Article available at http://www.parasite-journal.org or http://dx.doi.org/10.1051/parasite/2004113301
FRENKELIA PARASITES IN A SMALL MAMMAL COMMUNITY. DYNAMICS OF INFECTION AND EFFECT ON THE HOST ,
,
FICHET-CALVET E.******, KIA E.B.******, GIRAUDOUX P.***. QUÉRÉ J.P.*, DELATTRE P.* & ASHFORD R.W.**
Summary:
Résumé :
A community of small mammals, Clethrionomys glareolus, Arvicola terreslris, Microtus avalis, M. agrestis, M. subterraneus, Apodemus spp. and Sorex spp., was studied as hosts of Frenkelia glareoli and F. microti in Franche-Comte (France). They were monitored in spring, summer and autumn on an area of about 1,350 ha comprising open field, hedgerow network and forest. Among 1,714 small mammals examined between July 1992 and October 1993, 4 7 % (178/376) of C. glareolus, 9.9 % (14/139) of A. terreslris and 1.3 % (4/311) of Apodemus spp. were infected by F. glareoli. The prevalence of infection with F. microti was 9.2 % (66/716) in M. arvalis and 8.2 % (6/73) in M, agrestis. M. subterraneus and Sorex spp. were not infected. The maintenance of each parasite in a rural landscape is assured both by a forest and a grassland host. Multiple logistic regression showed that prevalence was highly agedependent, with an apparent seasonal pattern. Prevalence varied between 30 % in summer and 60 % in early spring for F. glareoli in C. glareolus and between 3 % in autumn to 30 % in early spring for F. microti in M. arvalis. The year, habitat, host sex, relative density had no impact on prevalence. In M arvalis only, sexually active voles were preferentially uninfected, indicating a possible impact of this parasitism on fertility.
MAMMIFÈRES : DYNAMIQUE DE L'INFESTATION ET IMPACT SUR L'HÔTE
KEY WORDS : voles, population dynamics, Frenkelia spp., Coccidia, prevalence, age effect, agroecosystem, mid-mountain. MOTS CLÉS: campagnol, dynamique de population, Frenkelia spp., coccidie, prevalence, effet de I'age, agroecosysteme, moyenne montagne.
INTRODUCTION
P
arasites have long b e e n considered as possible factors in the regulation o f rodent populations (Elton et al., 1935). Around 1980. hypothetical
* Centre de biologie et de gestion des populations. Campus de Baillarguet. Montferrier-sur-Lez, France. ** Liverpool school of tropical medicine. United Kingdom. *** Biologie environnementale EA 3184 use INRA, Université de Franche-Comté. Besançon. France. **** School of public health and Institute of public health research. Tehran University of medical sciences. Iran. ***** FRF. CNRS 269S "Origine, structure et évolution de la biodiversité". Laboratoire mammifères & oiseaux. Muséum national d'histoire naturelle, Paris. France Correspondence : Dr Elisabeth Fichel-Calvet. FRE CNRS 269=5 "Origine, structure et évolution de la biodiversité". Laboratoire mammifères & oiseaux, Muséum national d'histoire naturelle. îî, rue Buffon. 75005 Paris. France. Tel.: 33 (6)1 40 79 30 69 - Fax : 33 (0)1 40 79 30 63. E-mail:
[email protected] Parasite. 2 0 0 4 , 11. 3 0 1 - 3 1 0
D E S FRENKELLIA CHEZ UN PEUPLEMENT DE PETITS
L'infestation par Frenkelia glareoli ef F. microti a éfé étudiée au niveau d'un peuplement de petits mammifères composé de Clethrionomys glareolus, Arvicola terreslris, Microtus arvalis, M . agrestis, M . subterraneus, Apodemus spp. et Sorex spp. Les populations ont été suivies au printemps, en été et en automne dans un agroécosystème comprenant des champs ouverts, du bocage et de la forêt. Parmi les 1714 petits mammifères examinés entre juillet 1992 et octobre 1993. 47 % ( 1 7 8 / 3 7 6 ) des C. glareolus, 9,9 % ( 1 4 / 1 3 9 ) des A. terrestris ef 1,3 % ( 4 / 3 1 ) des Apodemus spp. étaient infestés par F. glareoli. La prévalence de F. microti était de 9,2 % 6 6 / 7 1 6 ) chez M . arvalis et 8,2 % ( 6 / 7 3 ) chez M . agrestis. Aucune infestation n'a été observée chez M . subterraneus et Sorex spp. Dans un tel paysage rural, la maintenance de chaque parasite est assurée par deux hôtes, l'un fréquentant les habitats prairiaux, l'autre les habitats forestiers. Une analyse par régression logistique multiple a montré que les prévalences sont étroitement liées à l'âge de l'hôte alors que les fluctuations saisonnières (30-60 % pour F. glareoli chez C. clethrionomys; 3-30 % pour F. microti chez M . arvalis,) de la prévalence ne sont qu'apparentes et ne dépendent que de la structure en âge de la population hôte. L'année, l'habitat, le sexe de l'hôte et sa densité relative n'ont pas d'influence sur les prévalences. Chez M . arvalis, les individus sexuellement actifs sont préférentiellement ceux qui sont indemnes de F. microti, suggérant ainsi un possible impact de ce parasitisme sur la fertilité de ces rongeurs in natura.
models were produced, considering host-parasite systems as a special case o f predator-prey interaction (Anderson & May, 1978; Holmes, 1982). Paradoxically, very few studies in nature have tested these models. This was true long ago (Wiger, 1977), and there have b e e n few relevant studies since that time. Ecological studies on the effects o f parasites on host populations are particularly appropriate in the applied study o f the management o f irruptive rodent species (Jäkel el a I.. 1999). W e have carried out a long-term study o f agricultural pest rodents in mid-mountain zones in France (Delattre etai, 1992, 1996. 1999). and have previously linked a study of the cestode parasites o f these rodents (Giraudoux. 1991; Le Pesteur et al.. 1992). Additional results are presented here, on the protozoan parasites Frenkelia spp., o f the brains o f these rodents. Frenkelia microti was first discovered (As "M organism", thought to b e closely related to Toxoplasma) in Wales,
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by Findlay & Middleton ( 1 9 3 4 ) , in Microtus agrestis at the time o f a population crash. Frenkelia spp are h e t e r o x e n o u s coccidia with sexual reproduction in the intestines o f birds o f prey, espe cially the Buzzard Buteo buteo, and asexual multipli cation in the brains o f rodents. The full life history was first described, for F. glareoli in the b a n k vole, Clethrionomys glareolus, by Rommel & Krampitz ( 1 9 7 5 ) . Resistant sporocysts are excreted in the faeces o f Buz zards from 7-9 days following infection, for a period of 7-57 days. Sporozoites emerge from the sporocysts w h e n these are ingested by a rodent, and migrate to the brain, w h e r e they produce cysts that are visible 17 to 18 days following infection (Geisel et al., 1978; Laarman et al., 1979). Over a period o f weeks, the cysts grow to 3 5 0 um in diameter, and contain many thou sand bradyzoites, w h i c h are infective to Buzzards w h e n the rodent host is eaten. Heavily infected rodents contain numerous cysts, w h i c h o c c u p y a considerable proportion o f all parts o f the brain, and infection lasts for the life o f the host (Tadros & Laarman, 1976; Laar man et al., 1979). The earliest age at which the rodents can b e infected is unknown. Two species o f Frenkelia are known to occur in Europe, F. glareoli, mainly in C. glareolus, and F. microti, mainly in Microtus spp. (Tadros & Laarman, 1982). Vorisek et al. ( 1 9 9 8 ) have shown evidence that infected rodents are m o r e likely to b e predated than uninfected individuals, as h a p p e n s in certain other host-parasite combinations (review by C o m b e s , 1995). Transmission of the parasite is thereby facilitated, and the longevity of infected rodents is reduced. Reduced longevity o f s o m e individuals does not necessarily have any regu latory effect on populations. In order to assess any regulatory effect o f the parasite on intermediate host populations, information is first required on the dis tribution o f the parasite in the host community at a local scale. T h e aim o f this study is to test for any effect o f extrinsic (year, s e a s o n , habitat) and intrinsic (host age, s e x and relative density) factors o n the infec tion rates in e a c h s p e c i e s o f the rodent c o m m u n i t y . T h e n , the possibility o f an effect o f the parasites on the hosts w a s investigated b y c o m p a r i n g the b o d y weight and sexual activity o f infected and uninfected individuals.
MATERIALS A N D METHODS STUDY SITE The
study area o c c u p i e s a b o u t 1,350 ha, in Franche-Comté, 10 km north-west o f Pontarlier (47.10° N, 6.24° E, 8 5 0 m a b o v e sea level) with m e a n annual rainfall o f 1,500 mm. T h e landscape is
302-
Fig. 1. - L a n d s c a p e c o m p o s i t i o n o f t h e study site l o c a t e d in t h e J u r a plateau.
c o m p o s e d o f forest and agricultural land. T h e forest is mostly semi-natural, c o m p o s e d o f m i x e d b e e c h Fagus sylvatica, oak Quercus robur, and fir Abies alba, and there are s o m e spruce Picea abies plantations. T h e agricultural land is either improved grassland or per manent pasture (Delattre et al, 1988; Giraudoux et al., 1 9 9 7 ) , and is either, o p e n over wide areas ( o p e n field), or e n c l o s e d by hedgerows in plots o f ca 1 ha (Fig. 1). TRAPPING A N D SAMPLING Trapping was carried out in forest (deciduous, mixed, coniferous), h e d g e r o w network (hedge, hedge edge, enclosures) and open field (permanent grassland) habi tats. B e c a u s e this study is part o f a rodent survey for outbreak management, small mammals w e r e sampled during the reproduction period: in July and O c t o b e r 1992, and April, July and O c t o b e r 1993. INRA (French Agronomic Research Institute) trap lines w e r e used (Spitz et al., 1 9 7 4 ) . Thirty-four traps were placed at 3 m intervals in each line o f about 100 m. T h e numbers o f lines set on e a c h occasion, and the distribution by habitat o f the 2 1 4 trap lines ( 2 1 , 8 2 8 trap nights) are shown in Table I. Traps were left in place for three consecutive nights, and w e r e visited twice daily. Ani mals were killed by cervical dislocation according to Mills et al. ( 1 9 9 5 ) .
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T a b l e I. - Distribution o f t h e 2 1 4 trap lines b y habitat a n d b y s e a s o n . Arrows d e s i g n the habitats in w h i c h the rodent ralative a b u n d a n c e s a r e c a l c u l a t e d . * c o r r e s p o n d s to a forest c l e a r i n g .
H O S T POPULATION PARAMETERS
STATISTICAL ANALYSIS
Relative abundance was estimated for each species as the number caught per trap line. For C. glareolus, cap tures in forest and in hedgerow network were analysed separately. For Microtus arvalis, captures in hedgerow network and in o p e n field were analysed separately. For Microtus agrestis, which was less abundant, cap tures in forest and in hedgerow network were c o m bined. Arvicola terrestris numbers were not estimated as only juveniles were sampled, adults being too big to enter the traps.
Year, season, habitat, host sex, age and relative density effects on prevalence were analysed with a multiple logistic regression using a binary factor (infected = 1, non infected = 0 ) as the dependent variable and year (two levels: 1992, 1993), season (three levels: spring, summer and autumn), habitat (three levels: open field, hedgerow network and forest), host sex (two levels), host age (continuous ELW) and host relative density (continuous abundance index) as independent variables. The strategy of data treatment was first to enter all the variables in a global model, and to perform a forward stepwise regres sion to select the non redundant variables. The second stage was to enter these selected variables with their interactions in a restricted model as recommended by Kleinbaum & Klein (2002). This analysis was performed with Systat 9. SAS Institute Inc. (1999). T h e effect o f infection on weight was analysed using ANCOVA with weight as the dependent variable and the host s e x and infection (two levels: infected, unin fected) as the independent variables. Host age (conti nuous ELW) was entered as the covariate in the model. T h e effect o f infection on sexual activity was analysed using multiple logistic regression including sexual acti vity (active = 1, inactive = 0 ) as the dependent variable and the infection (two levels: infected, uninfected), year (two levels), season (three levels), habitat (three levels), host sex, host age (continuous ELW) and host relative density (continuous abundance index) as the inde pendent variables. As the goal o f this analysis was to obtain a single estimate o f the Frenkelia infection, adjusted for year, season, habitat, host sex, host age, and host relative density, the interactions were not included in the model (Kleinbaum & Klein, 2 0 0 2 ) . T h e effect o f infection on fertility in each s e x was analysed using ANCOVA with seminal vesicle size or litter size as the dependent variable and infection (two levels: infected, uninfected), season (three levels) and host age (continuous ELW) as independent variables (Legendre & Legendre, 1998; Sokal & Rohlf. 1998).
T h e weight o f the desiccated e y e lens (ELW) gives the best indication o f age for small mammals (Lord, 1959; Martinet. 1966, rev. in Morris, 1971). Eyes were removed and preserved for a minimum o f two w e e k s in 10 % formalin, then the lenses were extracted, dried for two hours at 100° C, and weighed to a precision o f 0.1 mg. Females were classified as sexually active if they were pregnant or lactating, as w e r e males with seminal vesicles over 4 0 m m (length x breadth). Litter size was estimated by the number o f embryos. 2
PARASITES Carcasses were preserved in 10 % formalin before e x a mination. T h e brain was removed by dissection o f the skull, and stained for at least 24 h in undiluted Semic h o n ' s acetic carmine. T h e y w e r e then washed in dis tilled water, transferred to 1 % HC1 in 7 0 % ethanol to differentiate, until the brain material was very pale pink in colour (usually a few hours), and placed in glycerine to clear. T h e stained, cleared brains were then sliced with a scalpel and the slices were e x a mined with a dissecting m i c r o s c o p e ( x 100) to detect any parasites. L o b u l a t e d cysts w e r e identified as F. microti, and large round cysts as F. glareoli (Tadros et al., 1972; Tadros & Laarman, 1978). A few very small round cysts were regarded as unidentifiable e x c e p t in juvenile A. terrestris in which 2 / 1 0 2 were Toxoplasma gondii, and 3 / 1 0 2 w e r e Frenkelia, glareoli (Kia et al., in press). Parasite. 2 0 0 4 . 11. 3 0 1 - 3 1 0
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most important hosts, C. glareolus, lis and M. agrestis.
RESULTS HOST RANGE
O
Further analysis is restricted to the infection in the four Host species
glareoli
in Cletbrionomys
No. collected
No. e x a m i n e d
F. glareoli
(%)
376
1 7s I R ) lr (9.9)
Microtus
amalis
981
139 716
Microtus
agrestis
136
is su bterra
Apodemus
spp.
spp.
net is
1
761
2
flat icollis
p r e d o m i n a t e d , but A. sylvaticus
c i e s are g r o u p e d t o g e t h e r .
2
Sorex
coronatus
(%)
3(1) 0 66 (9.2) 6 (8.2)
1 (1.4)
311 87
203
F. microti
1 (2)
73 12
20
glareolus
2
537
Sorex
arva
The influence o f year, season, habitat, host sex, age and relative density on prevalence was analysed in a global model by forward stepwise regression. T h e main effect on prevalence was due to host age ( c h i = 19.204, p < 0.0001), whereas the other factors were not significant. Host age is highly significant with an odds ratio o f 1.043 (p < 0.0001), indicating an increase o f prevalence with
210
glareolus
Apodemus
1
• Frenkelia
terrestris
Cletbrionomys
Microti
M.
PREVALENCE
f 2,848 animals collected, 1,714 were e x a mined for Frenkelia infection (Table II). F. glareoli was mainly found in C. glareoli, secon darily in Arvicola terrestris, and rarely in M. arvalis, M. agrestis and Apodemus spp. F. microti was most fre quent in .M. arvalis and M. agrestis and was also found rarely in C. glareolus. Microtus subterraneus and Sorex spp were never found infected.
Articola
A. terrestris,
0
0
-i ( 1 . 3 )
0
0
0
a l s o o c c u r r e d : n o a t t e m p t w a s m a d e t o distinguish j u v e n i l e s p e c i m e n s , s o b o t h s p e
a n d .V. araneus
w e r e not d i s t i n g u i s h e d for t h e p r e s e n t study.
T a b l e II. - P r e v a l e n c e s a n d host r a n g e in a small m a m m a l c o m m u n i t y i n f e c t e d b y Frenkelia
glareoli
a n d F.
microti.
Fig. 2. - P r e v a l e n c e o f Frenkelia
glareoli
a b u n d a n c e o f its host Cletbrionomys
and glareolus
( n u m b e r o f c a p t u r e s p e r 1 0 0 m o f trap l i n e ) in t h e h e d g e r o w Numbers
network
and in t h e
forest.
u n d e r e a c h b a r c o r r e s p o n d to t h e
r o d e n t s e x a m i n e d for infection.
Frenkelia
glareoli
in Clethrionomys
glareolus
H e d q e r o w network & F o r e s t
Fig. 3. - Distribution o f Frenkelia
304
glareoli
in Clethrionomys
glareolus
b y e y e l e n s w e i g h t (F.LW in m g ) a n d s e x o f h o s t .
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age. The host age effect is illustrated in Figure 3 where the age structure is presented for each session. Infected individuals were present at each session and in each ELW class over 3 mg. Prevalence remained high throu ghout the year (Fig. 2) with the lowest prevalence in July 1993 (32 % ) and the highest in April 1993 (62 % ) . T h e s e seasonal variations were not significant w h e n host age was taken into account. The model explains only 4 % (250.475 - 2 4 0 . 5 7 7 / 2 5 0 . 4 7 5 ) o f the total varia tion, suggesting that many other factors than host age explain 9 6 % o f the variation o f prevalence o f F. glareoli in C. glareolus.
Residual deviance
Variables
Chi
Host age (1.2 < elw < "".5) Host relative density ( 2 . 8 < ai < 1 4 . 4 )
60.084 20.610
181.277 169.946
< 0.0001
165.854
0.004
Y e a r x host relative density
8.083 4.462
Year x host age x host relative density
3.969
163.815 161.922
Year x host a g e
0.093 0.028
161.922
0.035 0.046 0.-61
161.922
0.868
2
Null
P
208.882
Host age x host relative density
Year
T a b l e III. - Logistic r e g r e s s i o n results for Frenkelia in Microtus
arvalis
microti
< 0.0001
infection
in a h e d g e r o w n e t w o r k ( I I N W ) a n d o p e n field
m o d e l . F l w = e y e l e n s w e i g h t in mg, ai = a b u n d a n c e
i n d e x in
n u m b e r o f v o l e s t r a p p e d p e r 1 0 0 m. n = 6 7 0 .
• Frenkelia
glareoli
in Arvicola
terrestris
Because A. terrestris were captured almost exclusively in enclosed grassland, the variable "habitat" was excluded from this analysis. In addition, relative density o f this spe cies was not evaluated since only juvenile specimens were caught. Among the four remaining variables, year, season, host sex and host age (1.6 < ELW < 11.1), the multiple logistic regression shows that only the two last had an effect on prevalence (Chi = 3-983, p = 0.046 and Chi = 21.200. p < 0.0001 respectively). Infection was twice as common in females ( 8 / 6 l ) than in males ( 4 / 5 8 ) .
that these variables were not additive, and also that the combined effect o f host age and host density on preva lence has to be considered year by year. This restricted model explains 22 % (208.882-161.922/208.882) of the total variation, suggesting that other factors are involved in Frenkelia infection in M. arvalis. • Frenkelia
microti
in Microtus
agrestis
2
2
• Frenkelia
microti
in Microtus
arvalis
2
First, the forward stepwise regression showed host age, relative density, and year to b e significant variables having an effect on the Frenkelia infection. The other variables, season, host sex and habitat, were not corre lated with prevalence. Host age is highly significant with an odds ratio o f 1.069 (p O.OOOl), indicating increasing infection with age. This effect is illustrated in Figure 5 where the age structure is presented for each session. Infected individuals were present at each session, with very young ones in summer with ELW between 2 and 3 mg. In autumn, the youngest infected vole had ELW over 3-5 mg. Host relative density is significant with an odds ratio o f 0.807 (p < 0.001) indicating that the pre valence of F. microti is negatively correlated with the abundance of its host. The year effect is described by an OR o f 0.635 indicating a lower prevalence in 1993 than 1992. Figure 4 shows prevalence to b e highest in spring (29 % in April 1993), when the vole population was at its lowest; prevalence declined in the breeding season (17 % in July 1992; 3 % in July 1993). reaching its lowest in autumn (7 % in October 1992; 3 % in October 1993), when the host population was at its greatest. These sea sonal variations were not significant when host age was taken into account. In the restricted model containing the main factors, host age, host relative density and year, and their interactions, the 2-way interactions, i.e., "year x host relative density" and the 3-way interaction "year x host age x host density" were significant, whereas the main factor turned to non-significant (Table III). This means Parasite. 2 0 0 4 . 7 7 . 3 0 1 - 3 1 0
As the M. agrestis sample was not large enough to segre gate captures between hedgerow network and forest, the data were pooled, and habitat was excluded from the model. Here, the main effect on infection is ckte to host age only (Chi = 7.851, p = 0.005) whereas year, season, host sex and relative density are not significant. EFFECT OF PARASITES ON WEIGHT AND SEXUAL ACTIVITY OF THE HOST T o assess the possible impact o f parasitism on weight and sexLtal activity in rodents, infection in the two numerous and well sampled hosts, C7. glareolus and M. arvalis was analysed. • Frenkelia
glareoli
in C
glareolus
Table IV shows the effect o f F. glareoli infection on the weight o f C. glareolus with season, host s e x and C.
Source o f variation Host infection
glareolus model
F
P
0.019 9.857
Host s e x Season
36.474
Infection x s e x Infection x season Sex x season Infection x s e x x s e a s o n Host a g e ( e l w )
0.889 0.002 < 0.0001
0.882
0.348
4.649 26.024 0.668 184.219
M.
arvalls model
F 0.583
P
13.991
0.445 0.0002
131.277
< 0.0001 7
0.010
4.749 1.421
0.29" 0.242
< 0.0001
4.672
0.009
0.513 < 0.0001
0.856
0.425 < 0.0001
546.805
T a b l e IV. - Intrinsic a n d e x t r i n s i c s o u r c e s o f variation in the b o d y w e i g h t in Clelhrlonomys
glareolus
and
infected with
in Microlus
arratis
i n f e c t e d with Frenkelia Frenkelia
microti
glared! through
A N C O V A . H o s t a g e . e s t i m a t e d b y the e y e l e n s w e i g h t ( e l w ) . is e n t e r e d as a c o v a r i a t e in e a c h m o d e l .
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Fig. 4. - Prevalence of Frenkelia microti and abundance of its host Microtus arvalis (number of captures per 100 m of trap line) in the hedgerow network and in the open field habitats. Numbers under each bar cor respond to the rodents examined for infection.
Frenkelia
microti
in Microtus
arvalis
H e d g e r o w network & O p e n field
Fig.
5. - Distribution o f Frenkelia
microti
in Microtus
amalis
b y e y e l e n s w e i g h t ( E L W in m g ) a n d s e x o f host.
age taken into account through ANCOVA. Weight is significantly affected by age, s e x and season but not by Frenkelia infection. T h e two w a y interaction, infec tion x season, is significant. This is illustrated in Figure 6
showing that, in summer, the infected voles are hea vier than the uninfected o n e s . Using multiple logistic regression, sexual activity in both s e x e s is significantly correlated with year, season, host s e x and age, but not with habitat, relative den sity or Frenkelia infection (Table V ) . For males, the length x breadth o f the seminal vesicles is highly cor related with host age ( F = 58.980, p < 0 . 0 0 0 1 ) and 1 2 0 0
C.
glareolus
M.
model Variables
Chi
2
Chi
P
2
Infection
0.200
0.655
6.205
3.980
0.046
32.940
0.013 < 0.0001
Season
18.867
< 0.0001
97.888
< 0.0001
Habitat
0.151
0.697
< 0.0001
lc >st
sex
16.044
< 0.0001
17.105 0.470
Host age
16.578
< 0.0001
0.145
0.703
H o s t relative d e n s i t y
42.791 11.802
Fig. 6. - M e a n b o d y w e i g h t (in g with standard error b a r s ) in infected
T a b l e V. - Logistic r e g r e s s i o n results for s e x u a l activity in
a n d u n i n f e c t e d C/ethrionomys
nomys
glareolus
arvalis
infected with Frenkelia
glareolus,
by season. Number close
to e a c h s y m b o l i n d i c a t e s t h e s a m p l e size.
P
Year
I
306
arvalis model
Memoire
i n f e c t e d w i t h Frenkelia
glareoli
a n d in
0.493 < 0.0001 0.0006 ClethioMicrotus
microti.
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FRENKELIA INFECTIONS IN VOLES
season ( F = 53-222, p < 0 . 0 0 0 1 ) but not with Frenkelia infection (F, = 2.506, p = 0 . 1 1 5 ) . There were insufficient pregnant females in the sample to test for differences in litter size. 2 2 0 0
10 % prevalence is probably an underestimate o f the real prevalence. Apodemus spp, Microtus arvalis and Al. agrestris are incidental hosts, with low prevalence o f infection. A similar result was found in the Czech Republic by Vorisek et al. ( 1 9 9 8 ) . It is not clear whether these low prevalences are due to innate resistance in most indi viduals, l o w e r e x p o s u r e to infection (unlikely for Microtus spp, as these are infected with F. microti which has the same transmission mechanism), or high mortality o f infected animals. T h e main hosts o f F. microti are confirmed to be M. aivalis ( 9 % preva l e n c e ) and M. agrestis ( 8 % prevalence). C. glareolus is clearly an incidental host for this parasite. This study shows that each o f the Frenkelia species is maintained by two main intermediate hosts, which inhabit w o o d e d habitats such as forest or hedges in hedgerow network, and grassland such as o p e n field or fields in hedgerow network.
2 0 0
• Frenkelia
microti
in M.
atvalis
T h e s a m e analysis as a b o v e s h o w s that the variation in weight o f M. ari'alis was mainly due to host age and sex, and to season, but not to Frenkelia infection. The significant two way interaction, season x host sex, is due to the increased weight o f females in autumn (Table IV). T h e sexual activity in M. atvalis also showed a multi factorial d e p e n d e n c y pattern, significantly correlated with year, season, host age, relative density, habitat and infection (Table V ) . T h e most interesting correlations are those concerning habitat and Frenkelia infection. Their partial coefficients s h o w that o p e n field and infection are negatively correlated with sexual activity (r = - 0 . 1 3 1 . p < 0.0001 and r = - 0.069, p = 0 . 0 1 4 respectively). This suggests a lower sexual activity in o p e n field than in h e d g e r o w network, and in infected voles than in uninfected ones. Seminal vesicle size was not influenced by Frenkelia infection ( F , = 0.949, p = 0.330) but only by age ( F = 102.341, p < 0.0001) and season ( F , , = 22.567, p < 0 . 0 0 0 1 ) . Litter size was related to the season only ( F , = 10.478, p < 0 . 0 0 0 1 ) but not to Frenkelia infection (F, = 0.022, p = 0.871).
VARIATIONS IN PREVALENCE Host age is the main factor influencing the prevalence o f Frenkelia in voles (Table VI). This positive relation has b e e n pointed out in rodents infected with many parasites such as cestodes ( B e h n k e et al., 1993, 1999), trematodes (Duplantier & Sene, 2 0 0 0 ) , protozoa (Tur ner, 1986), bacteria ( G o d e l u c k el al., 1994; FichetCalvet et al, 2 0 0 0 ) and viruses (Mills et al, 1992). T h e s e results suggest that as the rodents age, the pro bability o f infection increases. In M. arvalis, the infec tion can o c c u r very early in its life, around 20-30 days in summer. This age was extrapolated from the FLW measures o f captive-bred animals (Martinet. 1966). In C. glareolus, host age is the only factor correlated with the prevalence o f F. glareoli whereas year and host relative density also showed a distinct influence on the prevalence o f F. microti in M. arvalis. F. microti was more prevalent in 1992 than 1993 and during this time, the density o f M. arvalis was stable. As the buzzard population declined in 1993 (pens, obs.), it is suggested
3 5 9
1 3 5 9
3
9
9 ]
9 1
DISCUSSION OCCURRENCE OF FRENKELIA SPP. IN INTERMEDIATE HOSTS The main host for F. glareoli is clearly C. glareolus, with almost 50 % prevalence overall. T h e other important host for this species is A. terrestris, which appears to be a n e w host record. Bearing in mind the fact that only juvenile animals o f this species were sampled, the
Prevalence
Sexual activity
Source of variation
Fg in Cg
Fg in At
Year
0
0
Season
0
0
Fin in Ma -
Fm in Mg
(1993) 0
0 0
Ma
Cg -
(1993)
+ (spring & s u m m e r )
- (1993) + (spring & s u m m e r ) - ( O p e n Field)
Habitat
0
N1
0
N1
0
Host s e x
0
+ (female)
(1
0
+ (male)
0
Host age
+
+
+
+
+
H o s t relative d e n s i t y
0
N1
-
0
0
+ +
0
- (infected)
Infection T a b l e VI. - S u m m a r i z e d effects v a r i a b l e s o n the p r e v a l e n c e o f Frenkelia
infection a n d o n s e x u a l activity. 0 = not significant, + positive,
- n e g a t i v e . I n f o r m a t i o n b e t w e e n b r a c k e t s i n d i c a t e s w h i c h level is s o u r c e o f variation
for n o m i n a l factors. Fg = Frenkelia
Frenkelia
aivalis.
microti.
Parasite. 2 0 0 4 . 11.
Cg = Clethrionomys
301-310
glareolus,
At = Articola
terrestris.
Ma = Microtus
Mg = Microtus
agrestis.
glareoli.
Fin =
X I = non included.
307
FICHET-CALVET F... KIA E.B.. GIRAUDOUX P. ET AL
that the reduction in prevalence may b e related to the decrease in density o f buzzards. T h e negative corre lation b e t w e e n prevalence and host relative density indicates that w h e n the voles are most numerous, the infection rate is lowest. T h e buzzard is a c o m m o n pre dator o f the two voles, but the density effect is not dis cernible in C. glareolus, probably b e c a u s e o f the rela tive stability o f their population. Even though prevalence fluctuates seasonally, season has no impact on prevalence w h e n host age is taken into account. In spring, w h e n the population o f C. gla reolus and M. arvalis was at its minimum, consisting only o f old adults that had survived the winter, pre v a l e n c e o f both Frenkelia spp. was maximal (F. glareoli: 62 %, F. microti: 2 9 % ) . Infected animals were then diluted by newly born individuals b e t w e e n spring and autumn and, as the older individuals died off, ove rall prevalence declined (F. glareoli: 3 2 %, F. microti: 3 % ) . T h e s e findings agree with those o f Laarman et al. ( 1 9 7 9 ) in the Netherlands where, in winter and early spring, most o f b a n k voles were infected whereas only 2 5 - 3 0 % were infected in summer. In the Czech Republic, Vorisek et al. ( 1 9 9 8 ) found a mean preva lence o f 16 % in spring which is similar to that obser ved here in the C. glareolus living in forest. T h e decli ning prevalence o f F. microti between July and October is explicable partly by the extension o f the breeding season into the autumn, and continuing dilution with young individuals. The data suggest that F. microti and F. glareoli are equally transmitted all through the year. T h e high pre valence o f F. glareoli in young o f both C. glareolus in July and A. terrestris in April, indicating a high rate o f transmission in spring and early summer, supports this hypothesis. In M. agrestis, the prevalence o f F. microti in April 1 9 9 3 ( 2 2 % ) was greater than that observed in Finland (6 % ) in the same season (Soveri et al., 2 0 0 0 ) . In the winter of 1 9 9 2 - 1 9 9 3 , buzzards were unusually abundant on the Jura plateau, which may explain this difference. Habitat had n o impact on the prevalence o f Frenkelia spp. The bank voles were equally infected in hedgerow network as in the forest. T h e irregularity o f the forest boundaries and clearings make the forest a mosaic where the permeability o f the parasite is equal to that in the h e d g e r o w network. A comparative study in a landscape with larger areas o f unbroken forest would b e necessary to s h o w any impact o f habitat on pre valence. Infection in Microtus spp. is equally preva lent in e n c l o s e d and o p e n grassland and, here again, a larger o p e n field would b e necessary to s h o w any impact o f the habitat in relation to the behaviour o f the buzzard, which spends more time on o p e n than closed habitats.
308
EFFECTS OF PARASITES ON HOST WEIGHT AND SEXUAL ACTIVITY In the overall samples, w h e n animals o f all ages were represented, and before correcting for age, infected voles o f both species were systematically heavier than uninfected individuals (C. glareolus: 1 9 . 3 ± 3 - 1 g, n = 1 7 2 vs 1 8 . 3 ± 3 . 5 g, n = 1 9 0 ; M. arvalis: 2 1 . 6 ± 6 . 3 g, n = 6 3 vs 1 8 . 2 ± 6 . 2 g. n == 6 0 7 ) . For C7. glareoli infected by F. glareoli, this trend was particularly true in sum mer w h e n voles were reproducing. H o o g e n b o o m & Dijkstra ( 1 9 8 7 ) found a similar effect in another heteroxenous coccidian in the muscles o f M. arvalis, Sarcocystis cernae, in which infection was associated with increased weight, but this study was not fully adjusted for age, and older, heavier animals are more likely to be infected. W h e n the sample is restricted to similar season and age cohort, as for Psammomys obesus infec ted with Bartonella spp. or Babesia spp. in Tunisia, the weight is equal in. infected and uninfected animals (Fichet-Calvet et al, 2 0 0 0 ) . In our study, infection with Frenkelia spp. had no impact on body weight w h e n season, s e x and age were taken into account. T h e s e last three factors are normally the main determinants of b o d y weight. More interesting are the results concer ning sexual activity, which also depends on season and age (Table VI). Sexual activity was also dependent on year, with a higher probability o f inactivity in 1 9 9 3 . This lack o f sexual activity could explain why the M. arvalis population crashed in 1 9 9 4 following a period o f high density lasting three years (unpublished data). M. arvalis was less sexually active in o p e n field than in hedgerow network, indicating that the crash began in o p e n field before continuing in hedgerow network. Sexual activity was positively related with density, reflecting continuation o f reproduction into the autumn and an accumulation o f several cohorts born in the previous spring and summer w h e n .M. arvalis was abundant. In males, the infection was not corre lated with the size o f the seminal vesicles. In pregnant females, the infection did not affect the litter size. However, infected individuals o f M. arvalis were less sexually active than uninfected ones. This suggests that F. microti may delay female sexual maturity. Mecha nisms such as a delay in the first pregnane) or an increasing time b e t w e e n litters have b e e n shown in C. glareolus infected with c o w p o x virus in UK ( F e o r e et al, 1 9 9 7 ) . Our result is consistent with a possible regulation o f host population by parasitism. An addi tional regulatory effect on the intermediate host popu lations could operate through an increased risk o f predation leading to reduced longevity and a reduced number o f litters produced by predated individuals. Against this, there is n o evidence o f reduced preva lence in the oldest animals, indicating that longevity is not reduced in infected individuals.
Mémoire
Parasite, 2 0 0 4 , 11, 3 0 1 - 3 1 0
FRENKELIA INFECTIONS IN VOIES
L'sammomys ohesus. in Tunisia, and effect on the host. Annals of Tropical Medicine and Parasitology, 2000, 94,
ACKNOWLEDGEMENTS
F
inancial support o f the Franche-Comté regional council is gratefully acknowledged. E. F-C b e n e fited from a grant from the Société de Secours des Amis d e s S c i e n c e s (Paris). Many thanks to the Réseau d'Observation Prédateurs-Rongeurs-Environne ment for providing data on buzzard dynamics. T h e authors are grateful to J-M. Duplantier for his useful c o m m e n t s on the earlier version o f the manuscript.
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