Susceptibility to Ticks and Lyme Disease Spirochetes Is Not Affected in Mice Coinfected with Nematodes

Macro-parasites and micro-parasites co-exist in rodent communities but are associated with different community-level parameters

Wildlife species are often heavily parasitized by multiple infections simultaneously. Yet research on sylvatic transmission cycles, tend to focus on host interactions with a single parasite and neglects the influence of co-infections by other pathogens and parasites. Co-infections between macro-parasites and micro-parasites can alter mechanisms that regulate pathogenesis and are important for understanding disease emergence and dynamics. Wildlife rodent hosts in the Lyme disease system are infected with macro-parasites (i.e., ticks and helminths) and micro-parasites (i.e., Borrelia spp.), however, there has not been a study that investigates the interaction of all three parasites (i.e., I. pacificus, Borrelia spp., and helminths) and how these co-infections impact prevalence of micro-parasites. We live-trapped rodents in ten sites in northern California to collect feces, blood, ear tissue, and attached ticks. These samples were used to test for infection status of Borrelia species (i.e., micro-parasite), and describe the burden of ticks and helminths (i.e., macro-parasites). We found that some rodent hosts were co-infected with all three parasites, however, the burden or presence of concurrent macro-parasites were not associated with Borrelia infections. For macro-parasites, we found that tick burdens were positively associated with rodent Shannon diversity while negatively associated with predator diversity, whereas helminth burdens were not significantly associated with any host community metric. Ticks and tick-borne pathogens are associated with rodent host diversity, predator diversity, and abiotic factors. However, it is still unknown what factors helminths are associated with on the community level. Understanding the mechanisms that influence co-infections of multiple types of parasites within and across hosts is an increasingly critical component of characterizing zoonotic disease transmission and maintenance.

NAD(P)H fluorescence lifetime imaging of live intestinal nematodes reveals metabolic crosstalk between parasite and host

Infections with intestinal nematodes have an equivocal impact: they represent a burden for human health and animal husbandry, but, at the same time, may ameliorate auto-immune diseases due to the immunomodulatory effect of the parasites. Thus, it is key to understand how intestinal nematodes arrive and persist in their luminal niche and interact with the host over long periods of time. One basic mechanism governing parasite and host cellular and tissue functions, metabolism, has largely been neglected in the study of intestinal nematode infections. Here we use NADH (nicotinamide adenine dinucleotide) and NADPH (nicotinamide adenine dinucleotide phosphate) fluorescence lifetime imaging of explanted murine duodenum infected with the natural nematode Heligmosomoides polygyrus and define the link between general metabolic activity and possible metabolic pathways in parasite and host tissue, during acute infection. In both healthy and infected host intestine, energy is effectively produced, mainly via metabolic pathways resembling oxidative phosphorylation/aerobic glycolysis features. In contrast, the nematodes shift their energy production from balanced fast anaerobic glycolysis-like and effective oxidative phosphorylation-like metabolic pathways, towards mainly anaerobic glycolysis-like pathways, back to oxidative phosphorylation/aerobic glycolysis-like pathways during their different life cycle phases in the submucosa versus the intestinal lumen. Additionally, we found an increased NADPH oxidase (NOX) enzymes-dependent oxidative burst in infected intestinal host tissue as compared to healthy tissue, which was mirrored by a similar defense reaction in the parasites. We expect that, the here presented application of NAD(P)H-FLIM in live tissues constitutes a unique tool to study possible shifts between metabolic pathways in host-parasite crosstalk, in various parasitic intestinal infections.

Environmental exposure to cadmium reduces the primary antibody-mediated response of wood mice (Apodemus sylvaticus) from differentially polluted locations in the Netherlands

The Wood mouse (Apodemus sylvaticus) is a widespread mammalian species that acts as a reservoir host for multiple infections, including zoonotic diseases. Exposure to immunotoxins, like for instance trace metals, may reduce the ability of the host to mount proper responses to pathogens, potentially increasing the transmission and prevalence of infections. Antibody-mediated responses are crucial in preventing and limiting infections, and the quantification of the primary antibody response is considered a sensitive predictor of immunosuppression. The current study aims to investigate effects of cadmium exposure on the antibody-mediated responses of wood mice inhabiting polluted and non-polluted areas in the Netherlands. Wood mice were captured alive at different locations and immunized to sheep red blood cells (SRBC) to induce a primary antibody response. SRBC-specific antibody-producing cells, or plaque forming cells (PFC), were quantified and related to kidney cadmium levels. Differential circulating main leukocyte populations were also characterised. Cadmium concentrations in mice kidneys differed between mice captured at different locations, and increased with individual body mass, likely associated with age-related time of exposure. Effect of cadmium was apparent on the percentages of B cell counts in blood. Because of potential natural immune heterogeneity between wild rodent populations, mice immune responses were analysed and compared grouped by captured locations. Capture location had significant effect on the total counts of white blood cells. Increasing cadmium exposure in wood mice captured from polluted sites was associated with a decrease of splenic PFC counts. This field research shows that wood mice antibody responses can be impaired by cadmium exposure, even at low environmental levels, by affecting B cell functioning mainly. Impaired B cell function can make exposed mice more susceptible to infections, potentially increasing the reservoir function of their populations. It also shows that immunomodulatory effects in the field should be assessed site specifically.

Sphingomyelinases in a journey to combat arthropod-borne pathogen transmission

Ixodes scapularis ticks feed on humans and other vertebrate hosts and transmit several pathogens of public health concern. Tick saliva is a complex mixture of bioactive proteins, lipids and immunomodulators, such as I. scapularis sphingomyelinase (IsSMase)-like protein, an ortholog of dermonecrotoxin SMase D found in the venom of Loxosceles spp. of spiders. IsSMase modulates the host immune response towards Th2, which suppresses Th1-mediated cytokines to facilitate pathogen transmission. Arboviruses utilize exosomes for their transmission from tick to the vertebrate host, and exosomes derived from tick saliva/salivary glands suppress C-X-C motif chemokine ligand 12 and interleukin-8 immune response(s) in human skin to delay wound healing and repair processes. IsSMase affects also viral replication and exosome biogenesis, thereby inhibiting tick-to-vertebrate host transmission of pathogenic exosomes. In this review, we elaborate on exosomes and their biogenesis as potential candidates for developing novel control measure(s) to combat tick-borne diseases. Such targets could help with the development of an efficient anti-tick vaccine for preventing the transmission of tick-borne pathogens.

Perpetuation of Borreliae

With one exception (epidemic relapsing fever), borreliae are obligately maintained in nature by ticks. Although some Borrelia spp. may be vertically transmitted to subsequent generations of ticks, most require amplification by a vertebrate host because inheritance is not stable. Enzootic cycles of borreliae have been found globally; those receiving the most attention from researchers are those whose vectors have some degree of anthropophily and, thus, cause zoonoses such as Lyme disease or relapsing fever. To some extent, our views on the synecology of the borreliae has been dominated by an applied focus, viz., analyses that seek to understand the elements of human risk for borreliosis. But, the elements of borrelial perpetuation do not necessarily bear upon risk, nor do our concepts of risk provide the best structure for analyzing perpetuation. We identify the major global themes for the perpetuation of borreliae, and summarize local variations on those themes, focusing on key literature to outline the factors that serve as the basis for the distribution and abundance of borreliae.

Susceptibility to infection with Borrelia afzelii and TLR2 polymorphism in a wild reservoir host

The study of polymorphic immune genes in host populations is critical for understanding genetic variation in susceptibility to pathogens. Controlled infection experiments are necessary to separate variation in the probability of exposure from genetic variation in susceptibility to infection, but such experiments are rare for wild vertebrate reservoir hosts and their zoonotic pathogens. The bank vole (Myodes glareolus) is an important reservoir host of Borrelia afzelii, a tick-borne spirochete that causes Lyme disease. Bank vole populations are polymorphic for Toll-like receptor 2 (TLR2), an innate immune receptor that recognizes bacterial lipoproteins. To test whether the TLR2 polymorphism influences variation in the susceptibility to infection with B. afzelii, we challenged pathogen-free, lab-born individuals of known TLR2 genotype with B. afzelii-infected ticks. We measured the spirochete load in tissues of the bank voles. The susceptibility to infection with B. afzelii following an infected tick bite was very high (95%) and did not differ between TLR2 genotypes. The TLR2 polymorphism also had no effect on the spirochete abundance in the tissues of the bank voles. Under the laboratory conditions of our study, we did not find that the TLR2 polymorphism in bank voles influenced variation in the susceptibility to B. afzelii infection.

Factors associated with diversity, quantity and zoonotic potential of ectoparasites on urban mice and voles

Wild rodents are important hosts for tick larvae but co-infestations with other mites and insects are largely neglected. Small rodents were trapped at four study sites in Berlin, Germany, to quantify their ectoparasite diversity. Host-specific, spatial and temporal occurrence of ectoparasites was determined to assess their influence on direct and indirect zoonotic risk due to mice and voles in an urban agglomeration. Rodent-associated arthropods were diverse, including 63 species observed on six host species with an overall prevalence of 99%. The tick Ixodes ricinus was the most prevalent species, found on 56% of the rodents. The trapping location clearly affected the presence of different rodent species and, therefore, the occurrence of particular host-specific parasites. In Berlin, fewer temporary and periodic parasite species as well as non-parasitic species (fleas, chiggers and nidicolous Gamasina) were detected than reported from rural areas. In addition, abundance of parasites with low host-specificity (ticks, fleas and chiggers) apparently decreased with increasing landscape fragmentation associated with a gradient of urbanisation. In contrast, stationary ectoparasites, closely adapted to the rodent host, such as the fur mites Myobiidae and Listrophoridae, were most abundant at the two urban sites. A direct zoonotic risk of infection for people may only be posed by Nosopsyllus fasciatus fleas, which were prevalent even in the city centre. More importantly, peridomestic rodents clearly supported the life cycle of ticks in the city as hosts for their subadult stages. In addition to trapping location, season, host species, body condition and host sex, infestation with fleas, gamasid Laelapidae mites and prostigmatic Myobiidae mites were associated with significantly altered abundance of I. ricinus larvae on mice and voles. Whether this is caused by predation, grooming behaviour or interaction with the host immune system is unclear. The present study constitutes a basis to identify interactions and vector function of rodent-associated arthropods and their potential impact on zoonotic diseases.

Toxoplasma Co-infection Prevents Th2 Differentiation and Leads to a Helminth-Specific Th1 Response

Nematode infections, in particular gastrointestinal nematodes, are widespread and co-infections with other parasites and pathogens are frequently encountered in humans and animals. To decipher the immunological effects of a widespread protozoan infection on the anti-helminth immune response we studied a co-infection with the enteric nematode Heligmosomoides polygyrus in mice previously infected with Toxoplasma gondii. Protective immune responses against nematodes are dependent on parasite-specific Th2 responses associated with IL-4, IL-5, IL-13, IgE, and IgG1 antibodies. In contrast, Toxoplasma gondii infection elicits a strong and protective Th1 immune response characterized by IFN-γ, IL-12, and IgG2a antibodies. Co-infected animals displayed significantly higher worm fecundity although worm burden remained unchanged. In line with this, the Th2 response to H. polygyrus in co-infected animals showed a profound reduction of IL-4, IL-5, IL-13, and GATA-3 expressing T cells. Co-infection also resulted in the lack of eosinophilia and reduced expression of the Th2 effector molecule RELM-β in intestinal tissue. In contrast, the Th1 response to the protozoan parasite was not diminished and parasitemia of T. gondii was unaffected by concurrent helminth infection. Importantly, H. polygyrus specific restimulation of splenocytes revealed H. polygyrus-reactive CD4⁺ T cells that produce a significant amount of IFN-γ in co-infected animals. This was not observed in animals infected with the nematode alone. Increased levels of H. polygyrus-specific IgG2a antibodies in co-infected mice mirrored this finding. This study suggests that polarization rather than priming of naive CD4⁺ T cells is disturbed in mice previously infected with T. gondii. In conclusion, a previous T. gondii infection limits a helminth-specific Th2 immune response while promoting a shift toward a Th1-type immune response.

Closely-related Borrelia burgdorferi (sensu stricto) strains exhibit similar fitness in single infections and asymmetric competition in multiple infections

Background

Wild hosts are commonly co-infected with complex, genetically diverse, pathogen communities. Competition is expected between genetically or ecologically similar pathogen strains which may influence patterns of coexistence. However, there is little data on how specific strains of these diverse pathogen species interact within the host and how this impacts pathogen persistence in nature. Ticks are the most common disease vector in temperate regions with Borrelia burgdorferi, the causative agent of Lyme disease, being the most common vector-borne pathogen in North America. Borrelia burgdorferi is a pathogen of high public health concern and there is significant variation in infection phenotype between strains, which influences predictions of pathogen dynamics and spread.

Methods

In a laboratory experiment, we investigated whether two closely-related strains of B. burgdorferi (sensu stricto) showed similar transmission phenotypes, how the transmission of these strains changed when a host was infected with one strain, re-infected with the same strain, or co-infected with two strains. Ixodes scapularis, the black-legged tick, nymphs were used to sequentially infect laboratory-bred Peromyscus leucopus, white-footed mice, with one strain only, homologous infection with the same stain, or heterologous infection with both strains. We used the results of this laboratory experiment to simulate long-term persistence and maintenance of each strain in a simple simulation model.

Results

Strain LG734 was more competitive than BL206, showing no difference in transmission between the heterologous infection groups and single-infection controls, while strain BL206 transmission was significantly reduced when strain LG734 infected first. The results of the model show that this asymmetry in competition could lead to extinction of strain BL206 unless there was a tick-to-host transmission advantage to this less competitive strain.

Conclusions

This asymmetric competitive interaction suggests that strain identity and the biotic context of co-infection is important to predict strain dynamics and persistence.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1964-9) contains supplementary material, which is available to authorized users.