Oo-complexity - a note on the Eimeria oocyst

As is well known among poultry professionals, subclinical coccidiosis is a huge challenge to the industry. Reinfection in poultry houses is the single most important route of disease sustenance in the population, especially in the deep-litter system. Caged and raised cage systems, although much more effective in keeping other infections low, do not completely isolate the birds from infective stages. Excellent husbandry practices and prophylactic medication are the mainstays of infection-reduction plans. To say that current control strategies are not perfect is an understatement. All that can be done is being done and yet we are getting nowhere. Given the production parameters on high throughput farms, the sheer number of birds raised poses a big economic challenge. Cost of control adds to the variable production cost and raises the cost of production per unit of meat/eggs. The biggest problem with coccidiosis control lies in our inability to control of the oocysts of the parasitic genus, Eimeria.

Eimeria is an obligate protozoan parasite that belongs to the phylum Apicomplexa. The lifecycle is complex and monoxenous (sexual and asexual development takes place in the same definitive host). The oocysts that are formed at the end of sexual development are shed in the feces. They sporulate in the environment and are infectious to susceptible birds.

(Source : http://www.ars.usda.gov/Main/docs.htm?docid=11018; Creative Commons License)

Host and site specificity is maintained during infection. The variability in the virulence of the parasite allows it to cause either hemorrhagic enteritis or malabsorptive enteritis. Now, if you have ever taken a college course in Protozoology, you might already know all that. You might also know that appropriate temperature and oxygen conditions are needed for sporulation, at the end of which the infectious oocyst contain four sporocysts with two sporozoites each (Eimeria sps.). But, let us move in a little deeper, beyond textbook material. What makes the parasite tick (no pun intended)? How is such a complex lifecycle so effortlessly maintained?

The answer lies in the utter complexity of the oocyst .

(Source : http://www.ars.usda.gov/Main/docs.htm?docid=11018; Creative Commons License)

Epidemiologically, it has long been recognized that altering the transmission stage of a pathogen can adversely affect it's survivability. This lends credence to the idea that changing the temperature, pH , oxygen availability etc among other environmental factors changes oocyst viability and transmissibility.

The oocyst is quite resistant, to dessication and hence destruction. In terms of parasitic survival, the oocyst occupies a marvellous functional niche. It can be favorably compared to the eggs of birds with the oocyst wall performing the same function as the calcareous shell, providing strength , protection and stability.
Equivalent to hatching is the process of excystation, by which infective sporozoites are released at the predilection site. The oocyst wall is sturdy enough to resist extreme challenges like the action of detergents (to which most bacteria are susceptible), mechanical force, enzyme action and strong chemical reagents such as bleach and potassium dichromate. Sodium hypochlorite at various strengths removes parts of the oocyst wall.

  In a recent study, Jenkins et al., compared the "Differing susceptibilities of Eimeria acervulina, E. maxima and E. tenella oocysts to dessication". The group concluded that Eimeria maxima oocysts displayed the highest resistance to dessication and were able to survive even in dry litter material. (Jenkins et al., 2013)

The protein rich oocyst wall starts out as electron dense wall forming bodies (WFB) in the endoplasmic reticulum and cytoplasm of the macrogamete. The veil forming bodies, WFB1 and 2 , are sequentially released after 'fertilization' of the macrogamete by the microgamete, to form the bilayered oocyst wall. [The layers are separated by a zone that spans 40 nm, (allowing the outer wall to be stripped away by bleach)] .
The wall forming bodies are chock full of proproteins that ultimately form the wall after post-translational modifications viz. N and C terminal modifications. Biochemical analysis of the wall proteins, using the combined gas chromatography- mass spectrometry technique, by Mai et al. revealed that the walls are mainly made up of proteins in which five amino acids predominate , viz. alanine , proline, valine, aspartic acid and isoleucine. Others have found that tyrosine also predominates and is important in dityrosine-protein cross linking. Lipids and carbohydrates contribute a little to the wall, with the predominant moieties being palmitic acid, stearic acid, oleic acid, linoleic acid, behenic acid, lignoceric acid, cholestane and cholesterol. (Mai et al., 2009)

So , what exactly confers the complexity ? It seems to me that the more we learn about the oocyst wall, the more we understand that there is more that we do not understand. Some questions remain. How do the dityrosine protein crosslinkings provide such enormous strength? What genes encode these proteins? Are there multiple genes that control these functions? Is the nature of the oocyst similar across the apicomplexans? Is it in anyway similar to egg walls of multicellular eukaryotic parasites? Comparative molecular parasitology might be the only way to answer these questions that deal with the very nature of the fabric of life of the oocyst.


References : 

1. Belli SI, Smith NC, Ferguson DJ. The coccidian oocyst: a tough nut to crack! Trends Parasitol 2006;22:416-423.

2. Jenkins MC, Parker C, O'Brien C, et al. Differing susceptibilities of Eimeria acervulina, Eimeria maxima, and Eimeria      

    tenella oocysts to desiccation. J Parasitol 2013;99:899-902.

3. Mai K, Sharman PA, Walker RA, et al. Oocyst wall formation and composition in coccidian parasites. Mem Inst        

   Oswaldo Cruz 2009;104:281-289.