Population, genetic, and antigenic diversity of the apicomplexanEimeria tenellaand their relevance to vaccine development

Abstract

The phylum Apicomplexa includes serious pathogens of humans and animals. Understanding the distribution and population structure of these protozoan parasites is of fundamental importance to explain disease epidemiology and develop sustainable controls. Predicting the likely efficacy and longevity of subunit vaccines in field populations relies on knowledge of relevant preexisting antigenic diversity, population structure, the likelihood of coinfection by genetically distinct strains, and the efficiency of cross-fertilization. All four of these factors have been investigated for Plasmodium species parasites, revealing both clonal and panmictic population structures with exceptional polymorphism associated with immunoprotective antigens such as apical membrane antigen 1 (AMA1). For the coccidian Toxoplasma gondii only genomic diversity and population structure have been defined in depth so far; for the closely related Eimeria species, all four variables are currently unknown. Using Eimeria tenella, a major cause of the enteric disease coccidiosis, which exerts a profound effect on chicken productivity and welfare, we determined population structure, genotype distribution, and likelihood of crossfertilization during coinfection and also investigated the extent of naturally occurring antigenic diversity for the E. tenella AMA1 homolog. Using genome-wide Sequenom SNP-based haplotyping, targeted sequencing, and single-cell genotyping, we show that in this coccidian the functionality of EtAMA1 appears to outweigh immune evasion. This result is in direct contrast to the situation in Plasmodium and most likely is underpinned by the biology of the direct and acute coccidian life cycle in the definitive host.Eimeria | coccidiosis | population structure | chickens | food security

Application of a new PCR-RFLP panel suggests a restricted population structure for Eimeria tenella in UK and Irish chickens

“…Experimental recombinant or subunit vaccines have been described for many apicomplexans with varying levels of efficacy obtained under controlled laboratory or experimental animal conditions. However, commercial translation has commonly been hindered by a range of factors including naturally occurring genetic diversity in field parasites, resulting in insufficient immunological protection in the vaccinated host and selection of resistant populations (Dutta et al, 2007, Blake et al, 2015). If vaccines are to be successful and remain effective in the long term it is essential to understand the impacts that parasite genetic (antigenic) diversity and population structures have on the selection of field populations capable of vaccine escape.…”

“…For most other apicomplexans details on diversity are scarce; for example for Eimeria , the cause of the enteric disease coccidiosis, rather little is known at the molecular level. Recently, populations of Eimeria tenella parasites isolated from chicken facilities in Egypt, Libya, India and Nigeria were genotyped using a Sequenom MassARRAY single nucleotide polymorphism (SNP) tool, revealing notable variation in haplotype diversity and population structure with a North/South regional divide (Blake et al, 2015). E. tenella populations sampled below the 30th parallel north (30°N latitude) were defined by extensive haplotype diversity with no significant signatures of selection.…”

Avian Coccidiosis, New Strategies of Treatment

“…They found that the chicken immune system recognizes the expressed viral proteins, which is a significant precedent to develop a vaccine in future, due to the chance to express antigens that allow recognition of the parasite. A study about the population structure or E. tenella as well as their genotype distribution and the presence of the Apical Membrane Antigen 1 (AMA1) showed that this antigen outweighs immune evasion [1].…”

“…About 60 billion chickens are produced worldwide every year. However, Eimeria protozoan parasites of Phylum Apicomplexa are considered the main risk to avian production since they are the causative agent of avian coccidiosis [1]. For Avian coccidiosis is caused by seven species of Eimeria, which parasite chickens intestine resulting in economic losses around $2.4 billion per year worldwide [2].…”

“…The efforts to prevent this disease have been focused on developing vaccines and drugs with coccidiostatic and coccidicidal activity [4]. So far, genomic studies have revealed the wide antigenic variability of species such as Eimeria tenella, one of the most pathogenic parasites to chickens, which leads to develop a vaccine resistance rapidly [1].…”