Coccidiosis in Chickens: Etiology, Diagnosis, and Treatment

Etiology and Parasite Biology

Coccidiosis in chickens is caused by obligate intracellular protozoan parasites of the genus Eimeria (phylum Apicomplexa). Seven species are recognized as pathogenic in domestic chickens (Gallus gallus domesticus): Eimeria acervulina, Eimeria maxima, Eimeria tenella, Eimeria necatrix, Eimeria brunetti, Eimeria mitis, and Eimeria praecox [1]. Each species exhibits a high degree of host specificity and a predilection for distinct regions of the intestinal tract [1]. E. tenella targets the ceca, E. necatrix the midgut, E. maxima the midgut, E. acervulina the duodenum, and E. brunetti the lower intestine and rectum [1, 2]. The life cycle is monoxenous and comprises an exogenous sporulation phase and an endogenous phase involving merogony (asexual replication) and gametogony (sexual replication) within the intestinal epithelium [1].

Sporulated oocysts, containing four sporocysts each with two sporozoites, are the infective stage [1]. Following ingestion, sporozoites excyst in the lumen and invade enterocytes. The parasite undergoes several generations of merogony, producing merozoites that rupture host cells, causing tissue damage and hemorrhage [3, 1]. The transition to gametogony produces macrogametes and microgametes; fertilization yields unsporulated oocysts that are shed in feces [1]. Sporulation in the environment requires appropriate temperature, humidity, and oxygenation [2]. The prepatent period ranges from 4 to 7 days depending on the species [1].

Recent proteomic analyses of E. tenella have provided a high-resolution map of proteins localized to key invasion organelles, including micronemes, rhoptries, and dense granules [4]. The microneme protein EtMIC2 promotes invasion by binding to the ITGAV receptor on host cells and inhibits host cell apoptosis [5]. Phosphoglycerate mutase 1 (PGM1) has been implicated in both maduramycin resistance and host cell invasion [6]. Comparative genomics and transcriptomics have identified surface antigen genes SAG17 and SAG23 as key determinants of early-stage virulence divergence in E. tenella strains [7]. Differential induction of host cell autophagy has been observed between virulent and precocious strains, suggesting a role for autophagy in pathogenesis [8].

Epidemiology and Transmission

Transmission occurs via the fecal-oral route. Chickens ingest sporulated oocysts from contaminated litter, feed, water, or soil [1, 2]. High stocking density, poor litter management, and warm, humid conditions facilitate oocyst accumulation and sporulation [2]. Environmental contamination modeling in broiler farms has demonstrated that oocyst burden in litter correlates with management practices and can be used to predict coccidiosis risk [2]. Layer pullets exposed to a mixed-species Eimeria challenge in combination with the feed-borne mycotoxin deoxynivalenol exhibit interactive effects on performance and gut health during the transition to lay [9].

Clinical Signs and Pathology

Clinical manifestations range from subclinical to severe depending on the Eimeria species, infective dose, host immunity, and concurrent stressors [1, 10]. Acute coccidiosis is characterized by diarrhea (often mucoid or hemorrhagic), dehydration, anorexia, ruffled feathers, depression, and reduced weight gain [1, 11]. E. tenella infection produces cecal hemorrhage and mortality in severe cases [3, 11]. E. necatrix causes intestinal hemorrhage and significant morbidity [12, 13]. E. acervulina infection results in whitish, transverse plaques in the duodenum and reduced nutrient absorption [1]. E. brunetti is associated with wet litter and subclinical impacts on feed conversion [1].

Pathological lesions are species-specific and scored using standardized systems. E. tenella induces cecal thickening, hemorrhage, and caseous cores [3, 11]. E. maxima produces petechiae and orange mucoid exudate in the midgut [1]. E. necatrix causes ballooning of the midgut with pinpoint hemorrhages and white necrotic foci [12, 13]. Histologically, meronts and gamonts are visible within enterocytes, accompanied by epithelial sloughing, villous atrophy, and inflammatory cell infiltration [3, 14]. Toll-like receptor-mediated innate immune responses correlate with the pathogenicity of E. tenella infection, with differential expression of TLRs and downstream cytokines [3]. TRAF6, a target of gga-miR-7b, promotes E. tenella-induced inflammation and apoptosis by activating the NF-kappaB pathway [14].

Diagnosis

Definitive diagnosis relies on a combination of clinical history, necropsy findings, and laboratory identification of Eimeria oocysts or species-specific lesions [15, 16, 1].

Fecal Examination and Oocyst Quantification

Flotation techniques using saturated sodium chloride or sucrose solutions are standard for recovering oocysts from feces or litter [15, 1]. Oocysts are identified by their size, shape, and morphology. Species differentiation based solely on oocyst morphology is unreliable due to overlap; molecular methods are preferred for definitive speciation [15, 16]. Quantitative oocyst counts (oocysts per gram of feces) are performed using McMaster counting chambers [15]. Optimized DNA extraction protocols from chicken feces have been developed for downstream real-time PCR quantification, improving sensitivity and reproducibility [15].

Molecular Diagnostics

Real-time PCR (qPCR) assays targeting species-specific genetic markers (e.g., internal transcribed spacer 1, ITS-1) enable sensitive and specific detection and quantification of individual Eimeria species [15, 16]. Cross-priming amplification (CPA) combined with lateral flow immunoassay (LFIA) biosensors has been developed for rapid, genus-level detection and identification of the four most economically important species (E. tenella, E. acervulina, E. maxima, E. necatrix) [16]. This technology offers field-deployable diagnostics without requiring thermocyclers [16].

Necropsy and Lesion Scoring

Postmortem examination is critical. Lesion scoring (0 to 4 scale) at specific intestinal sites provides a semi-quantitative measure of infection severity [1]. E. tenella lesions are confined to the ceca; E. necatrix lesions appear in the midgut; E. acervulina lesions in the duodenum; E. maxima lesions in the midgut; and E. brunetti lesions in the lower intestine [1].

In Vitro Assays

A bioluminescence-based in vitro assay using transgenic Eimeria parasites expressing luciferase has been developed for rapid, quantitative anticoccidial screening [17]. This assay measures parasite viability in real time and reduces reliance on animal experiments for drug susceptibility testing [17].

flowchart TD
    A[Clinical Signs: Diarrhea, Weight Loss, Hemorrhage], > B[Fecal Flotation / McMaster Count]
    B, > C{Oocyst Detection?}
    C, No, > D[Consider Other Enteric Pathogens]
    C, Yes, > E[Species Identification]
    E, > F[Option 1: qPCR / ITS-1 Assay]
    E, > G[Option 2: CPA-LFIA Biosensor]
    E, > H[Option 3: Lesion Scoring at Necropsy]
    F, > I[Confirm Species & Quantify Burden]
    G, > I
    H, > I
    I, > J[Select Anticoccidial Therapy]
    J, > K[Monitor Treatment Response & Resistance]

Chicken Coccidia Meds: Treatment and Anticoccidial Agents

Therapeutic management of coccidiosis relies on anticoccidial drugs classified into two main categories: ionophore antibiotics and synthetic chemicals [18, 19, 1, 20]. Resistance to both classes is widespread and represents a major clinical challenge [18, 6, 21].

Ionophore Antibiotics

Ionophores (e.g., monensin, salinomycin, narasin, maduramycin, lasalocid) disrupt ion gradients across parasite cell membranes, leading to metabolic failure [19, 6, 21]. Narasin is often used in combination with diclazuril as a feed additive for chickens for fattening [19]. Resistance to ionophores has been documented, including in cryptic species such as Eimeria zaria [21]. Maduramycin resistance in E. tenella is associated with upregulation of phosphoglycerate mutase 1 [6].

Synthetic Chemicals

Synthetic anticoccidials include triazines (toltrazuril), sulfonamides (sulfaclozine, sulfamidine), dihydrofolate reductase inhibitors (diaveridine), quinolones, and benzeneacetonitriles (diclazuril) [18, 19, 20]. Toltrazuril and sulfaclozine resistance has been reported in Vietnamese field isolates, with reduced intestinal recovery following treatment [18]. The combination of sulfamidine and diaveridine has shown therapeutic efficacy against Vietnamese field isolates of Eimeria spp. [20]. Diclazuril is used in combination with narasin in feed additive formulations [19].

Resistance Mechanisms

Anticoccidial resistance arises from repeated exposure to subtherapeutic concentrations and is characterized by reduced drug sensitivity, shorter prepatent periods, and increased oocyst output [18, 6, 21]. Molecular mechanisms include target site mutations, efflux pump upregulation, and metabolic bypass pathways [6]. Routine sensitivity testing using in vitro bioluminescence assays or in vivo dose-titration studies is recommended to guide drug selection [17].

Alternative and Phytogenic Agents

Phytogenic feed additives have been extensively investigated as alternatives or adjuncts to conventional anticoccidials [22, 23, 24, 25, 26, 27, 28, 29]. Curcumin modulates gut bacterial populations and NF-kappaB/NRF2 immune-redox responses in Eimeria-challenged broilers [22]. Lavender essential oil (Lavandula angustifolia) has demonstrated anticoccidial activity in vitro and in vivo [23]. Quercetin and thyme oil reduce oxidative stress biomarkers and modulate interleukin expression during E. tenella infection [24]. Gentiana scabra mitigates E. tenella-induced coccidiosis by regulating the gut microbiota-metabolome and strengthening the intestinal barrier [25]. Oregano extracts, alone or in combination with other biomolecules, improve growth performance and reduce parasitological parameters in Eimeria-challenged broilers [26]. Eucalyptus oil microcapsules and mangosteen extract have shown efficacy against E. tenella [27]. Phytogenic feed additives containing saponins and polyphenols improve growth performance and gut health in broilers exposed to multiple stressors including coccidiosis [10, 28]. Stemona tuberosa extracts exhibit anticoccidial activity and protect intestinal integrity [29]. Red osier dogwood extract improves growth performance, protein digestibility, and gut health in a coccidiosis vaccine challenge model [30].

Probiotics and Prebiotics

Probiotic supplementation with Lactobacillus acidophilus and Enterococcus faecium, delivered in ovo or via drinking water, reduces Eimeria infection severity in broilers [31]. A meta-analysis confirmed that probiotic supplementation improves growth performance and reduces oocyst shedding in broilers challenged with coccidiosis [32]. 5-Aminolevulinic acid supplementation suppresses body weight loss and reduces disease severity during E. tenella infection [11].

Vaccination and Immunoprophylaxis

Live vaccines containing attenuated (precocious) or non-attenuated Eimeria strains are used to induce protective immunity [30, 33, 34]. Recombinant subunit vaccines are under development. A tetravalent recombinant subunit vaccine provides protection against mixed challenges with four Eimeria species [34]. DNA vaccines based on E. maxima elongation factor 1-alpha (EF-1alpha) antigen combined with chicken XCL1 chemokine enhance immunoprotective effects [33]. A chimeric multi-antigen fusion vaccine, EimeriaBig, has been constructed and evaluated for immune response and protective effect against E. necatrix [12]. Microneme protein 3 from E. necatrix shows immunoprotective potential [13].

Control Strategies

Integrated control combines chemotherapy, vaccination, biosecurity, and management practices [1, 2]. Shuttle programs (rotating ionophores and chemicals across grow-out periods) and rotation between drug classes are used to delay resistance [1]. Litter management, including removal of wet litter and adequate ventilation, reduces oocyst sporulation [2]. Environmental contamination modeling can identify high-risk farms and guide targeted interventions [2]. Feed withdrawal prior to coccidiosis inoculation has temporal effects on disease outcome, and housing methods (floor pens vs. cages) influence experimental reproducibility [35].

References

[1] Nguyen BT, Flores RA, Kim T et al. Understanding Eimeria infection for the treatment and prevention of chicken coccidian parasites. Front Cell Infect Microbiol. 2026. https://pubmed.ncbi.nlm.nih.gov/41969659/

[2] Bachene MS, Hentabli S, Khelouia A et al. Environmental contamination by Eimeria spp. and coccidiosis risk modeling in broiler farms of Medea province, Algeria. Comp Immunol Microbiol Infect Dis. 2026. https://pubmed.ncbi.nlm.nih.gov/41793990/ *** Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.

[3] Zhu H, Zheng G, Wang D et al. Toll-like receptor-mediated innate immune response correlate with the pathogenicity of Eimeria tenella infection in SPF chickens. Parasit Vectors. 2026. https://pubmed.ncbi.nlm.nih.gov/42204750/

[4] Attree E, Barylyuk K, Noonan C et al. The spatial proteome of Eimeria tenella provides a high-resolution view of proteins localised to key invasion organelles. Int J Parasitol. 2026. https://pubmed.ncbi.nlm.nih.gov/42102995/

[5] Cui KL, Guo LL, Lei X et al. Pathogenic mechanism of Eimeria tenella EtMIC2 promotes Eimeria tenella invasion and inhibits host cell apoptosis through binding to the ITGAV receptor. Poult Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/42025007/

[6] Bai J, Zhao Q, Xiao K et al. Phosphoglycerate mutase 1 is implicated in maduramycin resistance and host cell invasion in Eimeria tenella. Vet Parasitol. 2026. https://pubmed.ncbi.nlm.nih.gov/41955695/

[7] He Y, Wan X, Wang X et al. Integrative comparative genomics and transcriptomics reveal key roles of SAG17 and SAG23 in early-stage virulence divergence of Eimeria tenella. Vet Res. 2026. https://pubmed.ncbi.nlm.nih.gov/42050698/

[8] Zhang L, Chen YY, Zhang HH et al. Differential induction of host cell autophagy by virulent and precocious strains of Eimeria tenella in vitro and in vivo. Vet World. 2026. https://pubmed.ncbi.nlm.nih.gov/41822570/

[9] Paneru D, Sharma MK, Shi H et al. Interactive effects of the feed-borne mycotoxin deoxynivalenol and a mixed-species Eimeria challenge on layer pullets during the transition to lay. Poult Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/41850060/

[10] Froebel LE, Rincker MJ, Dilger RN. Effects of dietary saponin and polyphenol supplementation in broiler chickens exposed to multiple mild stressors of cyclic elevated ambient temperature, feed withdrawal, and coccidiosis infection. Poult Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/41865655/

[11] Hanif TA, Matsubayashi M, Hatabu T. Supplementation of 5-Aminolevulinic Acid Suppressed Body Weight Loss and Reduced Disease Severity During Eimeria tenella Infection in Broiler Chickens. J Poult Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/42028132/

[12] Chen X, Cai H, Liao S et al. Construction of a chimeric multi-antigen fusion vaccine, EimeriaBig, and evaluation of immune response and protective effect in Eimeria necatrix. Poult Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/42214189/

[13] Feng Q, Yan D, Xue N et al. Molecular characterization and immunoprotective potential of microneme protein 3 from Eimeria necatrix. Parasit Vectors. 2026. https://pubmed.ncbi.nlm.nih.gov/42069683/

[14] Tang J, Zhang J, Tang M et al. TRAF6, a gga-miR-7b Target, Promotes Eimeria tenella-Induced Inflammation and Apoptosis in Chickens by Activating NF-kappaB Pathway. Biomolecules. 2026. https://pubmed.ncbi.nlm.nih.gov/42194006/

[15] Jung HR, Her M, Yun CS et al. Development of optimized DNA extraction protocols for the quantification of Eimeria spp. from chicken feces using real-time PCR. Poult Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/42068766/

[16] Wang YX, Wu ZX, Wang ZR et al. Cross-priming amplification strategy-assisted lateral flow immunoassay biosensors for the rapid detection of chicken Eimeria parasites at genus-level and identification of the four most economically important species. Poult Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/42048791/

[17] Felici M, de Hoest-Thompson C, Tugnoli B et al. Bioluminescence-based in vitro assay for rapid and quantitative anticoccidial screening. Front Cell Infect Microbiol. 2026. https://pubmed.ncbi.nlm.nih.gov/41834992/

[18] Na TT, Hoa NT, Hung PHS et al. Evaluation of Toltrazuril and Sulfaclozine resistance in chicken coccidiosis in Vietnam and its impact on intestinal recovery. Vet Res Commun. 2026. https://pubmed.ncbi.nlm.nih.gov/42171921/

[19] EFSA Panel on Additives and Products or Substances used in Animal Feed (FEEDAP), Villa RE, Azimonti G et al. Safety and efficacy of a feed additive consisting of narasin and diclazuril (Interban) for chickens for fattening and chickens reared for laying (Elanco GmbH). EFSA J. 2026. https://pubmed.ncbi.nlm.nih.gov/42016296/

[20] Pham HSH, Nguyen TH, Le DP et al. Evaluation of the therapeutic efficacy of the sulfamidine-diaveridine combination against Vietnamese field isolate of Eimeria spp. in broiler chickens. Vet Parasitol. 2026. https://pubmed.ncbi.nlm.nih.gov/41962192/

[21] Raffaelli M, Jaramillo-Ortiz JM, Vasilogianni M et al. Ionophore susceptibility of Eimeria zaria: First characterisation in a cryptic Eimeria species of chickens. Vet Parasitol. 2026. https://pubmed.ncbi.nlm.nih.gov/41930887/

[22] Abdulhusein HM, Taherpour K, Ghasemi HA et al. Curcumin modulates targeted gut bacterial populations and NF-kappaB/NRF2 immune-redox responses in Eimeria-challenged broilers fed soybean or canola oil. Sci Rep. 2026. https://pubmed.ncbi.nlm.nih.gov/42252365/

[23] Iqbal S, Tanveer S, Allaqaband SM et al. Lavender essential oil as a novel anticoccidial agent: First report from in-vitro and in-vivo studies of Lavandula angustifolia flowering plant grown in the Kashmir Himalayas. Microb Pathog. 2026. https://pubmed.ncbi.nlm.nih.gov/42061661/

[24] Mohamed RA, Ali HA, Salem GA et al. Assessment of Quercetin and Thyme Oil Effect on Oxidative Stress Biomarkers and mRNA Expressions of Interleukin 6, 2, and 16 During Eimeria tenella Infection. Avian Dis. 2026. https://pubmed.ncbi.nlm.nih.gov/41973004/

[25] Yuke Z, Chen X, Abuzeid AMI et al. Gentiana scabra mitigates Eimeria tenella-induced Coccidiosis by regulating the gut microbiota-metabolome and strengthening the intestinal barrier. Poult Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/41936291/

[26] Yousfi S, Fennouh C, Touhami NAK et al. Effects of oregano extracts, alone or in combination with other biomolecules, on growth performances and parasitological parameters of broiler chickens challenged with Eimeria spp.: a meta-analysis. Avian Pathol. 2026. https://pubmed.ncbi.nlm.nih.gov/41875322/

[27] Xia Q, Weng S, Li K et al. Exploration of the efficacy of eucalyptus oil (micro-capsules) and mangosteen extract against Eimeria tenella infection in chickens. Poult Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/41855807/

[28] Lee JH, Kim DH, Vu VA et al. Effects of phytogenic feed additive on growth performance, gut health, and antioxidant capacity in broiler chickens challenged with pathogenic Eimeria tenella. Poult Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/41855797/

[29] Ma Y, Dai L, Yu X et al. Anticoccidial activity and its potential Mechanisms of Stemona tuberosa against Eimeria tenella: In Vivo and In Vitro Investigations and Host Intestinal Protection. Vet Parasitol. 2026. https://pubmed.ncbi.nlm.nih.gov/41807897/

[30] Santos Reis Pereira I, I Adewole D. Effects of red osier dogwood extract on growth performance, protein digestibility, tibia breaking strength, immune response, and gut health of broiler chickens in a coccidiosis vaccine challenge model. Poult Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/42105386/

[31] Aydin R, Tegun E, Ozuicli M et al. Efficacy of in ovo and drinking water delivery of Lactobacillus acidophilus and Enterococcus faecium against Eimeria infection in broiler chickens. Exp Parasitol. 2026. https://pubmed.ncbi.nlm.nih.gov/42092525/

[32] Riaz R, Olmez N, Raza A et al. Efficacy of Probiotic Supplementation in Broilers Challenged with Coccidiosis: A Meta-Analysis. Probiotics Antimicrob Proteins. 2026. https://pubmed.ncbi.nlm.nih.gov/41801645/

[33] Lin XF, Wang XG, Fu CS et al. Evaluation of Immunoprotective Effects of DNA Vaccine Based on Eimeria maxima EF-1alpha Antigen and Chicken XCL1 Chemokine. Animals (Basel). 2026. https://pubmed.ncbi.nlm.nih.gov/41976087/

[34] Ma X, Zhang X, Li J et al. A Tetravalent Recombinant Subunit Vaccine Provides Protection Against Mixed Challenges with Four Eimeria Species in Chickens. Animals (Basel). 2026. https://pubmed.ncbi.nlm.nih.gov/41976065/

[35] Froebel LE, Watson BY, Rincker MJ et al. Temporal effects of a botanical feed additive and experimental housing methods in broilers exposed to an acute feed withdrawal period prior to coccidiosis inoculation. Poult Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/42019473/