The Eimeriid Coccidia

Alex Knight

Ph.D. Student, School of Biological Sciences, The University of Auckland, New Zealand

If you were asked to think of a prolific pathogenic parasite of humans and wildlife, malaria or maybe a helminth worm might come to mind, but probably few people would consider coccidia. In fact, in my experience many people have never even heard of this group of organisms, which is somewhat surprising as coccidia are, perhaps, the greatest hitchhikers the natural world has ever known. Coccidia are recognized as a class of protozoa that are known to parasitize a vast range of animals including birds, amphibians, reptiles, arthropods and mammals, including humans. This broad range of potential hosts has enabled this taxon to become globally distributed. The radiation of these freeloaders was probably an ancient affair, and their long co-evolutionary histories with their varied hosts have resulted in a complex and extremely speciose phylogenetic (evolutionary) tree. This phylogenetic complexity has led to taxonomical controversy that has been deftly reviewed by Tenter et al. (2002).        

 Figure 1. Coccidia extracted from hihi, a threatened passerine bird endemic to New Zealand. The coccidia on the left and right have undergone the process of sporulation and formed two sporocysts each containing the sporozoites, which are required to infect the next host. The coccidium in the centre has yet to sporulate.

Figure 1. Coccidia extracted from hihi, a threatened passerine bird endemic to New Zealand. The coccidia on the left and right have undergone the process of sporulation and formed two sporocysts each containing the sporozoites, which are required to infect the next host. The coccidium in the centre has yet to sporulate.

The Eimeriidae are a family within the coccidia, whose membership - although often revised - includes some of the taxon’s most notorious members. Different taxonomic classifications based on morphological characters of this family can include upward of 10 genera and thousands of species. I continue the focus of this article on two genera, Eimeria and Isospora. The two genera were classically distinguished using morphological features such as the number of sporocysts and sporozoites, structures that develop during the exogenous stage of the parasite (Fig. 1). Isospora spp. have two sporocysts containing four sporozoites, the invasive stage of the parasite, while Eimeria spp. have four sporocysts with two sporozoites. 

Coccidia of the genera Isospora and Eimeria live by invading the cells of their host and reproducing asexually and then sexually within the cell before being excreted back into the environment. It appears that some coccidia are restricted to invading epithelial cells of the gastro-intestinal tract of their host. Others, however, are capable of invading immune system cells and finding their way to the viscera (Box 1977). This latter type of infection may be much more pathogenic, but what causes this difference in infection location is not well understood. The development of infection at different sites in the host could be related to different strains of coccidia that are capable of infecting differing cell types or be influenced by the physical state of the host. One of the major symptoms of coccidiosis (the collective term for the pathologies these parasites cause) is an inability to absorb nutrients. This affects the body condition of the host by reducing their mass and inhibiting proper development or maintenance of the musculature. Severe infections with coccidia can be lethal, and pathogenicity varies among coccidial species.

Eimeria are often associated with domestic fowl and can cause substantial economic losses to the poultry industry. Indeed, if anyone is familiar with coccidia and coccidiosis, it is probably poultry farmers. The cost that these parasites impose on the poultry industry is estimated to be in the hundreds of millions of dollars annually (Allen & Fetterer 2002). Isospora are closely related to Eimeria; species of this genus are frequently found in wild birds and have been linked to reduced fitness and high mortality rates leading to concerns about their impact on species of conservation concern (Knight et al. 2018). For example, a captive population of the Critically Endangered blue-crowned laughingthrush had failed to rear chicks past 1 year-of-age for 17 consecutive years (Mohr et al. 2017). Investigations identified coccidia to be present in both adults and chicks and was potentially the cause of such high mortality among the chicks of the population. 

Notwithstanding the dangers associated with this taxon and their more cherished hosts, these organisms are a remarkable example of adaptation: they are passed from host to host via a faecal-oral pathway, i.e. one organism has to excrete it before another consumes it. While in the high density environments of chicken farms, frequent host to host transmission is easily imaginable, it is an incredible feat of transference that an organism about 20 to 30 microns in diameter with no locomotive ability in its exogenous stage can find its way among hosts in the great outdoors. Coccidia that infect passerine birds have evolved a clever adaptation to achieve such a feat. They appear to control the timing of their excretion from their hosts, so that the majority of coccidia are expelled in the late afternoon or evening (reviewed in Knight et al. 2018). This has a two-fold advantage, it increases the concentration of coccidia in the environment prior to peaks in avian feeding activity, the afternoon and the following morning, while avoiding the harmful UV rays of the midday sun. 

Knowledge regarding this taxon has remained somewhat esoteric to date. If the global biodiversity crisis continues, understanding the epidemiology of this parasite will become increasingly pertinent for managing threatened hosts. Parasites, despite having the potential to regulate host populations, are also a component of biodiversity that will be lost along with their hosts in the event of their extinction (Spencer & Zuk 2016). For more information please see our recent review of coccidia in passerines: “The Evolutionary Biology, Ecology and Epidemiology of Coccidia of Passerine Birds” published this year in Advances in Parasitology (Knight et al. 2018). 

About the Author

Alex Knight is currently working on the epidemiology of coccidian infection in the hihi (Notiomystis cintca), a threatened passerine bird endemic to New Zealand. Alex is a Ph.D. candidate in conservation biology at the University of Auckland whose professional interests include wildlife epidemiology, population genetics and population biology. He is supervised by Dr Anna Santure at the University of Auckland, and co-supervised by Dr John Ewen and Patricia Brekke at the Zoological Society of London.


Allen, P.C. & Fetterer, R.H. 2002. Recent advances in biology and immunobiology of Eimeria species and in diagnosis and control of infection with these coccidian parasites of poultry. Clinical Microbiology Reviews 15: 58–65.

Box, E.D. 1977. Life cycles of two Isospora species in the canary, Serinus canarius Linnaeus. Journal of Protozoology 24: 57–67.

Knight, A., J.G. Ewen, P. Brekke & A.W. Santure. 2018. The Evolutionary Biology, Ecology and Epidemiology of Coccidia of Passerine Birds. Advances in Parasitology. 99: 35–60.

Mohr, F., M. Betson & B. Quintard. 2017. Investigation of the presence of Atoxoplasma spp. in blue-crowned laughingthrush (Dryonastes courtoisi) adults and neonates. Journal of Zoo and Wildlife Medicine 48: 1–6.

Spencer, H.G. & M. Zuk. 2016. For Host's Sake: The Pluses of Parasite Preservation. Trends in Ecology and Evolution 31: 341–343.

Tenter, A.M., J.R. Barta, I. Beveridge, D.W. Duszynski, H. Mehlhorn, D.A. Morrison, R.C.A. Thompson & P.A. Conrad. 2002. The conceptual basis for a new classification of the coccidia. International Journal for Parasitology 32: 595–616.