Chickens are major species globally adapted to a range of climatic conditions in which humans live and play an important role in supplying animal-derived proteins to improve human nutrition .
Coccidiosis is recognized as a pandemic disease of broilers caused by the intracellular protozoan parasites of the genus Eimeria. Although coccidiosis is known for many years, it is still considered as the major economical parasitic condition affecting poultry industries [2,3]. The major part of these economical damages is due to losses in performance including mortality, weight loss and reduced nutrition digestibility resulting in malabsorption caused by gut damage. The minor part of these economical damages is due to the costs of therapy and prophylaxis [4–6].
For controlling coccidiosis in broilers, various preventive medications have been approved for use globally and approached, but resistance is increasingly important as no new anticoccidial compounds are known to be under development . Therefore, the purpose is to provide available information on coccidiosis in broilers and to highlight etiology, life cycle and preventive measures.
Coccidiosis in broilers is caused by the intracellular protozoan parasite in the genus Eimeria family Eimeridae order Eucoccidiorida and phylum Apicomplexa. Seven species of Eimeria (E. acervulina, E. brunetti, E. maxima, E. mitis, E. necatrix, E. praecox, and E. tenella) are recognized to infect chickens . All the Eimeria spp. are very host-specific and sites of development are certain sites of the intestinal tract in broilers with different pathogenicity  (Table 1). Concurrent infection with at least six species is prevalent in a single flock causing marked, independent, recognizable diseases leading to the subclinical intestinal infection to sub-acute mortality .
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Oocyst of Eimeria spp. is a robust wall resistant to both mechanical, chemical and proteolytic degradation. Its wall has been associated with a bilayer structure composed of lipid and protein. The protein layer provides great stability against extreme cold and heat while the lipid layer supplies a cushion against chemical damage [10,11]. There are two types of oocysts depending on the infectious ability. Sporulated oocysts are infectious while unsporulated oocysts are noninfectious . The sporulated oocyst can survive up to 602 days in the external environment of its host whereas unsporulated oocysts can survive for seven months in the host caecum . The oocyst is unsporulated and noninfectious in the majority of cases whereas unsporulated oocysts can be converted into sporulated oocysts with appropriate temperatures and humidity. This process is known as sporulation  and sporulated Eimeria oocyst is composed of four sporocysts and each with two sporozoites and the lsospora has two sporocysts, each with four sporozoites . Oocysts of distinct species show contrasts of size (area, diameter), contour (elliptic, ovoid, circular), thickness, internal structure and color of the oocyst wall among other morphological variations .
Eimeria species follow a complex life cycle, consisting of two developmental stages namely the exogenous phase (sporogony; occurs in the outer environment) and the endogenous phase (schizogony/merogony and gametogony) [5,15]. The life cycle of all Eimeria species entails two or more generations of an asexual development (schizogony), then merogony and followed by a sexual phase known as gametogony which contributes to the formation of the oocyst . Under suitable conditions of temperature and moisture unsporulated oocysts develops within 24–48 hours to form a sporulated oocyst. The sporulated oocyst possesses four sporocysts, each containing two sporozoites (the infective agent). These oocysts can remain viable in the litter for a prolonged period (months to years) under conditions upon ingestion by a suitable host. Birds intake the sporulated oocyst by pecking in the litter for bedding or on the ground [16,17]. This process is the action of mechanical and chemical (i.e., trypsin, bile salt, carbon dioxide) factors that leads to the release of sporocysts and then sporozoites within the intestinal lumen of birds [1,16].
Excystation contributes to the opening of the anterior cap of sporocysts for releasing sporozoites. Interactions between the apical complex and the plasma membrane of epithelial cells permit sporozoites to penetrate the host cell . The released sporozoites penetrate directly into host intestinal epithelial cells in a specific site of the intestine determined by the species of Eimeria. After the invasion, sporozoites convert into a growing stage named trophozoites, which get enlarged and divide asexually to generate a large number of merozoites (known as merogony). Merozoites are separated from the original infected host epithelial cell to infect new cells in both the small and large intestine, developing on the second cycle of merogony [16,19,20]. In the last merogonic cycle, the resulting merozoites invade adjacent epithelial cells undergoing the sexual phase of the life cycle known as gametogenesis. During gametogenesis, these merozoites develop into either microgametes or macrogametes. Microgametes give rise to the release of numerous minute biflagellate microgametes, that exit, seek, and fertilize the macrogamete then form a zygote. This stage is the immature oocyst [5,15,21].
The diploid zygote produced during fertilization forms the oocyst wall capacity strong resistance and invades the intestinal lumen. Ultimately, Sporulation arises within infective sporozoites that grow and pollute the environment to begin a new cycle . The life cycle of Eimeria generally takes 4–6 days [16, 22].
Pathogenicity in Eimeria deteriorates with boosting the number of sporulated oocysts ingested by the host . Infection of Eimeria impairs the intestinal epithelial cells and tissues, destabilizing the gut of chickens . As a result, coccidiosis leads to disorder digestion of feed and nutrient absorption, dehydration, blood loss and loss of skin pigmentation . Moreover, Eimeria spp. change the overall morphology of the intestinal microbiota in the gut, truncating intestinal villi which result in a decreased ability for the digestion and absorption of nutrients .
Coccidiosis of broilers affects two forms as sub-clinical and clinical . In sub-clinical coccidiosis, the symptom demonstrates poor weight gain and deteriorated feed efficiency, reduced growth rate and causes an enormous proportion of the economic loss . Clinical signs of coccidiosis are on account of the destruction of the intestinal epithelium and underlying connective tissue of the mucosa. It may go along with hemorrhage into the lumen of the intestine as symptoms advance. Accordingly, the clinical form of coccidiosis appears in weight loss, reduced feed intake, paleness, ruffled feathers, depression, pale color comb, wattles, huddling, closed eyes, diarrhea with bloody feces, dehydration and increased number of mortality may accompany [1,28].
Lesion scoring is the method to interpret poultry coccidiosis by observing each of the intestinal tracts for macroscopically visible lesions which are following a scoring system from 0 to 4 (i.e., 0 = healthy or no sign of infection, 1 = mild lesions, 2 = moderate lesions, 3 = severe lesions, 4 = extremely severe lesions) caused by Eimeria. This method is work-intensive by investigating post-mortem, occasionally subjective and only dependable when performed by veterinarians or highly trained individuals .
Eimeria oocysts can be acquired easily from the feces of infected birds hence using oocyst character is a non-invasive method to determine the causative species in chickens.
The most extensively used method for the concentration of parasite oocysts is flotation. A solution with a higher specific gravity (i.e., saturated NaCl + glucose) is used to float the relatively low specific gravity coccidial oocysts to the surface and remained other debris in feces at the bottom of the solution .
This method is carried out to determine the number of oocysts per gram of feces (OPG count) together with the percentage of sporulation and oocyst dimensions. Typically, the McMaster chamber method is used for counting Eimeria oocysts in the fecal solution and for establishing the individual oocysts shedding pattern of an infected bird . The floatation method of oocysts is identical to the quantitative technique and uses the McMaster sliced to facilitate easy counting through the light microscope . Nevertheless, expertise is required to identify the different species based on the morphology of oocysts .
Although traditional methods such as lesion scoring and oocyst detection in feces are still being used, molecular biological methods are applicable in most research since the above mentioned traditional methods are time-intensive and in the case of detection of oocyst, it is hard to distinguish what kind of oocysts are there .
Previous techniques distinguishing of different species were based on the isoenzyme patterns of oocysts and rRNA and rDNA proves [33,34]. Polymerase chain reaction (PCR) is a rapid, accurate and highly sensitive molecular diagnostic technique to identify the Eimeria spp. in chickens by inspecting their variations of genomic DNA . This method amplifies the chicken coccidian species-specific DNA sequence for the detection and discrimination to hundreds of millions for a few hours and the new copies can be divided by electrophoresis to visualize it under UV light by a fluorescent dye [23,36].
TREATMENT, CONTROL AND PREVENTION
Chemoprophylaxis for the treatment and prevention of coccidiosis has been accomplished by the inclusion of a range of anticoccidial drugs in the feed or drinking water to the chickens [8,37]. Anticoccidial drugs can be classified into three categories as follows: i) Synthetic compounds, ii) Ionophores/polyether antibiotics, iii) Mixed products of the synthetic compounds and ionophores [16,38]. Unfortunately, most of the anticoccidial drugs became ineffective due to drug resistance .
Synthetic compounds are manufactured by chemical synthesis and are often known as ‘chemicals’ . Synthetic drugs work by preventing different biochemical pathways of the developing parasite metabolism . Sulfonamides, nicarbazin, clopidol, quinolones, amprolium, halofuginone are chemical components’ examples of commonly used synthetic drugs [38,41].
Ionophores/polyether antibiotics are manufactured by the fermentation of Streptomyces spp. or Actinomadura spp. and arrest the ion (i.e. sodium, potassium) transport channels and hinder the osmotic balance of the coccidian species [38,42]. The groups of ionophores can be classified into three categories as follow: i) Monovalent ionophores (monensin, narasin, salinomycin), ii) Monovalent glycosidic ionophores (maduramicin, semduramycin), iii) Divalent ionophores (lasalocid) .
The intensive use of anticoccidial drugs which resulted in the development of resistance, chemical residues and environmental pollution of poultry products promoted research on alternative control methods such as early application of vaccines or the development of new drugs . There are two types of available live vaccines for the immunization of chickens as follows: i) virulent vaccines, ii) attenuated vaccines .
Virulent (Unattenuated) vaccines are comprised of Eimeria species originating from laboratory or field strains, which have not been modified in any way to change their pathogenicity . Although these vaccines induce a healthy immune activity in the poultry, the exposure to the oocytes of Eimeria must arise again through the litter to maintain the immune status to the same level as birds saved previous induced immune activity . Furthermore, it is necessary to inoculate the recommended dosage of virulent vaccines to avoid clinical coccidiosis from the birds .
Attenuated vaccines are produced downgrading the virulence of Eimeria species artificially, making minimum damage to the intestinal epithelium after one passage through the gut . Notwithstanding these vaccines are safe, the price of their production is high because of the decreasing fecundity of precocious parasites , and 2–3 cycles of Eimeria are requisite for maximum acquired immunity .
Phytogenic compounds are a wide range of plant-derived products that find use as prophylactic agents in enhancing performance, productivity as well as alleviating harmful effects of coccidiosis [47,48]. Furthermore, these compounds include antioxidant, antimicrobial, antiparasitic and antiviral properties . Currently, various studies related to phytochemicals are being conducted to replace the traditional methods (i.e., vaccination, use of anticoccidial drugs) for preventing poultry coccidiosis on account of safety, efficient price, drug resistance and drug residues [50–52].
Oregano extracts as phytogenic compounds are known to have positive effects on growth performance as well as the efficacy of cocci elimination of coccidiosis-infected broilers [53,54]. Among them, phenols such as carvacrol and thymol have been analyzed as they exhibit plentiful biological properties, including antioxidant, antifungal, antibacterial, and antiprotozoal effects against Eimeria spp. [52,55,56]. The high lipid solubility and hydrophobicity of carvacrol and thymol led to interaction with the phospholipid bilayer in the Eimeria species cell membrane, resulting in a change in membrane permeability for cations such as H+ and K+. Consequently, the collapse of the parasitic cells occurs through energy losses (i.e., ATPase inhibition) and ion leakage (particularly calcium), resulting in water imbalance [52,57].
3,4,5-Trihydroxybenzoic acid, also known as gallic acid, is found in several vegetables and fruits . Gallic acid is made up of a low molecular weight tri-phenolic compound. It possesses anti-inflammatory and anti-oxidative activities as well as several evident pharmacological effects including anti-tumor, anti-bacterial, anti-diabetes, anti-obesity, anti-microbial and anti-myocardial ischemia .
Coccidiosis is a major enteric parasitic disease in the poultry industry and is caused by seven Eimeria parasite species developing in a particular chick’s digestive tract. These species induce symptoms from subclinical enteric infection to subacute mortality. For these reasons, coccidiosis is still mightily important to prevent economic loss worldwide. Although diverse control measures have been accomplished against the disease, prophylactic use of anticoccidial drugs was an extensively used control approach which makes a problem of drug resistance recently. Hence, phytogenic compounds are emerging for the control and prevention of poultry coccidiosis instead of previous methods. Therefore, there need for various researches related to phytogenic compounds affecting coccidiosis in broilers.