3 Aug 2015
Connor Sheedy
Job Title
Figure 2. Haemorrhagic gastrointestinal tract of an animal suffering from clinical coccidiosis.
This article provides an initial introduction to coccidiosis in cattle and sheep with a brief overview of the pathogen and its life cycle. Incidence and epidemiology is then covered, along with route of transmission. The clinical and sub-clinical forms of the disease are compared, and their relative importance with regards to production. Treatment of clinical cases and instructions on how to handle an outbreak are then discussed, followed by methods of control with management and pharmaceutical techniques and their relative merits.
Coccidiosis is a parasitic gastrointestinal disease of cattle and sheep, present throughout the UK and caused by protozoa of the genus Eimeria, among others.
The protozoa cause damage by entering and reproducing within the host’s enterocytes. After several phases of reproduction they burst out, destroying the cell and passing out in the host’s faeces. This destruction of the gut mucosa reduces surface area and impairs the absorption of fluids and nutrients across the gut wall. Once out of the host, the immature oocysts undergo sporulation and are capable of infection after two to four days, dependent on environmental conditions.
After sporulation these oocysts become highly resistant due to their protective cell wall and can persist in the environment for years. The complete life cycle takes between 18 to 21 days and animals with severe disease can shed millions of oocysts a day, with as few as 50,000 oocysts ingested by a single host capable of causing clinical disease. It is one of the most economically significant diseases in cattle and sheep due to the considerable impact on production at a herd/flock level.
Coccidiosis is most commonly seen in young animals, especially those subject to environmental stressors or malnutrition (Daugschies and Najdrowski, 2005). Initial disease is usually seen once maternal passive immunity begins to decline, and peak incidence is between one to three months in calves and lambs, although animals of all ages can be affected. Adult animals rarely show signs of disease, but will commonly have low-level infection and so serve as a source of oocysts for younger animals, particularly the dam, which will show a periparturient rise in shedding.
Transmission is via the faeco-oral route and calves and lambs will usually ingest oocysts at an early age. Small burdens are tolerated well in healthy animals and continuous low-level exposure will result in a level of species-specific immunity. This immunity does not completely prevent the shedding of oocysts, but will generally result in a low level of shedding.
However, when a higher dose is encountered – such as in conditions of high stocking densities or poorly maintained housing, or when the host becomes immunosuppressed – the host may become overwhelmed, which results in either clinical or subclinical disease.
Even animals with a level of immunity can contract clinical disease if a large enough challenge is presented. In severe cases, the resulting destruction of the intestinal mucosa will result in watery diarrhoea in which blood and strands of intestinal mucosa may be present, along with other clinical signs that may include pyrexia, tenesmus, dehydration, abdominal pain and weight loss.
Secondary bacterial infections can cause further damage and the increased risk of blowfly strike, due to the heavy faecal contamination of lambs’ coats, can also add further complications (Figure 1).
When the infective dose is relatively smaller, but still significant, milder cases will be seen as the more common subclinical form, in which affected animals appear healthy apart from some potential vague clinical signs, such as a roughened coat. The importance of this form of the disease derives from the reduction in feed efficiency that is seen, and the shedding of oocysts by subclinically infected individuals, which contributes to environmental contamination.
This form of the disease is more significant from a production point of view as mortality is low, but morbidity can be as high as 100% in calves, so losses are largely due to a reduced growth rate and failure to thrive, which will account for the largest economic impact (Fitzgerald, 1980).
The extent of this impact are difficult to quantify as many variables are involved, but one study found an increase of 0.2kg daily liveweight gain in dairy calves up to 12 weeks when treated with coccidiostats compared to those that were not (Potter and Drysdale, 2011). This demonstrates the significant effect on production. Once subclinical disease is established in a population, the significant level of shedding will be such that a susceptible individual introduced to this volume of oocysts will likely suffer severe clinical disease. Also, the damage done to the intestinal tract in such cases can have permanent consequences, with severely affected individuals never doing as well as their pen mates even after treatment.
In sheep, coccidiosis is mostly seen during the months of spring, although this correlation is more likely to be due to the numbers of young lambs on the ground at this time of year than any other factors. In cattle, peaks are seen in the summer and winter months, but the disease is an all-year-round one.
Diagnosis of coccidiosis is often based on history and clinical signs, as well as faecal testing for the presence of oocysts. Faecal samples should be taken to assist diagnosis; however, as low numbers of oocysts are often present in healthy animals, and the number of oocysts does not always directly correlate to the severity of disease (Daugschies et al, 2007), the results require some interpretation. Also, the onset of clinical signs may precede the shedding of significant amounts of oocysts. To help clarify the results, samples should be taken from a number of individuals and quantitative oocyst counts carried out on each sample to help confirm the diagnosis.
As many species of Eimeria are non-pathogenic, speciation of coccidia can be performed to determine clinical relevance, but is usually unnecessary in first-line testing, provided the whole clinical picture is taken into account when making the diagnosis.
Faecal egg counts should also be performed, as other parasitic gastroenteritis will likely be the primary differential, and mixed infections with other gastrointestinal parasites can occur and cause more severe disease.
If the opportunity presents, postmortem can also be useful to confirm the diagnosis. Gross findings may include thickened, inflamed, haemorrhagic ileum (Figure 2), caecum and colon, and thick, white opaque patches in the small intestines, which can be sampled and inspected under microscopy to confirm the presence of oocysts.
Control in the face of a coccidiosis outbreak should be carried out by first isolating animals suffering from clinical signs due to the massive number of oocysts shed in their faeces. Treatment of clinical cases with a coccidiostatic oral drench is recommended, as well as treating at-risk animals not yet showing clinical signs. This especially applies to young animals in close proximity that are sharing the same contaminated water/feed supply.
Coccidiostats are preferred to coccidiocides as they have been shown to not interfere with the development of immunity (Agneessens et al, 2006). This will help reduce the number of subclinical cases that may develop into more severe disease and reduce the shedding of oocysts to minimise environmental contamination. Supportive therapy is also very important as the disease is described as self-limiting and individuals will often recover spontaneously as long as dehydration and secondary infections are addressed. Fluid loss caused by the diarrhoea is corrected using oral and parenteral fluid therapy as necessary, and secondary bacterial infections are treated using antibiotics.
Prevention is a more economical and effective approach than treatment of clinical cases, and preventing coccidiosis is primarily focused on management.
Stocking density should be reduced, particularly in housed animals, where possible, to lower both stress to the animals and the number of individuals shedding into the environment. Figures for minimum stocking densities can be found in Defra’s codes of recommendations – visit http://bit.ly/1fjrGM1 (sheep) and http://bit.ly/1RMP5ak (cattle).
Move feed and water troughs, where feasible, to prevent these areas becoming a haven for oocysts, as well as minimising the potential for faecal contamination of the food and water supply by either raising or covering these troughs. Keeping pens clean, well-drained and with ample bedding will also help reduce the build-up of eggs in the environment.
Calves should always be batched according to age, and mixing of different ages should be avoided to reduce exposure to the younger animals.
In the same vein, it is important to thoroughly disinfect pens in between batches of youngstock for the same reasons. The resistant nature of the oocysts means many standard disinfectants will not be effective against them, so ensure the chosen products are coccidiocidal.
These steps, along with other generally good practices such as good colostrum management and minimising environmental stressors, will greatly help reduce the incidence of coccidiosis and its impact on the herd/flock. Blanket treatment with coccidiostats in feed is another tool that can be used effectively to reduce disease incidence, but should not be viewed as an alternative to proper management.
It is most effective when used as a prophylactic treatment before clinical disease is seen after a period of known stress, such as ration change, weaning or travel.
To maximise the efficiency of these preventive treatments, they should be initiated one week after these anticipated times of stress. These are most commonly used in the form of hard feed containing decoquinate in lambs and beef calves. In young dairy calves where hard feed intake can be difficult to predict, milk replacer containing decoquinate can provide an alternative treatment option due to the intake being more predictable, so a more accurate dose can be administered (Potter and Drysdale, 2011).
Whichever preventive protocol is adopted, taking samples at regular intervals for routine monitoring should be encouraged to assess its effectiveness.
Coccidiosis is a significant disease of cattle and sheep, both from a health and economic standpoint. However, through good management, prompt detection and treatment of clinical cases, its impact can be greatly reduced.
