A common multi-host parasite shows genetic structuring at the host species and population levels.

Although individual parasite species commonly infect many populations across physical space as well as multiple host species, the extent to which parasites traverse physical and phylogenetic distances is unclear. Population genetic analyses of parasite populations can reveal how parasites move across space or between host species, including helping assess whether a parasite is more likely to infect a different host species in the same location or the same host species in a different location. Identifying these transmission barriers could be exploited for effective disease control. Here, we analysed population genetic structuring of the parasite Pasteuria ramosa in daphniid host species from different lakes. Outbreaks occurred most often in the common host species Daphnia dentifera and Daphnia retrocurva. The genetic distance between parasite samples tended to be smaller when samples were collected from the same lake, the same host species and closer in time. Within lakes, the parasite showed structure by host species and sampling date; within a host species, the parasite showed structure by lake and sampling date. However, despite this structuring, we found the same parasite genotype infecting closely related host species, and we sometimes found the same genotype in nearby lakes. Thus, P. ramosa experiences challenges infecting different host species and moving between populations, but doing so is possible. In addition, the structuring by sampling date indicates potential adaptation to or coevolution with host populations and supports prior findings that parasite population structure is dynamic during outbreaks.

Virulence evolution during a naturally occurring parasite outbreak.

Virulence, the degree to which a pathogen harms its host, is an important but poorly understood aspect of host-pathogen interactions. Virulence is not static, instead depending on ecological context and potentially evolving rapidly. For instance, at the start of an epidemic, when susceptible hosts are plentiful, pathogens may evolve increased virulence if this maximizes their intrinsic growth rate. However, if host density declines during an epidemic, theory predicts evolution of reduced virulence. Although well-studied theoretically, there is still little empirical evidence for virulence evolution in epidemics, especially in natural settings with native host and pathogen species. Here, we used a combination of field observations and lab assays in the Daphnia-Pasteuria model system to look for evidence of virulence evolution in nature. We monitored a large, naturally occurring outbreak of Pasteuria ramosa in Daphnia dentifera, where infection prevalence peaked at ~ 40% of the population infected and host density declined precipitously during the outbreak. In controlled infections in the lab, lifespan and reproduction of infected hosts was lower than that of unexposed control hosts and of hosts that were exposed but not infected. We did not detect any significant changes in host resistance or parasite infectivity, nor did we find evidence for shifts in parasite virulence (quantified by host lifespan and number of clutches produced by hosts). However, over the epidemic, the parasite evolved to produce significantly fewer spores in infected hosts. While this finding was unexpected, it might reflect previously quantified tradeoffs: parasites in high mortality (e.g., high predation) environments shift from vegetative growth to spore production sooner in infections, reducing spore yield. Future studies that track evolution of parasite spore yield in more populations, and that link those changes with genetic changes and with predation rates, will yield better insight into the drivers of parasite evolution in the wild. Supplementary Information: The online version contains supplementary material available at 10.1007/s10682-022-10169-6.

Genotypic variation in an ecologically important parasite is associated with host species, lake and spore size.

Genetic variation in parasites has important consequences for host­parasite interactions. Prior studies of the ecologically important parasite Metschnikowia bicuspidata have suggested low genetic variation in the species. Here, we collected M. bicuspidata from two host species (Daphnia dentifera and Ceriodaphnia dubia) and two regions (Michigan and Indiana, USA). Within a lake, outbreaks tended to occur in one host species but not the other. Using microsatellite markers, we identified six parasite genotypes grouped within three distinct clades, one of which was rare. Of the two main clades, one was generally associated with D. dentifera, with lakes in both regions containing a single genotype. The other M. bicuspidata clade was mainly associated with C. dubia, with a different genotype dominating in each region. Despite these associations, both D. dentifera- and C. dubia-associated genotypes were found infecting both hosts in lakes. However, in lab experiments, the D. dentifera-associated genotype infected both D. dentifera and C. dubia, but the C. dubia-associated genotype, which had spores that were approximately 30% smaller, did not infect D. dentifera. We hypothesize that variation in spore size might help explain patterns of cross-species transmission. Future studies exploring the causes and consequences of variation in spore size may help explain patterns of infection and the maintenance of genotypic diversity in this ecologically important system.

Carboplatin-pemetrexed in treatment of patients with recurrent/metastatic cancers of the head and neck; superior outcomes in oropharyngeal primaries.

BACKGROUND: Platinum-based therapy in combination with 5-fluorouracil with cetuximab has shown the best survival in pts with recurrent/metastatic squamous cell carcinoma of the head and neck (R/M SCCHN). The purpose of this study is to evaluate the efficacy and tolerability of carboplatin, pemetrexed and to assess differential outcomes in patients with oropharyngeal primary and HPV related disease. PATIENTS AND METHODS: The charts of consecutive patients with R/M SCCHN were reviewed. All patients receiving at least one cycle of the two-drug regimen (pemetrexed 500 mg/m(2), carboplatin area under the curve of five intravenously), were included for assessment of response, safety, toxicity, and survival. RESULTS: A total of 86 patients received this regimen between January 2008 and December 2012, of which, 63 were included in this analysis. Forty-one percent (26) of the patients had cancers of the oropharynx, and of those, 50% had HPV-positive disease, 32% (20) had cancers of the larynx, and 24% (15) of the oral cavity. Median number of cycles administered was 4 (range 1-14 cycles) with 50% of the patients receiving four or more cycles. Half the patients achieved stable disease as their best response, 8% (5) attained a partial response, 24% progressed on therapy, and the remaining patients (12) could not have their response assessed. On the basis of Kaplan-Meier analysis, median progression-free survival (PFS) was 5.1 months (95% CI 3.2, 6.2) and median overall survival (OS) was 9.4 months (95% CI 4.3, 13.1). Among pts with oropharyngeal primary (n = 26), median PFS was 6.4 months (95% CI 2.8, 7.9) and median OS was 16.6 months (95% CI 9.6, 19.5). Among HPV+ pts (n = 13), median PFS was 7.0 months (95% CI 4.8, ne) and median OS was 17.1 months (95% CI 11.2, 21.7). CONCLUSION: Combination carboplatin-pemetrexed is an effective and well-tolerated treatment, associated with a median PFS of 5.1 months and a clinical benefit in at least 57% of the patients treated.