Influence of mouse strain, infective dose and larval burden in the brain on activity in Toxocara-infected mice

Outbred LACA mice and inbred NIH mice were administered low (100 ova), medium (1000 ova), high (3000 ova) and trickle (4x250 ova) doses of Toxocara canis ova and the effect of infection on activity was examined with respect to: (i) the dose of ova administered and (ii) the number of larvae recovered from the brain. Larval recovery from the brain was significantly reduced in NIH mice compared to LACA mice for the 1000, 3000 and trickle doses. Mice from each strain were divided into larval intensity groupings based upon the number of larvae recovered from their brain. Activity for each mouse was measured pre- and post-infection by observing its behaviour in the home cage. Activity was assessed by monitoring six different independent categories of murine behaviour - ambulation, grooming, rearing, digging, climbing and immobility. Within each behavioural category, the duration of time spent at each behaviour per mouse within one thousandth of a second, the number of short bouts performed and the number of long bouts of behaviour performed were recorded over a 20 min period. Activity of LACA and NIH mice differed prior to infection. LACA mice spent more time immobile compared to NIH mice, which ambulated and climbed more. Variations in activity were also observed between groups of mice prior to infection. The effect of infection differed by strain, by dose and by larval intensity. Post-infection LACA mice became more immobile and ambulated less. NIH mice showed reduced immobility, but while ambulation decreased digging and climbing increased post-infection. Short bouts of activity remained unchanged among LACA mice post-infection but showed an increase for some behaviours in NIH mice.     

Toxocara in the mouse: a model for parasite-altered host behaviour?

The objective of this paper is to critically evaluate the significance of parasite-altered host behaviour in the Toxocara mouse model particularly in the light of the Manipulation Hypothesis. Murine behaviours were examined in both outbred and inbred strains of mice infected with different doses of Toxocara canis ova. Behaviours investigated included activity, exploration, response to novelty, anxiety, learning, memory and social behaviour. Subsequent modifications to the behaviour of infected mice were investigated with respect to dose administered and larval accumulation in the brain. There was substantial variation in the number of larvae recovered from brains of individual mice, which received similar doses of Toxocara ova. Furthermore, the numbers of larvae recovered at different doses differed significantly between an outbred and inbred strain of mouse. Alterations in infected host behaviour occurred and were related to the number of larvae recovered from the brain. For social behaviour in outbred mice, a high infection in the brain reduced levels of aggressive behaviour and increased levels of flight and defensive behaviours. In contrast, outbred mice with a low infection in the brain displayed a greater level of risk behaviour in respect of predator odour and the light/dark box compared to control or high infection mice. Post-infection, outbred mice were more immobile whereas inbred mice showed reduced immobility and increased digging and climbing. Impaired learning ability was observed in outbred mice with moderate and high levels of infection in the brain compared to control and low infection mice. Toxocara infection has an impact upon a diverse range of murine behaviours with little evidence for a specific and hence an adaptive alteration. Many of the effects on murine host behaviour by Toxocara are likely to be pathological side effects of infection rather than as a consequence of adaptive host-manipulation. Observed changes in murine behaviour may be relevant to human toxocariasis.     

Trickle infections with Nippostrongylus brasiliensis in rats: larval migration through the lungs

The effects of exposure of rats to repeated low-level (trickle) infections with Nippostrongylus brasiliensis were assessed by measuring intestinal and lung worm burdens. Worm recoveries from the intestine, made during a period of trickle infection in rats of different ages, showed a virtually complete rejection of intestinal worms in old rats and a partial rejection in young rats. Recoveries from lungs were made in young rats after challenge infection with 500 third-stage (L3) larvae, given after a 2- or 4-wk period of sensitization, during which rats were infected with 10 or 20 doses of 25 larvae. Such trickle infections elicited a strong host response to a challenge infection, manifested by low recoveries of larvae and an increased duration of larval retention in lungs. In another group of rats sensitized by a single dose of 250 L3 larvae, the recovery of larvae from challenge infection and their clearance from the lungs were similar to these observed in rats uninfected prior to challenge. The effect of trickle infections on preintestinal stages was most pronounced and consistent in rats exposed to larvae the greater numbers of times and over the longest period.     

The influence of inoculum size and time post-infection on the number and position of Toxocara canis larvae recovered from the brains of outbred CD1 mice

Outbred CD1 mice were administered doses of 1000 and 3000 Toxocara canis eggs and postmortem took place on days 7, 42 and 120 post-infection. Mice were killed by cervical dislocation and brains were sagitally bisected and fixed in 10% neutral buffered formalin prior to histological preparation and examination. The number of T. canis larvae were counted per brain and per section and the number of larvae cited for the first time per section were also recorded. These observations were compared by dose administered and by day of postmortem. The total number of larvae per brain and per section was higher for the 3000 dose compared to the 1000 dose. A different pattern emerged for the number of larvae observed in the brain over the three postmortem days depending upon the dose received. For the 1000 dose larval numbers increase from day 7 to day 120 whereas for the 3000 dose the opposite trend occurs. Larvae were assigned to one of five regions in the brain - the telencephalon, diencephalon, cerebellum, medulla, pons and brain stem and the olfactory bulb. Larvae did not show a random distribution in the brain. The majority of larvae were recorded from the telencephalon and the cerebellum. The percentage of sections with larvae in them is higher for the 3000 dose compared to the 1000 dose for all regions of the brain. For the majority of regions, the percentage of sections with larvae in them increases between day 7 and 42 and then decreases by day 120 and this is most pronounced for the cerebellum. For the telencephalon and diencephalon only, more larvae were detected on the right hand side of the brain compared to the left hand side. Statistical analysis revealed that dose and brain region are significant factors which influence the number of larvae observed in histological sections of the brain but day post-infection is not.     

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.     

Plagiorchis noblei (Plagiorchiidae) in Aedes aegypti: parasite acquisition and host mortality in trickle infections

The prevalence and intensity of experimental infections of Aedes aegypti with the digenean Plagiorchis noblei increased significantly with the level of trickle exposure to cercariae. Daily exposure to doses of 16 cercariae/day yielded a mean infection intensity of 13.0 metacercariae; doses of 1 cercaria/day resulted in only 2.4 metacercariae per infected mosquito larva. The prevalence of infection rose from 46% at an exposure of 1 cercaria/day to 99% at 16 cercariae/day. Host mortality rose concomitantly from 25% to 88%. Host mortality and parasite acquisition were independent of environmental temperatures (21-29 C), despite the fact that developmental times, and consequently the number of daily exposures, were more than 50% greater at the low end of the temperature scale. This may be attributable to low activity of mosquito larvae and the resulting decrease in the number of encounters with cercariae.     

Experimental infection with Ancylostoma caninum larvae in mice. V. Serum protein pattern during infection with various doses.

Serum protein patterns of Swiss albino mice during Ancylostoma caninum infection with varying dose levels were studied electrophoretically. There was a significant decrease in albumin associated with a corresponding increase in beta globulin in all the infected groups. Gamma globulin decreased significantly in all groups except the one infected with a challenge dose of 4000 larvae. Maximum changes occurred on day 31, 15, 9, 15 and 6 when mice were infected with 250, 500, 1000, 2000 and 4000 larvae, respectively. There was a significant negative correlation between albumin and the infective doses and a positive correlation between beta globulin and the infective doses and among the changes caused by different infective doses.     

Infectivity of two nematode parasites, Camallanus lacustris and Anguillicola crassus, in a paratenic host, the three-spined stickleback Gasterosteus aculeatus

Three-spined sticklebacks Gasterosteus aculeatus are frequent paratenic hosts of the nematode parasites Anguillicola crassus and Camallanus lacustris. As paratenic hosts, sticklebacks could spread infection by carrying high numbers of infective stages. In contrast, low infective ability of either parasite for the paratenic host could hinder the spread of infection. In the present study, G. aculeatus was, for the first time, infected under controlled laboratory conditions with defined doses of the parasites. Sticklebacks were exposed to 6, 12, 18 and 24 parasite larvae to determine the infective ability of the 2 nematode species. There were significantly higher infection rates for C. lacustris (18 to 49%) than for A. crassus (4 to 14%) at each exposure dose. In C. lacustris-infected sticklebacks, infection rates tended to be highest after exposure to 12 C. lacustris larvae and lowest after exposure to 24 parasites. In A. crassus-infected sticklebacks, no effect of parasite exposure dose on infection rates was observed. Immunity parameters such as respiratory burst activity and lymphocyte proliferation of head kidney leukocytes recorded 18 wk post exposure were not significantly affected by either parasite or exposure dose. Granulocyte:lymphocyte ratios were elevated only within the stickleback group showing the highest infection intensity of C. lacustris, i.e. to those exposed 18 parasites.     

Infectivity, speed of kill, and productivity of a baculovirus expressing the itch mite toxin txp-1 in second and fourth instar larvae of Trichoplusia ni

A cDNA clone of the gene coding for the paralytic neurotoxin (tox34) from the female straw itch mite, Pyemotes tritici, was created by RT-PCR and inserted into the genome of the Autographa californica nucleopolyhedrovirus (AcMNPV) under the control of the AcMNPV p10 promoter. This recombinant virus, AcTOX34.4, caused a rigid paralysis in infected larvae. The infectivity of AcTOX34.4 was compared to the wild-type parent strain, AcMNPV-C6, in second and fourth instar larvae of the cabbage looper, Trichoplusia ni. There were no significant differences in LD(50) values between the recombinant virus and its wild-type parent strain but, as expected, the LD(50) was lower for second instar larvae. The mean time to death and yield of occlusion bodies were measured in second and fourth instar T. ni larvae at a high (100% mortality) and low (<50% mortality) doses of the virus. The mean time to death of recombinant infected larvae was reduced by 50-60% compared to larvae infected with the wild-type strain, depending on virus dose and instar, with these larvae becoming paralysed after approximately 60 h and dying 10-20 h later. This is among the fastest speeds of kill recorded for recombinant baculoviruses. Fourth instar larvae were found to succumb to the recombinant virus more quickly than the second instar larvae. The increase in the speed of kill of the recombinant virus was accompanied by a large reduction of approximately 95% in the yield of progeny virus. The yield of virus showed a highly significant relationship with time to death, but this relationship was complex and varied between the different viruses, concentrations, and instars. The yield per unit weight of the larvae was found to be constant at a low virus dose and increased over time at a high virus dose, irrespective of instar and virus. It is predicted that these changes in the performance of the recombinant virus would act toward reducing its fitness, leading to it being outcompeted by the wild type in field situations.     

Trichinella spiralis: quantitative relationships between intestinal worm burden, worm rejection, and the measurement of intestinal immunity in inbred mice

The effect of widely different doses of Trichinella spiralis muscle larvae on time to rejection of intestinal adults and on host survival was assessed in mice of the three rejection phenotypes; strong, intermediate, and weak. Rejection is weak with doses of less than 50 larvae per mouse. At these doses all mice rejected worms at a similar rate and no phenotypic variation was evident among strains. In contrast, rejection time was shortest for all strains and phenotypic variation among strains was evident in the range 50-100 muscle larvae/mouse. Above this dose the time taken to rejection increases monotonically with dose for all mouse strains examined. Despite this, the relative strength of rejection (i.e., phenotype) of a given strain of mouse was not changed at higher doses. Based on an end point of 98% rejection of the infective dose, time to rejection was predictable to +/- 1 day for all mouse strains and doses tested over the range 100-1000 worms administered. The principal reason for the increased time to complete rejection with larger worm doses was a delay in the initiation of intestinal rejection. This delay was evident above a dose of 50-100 larvae per mouse and occurred in all strains. Once begun, rejection was faster and eliminated more worms in unit time at higher doses (400-800 more) than at lower doses of worms. This appeared to be due to a stronger immune response of the host at higher doses. However, the increase in the rate of rejection was still not as great as the increase in the dose. We postulate that the delay in rejection with increased dose is due to a requirement for a "critical mass" of effectors/worm required to cause rejection. As dose increases, more time is required to reach the level at which worm rejection commences. Deaths due to higher doses of worms also occurred in a strain-specific manner and were temporally biphasic. The intestinal phase of infection produced mortality from 1 to 5 days after infection and the strongest rejection phenotype (NFS) was also the most resistant to intestinal deaths. Deaths occurring after Day 5 were due to the parenterally migrating newborn larvae. The weakest rejection phenotype, that of the B10 congenics, was also the least resistant to intestinal deaths. An experimental formula describing 98% worm rejection time with different doses was derived from the data.(ABSTRACT TRUNCATED AT 400 WORDS)     

Trichinella spiralis: dose dependence and kinetics of the mucosal immune response in mice.

The role of the mucosal immune response in helminth infections is not clear. In this study, the dose dependence and kinetics of the mucosal immune response to Trichinella spiralis were determined in experimentally infected Swiss Webster and BALB/c mice. The primary mucosal isotype was sIgA, although IgG was also detected, and primary infections with 10 and 150 larvae produced an anamnestic response on challenge. The mucosal and systemic immunoglobulin responses were dose dependent in both primary and challenge infections. The fecundity and length of worms and the rate of expulsion from the gut were determined on Day 6 postchallenge in Swiss Webster mice. Adult worm recovery and fecundity were reduced by greater than 50% and worm length by 28% in mice infected and challenged with 10 larvae and by 90, 85, and 35%, respectively, in mice infected and challenged with 150 larvae. The rate of expulsion was correlated with the size of both primary and challenge doses and a reduction in fecundity was correlated with the size of the primary dose only. The reduction in worm length did not differ significantly between the infection doses, but the trend was similar to that for expulsion. In BALB/c mice the expulsion response was dissociated from a reduction in fecundity and worm length, the latter two being positively correlated with sIgA levels, supporting a role for sIgA and/or IgG in these effects. However, expulsion does not appear to be dependent on the mucosal immunoglobulin response.     

Low and high-dose intradermal infection with Leishmania major and Leishmania amazonensis in C57BL/6 mice

A model of skin infection with Leishmania amazonensis with low doses of parasites is compared to infection with high doses of L. amazonensis and low and high doses of Leishmania major. C57BL/6 mice were infected with 103 or 10(6) parasites in the ear and the outcome of infection was assessed. The appearance of lesions in mice infected with 103 parasites was delayed compared to mice infected with 10(6) Leishmania and parasites were detectable at the infection site before lesions became apparent. Mice infected with L. amazonensis displayed persistent lesions, whereas infection with L. major spontaneously healed in all groups, although lymphocytes persisted at the site of infection after healing. Macrophages persisted only in L. amazonensis-infected mice. High-dose L. amazonensis-infected mice produced lower levels of IFN-γ and TNF than mice infected with L. major. No correlation between the persistence of parasites and IL-10 levels and the production of nitric oxide or urea by macrophages was found. We conclude that infection with low doses of L. amazonensis in the dermis changes the course of infection by delaying the appearance of lesions. However, low-dose infection does not change the outcomes of susceptibility and cytokine production described for subcutaneous infection with high numbers of parasites.     

Movement of Pomphorhynchus bulbocolli larvae from the hemocoel to the peripheral circulation of Gammarus pseudolimnaeus

Acanthors and early acanthellae of Pomphorhynchus bulbocolli were observed in the peripheral circulation of experimentally infected laboratory-reared Gammarus pseudolimnaeus. This has not previously been reported for larval P. bulbocolli during development in the intermediate host. The timing of larval movement from hemocoel to vessels and sinuses was related to intensity of infection. In an experimental group with a low mean intensity of infection, the movement of larval P. bulbocolli into the peripheral circulation occurred later (9 days postfeeding) than in a group with a high mean intensity of infection (2 days postfeeding). Larvae were observed in the peripheral circulation for a longer period of time in amphipods from the group with the high mean intensity of infection. Dead and slightly melanized acanthors and early acanthellae were present in the hemocoel at the time living larvae moved into the peripheral circulation. Only 1 incidence of a hemocytic response to the larvae was observed in the first 20 days postfeeding.     

CD8+ T cells mediate recovery and immunopathology in West Nile virus encephalitis

C57BL/6J mice infected intravenously with the Sarafend strain of West Nile virus (WNV) develop a characteristic central nervous system (CNS) disease, including an acute inflammatory reaction. Dose response studies indicate two distinct kinetics of mortality. At high doses of infection (10(8) PFU), direct infection of the brain occurred within 24 h, resulting in 100% mortality with a 6-day mean survival time (MST), and there was minimal destruction of neural tissue. A low dose (10(3) PFU) of infection resulted in 27% mortality (MST, 11 days), and virus could be detected in the CNS 7 days postinfection (p.i.). Virus was present in the hypogastric lymph nodes and spleens at days 4 to 7 p.i. Histology of the brains revealed neuronal degeneration and inflammation within leptomeninges and brain parenchyma. Inflammatory cell infiltration was detectable in brains from day 4 p.i. onward in the high-dose group and from day 7 p.i. in the low-dose group, with the severity of infiltration increasing over time. The cellular infiltrates in brain consisted predominantly of CD8(+), but not CD4(+), T cells. CD8(+) T cells in the brain and the spleen expressed the activation markers CD69 early and expressed CD25 at later time points. CD8(+) T-cell-deficient mice infected with 10(3) PFU of WNV showed increased mortalities but prolonged MST and early infection of the CNS compared to wild-type mice. Using high doses of virus in CD8-deficient mice leads to increased survival. These results provide evidence that CD8(+) T cells are involved in both recovery and immunopathology in WNV infection.     

Amphetamine-induced c-fos mRNA expression in the caudate-putamen and subthalamic nucleus: interactions between dose,environment, and neuronal phenotype

When administered in a novel environment relatively low doses of amphetamine induce c-fos mRNA in the subthalamic nucleus (STN) and in preproenkephalin mRNA-containing (ENK+) neurons in the caudate-putamen (CPu). When administered at home, however, low doses of amphetamine do not produce these effects. Environmental novelty also facilitates the behavioral effects of acute and repeated amphetamine, but this is dose-dependent. The purpose of the present experiment therefore was to determine if the effect of context on amphetamine-induced c-fos expression is also dose-dependent. It was found that: (i) No dose of amphetamine tested (1-10 mg/kg) induced c-fos in many ENK+ cells when given at home. (ii) When given in a novel environment low to moderate doses of amphetamine (1-5 mg/kg) induced c-fos in substantial numbers of ENK+ cells, but the highest dose examined (10 mg/kg) did not. (iii) Environmental novelty enhanced the ability of low to moderate doses of amphetamine to induce c-fos in the STN, but the highest dose of amphetamine induced robust c-fos mRNA expression in the STN regardless of context. The results do not support the idea that engaging ENK+ cells, at least as indicated by c-fos mRNA expression, is critical to produce robust behavioral sensitization, but do suggest a possible role for the STN. Furthermore, the results highlight the importance of drug-environment interactions on the neurobiological effects of drugs, and have implications for thinking about the circuits by which context modulates the acute and long-lasting consequences of amphetamine treatment.     

Environmental effects on cytoplasmic incompatibility and bacterial load in Wolbachia-infected Drosophila simulans

The effects of high temperatures, antibiotics, nutrition and larval density on cytoplasmic incompatibility caused by a Wolbachia infection were investigated in Drosophila simulans. Exposure of larvae from an infected stock to moderate doses of tetracycline led to complete incompatibility when treated females were crossed to infected males; the same doses only caused a partial restoration of compatibility when treated males were crossed to uninfected females. In crosses with treated females, there was a strong correlation between dose effects on hatch rates and infection levels in embryos produced by these females. Ageing and rearing males at a high temperature led to increased compatibility. However, exposing infected females to a high temperature did not influence their compatibility with infected males. Male temperature effects depended on conditions experienced at the larval stage but not the pupal stage. Exposure to 25 °C reduced the density of Wolbachia in embryos compared with a 19 °C treatment. Low levels of nutrition led to increased compatibility, but no effect of larval crowding was detected. These findings show the ways environmental factors can influence the expression of cytoplasmic incompatibility and suggest that environmental effects may be mediated by bacterial density.     

An RNA virus in Autographa californica nuclear polyhedrosis virus preparations: Gross pathology and infectivity

The pathology and infectivity of an RNA virus infectious to Trichoplusia ni larvae was investigated. The enzyme-linked immunosorbent assay (ELISA) and weight depression were used as criteria for virus concentration in larval homogenates and live larvae, respectively. Infected larvae were severely stunted, weighing as little as 13 times less than uninfected individuals of the same age, yet appeared normal morphologically. The virus was found to cause only slight mortality at high concentrations. Infected larvae displayed the pathological stunting response down to a dose of 0.1 ng of virus. Larvae infected with doses 100 times lower did not show the weight response but such inapparent infections were detectable by ELISA. Because of these subtle gross pathological symptoms, particularly at low levels of infection, infected individuals could easily remain unde-tected in a group-reared colony.     

Kinetics of liver trapping of infective larvae in murine toxocariasis

Mice sensitized by prior infection with Toxocara canis eggs trap many larvae of a challenge infection within the liver. In this study the distribution of challenge larvae in sensitized mice was examined to determine the earliest onset of liver trapping and to establish if the previously described phenomenon truly represented larval trapping. In all experiments, C57BL/6J mice were infected with a sensitization dose of 125 infective T. canis eggs on day 0 postinfection (PI) and challenged with 500 infective eggs on day 28 PI. In the initial experiments, larval numbers were determined within the intestinal contents, intestinal wall, mesenteric tissues, liver, lungs, skeletal muscle, and brain of each mouse on days 0.5, 1, 2, 3, 5, and 6 postchallenge (PC). Migration patterns were similar among the test and control groups except the peak of larval numbers in the liver, seen at 1 day PC in control mice, was delayed until 3 days PC in the test group. Larval trapping occurred within the liver of test mice at least by day 5 PC. In subsequent experiments, larval numbers were determined within the liver, skeletal muscle, brain of each mouse, and within the eyes of each mouse group at 4, 8, 12, and 16 wk PC. Larval numbers within the liver of test mice were similar both at 5 days PC and 16 wk PC, implying that larvae were trapped in this organ rather than delayed in their migration to other body sites. Liver trapping did not protect the eyes or brain of sensitized mice from larval migration, nor did it result in larval killing.     

Heligmosomoides polygyrus: Simple recovery of post-infective larvae from mouse intestines

Mice infected orally with third-stage larvae of Heligmosomoides polygyrus were killed at various times after infection. Their small intestines were removed, tied at each end and incubated at 37 C in dilute culture medium. When intestines were taken from mice infected for a period of between 1 and 7 days, a number of developing larvae comprising up to 20% of the infective dose emerged within 60 min through the intestinal wall into the medium. The recovery of emergent larvae was highest using intestines from mice infected 36 to 120 hr previously. The proportion of parasites emerging from the intestines of 48-hr-infected mice was similar for doses of 100 to 2400 larvae. Significantly fewer larvae emerged from the intestines of mice resistant to reinfection and challenged with third-stage larvae 36–72 hr before necropsy.     

Low-dose meningeal worm (Parelaphostrongylus tenuis) infections in moose (Alces alces)

Parelaphostrongylosis has a rapid onset and is lethal in neonatal moose (Alces alces) when large numbers of third-stage Parelaphostrongylus tenuis larvae (L3) are given experimentally. Little is known, however, about the severity and prognosis of infections acquired naturally by accidentally ingesting terrestrial gastropods which are rarely infected and have few larvae. To investigate the relationship between infecting dose, age of moose, and severity of disease, five calves were given low doses of three to 10 L3 when five (n = 2) or 9.5 mo old (n = 3). Each of two animals initially given low doses were later challenged with a dose of 15 L3. As positive controls, two calves were given doses of 15 and 30 L3, considered to be high. All five calves given low doses showed abnormal locomotory signs at 20-28 days postinoculation (DPI) that progressively became more pronounced with hind quarter weakness and front lameness. However, after 77-130 DPI, signs diminished markedly in two of these animals and disappeared in another two. Challenge infections of 15 L3 given 199 days after initial infections had no noticeable effects although an immature worm, probably resulting from the challenge, was found in the spinal cord of one animal killed 51 days later. Two positive control animals given the high doses of 15 and 30 L3 showed moderate to severe, non-resolving, locomotory signs and had to be euthanized. Results demonstrate that single, low doses of three to 10 P. tenuis L3 cause moderate disease in moose calves but over time, some worms die and animals can recover. A degree of protection may develop against future infection.