapy. The purpose of this study was to evaluate parasite antigen detection as a means of noninvasively monitoring Brugia pahangi infection in jirds. A parasite antigen with Mr of 105-110 kDa was identified in sera from i.p.- and s.c.-infected jirds by immunoblot with a monoclonal antibody to phosphorylcholine. The same antibody was used in a direct sandwich enzyme immunoassay to measure antigen in jird sera. Parasite antigen was detectable as early as 2 wk after i.p. or s.c. injection of L3. Antigen titers increased between 2 and 12 wk and stabilized between 12 and 36 wk after infection in s.c.-infected animals. A different pattern was seen in i.p.-infected jirds with antigen titers peaking at 16 wk and falling significantly between 16 and 32 wk after infection. Parasite antigen titers correlated significantly with adult worm infection intensities in jirds with mature i.p. and s.c. infections. Antigenemia was also detectable in sera from jirds after i.p. implantation of adult parasites of either sex. However, antigen was not detected in sera from infant offspring of antigenemic infected mothers. We conclude that parasite antigen detection allows B. pahangi development and survival as well as infection intensity to be monitored in living animals with unprecedented sensitivity and accuracy. This technique should facilitate drug and vaccine studies in this important experimental filariasis model.     

Comparative susceptibility to anthelmintics of Brugia pahangi in jirds infected by different methods

It is possible to infect jirds with Brugia pahangi by three methods. Infective larvae (L3) can be injected either intraperitoneally (ip), when adults develop in the peritoneal cavity, or sub-cutaneously (sc), when they develop in the lymphatics or the heart and blood vessels associated with the lungs. Alternatively adult worms which have been grown in the peritoneal cavities of jirds can be implanted into the peritoneal cavities of other jirds. This latter system has been widely used for screening for new filaricides. We have compared the activity of 9 macrofilaricidal compounds against these 3 types of infection. Mebendazole and albendazole were more active against implanted adults than against L3 induced adults in the peritoneal cavity. Oxibendazole, flubendazole, CGP24588A and oxfendazole were equally active against both types of worm. CGP20376, Mel Ga and Mel Ni were more active against adult worms derived from inoculated L3 than implanted worms. When comparing intra-lymphatic and ip adults (both derived from L3 infections and in the same jirds) albendazole and CGP20376 were active at the same levels against both types of infection. Mebendazole, flubendazole, oxfendazole, CGP24588A, Mel Ga and Mel Ni were more active against ip adults than intra-lymphatic adults. No drug was more active against intra-lymphatic adults than against adults.     

Brugia malayi: antibody responses to larval antigens in infected and immunized jirds.

Vaccination with irradiated third stage Brugia malayi larvae (L3) has been reported to induce partial protective immunity to L3 challenge in jirds. The purpose of this study was to identify antigens that may be targets of protective immunity in this model. Jirds were immunized by s.c. injection of irradiated L3 and challenged either s.c. or i.p. Necropsy was performed 11 wk after challenge. Partial protection was achieved in s.c. challenged animals; worm recovery was only 41% of that observed in unvaccinated controls, and worms recovered from immunized animals were stunted. Worm recoveries in immunized animals that were challenged i.p. did not differ from those of unimmunized controls. Group differences in parasite antigen levels in sera collected 2-11 wk after larval challenge were consistent with parasitological findings obtained at necropsy. Antibody studies compared prechallenge sera from immunized animals to sera from infected (unimmunized) controls. Antibody responses to L3 surface antigens (assessed by IFA) were much stronger after immunization than after infection. Immunoblot studies showed preferential recognition of several L3 antigens (97, 54, 48, and 40 kDa) by antibodies in sera from immunized animals. Additional studies are needed to determine whether immunization with such preferentially recognized antigens can induce protection to larval challenge comparable to or better than that observed with live vaccines.     

Brugia pahangi: comparative susceptibility of the Mongolian jird, Meriones unguiculatus, and the PD4 inbred hamster, Mesocricetus auratus

The susceptibility of Mongolian jirds, Meriones unguiculatus, and PD4 hamsters, Mesocricetus auratus, to Brugia pahangi was compared based on the percentage adult worm recoveries, mean microfilaremia levels, and adult worm lengths. Fourteen male jirds and seventeen male PD4 hamsters were each inoculated subcutaneously in the left inguinal region with 90-100 L3 of B. pahangi and necropsied 130-150 days after inoculation. There were no significant differences between jirds and hamsters in mean adult worm recoveries (24.7 vs 25.4%) and prepatent periods (69.9 vs 77 days after inoculation). In hamsters, 85% of recovered worms were found in the heart and lungs and 15% were found in genital lymphatic vessels. In jirds, distribution of recovered worms was 66% in genital lymphatics, 23% in the heart and lungs, 8% in the peritoneal cavity, and 3% in lymphatic vessels in other sites. The mean microfilaremia level in jirds (16.5/20 microliter) was significantly higher than in hamsters (8.7/20 microliter. Female worms in the genital lymphatics of jirds were significantly longer than female worms in the genital lymphatics of PD4 hamsters (33.5 vs 27.3 mm). Lengths of worms in other locations were similar between the two species.     

Induction of protection against Brugia malayi infection in jirds by microfilarial antigens

The development of immunologic methods to reduce transmission of human lymphatic filariasis depends on measures that will enhance the host's ability to eliminate infective larvae, adult worms, or blood-borne microfilariae (mf). The present study was designed to assess the capacity of a crude extract of Brugia malayi mf to decrease the level of microfilaremia and adult worm burden in jirds inoculated with infective larvae, and to identify the filarial antigens that elicit antibody responses in these animals. Thirty weeks after subcutaneous inoculation with 75 infective larvae, 100% of control jirds were patent (i.e., had microfilaremia) compared with 60% of the group immunized with 10 micrograms of crude microfilarial extract (p less than 0.05). In addition, microfilaremia was lower in patent immunized animals compared with controls (p less than 0.05). The mean total number of adult female B. malayi per jird recovered at necropsy in control animals was 16.0 vs 7.0 in immunized jirds (p less than 0.05). Serum of immunized jirds contained anti-mf antibodies with an end titer of 1:8000, a value similar to that of animals with chronic B. malayi infection. Microfilarial antigens of Mr approximately 150,000, 75,000, 42,000, and 25,000 were identified in immunoblotting studies by reactivity with antibodies in sera of immunized jirds. Antibodies induced by immunization with microfilarial extract were not specific for this stage of the parasite life cycle, as jird anti-mf antibodies reacted with a Mr approximately 150,000 and several Mr 50,000 to 110,000 antigens derived from immature and mature adult parasites of both sexes. These data indicate that immunization of jirds with a water soluble microfilarial extract enhances the host's ability to eliminate adult worms and blood-borne mf. The filarial antigens that induce antibodies in immunized jirds have been identified.     

Brugia pahangi: effects of duration of infection and parasite burden on lymphatic lesion severity, granulomatous hypersensitivity, and immune responses in jirds (Meriones unguiculatus)

The effects of Brugia pahangi infection duration and parasite burden on parasite-associated inflammatory and immune responses were determined over a 181-day period in jirds receiving from one to eight inoculations of infective larvae. Multiple infections did not produce a protective resistance to reinfection as determined by adult worm recovery at necropsy. Intralymphatic granulomatous lesions, lymph thrombi, were first seen at 48 days post initial inoculation (DPI). The numbers of lymph thrombi reached peak levels in singly inoculated jirds at 90 DPI and significantly decreased to low levels by 160 DPI. The ratio of lymph thrombi to adult worms recovered from the spermatic cord lymphatics followed a similar pattern. Sizes of renal lymph nodes, which drain lymphatics containing parasites, followed a temporal pattern of increase and decrease similar to that of lymph thrombi numbers. Peak granuloma areas around antigen-coated beads embolized in lungs were seen at 27 DPI. Granuloma areas around antigen-coated beads began to decrease after 69 DPI and reached sizes not significantly different from uninfected controls by 118 DPI. Multiple inoculations of infective larvae and increasing worm burdens did not affect the pattern of granulomatous response to antigen-coated beads. Eosinophilia of singly and multiply infected jirds peaked at 26 DPI. Eosinophilia of singly infected jirds returned to normal levels by 103 DPI but those of multiply infected jirds remained elevated until 160 DPI. Lymph node cell blastogenic responses to antigen were greater than those of splenocytes at all time intervals measured. However, significant differences in stimulation indexes between groups with different infection durations were not seen with either cell type. Antibody responses to somatic adult worm antigen as measured by ELISA reached near peak levels by 48 DPI and remained elevated for the course of the study in all infected jirds. The decrease in lymphatic lesion severity seen in chronically infected jirds temporally corresponds to the decrease in granulomatous reactivity measured around antigen-coated beads embolized in the lungs. This observation suggests that host and/or parasite factors associated with these two phenomena may be similar. Although these decreases may be the result of down-regulated immune responses, corresponding decreases in antibody levels and blastogenesis of lymphocytes stimulated by crude worm extracts were not observed in chronic infections.     

Brugia pahangi: granulomatous lesion development in jirds following single and multiple infections

The development of adult worm burdens and microfilaremias were determined in jirds which received 2, 3, or 4 subcutaneous inoculations of 50 Brugia pahangi infective larvae. Parasite burdens in multiply inoculated jirds were compared to those in four different groups of jirds which received single inoculations of 50 infective larvae. One of each of these singly inoculated groups was infected on the same day that one of the inoculations was given to the multiply infected jirds. Thus, the duration of the infections in the four groups of jirds receiving one inoculation was 54, 118, 189, and 254 days. The development of lymphatic lesions and granulomatous hypersensitivity to B. pahangi antigen was assessed in all jirds at necropsy. The percentage recoveries of adult worms and their locations did not differ in the singly inoculated jirds with infections of different durations. A protective resistance to reinfection, as measured by adult worm recovery in multiply infected jirds, did not occur. The lymphatic lesion scores and numbers of intralymphatic thrombi was greatest in singly inoculated jirds examined 54 days after infection. Pulmonary granuloma areas around adult filarial antigen coated beads embolized in the lungs of jirds 3 days prior to necropsy were also greatest in singly inoculated jirds examined 54 days after infection. Using criteria of lesion scores and lymph thrombi numbers to assess lymphatic lesion severity, a decrease in lesion severity as well as pulmonary granuloma size around antigen coupled beads was seen by 118 days after infection in singly inoculated jirds.(ABSTRACT TRUNCATED AT 250 WORDS)     

Use of parasite antigen detection to monitor the success of drug therapy in Dirofilaria immitis-infected dogs

Recently the authors developed a monoclonal antibody-based enzyme immunoassay for circulating Dirofilaria immitis antigen and demonstrated its utility as a diagnostic tool for canine dirofilariasis. In the present study, serum parasite antigen measurements were used to monitor the success of thiacetarsamide therapy in 2 controlled trials that involved 24 naturally infected dogs. Parasite antigen levels correlated significantly with adult worm burdens in untreated control dogs. Antigen levels fell dramatically by 8 wk after treatment in successfully treated dogs and were undetectable 12 wk after treatment in dogs that were parasitologically cured. Microfilarial counts exhibited seasonal periodicity in both treated and control dogs and were not useful in monitoring the success of adulticide therapy. Parasite antigen detection is quite useful in monitoring the efficacy of adulticide therapy for dogs infected with D. immitis. This approach may lead to improved clinical use of thiacetarsamide, and it should facilitate evaluation of new drugs for this important infection.     

Delayed macrofilaricidal activity of diethylcarbamazine against Brugia pahangi in Mongolian jirds

The macrofilaricidal activity of diethylcarbamazine (DEC) was confirmed in jirds infected with Brugia pahangi. Seventy jirds were inoculated subcutaneously with 100 infective larvae. At 20 weeks post-infection, the microfilaraemic jirds were divided into two groups, untreated and treated. For the treated group, 200 mg kg(-1) of DEC was injected intraperitoneally for 5 consecutive days. One, 4, 8, 12, 16 and 27 weeks after the final treatment, 4-7 jirds in each group were sacrificed to measure adult worm burdens. The number of adult worms recovered from treated jirds was comparable to controls at earlier necropsy (1 and 4 weeks post-treatment). However, at late necropsy (8 weeks and later) the recovery rate of adult worms in treated jirds was significantly lower than that in untreated controls, indicating an adultcidal effect of DEC. The present study demonstrates that DEC requires 8 weeks to kill B. pahangi adult worms in jirds and that the Mongolian jird is a useful model for screening antifilarial activity.     

The chemotherapeutic effect of praziquantel against Schistosoma mansoni is dependent on host antibody response

To assess the role of host humoral immune responses in the mechanism of action of praziquantel (PZQ) against Schistosoma mansoni, the efficacy of the drug was compared in infected B cell-depleted (mu-suppressed) vs immunologically intact C3H/HeN mice. We found that PZQ was on the average only 20% as effective in eliminating adult schistosomes from mu-suppressed as compared with control animals. Indeed, in three of four experiments performed, the drug failed to significantly reduce adult worm burdens in the mu-suppressed mice. These results were not due to a delay in parasite death in the infected B cell-depleted animals, because adult worms recovered from these mice as late as 7 wk after chemotherapy were indistinguishable in number and appearance from those recovered from non-drug-treated animals. The efficacy of PZQ against schistosomes in mu-suppressed mice was completely restored by passive transfer of immune serum from donor mice infected for 6 wk and partially restored with IgG purified from the same sera. Moreover, IgG as well as IgM antibodies were detected by immunofluorescence on the surface of adult worms recovered from intact mice as early as 1 hr after administration of the drug in vivo. The tubercles of the male worms appeared to be a major site for antibody binding. These results formally demonstrate that the mechanism of action of PZQ, the most important anti-schistosomal compound in current use, involves a synergy between the drug and the humoral immune response of the host, and suggest that the relevant effector antibodies act directly against parasite antigens which become exposed on the surface of the worms as a consequence of interaction with the drug.     

Growth and development of Haemonchus contortus in jirds, Meriones unguiculatus.

Growth and development of Haemonchus contortus were examined in jirds and were compared to these processes in lambs. Number, sex, size, and stage of development were determined for worms recovered at necropsy at various times postinoculation (PI) from immunosuppressed jirds inoculated with approximately 1,000 exsheathed infective larvae (L3) of H. contortus. In addition, gastric tissue samples from jirds were examined histologically. Parallel studies were done in lambs inoculated with approximately 7,500 L3. Typically, 5-30% of the inoculum established and survived in jirds at reasonably stable numbers to day 14 PI. By day 21 PI, worm numbers in jirds decreased dramatically. Although the parasite was similar in size and development on day 4 PI in jirds and lambs, from day 7 PI on, worms were significantly smaller and less developed in jirds. On histological examination, the parasite was found only in the glandular portion of the stomach of jirds (anatomically similar to its predilection site in the abomasum of lambs), and histological changes were consistent for both host species. Although growth and development of H. contortus are slower and incomplete in jirds, the parasite establishes, grows, and develops (at anatomically comparable sites in both hosts) in this model. Thus, the model appears to provide a useful laboratory host to study H. contortus.     

Monoclonal antibodies to parasite antigens found in the serum of Dirofilaria immitis-infected dogs

We recently reported that parasite antigens are detectable in the serum of Dirofilaria immitis-infected dogs by counterimmunoelectrophoresis (CIE). Hybridoma cell lines that produce monoclonal antibodies specific for these antigens were obtained by immunizing mice with a partially purified antigen preparation, fusing spleen cells with SP-2 myeloma cells, and screening cell culture supernatants for antibody by ELISA and CIE inhibition. Antibodies specific for two epitopes shared by the two major circulating parasite antigens were identified. Immunoperoxidase studies showed that the epitopes recognized by the monoclonals were widely distributed in D. immitis, but the female uterus and eggs were particularly strongly labeled. A monoclonal antibody-based ELISA was developed to measure parasite antigens in dog sera. Parasite antigens were detected in 45 of 46 sera from infected dogs but were absent in sera from uninfected dogs and sera from dogs infected with Dipetalonema reconditum. Serum antigen content was significantly correlated with the number of female worms recovered from infected dogs (r = 0.82, p less than 0.001). Antigenemia was first detected 6 mo after infection, and antigen levels remained fairly stable between 9 and 21 mo after infection. Parasite antigen detection with this monoclonal antibody-based ELISA appears to be superior to previously described diagnostic methods for canine dirofilariasis in terms of sensitivity, specificity, and relation to infection intensity.     

Apodemus sylvaticus, a new host for Acanthocheilonema viteae (Nematoda: filarioidea)

Susceptibility of Apodemus sylvaticus and A. agrarius to infection with Acanthocheilonema viteae was compared with that of hamsters and jirds. Microfilaremia in A. sylvaticus was first noted on day 52 post-infection (p.i.) and lasted during the course of the study (up to day 150 p.i.). Maximum microfilaremic levels (female worm basis) of A. sylvaticus [mean +/- S.D. (n) = 690 +/- 1288(6)] were considerably higher than those of hamsters [16 +/- 18(6)] and jirds [51 +/- 25(5)]. Adult worm recovery in A. sylvaticus ranged from 2 to 40% of the number of infective larvae inoculated. Worm development in A. sylvaticus resembled that in hamsters and jirds. In contrast, microfilaremia was not detected in, nor adult worms recovered from A. agrarius throughout the study.     

Altered adult worm location in young male jirds infected with Brugia pahangi

Male jirds (Meriones unguiculatus) were inoculated subcutaneously with 100 Brugia pahangi L3 each at 2, 6, 10, and 15 wk of age to compare their susceptibility and pathologic reactivity to infection. Adult worm recoveries (mean +/- SD) ranged from 24.1 +/- 15.1 to 36.4 +/- 13.9 at 60 days postinfection. No significant difference in susceptibility was measured among the 4 age groups. Jirds infected at 2 wk of age had significantly fewer (alpha less than or equal to 0.025) testicular and intralymphatic worms than all other age groups. Numbers of intralymphatic thrombi were significantly lower (alpha less than or equal to 0.01) in jirds infected at 2 wk of age. Lymphatic lesion severity, expressed as the number of intralymphatic thrombi per intralymphatic worm, was similar between age groups. These data indicate no differences in susceptibility or lymphatic lesion formation following B. pahangi infection in 2-wk-old male jirds, despite altered adult worm location.     

The anthelmintic activity of a novel organic arsenical, R7/45, upon Brugia pahangi in Meriones unguiculatus

The new organic arsenical R7/45 is a rapidly acting and very potent anthelmintic against adult Brugia pahangi in jirds. Against adult worms implanted into the peritoneal cavity 5 subcutaneous (SC) injections at 2.5 mg/kg of R7/45 killed 100% of adult worms. A single dose SC of 20 mg/kg was 100% effective and 10 mg/kg 76% effective against adult worms. When jirds were autopsied at different times after treatment at 20 mg/kg SC 89% of worms were dead within three days. R7/45 was not active when given by stomach intubation. Pretreatment of jirds with R7/45 had no effect on adult worms subsequently implanted into jirds. R7/45 was highly active against third and fourth stage larvae of B. pahangi in jirds.     

Acanthocheilonema viteae: vaccination of jirds with irradiation-attenuated stage-3 larvae and with exported larval antigens

Jirds (Meriones unguiculatus) were immunized with irradiated (35 krad) stage-3 larvae (L3) of Acanthocheilonema viteae. The induced resistance against homologous challenge infection and the antibody response of the animals were studied. Immunization with 3, 2, or 1 dose of 50 irradiated L3 induced approximately 90% resistance. Immunization with a single dose of only 5 irradiated L3 resulted in 60.8% protection while immunization with a single dose of 25 L3 induced 94.1% protection. The protection induced with 3 doses of 50 irradiated L3 did not decrease significantly during a period of 6 months. Sera of a proportion, but not all resistant jirds, contained antibodies against the surface of vector derived L3 as defined by IFAT. No surface antigens of microfilariae or adult worms were recognized by the sera. Vaccinated animals had antibody responses against antigens in the inner organs of L3 and in the cuticle and reproductive organs of adult worms as shown by IFAT. Immunoblotting with SDS-PAGE-separated L3 antigens and L3-CSN revealed that all sera contained antibodies against two exported antigens of 205 and 68 kDa, and against a nonexported antigen of 18 kDa. The 205-kDa antigen easily degraded into fragments of 165, 140, 125, and 105 kDa which were recognized by resistant jird sera. Various antigens of adult worms, but relatively few antigens of microfilariae, were also recognized. To test the relevance of exported antigens of L3 to resistance, jirds were immunized with L3-CSN together with a mild adjuvant. This immunization induced 67.7% resistance against challenge infection and sera of the immunized animals recognized the 205- and 68-kDa antigens of L3.     

Concomitant immunity in a rodent model of filariasis: the infection of Meriones unguiculatus with Acanthocheilonema viteae

In an attempt to study the occurrence of concomitant immunity in filarial infections, jirds (Meriones unguiculatus) were experimentally infected with Acanthocheilonema viteae, and patent animals were superinfected with a defined dose of A. viteae stage 3 larvae (L3). Infected animals harbored significantly less worms deriving from the superinfection than the control group (P < 0.05, 56.2%, and 63.4% protection), as shown by analysis of female worms 6 wk after superinfection on the basis of their developmental status and their length. This protection was not due to contact with L3 antigens because a significant reduction of worm burdens deriving of a superinfection was also observed after subcutaneous implantation of a single female worm (P < 0.05, 40.2% and 64.9% protection). The induced protective responses target L3 and restrict their migration because an established infection resulted in a reduction of L3 recovery (95.6% and 94.3%, P < 0.001) from tissues of jirds at day 5 after superinfection. Other data show that L3 from a superinfection are trapped within eosinophil-rich granulomas, which is likely to create unfavorable conditions for the worms and to lead to later death. Taken together, established A. viteae-infections partially protect hosts against homologous superinfection by an immune-mediated mechanism and, thus, regulate the population density of the parasites within the host by concomitant immunity.     

Depletion of eosinophils by anti-IL-5 monoclonal antibody treatment of mice infected with Trichinella spiralis does not alter parasite burden or immunologic resistance to reinfection.

Mechanisms of parasite killing by eosinophils are widely studied and are often implicated in mediating resistance to parasitic infection, especially in conjunction with specific antibodies. Evidence for the eosinophil as an anti-parasite killer cell in vivo is limited and may not justify the belief that eosinophils engage and/or kill infective helminths. We reexamined this question in a mouse model of trichinosis in which antisera to eosinophils were previously used to show the requirement for eosinophils in resistance to this nematode. The current studies used mAb to IL-5 to suppress eosinophil levels in CF1 mice infected with Trichinella spiralis. In mice given a primary infection and injected with an isotype control mAb or left untreated, the medullary and peripheral blood eosinophil numbers peaked at 3 wk postinfection (PI) and returned to baseline levels by 4 wk PI. Peripheral blood eosinophil numbers in infected mice injected with anti-IL-5 were maintained at levels below those of uninfected normal mice through 4 wk of infection. Histologically, there was a prominent eosinophil accumulation in infected, untreated, or control-mAb-treated mice associated with nurse cell complexes containing infective juveniles in skeletal muscle at 3 and 4 wk PI. This was largely eliminated in mice treated with anti-IL-5 mAb. However, the number of muscle stage juvenile worms recovered 3 and 4 wk PI after acid pepsin digestion was unaffected by eosinophil depletion. Challenge infections, in which mice were infected at day 0 with 125 muscle stage worms and challenged at day 28 PI with 350 muscle stage worms, developed peak eosinophil numbers in bone marrow and peripheral blood 3 wk after primary infection and 2 wk after challenge infection in mice receiving either no treatment or control mAb. In challenged mice receiving anti-IL-5 mAb, medullary and peripheral blood eosinophil numbers remained at or below those of uninfected animals. Although all groups exhibited significant resistance measured as muscle stage worm burdens 56 days PI, eosinophil depletion did not affect resistance of muscle worm recovery. These results suggest that eosinophils are not essential in the control of T. spiralis in either primary or challenge infections of CF1 mice. This in vivo study illustrates the questionable value of in vitro killing assays to assign effector function to any single inflammatory cell type.