Secondary infection of Nippostrongylus brasiliensis in lactating rats is sensitive to dietary protein content

The energetic costs of sub-lethal helminth parasites in mammals: a meta-analysis

Parasites, by definition, have a negative effect on their host. However, in wild mammal health and conservation research, sub-lethal infections are commonly assumed to have negligible health effects unless parasites are present in overwhelming numbers. Here, we propose a definition for host health in mammals that includes sub-lethal effects of parasites on the host's capacity to adapt to the environment and maintain homeostasis. We synthesized the growing number of studies on helminth parasites in mammals to assess evidence for the relative magnitude of sub-lethal effects of infection across mammal taxa based on this expanded definition. Specifically, we develop and apply a framework for organizing disparate metrics of parasite effects on host health and body condition according to their impact on an animal's energetic condition, defined as the energetic burden of pathogens on host physiological and behavioural functions that relate directly to fitness. Applying this framework within a global meta-analysis of helminth parasites in wild, laboratory and domestic mammal hosts produced 142 peer-reviewed studies documenting 599 infection-condition effects. Analysing these data within a multiple working hypotheses framework allowed us to evaluate the relative weighted contribution of methodological (study design, sampling protocol, parasite quantification methods) and biological (phylogenetic relationships and host/parasite life history) moderators to variation in the magnitude of health effects. We found consistently strong negative effects of infection on host energetic condition across taxonomic groups, with unusually low heterogeneity in effect sizes when compared with other ecological meta-analyses. Observed effect size was significantly lower within cross-sectional studies (i.e. observational studies that investigated a sub-set of a population at a single point in time), the most prevalent methodology. Furthermore, opportunistic sampling led to a weaker negative effect compared to proactive sampling. In the model of host taxonomic group, the effect of infection on energetic condition in carnivores was not significant. However, when sampling method was included, it explained substantial inter-study variance; proactive sampling showing a strongly significant negative effect while opportunistic sampling detected only a weak, non-significant effect. This may partly underlie previous assumptions that sub-lethal parasites do not have significant effects on host health. We recommend future studies adopt energetic condition as the framework for assessing parasite effects on wildlife health and provide guidelines for the selection of research protocols, health proxies, and relating infection to fitness.

Dietary protein supplementation results in molecular and cellular changes related to T helper type 2 immunity in the lung and small intestine in lactating rats re-infected with Nippostrongylus brasiliensis

Acquired immunity to gastrointestinal nematodes reduces during late pregnancy and lactation which is known as periparturient relaxation of immunity (PPRI). Protein supplementation reduces the degree of PPRI in a rat model re-infected with Nippostrongylus brasiliensis, but the underlying molecular mechanisms have yet to be elucidated. Here, we hypothesized that protein supplementation will enhance T helper type 2 immunity (Th2) in the lung and small intestine. Nulliparous Sprague-Dawley rats were given a primary infection of N. brasiliensis prior to mating and restrictedly fed diets with either low protein (LP) or high protein (HP) during pregnancy and lactation. Dams were secondary infected with N. brasiliensis on day 2 post-parturition, and histology and gene expression were analysed for tissue samples collected at days 5, 8 and 11. Genes related to Th2 immunity in the lung, Retnla, Il13 and Mmp12, and in the intestine, Retnlb, were upregulated in HP dams compared to LP dams, which indicates the effect of dietary protein on Th2 immunity. HP dams also had increased splenic CD68+ macrophage populations compared to LP dams following secondary infection, suggesting enhanced immunity at a cellular level. Our data assist to define strategic utilization of nutrient supply in mammals undergoing reproductive and lactational efforts.

Body Protein Reserves Sustain Maternal Performance in Early Lactation but Dietary Protein Is Necessary to Maintain Performance and Immune Responses to Nippostrongylus brasiliensis in Lactating Rats

Background

It has been shown that dietary protein supplementation during lactation boosts immunity in Nippostrongylus brasiliensis-infected periparturient rats. It is not known whether body protein reserves accumulated during gestation have a similar effect during lactation.

Objective

This study aimed to quantify the impact of body protein reserves and dietary protein supplementation on maternal performance and immune responses to N. brasiliensis during lactation.

Methods

Multiparous female Sprague-Dawley rats were administered a primary infection of N. brasiliensis before mating and were restriction-fed either 60 g [low-protein diet gestation (Lge)] or 210 g [high-protein diet gestation (Hge)] crude protein (CP) per kilogram of dry matter (DM) until parturition. From parturition onward, dams were restriction-fed either 100 g [low-protein diet lactation (Lla)] or 300 g [high-protein diet lactation (Hla)] CP per kilogram of DM, generating 4 different dietary treatments. A subset of rats was sampled before parturition; postparturition, dams were secondarily infected with N. brasiliensis and samples were collected at days 5 and 11 postparturition.

Results

Maternal performance until parturition, as measured by pup weight, was better in Hge rats than in Lge rats [Lge: 4.84 g; Hge: 6.15 g; standard error of the difference (SED): 0.19]. On day 11, pup weights of dams with reduced protein reserves fed protein during lactation (Lge-Hla; 20.28 g) were higher than their counterparts from Hge-Lla dams (17.88 g; SED: 0.92). Worm counts were significantly different between Lge-Lla-fed (253; 95% CI: 124, 382) and Hge-Hla-fed (87; 95% CI: 22, 104) dams on day 11 (P = 0.024). The expression of splenic interleukin 13 (Il13) and arachidonate 15-lipoxygenase (Alox15) was significantly higher (P < 0.05) in Hge-Hla dams compared with Lge-Lla dams on day 5.

Conclusions

Although protein reserves were adequate to maintain maternal performance in the early stage of lactation in dams infected with N. brasiliensis, they were not adequate to maintain maternal performance and effective immune responses at later stages. Dietary protein supplementation was required to achieve this.

Temporal effects of protein supply on expression of immunity to Nippostrongylus brasiliensis during lactation in rats

Expression of acquired immunity to gastrointestinal parasites usually breaks down during the periparturient period, which is characterised by an increased worm burden and nematode egg excretion. A possible nutritional basis of this phenomenon has been addressed in a rat model, as lactating rats exhibit a breakdown of immunity to the gastrointestinal nematode Nippostrongylus brasiliensis (Houdijk et al., 2005). Indeed, increasing protein supply and reducing protein demand at times of protein scarcity improved resistance to N. brasiliensis (Houdijk et al., 2005; Normanton et al., 2006) but did not affect associated local immune responses. The latter may have been due to the single sampling point used. Therefore, the objective of the current experiment was to assess temporal effects of increased protein supply on resistance and immune responses to N. brasiliensis in lactating rats.

Amelioration of the periparturient relaxation of immunity to parasites through a reduction in mammalian reproductive effort

The degree of periparturient relaxation of immunity to gastrointestinal parasites has a nutritional basis, as overcoming protein scarcity through increased protein supply improves lactational performance, enhances local immune responses and reduces worm burdens. Herein lactating rats, re-infected with Nippostrongylus brasiliensis, are used to test the hypothesis that a similar and rapid improvement of immunity can be achieved through reducing nutrient demand at times of dietary protein scarcity. Reducing litter size from 12 to three pups during lactation resulted, as expected, in cessation of maternal body weight loss and increased pup body weight gain compared with dams which continued to nurse 12 pups. This increase in performance concurred with a rapid decrease in parasitism; within 3 days post nutrient reduction, a 87% reduction in the number of worm eggs found in the colon and 83% reduction in worm burdens was observed, which concurred with increased local immune responses, i.e. 70% more mast cells and 44% more eosinophils in the small intestinal mucosa, to levels similar to those in dams nursing three pups throughout. However, there were no concurrent changes in goblet cell hyperplasia, serum anti-N. brasiliensis-specific antibody levels or mRNA expression of IL-4, IL-10 or IL-13 in the mesenteric lymph nodes. To our knowledge the current study is the first to employ a litter reduction strategy to assess the rate of immune improvement upon overcoming nutrient scarcity in a non-ruminant host. These data support the hypothesis that periparturient relaxation of immunity to gastrointestinal nematodes can be reduced by restoring nutrient adequacy and, importantly, that this improvement can occur very rapidly.

Leucine and methionine deficiency impairs immunity to gastrointestinal parasites during lactation

Lactating rats reinfected with Nippostrongylus brasiliensis fed low-crude protein (CP) foods show reduced lactational performance and less resistance to parasites compared with their high-CP counterparts. Here, we hypothesised that feeding high-CP foods deficient in specific essential amino acids (AA) would result in similar penalties. Second-parity lactating rats, immunised with 1600 N. brasiliensis infective larvae before mating, were fed foods with either 250 (high protein; HP) or 150 (low protein; LP) g CP/kg, or were HP deficient in either leucine (HP-Leu) or methionine (HP-Met). On day 1 of lactation, litter size was standardised at twelve pups. On day 2, dams were either reinfected with 1600 N. brasiliensis larvae or sham-infected with PBS. Dams and litters were weighed daily until either day 8 or 11, when worm burdens, and inflammatory cells and systemic levels of N. brasiliensis-specific Ig isotypes were assessed. Data from five out of sixteen HP-Met rats were omitted due to very high levels of food refusals from parturition onwards. Relative to feeding HP foods, feeding LP, HP-Met and HP-Leu foods reduced dam weight gain and, to a lesser extent, litter weight gain, and increased the number of worm eggs in the colon, indicative of a reduction in resistance to parasites. However, only feeding LP and HP-Leu foods resulted in increased worm numbers, while none of the feeding treatments affected systemic Ig, mast and goblet cells, and eosinophil numbers. The present results support the view that resistance to parasites during lactation may be sensitive to specific essential AA scarcity.

Interactive effects of protein nutrition, genetic growth potential and Heligmosomoides bakeri infection pressure on resilience and resistance in mice

The ability of animals to cope with an increasing parasite load, in terms of resilience and resistance, may be affected by both nutrient supply and demand. Here, we hypothesized that host nutrition and growth potential interact and influence the ability of mice to cope with different parasite doses. Mice selected for high (ROH) or low (ROL) body weight were fed a low (40 g/kg; LP) or high (230 g/kg; HP) protein diet and infected with 0, 50, 100, 150, 200 or 250 L3 infective Heligmosomoides bakeri larvae. ROH-LP mice grew less at doses of 150 L3 and above, whilst growth of ROH-HP and of ROL mice was not affected by infection pressure. Total worm burdens reached a plateau at doses of 150L3, whilst ROH mice excreted fewer worm eggs than ROL mice. Serum antibodies increased with infection dose and ROH mice were found to have higher parasite-specific IgG1 titres than ROL mice. In contrast, ROL had higher total IgE titres than ROH mice, only on HP diets. The interaction between host nutrition and growth potential appears to differentially affect resilience and resistance in mice. However, the results support the view that parasitism penalises performance in animals selected for higher growth.

Genome-wide transcriptomic analysis of intestinal tissue to assess the impact of nutrition and a secondary nematode challenge in lactating rats

Background

Gastrointestinal nematode infection is a major challenge to the health and welfare of mammals. Although mammals eventually acquire immunity to nematodes, this breaks down around parturition, which renders periparturient mammals susceptible to re-infection and an infection source for their offspring. Nutrient supplementation reduces the extent of periparturient parasitism, but the underlying mechanisms remain unclear. Here, we use a genome wide approach to assess the effects of protein supplementation on gene expression in the small intestine of periparturient rats following nematode re-infection.

Methodology/Principal Findings

The use of a rat whole genome expression microarray (Affymetrix Gene 1.0ST) showed significant differential regulation of 91 genes in the small intestine of lactating rats, re-infected with Nippostrongylus brasiliensis compared to controls; affected functions included immune cell trafficking, cell-mediated responses and antigen presentation. Genes with a previously described role in immune response to nematodes, such as mast cell proteases, and intelectin, and others newly associated with nematode expulsion, such as anterior gradient homolog 2 were identified. Protein supplementation resulted in significant differential regulation of 64 genes; affected functions included protein synthesis, cellular function and maintenance. It increased cell metabolism, evident from the high number of non-coding RNA and the increased synthesis of ribosomal proteins. It regulated immune responses, through T-cell activation and proliferation. The up-regulation of transcription factor forkhead box P1 in unsupplemented, parasitised hosts may be indicative of a delayed immune response in these animals.

Conclusions/Significance

This study provides the first evidence for nutritional regulation of genes related to immunity to nematodes at the site of parasitism, during expulsion. Additionally it reveals genes induced following secondary parasite challenge in lactating mammals, not previously associated with parasite expulsion. This work is a first step towards defining disease predisposition, identifying markers for nutritional imbalance and developing sustainable measures for parasite control in domestic mammals.

Dietary protein and energy supplies differentially affect resistance to parasites in lactating mammals

Periparturient relaxation of immunity (PPRI) to parasites in mammals results in higher worm burden and worm egg excretion and may have a nutritional basis. Nippostrongylus brasiliensis re-infected lactating rats fed low-crude protein (CP) diets show an augmented degree of PPRI compared with their high CP-fed counterparts. However, such effects of CP scarcity have been confounded by metabolisable energy (ME) scarcity due to increased intake of the high-CP foods. Here, we independently assessed the effects of dietary CP and ME scarcity on the degree of PPRI. Second, parity rats were infected with N. brasiliensis larvae before mating. Upon parturition, dams were allocated to one of six feeding treatments (1-6), consisting of two levels of dietary ME supply, each with three levels of CP supply. On day 2 of lactation, dams were either re-infected with 1600 N. brasiliensis larvae or sham-infected with PBS, while litter size was standardised at ten pups. Dams and litters were weighed daily until either day 8 or 11 of lactation, when worm burdens were assessed as a proxy for PPRI. Increased CP and ME supply independently improved lactational performance. While ME supply did not affect parasitism, increasing CP supply reduced worm burden and the percentage of female worms in the small intestine; the latter was especially pronounced at the lower level of ME supply. The present results support the view that PPRI to parasites may be sensitive to CP scarcity, but not to moderate ME scarcity.

Dissecting the impact of protein versus energy host nutrition on the expression of immunity to gastrointestinal parasites during lactation

Many mammals exhibit a periparturient relaxation of previously established immune responses (PPRI) to gastrointestinal nematodes culminating in increased worm burdens. It has been suggested that the extent of PPRI may have a nutritional basis as it is considerably augmented when protein supply is scarce. Subsequent studies have shown that increased dietary protein intake can ameliorate this phenomenon. However, this effect is often confounded with increased food intake and thus increased energy levels. Herein, we aimed to dissect the effects of protein and energy nutrition on the immune status and resistance to re-infection with gastrointestinal nematodes in the periparturient host. The lactating, Nippostrongylus brasiliensis re-infected rat was utilised as an established model for mammalian PPRI. Experimental animals were assigned to restricted feeding regimens designed to achieve four pre-determined levels of crude protein (CP) at one of two levels of metabolisable energy (ME) and parasitological and immunological measurements taken at either day 6 or day 9 post re-infection. We clearly show that increased supply of dietary CP, but not increased dietary ME, significantly reduced worm burdens. The increased magnitude of worm expulsion with increased dietary CP supply strongly correlated with mucosal mast cell accumulation in the small intestine. In addition, increased CP and not ME supply increased mucosal eosinophil numbers. Furthermore, increased CP led to higher levels of total IgG at high ME only and there were interactive effects of CP and ME on serum levels of IgG1 and IgG2a. Perhaps surprisingly, CP nutrition did not affect expression of either Th1 (IFN-γ) or Th2 (IL-4, IL-13) cytokines in the mesenteric lymph nodes. These data emphasise the role of immunonutrition, and particularly dietary protein, in combating infectious disease such as gastrointestinal parasitism.

Immunomodulatory effects of dietary protein during Nippostrongylus brasiliensis re-infection in lactating rats

Periparturient relaxation of immunity (PPRI) to secondary infection with nematodes is believed to have a nutritional basis due to differential partitioning of scarce nutrient resources, particularly protein, to reproductive rather than immune functions. At times of protein scarcity, an increase in protein supply has been reported to assuage this phenomenon. The Nippostrongylus brasiliensis reinfected lactating rat model is now being utilized to investigate the immune reactions underlying the modifying role of dietary protein on PPRI. Herein, we demonstrate that lactating rats reinfected with N. brasiliensis under high protein (HP) dietary conditions exhibit decreased worm burdens and reduced colon egg counts compared to their low protein (LP) counterparts. These reductions correlated with increased mastocytosis and greater goblet cell hyperplasia. Additionally, the local antibody profile revealed that HP reinfected lactating rats developed a stronger antigen specific IgG2b response earlier in infection in comparison with their LP counterparts. Our study provides evidence that increased dietary protein content reduces the PPRI to N. brasiliensis re-infection in the lactating rat through improved mucosal immune responses.

Consequences of protein supplementation for anorexia, expression of immunity and plasma leptin concentrations in parasitized ewes of two breeds

The periparturient relaxation of immunity (PPRI) against parasites in ewes has a nutritional basis. We investigated whether ewes experience a reduction in food intake (anorexia) during PPRI and if the magnitude of anorexia is affected by host production potential and dietary protein supplementation. We also investigated whether nematode infection is linked to plasma leptin concentrations in periparturient ewes. The experiment was a 2 × 2 × 2 factorial design. Two breeds of twin-bearing/lactating ewes (Greyface cross, G (n 32) and Scottish Blackface, B (n 32)) were used. Half of the ewes were trickle infected with 30 000 larvae of the abomasal parasite Teladorsagia circumcincta per week and the other half were not. During the experiment, all ewes had ad libitum access to a low-protein diet that provided less protein than the recommended allowance. In addition, half of the ewes received a protein supplement that resulted in protein intakes that exceeded recommendations. Nematode infection resulted in a breakdown of immunity to parasites and a reduction in food intake in both breeds. The breeds differed in the extent of PPRI (G ewes having higher faecal egg counts than B ewes), but not in the magnitude of anorexia. Protein supplementation resulted in a reduction in faecal egg counts, but had no effect on the magnitude of anorexia. Plasma leptin concentrations changed significantly over time, but were not affected by protein supplementation or infection. It is concluded that infection with T. circumcincta in periparturient ewes results in anorexia that is not alleviated by protein supplementation and seems unrelated to plasma leptin concentrations.

Influence of periparturient nutritional demand on resistance to parasites in livestock

Periparturient relaxation of immunity (PPRI) to gastrointestinal nematode parasites may have a nutritional basis, as the increasing nutrient demand at times of nutrient scarcity during late pregnancy and subsequent lactation may result in a penalty on expression of acquired immunity to parasites. This nutritional basis implies that lowering nutritional demand, at constant nutrient supply, should reduce the degree of PPRI. Evidence to support the latter is reviewed through exploration of the effects of reproductive effort on periparturient resistance to parasites. A large body of evidence shows that a lower nutritional demand arising from rearing single rather than multiple lambs and/or kids consistently reduces the degree of PPRI in small ruminants, as manifested by reduced worm burdens and/or nematode egg excretions. Such variation in reproductive effort may also account, at least to some extent, for the often observed between-breed differences in PPRI, which may arise from differences in production potential, and thus nutritional demand. A reduction in nutrient demand can reduce the degree of PPRI in a matter of days. However, host immune responses associated with this nutritional sensitivity of host resistance remain to be elucidated, which may be achieved through a recently established periparturient rodent model.

The effects of changes in nutritional demand on gastrointestinal parasitism in lactating rats

Lactating rats experience a breakdown of immunity to parasites, i.e. they carry larger worm burdens after re-infection compared to their non-lactating counterparts. Feeding high-protein foods to lactating rats results in reduced worm burdens. This could be attributed to changes in gastrointestinal environment or to overcoming effects of nutrient scarcity on host immunity. The latter hypothesis was addressed through a manipulation of nutrient demand by manipulating litter size. Twenty-three rats were immunized prior to mating and re-infected on day 2 of lactation with 1600 infective Nippostrongylus brasiliensis larvae. From parturition onwards, rats received ad libitum low-protein food (100 g crude protein/kg). Litter size were standardised to nine (LS9), six (LS6) or three (LS3) pups, by day 2 of lactation. After a further 10 d, LS9 and LS6 rats carried more worms than LS3 rats. However, feeding treatment did not affect concentrations of mucosal inflammatory cells. Achieved feed intake did not differ consistently between the treatment groups. However, LS9 and LS6 rats lost weight, whilst LS3 rats gained weight during lactation. The results support the view that resistance to N. brasiliensis is sensitive to changes in nutrient demand, and the improved resistance to N. brasiliensis is likely due to effects of overcoming nutrient scarcity on host immunity.