Humoral immune response of dogs immunized with salivary gland, midgut, or repeated infestations with Rhipicephalus sanguineus.

The antibody (Ab) responses of dogs immunized with adult tick salivary gland (TSG), midgut (TMG), or repeated infestations of Rhipicephalus sanguineus were monitored to determine if there is an association between Ab production and R. sanguineus performance. Tick-naïve dogs were immunized with TSG or TMG and subjected to two challenge infestations. The control group was infested five times at 21-day intervals. The ELISA technique was used to measure Ab levels in sera from these dogs, which expressed different forms of resistance against R. sanguineus. In dogs immunized with TSG or TMG, similar Ab levels were detected against TMG, TSG, muscle, synganglion, and reproductive organs. However, these sera had different Ab levels against egg mass, unfed larvae, fed larvae, and nymph antigens. Ab levels to muscle, nerve, and reproductive antigens were lower than those observed when TMG or TSG antigens were used. Sera from dogs immunized with TMG or TSG responded to most tick stages or tissue antigens, whereas repeated infestation sera showed the lowest response among the three groups.

Performance of female Rhipicephalus sanguineus (Acari: Ixodidae) fed on dogs exposed to multiple infestations or immunization with tick salivary gland or midgut tissues.

This investigation compared the effects of repeated infestations to immunization of dogs with tick salivary gland or midgut extracts on the feeding and fecundity performances of female Rhipicephalus sanguineus (Latrielle). In each immunized group, three tick-naive dogs were immunized three times with tick salivary gland or midgut extracts, and twice challenged at 21-d intervals by allowing 80 female and 40 male adult ticks to feed on each host. The repeated infestation group of three naive dogs was infested five times at 21-d intervals by the same numbers of ticks. The repeated infestation group showed a trend of reduced tick performance after the third infestation, but some of the tick performance parameters had recovered by the fifth infestation. Tick attachment was reduced by immunization with either tick salivary gland or midgut extract. Immunization with tick salivary gland extract had the greatest impact on the feeding period and engorgement weight of the female ticks. Immunization with tick midgut extract resulted in the greatest reduction of tick fecundity parameters, which included preoviposition, oviposition, and egg-incubation periods in addition to reduced egg production and egg viability. These results confirm that dogs can become resistant to R. sanguineus, and demonstrate that immunization with tick salivary gland or midgut extract has different effects on tick feeding and fecundity.

Demonstration of anti-muscle immunity in murine trichinellosis.

Swiss mice infected with Trichinella spiralis (I mice), or immunized with straight Freund's complete adjuvant (A mice) or with rat muscle extract (RME) in Freund's complete adjuvant (MA mice) were tested for skin reactivity against RME and compared with uninfected, unimmunized, tested controls (C mice). C, A and I mice infected for 32 days did not show any reaction; MA mice exhibited significant enlargement of the foot-pads at 24 h of the skin inoculation when tested 7 days after immunization and at 0.5, 5, 24 and 48 h of inoculation when tested 18 days after immunization. I mice infected for 3 months produced skin reactions at 0.5, 5 and 48 h of the inoculation. RME is antigenic in mice when injected with adjuvant and 3 month old T. spiralis infection elicits immediate, intermediate and delayed skin reactivities to heterologous muscle components.

Immunity to Toxocara vitulorum repeated infections in a rabbit model.

Eleven rabbits were infected with 10 embryonated eggs of Toxocara vitulorum per g body weight on Days 0, 35 and 72. Embryonated eggs and larvae were enumerated in feces on Days 1-3 after each infection. Two rabbits were killed and larvae were enumerated in small intestine, liver, lungs, skeletal muscles, heart, kidney, brain, eye, uterus, and mammary glands on Days 5, 15, 30, 65 and 101. Serum was obtained on Days 0, 5, 15, 30, 42, 50, 65, 78, 86 and 101 to perform enzyme-linked immunosorbent assays (ELISAs) and Western blots against an extract of embryonated eggs. Between 4 and 10% of the administered parasites, almost all embryonated eggs, were found in the feces after the first or second infection, but 32% (27% of them larvae) after the third. Yields of tissue parasites were 4.1% of the administered dose on Day 5, 2% on Day 15, and 0.8% on Day 30 of the first infection, 0.1% on Day 30 of the second infection, and 0.06% on Day 30 of the third. Larvae were found only in liver, lungs and muscle, including heart. Larva content declined steadily in liver and lungs from Day 5 to 30 of the first infection, was absent in the liver at Day 30 of the second, and in both organs at Day 30 of the third. Muscle larva content increased from Day 15 to 30 of the first infection, and persisted throughout the third infection. Production of IgM antibody was minimal, IgG and the sum of IgMGA antibodies increased slightly or moderately after the first and second infections, but dramatically after the third. Western blots revealed the first antigens (12) by Day 15 of the first infection. Their total number increased with time and number of infections, but some antigens disappeared, whereas new antigens appeared in the course of the observations. Four antigens (32,500-41,000 mol.wt.) may be related to protection. Comparison of the Western blot patterns of two rabbits showed differences in the antigens, recognizable for each rabbit.

Transfer of infection-induced immune protection to Toxocara canis in a mouse model.

Groups of mice were vaccinated twice with soluble extracts of embryonated eggs, females or males of Toxocara canis, or horse serum, and infected with 2,000 homologous embryonated eggs. Recovery of larvae on the fifth day by digestion of mesenteric lymph nodes, liver, lung, brain, and carcass revealed a slight but nonsignificant protection elicited by the parasite materials. Other groups were immunized by homologous infections. A single, 200-day-old infection increased importantly the number of larvae resulting from a challenge, possibly by inducing an immunosuppression in the host. Two infections given within 11 months protected partially against the larvae of a challenge, particularly by trapping the parasites in the liver. Transfer of mesenteric lymph node cells from twice infected mice reduced the total number of parasites, and the liver and lung parasitism of a challenge in the recipients, whereas transfer of serum decreased the total number of parasites and the brain and carcass parasitism. The combination of cells and serum acted synergistically in lungs and brain but antagonistically in liver and carcass.

Humoral immunity in the prepatent primary infection of dogs with Echinococcus granulosus.

Twenty-one parasite-naive dogs were infected with 60,000 protoscolices of Echinococcus granulosus. Transformation of peripheral lymphocytes was investigated before and 29 days after the infection, immunoglobulin concentration and anti-hydatid fluid protein (HFP) titers in serum and feces before and at 35 days of infection, skin reactivity to HFP at 36 days, and characteristics of the parasites at 40 days. The infection caused a significant depression of the spontaneous, lipopolysaccharide-stimulated, and purified protein derivative-stimulated blastogenesis. Responses to phytohemagglutinin were unchanged and reactivity to concanavalin A was enhanced with the infection. Only the concentrations of IgG and IgA in the serum and IgA in the feces increased significantly after infection. Fifteen (71%) dogs produced significant serum titers of anti-HFP hemagglutinins but copro-antibodies were detectable in only 3 dogs at minimum titers. Titers were abolished by treatment with 2-mercaptoethanol. The serum of 11 (52%) dogs transferred passive cutaneous anaphylaxis to guinea pigs but none transferred skin reactivity to pups or rabbits. Five and 1 (but not 0.2) micrograms of HFP caused skin reactivity in 4 parasite-naive dogs. Nineteen (90.5%) infected dogs reacted significantly to skin inoculation of 0.2 microgram of HFP at 0.5 hours and 13 (62%) at 6 hours. The 7 dogs with the highest anti-HFP serum titers or the greatest skin reactivity at 6 hours had significantly less mature or fewer tissue parasites, respectively, than the 7 dogs with the smallest responses. Since there was evidence that the specific immunity was still developing at the time of the study, these results indicate that immunological diagnosis of, and artificial immunization against, canine echinococcosis are feasible.

Cell-mediated immunity in the prepatent primary infection of dogs with Echinococcus granulosus.

Cell-mediated immunity was studied in 21 parasite-naive dogs by transformation of peripheral lymphocytes with phytohemagglutinin (PHA), concanavalin A (Con A), or hydatid fluid protein (HFP) at 29 days of a primary infection with Echinococcus granulosus, and by delayed cutaneous hypersensitivity to HFP at 36 days of the infection. Infection depressed significantly the ability of unstimulated cells to proliferate but enhanced the response to Con A. The lymphocytes of four dogs reacted to HFP significantly above the average, and one dog exhibited cutaneous reactivity to 0.2 micrograms of HFP. Five or 1 but not 0.2 micrograms of HFP produced non-specific skin reactions in parasite-naive dogs. The 7 dogs with the highest reactivity to PHA or Con A had significantly fewer parasites than the 7 less reactive dogs. The differences in other parasite characteristics were not significant in the 7 dogs with the greatest or smallest reactivities. Correlation tests showed an inverse correlation of PHA reactivity with establishment and maturation of parasites, of Con A reactivity with growth and maturation, and of HFP with maturation.

Evidence of immunosuppression by Demodex canis.

Three clinically normal beagles, 3 beagles with localized demodectic mange (LDM), and 3 beagles with generalized demodectic mange (GDM) were investigated simultaneously 1-3 and 4-6 weeks from the appearance of the clinical signs. Blood clinical examination and reactivity of peripheral lymphocytes to Con A and PHA were investigated in the first instance, and reactivity to Con A, PHA, and LPS in the second. Eight aliquots were used in each blastogenesis assay for each dog. All dogs were negative for rheumatoid factor. The results of blastogenesis showed that many observations were distributed non-normally, and that not all dogs in each group responded homogeneously. Comparison of blastogenesis results between dogs demands careful statistical analysis. Responses to mitogens were normal in all dogs at 1-3 weeks except for the LDM dogs that showed an increased response to PHA. Only the response to Con A was moderately inhibited in the LDM dogs at 4-6 weeks. All responses were severely depressed in the GDM dogs at 4-6 weeks. This means that immunosuppression follows rather than precedes the clinical manifestations of GDM, and implies that the phenomenon is induced by the parasite or the host's reaction to it.

A critical look at the importance, prevalence and control of toxocariasis and the possibilities of immunological control.

The visceral infection of humans with Toxocara canis is particularly prevalent in children and may cause a variety of symptoms that commonly persist for 6-24 months. The ocular infection usually causes permanent loss of visual acuity. Human infection is acquired by ingestion of embryonated T. canis eggs with contaminated dirt. Review of recent reports indicates that patent T. canis infection is widely prevalent in the general population of dogs all over the world (3-81%) and results in a substantial contamination of the ground (0.3-87%). The results of sensitive and specific serological tests suggest that about 7% of the clinically healthy human population of the United States, about 5% of that of Canada, and about 4% of that in Great Britain is infected with the parasite. Control of transmission of the parasite to man is often attempted by eliminating the infection in dogs, reducing the population of dogs and the environmental contamination with their feces, and educating the public about the zoonotic potential of toxocariasis. The evidence reviewed indicates that these methods are only marginally effective. Because T. canis relies on congenital and lactogenic transmission to persist in nature, only a procedure that effects the sustained killing of the reservoir larvae in the tissues of the bitch, or of newly-acquired parasites, is expected to be successful. Research with mice, rabbits and dogs demonstrated that prior infections of the host induce the development of protective immunity to reinfections. This procedure, however, leaves remnant populations of larvae from the immunizing infections that are resistant to anthelmintics and to the effect of prior irradiation. Hyperimmunization with partially-purified extracts of T. canis larvae induced 37% resistance to a challenge in mice when the extract was administered alone, and 76% resistance when administered with lipopolysaccharide adjuvant. Production of complete resistance, however, will probably require the prior control of the immunosuppression induced by the parasite. T. canis infections inhibit the production of homologous protective immunity and antibody responses to heterologous antigens, probably by interfering with the activity of helper T-cells, competing with protective antigens, and suppressing antibody synthesis. The evidence indicates, however, that an anti-T. canis vaccine to eliminate the parasite in dogs is feasible.

Immunomodulation by nematodes: a review.

There is current evidence that infections with Trichinella spiralis, Ascaris suum, Nippostrongylus brasiliensis, Nematospiroides dubius (syn. Heligmosomoides polygyrus) and diverse filariae affect the immune responsiveness of their hosts. T. spiralis, or its extracts, can depress or enhance the heterologous humoral or cell-mediated immunities, and affect macrophage activity or the response to other invaders. These effects are induced by products of the migratory and early muscle larvae and appear to obey more than one single-mechanism. A suum acute infections or extracts depress responses involving T cell activity, but stimulate polygonal expansion of B-cells. Nippostrongylus brasiliensis causes polyclonal stimulation of IgE-producing cells, enhances immune responses during the first week of infection and inhibits them later on. Nematospiroides dubius depresses homologous and heterologous immunity and facilitates the permanence of other intestinal nematodes. Filarial worms appear to depress the homologous cell-mediated immunity and the heterologous humoral response by induction of suppressor cells and humoral factors. These phenomena are probably the result of evolutionary pressures on the parasites that facilitate their survival. In the host, they are likely to aggravate the homologous infection, facilitate intercurrent conditions and interfere with immunoprophylaxis procedures.

A review on vaccination against protozoa and arthropods of veterinary importance.

The recent advances in immunology and biotechnology have stimulated much research on the control of parasitic diseases through vaccination. This is a review of the state of the art regarding important protozoan and arthropod veterinary parasites. A live oocyst vaccine for avian coccidiosis is still in use but much work has been done on the identification, cloning, and assay of protective antigens. The sporozoites of Eimeria tenella have been the preferred subject and at least four recombinant antigens have already been tested with partial success. Premunization against babesiosis is still widely used in Latin America as is a live vaccine with attenuated parasites in Australia. At least three Babesia bovis and three Babesia bigemina antigens that generate partial protection have been produced as recombinant proteins. A vaccine against canine babesiosis is being commercialized in France. Infection-treatment is still used to vaccinate against Theileria parva and a schizont vaccine against Theileria annulata. Recombinant sporozoite antigens have been assayed with partial success against both species but the identification and administration of protective schizont antigens, regarded as the most important, still requires considerable work. The immunological control of African trypanosomoses is still impaired by the antigenic variation that the parasites experience during the infection. Although some possibilities exist, most specialists are pessimistic about the promise of developing a vaccine in the near future. Control of Boophilus ticks with an occult tick intestine recombinant antigen seems to have potential in inhibiting reproduction of the tick but salivary antigens appear to be more effective at inhibiting feeding and pathogen transmission. Vaccination with a Hypoderma protein, recently cloned, has induced 90% protection against subsequent infestations. It is very likely that effective vaccines against veterinary parasites will become available in the near future.

Potentiation of an IgE-like response to Bordetella bronchiseptica in pigs following Ascaris suum infection.

Ascaris suum-infected and uninfected pigs were vaccinated with Bordetella bronchiseptica bacterin at 8 and 28 days of age. Only the infected pigs gave Type I cutaneous hypersensitivity to the bacterin when tested at 80 days of age. Only the native sera of infected pigs caused passive cutaneous anaphylaxis 24 h after inoculation into guinea pigs. Heating the sera at 56 degrees C for 4 h abolished this property. Mice infected with A. suum for 34 days did not react to the bacterin. An A. suum infection induces an IgE-like response to B. bronchiseptica in pigs that is not explained by cross-reactive antigens. This finding may have practical implications for the immunization against atrophic rhinitis of swine.

Biology and pathophysiology of Toxocara vitulorum infections in a rabbit model.

Ten female New Zealand rabbits were infected via stomach intubation with eggs of Toxocara vitulorum at a dosage of 10 embryonated eggs per gram of body weight on Days 0, 35 and 72. Ten or 4% of the administered parasites passed in the feces during the 3 days following the first or second infection, but 32% after the third infection. Many larvae were passed in the third infection, but not in the first or second. Tissue parasite yields were 4.1% on Day 5, 2% on Day 15, 0.8% on Day 30, 0.1% on Day 65 and 0.06% on Day 101. Five hundred and ninety-three larvae were recovered from liver, 243 from lungs and 0 from muscles on Day 5; 282 from liver, 138 from lungs and 21 from muscles on Day 15; 151 from liver, 21 from lungs and 50 from muscles on Day 30; 0 from liver, 26 from lungs and 15 from muscles on Day 65; 0 from liver, 0 from lungs and 9 from muscles on Day 101. No larvae were found in other tissues. The size of the muscle larvae at 30, 65 and 101 days indicated that the parasites did not develop beyond the infective stage and suggested that they were probably hypobiotic organisms. Erythrocytes, packed cell volume and monocytes decreased, but eosinophils and basophils increased, after each infection. Serum enzyme levels indicated that liver damage occurred only after the first infection, but muscle injury occurred after each infection and was increasingly more precocious after each infection.

Comparative immunizing power of infections, salivary extracts, and intestinal extracts of Hyalomma marginatum marginatum in cattle.

Tick concealed antigens have been successful in producing immunity that inhibits tick fertility, but require periodic revaccination and are little effective in preventing tick feeding, which is critical to stop pathogen transmission. Tick natural salivary antigens also induce important immunity, but revaccination may be unnecessary in enzootic areas. In addition these antigens may inhibit tick feeding. We immunized groups of three tick-naive calves with four prior infestations with Hyalomma marginatum marginatum, a salivary extract (SE), or an intestinal extract (IE) of the ticks. The calves were challenged with 100 pairs of homologous ticks and characteristics representing tick feeding or fertility were recorded and compared between groups. The percentage of attachment was inhibited by 46% by the infestation-generated immunity, 47% by the SE-generated immunity, and 0% by the IE-generated immunity. The percentage of engorgement was reduced 40% by the infestations, 57% by the SE, and 29% by the IE. The length of feeding was prolonged 92% by the infestations, shortened 44% by the SE, and not affected by the IE. The weight of the engorged females was decreased 67% by the infestations, 64% by the SE, and 31% by the IE. The percentage of engorged ticks that oviposited was inhibited 52% by the infestations, 27% by the SE, and 63% by the IE. The preoviposition period was prolonged 160% by the infestations, 80% by the SE, and 140% by the IE. The egg weight was reduced 60% by the infestations, 60% by the SE, and 66% by the IE. Taking into account mortality before oviposition, fertility was inhibited 88.2% by the infestations, 87.5% by SE, and 91.4% by the IE. The effect of IE immunization on tick feeding was not significant statistically.

Relationships and influences between Boophilus microplus characteristics in tick-naive or repeatedly infested cattle.

Six tick-naive male Hereford calves were infested once a month for 6 months with 18,000 Boophilus microplus larvae on the back and with 400 larvae in a cloth bag glued on the lumbar region. Working with the bag ticks, 12 tick characteristics were recorded for each infestation. Each tick attribute was analyzed for significant differences with those of the first infestation (analysis of variance), and for similarity (clustering), degree of relationship (correlation), and concomitant variation (regression) against all the other attributes during the first, third, and sixth infestations. Some attributes were affected maximally by host immunity about the third infestation but recovered later (length of feeding, detachment weight, egg weight, start of oviposition, fertility efficiency index), whereas others continued to be affected until the last infestation (length of oviposition, corpse weight, start of hatching, feeding efficiency index). All analyses showed that weight at detachment and egg weight were closely related, and corpse weight was partially related to these two. All other natural characteristics were largely independent. Length of feeding showed no significant relation with weight at detachment nor length of oviposition with egg weight. These findings suggest that different tick functions are independently affected by host immunity and recommends against estimating general anti-tick resistance by the evaluation of only a few tick characteristics.

Identification by transfer blot of antigens reactive in the enzyme-linked immunosorbent assay (ELISA) in rabbits immunized and a calf infected with Cryptosporidium sp.

Soluble and particulate fractions of Cryptosporidium sp. oocysts from cattle were obtained by homogenization and sonication. Electrophoresis of the soluble fraction in polyacrylamide gels with sodium dodecyl sulfate and silver staining revealed the presence of 41 bands. Enzyme-linked immunosorbent assay (ELISA) of sera from rabbits immunized with either fraction and from a calf 40 days after infection showed that the animals produced specific antibodies. Enzyme-linked immunoelectrotransfer blot tests revealed the presence of five antigens with the rabbit sera and nine with the calf serum. ELISA proved to be an appropriate test for diagnosis of cryptosporidiosis. Selection of reactive antigens may improve the quality of diagnosis and/or reveal the presence of protective materials in the parasite.

IgG response in guinea pigs to Trichinella spiralis infection.

An enzyme-linked immunosorbent assay (ELISA) using excretory-secretory antigens was developed to study the dynamics of the IgG antibody response to varying levels of Trichinella spiralis infection in the guinea pig. Four groups of four Hartley guinea pigs each were infected with 1250, 250, 50 or 10 T. spiralis infective muscle larvae. They were bled every 15 days for 6 months and the IgG antibody response determined by ELISA. The time of seroconversion was dose dependent as the larger the dose, the earlier the response occurred. Significant differences in antibody response between the dose groups were evident at 30 days post-infection (P less than 0.05). Beyond 60 days post-infection, the response was similar in the four groups. The antibody response in the groups infected with 250 and 50 infective larvae was similar, but was significantly different from that of the high (1250) and low (10) dose groups from 30 days post-infection (P less than 0.01). Once seroconversion occurred, the antibody titer rose to the same level, irrespective of the initial dose. To compare the antibody response according to muscle larvae recovered, the guinea pigs were grouped into four categories: less than 10 larvae; 10-25 larvae, 50-80 larvae, greater than 100 larvae. A significant positive correlation (P less than 0.05) was observed at 60 days post-infection when these groups were compared.

Modification of immune competence by parasitic infections. I. Responses to mitogens and antigens in mice treated with Trichinella spiralis extract.

Spleen cells from mice pretreated with a Trichinella spiralis extract (TsE-mice) showed severe depression of the response to lipopolysaccharide (LPS) and to concanavalin A (Con A), slight depression to phytohemagglutinin (PHA) and normal response to tuberculin purified protein derivative (PPD) as compared to saline-pretreated controls. Mice pretreated with bovine serum albumin (BSA-mice) revealed greatly reduced responses to LPS, somewhat reduced response to Con A, and normal responses to PHA and to PPD. Only TsE-mice showed significant reduction in the number of rosette-forming cells and of direct and indirect plaque-forming cells (DPFC and IPFC). BSA-mice exhibited some reduction of the DPFC only. Direct hemagglutinating (HA) titers were equivalent in the 3 groups after immunization with sheep erythrocytes but facilitated HA titers were depressed in TsE-mice. The total number and the number of viable cells were similar in the spleens of all animals. TsE treatment causes a reduction in the number of T1 lymphocytes and an inhibition of the late differentiation of B cells in the spleen. Suppressor T-cells apparently play a major but not exclusive role in T. spiralis-induced nonspecific immunodepression.

Responses of B-cells to mitogens and antigen in mice receiving isogenic splenocytes from animals treated with Trichinella extract.

Splenocytes of C57BL/6J mice injected with a Trichinella spiralis larval extract for 7 consecutive days were transferred in two doses into isogenic, immunocompetent mice. On the 3rd day, some recipients were immunized with 10(9) sheep red blood cells and others were killed to investigate blastogenic response of their splenocytes to concanavalin A (Con A), Escherichia coli lipopolysaccharide (LPS), and Mycobacterium's purified protein derivative (PPD). On the 8th day of immunization, the corresponding mice were killed to study rosette-forming cells (RFC) and direct and indirect plaque-forming cells (D- and I-PFC) in their spleens. Transfer of 10(6) cells depressed the Con A reactivity and the number of RFC and 1-PFC, but increased the PPD reactivity and the number of D-PFC in the recipients, as compared to control mice receiving splenocytes from donors injected with a saline solution. Ten million cells inhibited only the Con A reactivity, but enhanced the number of LPS- and PPD-responding cells and of D-PFC in the recipients over the controls. Inoculation of cells from mice injected with bovine serum albumin did not reproduce the same effects. Splenocytes of mice treated with T. spiralis extract simultaneously inhibit and enhance diverse functions of the immune system. Stimulation is exerted on IgG antibody production and appears to be mediated by suppressor T-cells. Stimulation is exerted mainly on IgM antibody formation. Depression seems to be antigen-specific; it is partially compensated by the concurrent suppression, and it is probably a result of macrophage activation.

Effects of irradiation on the biology of the infective larvae of Toxocara canis in the mouse.

Mice were infected with either 2,000 normal or irradiated embryonated eggs of Toxocara canis and the number of larvae in their livers, lungs, brains, and carcasses investigated at 5, 20, and 33 days of infection. Mortality of mice infected with normal eggs was 33% between day 4 and 8 postinfection but there was no mortality among mice infected with irradiated eggs. Irradiation with 60, 90, or 150 kr of X-rays inhibited the migration of larvae from the livers and lungs and their accumulation in brain and carcass in proportion to the irradiation dose. By day 33 of infection, the ratio of larvae in liver and lungs to larvae in brain and carcass was 0.16 in normal mice, 0.42 in 60-kr mice, 0.98 in 90-kr mice, and 23.3 in 150-kr mice. Irradiated larvae, particularly those migrating through the peritoneal cavity, died faster than normal larvae until day 20. Irradiation favored survival after day 20. By days 20 and 33 postinfection the total parasite load was 29% and 8%, respectively, of the administered dose in control mice, 18% and 12% in 60-kr mice, 8% and 4% in 90-kr mice, and 0.9% and 0.3% in 150-kr mice. Irradiation of infective T. canis larvae, then, reduces their pathogenicity, inhibits their migration from liver and lungs, kills some of the parasites during the first 3 weeks of infection, but favors their late survival in the host.