Emerging threat of Eimeria operational taxonomic units (OTUs) on poultry production

Coccidiosis is an antagonistic poultry disease which negatively impacts animal welfare and productivity. The disease is caused by an obligate, intracellular protozoon known as Eimeria. Several Eimeria species known to infect chickens have been well documented. However, recent studies have elucidated the emergence of three novel genetic variants or operational taxonomic units (OTUs). The discovery of OTUx, OTUy and OTUz complicates the identification and diagnosis of coccidiosis. OTUs are clusters of unknown or uncultivated organisms that are grouped according to a similarity in DNA sequence to a set of specific gene markers. OTUs have been reported in the Earth's Southern Hemisphere, including Australia, Venezuela, India, Zambia, Uganda, Tanzania, China and Ghana. Elucidating their impact on the poultry industry is fundamental in preventing anticoccidial resistance and to access the potential of OTUs as vaccine candidates to provide cross-protection against similar Eimeria species. The identification of OTUs further decreases the risk of false negative coccidial diagnosis. Therefore, this article reviews the importance and risk imposed by OTUs, coupled with their prevalence and geographical distribution in chickens globally.