Eimeria acervulina: Duodenal Coccidiosis – The Most Prevalent Eimeria Species in Chickens

Etiology and Taxonomic Classification

Eimeria acervulina is an obligate intracellular apicomplexan parasite belonging to the phylum Apicomplexa, class Conoidasida, order Eucoccidiorida, and family Eimeriidae. It is one of seven recognized species of Eimeria that infect domestic chickens (Gallus gallus domesticus). The parasite exhibits a high degree of host specificity, infecting only galliform birds, and demonstrates strict site specificity within the intestinal tract. E. acervulina preferentially colonizes the duodenum and upper jejunum, where it undergoes a defined life cycle comprising both asexual (schizogony) and sexual (gametogony) phases within enterocytes [1, 2].

The oocysts of E. acervulina are ovoid to ellipsoid, measuring approximately 17.7 to 20.2 micrometers by 13.7 to 16.3 micrometers, with a mean size near 18.3 by 14.6 micrometers. They lack a micropyle and oocyst residuum, features that aid in morphological differentiation from other species. The sporulation time under optimal conditions (25 to 30 degrees Celsius with adequate aeration) is approximately 17 to 24 hours, which is among the shortest for chicken Eimeria species. This rapid sporulation contributes to its high prevalence in litter-based housing systems [2, 3].

Epidemiology: The Most Prevalent Eimeria Species

Global epidemiological surveys consistently identify E. acervulina as the most prevalent Eimeria species in commercial and backyard chicken flocks. A large-scale investigation in Korean chicken farms reported an overall Eimeria prevalence of 75% (291/388), with E. acervulina detected in 98.6% of positive samples, making it the dominant species [4]. Similarly, studies in Greece found E. acervulina to be among the most frequently identified species, often co-occurring with E. tenella and E. maxima [5]. In China, a molecular epidemiological survey of broiler chickens immunized with live anticoccidial vaccines reported an Eimeria prevalence of 86.12% (546/634), with E. acervulina present in 65.62% of samples, again the highest among all species detected [6].

The predominance of E. acervulina is not limited to Asia. Investigations in Algeria identified E. acervulina as the most prevalent species (32.05%), followed by E. tenella (26.92%) [7]. In Brazil, a study of broiler houses with positive and negative pressure ventilation systems found E. acervulina to be the most prevalent species, with mixed infections involving E. acervulina, E. maxima, and E. tenella being the most common combination [8]. Reports from Iran, Nigeria, and Ethiopia also confirm the high prevalence of this species, often exceeding 50% of positive samples in some regions [9, 10, 11, 12, 13].

Mixed-species infections are the rule rather than the exception. Positive fecal samples frequently contain multiple Eimeria species, with an average of 3.4 to 4.2 species per sample [4, 14]. The most common coinfection combinations involve E. acervulina with E. tenella and E. acervulina with E. necatrix [15, 6]. This high prevalence and frequent coinfection complicate diagnosis and control, as different species may exhibit varying susceptibilities to anticoccidial drugs and vaccines.

Pathogenesis and Clinical Signs

E. acervulina is considered moderately pathogenic compared to E. tenella and E. necatrix, but its economic impact is substantial due to its high prevalence and effects on nutrient absorption [2]. The parasite invades the epithelial cells of the duodenum and upper jejunum, causing villous atrophy, crypt hyperplasia, and fusion of villi. These histopathological changes reduce the absorptive surface area of the small intestine, leading to malabsorption of nutrients, particularly carotenoids and fats [1, 15].

Clinical signs of duodenal coccidiosis caused by E. acervulina are often subclinical or mild in low-dose infections. In heavier infections, affected chickens exhibit decreased feed intake, reduced body weight gain, poor feed conversion ratios, and diarrhea. A characteristic sign is the presence of whitish, transverse band-like lesions (often described as "ladder-like" or "snowflake" lesions) on the mucosal surface of the duodenum. These lesions correspond to areas of heavy schizont and gametocyte aggregation [1, 16]. Unlike E. tenella, which causes hemorrhagic cecal coccidiosis, E. acervulina infections rarely produce frank blood in the feces. Instead, the droppings may appear watery or contain mucus.

Coinfection with other Eimeria species, particularly E. tenella, can exacerbate clinical signs. A study on dual infection with E. acervulina and E. tenella demonstrated that coinfected birds exhibited higher weight loss and oocyst excretion than those infected with E. acervulina alone, although duodenal lesion scores were paradoxically lighter in the early phase of coinfection [15]. This interaction highlights the complexity of pathogenesis in field conditions where mixed infections are ubiquitous.

Pathology and Lesion Scoring

Gross pathological lesions are confined to the duodenum and upper jejunum. The characteristic white, transverse plaques are pathognomonic for E. acervulina infection. These lesions are graded on a scale of 0 to 4 based on the Johnson and Reid scoring system, where 0 indicates no lesions, 1 indicates small scattered white plaques, 2 indicates plaques covering up to half the duodenal mucosa, 3 indicates plaques covering more than half the mucosa with thickening, and 4 indicates confluent lesions with a thickened, corrugated mucosa [16].

Histologically, the lesions correspond to the presence of developing schizonts and gametocytes within enterocytes. Villous atrophy is a consistent finding, with a significant reduction in the villus height-to-crypt depth ratio. This ratio is a key indicator of intestinal health and absorptive capacity. Berberine treatment in E. acervulina-infected chickens has been shown to increase duodenal villus height, suggesting a protective effect on intestinal morphology [1]. The inflammatory response involves infiltration of lymphocytes, macrophages, and heterophils into the lamina propria. Transcriptomic analysis reveals that at day 4 post-infection, there is a downregulation of metabolism-related genes and an upregulation of cell integrity-related genes. By day 6 post-infection, immune-related genes and cellular homeostasis genes are upregulated, reflecting the host's adaptive immune response [1].

Diagnostic Approaches

Accurate diagnosis of E. acervulina infection requires a combination of clinical, pathological, and laboratory methods.

Microscopic Examination

Traditional diagnosis relies on the morphological identification of oocysts in fecal samples using flotation techniques (e.g., Sheather's sugar solution or saturated sodium chloride). Oocysts are identified based on size, shape, color, and the presence or absence of a micropyle and oocyst residuum. E. acervulina oocysts are ovoid, lack a micropyle, and measure 17.7 to 20.2 by 13.7 to 16.3 micrometers [2, 3]. However, morphological identification is subjective and requires significant expertise, especially in mixed infections where oocyst dimensions overlap between species.

Molecular Diagnostics

Polymerase chain reaction (PCR) targeting the internal transcribed spacer 1 (ITS-1) region of ribosomal DNA is the gold standard for species-specific identification. Species-specific primers allow for the detection and differentiation of all seven Eimeria species in chickens [3, 14]. Multiplex PCR assays are particularly useful for detecting mixed infections in a single reaction. Quantitative PCR (qPCR) can also be used to estimate the relative abundance of each species in a sample, providing valuable epidemiological data.

Postmortem Examination

Necropsy with examination of the intestinal tract for characteristic lesions is a rapid and reliable diagnostic method. The presence of white, transverse plaques in the duodenum is highly suggestive of E. acervulina infection. Mucosal scrapings from affected areas can be examined microscopically for the presence of schizonts, gametocytes, and oocysts [17, 18].

Serological Assays

Enzyme-linked immunosorbent assays (ELISAs) for the detection of antibodies against Eimeria species are available for research and surveillance purposes. These assays detect antibodies against sporozoite or merozoite antigens and can indicate past or current exposure. However, they do not differentiate between Eimeria species and are less commonly used in routine clinical diagnosis compared to molecular methods.

Treatment and Anticoccidial Resistance

Control of E. acervulina has historically relied on the prophylactic use of anticoccidial drugs (coccidiostats) in feed. Commonly used compounds include ionophores (monensin, salinomycin, maduramycin) and chemical coccidiostats (clopidol, diclazuril, toltrazuril, nicarbazin). However, widespread and prolonged use has led to the development of resistance in field isolates.

A study in Korean chicken farms tested nine field samples against six anticoccidial drugs and found that all samples exhibited strong resistance to all tested drugs based on the anticoccidial index, percent optimum anticoccidial activity, relative oocyst production, and reduced lesion score index [4]. Similarly, a study in Thailand reported that E. tenella field isolates were resistant to salinomycin but sensitive to nicarbazin, monensin, and a monensin-nicarbazin combination [19]. These findings underscore the critical need for ongoing surveillance of anticoccidial sensitivity and the development of alternative control strategies.

Alternative Therapeutics

Natural products are being investigated as alternatives to conventional anticoccidials. Berberine, an isoquinoline alkaloid derived from plants of the genus Berberis, has demonstrated anticoccidial activity against E. acervulina. In a controlled study, berberine administration to infected chickens significantly reduced fecal oocyst production and intestinal lesion scores while increasing duodenal villus height. Transcriptomic analysis revealed that berberine modulated the host immune response, inducing cytokine-cytokine receptor interactions at day 4 post-infection, which were not observed in untreated infected chickens [1]. These findings suggest that berberine may have therapeutic or prophylactic potential as a feed additive.

Control and Prevention

An integrated control strategy is essential for managing E. acervulina and other Eimeria species in commercial poultry operations.

Vaccination

Live anticoccidial vaccines containing attenuated or non-attenuated oocysts of multiple Eimeria species are widely used. Vaccination induces protective immunity by exposing birds to controlled doses of oocysts, allowing the development of species-specific and cross-species immunity. However, vaccine efficacy can be compromised by the presence of antigenically diverse field strains. A study in Korea found that cross-immunity among E. maxima isolates varied from 54.3% to 100% against heterologous isolates, indicating that antigenic variation is a significant challenge [14]. In China, molecular epidemiological surveys of vaccinated flocks revealed that E. acervulina remained highly prevalent, suggesting that current vaccines may not provide complete protection against all field strains [6].

Management Practices

Biosecurity measures are critical for reducing environmental oocyst loads. These include all-in/all-out production systems, thorough cleaning and disinfection between flocks, proper litter management, and control of mechanical vectors such as beetles and flies. Litter treatment with quicklime has been shown to reduce the risk of E. maxima and E. tenella presence, although its effect on E. acervulina is less pronounced [8]. Ventilation system design also influences prevalence; negative pressure ventilation systems have been associated with better feed conversion and daily weight gain compared to positive pressure systems, possibly due to reduced oocyst accumulation [8].

Anticoccidial Rotation and Shuttle Programs

To delay the development of resistance, anticoccidial drugs are often used in rotation or shuttle programs, where different drugs are used in starter, grower, and finisher feeds. However, the high prevalence of multidrug-resistant E. acervulina field isolates indicates that these strategies are not always effective [4]. Continuous monitoring of drug sensitivity through in vivo anticoccidial sensitivity tests (ASTs) is recommended to guide drug selection.

Conclusion

Eimeria acervulina is the most prevalent Eimeria species in chickens worldwide, causing duodenal coccidiosis that results in significant economic losses due to reduced growth performance and feed efficiency. Its high prevalence, frequent involvement in mixed-species infections, and widespread resistance to anticoccidial drugs make it a persistent challenge for the poultry industry. Accurate diagnosis requires a combination of postmortem examination, microscopic oocyst identification, and molecular methods such as ITS-1 PCR. Control strategies must integrate vaccination, biosecurity, and judicious use of anticoccidials, with ongoing surveillance for drug resistance. Natural products such as berberine represent promising alternatives that warrant further investigation. Understanding the epidemiology and pathogenesis of E. acervulina is essential for developing effective, sustainable control programs.

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