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Season had a strong influence for both LL and NN (Tables 1 and 2), with lowest records in autumn and greatest in summer (Figs. 1 and 2), although in the second year the larval A. triste burden was slightly greater in spring (Fig. 1).Larval A. triste burdens were generally greater in sites with cattle than in those without, but this effect was largely dependent on rodent abundance (Table 1). When the number of rodents was low, the difference was very large, but it rapidly decreased as rodent abundance increased, to the point of larval abundance becoming greater at sites without cattle at the highest rodent abundance records (Fig. 3). Independently of other studied variables, NN showed significantly greater mean abundance in sites with cattle.Larval A. triste burdens were greater in sites with implanted forests than in natural grasslands, but this was only observed in individuals that also had I. loricatus (Table 1, Fig. 4). The difference became more significant and exponentially larger the more I. loricatus larvae were co-parasitizing the host (Fig. 4).Females had greater larval A. triste burdens than males, but this difference was only observed among individuals parasitized by lice. The more lice were co-infecting the host the greater the female-biased parasitism (Table 1, Fig. 5).Natural dub ubiquitin were positively correlated with LL (Table 1).Body length was strongly negatively correlated with nymphal tick burden, but only for individuals that were parasitized with I. loricatus (Table 2, Fig. 2). The larger the I. loricatus burden, the stronger the negative correlation. Body condition was also associated with nymphal A. triste burden, but again this depended on co-infestation with I. loricatus (Table 2, Fig. 2).Fleas and lice were associated with larval A. triste abundance, but the magnitude and direction of their associations depended on each other (Table 1, Fig. 5). As flea burdens increased, LL also increased, but this was reversed if lice were present (Fig. 5). Similarly, but only in females, louse burden was strongly positively associated with A. triste LL, but especially when no or few fleas were present (Fig. 5). In individuals with larger flea burdens the association reversed. In males, lice were not associated with LL, unless they had fleas, in which case the association was negative (Fig. 5). Ixodes loricatus was positively associated with LL, but only in implanted forests (Table 1, Fig. 4). Regarding NN, I. loricatus was found to be strongly associated and explained much of the variability in nymphal tick abundance. It also interacted with three of the other significant independent variables (see significant interaction terms in Table 2). As I. loricatus burdens increased, the mean abundance of NN was higher, especially in spring, and in small (hence young) individuals.DiscussionThe ecology of A. triste does not appear to be simple. Along the Parana River, a corridor where ecological conditions for A. triste seem to be favorable, its distribution is not consistent with what might be predicted; the tick was present in unexpected localities, and absent in many areas where its presence was presumed (Guglielmone et al., 2013). To better understand A. triste ecology, we assessed diverse factors associated with this tick occurrence in one of the main hosts of immature stages, A. azarae, in an area where the tick is known to occur.Host factors may be important drivers of the population dynamics of ticks, as they can influence the probability of encounter with a questing tick and/or affect the performance of the parasite on a host (Brunner and Ostfeld 2008). It has been found that, in rodents, testosterone depresses the innate and acquired resistance to Ixodes ricinus, which could result in sex-biased parasitism (Hughes and Randolph, 2001). This may explain that the patterns reported for ixodid tick parasitism in rodents are in general male-biased (Hughes and Randolph 2001; Soliman et al., 2001; Krasnov et al., 2005; Boyard et al., 2008; Harrison et al., 2010; Krasnov et al., 2012; Lutermann et al., 2015). However, field data from A. azarae failed to find a difference between I. loricatus burdens of males and females (Beldomenico et al., 2005; Colombo et al., 2014a,b). Moreover, here we found no effect of sex among individuals with null or very low louse infestations, whereas females infected with lice had greater burdens of A.triste than males with similar louse burdens, which runs counter the hypothesis of immunological handicap of males. This is further supported by the positive association between LL and NAb. It is worth mentioning that a raw analysis (uni-variable), comparing males and females, produces an apparent male-biased result: mean larval abundance in males was 2.7, but only reached 1.2 in females. This highlights the importance of controlling for potential confounders and effect modifiers when conducting analysis of field data.