Eimeria was the last major group of apicomplexa to be fully analysed. A branch of the phylogenetic (evolutionary) tree, the apicomplexa radiate outwards into various categories, including two of eimeria’s more famous cousins: plasmodia, the parasites behind malaria, which kills hundreds of thousands of people every year; and Toxoplasma gondii, a mind-altering protist that infects felines, rodents and a variety of dead-end hosts (including humans).
Eimeria may not have quite the same qualities as plasmodia and toxoplasma, both of which are intriguing enough to have been featured in National Geographic, but it does have some qualities that make it interesting not just to the poultry industry, but to the scientific community at large – as a model organism for comparative work on apicomplexa in general.
Unlike the life cycles of plasmodium and toxoplasma, which involve multiple hosts, eimeria only requires one. This makes it a convenient model to work with, because it can be propagated to high numbers in a single life cycle with relative ease.
It’s also particularly interesting because it represents an extreme end of the spectrum in terms of its host specificity. At one end, toxoplasma which is capable of infecting virtually any warm-blooded animal and any type of nucleated cell. It is so liberal that there is only one species within the genera – Toxoplasma gondii.
Eimeria, on the other hand, are so specific that there are over 1,200 known species, virtually all of which are specific to a single host. This goes right down to tissue level; each of the seven eimeria species which infect chickens are known to colonise their own preferred region within the animal’s gut.
Now that all seven avian eimeria genomes have been sequenced and to a large extent annotated, comparisons can made among the genomes of toxoplasma, plasmodia and eimeria. Any major variations could offer insights into the mechanisms behind their differing host specificities.
The importance of emerging zoonosis from parasites such as Plasmodium knowlesi, a malarial parasite that was recently found to be capable of infecting both monkeys and humans, and which accounts for over 50% of malaria cases in Malaysia, is just one example of how insights gained from research on eimeria have uses beyond the narrow field of poultry farming.
Already, research has thrown up some interesting clues. Prof Dr Wan Kiew Lian says two things stand out.
“Evidence suggests that there is a unique banding pattern that cuts across all seven species. These are extremely unusual. Such repeats normally occur between genes (the coding regions of a genome), but in eimeria, they occur within genes and yet do not disturb their function,” he explains.
Secondly, the genes that code for the parasites’ surface antigens, which are the first point of contact between parasite and host, are very different from those found in other parasites. Most genes are similar in closely related species, indicating a common evolutionary origin.
“But these surface antigen genes in the seven species of eimeria are completely different from their closest relatives,” says Wan. “This may unlock the question of whether there is one or multiple ‘keys’ involved in recognising different host cells. This could eventually tell us something about why toxoplasma will bind to anything, but eimeria are very specific.”