Functional Ecology 2006

20, 637–646

637

© 2006 The Authors.Journal compilation© 2006 British Ecological Society

Blackwell Publishing LtdImmunocompetence and flea parasitism of a desert rodent

J. GOÜY DE BELLOCQ,*†‡ B. R. KRASNOV,* I. S. KHOKHLOVA,§ L. GHAZARYAN*§ and B. PINSHOW¶*Ramon Science Center and Mitrani Department of Desert Ecology, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, 84990 Midreshet Ben-Gurion, Israel, ‡Centre de Biologie et de Gestion des Populations (CBGP), Campus International de Baillarguet, CS 30016,34988 Montferrier-sur-Lez, Cedex, France, §Wyler Department of Dryland Agriculture, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, 84990 Midreshet Ben-Gurion, Israel, and ¶Mitrani Department of Desert Ecology, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, 84990 Midreshet Ben-Gurion, Israel

Summary

1. Immunocompetence is the general capacity of an organism to mount an immuneresponse against pathogens and parasites. We studied the relationship between theimmunocompetence of a rodent host, Meriones crassus (Sundevall’s Jird) and para-sitism by the flea Xenopsylla ramesis. We hypothesized that flea parasitism affectsphysiological and immunological variables of the host, and that the host’s level ofimmunocompetence affects fitness components in the flea parasite. We wanted to findout (a) the effect of flea parasitism on the haematocrit (Hct), leucocyte concentration(LC) and response to phytohaemagglutinin (PHA) of the host; (b) if and how the levelof immunocompetence of the host is related to the fitness of fleas exploiting it; and (c)the relationship between the immunological and behavioural defences of the host.2. We measured Hct and LC and response to PHA before and after 16 days of fleainfestation in Sundevall’s Jird and compared these variables between jirds subjected toflea parasitism and non-parasitized (control) animals. We evaluated egg production andhatching success in fleas exploiting hosts with different levels of immunocompetence.3. The effect of flea parasitism on Hct and LC of rodents was manifested by anincrease in among-host variation in the temporal changes of these variables. Responseto PHA injection was significantly lower in parasitized than in control animals.4. Flea egg production and hatching success were not related to either LC, prior toexperimental treatments, or to the PHA response of rodents. However, both of theseflea fitness traits were negatively correlated with changes in LC between the 1st and the16th days of infestation.5. At the end of the experiment, blood consumption of fleas was measured in jirds ofboth treatment groups. Fleas consumed significantly more blood when they fed onpreviously parasitized than on non-parasitized animals. Among fleas that fed onpreviously parasitized animals, blood consumption was positively correlated with theinitial LC of the hosts, and negatively correlated with differences in LC between the1st and the 16th days of flea infestation. We found no correlation between bloodconsumption in fleas that fed on control animals and either immunological variable ofthe hosts.6. No trade-off was found between behavioural and immune defences of the rodenthosts.

Key-words: Behavioural defence, haematocrit, immune response, leucocyte concentration, PHA test

Functional Ecology (2006) 20, 637–646 doi: 10.1111/j.1365-2435.2006.01140.x

Introduction

One of the most important direct detrimental effects ofparasites on their hosts is depletion of the host’s nutrientand energy reserves. This probably affects the host’svigour via changes in blood composition. For example,

†Author to whom correspondence should be addressed.E-mail: bellocq@ensam.inra.fr, joelle@bgu.ac.il

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haematocrit (Hct) in host animals decreases withincreasing ectoparasite load (Hurtrez-Boussès et al.1997; Potti et al. 1999; Szabó et al. 2002). However,hosts are not necessarily passive victims of theirparasites, but defend themselves actively using specificbehavioural, physiological and/or immunologicalmechanisms. Therefore, parasites also exert indirectinjurious effects on their hosts by triggering energy-consuming defence mechanisms. Of these, the immunesystem is the crucial means by which the host canprotect itself from parasites (Wakelin & Apanius 1997).Thus, a host’s immunocompetence, i.e. its generalcapacity to mount an immune response against path-ogens and parasites (Schmid-Hempel 2003), is likelyto be positively correlated with its fitness. However,immunological defence is costly both in terms ofactivation and maintenance and is therefore party totrade-offs among the organism’s competing energyrequirements (Sheldon & Verhulst 1996; Owens &Wilson 1999).

Within a host population, immunocompetencevaries among individuals depending on age, sex andnutritional and/or reproductive status (Lochmiller,Vestey & Boren 1993; Nelson & Demas 1996; Hum-phreys & Grencis 2002). As a result, parasite loadshould vary among hosts characterized by differentlevels of immunocompetence. For example, the numberof mites, Spinturnix myoti, was negatively correlatedwith immunocompetence in lactating female greaterMouse-Eared Bats, Myotis myotis (Christe, Arlettaz &Vogel 2000). While feeding on blood, an ectoparasite’ssaliva-transmitted proteins elicit an immune responseby the host (Jones 1996). This immune response mayaffect reproduction and survival of the ectoparasite.For example, Hen Fleas, Ceratophyllus gallinae, feedingon previously flea-exposed Great Tits, Parus major,had lower hatching success and fewer larvae than henfleas fed on immune-naive hosts (Walker et al. 2003).Thus, fitness components of a parasite should also berelated to the level of immunocompetence in its host.

Although immune responses are the main mech-anisms that cope with parasites and pathogens, thefirst line of defence against many parasites, especiallyectoparasites, remains behavioural. Anti-parasiticbehaviours include changing den, nest sanitationand self-grooming (Hart 1990, 1994, 1997). The lastis effective for reducing ectoparasites in birds andmammals but entails energetic costs (Hart 1994; Giorgiet al. 2001). For example, grooming causes significantwater loss in rats (see Hart 1994 for review) and affectstime and energy budgets in bats (Giorgi et al. 2001).Given that both behavioural and immunologicaldefences are energetically costly, one may expectoptimization of energy allocation for general resistanceagainst parasites. In other words, one may expect atrade-off between behavioural and immunologicaldefences.

Techniques for the assessment of immunocompetencecan be divided into non-functional and functional.

Non-functional techniques consist of measurementsof immuno-related structures including leucocyteconcentration (LC) and mass of lymphoid organs suchas the spleen. An individual host with a relatively highLC or a large spleen presumably possesses betterimmune defences than an individual with a low LCor with a smaller spleen. A functional way to assessimmunocompetence is to challenge individual hostswith antigens that trigger cellular or humoral immuneresponses. The phytohaemagglutinin injection assay(PHA test) is commonly used to measure immuno-competence in birds and mammals (Smits, Bortolotti& Tella 1999). The test is by subcutaneous injection ofvegetal lectin, a phytohaemagglutinin that induceslocal T-cell stimulation and proliferation that causesswelling. The degree of swelling gives a measure of thepotential proliferative response of circulating T cells. ThePHA test has become popular in immunoecologicalfield studies because it does not need sophisticatedequipment and the response is seen soon after PHAinjection (Smits et al. 1999). In addition, the PHAresponse is apparently independent of experimentallycreated levels of parasitism, suggesting that it is aninherent and objective measure of immunocompetence(Saino, Calza & Møller 1998; Brinkhof et al. 1999).However, in concomitant infections, infection byone parasite influences the outcome of an infection byanother through depression or stimulation of theimmune response (Cox 2001). As the host mounts animmune reaction against phytohaemagglutinin,one might expect that individuals suffering parasiticinfection will mount a different response against thephytohaemagglutinin than parasite-free individuals.

In this study, we investigated the relationship betweenimmunocompetence of a rodent, Meriones crassus(Sundevall’s Jird), that is host to the flea Xenopsyllaramesis. Fleas (Siphonaptera) are obligate haemat-ophagous ectoparasites of higher vertebrates, beingmost abundant and diverse on small to medium-sizedmammal species. We hypothesized that flea parasitismaffects physiological and immunological variables inM. crassus and that the level of immunocompetence ofthe host affects the fleas’ fitness. Specifically, we askedthe following questions:

1. What is the effect of flea parasitism on specificblood (Hct) and immune (LC and response to PHA)variables of a host?2. If and how the level of immunocompetence of ahost is related to the fitness of fleas exploiting it?3. What is the relationship between the immunologicaland behavioural defences of the host against fleas?

We predicted that the Hct of host individuals parasi-tized by fleas will be lower than that of non-parasitizedindividuals. Given that infestation by one parasitespecies affects the outcome of a subsequent infestationby another parasite species (see above), we expected animmunomodulatory effect of fleas on preparasitized

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individuals. Also, given that immunocompetencereflects a capacity to resist parasites, we further predictedthat fleas exploiting host individuals with higherimmunocompetence will produce fewer eggs than fleasthat fed on less immunocompetent hosts. Finally, wepredicted a negative relationship between behaviouraland immune defences, as these two ways to cope withthe parasitism should be optimized.

Materials and methods

Sundevall’s Jird is a rodent species common insouthern Israel. It occupies a variety of habitats and isparasitized by several flea species including Xenopsyllaramesis (Krasnov et al. 1996, 1997). We used animalsfrom our laboratory colony. Progenitors of the colonywere captured in the Ramon erosion cirque, NegevHighlands, Israel (30°35′ N, 34°45′ E). The study wasdone under permits from the Israel Nature andNational Parks Protection Authority and Ben-GurionUniversity Committee for the Ethical Care and Use ofAnimals in Experiments.

We used 19 adult jirds (9 females and 10 males), all5–7 months old, which had never been in contact withfleas. The jirds were individually housed in plasticcages (60 × 50 × 40 cm3) that contained 1 cm of sandcovered by a steel screen on the floor and a small plasticnestbox. Animals were maintained at 25 °C with a photo-period of 12:12 h (light : dark). They were fed milletseeds and alfalfa (Medicago sp.) leaves ad libitum.

Xenopsylla ramesis is a common ectoparasite of gerbilsand jirds throughout the Middle East and occursmainly in sandy-gravel and loess habitats (Krasnovet al. 1997). Fleas were obtained from our laboratorycolony that we started in 1998 from field-collectedspecimens on M. crassus using rearing proceduresdescribed by Khokhlova et al. (2002) and Krasnovet al. (2004). Colonies of fleas were maintained at25 °C and 75% r.h. with a photoperiod of 12:12 h.

Jirds used in the experiments were nine pairs ofsiblings (five male and four female pairs), all raisedfrom different parent pairs, and one female that wasnot related to any of the other animals. Individuals fromeach sibling pair were randomly assigned to eitherthe experimental (later subjected to flea parasitism),or the control (non-parasitized) group. On the firstday of the experiment, we took an initial bloodsample (see below) to measure Hct and LC. Then, 150newly emerged male and female X. ramesis wereplaced in the cages of each rodent in the experimentalgroup.

Every four days (i.e. on the 5th, 9th, 13th and 17thdays of the experiment), we removed all the fleas fromone of the jirds, and from its cage. Fleas were removedby holding the rodent over a white plastic can andblowing over its fur until no more fleas were detected.The sand from a cage was thoroughly sieved andchecked for remaining fleas that were collected with anaspirator. After removal of the fleas from the rodent’sbody and its cage, 100 new newly emerged fleas wereadded directly to the rodent, except after removalon day 17, when no fleas were added. Preliminaryexperiments showed that this design ensured continuous4-day flea pressure on a rodent (J. Goüy de Bellocqet al., unpublished data). After 18 h (see below), thefleas recovered during the removal were placed on theiroriginal host, except on day 17. For each of the fourremovals, we calculated an index of grooming success(initial number of fleas minus number of recoveredfleas/initial number of fleas) that was assumed toreflect the efficiency of an individual’s behaviouraldefences against fleas. Thus, four values of the index ofgrooming success were obtained for each jird. Themeans of these four values were used in subsequentanalyses. The initial number of fleas was either 150 forthe first index value or, for the second, third and fourthvalues, the number of fleas recovered in the previousremoval, and replaced, plus 100.

We identified the sex of each flea removed from thejirds and their cages by examination it under a lightmicroscope. The fleas were then placed in Petri dishes,transferred into an incubator, maintained for 18 h atan air temperature of 25 °C and relative humidity of92–95%, and returned to their original host (see above;except for day 17). We counted the number of eggs laidby the fleas after 18 h and calculated the egg pro-duction per female. A thin layer of sand was added toeach Petri dish before returning them to the incubator.Petri dishes with eggs were checked daily until alllarvae hatched. An egg was considered dead if it didnot hatch in 30 days. Hatching success was calculatedas the proportion of hatched eggs. Four values of eggproduction and hatching success were thus obtainedfor each individual jird. The means of these four valueswere used in the subsequent analyses. Individuals fromthe control group were treated as the experimentaljirds; they were taken from their cages, held over aplastic can, and their fur was blown over for thesame amount of time as for parasitized individuals.Individuals were then returned to their cages.

We took a second blood sample from the jirds onday 16, and measured Hct and LC. We then injectedPHA in the hind left foot pad of each animal (seebelow). On day 23, we measured blood consumptionby fleas that fed on experimental and control rodents.The rodents were placed in wire mesh (5 × 5 mm2) tubes(15 cm long × 5 cm diameter) that limited movementand did not allow self-grooming. These tubes wereplaced in individual plastic pans, and 30 newly emerged,preweighed (to the nearest 0·01 mg; Precisa Balance,

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model 290 SCS, Precisa Instruments AG, Dietikon,Switzerland) female fleas were placed on each jird for2 h. Then, all fleas were removed and re-weighed.Mean flea blood consumption was calculated asdifference in flea body mass before and after the feedingbout, divided by the total number of fleas.

Blood samples were collected from the infraorbitalsinus using heparinized capillary tubes. The animalsrecovered fully within 2 min of blood sampling. Thecapillaries were centrifuged for 10 min in a standardHct centrifuge (ALC Haematocrit centrifuge 4203,Milano, Italy) and Hct was measured with digitalcallipers (Mitutoyo digimatic calliper, 500–12, Japan)to the nearest 0·01 mm. We calculated the differencein Hct between the first and the second blood sampling(further denoted as ∆Hct16−1).

We measured two components of immunity, namelynon-specific immunity by the LC in the blood, and T-cell-mediated immunity by the PHA test. To measureLC, we diluted 4 µl of the whole blood with Türksolution (1:25) to ensure erythrocyte lysis and countedleucocytes under a light microscope at 40× magnificationusing a Bürker-Türk hæmocytometer, and expressedconcentration as number of cells mm−3. In addition, wecalculated the difference in LC between day 1 and day16 (∆LC16−1).

To measure cell-mediated immunity, we used asimplified protocol of the PHA skin test (Smits et al.1999). We measured the left footpad thickness with ascrew micrometer (Mitutoyo digimatic thickness gauge,547–301) to the nearest 0·01 mm and immediatelyinjected 0·1 mg of PHA dissolved in 0·03 ml of salinesubcutaneously in the middle. We measured the footpadthickness again after 24 h (see Mendenhall et al. 1989).The micrometer exerts a constant pressure on thefootpad; to standardize the measurements we tookreadings 3 s after its application. We used the differencebetween post- and preinjection measures of footpadthickness as an indicator of PHA response. The PHAresponse provides a measure of the proliferativeresponse potential of circulating T cells to an injectedmitogen and it is a reliable measure of T-cell-dependentimmunocompetence in rodents (Mendenhall et al.1989).

Hct and LC measurements were made in duplicate,and PHA response measurements in triplicate to testrepeatability that was calculated as the intraclasscorrelation coefficient based on variance componentsderived from a one-way (Lessels & Boag 1987).Since repeatability was high (r = 0·85–0·95 for Hct;

r = 0·82–0·85 for LC; and r = 0·91–0·98 for PHAresponse; P < 0·0001 for all), mean values of measure-ments were used for analyses. All dependent variableswere normally distributed (Kolmogorov–Smirnovtests, P > 0·20 for all), therefore parametric statisticswere applied. Since some laboratory animals wereoffspring of the same mother, we corrected for within-litter non-independence by running an for eachdependent variable with a litter identification numberas an independent factor. No between-litter differenceswere found in any of the variables (F7,12 = 0·63–1·88,P > 0·16 for all). In other words, no statistical differencein maternal effect on any variable was found amongindividuals. No correlation was found between eitherPHA response and initial body mass of jirds (r = 0·24,P = 0·32), or PHA response and body mass differenceprior to and after the experiment (r = 0·13, P = 0·60).This suggests that the relationship between PHAresponse and body mass reported for birds (Alonzo-Alvarez & Tella 2001) may not be valid for rodents.Consequently, our analyses of the PHA response in thepresent study were done without correction for bodymass differences.

To test for differences in initial Hct and LC betweenanimals from the experimental and control groups andbetween males and females we used two-way ,with group and sex as independent variables. Differencesin Hct and LC (see above) were analysed with repeatedmeasures s. In these analyses, the repeated mea-sures were the Hct and LC of each individual; subjectswere the individual animals; and factors were groupand sex. The effects of flea parasitism and sex on thePHA response were analysed using two-way .Pearson’s correlation was used to test the relationshipsamong host immune variables (initial LC, ∆LC16–1 andPHA response). The relationships (a) between hostimmune variables and flea fitness traits, namely meanegg production and mean hatching success, or bloodconsumption, and (b) between host grooming successand host immune responses were tested by regressionanalyses. Data are presented as means ± SE. Analyseswere done using STATISTICATM 7·0. α = 0·05 waschosen as the lowest acceptable level of significance.

Results

No significant differences in the initial Hct and LCwere found between jirds from the experimental andcontrol groups (F1,15 = 0·03 and F1,15 = 0·03, respectively;P > 0·80 for both). Similarly, no sex difference in thesevariables was found (F1,15 = 2·44 and F1,15 = 2·33,respectively; P > 0·14 for both).

After 15 days of the experiment, no changes betweenthe experimental and control groups were found ineither Hct or LC, compared with their initial levels(Table 1). However, there were, albeit non-significant,trends towards a decrease in Hct and an increase in

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LC in the experimental vs the control groups (∆Hct =−0·95 ± 1·11% vs ∆Hct = 0·11 ± 0·35% and ∆LC =1573·37 ± 1294·08 cell mm−3 vs ∆LC = 504·63 ± 518·21cell mm−3, respectively). There was a slight, but signif-icant effect of sex on Hct (Table 1) on day 16 of theexperiment (52·10 ± 0·84% in males vs 49·58 ± 0·71%in females), although initial Hct did not differ betweenmales and females (51·99 ± 0·47% vs 50·74 ± 0·63%,respectively).

In 8 of 19 rodents Hct increased, while in the other11 it decreased. Similarly, in 7 of the 19 animals LCincreased, while it decreased in the remainder. Therewas no difference in the proportion of individuals inwhich Hct or LC increased or decreased betweenthe experimental and control groups (χ2 = 0·54 andχ2 = 0·02, respectively, P = 0·40 for both). However, thevariance in ∆Hct16−1 was significantly higher in theexperimental than in the control group (12·41 vs 1·31%,respectively, Levene’s test: F1,17 = 7·95, P = 0·01). Thevariance of ∆LC16–1 was also significantly higher inthe experimental than in the control group (16,746,325vs 2,416,860 cell mm−6; Levene’s test: F1,17 = 6·0,P = 0·03). PHA response was significantly lower in theexperimental than in the control group (0·55 ± 0·04 vs0·80 ± 0·07 mm, respectively; , F1,15 = 21·80, P =0·0003), being significantly higher in control femalesthan in control males and experimental males andfemales (, F1,15 = 11·80, P < 0·01) (Fig. 1).

No relationship was found between PHA response andinitial LC or ∆LC16−1 for either experimental (r = 0·22

and r = 0·30, respectively; P > 0·4 for both) or control(r = 0·24 and r = 0·13, respectively; P > 0·5 for both)groups. Initial LC was correlated with LC on day 16 ofthe experiment in control animals (r = 0·94, P = 0·001),but not in experimental ones (r = 0·51, P = 0·13).However, no correlation between ∆LC16−1 and initialLC was found in animals from either group (r = 0·50for the experimental and r = 0·38 for the control group;P > 0·1 for both).

Mean egg production and mean hatching success offleas were not related either to initial LC (r2 = 0·04 andr2 = 0·08, respectively; P > 0·6 for both) or to PHAresponse (r2 = 0·24 and r2 = 0·04, respectively; P > 0·1for both) of the rodent hosts. Mean flea egg productionwas significant and negatively correlated with theirhosts’ ∆LC16−1 (r

2 = 0·57, P = 0·01) (Fig. 2a). In contrast,no significant relationship was found between meanhatching success of fleas and the hosts’ ∆LC16−1

(r2 = 0·17, P = 0·24). However, for unknown reasons,fleas from one rodent had extremely low hatchingsuccess. When the data related to this outlier wereomitted from the analysis, the relationship betweenmean hatching success and the hosts’ ∆LC16−1 wassignificant and negative (r2 = 0·52, P = 0·03) (Fig. 2b).

Blood consumption of fleas differed depending on thegroup (F1,17 = 8·51, P = 0·01) being significantly higherin fleas that fed on experimental animals than infleas that fed on control animals (0·11 ± 0·006 mg vs0·08 ± 0·005 mg, respectively). Among fleas that fedon the experimental animals, blood consumption wassignificantly and positively correlated with the initial

Table 1. Summary of the repeated measures s ofhaematocrit (Hct) and leucocyte concentration (LC) of M.crassus in dependence of sex and parasitological experience

Effect df MS F P

HctTreatment 1 3·50 0·79 0·39Sex 1 32·83 7·38 0·016Treatment × Sex 1 0·76 0·17 0·69Error 15 4·45Hct 1 2·21 0·58 0·46Hct × Treatment 1 2·00 0·53 0·48Hct × Sex 1 3·38 0·89 0·36Hct × Treatment × Sex 1 0·64 0·17 0·69Error 15 3·79

LCTreatment 1 7 522 922 0·244 0·63Sex 1 38 384 755 1·24 0·28Treatment × Sex 1 13 938 707 0·45 0·51Error 15 30 883 924LC 1 10 084 553 2·23 0·16LC × Treatment 1 2 726 711 0·60 0·45LC × Sex 1 8 638 530 1·91 0·19LC × Treatment × Sex 1 7 425 770 1·64 0·22Error 15 4 530 686

Fig. 1. Swelling response (PHA response) in male and femaleM. crassus injected with a vegetal lectin (phytohaemagglutinin).Animals were either subjected to flea parasitism (experimental)or not parasitized (control). Values are means ± SE.

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LC of their hosts and significantly negatively correlatedwith the hosts’ ∆LC16–1 (r

2 = 0·45 and r2 = 0·41, respec-tively; P < 0·05 for both) (Fig. 3). Blood consumption offleas and PHA response of the experimental animalswere not correlated (r2 = 0·10, P = 0·37). Among fleasthat fed on control animals, blood consumption was notcorrelated with either measured host immunologicalvariable (r2 = 0·01 for initial LC, r2 = 0·02 for ∆LC16−1

and r2 = 0·34 for PHA response; P > 0·10 for all)(Fig. 3).

Over 4 days, grooming decreased the number of fleasindividual jirds by 52·11 ± 2·49%, on average. Ourrecovery of fleas was similar for male and female jirds(, F1,8 = 2·15, P = 0·18). Consequently, we pooledthe data for analysis. No relationship was foundbetween grooming index and the initial LC, ∆LC16−1 orPHA response (r2 = 0·24, 0·005 and 0·002, respectively;P > 0·1 for all).

Discussion

Our hypotheses were partly supported by our results.As we predicted, flea infestation impaired the responseto PHA injection. Moreover, flea infestation wasassociated with increased variance in changes in Hct and

LC between the 1st and 16th days of the experiment,although no significant difference was found betweenthe experimental and control groups between theinitial and day 16 measures. Changes in LC after 15 daysof flea infestation of the jirds were related to fitnesstraits and the quantity of blood consumed by fleas.Finally, in contrast with our predictions, behaviouraldefence was apparently independent of immunologicaldefence.

Hct measures the cell fraction of the total blood volume.Hct is assumed to reflect body condition and diseasestatus of an animal, such as nutritional stress, dehydra-tion, erythrocyte destruction by blood parasites ordirect blood cell loss to blood-sucking parasites(e.g. Franzmann & LeResche 1978; but see Dawson &Bortolotti 1997). The last has been shown to induce adecrease in Hct (Møller 1991). Furthermore, Hct hasbeen commonly used to test the impact of ectoparasitesor blood parasites on the physiology of the host (e.g.Ots & Horak 1998; Potti et al. 1999; Gauthier-Clercet al. 2003). Indeed, some studies reported that hostHct decreased with an increase of ectoparasite orblood parasite load (Lehmann 1992; Hurtrez-Boussès

Fig. 2. Relationship between the change in leucocyteconcentration of M. crassus before and after 16 days (∆LC16–1)of infestation by X. ramesis and (a) mean egg production perfemale flea and (b) mean hatching success of flea eggs.

Fig. 3. Relationships between the mean amount of bloodconsumed by X. ramesis from their M. crassus hosts and (a)host leucocyte concentration (LC) prior to flea infestation,and (b) difference in the leucocyte concentration between the16th day of flea infestation and the initial value (∆LC16–1) inhosts that were previously either subjected to flea parasitism(closed circles, solid line) or not parasitized (open squares,dashed line).

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et al. 1997; Potti et al. 1999; Szabó et al. 2002). However,numerous other studies failed to find any relationshipbetween Hct and parasite infestation (Johnson &Albrecht 1993; Ots & Horak 1998; O’Brien, Morrison& Johnson 2001; Gauthier-Clerc et al. 2003; Valera,Hoi & Kristin 2005). Indeed, we did not find a decreasein Hct of flea-infested rodents. This suggests that bloodloss caused by X. ramesis in its host was negligible,and did not affect Hct. The reason for this could be arapid production of new cells in response to blood lossas well as swelling of individual cells left in the blood-stream after parasite feeding (Christie 1978; Schindleret al. 1987). In other words, rapid erythropoiesis maymask the real impact of haematophagous parasites(O’Brien et al. 2001). Indeed, research on the responseof the House Wren (Troglodytes aedon) to chronic bloodloss due to parasitism by Blow Fly (Protocalliphoraparorum) larvae demonstrated that parasitized nest-lings had normal volumes of red blood cells (O’Brienet al. 2001). Moreover, previous study on parasitismby X. ramesis on Wagner’s gerbil Gerbillus dasyurus,demonstrated that the impact of fleas in terms of bloodconsumed was extremely low although fleas affectedthe energy requirement of their hosts (Khokhlovaet al. 2002). In other words, the major effects of fleas onthe energy expenditure of the G. dasyurus is apparentlythrough factors other than direct blood loss, such asenergy allocation to immune reactions (Khokhlova et al.2002). Nonetheless, we did find that flea infestationincreased among-individual variation of Hct changes.Thus, although flea infestation apparently affectsthe physiology of the host, Hct does not seem to bea particularly useful variable for the detection of theeffects of parasitism (Dawson & Bortolotti 1997).Alternatively, our inability to detect effects fleas onHct might be due to the relatively short period ofinfestation in our experiments.

One of the main advantages of using the PHA test toassess host immunocompetence is that, in contrast toother immunological variables such as immunoglobulinlevel, the level of parasite infestation does not confoundthe PHA response (Saino et al. 1998; Brinkhof et al.1999). However, a previous infection with one parasitecan modulate or suppress the immune response tounrelated antigens or pathogens (Cox 2001). Our resultssupport the prediction that the PHA response in jirdsis affected by flea infestation. Indeed, flea-infestedrodents had a lower PHA response than non-parasitizedanimals. The attenuating effect of parasites on PHAresponse was found in birds as well; the PHA responseof Red Jungle Fowl, Gallus gallus, parasitized by theintestinal nematode Ascaridia galli was lower than incontrol birds (Johnsen & Zuk 1999).

Two possible mechanisms explain the decrease inPHA response in flea-infested jirds. First, antigens pro-duced by fleas have a depressing effect on the immune

cells and, thus, the recruitment of T cells involved inthe PHA response is low. Second, the energy requiredto withstand flea infestation is allocated to other antipar-asite defences, grooming for example (but see below),or other types of immune responses and, consequently,less energy is available to mount an immune responseagainst the new antigen (phytohaemagglutinin). Thesetwo explanations are not necessarily mutually exclusive.The depressive effect on T cells could be an adaptiveresponse to a lack of sufficient energy to mount a PHAresponse. In another desert gerbillid, Gerbillus dasyurus,flea infestation causes an increase in maintenanceenergy requirements (Khokhlova et al. 2002). However,parasitized and non-parasitized gerbils did notshow any significant differences in food consumption(Khokhlova et al. 2002). This means that, when infestedwith fleas, any increase in the host’s energy expenditure,owing to activities such as grooming and mounting animmune defence, occurred without an increase in foodintake. Moreover, a previous study on the immuneresponse of M. crassus against fleas showed that the invitro stimulation of leucocytes by phytohaemagglutininwas higher for flea-experienced than non-parasitizedindividuals (Khokhlova et al. 2004a). The measurementsof the immune response in this experiment were donein a culture medium with glucose present. The level ofglucose consumption provided an assay of leucocyteblast transformation. Thus, energy was not limitingin this in vitro measurement of blast transformation.Consequently, energy allocation between differentfunctions, or between different types of immuneresponses, seems to be the more likely explanation ofthe depressing effect of flea infestation on host energyrequirements (Kyriazakis, Tolkamp & Hutchings1998).

We expected that the strength of the PHA responsewould be related to host antiparasite resistance thatcan be considered and measured as a host-mediatedloss of fitness in a parasite (Combes 2001). However,we found no relationship between PHA response andfitness components (egg production, hatching success),and blood consumption of fleas. Previous studiesreported relationships between the PHA response andhost resistance against parasites. For example, Christeet al. (2000) found a negative relationship between thenumber of mites Spinturnix myoti, on the bat Myotismyotis and its PHA response, and, in House Sparrows,Passer domesticus, the highest intensity and prevalenceof Haemoproteus infections were found for individualswith the lowest PHA response (Gonzalez et al. 1999;Navarro et al. 2003). However, in these studies, hostresistance was estimated in terms of parasite loadrather than in terms of direct fitness components of aparasite. The absence of a relationship between PHAresponse and flea fitness components may be due tothe fact that the PHA response is itself sensitive toflea infestation rate. Thus, the strength of the PHAresponse does not reflect the overall immunocom-petence of individuals. Therefore, to test the relationship

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between fitness components of fleas and immunocompe-tence assessed by the PHA response, further experimentsare required. These future experiments should be aimedat measuring flea fitness components after applyingthe PHA test. This will permit avoidance of the immuno-depressing effect of flea infestation. Note, however,that the results of laboratory experiments on theimmunocompetence should be interpreted cautiously.In particular, wild and captive animals of the samespecies could respond differently to the same manip-ulations with PHA because of the effect of diet (e.g.protein content) on immunocompetence (Lochmilleret al. 1993; Alonzo-Alvarez & Tella 2001).

No significant difference in LC was found betweenjirds in the experimental and control groups, both atthe beginning and after 15 days of the experiment.However, as with Hct, flea infestation resulted in anincrease in among-individual variation in ∆LC116−1.Depending on an individual, flea parasitism had eithera depressing or a stimulating effect on LC level. Differentpatterns of variation in LC in rodents due to fleaparasitism have been reported in other studies, as well(Khokhlova et al. 2004a,b).

Correlation between changes in rodent LC after15 days of flea parasitism and flea fitness variables, orthe amount of blood consumed by fleas, imply thathost’s immune response affected the reproductivephysiology of the fleas. Taken together, the two patterns,namely the relationship between ∆LC16−1 and fleareproductive/feeding variables, and intrahost hetero-geneity in the leucocyte response to flea parasitism,suggest that in natural populations fleas could selecthost individuals with the lowest level of immuno-competence and thus minimize the negative impact ofthe host’s immune response on themselves. This isanalogous to the mechanisms of parasite aggregationinferred by the ‘tasty chick hypothesis’ (Christe, Møller& De Lope 1998) that states that the last-hatched chickin an asynchronously hatching brood would have theleast efficient immune system, resulting in subsequentparasite aggregation on this chick.

The initial LC of the jirds was related neither tochanges in LC after infestation nor to fitness componentsof fleas. This result emphasizes that non-functionaltechniques of measurement of immuno-related struc-tures (i.e. LC or spleen mass) should be cautiouslyused for the assessment of immunocompetence. Therelationship between initial LC and blood consumptionby fleas can be explained by the fact that initial LC mayreflect the body condition of the host. However, thisrelationship was found only for the experimentalgroup that experienced flea infestation for 15 daysbefore measurement of blood consumption and couldbe because the jirds did not acquire resistance to fleasduring the first 15 days of flea infestation.

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We found no relationship between LC and PHAresponses. LC refers to the concentration of two typesof cells associated with immunity, phagocytoticgranylocytes and lymphocytes, represented mainly byT and B cells related to the acquired immune system.The PHA response provides a measure of T-cellmediated immunocompetence and, thus, reflects thelevel of acquired immunity. Our results support theidea that these two measures of immune defence areindependent.

We did not find a negative relationship betweenbehavioural and immunological defences. Behaviouris the first line of resistance to parasites in highervertebrates. Mechanisms such as grooming are costlybut efficient (Hart 1997; Eckstein & Hart 2000; Giorgiet al. 2001). If the availability of resources limitsdefence against parasites, then trade-offs betweenthese two means of defence could be important indetermining optimal antiparasitic defence. We assessedbehavioural defence by counting fleas remaining inrodent cages and on rodent bodies, assuming that‘cleaning’ behaviour consisted mainly of self-grooming.Although ectoparasite removal is one of the mostimportant functions of grooming, grooming has otherfunctions as well, such as maintenance of insulation,and maintenance of social relationships (Mooring,Blumstein & Stoner 2004). The multiple functions ofgrooming could mask the potential relationshipbetween immune and behavioural defences. Furtherexperiments combining behavioural observations withimmune measurements are required to investigate thisissue.

Conclusion

An important immunoecological question that has notyet been completely elucidated is whether the differentways to assess the immune status of individuals aregood indices of the ability of a potential host to resistparasites (Owens & Wilson 1999). Among the threeimmunological variables measured in the presentstudy to assess immunocompetence in rodents (initialLC, differences in LC after 15 days of infestation, andthe PHA response), only LC after 15 days of infestationappeared to be linked with the resistance against fleas.Moreover, we have shown that the immune responseassessed by the PHA test that is used routinely inimmuno-ecological studies is not independent of fleainfestation. These results emphasize the difficulty inassessing general immunocompetence. This difficultyresults from the extreme complexity of both the struc-ture and the function of the vertebrate immune systemwhich is composed of many different cell types andeffectors. This study supports the view that severalimmune variables, both cellular and humoral, relatedto both innate and acquired immunity, should be

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measured when attempting to characterize immuno-competence of an animal (Sheldon & Verhulst 1996;Norris & Evans. 2000; Tella, Scheuerlein & Ricklefs2002).

Acknowledgements

JGB received financial support from the BlausteinCenter for Scientific Cooperation and the Council forHigher Education of Israel. LG received financialsupport from the Albert Katz International School forDesert Studies. This study was partly supported byIsrael Science Foundation Grant no. 249/04 to BRKand ISK and is publication number 206 of the RamonScience Center and number 527 of the Mitrani Depart-ment of Desert Ecology.

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Received 10 January 2006; revised 17 April 2006; accepted 18April 2006 Editor: Peeter Hõrak