Coccidiosis – impact, treatment and prevention

Crossing the halfway point of calving results in a mix of calf ages in sheds along with an increasing density of stock numbers indoors, while on drier farms with early turn out of calves, small calf paddocks close to sheds are often targeted first for grazing. Either way, we are in a risk period for coccidiosis to occur on farms.

What is coccidiosis?

Coccidiosis is a common disease of the intestine that is caused by a parasite known as Eimeria. The Eimeria family of parasites has around 20 different species, but only three of them are known to cause significant illness. Calves from the ages of three weeks to six months are particularly susceptible as they have little immunity in comparison to older animals. Young calves become infected by ingesting Eimeria eggs, known as oocysts, that are in the environment. These tiny oocysts are very robust and can survive from year to year in infected faeces, often being completely unaffected by most disinfectants due to their thick outer wall. 

Through feeding equipment, water sources or licking, these microscopic oocysts are ingested and invade the gut lining of the calf. Once inside the cells of the gut wall, the parasite undergoes cycles of development and replication resulting in damage to the gut lining as the cells rupture. As coccidia are highly prolific, the replication process results not only in damage to the gut, but also the release new oocysts in high numbers into the environment for the next susceptible animal to become infected. A calf can shed more than a million oocysts per gram of faeces during an initial infection demonstrating how quickly calf pens can become highly contaminated. 

How does coccidiosis affect calves?

The outcome of infection with coccidia depends on a number of factors: 

• The number of oocysts ingested;
• The immune response of the calf;
• Stressors (poor weather, transport, regrouping, dehorning, weaning); and
• Management practices (nutrition, stocking density, hygiene).

Other disease pressures

Calves presenting with clinical coccidiosis show signs of watery, sometimes bloody scour accompanied with weakness, dehydration, straining and weight loss. Death can result in severe cases, but chronic ill thrift is often seen in calves following infection due to permanent damage caused to the gut. The lining of the gut is where a calf absorbs all the nutrients required for growth and it is easy to understand how damage here can impact on productivity. Subclinical infection is of much greater significance as the long-term effects of reduced weight gain, feed intake and poor performance are considerable. Over a 21-day period, research studies have demonstrated a 2-6kg difference in weight gain across a group of calves due to subclinical coccidiosis. 

Treatment and prevention options 

It’s important to reach the correct diagnosis on any illness that causes scour in calves as this is key for successful treatment options. Your vet might use a combination of clinical signs along with pooled faecal samples from groups of affected calves to reach a working diagnosis of coccidiosis. In terms of treatment, prevention is the best option, as once clinical signs are seen, gut damage and production losses are already suffered. Prevention needs to be considered as a three-pronged approach combining:

• Calf health;
• Hygiene; and
• Strategic use of anticoccidial medication.

Firstly, the basic foundations of good calf health are essential following best practice around colostrum intake, minimising stress and optimising nutrition. Secondly, in order to limit the build-up of oocyst contamination in the environment, hygiene is paramount. This involves limiting overcrowding, clean feeding equipment, good bedding management as well as looking at the out of season house hygiene in order to begin the spring with low levels of contamination. It is important to understand that 100 per cent environmental elimination of the parasite is difficult to achieve and explains why we see this issue annually on farms.
Finally, in order to develop immunity, some low-level exposure to coccidia is necessary but this needs to be limited in order to prevent disease risk. The ingestion of oocysts allows time to stimulate immunity, but the parasite is removed through strategic dosing with an anticoccidial medication around two to three weeks following exposure. It is necessary to treat all calves in the group due to the nature of the infection; if one animal is exposed it is safe to assume all are. Use of anticoccidial medication has not only shown to prevent clinical and subclinical disease by removing parasites within the gut, but also reduces subsequent environmental contamination with oocysts. This preventative approach, combining farm management, calf health and the use of anticoccidial medication, reduces the considerable annual economic losses that occur due to both clinical and subclinical coccidiosis.