Coccidiosis is an intestinal disease that causes destruction of the enterocytes of the gut wall, leading to enteritis.
Clinical signs range from depression, ruffled feathers, arched body, diarrhoea and bloody droppings to the death of the bird.
For the broiler industry, what has a greater impact is subclinical coccidiosis which undermines the efficiency with which feed is digested, leading to poor growth rates, poor feed conversion and a decrease in body weight.
The causative agent of coccidiosis in chickens is a protozoan parasite of the phylum Apicomplexa, genus Eimeria.
Seven species of Eimeria (E. acervulina, E. brunetti, E. maxima, E. mitis, E. necatrix, E. praecox and E. tenella) are recognized as being able to affect chickens of the genus Gallus gallus.
The lifecycle of all these species of Eimeria is homoxenous and hosts are infected by the oral-faecal route.
Chickens ingest sporulated oocysts, which contain four sporocysts – each containing two sporozoites – from contaminated litter and which then pass into the gizzard, where sporocysts excyst from the oocysts, thanks to the mechanical grinding of the gizzard together with the feed and its acid content.
Subsequently, the sporocysts reach the intestine, where – due to enzymes, bile and CO2 – they release the sporozoites which invade the cells of the intestinal wall.
Each species of Eimeria has a specific gut localization where it undergoes schizogony, causing different degrees of lesions depending on the seriousness of the infection.
Sporozoites penetrate into the enterocytes’ core and start nuclear division followed by cytoplasmic differentiation.
This replicative phase, called schizogony or asexual replication, leads to cellular damage in the epithelium and in fact ends with the formation of a mother cell, called a schizont, with the first generation of merozoites inside.
The schizont grows, breaks the enterocytes and releases the first generation merozoites that penetrate other host cells. During the 2nd and 3rd phase of asexual replication, gut damage becomes evident because of the high number of merozoites infecting enterocytes.
After at least two generations of asexual reproduction, Eimeria merozoites enter sexual replication or gamogony; in fact they invade the enterocytes and differentiate into either male (microgamonts) or female (macrogamonts).
The microgamonts release many microgametes that exit, seek and fertilize the macrogamonts. This fusion produces a zygote (immature oocyst) that is subsequently excreted in the faeces. The entire Eimeria cycle usually takes 4-7 days depending on the species.
Given the correct environmental conditions (warmth, oxygen and moisture), the oocyst sporulates and becomes infective, undergoing sporogony (a meiotic process) that takes about 24-48 hours.
This short life-cycle combined with the potential for massive reproductive capability during the intracellular phase, makes this group of parasites a serious problem under intensive farming conditions.
This video explains the key points of the Eimeria biological cycle and principles of coccidiosis immunology: https://www.youtube.com/watch?v=euHBghFhJRc&list=PL7fOBlEcrBF_4ve0-_A6ytcZPt2Gyj5U2&index=2
The oocyst count, commonly referred to as OPG (oocysts per gram of faeces), has been used for decades as the most common way to detect Eimeria oocysts on farms and it is still widely performed with the same purpose.
However, the simple presence of oocysts does not necessarily mean that a coccidiosis outbreak will occur soon on that farm, as Eimeria oocysts are ubiquitous.
Therefore, the significance of this finding must be weighed against the level of OPG first and then other indicators of disease, such as the presence of clinical signs in live animals, as well as macro- and microscopic lesions in the intestine at necropsy.
In spite of this, the oocyst count has been shown to be very useful when monitoring the farm with weekly sampling, as the results can give us an indication of either the development of anticoccidial resistance or of the vaccine intake and onset of immunity after coccidiosis vaccination.
The PCR technique for Eimeria – recently introduced as a routine test in more and more diagnostic labs – also has advantages and limitations.
The main advantage is the possibility of an accurate species identification of the Eimeria spp. contained in the sample.
In fact, field samples may contain debris and impurities and even if they are well preserved (refrigeration at 4-8ºC) Eimeria spp. morphological identification may be difficult.
Whereas the main disadvantage of this technique lies in the fact that we do not know whether the Eimeria DNA that has been amplified comes from a live and infective parasite or from DNA debris contained in the sample.
This is why, the above having been said, lesion scoring using the Johnson & Reid method (1970) still remains the earliest method of diagnosis for coccidiosis outbreaks together with clinical signs.
In fact, we know from the biological cycle of Eimeria that schizogony (the asexual replication of the parasite that leads to gut damage) constitutes the first stage of replication of the parasite and it is only after three or four rounds of asexual reproduction and sexual replication (or gamogony) that formation of immature oocysts occurs, these being excreted in the external environment together with faeces.
This said, it is easy to understand why lesion scoring probably still represents the quickest way of diagnosing avian coccidiosis, and the method that allows diagnosis at the earliest stage, at least for those Eimeria spp. with pathognomonic lesions, such as Eimeria acervulina, E. brunetti, E. maxima, E. necatrix and E. tenella.
To sum up, in order to perform a careful coccidiosis diagnosis, we need to work with all three above-mentioned methods together and be well aware of the advantages and limitations of each.
Anticoccidial feed additives or anticoccidial drugs have been and still are the most widely used prevention tool in broilers.
These products have many merits but the two main issues associated with their use are (i) reduced sensitivity of Eimeria parasites when a certain product is used for too long or too often and (ii) cross resistance between certain compounds.
Therefore, there is an intensified need for alternative approaches to Eimeria prevention.
With the increasing problems of drug resistance and pressure from consumers to ban drugs from animal feeds, there is a pressing need to move away from chemotherapeutic control towards vaccination against coccidiosis in chickens.
Furthermore, vaccines are the only products that are able to generate a good level of specific immunity from the very beginning.
When it comes to breeders, the situation is completely different; in fact, almost all breeders use coccidiosis vaccines as the only method of prevention.
In this category of animals which live much longer compared to a broiler, the use of anticoccidials would result in an expensive and in most cases inefficient method of prevention.