Disease tolerance alters host competence in a wild songbird

Individuals can express a range of disease phenotypes during infection, with important implications for epidemics. Tolerance, in particular, is a host response that minimizes the per-pathogen fitness costs of infection. Because tolerant hosts show milder clinical signs and higher survival, despite similar pathogen burdens, their potential for prolonged pathogen shedding may facilitate the spread of pathogens. To test this, we simulated outbreaks of mycoplasmal conjunctivitis in house finches, asking how the speed of transmission varied with tissue-specific and behavioural components of tolerance, milder conjunctivitis and anorexia for a given pathogen load, respectively. Because tissue-specific tolerance hinders pathogen deposition onto bird feeders, important transmission hubs, we predicted it would slow transmission. Because behavioural tolerance should increase interactions with bird feeders, we predicted it would speed transmission. Our findings supported these predictions, suggesting that variation in tolerance could help identify individuals most likely to transmit pathogens.

Complexity and critical thresholds in the dynamics of visceral leishmaniasis

We study a general multi-host model of visceral leishmaniasis including both humans and animals, and where host and vector characteristics are captured via host competence along with vector biting preference. Additionally, the model accounts for spatial heterogeneity in human population and heterogeneity in biting behaviour of sandflies. We then use parameters for visceral leishmaniasis in the Indian subcontinent as an example and demonstrate that the model exhibits backward bifurcation, i.e. it has a human infection and a sandfly population threshold, characterized by a bi-stable region. These thresholds shift as a function of host competence, host population size, vector feeding preference, spatial heterogeneity, biting heterogeneity and control efforts. In particular, if control is applied through human treatment a new and lower human infection threshold is created, making elimination difficult to achieve, before eventually the human infection threshold no longer exists, making it impossible to control the disease by only reducing the infection levels below a certain threshold. A better strategy would be to reduce the human infection below a certain threshold potentially by early diagnosis, control animal population levels and keep the vector population under check. Spatial heterogeneity in human populations lowers the overall thresholds as a result of weak migration between patches.

General Microbiota of the Soft Tick Ornithodoros turicata Parasitizing the Bolson Tortoise (Gopherus flavomarginatus) in the Mapimi Biosphere Reserve, Mexico

The general bacterial microbiota of the soft tick Ornithodoros turicata found on Bolson tortoises (Gopherus flavomarginatus) were analyzed using next generation sequencing. The main aims of the study were to establish the relative abundance of bacterial taxa in the tick, and to document the presence of potentially pathogenic species for this tortoise, other animals, and humans. The study was carried-out in the Mapimi Biosphere Reserve in the northern-arid part of Mexico. Bolson tortoises (n = 45) were inspected for the presence of soft ticks, from which 11 tortoises (24.4%) had ticks in low loads (1–3 ticks per individual). Tick pools (five adult ticks each) were analyzed through 16S rRNA V3–V4 region amplification in a MiSeq Illumina, using EzBioCloud as a taxonomical reference. The operational taxonomic units (OTUs) revealed 28 phyla, 84 classes, 165 orders, 342 families, 1013 genera, and 1326 species. The high number of taxa registered for O. turicata may be the result of the variety of hosts that this tick parasitizes as they live inside G. flavomarginatus burrows. While the most abundant phyla were Proteobacteria, Actinobacteria, and Firmicutes, the most abundant species were two endosymbionts of ticks (Midichloria-like and Coxiella-like). Two bacteria documented as pathogenic to Gopherus spp. were registered (Mycoplasma spp. and Pasteurella testudinis). The bovine and ovine tick-borne pathogens A. marginale and A. ovis, respectively, were recorded, as well as the zoonotic bacteria A. phagocytophilum,Coxiella burnetii, and Neoehrlichia sp. Tortoises parasitized with O. turicata did not show evident signs of disease, which could indicate a possible ecological role as a reservoir that has yet to be demonstrated. In fact, the defense mechanisms of this tortoise against the microorganisms transmitted by ticks during their feeding process are still unknown. Future studies on soft ticks should expand our knowledge about what components of the microbiota are notable across multiple host–microbe dynamics. Likewise, studies are required to better understand the host competence of this tortoise, considered the largest terrestrial reptile in North America distributed throughout the Chihuahuan Desert since the late Pleistocene.

Experimental comparison of Baylisascaris procyonis definitive host competence between domestic dogs and raccoons (Procyon lotor)

Domestic dogs can function as either paratenic or definitive hosts for the zoonotic raccoon roundworm Baylisascaris procyonis. However, factors leading to development of patent infections in dogs are under-studied. Here we compared infection dynamics of B. procyonis in dogs vs the natural raccoon host. Dogs and raccoons were inoculated 5000 or 500 B. procyonis eggs (n = 3 per dose) or were fed B. procyonis-infected laboratory mice (n = 3 per dose; mice inoculated with 1000 or 250 eggs). Fecal samples were analysed via flotation and a commercial coproantigen ELISA designed for detection of Toxocara spp. Two of 12 dogs (both received low dose larvae) developed patent infections; all 12 raccoons became infected with 10 developing patent infections. Compared with dogs, prepatent periods were shorter in raccoons and maximum egg outputs were much greater. Baylisascaris procyonis coproantigens were detectable via ELISA in all raccoons and the patently infected dogs. Finally, dogs spontaneously lost infections while all patently infected raccoons shed eggs until conclusion of the study. Our results demonstrate that dogs are clearly suboptimal hosts showing limited parasite establishment and fecundity vs raccoons. Despite the low competence, patently infected dogs still pose a risk for human exposure, emphasizing the importance of control measures.

Bird Species Involved in West Nile Virus Epidemiological Cycle in Southern Québec

Despite many studies on West Nile Virus (WNV) in the US, including the reservoir role of bird species and the summer shifts of the Culex mosquito, feeding from birds to mammals, there have been few equivalent studies in the neighboring regions of Canada where WNV is endemic. Here, a priority list of bird species likely involved in WNV transmission in the greater Montréal area is constructed by combining three sources of data: (i) from WNV surveillance in wild birds (2002–2015); (ii) blood meal analysis of Culex pipiens–restuans (CPR), the primary enzootic vectors of WNV in the region, collected from surveillance in 2008 and 2014; (iii) literature review on the sero-prevalence/host competence of resident birds. Each of these data sources yielded 18, 23 and 53 species, and overall, 67 different bird species were identified as potential WNV amplifiers/reservoirs. Of those identified from CPR blood meals, Common starlings, American robins, Song sparrows and House sparrows ranked the highest and blood meal analysis demonstrated a seasonal shift in feed preference from birds to mammals by CPR. Our study indicates that there are broad similarities in the ecology of WNV between our region and the northeastern US, although the relative importance of bird species varies somewhat between regions.

On the Fly: Interactions Between Birds, Mosquitoes, and Environment That Have Molded West Nile Virus Genomic Structure Over Two Decades

West Nile virus (WNV) was first identified in North America almost 20 yr ago. In that time, WNV has crossed the continent and established enzootic transmission cycles, resulting in intermittent outbreaks of human disease that have largely been linked with climatic variables and waning avian seroprevalence. During the transcontinental dissemination of WNV, the original genotype has been displaced by two principal extant genotypes which contain an envelope mutation that has been associated with enhanced vector competence by Culex pipiens L. (Diptera: Culicidae) and Culex tarsalis Coquillett vectors. Analyses of retrospective avian host competence data generated using the founding NY99 genotype strain have demonstrated a steady reduction in viremias of house sparrows over time. Reciprocally, the current genotype strains WN02 and SW03 have demonstrated an inverse correlation between house sparrow viremia magnitude and the time since isolation. These data collectively indicate that WNV has evolved for increased avian viremia while house sparrows have evolved resistance to the virus such that the relative host competence has remained constant. Intrahost analyses of WNV evolution demonstrate that selection pressures are avian species-specific and purifying selection is greater in individual birds compared with individual mosquitoes, suggesting that the avian adaptive and/or innate immune response may impose a selection pressure on WNV. Phylogenomic, experimental evolutionary systems, and models that link viral evolution with climate, host, and vector competence studies will be needed to identify the relative effect of different selective and stochastic mechanisms on viral phenotypes and the capacity of newly evolved WNV genotypes for transmission in continuously changing landscapes.