Warm winters are associated to more intense West Nile virus circulation in southern Spain.

West Nile virus (WNV) is the most widely distributed mosquito-borne flavivirus in the world. This flavivirus can infect humans causing in some cases a fatal neurological disease and birds are the main reservoir hosts. WNV is endemic in Spain, and human cases have been reported since 2004. Although different studies analyse how climatic conditions can affect the dynamics of WNV infection, very few use long-term datasets. Between 2003 and 2020 a total of 2,724 serum samples from 1,707 common coots (Fulica atra) were analysed for the presence of WNV-specific antibodies. Mean (SD) annual seroprevalence was 24.67% (0.28) but showed high year-to-year variations ranging from 5.06% (0.17) to 68.89% (0.29). Significant positive correlations (p < 0.01) were observed between seroprevalence and maximum winter temperature and mean spring temperature. The unprecedented WNV outbreak in humans in the south of Spain in 2020 was preceded by a prolonged period of escalating WNV local circulation. Given current global and local climatic trends, WNV circulation is expected to increase in the next decades. This underscores the necessity of implementing One Health approaches to reduce the risk of future WNV outbreaks in humans. Our results suggest that higher winter and spring temperatures may be used as an early warning signal of more intense WNV circulation among wildlife in Spain, and consequently highlight the need of more intense vector control and surveillance in human inhabited areas.

Modelling the spatial risk of malaria through probability distribution of Anopheles maculipennis s.l. and imported cases.

Malaria remains one of the most important infectious diseases globally due to its high incidence and mortality rates. The influx of infected cases from endemic to non-endemic malaria regions like Europe has resulted in a public health concern over sporadic local outbreaks. This is facilitated by the continued presence of competent Anopheles vectors in non-endemic countries.We modelled the potential distribution of the main malaria vector across Spain using the ensemble of eight modelling techniques based on environmental parameters and the Anopheles maculipennis s.l. presence/absence data collected from 2000 to 2020. We then combined this map with the number of imported malaria cases in each municipality to detect the geographic hot spots with a higher risk of local malaria transmission.The malaria vector occurred preferentially in irrigated lands characterized by warm climate conditions and moderate annual precipitation. Some areas surrounding irrigated lands in northern Spain (e.g. Zaragoza, Logroño), mainland areas (e.g. Madrid, Toledo) and in the South (e.g. Huelva), presented a significant likelihood of A. maculipennis s.l. occurrence, with a large overlap with the presence of imported cases of malaria.While the risk of malaria re-emergence in Spain is low, it is not evenly distributed throughout the country. The four recorded local cases of mosquito-borne transmission occurred in areas with a high overlap of imported cases and mosquito presence. Integrating mosquito distribution with human incidence cases provides an effective tool for the quantification of large-scale geographic variation in transmission risk and pinpointing priority areas for targeted surveillance and prevention.

Re-Emergence of a West Nile Virus (WNV) Variant in South Spain with Rapid Spread Capacity.

West Nile Virus (WNV) is a mosquito vector-borne zoonosis with an increasing incidence in Europe that has become a public health concern. In Spain, although local circulation has been known for decades, until 2020, when a large outbreak occurred, West Nile Virus cases were scarce and mostly occurred in southern Spain. Since then, there have been new cases every year and the pathogen has spread to new regions. Thus, monitoring of circulating variants and lineages plays a fundamental role in understanding WNV evolution, spread and dynamics. In this study, we sequenced WNV consensus genomes from mosquito pools captured in 2022 as part of a newly implemented surveillance program in southern Spain and compared it to other European, African and Spanish sequences. Characterization of WNV genomes in mosquitoes captured in 2022 reveals the co-circulation of two WNV lineage 1 variants, the one that caused the outbreak in 2020 and another variant that is closely related to variants reported in Spain in 2012, France in 2015, Italy in 2021-2022 and Senegal in 2012-2018. The geographic distribution of these variants indicates that WNV L1 dynamics in southern Europe include an alternating dominance of variants in some territories.

Two avian Plasmodium species trigger different transcriptional responses on their vector Culex pipiens.

Malaria is a mosquito-borne disease caused by protozoans of the genus Plasmodium that affects both humans and wildlife. The fitness consequences of infections by avian malaria are well known in birds, however, little information exists on its impact on mosquitoes. Here we study how Culex pipiens mosquitoes transcriptionally respond to infection by two different Plasmodium species, P. relictum and P. cathemerium, differing in their virulence (mortality rate) and transmissibility (parasite presence in exposed mosquitoes' saliva). We studied the mosquito response to the infection at three critical stages of parasite development: the formation of ookinetes at 24 h post-infection (hpi), the release of sporozoites into the hemocoel at 10 days post-infection (dpi), and the storage of sporozoites in the salivary glands at 21 dpi. For each time point, we characterized the gene expression of mosquitoes infected with each P. relictum and P. cathemerium and mosquitoes fed on an uninfected bird and, subsequently, compared their transcriptomic responses. Differential gene expression analysis showed that most transcriptomic changes occurred during the early infection stage (24 hpi), especially when comparing P. relictum and P. cathemerium-infected mosquitoes. Differentially expressed genes in mosquitoes infected with each species were related mainly to the metabolism of the immune response, trypsin, and other serine-proteases. We conclude that these differences in response may partly play a role in the differential virulence and transmissibility previously observed between P. relictum and P. cathemerium in Cx. pipiens.

Long-term serological surveillance for West Nile and Usutu virus in horses in south-West Spain.

West Nile virus (WNV) is a re-emerging zoonotic pathogen with increasing incidence in Europe, producing a recent outbreak in 2020 in Spain with 77 human cases and eight fatalities. However, the factors explaining the observed changes in the incidence of WNV in Europe are not completely understood. Longitudinal monitoring of WNV in wild animals across Europe is a useful approach to understand the eco-epidemiology of WNV in the wild and the risk of spillover into humans. However, such studies are very scarce up to now. Here, we analysed the occurrence of WNV and Usutu virus (USUV) antibodies in 2102 samples collected between 2005 and 2020 from a population of feral horses in Doñana National Park. The prevalence of WNV antibodies varied between years, with a mean seroprevalence of 8.1% (range 0%-25%) and seasonally. Climate conditions including mean minimum annual temperatures and mean rainy days per year were positively correlated with WNV seroprevalence, while the annual rainfall was negatively. We also detected the highest incidence of seroconversions in 2020 coinciding with the human outbreak in southern Spain. Usutu virus-specific antibodies were detected in the horse population since 2011. The WNV outbreak in humans was preceded by a long period of increasing circulation of WNV among horses with a very high exposure in the year of the outbreak. These results highlight the utility of One Health approaches to better understand the transmission dynamics of zoonotics pathogens.

Genomic Analysis of West Nile Virus Lineage 1 Detected in Mosquitoes during the 2020-2021 Outbreaks in Andalusia, Spain.

Emerging infectious diseases are one of the most important global health challenges because of their impact on human and animal health. The vector-borne West Nile virus (WNV) is transmitted between birds by mosquitos, but it can also infect humans and horses causing disease. The local circulation of WNV in Spain has been known for decades, and since 2010, there have been regular outbreaks in horses, although only six cases were reported in humans until 2019. In 2020, Spain experienced a major outbreak with 77 human cases, which was followed by 6 additional cases in 2021, most of them in the Andalusian region (southern Spain). This study aimed to characterize the genomes of the WNV circulating in wild-trapped mosquitoes during 2020 and 2021 in Andalusia. We sequenced the WNV consensus genome from two mosquito pools and carried out the phylogenetic analyses. We also compared the obtained genomes with those sequenced from human samples obtained during the outbreak and the genomes obtained previously in Spain from birds (2007 and 2017), mosquitoes (2008) and horses (2010) to better understand the eco-epidemiology of WNV in Spain. As expected, the WNV genomes recovered from mosquito pools in 2020 were closely related to those recovered from humans of the same outbreak. In addition, the strain of WNV circulating in 2021 was highly related to the WNV strain that caused the 2020 outbreak, suggesting that WNV is overwintering in the area. Consequently, future outbreaks of the same strain may occur in in the future.

A One Health view of the West Nile virus outbreak in Andalusia (Spain) in 2020.

Reports of West Nile virus (WNV) associated disease in humans were scarce in Spain until summer 2020, when 77 cases were reported, eight fatal. Most cases occurred next to the Guadalquivir River in the Sevillian villages of Puebla del Río and Coria del Río. Detection of WNV disease in humans was preceded by a large increase in the abundance of Culex perexiguus in the neighbourhood of the villages where most human cases occurred. The first WNV infected mosquitoes were captured approximately one month before the detection of the first human cases. Overall, 33 positive pools of Cx. perexiguus and one pool of Culex pipiens were found. Serology of wild birds confirmed WNV circulation inside the affected villages, that transmission to humans also occurred in urban settings and suggests that virus circulation was geographically more widespread than disease cases in humans or horses may indicate. A high prevalence of antibodies was detected in blackbirds (Turdus merula) suggesting that this species played an important role in the amplification of WNV in urban areas. Culex perexiguus was the main vector of WNV among birds in natural and agricultural areas, while its role in urban areas needs to be investigated in more detail. Culex pipiens may have played some role as bridge vector of WNV between birds and humans once the enzootic transmission cycle driven by Cx. perexiguus occurred inside the villages. Surveillance of virus in mosquitoes has the potential to detect WNV well in advance of the first human cases.

Widespread resistance to anticoagulant rodenticides in Mus musculus domesticus in the city of Barcelona.

Control of rodent populations is a big challenge because of the rapid evolution of resistance to commonly used rodenticides and the collateral negative impacts that these products may have on biodiversity. Second-generation anticoagulants are very efficient but different single nucleotide polymorphisms (SNPs) in the Vkorc1 gene may confer resistance in rodents. We sequenced exons 1, 2 and 3 of the Vkorc1 gene from 111 mice (Mus musculus domesticus) captured across the city of Barcelona and found SNPs associated with resistance to first- and second-generation anticoagulants in all of them. Although most of the SNPs were associated with resistance to bromadiolone, we also found SNPs associated with resistance to brodifacoum. Out of all the individuals analyzed, 94.59 % carried mutations associated to introgression events with Mus spretus, a sympatric rodent species. Currently most of the chemical products for rodent control commercialized in the area are based on bromadiolone, although recent public control campaigns have already shifted to other products. Thus, the widespread occurrence of resistant mice to bromadiolone represents a challenge for rodent control in Barcelona and may increase the risk of secondary poisoning of animals preying on this species. Public health managers, pest control companies and citizens should be aware that the use of bromadiolone based products is ineffective and represents a risk for the environment, including human and animal health.

Implications of diet on mosquito life history traits and pathogen transmission.

The environment, directly and indirectly, affects many mosquito traits in both the larval and adult stages. The availability of food resources is one of the key factors influencing these traits, although its role in mosquito fitness and pathogen transmission remains unclear. Larvae nutritional status determines their survivorship and growth, having also an impact on adult characteristics like longevity, body size, flight capacity or vector competence. During the adult stage, mosquito diet affects their survival rate, fecundity and host-seeking behaviour. It also affects mosquito susceptibility to infection, which may determine the vectorial capacity of mosquito populations. The aim of this review is to critically revise the current knowledge on the effects that both larval and adult quantity and quality of the diet have on mosquito life history traits, identifying the critical knowledge gaps and proposing future research lines. The quantity and quality of food available through their lifetime greatly determine adult body size, longevity or biting frequency, therefore affecting their competence for pathogen transmission. In addition, natural sugar sources for adult mosquitoes, i.e., specific plants providing high metabolic energy, might affect their host-seeking and vertebrate biting behaviour. However, most of the studies are carried out under laboratory conditions, highlighting the need for studies of feeding behaviour of mosquitoes under field conditions. This kind of studies will increase our knowledge of the impact of diets on pathogen transmission, helping to develop successful control plans for vector-borne diseases.

Mosquito Behavior and Vertebrate Microbiota Interaction: Implications for Pathogen Transmission.

The microbiota is increasingly recognized for its ability to influence host health and individual fitness through multiple pathways, such as nutrient synthesis, immune system development, and even behavioral processes. Most of these studies though focus on the direct effects microbiota has on its host, but they do not consider possible interactions with other individuals. However, host microbiota can change not only host behavior but also the behavior of other individuals or species toward the host. For example, microbes can have an effect on animal chemistry, influencing animal behaviors mediated by chemical communication, such as mosquito attraction. We know that host skin microbes play a major role in odor production and thus can affect the behavior of mosquitoes leading to differences in attraction to their hosts. Ultimately, the vector feeding preference of mosquitoes conditions the risk of vertebrates of coming into contact with a vector-borne pathogen, affecting its transmission, and thus epidemiology of vector-borne diseases. In this mini review, I provide an overview of the current status of research on the interaction between mosquito behavior and host skin microbiota, both in humans and other vertebrates. I consider as well the factors that influence vertebrate skin microbiota composition, such as sex, genetic makeup, and infection status, and discuss the implications for pathogen transmission.

Genomic architecture of gapeworm resistance in a natural bird population.

Parasites are recognized to be some of the strongest agents of natural selection, sometimes causing major changes in the phenotypes of their hosts. Understanding the genomic determinants leading to these adaptive processes is key to understand host-parasite interactions. However, dissecting the genetic architecture of host resistance in natural systems is difficult because of the multiple factors affecting these complex traits in the wild. In this issue of Molecular Ecology, Lundregan et al. (2020) use an impressive long-term data set to analyse the genomic architecture of host resistance to gapeworm in a metapopulation of house sparrows. The authors elegantly combine different approaches (variance component analyses, genome partitioning and genome-wide associations) to reveal that resistance to gapeworm is under polygenic control and can have both a significant additive genetic and dominance variance. This study is one of the first to simultaneously determine genomic architecture and assess additive genetic and dominance genetic variance in parasite resistance in natural populations.

Abundance, density, and social structure of African forest elephants (Loxodonta cyclotis) in a human-modified landscape in southwestern Gabon.

Population monitoring is critical to effective conservation, but forest living taxa can be difficult to directly observe. This has been true of African forest elephants (Loxodonta cyclotis), for which we have limited information regarding population size and social behavior despite their threatened conservation status. In this study, we estimated demographic parameters using genetic capture-recapture of forest elephants in the southern Industrial Corridor of the Gamba Complex of Protected Areas in southwestern Gabon, which is considered a global stronghold for forest elephants. Additionally, we examined social networks, predicting that we would find matrilineal structure seen in both savanna and forest elephants. Given 95% confidence intervals, we estimate population size in the sampled area to be between 754 and 1,502 individuals and our best density estimate ranges from 0.47 to 0.80 elephants per km2. When extrapolated across the entire Industrial Corridor, this estimate suggests an elephant population size of 3,033 to 6,043 based on abundance or 1,684 to 2,832 based on density, approximately 40-80% smaller than previously suggested. Our social network analysis revealed approximately half of network components included females with different mitochondrial haplotypes suggesting a wider range of variation in forest elephant sociality than previously thought. This study emphasizes the threatened status of forest elephants and demonstrates the need to further refine baseline estimates of population size and knowledge on social behavior in this taxon, both of which will aid in determining how population dynamics in this keystone species may be changing through time in relation to increasing conservation threats.

Evaluación ecográfica de la orquiepididimitis complicada / Ultrasound findings in complicated orchiepididymitis

Para describir los hallazgos ecográficos de la orquiepididimitis aguda complicada diseñamos un estudio descriptivo retrospectivo de casos entre 2006-2013. Registramos 75 casos de orquiepididimitis aguda severa, presentando complicaciones 15 pacientes (20%). La media de edad fue 54 años (rango 15-84). Las principales manifestaciones clínicas fueron dolor escrotal y edema testicular, encontrando: orquiepididimitis crónica, absceso escrotal, absceso intratesticular, piocele, isquemia testicular y atrofia testicular en el contexto de una orquitis crónica. Fue preciso realizar cirugía en 9 casos. La ecografía identifica lesiones específicas cuyo manejo precoz permite el tratamiento conservador y un seguimiento del proceso sin recurrir a exploraciones quirúrgicas innecesarias. Se revisan los datos clínicos y su correlación con los patrones ecográficos encontrados en nuestro estudio. La orquiepididimitis es un problema habitual que puede derivar en complicaciones de variada gravedad. El conocimiento de los signos ecográficos nos permitirá establecer un diagnostico precoz y fiable, eligiendo el tratamiento adecuado (AU)

A retrospective descriptive study was conducted on patients diagnosed between 2006 and 2013, in order to describe the ultrasound findings in complicated acute orchiepididymitis (orchitis). There have been 75 cases of severe acute orchitis, with 15 (20%) of them having complications. The mean age was 54 years (range 15-84), and the main clinical manifestations were scrotal pain and testicular swelling. Other findings included, chronic orchitis, scrotal abscess, intra-testicular abscess, pyocele, testicular ischaemia, and testicular atrophy. Surgery was performed in 9 cases. Ultrasound can reliably identify specific lesions, for which early treatment that includes conservative management and monitoring of the process without resorting to unnecessary surgical exploration. Clinical data and its correlation with the main ultrasound patterns found in the study were reviewed. The diagnosis of orchitis is a common problem that can lead to complications of varying severity. Knowledge of the ultrasound signs of these conditions will allow us to establish a reliable early diagnosis and the choice of appropriate conservative or surgical treatment (AU)

Rapid Identification of Major Histocompatibility Complex Class I Haplotypes Using Deep Sequencing in an Endangered Old World Monkey.

Immunogenetic data from wild primate populations have been difficult to obtain, due to logistic and methodological constraints. We applied a well-characterized deep sequencing method for MHC I typing, developed for macaques, to a population of wild red colobus to assess the feasibility of identifying MHC I-A/B haplotypes. Ten individuals produced sufficient data from blood and tissue samples to assign haplotypes. Eighty-two sequences were classified as red colobus MHC I alleles distributed across six MHC I loci. Individuals averaged ~13k reads across six MHC I loci, with 83% of all alleles representing either MHC I-A or MHC I-B loci. This study not only represents an important advance in the identification and genotyping of MHC in the endangered red colobus but also shows the potential for using this approach in other endangered wild primates.

Genomic resources notes accepted 1 December 2014 - 31 January 2015.

This article documents the public availability of (i) transcriptome sequence data and assembly for the rostrum dace (Leuciscus burdigalensis) naturally infected by a copepod ectoparasite (Tracheliastes polycolpus) and (ii) SNPs identified and validated from RAD sequencing for the Ugandan red colobus (Procolobus rufomitratus tephrosceles) using RAD sequencing.

Major Histocompatibility Complex, demographic, and environmental predictors of antibody presence in a free-ranging mammal.

Major Histocompatibility Complex (MHC) variability plays a key role in pathogen resistance, but its relative importance compared to environmental and demographic factors that also influence resistance is unknown. We analyzed the MHC II DRB exon 2 for 165 raccoons (Procyon lotor) in Missouri (USA). For each animal we also determined the presence of immunoglobulin G (IgG) and immunoglobulin M (IgM) antibodies to two highly virulent pathogens, canine distemper virus (CDV) and parvovirus. We investigated the role of MHC polymorphism and other demographic and environmental factors previously associated with predicting seroconversion. In addition, using an experimental approach, we studied the relative importance of resource availability and contact rates. We found important associations between IgG antibody presence and several MHC alleles and supertypes but not between IgM antibody presence and MHC. No effect of individual MHC diversity was found. For CDV, supertype S8, one allele within S8 (Prlo-DRB(∗)222), and a second allele (Prlo-DRB(∗)204) were positively associated with being IgG+, while supertype S4 and one allele within the supertype (Prlo-DRB(∗)210) were negatively associated with being IgG+. Age, year, and increased food availability were also positively associated with being IgG+, but allele Prlo-DRB(∗)222 was a stronger predictor. For parvovirus, only one MHC allele was negatively associated with being IgG+ and age and site were stronger predictors of seroconversion. Our results show that negative-frequency dependent selection is likely acting on the raccoon MHC and that while the role of MHC in relation to other factors depends on the pathogen of interest, it may be one of the most important factors predicting successful immune response.

Hallazgos ecográficos asociados al cáncer de vesícula biliar / Ultrasound findings associated to gallbladder carcinoma

OBJETIVO: Revisar los hallazgos ecográficos más relevantes asociados al cáncer de vesícula biliar. MATERIAL Y MÉTODO: Estudio descriptivo y retrospectivo de las características clínicas y estudios de imagen en pacientes con neoplasia de vesícula biliar en el periodo 2000-2011 en el Hospital General Reina Sofía de Murcia. RESULTADOS: Fueron hallados 15 casos de cáncer vesicular, de los que 9 eran mujeres. La edad media fue 77 años (rango 61-96). El dolor fue el principal motivo de consulta. Presentaban colelitiasis 13 casos, tabaquismo 2 casos y obesidad 3 casos. La ecografía comprobó engrosamiento de la pared vesicular (> 4 mm) en 8 casos, masa intraluminal en 4, vesícula escleroatrófica en 2 y masa que reemplaza la vesícula en uno. Solo en 4 casos hubo la sospecha preoperatoria de carcinoma vesicular. Siguiendo la estadificación pTNM, 4 pacientes presentaban un carcinoma in situ (Tis), un caso T1a, 6 casos T2, 3 casos T3 y un caso T4. En 7 casos, la única prueba radiológica preoperatoria fue la ecografía y en otros 8 se completó el estudio con una TC abdominal. CONCLUSIÓN: El diagnóstico precoz del cáncer vesicular es raro. La orientación diagnóstica por ecografía es limitada; solo el engrosamiento parietal localizado coexistente con litiasis vesicular parece significativo en estadios tempranos. La imagen de masa y cálculo ocupando la vesícula se asocia a etapas avanzadas de la enfermedad

OBJECTIVE: This article reviews the most relevant ultrasound findings associated with gallbladder cancer. MATERIALS AND METHODS: A descriptive and retrospective study was made of clinical features and imaging studies in patients subjected to surgery for gallbladder neoplasm in the Reina Sofía General University Hospital (Murcia) during the time period 2000-2011. RESULTS: A total of 15 cases of gallbladder cancer were found during the study period, 9 of whom were women. The mean age was 77 years (range 61-96). Pain was the principal complaint. The patients had cholelithiasis in 13 cases, smoking in 2 cases, and obesity in 3 cases. The ultrasound showed gallbladder wall thickening (> 4 mm) in 8 cases, intraluminal mass in 4, scleroatrophic gallbladder in 2, and mass replacing the gallbladder in one. Only in 4 cases was the suspicion of gallbladder carcinoma established preoperatively. According to the pTNM staging, 4 patients were carcinoma in situ (Tis), one case T1a, 6 cases T2, 3 cases T3 and one case T4. In 7 cases, the only evidence was the preoperative ultrasound, and in 8 the study was completed with an abdominal CT. CONCLUSION: Early diagnosis of gallbladder cancer is rare. The ultrasound diagnostic approach is difficult; only a localized thickening coexisting with gallstones seems to be significant, and requires a biopsy. The image of a mass and a stone occupying the gallbladder is associated with later stages of the disease

Ultrasound findings associated to gallbladder carcinoma.

OBJECTIVE: This article reviews the most relevant ultrasound findings associated with gallbladder cancer. MATERIAL AND METHOD: A descriptive and retrospective study was made of clinical features and imaging studies in patients subjected to surgery for gallbladder neoplasm in the Reina Sofía General University Hospital (Murcia) during the time period 2000-2011. RESULTS: A total of 15 cases of gallbladder cancer were found during the study period, 9 of whom were women. The mean age was 77 years (range 61-96). Pain was the principal complaint. The patients had cholelithiasis in 13 cases, smoking in 2 cases, and obesity in 3 cases. The ultrasound showed gallbladder wall thickening (>4mm) in 8 cases, intraluminal mass in 4, scleroatrophic gallbladder in 2, and mass replacing the gallbladder in one. Only in 4 cases was the suspicion of gallbladder carcinoma established preoperatively. According to the pTNM staging, 4 patients were carcinoma in situ (Tis), one case T1a, 6 cases T2, 3 cases T3 and one case T4. In 7 cases, the only evidence was the preoperative ultrasound, and in 8 the study was completed with an abdominal CT. CONCLUSION: Early diagnosis of gallbladder cancer is rare. The ultrasound diagnostic approach is difficult; only a localized thickening coexisting with gallstones seems to be significant, and requires a biopsy. The image of a mass and a stone occupying the gallbladder is associated with later stages of the disease.

The effect and relative importance of neutral genetic diversity for predicting parasitism varies across parasite taxa.

Understanding factors that determine heterogeneity in levels of parasitism across individuals is a major challenge in disease ecology. It is known that genetic makeup plays an important role in infection likelihood, but the mechanism remains unclear as does its relative importance when compared to other factors. We analyzed relationships between genetic diversity and macroparasites in outbred, free-ranging populations of raccoons (Procyon lotor). We measured heterozygosity at 14 microsatellite loci and modeled the effects of both multi-locus and single-locus heterozygosity on parasitism using an information theoretic approach and including non-genetic factors that are known to influence the likelihood of parasitism. The association of genetic diversity and parasitism, as well as the relative importance of genetic diversity, differed by parasitic group. Endoparasite species richness was better predicted by a model that included genetic diversity, with the more heterozygous hosts harboring fewer endoparasite species. Genetic diversity was also important in predicting abundance of replete ticks (Dermacentor variabilis). This association fit a curvilinear trend, with hosts that had either high or low levels of heterozygosity harboring fewer parasites than those with intermediate levels. In contrast, genetic diversity was not important in predicting abundance of non-replete ticks and lice (Trichodectes octomaculatus). No strong single-locus effects were observed for either endoparasites or replete ticks. Our results suggest that in outbred populations multi-locus diversity might be important for coping with parasitism. The differences in the relationships between heterozygosity and parasitism for the different parasites suggest that the role of genetic diversity varies with parasite-mediated selective pressures.