Host genetic diversity and body condition influence parasite resistance and clearance in a wild marine mammal population.

Host genetic variability can modulate infection resistance, although its role in infection clearance remains unclear. Hookworm disease (Uncinaria sp.) is the leading cause of pup mortality in several otariid species, although the parasite can be cleared through immune-mediated processes. We evaluated the association of host genetic diversity, body condition and immune response with hookworm resistance and/or clearance in the South American fur seal (Arctocephalus australis). Uninfected pups had higher heterozygosity than parasitized individuals, indicating a negative relationship between heterozygosity and the chances of infection. Likewise, pups that died of hookworm infection had lower heterozygosity than those that died of non-infectious causes. Interestingly, once infected, pups that survived hookworm infection had heterozygosities similar to pups that died of hookworm disease. However, pups that cleared the infection had a higher body mass and parasite-specific immunoglobulin G levels than those that did not recover or died of hookworm disease. Thus, although heterozygosity predicted resistance to and mortality from hookworm infections, it did not affect parasite clearance, which was facilitated by better body condition and adaptive immune responses. This demonstrates that host genetic variability and host-environment interactions influence disease dynamics, acting at different, well-defined stages of infection.

A systematic review of the diversity and virulence correlates of metastrongyle lungworms in marine mammals.

Metastrongyle lungworms could be particularly detrimental for diving animals such as marine mammals; however, little is known of the drivers of pathogenic host­parasite relationships in this group. This systematic review analysed the diversity of metastrongyles in marine mammals and the host and parasite traits associated with virulence. There have been at least 40 species of metastrongyles described in 66 species of marine mammals. After penalization for study biases, Halocercus hyperoodoni, Otostrongylus circumlitus, Parafilaroides gymnurus, Halocercus brasiliensis and Stenurus minor were the metastrongyles with the widest host range. Most studies (80.12%, n = 133/166) reported that metastrongyles caused bronchopneumonia, while in the cardiovascular system metastrongyles caused vasculitis in nearly half of the studies (45.45%, n = 5/11) that assessed these tissues. Metastrongyles were associated with otitis in 23.08% (n = 6/26) of the studies. Metastrongyle infection was considered a potential contributory to mortality in 44.78% (n = 90/201) of the studies while 10.45% (n = 21/201) of these studies considered metastrongyles the main cause of death. Metastrongyle species with a wider host range were more likely to induce pathogenic effects. Metastrongyles can cause significant tissue damage and mortality in marine mammals although virulent host­parasite relationships are dominated by a few metastrongyle species with wider host ranges.

Helminth-associated changes in host immune phenotype connect top-down and bottom-up interactions during co-infection.

1. Within-host parasite interactions can be mediated by the host and changes in host phenotypes often serve as indicators of the presence or intensity of parasite interactions. 2. Parasites like helminths induce a range of physiological, morphological, and immunological changes in hosts that can drive bottom-up (resource-mediated) or top-down (immune-mediated) interactions with co-infecting parasites. Although top-down and bottom-up interactions are typically studied in isolation, the diverse phenotypic changes induced by parasite infection may serve as a useful tool for understanding if, and when, these processes act in concert. 3. Using an anthelmintic treatment study of African buffalo (Syncerus caffer), we tracked changes in host immunological and morphological phenotypes during helminth-coccidia co-infection to investigate their role in driving independent and combinatorial bottom-up and top-down parasite interactions. We also examined repercussions for host fitness. 4. Clearance of a blood-sucking helminth, Haemonchus, from the host gastrointestinal tract induced a systemic Th2 immune phenotype, while clearance of a tissue-feeding helminth, Cooperia, induced a systemic Th1 phenotype. Furthermore, the Haemonchus-associated systemic Th2 immune phenotype drove simultaneous top-down and bottom-up effects that increased coccidia shedding by changing the immunological and morphological landscapes of the intestine. 5. Higher coccidia shedding was associated with lower host body condition, a lower chance of pregnancy, and older age at first pregnancy, suggesting that coccidia infection imposed significant condition and reproductive costs on the host. 6. Our findings suggest that top-down and bottom-up interactions may commonly co-occur and that tracking key host phenotypes that change in response to infection can help uncover complex pathways by which parasites interact.

Shifts in maternal foraging strategies during pregnancy promote offspring health and survival in a marine top predator.

The success of maternal foraging strategies during the rearing period can greatly impact the physiology and survival of dependent offspring. Surprisingly though, little is known on the fitness consequences of foraging strategies during the foetal period. In this study, we characterized variation in maternal foraging strategy throughout pregnancy in a marine top predator (South American fur seal, Arctocephalus australis), and asked if these shifts predicted neonatal health and postnatal survival. We found that during early pregnancy all pregnant females belonged to a single, homogenized foraging niche without evident clusters. Intriguingly though, during late pregnancy, individual fur seal mothers diverged into two distinct foraging niches characterized by a benthic-nearshore and a pelagic-offshore strategy. Females that shifted towards the benthic-nearshore strategy gave birth to pups with greater body mass, higher plasmatic levels of glucose and lower levels of blood urea nitrogen. The pups born to these benthic females were eight times more likely to survive compared to females using the pelagic-offshore foraging strategy during late pregnancy. These survival effects were mediated primarily by the impact of foraging strategies on neonatal glucose independent of protein metabolic profile and body mass. Benthic-nearshore foraging strategies during late pregnancy potentially allow for the greater maternal transfer of glucose to the foetus, leading to higher chances of neonatal survival. These results call for a deeper understanding of the balance between resource acquisition and allocation provided by distinct foraging polymorphisms during critical life-history periods, and how this trade-off may be adaptive under certain environmental conditions.

Neuropathology of feral conures with bromethalin toxicosis.

Bromethalin is a widely used neurotoxic rodenticide sometimes affecting nontarget wildlife. However, the effects of bromethalin on avian species are largely unknown. Here, we report the neuropathology of 14 feral conures (Psittacara sp.) with bromethalin toxicosis. Clinically, all birds presented with different degrees of paraparesis that sometimes progressed to dysphagia, ataxia, and tetraparesis. Histologically, there was astrogliosis, pallor, and vacuolation of white matter in the brain. This was usually more prominent in the medial longitudinal fasciculus, pons, optic tectum, cerebellar peduncle, and ventral funiculus. In most affected areas, there was loss of oligodendrocytes, and axons had extensive myelin loss or marked intramyelinic edema with splitting of myelin sheaths at the intraperiod line. Conures with bromethalin toxicosis had neuropathological changes similar to those of mammals exposed to bromethalin but with a characteristic distribution, probably related to higher susceptibility to cytotoxic edema in certain regions of the avian brain.

Lower marine productivity increases agonistic interactions between sea lions and fur seals in Northern Pacific Patagonia.

Interspecific interactions are key drivers of individual and population-level fitness in a wide range of animals. However, in marine ecosystems, it is relatively unknown which biotic and abiotic factors impact behavioral interactions between competing species. We assessed the impact of weather, marine productivity, and population structure on the behavioral agonistic interactions between South American fur seals (SAFSs), Arctocephalus australis, and South American sea lions (SASLs), Otaria byronia, in a breeding colony of SAFS. We hypothesized that agonistic interactions between SAFSs and SASLs respond to biotic and abiotic factors such as SAFS population structure, marine productivity, and weather. We found that SASL and SAFS interactions almost always resulted in negative impacts on the social structure or reproductive success of the SAFS colony. SASL adult males initiated stampedes of SAFS and/or abducted and predated SAFS pups. Adult SAFS males abundance and severe weather events were negatively correlated with agonistic interactions between species. However, proxies for lower marine productivity such as higher sea surface temperature and lower catches of demerso-pelagic fish were the most important predictors of more frequent agonistic interactions between SAFS and SASL. Under the current scenario of decline in marine biomass due to global climate change and overfishing, agonistic interactions between competing marine predators could increase and exacerbate the negative impacts of environmental change in these species.

Clinical signs, treatment, and outcome for California sea lions (Zalophus californianus) with Sarcocystis-associated polyphasic rhabdomyositis.

OBJECTIVE: To describe clinical signs, treatment, and outcome for California sea lions (Zalophus californianus) with Sarcocystis-associated polyphasic rhabdomyositis. ANIMALS: 38 free-ranging juvenile to adult California sea lions examined at a rehabilitation center in California between September 2015 and December 2017. PROCEDURES: Medical records at The Marine Mammal Center were reviewed to identify sea lions in which sarcocystosis had been diagnosed. RESULTS: Clinical signs were highly variable and associated with polyphasic rhabdomyositis attributed to Sarcocystis neurona infection. Generalized severe muscle wasting, respiratory compromise, and regurgitation secondary to megaesophagus were the most profound clinical findings. Respiratory compromise and megaesophagus were associated with a poor prognosis. Eight of the 38 sea lions were treated and released to the wild, and 2 subsequently restranded and were euthanized. Two additional animals received no targeted treatment and were released. The remaining 28 animals were either euthanized or died during treatment. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggested that unlike other marine mammals, which typically develop encephalitis, California sea lions with sarcocystosis often have polyphasic rhabdomyositis with highly variable clinical signs and that extensive diagnostic testing may be required to confirm the diagnosis. Treatment with an antiprotozoal drug in combination with corticosteroids may resolve clinical disease, but the prognosis is guarded.

Adenovirus Diversity in Fur Seal and Penguin Colonies of South America.

Adenoviruses are medium size nonenveloped viruses with a trend of coevolution with their hosts. We surveyed South American fur seals (Arctocephalus australis) and Humboldt penguins (Spheniscus humboldti) for adenoviruses at two sites from 2009 to 2012. Despite the common pattern of host specificity, some of the adenoviruses in our study were present in samples from unexpected host species. We identified mastadenoviruses, aviadenoviruses, and siadenoviruses in A. australis from Peru and Chile and in S. humboldti from Peru. The El Niño Southern Oscillation (ENSO) significantly reduces the productivity of the Humboldt upwelling system, which can change trophic and other ecological interactions, facilitating exposure to new pathogens. One aviadenovirus was detected in both the penguins and the fur seals, an interclass distance. This finding occurred only during the 2009 ENSO and not in 2010 or 2012. Further studies of viral diversity in sites with high-density mixed species populations are necessary to better understand viral evolution and the effect of environmental change on viral evolution and host specificity.

A Retrospective Study of Pathology in Bats Submitted to an Exotic and Zoo Animal Diagnostic Service in Georgia, USA (2008-2019).

Pathology records of bats submitted to the University of Georgia from managed care settings were reviewed to identify naturally occurring diseases. Fifty-nine cases were evaluated during an 11-year period (2008-2019), including representatives from four families: Pteropodidae (Yinpterochiroptera), Phyllostomidae, Vespertilionidae and Molossidae (Yangochiroptera). Pathology reports were reviewed to determine the primary pathological process resulting in death or the decision to euthanize. Cases were categorized as non-infectious (34/59; 58%), infectious/inflammatory (17/59; 29%) or undetermined due to advanced autolysis (8/59; 14%). Musculoskeletal diseases and reproductive losses were the most frequent pathological processes. Among the infectious processes identified, bacterial infections of the reproductive and haemolymphatic systems were most frequently observed. The first two reports of neoplasia in small flying foxes (Pteropus hypomelanus) are described. Bats under managed care present with a wide range of histopathological lesions. In this cohort, non-infectious disease processes were common.

Towards understanding the effects of oceanic plastic pollution on population growth for a South American fur seal (Arctocephalus australis australis) colony in Chile.

Entanglement of pinnipeds with plastic debris is an emerging conservation and animal welfare issue worldwide. However, the origins and long-term population level consequences of these entanglements are usually unknown. Plastic entanglement could produce a combination of wounds, asphyxiation, or inability to feed that results in the death of a certain percentage of individuals from the total population. In this research, we report on the consequent effect of plastic entanglement on population growth demographics in a South American fur seal (Arctocephalus australis australis) colony on Guafo Island, southern Chile. Using a stochastic matrix population model structured according to age and sex, and assuming an otherwise stable population, we explored population growth rates under five scenarios with differing rates of entanglement: A) a zero rate of plastic entanglement, B) entanglement rates (number of entangled individuals as a proportion of the total number of individuals) as observed in our study population (overall entanglement ratio of 1.2 × 10-3); and for the other scenarios, entanglement ratios as reported in the literature for other pinniped colonies around the world: C) 3.04 × 10-3, D) 4.42 × 10-2, and E) 8.39 × 10-2. Over the 30 years forecasting period and starting with a population size of ∼2950 individuals, the population growth rate was lower under all scenarios with rates of entanglement greater than zero (scenarios B-E). In comparison with scenario A, at the end of the 30-year period forecasted, we calculated a projected decrease in population size of between 20.34% (scenario B) and 91.38% (scenario E). These results suggest that even the lowest levels of entanglement in pinnipeds as reported in the literature might have significant effects over time on population-level dynamics. Our research offers potential insight when devising policy for the management and limitation of plastic pollution in the oceans, and indeed for the conservation and management policy of affected marine species. Furthermore, whilst there are some limitations to our methodology, it offers a straightforward and potentially useful approach for the standardized prediction of impacts at a population level of different rates of plastic pollution and entanglement and could be applied in distinct populations of the same species around the world.

Natural resistance to worms exacerbates bovine tuberculosis severity independently of worm coinfection.

Pathogen interactions arising during coinfection can exacerbate disease severity, for example when the immune response mounted against one pathogen negatively affects defense of another. It is also possible that host immune responses to a pathogen, shaped by historical evolutionary interactions between host and pathogen, may modify host immune defenses in ways that have repercussions for other pathogens. In this case, negative interactions between two pathogens could emerge even in the absence of concurrent infection. Parasitic worms and tuberculosis (TB) are involved in one of the most geographically extensive of pathogen interactions, and during coinfection worms can exacerbate TB disease outcomes. Here, we show that in a wild mammal natural resistance to worms affects bovine tuberculosis (BTB) severity independently of active worm infection. We found that worm-resistant individuals were more likely to die of BTB than were nonresistant individuals, and their disease progressed more quickly. Anthelmintic treatment moderated, but did not eliminate, the resistance effect, and the effects of resistance and treatment were opposite and additive, with untreated, resistant individuals experiencing the highest mortality. Furthermore, resistance and anthelmintic treatment had nonoverlapping effects on BTB pathology. The effects of resistance manifested in the lungs (the primary site of BTB infection), while the effects of treatment manifested almost entirely in the lymph nodes (the site of disseminated disease), suggesting that resistance and active worm infection affect BTB progression via distinct mechanisms. Our findings reveal that interactions between pathogens can occur as a consequence of processes arising on very different timescales.

Molecular survey of Bartonella spp. in rodents and fleas from Chile.

The aim of this study was to molecularly survey Bartonella spp. in rodents from the Valdivia Province, Southern Chile and from wild black rat-fleas in Guafo Island, Chilean Patagonia. Thrity-three spleens from synanthropic (Mus musculus, Rattus novergicus and Rattus rattus) and wild (Abrothrix longipilis, Oligoryzomys longicaudatus, Abrothrix sp.) rodents from Valdivia and 39 fleas/flea-pools (Plocopsylla sp. and Nosopsyllus sp.) from R. rattus in Guafo Island were obtained. All samples were screened by high-resolution melting (HRM) real-time PCR for Bartonella ITS locus (190 bp). ITS-Positive samples were further analyzed for two HRM real-time PCR assays targeting Bartonella rpoB (191 bp) and gltA (340 bp) gene fragments. All positive ITS, gltA and rpoB real-time PCR products were purified and sequenced. Bayesian inference trees were built for the gltA and rpoB gene fragments. Bartonella-ITS DNA was detected in 36.3% (12/33) [95% CI (22-53%)] of the tested rodents from Valdivia, being identified in all but O. longicaudatus rodent species captured in this study. ITS DNA was detected in 28% (11/39) [95% CI (16-43%)] of fleas/flea-pools from Guafo Island and identified in both Plocopsylla and Nosopsyllus genera. Sequencing and phylogenic analyses targeting three loci of Bartonella spp. allowed the identification of five genotypes in rodents from Southern Chile, potentially belonging to three different Bartonella spp. Those included Bartonella tribocorum identified from R. rattus, Bartonella rochalimae detected from Abrothix sp., and one novel genotype from uncharacterized Bartonella sp. identified in M. musculus, R. norvegicus, A. longipilis, and Abothrix sp., related to strains previously isolated in Phyllotis sp. from Peru. Additionally, two genotypes of B. tribocorum were identified in fleas from Guafo. In a nutshell, highly diverse and potentially zoonotic Bartonella spp. are described for the first time in wild and synanthropic rodents from Chile, and B. tribocorum was detected in wild back rat fleas from Guafo Island.

Antimicrobial resistance patterns of Acinetobacter spp. of animal origin reveal high rate of multidrug resistance.

Antimicrobial resistance has been declared by the World Health Organization as one of the biggest threats to public health and Acinetobacter baumannii is a notable example. A. baumannii is an important human nosocomial pathogen, being along with other multidrug resistant (MDR) bacteria, one of the biggest public health concerns worldwide. In Veterinary Medicine, resistance patterns of Acinetobacter species other than A. baumanii are unclear, and the scarce information available is limited and fragmented. We applied a statistical modeling approach to investigate the occurrence, clinical relevance and antimicrobial resistant phenotypes of Acinetobacter spp. originated from animals. Seven Acinetobacter species were identified in clinical specimens of more than 15 different domestic, zoo and exotic animal species. We found a high rate of MDR A. baumannii of canine origin with some of these isolates originating from serious systemic or wound infections, which highlights their potential pathogenic profile and spread in the human environment. Data also revealed different antimicrobial resistance patterns of animal-origin Acinetobacter species, emphasizing the necessity to implement specific antimicrobial susceptibility recommendations for animal isolates as there are no such clinical breakpoints currently in place. This study provides substantial advancing in our understanding of Acinetobacter spp. in animal clinical specimens, and highlights the role of animals in the dynamics of multidrug resistance in bacteria. The data presented here is a valuable source of information for further establishment of clinical breakpoints for susceptibility testing of animal-associated Acinetobacter isolates.

Monitoring the occurrence of microplastic ingestion in Otariids along the Peruvian and Chilean coasts.

Repeated reports of microplastic pollution in the marine pinniped diet have emerged in the last years. However, only few studies address the drivers of microplastics presence and the potential implications for monitoring microplastic pollution in the ocean. This study monitored their in the scats (N = 205) of four pinniped species/subspecies at five different locations in the southern Pacific Ocean (Peru and Chile). Samples from all rookeries contained microplastics, and overall, 68% of the examined scats contained fragments/fibers, mostly blue colored. We confirmed that 81.5% of the fragments/fibers were anthropogenic in origin , but only 30% were polymers. Scats from Juan Fernández Archipelago presented higher microplastic concentrations than continental rookeries. Also, the common diet in each location may influence the levels found in the samples. This study presents a useful non-invasive technique to track plastic pollution in top predator diets as bioindicators for future surveillance/management plans applied to different location.

Immune stability predicts tuberculosis infection risk in a wild mammal.

Immunity is one of the most variable phenotypic traits in animals; however, some individuals may show less fluctuation in immune traits, resulting in stable patterns of immune variation over time. It is currently unknown whether immune variation has consequences for infectious disease risk. In this study, we identified moderately stable immune traits in wild African buffalo and asked whether the stability of these traits affected bovine tuberculosis (TB) infection risk. We found that adaptive immune traits such as the level of interferon-γ (IFN-γ) released after white blood cell stimulation, the number of circulating lymphocytes and the level of antibodies against bovine adenovirus-3 were moderately repeatable (i.e. stable) over time, whereas parameters related to innate immunity either had low repeatability (circulating eosinophil numbers) or were not repeatable (e.g. neutrophil numbers, plasma bacteria killing capacity). Intriguingly, individuals with more repeatable IFN-γ and lymphocyte levels were at a significantly higher risk of acquiring TB infection. In stark contrast, average IFN-γ and lymphocyte levels were poor predictors of TB risk, indicating that immune variability rather than absolute response level better captured variation in disease susceptibility. This work highlights the important and under-appreciated role of immune variability as a predictor of infection risk.

Polyphasic Rhabdomyositis in California Sea Lions (Zalophus Californianus): Pathology and Potential Causes.

A myositis syndrome has been recognized for more than a decade in California sea lions (CSLs; Zalophus californianus) but a detailed description of the lesions and potential causes of this condition is lacking. The tissues of 136 stranded CSLs with rhabdomyositis were examined. Rhabdomyositis was considered incidental in 67% (91/136) of the CSLs, and a factor contributing to the animal stranding (significant rhabdomyositis) in 33% (45/136). Of the 91 cases with incidental rhabdomyositis, lesions consisted of a few small foci of lymphohistiocytic inflammation. Of the 45 cases with significant rhabdomyositis, 28 (62%) also presented with major comorbidities such as leptospirosis (2 animals) and domoic acid toxicosis (6 animals), whereas 17 (38%) had severe polyphasic rhabdomyositis as the only major disease process associated with mortality. In these animals, most striated muscles had multiple white streaks and diffuse atrophy. Microscopically, there was myofiber necrosis surrounded by lymphocytes and histiocytes admixed with areas of myofiber regeneration, and/or moderate to severe rhabdomyocyte atrophy usually adjacent to intact Sarcocystis neurona cysts. At the interface of affected and normal muscle, occasional T lymphocytes infiltrated the sarcoplasm of intact myocytes, and occasional myofibers expressed MHCII proteins in the sarcoplasm. S. neurona antibody titers and cyst burden were higher in animals with significant polymyositis antibody titers of (26125 ± 2164, 4.5 ± 1.2 cysts per section) and active myonecrosis than animals with incidental rhabdomyositis antibody titers of (7612 ± 1042, 1.7 ± 0.82 cysts per section). The presented findings suggest that S. neurona infection and immune-mediated mechanisms could be associated with significant polyphasic rhabdomyositis in CSLs.

Canine infectious respiratory disease: New insights into the etiology and epidemiology of associated pathogens.

Canine infectious respiratory disease (CIRD) is a syndrome where multiple viral and bacterial pathogens are involved sequentially or synergistically to cause illness. There is limited information regarding the prevalence of pathogens related to CIRD in the United States as well as the role of co-infections in the pathogenesis of the syndrome. We aimed to conduct a comprehensive etiologic and epidemiologic study of multiple CIRD agents in a diverse dog population using molecular methods and statistical modeling analyses. In addition, a novel probe-based multiplex real-time PCR was developed to simultaneously detect and differentiate two species of Mycoplasma (M. canis and M. cynos). Canine adenovirus, canine distemper virus, canine parainfluenza virus, coronavirus, influenza A virus (H3N2 and H3N8), Bordetella bronchiseptica, M. canis, M. cynos and Streptococcus equi subsp. zooepidemicus were investigated in specimens from clinically ill and asymptomatic dogs received at the Athens Veterinary Diagnostic Laboratory. Results showed low occurrence of classical CIRD agents such as B. bronchiseptica, canine adenovirus and distemper virus, while highlighting the potential role of emerging bacteria such as M. canis and M. cynos. Statistical modeling analyses of CIRD pathogens emphasized the impact of co-infections on the severity of clinical presentation, and showed that host factors, such as animal age, are the most important predictors of disease severity. This study provides new insights into the current understanding of the prevalence and role of co-infections with selected viruses and bacteria in the etiology of CIRD, while underscoring the importance of molecular diagnosis and vaccination against this disease.

Parasitism Elicits a Stress Response That Allocates Resources for Immune Function in South American Fur Seals (Arctocephalus australis).

Parasites can cause chronic stress in some animal species, and this type of stress response has been associated with adverse consequences for the host. In order to know whether parasitism elicited a stress response associated with decreased host fitness, hookworm (Uncinaria sp.) infection was studied in a colony of South American fur seals (Arctocephalus australis) in which hookworms infect nearly all pups born in a reproductive season. A parasite-free group was generated by treating a subset of pups with an antiparasitic drug before they developed patent hookworm infection. Stress and metabolic hormones, energy balance, and humoral and cellular immune parameters were measured in this group and hookworm-infected pups. Hookworms elicited a marked increase in plasma cortisol levels in fur seal pups. These hookworm-infected pups were able to maintain constant glucose levels, despite losing body mass over the course of infection potentially because of increased protein catabolism. Infected pups were able to mount an effective immune response against the parasite and eliminated hookworms from the intestine, recovering partial body mass lost as a result of hookworm infection at the end of the study period. As shown in previous studies, adequate glucose levels are critical for proper T lymphocyte reactivity, and it is possible that, through activation of a stress response, energy can be readily available for immune response against the parasite contributing to early recovery from infection. Although there are potential fitness costs to mounting a sustained stress response, these could also be adaptive and promote survival during critical life-history stages.

The contrasting hidden consequences of parasitism: Effects of a hematophagous nematode (Uncinaria sp.) in the development of a marine mammal swimming behavior.

Parasites are an important part of ecosystems, playing a critical role in their equilibrium. However, the consequences of parasitism beyond the direct effects associated with disease and mortality are not completely understood. This gap in knowledge is in part due to the difficulties to isolate the effect of single parasite species on physiological and behavioral traits in natural systems.The South American fur seal (Arctocephalus australis)-hookworm (Uncinaria sp.) interaction offers an ideal system to overcome these difficulties and study the behavioral and physiological effects of parasites in their hosts.Hookworms cause stunted growth and anemia in pinniped pups, which could affect early life active behaviors such as swimming. The aim of this study was to determine the effects of hookworms (Uncinaria sp.) on the development of swimming capabilities in A. australis through physiological and ethological analyses.Higher parasite burden was associated with reduced growth rates and lower blood hemoglobin concentrations, whereas scaled body mass and blood hemoglobin levels had an important positive effect on the water activity of the pups. However, antihookworm treatment did not affect the level of water activity of the pups, and pups with high hookworm burden increased their time budget in water. This was probably related to lower maternal attendance in heavily parasitized pups, leaving these pups more time to perform water activities. Therefore, pups with heavy hookworm burden, despite having decreased growth rates and blood hemoglobin concentrations, compensated for their handicap in physiological traits related to swimming by spending more time in the water.This work offers new insights to understand the contrasting effects of parasites on aquatic organisms, and the compensatory mechanisms employed by infected animals to avoid the worst consequences of parasitism.