Eliot Roman a publié une note il y a 6 jours et 1 heure
The age of a rodent is highly correlated with its size (Zullinger et al., 1984). As in observational studies (like this one), age cannot be easily estimated. We used ‘body length’ as a factor indicating both indivisible variables: size and age. Although host age is considered a key factor for host-parasite interactions (Soliman et al., 2001; Hudson et al., 2002; Brunner and Ostfeld, 2008; Cardon et al., 2011), previous studies assessing associations between age and tick burdens in wild rodents failed to find an association (Beldomenico et al., 2005; Hawlena et al., 2006). However, we found a strong negative correlation between body length and NN loads, but again conditional on the parasitism by other ectoparasite. Burdens of NN increased with age/size, but this association was stronger the greater the burden of I. loricatus. This, again, might reveal an interaction between ectoparasites, in this case taking place predominantly at small sizes/young ages.Host body condition was found associated with NN counts, but, once again, this depended on other ectoparasite. The larger the I. loricatus burden, the greater the negative correlation between body condition and NN. Larger burdens of parasites can cause the body condition to decline by extracting resources from the host (Whiteman and Parker, 2004) or due to dub inhibitor invested in the immune response (Sheldon and Verhulst, 1996), especially when other parasite is acting synergistically to cause physiological deterioration (Marzal et al., 2008). On the other hand, a host with poor body condition might have dampened resistance to parasites (Nelson et al., 1975; Simon et al., 2003; Krasnov et al., 2005). Longitudinal or experimental approaches are needed to confirm the cause-effect direction of this association, as well as to evaluate the possibility that they are engaged in a circular synergistic process where both are cause and effect (Beldomenico and Begon, 2010).To assess whether the immune system of the host was associated with A. triste burdens, we used generic indicators of cellular (WBC) and humoural (NAb) immunity. Only NAb were positively correlated with LL burdens. NAb provide unspecific humoural immunity independent of antigenic stimulation and are considered to be stable over time (Ochsenbein and Zinkernagel, 2000). This confers them potential to be good indicators of immune competence in wild animals (Racca et al., 2014), but it has not been established as yet whether they co-vary with specific antibodies, or there are trade-offs between innate and specific immunity, or what is the behaviour of NAb under particular physiological conditions. For example, while antibody production is known to be suppressed by chronic stress (Padgett and Glaser, 2003), it has been shown in capybaras that prolonged food restriction increases NAb levels (Eberhardt et al., 2013). Therefore, the elucidation of the meaning of this association requires further investigation of the biology of NAb.Relating to environmental factors, we found that burdens of A. triste on A. azarae were strongly associated with the environmental variables studied. The seasonality of A. triste was previously studied in Uruguay (Venzal et al., 2008) and in the same locality where the current study was performed (Nava et al., 2011), and in general terms, our results are in agreement with the seasonal pattern described by Venzal et al. (2008) and Nava et al. (2011). In addition, including season in our analysis allowed us to control for the potential confounding and effect modification that may result if that variable is ignored.Regarding host abundance, with many hosts collecting questing ticks, the burden on each is expected to decrease (Sorci et al., 1997; Stanko et al., 2002; Brunner and Ostfeld, 2008). At the same time, because adult stages of A. triste use different hosts than LL and NN, there is opportunity for intricate interactions to occur. A paucity of suitable hosts for immature stages would render few questing adults to infest larger mammals. Conversely, lack of suitable hosts for adult stages would result in low infestation by immature ticks in their rodent hosts, independently on their abundance. In turn, more hosts for adult stages would represent greater chance for increasing the tick reproductive output, and consequently there will be more questing larvae seeking for rodent hosts. Studies of systems with a tick that has a similar dissociation in host preference, Ixodes scapularis, showed that the burden of LL and NN on rodent hosts depend on the density of the preferred host for adult stages, Odocoileus virginianus (Stafford, 1993; Kilpatrick et al., 2014). In the Parana Delta region, the hosts recorded for adult stages of A. triste were B. dichotomus, H. hydrochaeris, cattle, horses and dogs (Nava et al., 2011). Of those domestic animals, cattle are frequently parasitized by A. triste adults in the study area (PM Beldomenico, unpublished data). Engorged females feeding on cattle successfully lay eggs (Nava et al., 2011). Our findings show that the presence of cattle was positively correlated with A. triste burdens, both for LL and NN. For larvae, the effect of cattle depended on the abundance of rodents. When rodent trappability was low, larval A. triste burdens were much higher in sites with cattle, but this difference disappeared and tended to reverse as rodent captures increased. The interaction between cattle and rodent abundance might be explained by a dilution of existing questing immature stages as their host populations grow, but the burdens where cattle was absent were independent of rodent density, perhaps indicating low availability of suitable hosts for adult stages. In the study area, B. dichotomus and H. hydrochaeris are among the few large wild mammals species known to be suitable hosts for A. triste (Nava et al., 2011). Despite there is no available data neither on the densities of B. dichotomus and H. hydrochaeris, nor information about their tick burdens during the development of the present study, we can speculate that these hosts may play an important role in the ecology of A. triste. However, from anecdotal evidence and direct observation, cows are at higher densities than deer and capybaras in the study area; so, the effect of these domestic animals on A. triste burdens could be stronger. More studies integrating data from marsh deer, capybara, cattle, rodents and questing ticks are needed to understand the whole scenario.