Importance of coccidiosis in ruminants

Coccidiosis in ruminants, is an enteric disease caused by unicellular parasites called coccidia. These microorganisms produce significant economic losses in ruminant farms, particularly those with high densities.

The economic impact is due to how easily this disease is disseminated in a group of animals, as every oocyst or parasite egg has the potential to produce around 23 million oocysts in 21 days (Keeton & Navarre, 2018), which means that they have the capacity to increase environmental contamination and cause severe infestations.

In fact, even mild or subclinical infestations, which do not cause signs in animals, have productive consequences because of the damage in the digestive mucosa, which decreases diet utilization and, therefore, causes growth stunt, worsens feed conversion and increases drug expenses, among others.

For these reasons, coccidiosis is a disease to consider, particularly in farms and areas where many animals congregate, such as feed lots, high-density pastures and feeding and drinking areas.

Etiology

Coccidiosis in ruminants is caused by coccidia of the genus Eimeria, from the Eimeriidae family, Eucoccidiorida order, Sporozoasida class and Apicomplexa phylum. Eimeria species are identified, among other characteristics, because sporulated oocysts have four sporocysts with two sporozoites each, a total of eight infective units.

They are species-specific, which means that each Eimeria species can infect only one animal species; as an example, Eimeria species affecting cows cannot infect sheep or goats. Additionally, some species have a predilection for certain areas of the digestive tract, so that the location of the lesions in the necropsies can help to reach a diagnosis. Despite this, animals often suffer from infestations caused by different Eimeria species, pathogenic and non-pathogenic, at the same time, making diagnosis much more complex.

Main pathogenic species of Eimeria and location of the lesions in cattle, sheep and goats

Life cycle and pathogenesis

Life cycle of Eimeria species should be considered to design effective treatment and prevention programs. The cycle lasts for two to four weeks, depending on the involved Eimeria species and the environmental conditions, and starts when an animal eats sporulated oocysts, whether from the floor, feces or other sources. Once in the gut, sporozoites are released from the inside of the oocyst and invade the cells of the gut mucosa.

Inside these cells, sporozoites evolve into schizonts, which asexually multiply to generate merozoites that invade other cells of the gut mucosa and multiply to produce macro and microgametes. During the next stage (gametogony), macrogametes are fertilized by microgametes and form an unsporulated oocyst, which will be excreted with dejections.

Unsporulated oocysts are not infective and need to sporulate in the environment to be able to infect the next animal. This process, called sporogony, depends on different factors, such as temperature, oxygen levels and environmental humidity. These oocysts can resist in the environment for weeks or months, depending on the environmental conditions.

The pathogenesis of this disease in based on the destruction of the intestinal epithelial cells because of the multiplication of the parasites inside them, which causes loss of mucosa’s integrity, decrease in nutrient absorption and worsening of the productive parameters. This loss of integrity significantly increases susceptibility to suffer from secondary infections, caused by pathogens that take advantage of the tissue weakness to cause disease.

Usually, adult ruminants get regularly infested with a combination of pathogenic and non-pathogenic coccidia species, but they do not suffer the clinical disease because they developed immunity in previous infestations. On the contrary, young animals are not immunized and get infested from the oocysts excreted by adults. This way, the disease remains in the farm.

Signs and lesions

Clinical signs depend on the age of the animal, in fact, they usually appear in young animals, particularly during stress periods such as weaning, transportation, sick periods or in malnourished animals. They also depend on the involved Eimeria species, the infective pressure in the environment and immunity.

Severity of the infestation can vary from subclinical or self-limited, in animals that recover without treatment, to severe, in cases that may cause death. Survivors can become chronic carriers and contribute to increasing the number of oocysts in the environment.

Calves from 6 to 12 months old and lambs and goatlings from 1 to 6 months of age are the most susceptible ruminants, although most of the clinical signs appear between weeks 4 and 8 of life (Keeton & Navarre, 2018). This is because they usually get first infested at these ages and have no immunity yet.

The most frequent clinical signs are bloody or mucous diarrhea, depression and weakness, abdominal pain, dehydration, paleness of the mucous membranes, weight loss and, in severe cases, constipation and rectal prolapse. In subclinical infestations, frequent in adult ruminants, only a decrease in the feed intake and productivity will be observed.

Usual findings in necropsies include hemorrhages, white to greyish plates or nodules in the mucosa, especially in the cecum, colon and distal ileum, as well as edema in the spiral colon. Hemorrhages can go from petechial to diffuse, depending on the severity of coccidiosis. Some areas of the submucosa can also be affected and covered by a membrane.

Ruminants can also suffer from nervous coccidiosis, which usually takes place in calves with severe infections by E. zuernii. In these cases, clinical signs include muscular tremors, convulsions, nystagmus and other nervous signs, such as prostration and intermittent neurological sings. It can cause up to 80% mortality (Keeton & Navarre, 2018). The pathogenesis of this type of coccidiosis has not been described, although several theories contemplate vitamin A deficiency or electrolytic imbalances (Jolley & Bardsley, 2006; Reppert & Kemp, 1972). Enteric lesions are the only ones observed in the necropsies and, in some cases, brain edema.

Diagnosis

Diagnosis of ruminant coccidiosis is based on the observation of clinical signs, bloody to mucous feces and the oocyst counting per gram of feces. Identification of lesions can help to reach a diagnosis in necropsied animals.

• Clinical signs: productivity loss, growth stunt, diarrhea and, in some cases, nervous signs.
• Feces: pastry to liquid and presence of mucus and/or blood.
• Oocyst counting: it can be conducted in feces using the salt flotation method. To confirm the diagnosis, involved Eimeria species need to be identified, since many are non-pathogenic. The counting can be negative if samples were taken during the prepatent period, before the animal starts excreting oocysts. For this reason, taking samples of feces from more than one animal is strongly recommended, if possible, from animals that have been showing signs for two or three days.
• Lesions: mild to severe hemorrhages and whitish nodules or plates in the cecum, colon and distal ileum. Nodules are characteristic of coccidiosis.
• Mucosa scraping: different stages of the parasite can be observed, such as macroschizonts, gametocytes or oocysts.

In nervous coccidiosis, diagnosis should be based on the clinical signs, presence of digestive lesions, absence of lesions in the central nervous system and identification of E. zuernii oocysts.

Prevention and treatment

Prevention is key to control coccidiosis in ruminants, since even subclinical cases cause important productive losses and are a source of infestation for susceptible animals. Preventive strategies include minimizing oocysts exposure and stress and the administration of preventive products during the risk periods.

Minimization of oocyst exposure

It is an important measure, especially in young animals because they are more susceptible. Exhaustive cleaning of the drinking system and feeders, as well as exposure to sunlight and desiccation of feed to reduce the presence of oocyst are strongly recommended.

Minimization of stress

Avoid stressful factors such as high densities or other diseases that lead to immunosuppression, which makes animals more susceptible to get coccidiosis.

Preventive products during risk periods

Products that prevent coccidiosis should be administered during the periods of major risk, particularly to young animals after weaning or in ruminants in feed lots. Several tools can be used:

• Coccidiostats: numerous drugs inhibit coccidian growth and reproduction, but many are not approved for ruminants. This is because they act as antibiotics and can alter the ruminal flora. Moreover, resistances have been described, for which regulations worldwide are limiting their use.
• Pronutrients: active molecules from plants that stimulate cellular physiology. In the case of gut optimizer pronutrients, these molecules activate the local immune system in the gut in charge of defending the animal against parasitic infestations, such as coccidiosis.

The administration of these pronutrients in the drinking water or the milk replacer in young ruminants is strongly recommended, as it optimized the protection mechanisms of the animals, making them able to interrupt the life cycle of coccidia and avoid their multiplication.

• Vaccines: no effective vaccine has been developed yet for ruminants, which limits prevention to the aforementioned tools.

Treatment

Supportive treatment is the main treatment, since only a few drugs were proved effective to treat coccidiosis in ruminants. In sick animals, dehydration, secondary infections, malnourishment and exposure to extreme temperatures must be avoided.

Conclusions

Coccidiosis is a parasitic disease causing severe economic losses in farms worldwide, and ruminants are no exception. For this reason, it is important to know how to identify it and to apply effective prevention and control measures, as treatments are little effective.

These measures are based on the elimination of infective oocysts from the environment and the control of stressful factors that affect the immune system of ruminants and, consequently, make animals more susceptible to suffer from coccidiosis.

In this sense, there are natural molecules, called gut optimizer pronutrients, that stimulate the physiological activity of the immune system in the gut, so that the animal is capable to eliminate coccidiosis and prevent the infestation.

Bibliography
Jolley, W. R., & Bardsley, K. D. (2006). Ruminant Coccidiosis. Veterinary Clinics of North America – Food Animal Practice, 22(3), 613–621. https://doi.org/10.1016/j.cvfa.2006.07.004
Keeton, S. T. N., & Navarre, C. B. (2018). Coccidiosis in Large and Small Ruminants. Veterinary Clinics of North America – Food Animal Practice, 34(1), 201–208. https://doi.org/10.1016/j.cvfa.2017.10.009
Reppert, J., & Kemp, R. (1972). Nervous Coccidiosis. Iowa State University Veterinarian, 34(1), 3.