Coccidiosis in poultry: Causes, clinical findings and diagnosis

Coccidiosis in poultry: Causes, clinical findings and diagnosis

Sonam Sarita Bal, Rajat Sood, Geeta Devi Leishangthem and Nittin Dev Singh

Department of veterinary Pathology, Guru Angad Dev Veterinary and Animal Sciences University, Ludhiana, Punjab

Abstract: Coccidiosis primarily affects the intestinal tract of mammals and birds. Coccidiosis has a huge impact on weight gain and feed conversion rate (FCR) in commercial poultry farming. E. tenella is the commonest of the highly pathogenic species, whereas E. acervulina and E. maxima are the most prevalent in poultry. The infective stage for young birds is the sporulated oocyst. Chemoprophylaxis and vaccination are the two primary methods for preventing coccidiosis.

Coccidiosis is a disease caused by Eimeria and Isospora parasites, affecting the intestinal tract of mammals and birds. The parasite poses significant economic risks to poultry industry, particularly in commercial poultry rearing, due to their high stock densities and increased mortality. Seven species of Eimeria (E. acervulina, E. brunetti, E. maxima, E. mitis, E. necatrix, E. praecox and E. tenella) are recognized as infecting chickens. Coccidiosis has a huge impact on weight gain and feed conversion rate (FCR) in commercial poultry farming and worldwide this disease costs around 2 billion dollars annually, major part of it being contributed by subclinical coccidiosis. The difficult diagnosis of subclinical coccidiosis may hinder the industry from evaluating the best control strategies.

The Eimeria are highly host specific. Coccidiosis primarily affects young birds. In these birds immunity develops quickly after exposure and provides protection against subsequent outbreaks. There is no cross-immunity between different Eimeria species, and later outbreaks may result from different Eimeria species. This disease can be mild or severe depending upon the ingestion of numbers of oocysts. E. tenella is the commonest of the highly pathogenic species, whereas E. acervulina and E. maxima are the most prevalent.

Susceptibility

Any age or breed of chicken is prone to coccidiosis. The type of Eimeria and the dose of oocysts determine the severity. Oocysts remain in buildings and are carried by humans, and due to the short prepatent period, the number of oocysts in the litter increases rapidly. Chickens reared on litter are relatively more susceptible of developing coccidiosis. Outbreaks are common between 3-6 weeks of age, and are rarely seen in flocks of less than 3 weeks due to insufficient chymotrypsin and bile salts in the intestines to cause excystation. Breeder pullets and layer pullets are at greatest risk due to their litter-keeping period of 20 weeks. Usually, infections with E. acervulina, E. tenella, and E. maxima are seen between 8-18 weeks of age.

Pathogenesis

The infective stage for young birds is the sporulated oocyst. After ingestion of this sporulated oocyst, sporozoites are released from it and enter the intestinal mucosa, undergo 2 generation of asexual (schizogony) and one generation of sexual cell cycle leading to formation of microgametes and macrogametes and ultimately zygote, which later on matures into an oocyst and sheds in faeces. In some species like E. tenella, E. necatrix, the maximum tissue damage occurs when second-generation schizonts rupture to release merozoites. The oocysts become infective only after undergoing sporulation, which requires warmth, moisture, and oxygen. Sporulated oocysts are resistant to various environmental conditions and can survive for months or years.

Clinical findings

Highly pathogenic group– E. brunetti affects the lower small intestine. It causes loss of weight, decreases feed conversion ratio, causes moderate mortality. E. necatrix, found in the middle small intestine of older birds.  Infected birds’ droppings typically contain blood, fluid, and mucus, and the intestine is distended to twice its normal size. Survivors may be emaciated and suffer secondary infections. E. tenella, the most pathogenic species, inhabits the caeca and adjacent intestinal tissues, causing severe disease characterized by bleeding, high morbidity, reduced weight gain, emaciation and very high mortality (upto 25% of the flock). The onset of mortality in a flock is rapid, occurring between 5 and 6 days after infection. Blood loss may reduce erythrocyte count and haematocrit value by 50%.

Moderately pathogenic group– E. acervulina is found in the duodenum. Light to moderate infections may have little effect on weight gain and feed conversion due to thickened intestinal mucosa. E. maxima parasitizes the middle small intestine and has characteristic large yellowish brown oocysts. It is moderately pathogenic.

Low pathogenic group– E. mitis is found in the duodenum and has slight lesions, often overlooked. E. praecox is also found in the duodenum and is named for its short prepatent period. It also produces no prominent lesions however, it may cause reduced weight gain, extreme fluid loss, and poor feed conversion.

Diagnosis

From clinical sign

Coccidiosis in birds can be detected by dysentery, diarrohea, or soft mucoid feces. The only sign in less pathogenic species is poor growth and impaired feed conversion. However in highly pathogenic species the earliest signs may be a sudden increase in daily mortality.

By post-mortem examination

Postmortem examination is crucial for diagnosis, with a few sick birds sacrificed for fresh material. Examining the entire intestinal tract, including mucosal surfaces, is essential to demonstrate the parasite in association with lesions. Wet smears examination of mucosal scrapping is sufficient to detect schizonts, gamonts, and oocysts. Histopathological examination of intestinal sample from affected poultry bird also showed schizonts (both macroschizont, microschizont), gamonts, oocysts (Fig. 1).

Coccimorph

A computational approach for parasite diagnosis, specifically Eimeria spp. from chicken and rabbit, uses images from sporulated oocysts to assess features like curvature, size, symmetry, and internal structure. Users can upload digital images from unidentified oocysts, identifying the species. This low-cost method is accessible but only identifies sporulated oocysts, limiting its use to litter sample identification.

Biochemical and molecular methods like multilocus enzyme electrophoresis, southern blot analysis, pulsed-field gel electrophoresis, and PCR techniques accurately identify different coccidial species.

Anticoccidial sensitivity testing (AST) is a widely recognized method used to evaluate the resistance of a specific coccidial isolate to various anticoccidial drugs.

Prevention and control of coccidiosis

Chemoprophylaxis and vaccination are the two primary methods for preventing coccidiosis, with chemoprophylaxis being the most popular, with 95% of broiler production receiving anticoccidials in their ration.

Chemoprophylaxis

Anticoccidials are divided into two groups: ionophorous antibiotics (ionophores) and synthetically produced drugs (chemicals). Currently available chemicals include amprolium, nicarbazin, robenidin, diclazuril, zoalene, decoquinate, and halofuginone. Among these nicarbazin is the most widely used synthetic chemical primarily added to the starter feed of broilers. The most popular anticoccidial products (ACPs) are carboxylic true ionophores, mainly because they pose a relatively limited risk of complete resistance compared to chemicals. The widely marketed ionophores include monensin, salinomycin, narasin, maduramicin, and semduramicin (all monovalent), lasalocid (divalent).

Vaccination

Live vaccination is less commonly used in broiler production today. There are two types of vaccines: attenuated and Non-attenuated/ virulent. Attenuated vaccines, such as Paracox and Livacox trigger an immune response and are less pathogenic than non-attenuated vaccines due to their reduced reproductive capacity. Non-attenuated vaccines, like Coccivac and Immucox, include wild-type strains of Eimeria. Live anticoccidial-sensitive vaccines can help modify resistance levels within a coccidial population.

Rotation/ shuttle program

Rotation/ shuttle program is the practice of switching from one anticoccidial to another after a certain grow-out period to manage and prevent coccidiosis in poultry. In this program, two drugs with different modes of action are used in succession within a single flock’s feed, to enhance coccidiosis control. A successful recent combination is Maxiban, which includes nicarbazin and narasin.

Management Considerations

Improvements in poultry management, such as replacing curtains with tunnel ventilation in hot climates and using efficient water systems like nipple drinkers to reduce spillage, have significantly enhanced control of environmental conditions and reduced coccidiosis risk. Poor management practices, such as difficult-to-clean feeding and drinking systems, lack of biosecurity measures, and presence of other animals, increase coccidiosis risk. Other less common risk factors include intermittent lighting, low initial temperatures, high ammonia levels, wet litter, and larger poultry house sizes. Maintaining optimal environmental conditions, including proper ventilation and avoiding wet litter, controlling humidity is crucial. Some disinfectants can destroy oocysts, and longer intervals between flocks can reduce viable oocysts in litter.

Conclusion

The poultry industry’s growth, vital for providing protein to the growing human population, is threatened by diseases like coccidiosis, which can impact human health. Coccidiosis is a global concern. Therefore, ongoing basic and applied research into the biology of these organisms is essential for sustainable control. Research focuses on enhancing molecular techniques for better diagnostics and developing recombinant vaccines. However, significant research is still needed to address practical questions about appropriate prevention methods for specific poultry production units.

Figure 1. Intestine: Different stages of coccidia (H & E, 40X).

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