Clinical coccidiosis is one of the main causes of growth retardation, poor feed conversion and high mortality
Photo*: Avian coccidiosis by Eimeria necatrix
Avian coccidiosis is a parasitic disease of global importance in aviculture due to its negative effects on production and development of birds. The name of this disease is used as a general term to describe clinical signs and lesions caused by protozoa of the genus Eimeria.
These parasites cause tissue damage in the intestine, altering the absorption of nutrients and producing diarrhea of different degrees and decrease in production.
Clinical coccidiosis is one of the main causes of growth retardation, poor feed conversion and high mortality in intensive farming of broilers, cage-free pullets and breeders. In addition, subclinical infections contribute to increase the economic impact of this disease.
Eimeria spp. is an obligate intracellular parasite that replicates in the intestinal epithelium of the host and produces different enteritis of a different severity depending on the specie.
This protozoon is host specific, and there are seven species of Eimeria that can affect chickens and hens (Gallus gallus domesticus). In order to classify them, different parameters are used, such as the duration of their biological cycle, the morphology of the oocyst and their intracellular forms, the number of parasitized cells and the location and types of lesions caused.
These seven species are: E. acervulina, E. maxima, E. necatrix, E. brunetti, E. mitis, E. praecox, found in the small intestine, and E. tenella, that has cecal location.
BIOLOGICAL CYCLE AND TRANSMISSION
Transmission is done by feco-oral route. The ingestion of sporulated oocysts, which is the resistance form with infective capacity, present in the litter from faeces of sick animals, is the usual mode of infection. There is also the possibility of mechanical transmission by dust, tools and clothes of farm workers, or animals like flies or larvae of the beetle Alphitobius spp.
Thanks to the bird digestion process, sporozoites are released from the interior of the oocyst and penetrate the intestinal cells, where they perform various cycles of asexual and sexual reproduction, destroying the epithelium and producing a large number of new oocysts that are later released in feces. It is a fast cycle, which lasts from 4 to 7 days depending on the Eimeria specie.
Oocysts can survive for a long time and under adverse environmental conditions. For oocysts to be infective again, that means to be sporulated, adequate conditions of oxygenation, temperature and humidity in the environment must occur. Wet litter and warm conditions in the production facilities favor sporulation and, therefore, coccidiosis outbreaks.
Pathogenicity and clinical presentation of each species are determined by several factors:
- Factors that depend on the host: Age, nutritional and immune status of the animal and existence of concomitant infections that are immunosuppressive or that attack the intestine. Animals that are not previously immunized, either because of their age or previous treatments with coccidiostats, are more susceptible to the disease.
- Factors that depend on the parasite: Number of ingested oocysts and species involved. Depending on the specie, penetration depth into the intestinal mucosa and the number of cycles of asexual reproduction performed vary. Because of these mentioned factors, tenella is the most pathogenic, followed by E. necatrix and E. maxima.
- External factors: Poor ventilation, wet litter and any stressful situation that decreases feed intake, such as overpopulation, heat stress or musculoskeletal problems.
CLINICAL SIGNS AND INJURIES
Presentation of the disease varies from an acute form with high mortality and great economic losses to a subclinical process that decreases productivity and is difficult to diagnose.
Intestinal epithelium destruction is the source of symptomatology and nutrients absorption alteration, which leads to weight loss, decrease in the laying rate and alterations in meat and eggs quality.
General clinical picture of the disease includes ruffled feathers, drowsiness, mucous or bloody stools that stain the area of the cloaca, dehydration and possible anemia.
Clinical signs and serious injuries are usually caused by five of the seven Eimeria species mentioned above, as described below:
- E. acervulina is related to catarrhal duodenitis, with spotted lesions in the mucosa that can fuse as white transverse lines.
- E. brunetti and E. maxima cause a bloody diarrhea that can become hemorrhagic, so that necropsy shows catarrhal ileitis or catarrhal jejunitis, respectively, with possible evolution to hemorrhagic.
- E. necatrix and E. tenella are the ones causing the highest mortality. The first causes hemorrhagic jejunitis with petechiae or visible hemorrhages in the mucosa, while the second one develops hemorrhagic typhlitis, with enlarged ceca containing hemorrhages or blood clots. These clots may be solidified in animals that overcome the disease and are called cecal casts.
It should be mentioned that most of the clinical outbreaks are caused by mixed infections of several species, so lesions in different parts of the intestine are commonly found. Species that parasitize the same intestinal region compete for localization and do not increase their pathogenicity due to the combined infection. In contrast, species that parasitize different areas of the intestine enhance their pathogenic effect when acting simultaneously.
Diagnosis is carried out from an evaluation of the clinical picture, an anatomopathological and histological study and laboratorial tests from fecal samples or intestinal scrapings, which can be subjected to flotation tests and subsequent microscopical evaluation. It is also advisable to study bed samples to perform oocyst counting.
To identify individual species, RNA and DNA tests, recombinant DNA techniques or PCR may be used.
Bloody stools are usually related to an outbreak of coccidiosis or necrotic enteritis, which can often be differentiated by a macroscopic examination of necropsy lesions of sick animals. Hemorrhagic enteritis is usually caused by Eimeria spp., while fibrin-necrotic enteritis is often related to Clostridium perfringens infections.
TREATMENT AND PREVENTION
One of the main preventive strategies that allows to break the cycle of the infection is the use of the system called “All in – all out”. This system is based on the complete emptying of the facilities, which ensures proper cleaning and disinfection to reduce microbial load before introducing a new batch of animals and, in addition, avoids contact between animals of different ages. In case this system cannot be performed, old birds should be kept as far as possible from new birds.
Other management measures to avoid humid litter during the productive stage, and thus prevent the sporulation of possible oocysts present in it, include the drinking system maintenance, in order to avoid water leakage, and ensure good ventilation, which prevents high humidity conditions and roofing condensation.
These biosecurity and preventive management measures mentioned allow to reduce the risk of coccidiosis transmission, but additional measures are always necessary to control the disease, such as the use of coccidiostats, vaccines, or natural products.
Coccidiostats are usually used mixed in feed and are classified into two categories, chemical anticoccidials and ionophores. These products act in the first phases of the parasite cycle and, therefore, prevent the symptomatology of the disease. Both require a withdrawal period to avoid presence of residues in meat or eggs.
In order to prevent the emergence of resistance to these agents, shuttle programs with different products have been used within the same cycle or between production cycles. As these strategies do not fully avoid the appearance of resistances, added to the increasing pressure for banning the use of drugs in animals for human consumption, alternatives such as vaccination or natural alternatives, such as pronutrients, have been developed.
Stimulation of immunity through live vaccines
Development of immunity against Eimeria spp. is produced by initial stimulation with low doses of oocysts and re-exposure to oocysts present in litter. These vaccines are administered in drinking water or in feed and, due to their price, their use is more widespread in breeders than in broilers. Vaccination may not completely prevent the disease, since vaccines may include different species of Eimeria from those present on the farm, and there is no cross-immunity between species.
Stimulation of immunity by pronutrients (natural products)
Nowadays, there is a growing interest in the use of natural products, used as dietary supplements, whose function varies between the stimulation of immunity or the anti-inflammatory and antioxidant effect.
Some studies revealed that the use of antioxidants of natural origin, such as vitamin E or plants such as oregano, improve the general condition of sick animals due to coccidiosis. Other studies indicated that the use of probiotics, such as diets enriched with lactobacilli, improve the intestinal microbial ecology, acting as immunomodulators and stimulating humoral immunity.
Different studies showed that the use of products based on pronutrients with local immunostimulant properties improves the productivity of animals challenged by coccidia. Pronutrients are active molecules derived from plants, which induce the activation of genes related to protein synthesis, such as interleukins, with the aim to promote the activity of the local immune system of the intestine. Sporozoites are the most sensitive stage of the cycle to immunity, so, when pronutrients are fed, coccidia elimination occurs at the beginning of the parasite cycle, thus avoiding the appearance of symptoms of the disease. As the product does not act directly on the pathogen, because it stimulates animal physiology, it does not create resistances and it can be administered in continuous programs. As it is of natural origin, it leaves no residues and does not require withdrawal period.
Lastly, it should be highlighted that injuries caused by coccidiosis, when not adequately controlled, are a gateway for other infectious processes, among them, necrotic enteritis, a main topic in future publications.
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