Intake and selection for white clover by grazing lambs in response to gastrointestinal parasitism

Fungal infection alters collective nutritional intake of ant colonies

In animals, parasitic infections impose significant fitness costs.1,2,3,4,5,6 Infected animals can alter their feeding behavior to resist infection,7,8,9,10,11,12 but parasites can manipulate animal foraging behavior to their own benefits.13,14,15,16 How nutrition influences host-parasite interactions is not well understood, as studies have mainly focused on the host and less on the parasite.9,12,17,18,19,20,21,22,23 We used the nutritional geometry framework24 to investigate the role of amino acids (AA) and carbohydrates (C) in a host-parasite system: the Argentine ant, Linepithema humile, and the entomopathogenic fungus, Metarhizium brunneum. First, using 18 diets varying in AA:C composition, we established that the fungus performed best on the high-amino-acid diet 1:4. Second, we found that the fungus reached this optimal diet when given various diet pairings, revealing its ability to cope with nutritional challenges. Third, we showed that the optimal fungal diet reduced the lifespan of healthy ants when compared with a high-carbohydrate diet but had no effect on infected ants. Fourth, we revealed that infected ant colonies, given a choice between the optimal fungal diet and a high-carbohydrate diet, chose the optimal fungal diet, whereas healthy colonies avoided it. Lastly, by disentangling fungal infection from host immune response, we demonstrated that infected ants foraged on the optimal fungal diet in response to immune activation and not as a result of parasite manipulation. Therefore, we revealed that infected ant colonies chose a diet that is costly for survival in the long term but beneficial in the short term-a form of collective self-medication.

Macronutrients and infection in fruit flies

Nutrition and infection are closely linked. While it is now well established that hosts can modulate their nutrition after being infected, the extent to which this change in foraging provides the host with a greater fitness remains to be fully understood. Our study explored the relationships between dietary choice, macronutrients intake [i.e., protein (P) and carbohydrate (C)], infection, survival rate and growth of pathogenic bacterial population in the true fruit fly Bactrocera tryoni. Results showed that flies injected with the bacterium Serratia marcescens decreased their macronutrient intake and shifted their diet choice to carbohydrate-biased diet compared to naïve individuals. Interestingly, flies injected with either PBS (i.e., sham-infected) or heat-killed bacteria also reduced food intake and modulated diet choice but only for a day after injection. When infected flies were restricted to the diet they selected (i.e., PC 1:8), they survived better the infection than those restricted to a protein-biased diet (i.e., PC 1:5). In addition, we did not observe any growth of pathogen load in infected flies fed PC 1:8 for the first 3 days post-infection. Finally, a decrease in lipid body reserves was found in flies injected with live bacteria and, interestingly, this loss of body lipid also occurred in flies injected with heat-killed bacteria, but in a diet-dependent manner. Our results indicated that B. tryoni flies modulated their macronutrient intake and decreased the negative effects of the infection on their survival ("nutritional self-medication") the first days following the infection.

Phytochemicals in animal health: diet selection and trade-offs between costs and benefits

Many plant tissues contain plant secondary compounds (PSC), which have long been recognised as defensive chemicals that deter herbivory via their toxic effects. However, herbivores may also benefit from including PSC into their diets. Plant-derived phenolics, terpenes and alkaloids have antiparasitic properties and sesquiterpene lactones have antibacterial, antifungal and antiparasitic properties. These actions are in part a consequence of the negative actions that PSC exert across several trophic levels, including the bacteria, parasites and fungi that inhabit herbivores' bodies. Given the dual action, toxin and medicine, it is possible to hypothesise that self-selection of PSC by herbivores should occur when the benefits outweigh the costs of PSC ingestion. Recent research suggests that sheep and goats self-medicate against parasitic infections. They increase preference for condensed tannin-containing foods when experiencing a parasitic burden. This behaviour improves health; it is triggered by parasitism and weakens when parasitism subsides. However, the causes underlying these responses are not straightforward when viewed under a unidimensional cost-benefit analysis. This is because the intensity of antinutritional/toxic and medicinal effects of PSC is not static or just dependent upon the isolated post-ingestive effects of single PSC. Nutrient-PSC and PSC-PSC interactions, social models, as well as feeding patterns, all influence the perceived net benefit of incorporating medicines into a diet. A better understanding of the net benefit of self-medication in complex feeding environments will allow for the development of innovative managing strategies aimed at providing the food alternatives and conditions for improving the nutrition, health and welfare of grazing animals.

Maize supplementation of Pelibuey sheep in a silvopastoral system: fodder selection, nutrient intake and resilience against gastrointestinal nematodes

This trial evaluated the effect of maize supplementation on the ingestive behavior, nutrient intake and the resilience against gastrointestinal nematode (GIN) infection of hair sheep in a silvopastoral system containing tropical grasses and legume trees. In addition, it attempted to determine the metabolic cost of the natural GIN infection in supplemented and non-supplemented animals. Twenty-nine 3-month-old lambs (male and female), raised nematode free, were allocated to four groups: I-NS (infected, not supplemented, n = 8), I-S (infected, supplemented with maize at 1.5% live weight (LW), n = 7), T-NS (treated with moxidectin 0.2 mg/kg LW every 28 days, and not supplemented, n = 7) and T-S (treated with moxidectin and supplemented with maize at 1.5% LW, n = 7). During the 70-day trial, fodder intake, fodder selection, LW change (LWC), red blood cell counts (RBC), hemoglobin (Hb), hematocrit (Ht) and eggs per gram of feces (EPG) were measured every 14 days. Supplement consumption was recorded daily. Metabolizable energy (ME) and protein (MP) consumption from the feeds were estimated. Maize supplementation helped to improve the resilience of hair sheep lambs against GIN infections. The I-S and T-NS groups showed similar LWC, RBC, Hb and Ht (P > 0.05) and both were higher than those in the I-NS group (P < 0.05). No difference was found in EPG between the I-NS and the I-S groups (P > 0.05). No effect of sex was observed in the different variables. Although all groups showed low dry matter intake (DMI) (< 2% LW), supplemented groups (T-S and I-S) showed higher total DMI (fodder + maize; P < 0.05), hence higher ME and MP intakes than the non-supplemented groups (T-NS and I-NS). All groups showed similar fodder selection patterns. The estimated metabolic cost of parasitism was ME = 0.70 MJ/day and MP = 9.2 g/day in the I-S animals. Meanwhile, the cost in the I-NS animals was ME = 1.46 MJ/day and MP = 12.71 g/day. Maize supplementation was an economically viable strategy to control GIN compared with no intervention.

Nutritional PharmEcology: Doses, nutrients, toxins, and medicines

The synthesis of pharmacological techniques and concepts into ecology holds considerable promise for gaining new insights into old questions, uncovering new priorities for research and, ultimately, for consolidating a new sub-discipline within the ecological sciences-PharmEcology. We argue that this potential will best be realized if the boundaries of PharmEcology are drawn broadly to encompass not only toxins and medicines, but also nutrients. The hub of our argument is that PharmEcology shares with the established discipline of nutritional ecology an organismal focus, at the core of which is the notion of evolutionary function. From this functional viewpoint the dividing lines between chemicals traditionally considered as "toxins," "medicines," and "nutrients" are often thin, vague, heavily contingent and non-stationary, and thus provide a poor footing for an emerging sub-discipline. We build our argument around three points: nutrients and toxins are not so different, medicines and nutrients are not so different, and even in cases in which nutrients, medicines and toxins can be categorically distinguished, the biological actions of these compounds are heavily interdependent.

The effect of mixed or sequential grazing of cattle and sheep on the faecal egg counts and growth rates of weaned lambs when treated with anthelmintics

An experiment was conducted over two successive years (2002 and 2003) to investigate the effects of grazing improved permanent pasture (mainly perennial ryegrass/white clover) by cattle or sheep, either sequentially or mixed, on the faecal egg counts and growth rates of weaned lambs when treated with anthelmintics. The grazing season was divided into two parts, May-July then July-October, relating to the pre- and post-weaning of the lambs. Four grazing regimes, replicated three times, were compared: (1) sheep only from May to October (SS); (2) cattle May to July followed by lambs until October (C/S); (3) cattle and sheep May to July followed by lambs until October (C+S/S); and (4) cattle and sheep May to July followed by cattle and lambs until October (C+S/C+S). Sward height was maintained at 6 cm using a "put and take" stocking system. At weaning, lambs were weighed and treated with an anthelmintic (0.08% ivermectin drench, Oramec) before being allocated to plots. They were then weighed and drenched every 28 days until the end of the experiment (Day 84). Faecal egg counts (FEC) were measured in all lambs immediately prior to each anthelmintic treatment. In 2002 and 2003, there were differences between the groups in FEC, with the SS lambs having the highest values and C/S lambs the lowest (P<0.01). There were also differences in the rate of liveweight gain of the lambs in each of the study years: for this parameter SS lambs had the lowest growth rate but the fastest growth was in C+S/C+S lambs not C/S lambs (P<0.01), indicating that these differences were due to factors other than parasite infection. Overall, sequential grazing of pastures with cattle then sheep reduced the faecal egg counts in lambs regularly treated with anthelmintics when compared with lambs grazing in mixed systems with cattle and sheep or with sheep only systems, however, the highest growth rates were observed in lambs in the mixed cattle/sheep grazing system throughout.

Parasite-induced anorexia in a free-ranging mammalian herbivore: an experimental test using Soay sheep

Theory suggests that gastrointestinal parasites can influence herbivore population dynamics by increasing host mortality rates. In addition, parasites may have a non-lethal range of both physiological and behavioural effects on their hosts. Two potential behavioural effects are parasite-induced anorexia and the alteration of diet selection patterns — both of which may influence plant communities without necessarily causing herbivore mortality. We report here the results of an experiment carried out in August–September 2001 to examine herbivore response to parasitism using feral Soay sheep ( Ovis aries L., 1758) living on Hirta, St. Kilda (Scotland, UK), as a model system. The aim of the experiment was to determine whether the sheep exhibit significant parasite-induced anorexia and whether diet selectivity was also altered in response to parasitism. The n-alkane technique was used to estimate forage intake rates and diet composition. Overall, intake rate increased with body mass and the mean (±SE) over both sexes was 737 ± 40 g DM / day (range 458–1241 g DM / day). However, we found no effect of parasite burden upon intake rate or selection patterns. The results of a power analysis imply that any reduction in intake rate was less than 30%, which is less than the 30%–60% reduction in intake rate recorded for domestic sheep under moderate parasite burdens. This suggests that Soay sheep are more resilient to parasitism than domestic breeds. Despite evidence to the contrary from simple farm-based studies, the lack of any effect on selection patterns in this case suggests that parasites do not cause significant alterations to the selection patterns of herbivores in complex non-agricultural environments.

Can animals use foraging behaviour to combat parasites?

Host-parasite interactions are often seen as an arms race, with parasites attempting to overcome host resistance to infection. Herbivory is a common route of transmission of parasites that represents the most pervasive challenge to mammalian growth and reproduction. The present paper reviews the foraging skills of mammalian herbivores in relation to their ability to exploit plant properties to combat parasites. The starting point is that foraging behaviour may ameliorate the impact of parasitism in three ways; hosts could: (1) avoid foraging in areas contaminated with parasites; (2) select diets which increase their resistance to parasites; (3) select for foods containing anti-parasitic properties (self-medication). Details are given of the pre-requisite skills needed by herbivores if they are to combat parasitism via behaviour, i.e. herbivores are able to: (a) determine their parasitic state and alter their behaviour in relation to that state (behaviours 1, 2 and 3); (b) determine the environmental distribution of parasites (behaviour 1); (c) distinguish plant species or plant parts that increase their resistance to parasites (behaviour 2) or have anti-parasitic properties (behaviour 3). Mammalian herbivores cannot detect the presence of the parasites themselves and must rely on cues such as faeces. Despite the use of these cues contacting parasites may be inevitable and so mechanisms to combat parasitism are necessary. Mammalian herbivores have the foraging skills needed to exploit the heterogeneous distributions of nutrients and parasites in complex foraging environments in order to avoid, and increase their resistance to, parasites. Current evidence for the use of plant secondary metabolites (PSM) by herbivores for self-medication purposes remains equivocal. PSM have both positive (anti-parasitic) and negative (toxic) effects on herbivores. Here details are given of an experimental approach using tri-trophic (plant-herbivore-parasite) interactions that could be used to demonstrate self-medication in animals. There is strong evidence suggesting that herbivore hosts have developed the foraging skills needed to take advantage of plant properties to combat parasites and thus use behaviour as a weapon in the host-parasite arms race.

The effect of pasture species on parasitism and performance of lambs grazing one of three grass-white clover pasture swards

Lamb parasitism and performance were compared on three pasture treatments: browntop/white clover, Yorkshire fog/white clover, and ryegrass/white clover swards maintained at a sward height of 5cm in each of two years by using continuous variable stocking management. In year 1, lamb performance (both suppressively drenched (SD) and trigger drenched (TD)) was similar on ryegrass and browntop swards but was poorer (P<0.05) on Yorkshire fog swards. In year 2, lamb performance (both SD and TD) was markedly better on ryegrass than on Yorkshire fog or browntop swards. This coincided with an increase in white clover content in the ryegrass/white clover swards and a decrease in white clover content in the browntop/white clover swards. Lamb performance was related to the white clover content of the swards (R2=85.4, P<0.001 and R2=77.5, P<0.001 for SD lambs and TD lambs, respectively). Differences among pasture treatments in faecal egg count (FEC) of TD lambs were greater in year 2 than in year 1 when no significant pasture effects were observed. In year 2, FEC was lowest in lambs which grazed ryegrass/white clover, intermediate in those which grazed Yorkshire fog/white clover and highest in lambs which grazed browntop/white clover, despite herbage allowance (kg herbage mass/kg lamb mass) being greater on the browntop/white clover and Yorkshire fog/white clover swards than the ryegrass/white clover swards. Circulating antibodies to gastrointestinal parasites were measured at the end of year 2. Antibody titres were highest (P<0.05) in the plasma of lambs which grazed ryegrass/white clover swards, and lowest in lambs which grazed browntop/white clover swards due to differences in levels of antibody to (P<0.05) Haemonchus contortus, Ostertagia circumcincta larvae and adults, but not Trichostrongylus colubriformis larvae or adults. These findings indicate that pasture composition, especially, the proportion of white clover, can help alleviate the production losses due to gastrointestinal parasitism. This information can provide a quick and readily adoptable practice for farmers who aim to reduce drench reliance and could become a key component of any integrated parasite control programme.

The effect of feeding sulla (Hedysarum coronarium) or lucerne (Medicago sativa) on lamb parasite burdens and development of immunity to gastrointestinal nematodes

Two experiments investigated the efficacy of the legume Hedysarum coronarium (sulla), which contains condensed tannins (CTs), for reducing gastrointestinal nematode infections relative to lucerne. Experiment 1 was aimed to show whether the lower faecal egg count (FEC) and larval establishment previously reported in lambs grazing sulla were due to direct effects of the forage on Ostertagia circumcincta and Trichostrongylus colubriformis or were mediated through an enhanced immune response. Experiment 2 evaluated the impact of feeding sulla relative to feeding lucerne (Medicago sativa), before, at, or after larval challenge on subsequent FECs and nematode burdens. In experiment 1, 64 Romney lambs were fed either freshly cut lucerne or sulla (32 lambs per herbage) for the duration of the trial. Within each herbage there were four treatment groups (n=8 per group). Initial levels of immunity were assessed in uninfected (UN) lambs which were maintained parasite-free until challenged with 15,000 O. circumcincta and 15,000 T. colubriformis larvae on day 63, and slaughtered on day 81. The other three treatment groups were trickle-infected with each of 5000 O. circumcincta and 5000 T. colubriformis larvae three times per week from day 1 to 35. Non-steroid infected (CONTROL) and steroid-treated (STER) groups were treated with anthelmintic on day 49 and challenged with 15,000 O. circumcincta and 15,000 T. colubriformis on day 63 and slaughtered on day 81. The STER lambs were given dexamethasone trimethylacetate from day 49 to 81 to determine effects of immunity on parasite infection. From day 35 an establishment group (EST) on each herbage was fed a common pelleted lucerne diet and slaughtered on day 56 to determine nematode establishment during trickle-infection. Diet did not affect FECs but feeding lucerne increased (P<0.05) numbers of T. colubriformis in CONTROL lambs compared to those fed sulla. O. circumcincta numbers were lower (P<0.05) in UN lambs fed sulla than lucerne. The sulla diet was associated with higher (P<0.05) antibody titres against secretory-excretory antigens to adult O. circumcincta and to adult and larval T. colubriformis, so there appeared to be some immunogenic response to the sulla diet but effects upon T. colubriformis numbers were not significant. The second experiment involved 48 Romney lambs grazing conventional pasture which were infected with 10,000 each of O. circumcincta and T. colubriformis larvae either 7 days before, 7 days after, or at the time they commenced grazing either sulla or lucerne. Lambs which grazed sulla had lower (P<0.05) FEC and lower (P<0.05) O. circumcincta burdens than lambs which grazed lucerne but timing of infection had no effect on FEC or worm burdens. T. colubriformis numbers were not affected by treatment or herbage. In conclusion, the sulla diet resulted in lower O. circumcincta numbers compared to lucerne outdoors and some evidence of an immunogenic response was obtained indoors. However, neither the herbage nor the immunogenic response reduced T. colubriformis numbers in either experiment.