EIMERIA is a parasite of young animals, found in almost every species of mammal, bird, amphibian and fish. There is a vast array of species, often infecting the same host, but with varying degrees of pathogenicity.
Eimeria spp. can occur in mixed infections and the presence of oocysts in the host’s faeces does not always coincide with clinical signs. In addition, clinical signs may not always be forthcoming and subclinical disease is also a cause of major economic loss. Diagnosing coccidiosis can be a minefield and not only requires lab information but a full clinical picture.
When Antoni Van Leeuwenhoek first laid eyes upon an oocyst in the bile of a rabbit back in 1674, little did he know the importance of this little parasite’s cousins, to the future lamb and calf population of the modern farming world.
Cows can be infected with 13 different species of coccidia, but only three are considered pathogenic: E. zuernii, E. bovis and E. alabamensis. Eleven different species infect sheep, but only two are pathogenic in this host: E. crandallis and E. ovinoidalis.
Clinical signs of coccidiosis are typically seen in young lambs from four to six weeks of age, and in calves from three to six months old. After high numbers of infective oocysts from the environment are ingested, the parasite invades epithelial cells lining both the small and large intestine.
It rapidly multiplies, disrupting the absorptive capacity of the gut and causing profuse, watery, dark green diarrhoea, containing blood and strips of mucosa. Rectal prolapse may occur as a consequence of tenesmus. Some cases may have a fatal outcome. New oocysts appear in the faeces two to three weeks after infection, and persist for only five to 10 days.
The subclinical form arises in 95% of cases, when oocyst challenge is lower, but is probably of greater economic importance.
The subclinical form is characterised by anorexia, weight loss, 10-30% reduction in growth rate, dull broken hair and poor general condition. Sometimes there may be intermittent grey diarrhoea or dark faeces clumped around the tail.
Key to diagnosis
In clinical cases of coccidiosis, diarrhoea may begin two days before oocyst shedding and continue for five to seven days afterwards, so the absence of oocysts does not mean that coccidiosis can be ruled out.
Oocysts may be missed if they are trapped within threads of fibrin or mucosa within the faecal sample. The subclinical form is equally challenging to diagnose, oocyst excretion peaks are transient, persisting for only half a day to two days and spread over three weeks. Therefore false negatives are a common problem.
In cattle the pathogenic species of Eimeria can be identified on oocyst morphology. However, the oocysts of sheep species require sporulation for speciation. Oocyst counts combined with identification of the Eimeria species are needed to see if coccidial infection is an underlying problem.
This lab information should be combined with epidemiological data, for example the age of the affected animals. Associated husbandry risk factors such as a history of recent stress, environmental contamination levels, overcrowding, mixing of different ages, etc. will also help confirm a diagnosis.
The differential diagnoses to rule out include E. coli, Salmonellosis, Rotavirus, Coronavirus, Cryptosporidiasis and nematodes. If there is still uncertainty as to the exact role of coccidiosis infection in a group of animals, it may be prudent to treat the animals with an anti-coccidial such as Vecoxan 2.5mg/ml Oral Suspension (diclazuril) and assess any “animal production” responses. These include an improvement in general condition, and increases in live weight gain.
Clinical and subclincal coccidiosis are highly prevalent in lambs and calves but the significance of oocyst faecal counts depend on the numbers and species of the parasites involved.
Diagnosis can be a challenge as false negatives are common but treating the animals and measuring any subsequent improvements is another option available to practitioners on farm.