When is a parasite not a parasite? Effects of larval tick burdens on white-footed mouse survival

A multi-omics approach for understanding blood digestion dynamics in Ixodes scapularis and identification of anti-tick vaccine targets

Ixodes scapularis, the black-legged tick, is a major arthropod vector that transmits the causative agents of Lyme disease and several other pathogens of human significance. The tick midgut is the main tissue involved in blood acquisition and digestion and the first organ to have contact with pathogens ingested through the blood meal. Gene expression in the midgut before, during, and after a blood meal may vary in response to the physiological changes due to blood feeding. A systems biology approach based on RNA and protein sequencing was used to gain insight into the changes in tick midgut transcripts and proteins during blood ingestion (unfed and partially fed) and digestion (1-, 2-, 7-, and 14 days post detachment from the host) by the Ixodes scapularis female ticks. A total of 2,726 differentially expressed transcripts, and 449 proteins were identified across the time points. Genes involved in detoxification of xenobiotics, proteases, protease inhibitors, metabolism, and immunity were differentially expressed in response to blood feeding. Similarly, proteins corresponding to the same groups were also differentially expressed. Nine genes from major gene categories were chosen as potential vaccine candidates, and, using RNA interference, the effect of these gene knockdowns on tick biology was investigated. Knockdown of these genes had variable negative impacts on tick physiology, such as the inability to engorge fully and to produce eggs and increased mortality. These and additional gene targets provide opportunities to explore novel tick control strategies.

Omnivore diet composition alters parasite resistance and host condition

Diet composition modulates animals' ability to resist parasites and recover from stress. Broader diet breadths enable omnivores to mount dynamic responses to parasite attack, but little is known about how plant/prey mixing might influence responses to infection. Using omnivorous deer mice (Peromyscus maniculatus) as a model, we examine how varying plant and prey concentrations in blended diets influence resistance and body condition following infestation by Rocky Mountain wood ticks (Dermacentor andersoni). In two repeated experiments, deer mice fed for 4 weeks on controlled diets that varied in proportions of seeds and insects were then challenged with 50 tick larvae in two sequential infestations. The numbers of ticks successfully feeding on mice declined by 25% and 66% after the first infestation (in the first and second experiments, respectively), reflecting a pattern of acquired resistance, and resistance was strongest when plant/prey ratios were more equally balanced in mouse diets, relative to seed-dominated diets. Diet also dramatically impacted the capacity of mice to cope with tick infestations. Mice fed insect-rich diets lost 15% of their body weight when parasitized by ticks, while mice fed seed-rich diets lost no weight at all. While mounting/maintaining an immune response may be energetically demanding, mice may compensate for parasitism with fat and carbohydrate-rich diets. Altogether, these results suggest that a diverse nutritional landscape may be key in enabling omnivores' resistance and resilience to infection and immune stressors in their environments.

Context-dependent host dispersal and habitat fragmentation determine heterogeneity in infected tick burdens: an agent-based modelling study

As the incidence of tick-borne diseases has sharply increased over the past decade, with serious consequences for human and animal health, there is a need to identify ecological drivers contributing to heterogeneity in tick-borne disease risk. In particular, the relative importance of animal host dispersal behaviour in its three context-dependent phases of emigration, transfer and settlement is relatively unexplored. We built a spatially explicit agent-based model to investigate how the host dispersal process, in concert with the tick and host demographic processes, habitat fragmentation and the pathogen transmission process, affects infected tick distributions among hosts. A sensitivity analysis explored the impacts of different input parameters on infected tick burdens on hosts and infected tick distributions among hosts. Our simulations indicate that ecological predictors of infected tick burdens differed among the post-egg life stages of ticks, with tick attachment and detachment, tick questing activity and pathogen transmission dynamics identified as key processes, in a coherent way. We also found that the type of host settlement strategy and the proportion of habitat suitable for hosts determined super-spreading of infected ticks. We developed a theoretical mechanistic framework that can serve as a first step towards applied studies of on-the-ground public health intervention strategies.

Context-dependent host dispersal and habitat fragmentation determine heterogeneity in infected tick burdens: an agent-based modelling study

As the incidence of tick-borne diseases has sharply increased over the past decade, with serious consequences for human and animal health, there is a need to identify ecological drivers contributing to heterogeneity in tick-borne disease risk. In particular, the relative importance of animal host dispersal behaviour in its three context-dependent phases of emigration, transfer and settlement is relatively unexplored. We built a spatially explicit agent-based model to investigate how the host dispersal process, in concert with the tick and host demographic processes, habitat fragmentation and the pathogen transmission process, affects infected tick distributions among hosts. A sensitivity analysis explored the impacts of different input parameters on infected tick burdens on hosts and infected tick distributions among hosts. Our simulations indicate that ecological predictors of infected tick burdens differed among the post-egg life stages of ticks, with tick attachment and detachment, tick questing activity and pathogen transmission dynamics identified as key processes, in a coherent way. We also found that the type of host settlement strategy and the proportion of habitat suitable for hosts determined super-spreading of infected ticks. We developed a theoretical mechanistic framework that can serve as a first step towards applied studies of on-the-ground public health intervention strategies.

Tick infestation effects on haemoglobin levels of deer mice (Peromyscus maniculatus)

Deer mice (Peromyscus maniculatus) are hosts to ixodid ticks as well as the associated tick-borne pathogens they can spread. As the ranges of black-legged ticks (Ixodes scapularis) and American dog ticks (Dermacentor variabilis) expand northwards, naïve host populations of deer mice are likely to become infested by ticks and experience the physiological effects that ticks can have on them via blood-feeding. The prevalence of these haematophagous ticks can affect the haemoglobin levels of the mice they infest. Haemoglobin levels were compared and analysed in deer mice populations at three different sites with varying tick exposure. These results suggested that without confounding effects, the abundance of black-legged and American dog ticks on individual mice had a significant negative effect on the hosts' haemoglobin levels, but only in an area with high tick infestation. This was seen across the average haemoglobin levels between populations, where there was a significant difference between the source population with the longest established tick populations and the source population where neither black-legged nor American dog ticks were prevalent. As the ticks' ranges expand and they become more abundant, it is important to understand how their prevalence and intensity can alter host physiology, potentially affecting their own range expansion and the spread of the diseases they may carry.

Effects of physical impairments on fitness correlates of the white-footed mouse, Peromyscus leucopus

Physical impairments are widely assumed to reduce the viability of individual animals, but their impacts on individuals within natural populations of vertebrates are rarely quantified. By monitoring wild populations of white-footed mice over 26 years, we assessed whether missing or deformed limbs, tail or eyes influenced the survival, body mass, movement and ectoparasite burden of their bearers. Of the 27 244 individuals monitored, 543 (2%) had visible physical impairments. Persistence times (survival) were similar between mice with and without impairments. Mice with eye and tail impairments had 5% and 6% greater mass, respectively, than unimpaired mice. Mice with tail impairments had larger home ranges than did unimpaired mice. Burdens of black-legged ticks (Ixodes scapularis) were higher among mice with tail and limb impairments while burdens of bot fly larvae (Cuterebra) were higher among mice with cataracts compared to mice without impairments. Our findings do not support the presupposition that physical impairments reduce viability in their bearers and are inconsistent with the devaluation of impaired individuals that pervaded early thinking in evolutionary biology.

Effects of ticks on community assemblages of ectoparasites in deer mice

Ectoparasites are fundamental to ecosystems, playing a key role in trophic regulation. Fleas, mites, and ticks are common hematophagous ectoparasites that infest shared mammalian hosts. One common host in Ontario, Canada, is the deer mouse (Peromyscus maniculatus). As the climate warms and the geographic ranges of blacklegged ticks (Ixodes scapularis) and American dog ticks (Dermacentor variabilis) expand, their introduction to new ecosystems may alter current ectoparasite communities. At three different sites where exposure to ticks varied (both in terms of tick diversity and abundance), ectoparasite community structures found on deer mouse hosts were examined, focusing on species co-occurrences and habitat partitioning on the host. We predicted that when tick species were prevalent, ticks would dominate the micro-habitat attachment sites often inhabited by other parasites, thereby significantly altering parasite community structure. Our results suggest that blacklegged ticks and American dog ticks could have a positive association with each other, but a negative or random association with other ectoparasite species, even when they do not occupy the same attachment site. Sampling site played a significant role in community assemblages as well, possibly due to the differences in tick exposure. As the ticks' ranges expand and they become more abundant, it is important to understand how their prevalence can potentially alter the dynamics in an ectoparasite community, affecting the transmission of pathogens that may spread within an ecosystem, from one host to another.

Ixodes scapularis: Vector to an Increasing Diversity of Human Pathogens in the Upper Midwest

INTRODUCTION

The black-legged tick, Ixodes scapularis (I scapularis), is now recognized as the deadliest tick vector in the United States. The Upper Midwest, particularly Wisconsin and Minnesota, are endemic to a diversity of tick-transmitted infectious diseases. Although Borrelia burgdorferi, the agent of Lyme disease, still accounts for the majority of diagnosed infections, I scapularis is known to transmit other bacterial, viral, and parasitic agents.

OBJECTIVE

To provide an overview of the array of pathogenic microorganisms carried by I scapularis ticks in the Upper Midwest.

METHODS

A literature review was conducted to collect and analyze current information about I scapularis lifestyle, transmission, microorganisms carried by the arthropod vector, and the diseases that occur as a result of infections with these microorganisms in the Upper Midwest.

RESULTS

Diagnosis of co-infection from tick-borne zoonosis in humans has increased over the last 2 decades. Since I scapularis can transmit multiple pathogens, it is clinically important because different diagnostic testing and treatment strategies may need to be implemented for a patient with I scapularis-borne infection(s).

CONCLUSIONS

This review has concentrated on I scapularis-transmitted diseases affecting the Upper Midwest and has explored the ecology of the I scapularis vector and its role in pathogen transmission.

Prevalence of Tick-Borne Pathogens in Two Species of Peromyscus Mice Common in Northern Wisconsin

Two species of mice, the white-footed mouse, Peromyscus leucopus (Rafinesque; Rodentia: Cricetidae) and the woodland deer mouse, Peromyscus maniculatus (Wagner; Rodentia: Cricetidae), serve as reservoirs of tick-borne pathogens in many parts of North America. However, the role P. maniculatus plays in the amplification and maintenance of Anaplasma phagocytophilum (Rickettsiales: Ehrlichiaceae) and Borrelia burgdorferi (Spirochaetales: Spirochaetaceae) is not well understood. In northern Wisconsin, from 2012 to 2014, 560 unique mice were captured at 83 sites distributed across five forests. P. leucopus was more likely infested with immature Ixodes scapularis compared to P. maniculatus (60.1 vs. 28.3%). Abundance of immature I. scapularis on P. leucopus (M = 2.69; SD = 3.53) was surprisingly low and even lower for P. maniculatus (M = 0.717; SD = 1.44). Both P. leucopus and P. maniculatus were infected with B. burgdorferi, 24.0 and 16.8%, respectively. The prevalence of A. phagocytophilum infection in P. leucopus (1.69%) was similar to that observed in P. maniculatus (4.73%). Nine of 10 mice co-infected with both pathogens were P. maniculatus, and there were more co-infections in this species than expected by chance (3.07 vs. 0.82%). Differences in the behavior and biology between these two mice species may contribute to the variation observed in the abundance of host-attached ticks and pathogen prevalence. These differences highlight a potential hazard of the failure to differentiate between these visually similar mice, but there is evidence that these two mice species can each serve as reservoirs for tick-borne pathogens that cause human disease in Wisconsin.

Acquisition of Borrelia burgdorferi Infection by Larval Ixodes scapularis (Acari: Ixodidae) Associated With Engorgement Measures

Measuring rates of acquisition of the Lyme disease pathogen, Borrelia burgdorferi sensu lato Johnson, Schmid, Hyde, Steigerwalt & Brenner, by the larval stage of Ixodes scapularis Say is a useful tool for xenodiagnoses of B. burgdorferi in vertebrate hosts. In the nymphal and adult stages of I. scapularis, the duration of attachment to hosts has been shown to predict both body engorgement during blood feeding and the timing of infection with B. burgdorferi. However, these relationships have not been established for the larval stage of I. scapularis. We sought to establish the relationship between body size during engorgement of larval I. scapularis placed on B. burgdorferi-infected, white-footed mice (Peromyscus leucopus Rafinesque) and the presence or absence of infection in larvae sampled from hosts over time. Body size, time, and their interaction were the best predictors of larval infection with B. burgdorferi. We found that infected larvae showed significantly greater engorgement than uninfected larvae as early as 24 h after placement on a host. These findings may suggest that infection with B. burgdorferi affects the larval feeding process. Alternatively, larvae that engorge more rapidly on hosts may acquire infections faster. Knowledge of these relationships can be applied to improve effective xenodiagnosis of B. burgdorferi in white-footed mice. Further, these findings shed light on vector-pathogen-host interactions during an understudied part of the Lyme disease transmission cycle.

Stress level, parasite load, and movement pattern in a small-mammal reservoir host for Lyme disease

Occurrence of Lyme disease has increased rapidly in Canada in the past 5 years. The emergence of Lyme disease coincides with the range expansion of the primary host, the white-footed mouse (Peromyscus leucopus (Rafinesque, 1818)), in the region. We evaluated the effects of stress level, parasite load, and forest-patch characteristics on P. leucopus movement pattern. We found negative relations between on the one hand the adrenal gland size, a proxy for stress level, and population density, and on the other hand, home-range area and movement rate of mouse individuals, suggesting that stressed mice cannot maintain a large home range. Population density was also related with excursion (outside the forest patch) and exploration (outside the home range) rates, either directly or through its effect on home-range area and movement rate. Finally, movement rate and excursion rate were lower in individuals infested with more black-legged ticks (Ixodes scapularis Say, 1821). Our results have implication for the mechanism of Lyme disease emergence in the region: individual hosts that carry more ticks and are thus more likely to be spreading the bacterium responsible for Lyme disease are dispersing less than tick-free individuals. Monitoring of Lyme disease should thus consider how the characteristics of host communities modulate the spread of the disease across the landscape.

Ectoparasites and fitness of female Columbian ground squirrels

Parasites play an important role in the evolution of host traits via natural selection, coevolution and sexually selected ornaments used in mate choice. These evolutionary scenarios assume fitness costs for hosts. To test this assumption, we conducted an ectoparasite removal experiment in free-living Columbian ground squirrels (Urocittelus columbianus) in four populations over three years. Adult females were randomly chosen to be either experimentally treated with anti-parasite treatments (spot-on solution and flea powder, N = 61) or a sham treatment (control, N = 44). We expected that experimental females would show better body condition, increased reproductive success and enhanced survival. Contrary to our expectations, body mass was not significantly different between treatments at mating, birth of litter or weaning of young. Further, neither number nor size of young at weaning differed significantly between the two treatments. Survival to the next spring for adult females and juveniles was not significantly different between experimental and control treatments. Finally, annual fitness was not affected by the treatments. We concluded that females and their offspring were able compensate for the presence of ectoparasites, suggesting little or no fitness costs of parasites for females in the different colonies and during the years of our experiments.

The generalist tick Ixodes ricinus and the specialist tick Ixodes trianguliceps on shrews and rodents in a northern forest ecosystem--a role of body size even among small hosts

Background

Understanding aggregation of ticks on hosts and attachment of life stages to different host species, are central components for understanding tick-borne disease epidemiology. The generalist tick, Ixodes ricinus, is a well-known vector of Lyme borrelioses, while the specialist tick, Ixodes trianguliceps, feeding only on small mammals, may play a role in maintaining infection levels in hosts. In a northern forest in Norway, we aimed to quantify the role of different small mammal species in feeding ticks, to determine the extent to which body mass, even among small mammals, plays a role for tick load, and to determine the seasonal pattern of the two tick species.

Methods

Small mammals were captured along transects in two nearby areas along the west coast of Norway. All life stages of ticks were counted. Tick load, including both prevalence and intensity, was analysed with negative binomial models.

Results

A total of 359 rodents and shrews were captured with a total of 1106 I. ricinus (60.0 %) and 737 I. trianguliceps (40.4 %), consisting of 98.2 % larvae and 1.8 % nymphs of I. ricinus and 91.2 % larvae, 8.7 % nymphs and 0.1 % adult females of I. trianguliceps. Due to high abundance, Sorex araneus fed most of the larvae of both tick species (I. ricinus 61.9 %, I. trianguliceps 64.9 %) with Apodemus sylvaticus (I. ricinus 20.4 %, I. trianguliceps 10.0 %) and Myodes glareolus (I. ricinus 10.9 %, I. trianguliceps 9.5 %) as the next most important hosts. Individual A. sylvaticus and M. glareolus had higher infestation intensity than S. araneus, while Sorex minutus had markedly lower infestation intensity. The load of I. ricinus larvae and nymphs was related to body mass mainly up to ~10 g, while the load of I. trianguliceps was less dependent of body mass. The load of I. trianguliceps was higher in spring than in fall, while the seasonal pattern was reversed for I. ricinus with higher loads in fall.

Conclusions

Body mass was important for explaining load of I. ricinus mainly up to a body mass of ~10 g across a range of smaller mammalian hosts. Consistent with earlier work elsewhere in Europe, we found the highest tick infestation intensity on the wood mouse A. sylvaticus. However, this rodent species fed only 20.4 % of all I. ricinus larvae, while the much more abundant S. araneus fed 61.9 %. Our study emphasizes an important quantitative role of the common shrew S. araneus as a main host to I. ricinus larvae and to both I. trianguliceps larvae and nymphs. The partly seasonal distinct attachment pattern of I. ricinus and I. trianguliceps is evidence for niche separation.

Parasite load and seasonal migration in red deer

Northern deer populations are typically partially migratory, but the relationship between migratory movements and parasites has received little attention. Migration often involves movement from a low-elevation winter range towards a summer range at higher elevation. In Europe these movements may also involve a gradient in abundance of Ixodes ricinus ticks, but whether tick loads on deer differ depending on migration tactic has not been quantified. Based on the examination of ears from 49 red deer (Cervus elaphus) marked with global positioning system collars, we provide the first evidence that the tick loads of deer covering longer distances between their winter and summer range, resulting in higher difference in elevation, are lower. Our study highlights that only the resident part of the red deer population will be available as year-round hosts to ticks, while a large part of the red deer population is unavailable to ticks for most of the tick questing season due to seasonal migration to higher elevation. Predicted changes in the migratory behaviour of ungulates could hence affect the proportion of the host population available to ticks in the future.

The context of host competence: a role for plasticity in host-parasite dynamics

Even apparently similar hosts can respond differently to the same parasites. Some individuals or specific groups of individuals disproportionately affect disease dynamics. Understanding the sources of among-host heterogeneity in the ability to transmit parasites would improve disease management. A major source of host variation might be phenotypic plasticity - the tendency for phenotypes to change across different environments. Plasticity might be as important as, or even more important than, genetic change, especially in light of human modifications of the environment, because it can occur on a more rapid timescale than evolution. We argue that variation in phenotypic plasticity among and within species strongly contributes to epidemiological dynamics when parasites are shared among multiple hosts, which is often the case.

Factors affecting larval tick feeding success: host, density and time

BACKGROUND

Ectoparasites rely on blood-feeding to sustain activity, support development and produce offspring. Blood-feeding is also a route for transmission of diverse vector-borne pathogens. The likelihood of successfully feeding is thus an important aspect of ectoparasite population dynamics and pathogen transmission. Factors that affect blood-feeding include ectoparasite density, host defenses, and ages of the host and ectoparasite. How these factors interact to affect feeding success is not well understood.

METHODS

We monitored blood-feeding success of larval Rocky Mountain wood ticks (RMWTs; Dermacentor andersoni) on deer mice (Peromyscus maniculatus) in several experiments to determine how tick density, host defense, and ages of mice and ticks interact to influence feeding success. In the first experiment, tick-naive deer mice were infested with one of several densities of RMWT larvae, while a second cohort of mice were infested with 50 larvae each. Two weeks after ticks dropped off, mice in the first cohort were re-exposed to 50 larvae each and mice in the second cohort were re-exposed to varying densities of larvae. In the second experiment mice of different ages (45-374 days old) were exposed to 50 larvae each. Two weeks later mice were re-exposed to 50 larvae each. We combined data from these and several similar experiments to test the generality of the patterns we observed. Lastly, we tested whether tick feeding success was consistent on individual mice that were challenged on four occasions.

RESULTS

Mice acquired resistance such that feeding success declined dramatically from the first to the second infestation. Feeding success also declined with tick density and tick age. Mice, however, became more permissive with age. The sizes of these effects were similar and additive. Surprisingly, over successive infestations the relative resistance among mice changed among hosts within a cohort.

CONCLUSIONS

We predict that larval blood-feeding success, and thus development to the nymph stage, will change due to variation in tick age and density, as well as the age and history of the host. Incorporating these biotic factors into modeling of tick population dynamics may improve predictions of tick-borne pathogen transmission.

Canine vector-borne infections in Mauritius

Background

Canine vector-borne diseases have a worldwide distribution, but to the best of our knowledge, no research has been carried out to evaluate their presence on the Indian Ocean island of Mauritius. An investigation into canine vector-borne infections was conducted in dogs (n = 78) resident at an animal shelter in Port Louis, Mauritius using a combination of traditional microscopy and serological methods.

Methods

Ticks were manually collected from the stray dog population for identification as well as for quantifying tick burdens. Blood was also collected from each dog via either the jugular vein or the cephalic vein, and was stored in EDTA tubes. The stored blood was then used to measure PCV values, make blood smears for the identification of parasites, and used for serological testing of vector-borne disease.

Results

A total of 178 ticks were collected from 52 dogs and identified as Rhipicephalus sanguineus (175/178) or Amblyomma variegatum (3/178). Twenty-six (33%; 95% CI 23, 45) dogs were seropositive for Ehrlichia spp., and 12 (15%; 95% CI 8, 25) for Anaplasma spp., Dirofilaria antigen was detected in 14 (18%; 95% CI 10, 28), and nine (12%; 95% CI 5, 21) dogs had Hepatozoon canis gamonts observed in blood films during microscopic examination. Eleven (14%; 95% CI 7, 24) dogs were co-infected with two pathogens. Borrelia burgdorferi antibodies were not detected in any dogs.

Conclusions

Infection with these pathogens had no significant effect on the packed cell volume (PCV), but high tick burdens were significantly associated with the presence of a tick-borne pathogen. This is the first study of its kind on the dog population in Mauritius and demonstrates the presence of previously undocumented canine vector-borne infections on the island. The relatively high proportion of infected dogs within the study should alert clinicians to the presence of canine vector-borne diseases on the island of Mauritius.