Biotechnological advances in the diagnosis of avian coccidiosis and the analysis of genetic variation in Eimeria

Abstract

Coccidiosis is an intestinal disease of chickens caused by various species of protozoan parasites within the genus Eimeria. This disease has a major economic impact to growers and to the poultry industry world-wide. The diagnosis and genetic characterization of the different species of Eimeria are central to the prevention, surveillance and control of coccidiosis, particularly now given the major problems with wide-spread resistance of Eimeria species against anticoccidial drugs (coccidiostats) and the residue problems associated with these compounds. While traditional methods have had major limitations in the specific diagnosis of coccidiosis, there have been significant advances in the development of molecular-diagnostic tools. The present article provides a background on coccidiosis, reviews the main molecular methods which have been used and describes recent advances in the establishment of polymerase chain reaction (PCR)-coupled electrophoretic approaches for the specific diagnosis of coccidiosis as well as the genetic characterization of species of Eimeria. These biotechnological advances are considered to represent a significant step toward the improved prevention and control of this important disease of poultry.

Introduction

Coccidiosis of chickens is an enteric parasitic disease caused by multiple species of the protozoan parasite genus Eimeria (Apicomplexa: Eucoccidia: Eimeriidae) and is one of the commonest and economically most important diseases of poultry world-wide (Shirley et al., 2005). It causes production losses, and high morbidity (due to acute, bloody enteritis) and mortality rates. While the control of coccidiosis has relied mainly on the preventative use of anticoccidial drugs (coccidiostats), together with the induction of species-specific natural immunity in chicken flocks (Shirley et al., 2004, Shirley et al., 2005), this widely used approach is costly and has led to serious problems with drug resistance in Eimeria populations (Williams, 1998). Due to these drug resistance problems, live, virulent vaccines have been utilized to protect chicken flocks against coccidiosis, particularly in intensive establishments (Williams, 2002b), and attenuated or precocious live vaccines are now finding wide application (Shirley et al., 2004, Shirley et al., 2005). In addition to the use of preventative chemotherapy and vaccination, the specific diagnosis of infection plays a key role in the prevention, surveillance and control of coccidiosis. Traditionally, diagnosis has been achieved by detecting and/or enumerating Eimeria oocysts excreted in the faeces from infected chickens and measuring oocyst/sporocyst dimensions, and (at post-mortem) assessing the site and extent of the pathological lesions caused by Eimeria in the intestine of chickens (Long and Joyner, 1984). However, these approaches can be unreliable, particularly given that multiple species of Eimeria can simultaneously infect the host and because there can be an “overlap” in the sizes of oocysts and the sites of infection in the intestines for some species (Long and Joyner, 1984). Increasingly, molecular tools have been developed and relied upon for the diagnosis of coccidiosis. The purpose of this article is to provide a brief background on the biology and life-cycle of Eimeria species of chickens and the disease they cause, to briefly describe methods of control, including vaccination, and to review traditional and molecular methods for the diagnosis of coccidiosis in chickens and the analysis of genetic variation within and among species of Eimeria, considering their relative merits, and emphasizing the prospects that these methods provide toward a better understanding and control of coccidiosis.