Mammal density and patterns of ectoparasite species richness and abundance

Patterns of species richness, prevalence and abundance of ectoparasites have rarely been investigated at both the levels of populations and species of hosts. Here, we investigated the effects in changes in small mammal density on species richness, abundance and prevalence of ectoparasitic fleas. The comparative analyses were conducted for different small mammal species and among several populations during a long-term survey. We tested the hypothesis that an increase in host density should be linked with an increase in parasite species richness both among host species and among populations within host species, as predicted by epidemiological models. We also used host species density data from literature. We found that host density has a major influence on the species richness of ectoparasite communities of small mammals among host populations. We found no relationship between data of host density from the literature and parasite species richness. In contrast with epidemiological hypotheses, we found no relationships between abundance, or prevalence, and host density, either among host species or among host populations. Moreover, a decrease in abundance of fleas in relation with an increase in host density was observed for two mammal species (Apodemus agrarius and A. flavicollis). The decrease or the lack of increase in flea abundance in relation with an increase in host density suggests anti-parasitic behavioural activities such as grooming.     

Distribution and abundance of parasite nematodes: ecological specialisation,phylogenetic constraint or simply epidemiology?

We investigate the patterns of abundance‐spatial occupancy relationships of adult parasite nematodes in mammal host populations (828 populations of nematodes from 66 different species of terrestrial mammals). A positive relationship between mean parasite abundance and host occupancy, i.e. prevalence, is found which suggests that local abundance is linked to spatial distribution across species. Moreover, the frequency distribution of the parasite prevalence is bimodal, which is consistent with a core‐satellite species distribution. In addition, a strong positive relationship between the abundance (log‐transformed) and its variance (log‐transformed) is observed, the distribution of worm abundance being lognormally distributed when abundance values have been corrected for host body size.
Hanski et al. proposed three distinct hypotheses, which might account for the positive relationship between abundance and prevalence in free and associated organisms: 1) ecological specialisation, 2) sampling artefact, and 3) metapopulation dynamics. In addition, Gaston and co‐workers listed five additional hypotheses. Four solutions were not applicable to our parasitological data due to the lack of relevant information in most host‐parasite studies. The fifth hypothesis, i.e. the confounded effects exerted by common history on observed patterns of parasite distributions, was considered using a phylogeny‐based comparison method. Testing the four possible hypotheses, we obtained the following results: 1) the variation of parasite distribution across host species is not due to phylogenetic confounding effects; 2) the positive relationship between mean abundance and prevalence of nematodes may not result from an ecological specialisation, i.e. host specificity, of these parasites; 3) both a positive abundance‐prevalence relationship and a negative coefficient of variation of abundance‐prevalence relationship are likely to occur which corroborates the sampling model developed by Hanski et al. We argue that demographic explanations may be of particular importance to explain the patterns of bimodality of prevalence when testing Monte‐Carlo simulations using epidemiological modelling frameworks, and when considering empirical findings. We conclude that both the bimodal distribution of parasite prevalence and the mean‐variance power function simply result from demographic and stochastic patterns (highlighted by the sampling model), which present compelling evidence that nematode parasite species might adjust their spatial distribution and burden in mammal hosts for simple epidemiological reasons.     

Bot fly parasitism of the red-backed vole: host survival,infection risk,and population growth

Parasites can play an important role in the dynamics of host populations, but empirical evidence remains sparse. We investigated the role of bot fly (Cuterebra spp.) parasitism in red-backed voles (Myodes gapperi) by first assessing the impacts of the parasite on the probability of vole survival under stressful conditions as well as on the reproductive activity of females. We then identified the main factors driving both the individual risk of infection and the abundance of bot flies inside red-backed voles. Finally, we evaluated the impacts of bot fly prevalence on the growth rate of vole populations between mid-July and mid-August. Thirty-six populations of red-backed voles were sampled in the boreal forest of Québec, Canada. The presence and the abundance of parasites in voles, two host life history traits (sex and body condition), three indices of habitat complexity (tree basal area, sapling basal area, coarse woody debris volume), and vole abundance were considered in models evaluating the effects of bot flies on host populations. We found that the probability of survival of red-backed voles in live traps decreased with bot fly infection. Both the individual risk of infection and the abundance of bot flies in red-backed voles were driven mainly by vole abundance rather than by the two host life history traits or the three variables of habitat complexity. Parasitism had population consequences: bot fly prevalence was linked to a decrease in short-term growth rate of vole populations over the summer. We found that bot flies have the potential to reduce survival of red-backed voles, an effect that may apply to large portions of populations. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Metazoan parasites of freshwater cyprinid fish ( Leuciscus cephalus): testing biogeographical hypotheses of species diversity

The diversity and similarity of parasite communities is a result of many determinants widely considered in parasite ecology. In this study, the metazoan parasite communities of 15 chub populations (Leuciscus cephalus) were sampled across a wide geographical range. Three hypotheses of biogeographical gradients in species diversity were tested: (1) latitudinal gradient, (2) a 'favourable centre' versus 'local oasis' model, and (3) decay of similarity with distance. We found that the localities in marginal zones of chub distribution showed lower parasite species richness and diversity. A latitudinal gradient, with increasing abundance of larvae of Diplostomum species, was observed. There was a general trend for a negative relationship between relative prevalence or abundance and the distance from the locality with maximum prevalence or abundance for the majority of parasite species. However, statistical support for a 'favourable centre' model was found only for total abundance of Monogenea and for larvae of Diplostomum species. The phylogenetic relatedness of host populations inferred an important role when the 'favourable centre' model was tested. Testing of the hypothesis of 'decay of similarity with geographical distance' showed that phylogenetic distance was more important as a determinant of similarity in parasite communities than geographical distance between host populations.     

Spatial and temporal patterns in the occurrence of peritrich ciliates as epibionts on calanoid copepods in the Chesapeake Bay, USA

We investigated temporal and spatial patterns of distribution in two peritrich ciliates (i.e. Zoothamnium intermedium and Epistylis sp.) living as epibionts on calanoid copepods (i.e. Acartia tonsa and Eurytemora affinis) in Chesapeake Bay. Net tow samples collected along the main axis of the Bay were analyzed to estimate the occurrence of epibionts on copepods and to explore relationships among infestation prevalence, host abundance, and environmental variables. Zoothamnium intermedium and Epistylis sp. colonized populations of A. tonsa during spring and summer months, while only Z. intermedium colonized E. affinis during spring. Occurrence of epibionts on copepods showed high interannual variation, marked seasonality, and geographic heterogeneity. Extensive statistical analyses rejected simple scenarios of interactions between epibiosis, environmental variables, and host density, suggesting a more complex dynamics for the system. Analyses of epibiont colonies and zooids per host area (i.e. the sum of width and length of the body including antennae and swimming legs calculated assuming a cylindrical shape) were also performed. Overall, epibiont infestation prevalence (i.e. colonies/host area) and load (i.e. zooids/host area) were higher on copepodites than on adults for both host species, suggesting a preferential attachment to juveniles, or a higher predation pressure on adult stages. Infestation density and loads of both epibiont species were higher on the cephalothorax and abdomen of A. tonsa and E. affinis in comparison to the antennae and swimming legs, suggesting that ciliates can more easily colonize less active parts of the host.     

Distribution of the cestode Caryophyllaeides fennica (Cestoda: Lytocestidae) in the population of cyprinid fish in the Rybinsk water reservoir

The distribution of the cestode Caryophyllaeides fennica in populations of the ide, roach, white bream and bream from the Rybinsk reservoir has been analyzed in regard to fish age. The relative abundance of C. fennica in populations of each host species was calculated as product of the helminthes abundance by the fish number in age groups. The highest prevalence and abundance of cestodes was found in the ide. All age classes of this species were infected, with the maximum in fishes of the age 3+ ... 5+. The roach is infected with C. fennica till only 10-year age; brean is infected till the age 4+. In the white bream C. fennica is an occasional parasite. The cestode number among hosts was as follows: bream--68%; roach--26%, ide--5%, white bream--1%. Different approaches to the estimation of the parasite abundance distribution among several host species (in terms of mean prevalence and intensity of the relative abundance of parasites) are discussed.     

A temporal dilution effect: hantavirus infection in deer mice and the intermittent presence of voles in Montana

The effect of intermittently occurring, non-reservoir host species on pathogen transmission and prevalence in a reservoir population is poorly understood. We investigated whether voles, Microtus spp., which occur intermittently, influenced estimated standing antibody prevalence (ESAP) to Sin Nombre hantavirus (SNV, Bunyaviridae: Hantavirus) among deer mice, Peromyscus maniculatus, whose populations are persistent. We used 14 years of data from central Montana to investigate whether ESAP among deer mice was related to vole presence or abundance while controlling for the relationship between deer mouse abundance and ESAP. We found a reduction in deer mouse ESAP associated with the presence of voles, independent of vole abundance. A number of studies have documented that geographic locations which support a higher host diversity can be associated with reductions in pathogen prevalence by a hypothesized dilution effect. We suggest a dilution effect may also occur in a temporal dimension at sites where host richness fluctuates. Preservation of host diversity and optimization of environmental conditions which promote occurrence of ephemeral species, such as voles, may result in a decreased ESAP to hantaviruses among reservoir hosts. Our results may extend to other zoonotic infectious diseases.     

Epidemiological prediction of the distribution of insects of medical significance: comparative distributions of fleas and sucking lice on the rat host Rattus norvegicus in Yunnan Province, China

Determining the distribution patterns of ectoparasites is important for predicting the spread of vector-borne diseases. A simple epidemiological model was used to compare the distributions of two different taxa of ectoparasitic insects, sucking lice (Insecta: Siphonaptera) and fleas (Insecta: Anoplura), on the same rodent host, Rattus norvegicus Berkenhout (Rodentia: Muridae), in Yunnan Province, China. Correlations between mean abundance and prevalence were determined. Both fleas and sucking lice were aggregated on their hosts, and sucking lice showed a higher degree of aggregation than fleas. The prevalence of both fleas and sucking lice increased with log-transformed mean abundance and a highly linear correlation and modelling efficiency of predicted prevalence against observed prevalence were obtained. The results demonstrate that prevalence can be explained simply by mean abundance.     

Patterns of parasite abundance and distribution in island populations of Galápagos endemic birds

Parasite life-history characteristics, the environment, and host defenses determine variation in parasite population parameters across space and time. Parasite abundance and distribution have received little attention despite their pervasive effects on host populations and community dynamics. We used analyses of variance to estimate the variability of intensity, prevalence, and abundance of 4 species of lice (Insecta: Phthiraptera) infecting Galápagos doves and Galápagos hawks and 1 haemosporidian parasite (Haemosporida: Haemoproteidae) infecting the doves across island populations throughout their entire geographic ranges. Population parameters of parasites with direct life cycles varied less within than among parasite species, and intensity and abundance did not differ significantly across islands. Prevalence explained a proportion of the variance (34%), similar to infection intensity (33%) and parasite abundance (37%). We detected a strong parasite species-by-island interaction, suggesting that parasite population dynamics is independent among islands. Prevalence (up to 100%) and infection intensity (parasitemias up to 12.7%) of Haemoproteus sp. parasites varied little across island populations.     

Relative importance of demographics, locale, and seasonality underlying louse and flea parasitism of raccoons (Procyon lotor)

A variety of demographic, seasonal, and site-specific variables may influence parasitism, but the relative importance of these variables is generally unclear. We measured the relative ability of host characteristics, season, and site to explain louse (Trichodectes octomaculatus) and flea (Orchopeas howardi) infestation across 10 populations of raccoons (Procyon lotor). Lice are highly dependent on specific hosts and are predicted to display a relatively strong relationship with factors intrinsic to the host, when compared to fleas, which can infest multiple species and survive off-host for weeks without feeding. We developed a priori models that represented explicit hypotheses and contrasted their ability to predict infestation patterns. While the abundance of lice was seasonal, models that included solely host age and sex best predicted prevalence and abundance, in part because males were infested with 3 times the number of lice than were females. Conversely, flea prevalence and abundance, which peaks sharply in the spring, was best predicted by season; factors intrinsic to the host were relatively unimportant for predicting abundance. These, and other, recent findings emphasize the need to simultaneously assess the relative importance of multiple ecological variables between parasite species when attempting to describe general trends and constraints of host-parasite associations.     

Spatial variation in population density across the geographical range in helminth parasites of yellow perch Perca flavescens

The abundance of a species is not constant across its geographical range; it has often been assumed to decrease from the centre of a species’ range toward its margins. The central assumption of this “favourable centre” model is tested for the first time with parasites, using different species of helminth parasites exploiting fish as definitive hosts. Data on prevalence (percentage of hosts that are infected) and abundance (mean no. parasites per host) were compiled for 8 helminth species occurring in 23 populations of yellow perch Perca flavescens, from continental North America. For each parasite species, correlations were computed between latitude and both local prevalence and abundance values. In addition, the relationships between the relative prevalence or abundance in one locality and the distance between that locality and the one where the maximum value was reported, were assessed separately for each species to determine whether abundance tends to decrease away from the presumed centre of the range, where it peaks. For both the cestode Proteocephalus pearsei and the acanthocephalan Leptorhynchoides thecatus, there was a positive relationship between prevalence or abundance and the latitude of the sampled population. There was also a significant negative relationship between relative prevalence and the distance from the locality showing the maximum value in P. pearsei, but no such pattern was observed for the other 7 parasite species. Since this single significant decrease in prevalence with increasing distance from the peak value may be confounded by a latitudinal gradient, it appears that the distribution of abundance in parasites of perch does not follow the favourable centre model. This means that the environmental variables affecting the density of parasites (host availability, abiotic conditions) do not show pronounced spatial autocorrelation, with nearby sites not necessarily providing more similar conditions for the growth of parasite populations than distant sites.     

Spatial patterns of distribution and abundance of Harrisia portoricensis,an endangered Caribbean cactus

Aims The spatial distribution of biotic and abiotic factors may play a dominant role in determining the distribution and abundance of plants in arid and semiarid environments. In this study, we evaluated how spatial patterns of microhabitat variables and the degree of spatial dependence of these variables influence the distribution and abundance of the endangered cactus Harrisia portoricensis.Methods We used geostatistical analyses of five microhabitat variables (e.g. vegetation cover, soil cover and light incidence) and recorded the abundance of H. portoricensis in 50 permanent plots established across Mona Island, Puerto Rico, by the United States Department of Agriculture Forest Service as part of the Forest Inventory and Analysis (USDA–FIA). We also used partial Mantel tests to evaluate the relationships between microhabitat variables and abundance of H. portoricensis, controlling for spatial autocorrelation.Important findings Abundance of H. portoricensis showed strong affinities with microhabitat variables related to canopy structure, soil cover and light environment. The distribution of this cactus species throughout the island was consistent with the spatial variation patterns of these variables. In general, landscape-level analyses suggested a predictive value of microhabitat traits for the distribution and abundance of this endangered species. For sensitive cacti species, wherein abundance may be influenced by similar variables, these types of analyses may be helpful in developing management plans and identifying critical habitats for conservation.     

In vivo and in vitro complementation study comparing the function of DnaK chaperone systems from halophilic lactic acid bacterium Tetragenococcus halophilus and Escherichia coli

In this study, we characterized the DnaK chaperone system from Tetragenococcus halophilus, a halophilic lactic acid bacterium. An in vivo complementation test showed that under heat stress conditions, T. halophilus DnaK did not rescue the growth of the Escherichia coli dnaK deletion mutant, whereas T. halophilus DnaJ and GrpE complemented the corresponding mutations of E. coli. Purified T. halophilus DnaK showed intrinsic weak ATPase activity and holding chaperone activity in vitro, but T. halophilus DnaK did not cooperate with the purified DnaJ and GrpE from either T. halophilus or E. coli in ATP hydrolysis or luciferase-refolding reactions under the conditions tested. E. coli DnaK, however, cross-reacted with those from both bacteria. This difference in the cooperation with DnaJ and GrpE appears to result in an inability of T. halophilus DnaK to replace the in vivo function of the DnaK chaperone of E. coli.     

Cryptic microsporidian parasites differentially affect invasive and native Artemia spp.

We investigated the host specificity of two cryptic microsporidian species (Anostracospora rigaudi and Enterocytospora artemiae) infecting invasive (Artemia franciscana) and native (Artemia parthenogenetica) hosts in sympatry. Anostracospora rigaudi was on average four times more prevalent in the native host, whereas E. artemiae was three times more prevalent in the invasive host. Infection with An. rigaudi strongly reduced female reproduction in both host species, whereas infection with E. artemiae had weaker effects on female reproduction. We contrasted microsporidian prevalence in native A. franciscana populations (New World) and in both invaded and non-invaded Artemia populations (Old World). At a community level, microsporidian prevalence was twice as high in native compared with invasive hosts, due to the contrasting host-specificity of An. rigaudi and E. artemiae. At a higher biogeographical level, microsporidian prevalence in A. franciscana did not differ between the invaded populations and the native populations used for the introduction. Although E. artemiae was the only species found both in New and Old World populations, no evidence of its co-introduction with the invasive host was found in our experimental and phylogeographic tests. These results suggest that the success of A. franciscana invasion is probably due to a lower susceptibility to virulent microsporidian parasites rather than to decreased microsporidian prevalence compared with A. parthenogenetica or to lower microsporidian virulence in introduced areas.     

Impact of waist circumference on cardiac phenotype in hypertensives according to gender

Our aim was to assess the differential effect of waist circumference on left-ventricular (LV) structural and functional alterations, in hypertensive males and females. One thousand seven hundred and eighty nine consecutive, nondiabetic, essential hypertensives (aged 55.8 +/- 13.5 years, 966 females), included in the 3H Study, an ongoing registry of hypertension-related-target-organ damage, were classified to obese and nonobese groups according to Adult Treatment Panel III criteria. All participants underwent complete echocardiographic study including LV diastolic function evaluation by means of conventional and tissue Doppler imaging (TDI) methods, averaging early and late diastolic mitral annular peak velocities (Em, Am, Em/Am) from four separate sites of measurement. Hypertensive obese women compared with nonobese exhibited significantly greater LV mass index and prevalence of LV hypertrophy (by 5.5 g/m(2), P = 0.003, and 8.8%, P = 0.005, respectively), while such differences were not present among men. Obese women compared to nonobese ones were accompanied by lower transmitral E/A (by 0.08, P < 0.001), TDI-derived Em/Am (by 0.12, P < 0.001), and higher E/Em ratio (by 0.8, P = 0.016). In contrast, hypertensive obese men compared to nonobese ones exhibited lower E and Em (by 0.04 m/s and 0.6 cm/s, both P < 0.05). A significant interaction between sex and abdominal obesity was observed only regarding TDI-derived Am and Em/Am. Furthermore, waist circumference was a predictor of E/A (beta = -0.097, P = 0.002) and Em/Am (beta = -0.116, P = 0.001), independently of body size, in females but not in males. The adverse effect of abdominal obesity on LV alterations is more pronounced among female hypertensives, suggesting that routine measurement of waist circumference provides additional information on cardiac phenotype especially in women.     

Ecological and biological factors involved in the transmission of Echinococcus multilocularis in the French Ardennes

In order to identify the respective importance of the ecological and biological factors involved in the transmission of Echinococcus multilocularis, we estimated grassland vole intermediate host (Microtus sp. and Arvicola terrestris) population densities, in relation to the diet of the definitive host (red fox, Vulpes vulpes) and with the prevalence of E. multilocularis in the fox population. The study was conducted in the Ardennes, north-eastern France, which is an area with a high incidence of alveolar echinococcosis. Surface index methods showed that Microtus was the most abundant intermediate host in the area. Furthermore, Microtus was present in one-third of the 144 faeces and 98 stomach content samples examined and represented more than two-thirds of the rodent occurrences. Red fox predation on Microtus was significantly correlated with Microtus relative abundance. In contrast, the relative abundance of A. terrestris was very low. This species, as well as Clethrionomys glareolus and Apodemus sp., was little consumed. E. multilocularis prevalence in foxes was determined from carcasses and reached 53% (95% confidence interval 45-61%). Intensity of infection varied from 2 to 73,380 worms per fox, with 72% of the sampled worm burden harboured by 8% of the sampled foxes. The selected explanatory variables (sex, year, age class, health and nutritional condition, and season) failed to predict prevalence rate and worm burden. The high prevalence rate in foxes indicates the possibility of intense E. multilocularis transmission, apart from periods, or in landscapes, favourable to large population outbreaks of grassland rodents.     

Introduced pathogens follow the invasion front of a spreading alien host

1.?When an invasive species first colonizes an area, there is an interval before any host-specific natural enemies arrive at the new location. Population densities of newly invading species are low, and the spatial and temporal interactions between spreading invasive species and specific natural enemies that follow are poorly understood. 2.?We measured infection rates of two introduced host-specific pathogens, the entomophthoralean fungus Entomophaga maimaiga and the baculovirus Lymantria dispar nucleopolyhedrovirus (LdNPV), occurring in spreading populations of an invasive forest defoliator, L. dispar (gypsy moth), in central Wisconsin. 3.?Over 3 years, we found that host density was closely associated with the presence and prevalence of both pathogens. The fungal and viral pathogens differed in the sensitivity of their response as E. maimaiga was present in lower-density host population than LdNPV. 4.?We examined the relationship between weather conditions and infection prevalence and found that activity of both the fungus and virus was strongly seasonally influenced by temperature and rainfall or temperature alone, respectively. 5.?Purposeful releases of pathogens (median distances of study sites from release sites were 65·2 km for E. maimaiga and 25·6 km for LdNPV) were not associated with pathogen prevalence. 6.?A generalist fly parasitoid, Compsilura concinnata, also killed L. dispar larvae collected from the study sites, and parasitism was greater when infection by pathogens was lower. 7.?Our results demonstrated that although infection levels were low in newly established host populations, host-specific pathogens had already moved into host populations close behind advancing populations of an invasive host; thus, spreading hosts were released from these enemies for only a relatively short time.     

Distribution patterns of Temnocephalids commensal with Crustacea and Mollusca from Argentina

An analysis and a comparison of abundance, prevalence and distribution patterns for three temnocephalid species are presented. Three temnocephalid populations from Río de la Plata river, República Argentina, were studied: Temnocephala digitata Monticelli, 1902 associated with Palaemonetes argentinus Nobilli, 1901 (Crustacea Caridea), T. axenos Monticelli, 1899 associated with Aegla uruguayana Schmitt, 1942, and A. platensis Schmitt, 1942 (Crustacea Anomura) and T. iheringi Haswell, 1893 associated with Pomacea canaliculata (Lamarck, 1822) (Mollusca Gastropoda). These host species provide different environments and consequently different opportunities for the development of their symbionts. Increasing gradients of abundance and prevalence, and a decrease in the aggregation index (k) were observed. The distribution pattern of the three symbiotic species is aggregated. The original distributions were compared with the parameters of the negative binomial. The observed overdispersion may be due to heterogeneity of the host population and reproduction of the commensal within the host. This revised version was published online in July 2006 with corrections to the Cover Date.     

The role of vole populations in prevalence of the parasite (Echinococcus multilocularis) in foxes

Effects of population fluctuation of the gray-sided vole(Clethrionomys rufocanus) on the prevalence (infection rates) of the parasiteEchinococcus multilocularis in red fox(Vulpes vulpes) populations was investigated from 1985 to 1992 in eastern Hokkaido (Abashiri, Nemuro, and Kushiro area), Japan. This parasite needs two hosts to complete its life cycle; the gray-sided vole as its intermediate host and the red fox as its final host. We found that: (1) Infection rates in foxes depended on the current-year abundance of voles in all three study areas, particularly in Abashiri. (2) In addition to this direct density-dependence, delayed density-dependence between the infection rate and the prior-year abundance of voles was detected in Nemuro and in Kushiro. (3) The regional differences in density-dependence pattern were related to regional differences in the winter food habits of red foxes: in Abashiri the proportion of voles in the fox’s diet greatly decreases in winter, while the proportion remains high in winter in Nemuro and in Kushiro, probably because of shallower snowpack. These results suggest that infection rates in foxes in Abashiri were less influenced by the prior-year prevalence, since the infection cycle might be interrupted in winter, when voles became less important in fox’s diet. In contrast, the state of the prevalence may carry over from year to year in Nemuro and in Kushiro, because red foxes continue to eat a considerable amount of voles throughout year. The regionally contrasted results for the relationship between infection rate in foxes and vole abundance were parallel to the regional difference in fluctuation pattern of vole populations, which are highly variable in Abashiri area, but less variable in Kushiro-Nemuro area. Drastic change in vole populations appears to affect the host-parasite system.     

Cuterebrid parasitism (Rogenhofera bonaerensis) on the shrubland mouse (Akodon molinae), in Argentina.

We evaluated the influence of temperature, rainfall, and host relative abundance on Rogenhofera bonaerensis (Diptera, Cuterebridae) parasitism prevalence in shrubland mouse (Akodon molinae) populations in central Argentina, from February 1983 to December 1987. Parasitism did not vary significantly with host age: juvenile-subadults (32%), adults (26%), and old adults (29%). Females were more frequently parasitized (36%) than were males (20%). There was no correlation between parasitism and reproductive activity. Infested hosts were recorded most commonly in summer (January to March, 19%), and in fall (April to June, 30%). During the dry season, July through November, cuterebrid parasitism averaged only 3%. The monthly prevalence of parasitism throughout the year was not associated either with monthly precipitation nor with mean monthly temperature at the time of sampling. But a 2 to 3 month time-lag effect of both climatic variables on parasitism was recorded. Bot fly prevalence was correlated to an index of host density. We propose climate and host availability as important factors affecting bot fly parasitism in the semiarid shrubland of central Argentina.