Direct sequencing of Cryptosporidium in stool samples for public health

The protozoan parasite Cryptosporidium is an important cause of diarrheal disease (cryptosporidiosis) in humans and animals, with significant morbidity and mortality especially in severely immunocompromised people and in young children in low-resource settings. Due to the sexual life cycle of the parasite, transmission is complex. There are no restrictions on sexual recombination between sub-populations, meaning that large-scale genetic recombination may occur within a host, potentially confounding epidemiological analysis. To clarify the relationships between infections in different hosts, it is first necessary to correctly identify species and genotypes, but these differentiations are not made by standard diagnostic tests and more sophisticated molecular methods have been developed. For instance, multilocus genotyping has been utilized to differentiate isolates within the major human pathogens, Cryptosporidium parvum and Cryptosporidium hominis. This has allowed mixed populations with multiple alleles to be identified: recombination events are considered to be the driving force of increased variation and the emergence of new subtypes. As yet, whole genome sequencing (WGS) is having limited impact on public health investigations, due in part to insufficient numbers of oocysts and purity of DNA derived from clinical samples. Moreover, because public health agencies have not prioritized parasites, validation has not been performed on user-friendly data analysis pipelines suitable for public health practitioners. Nonetheless, since the first whole genome assembly in 2004 there are now numerous genomes of human and animal-derived cryptosporidia publically available, spanning nine species. It has also been demonstrated that WGS from very low numbers of oocysts is possible, through the use of amplification procedures. These data and approaches are providing new insights into host-adapted infectivity, the presence and frequency of multiple sub-populations of Cryptosporidium spp. within single clinical samples, and transmission of infection. Analyses show that although whole genome sequences do indeed contain many alleles, they are invariably dominated by a single highly abundant allele. These insights are helping to better understand population structures within hosts, which will be important to develop novel prevention strategies in the fight against cryptosporidiosis

Identifying and resolving genome misassembly issues important for biomarker discovery in the protozoan parasite, cryptosporidium

Cryptosporidium is a protozoan parasite that causes a diarrhoeal disease in humans, and which may be spread by swimming pools or infected municipal water supplies. It can be a serious health risk for individuals with weakened immune systems. Genomics has the potential to help control this pathogen, but until recently, it has not been possible to perform whole genome sequencing directly from human stool samples. This is no longer the case, and there are now at least a dozen high quality genomes available via resources like CryptoDB and NCBI, with other isolates being sequenced. The analysis of these genomes will improve current approaches for tracking sources of contamination and routes of transmission by allowing the identification of biomarkers, such as multiple-locus variable tandem repeat regions (VNTRs). However, problems remain due to highly uneven sequence coverage, which causes serious errors and artefacts in the genome assemblies produced by a number of popular assemblers. He re we discuss these assembly issues, and describe our strategy to generate genome assemblies of sufficient quality to enable the discovery of new VNTR biomarker