Coccidiosis is a parasitic gut disease in cattle caused by single-celled organisms called Eimeria. These microscopic parasites invade and destroy the cells lining the intestine, leading to diarrhea (often bloody), poor growth, and sometimes death. While many cattle carry low levels of the parasite without showing symptoms, clinical outbreaks can be devastating, particularly in young, stressed, or overcrowded animals.
What Causes It
Cattle can be infected by several species of Eimeria, but two are responsible for most clinical disease: Eimeria bovis and Eimeria zuernii. Both target the lower small intestine, cecum, and colon, and both cause significant tissue damage. Other species like E. auburnensis and E. alabamensis exist widely but tend to cause less severe illness on their own.
The parasite spreads through a fecal-oral route. Infected cattle shed microscopic egg-like structures called oocysts in their manure. These oocysts need warmth, oxygen, and moisture to become infectious, a process called sporulation. Once conditions are right (which doesn’t take long in a typical feedlot, barn, or moist pasture), other cattle pick them up while eating, drinking, or grooming.
How the Parasite Damages the Gut
Once an animal swallows infectious oocysts, the parasites hatch in the intestine and immediately begin invading the cells that line the gut wall. Inside those cells, they reproduce asexually, multiplying rapidly. When the host cell can no longer contain the growing parasites, it ruptures, releasing a new wave of organisms that invade neighboring cells. This cycle repeats through multiple generations.
The first round of reproduction causes relatively little damage and few visible signs. But as each generation produces more parasites and destroys more cells, the intestinal lining starts to break down. Blood and fluid leak into the gut. Eventually the intestinal surface is stripped away, causing hemorrhage and the characteristic bloody diarrhea. To put the scale of destruction in perspective, an infection starting from just 1,000 ingested oocysts can ultimately destroy 24 billion intestinal cells.
After several rounds of asexual reproduction, the parasites shift to sexual reproduction, producing new oocysts that pass out in the feces and restart the cycle in the environment. The entire process from ingestion to oocyst shedding takes roughly two to four weeks depending on the species.
Signs to Watch For
Clinical coccidiosis typically shows up as watery diarrhea that progresses to bloody, foul-smelling feces. Affected animals often have blood-stained manure smeared around the tail and hindquarters. They lose appetite, become dehydrated (sunken eyes are a telltale sign), and lose weight rapidly. In one documented farm outbreak among adult cattle, sick animals ran fevers between 103 and 105°F with elevated heart rates and markedly reduced milk production. Some animals passed visible blood clots and strained repeatedly while trying to defecate.
Subclinical infection is actually more common and more economically insidious. These animals never develop obvious bloody diarrhea, but they eat less, gain weight more slowly, and convert feed less efficiently. Because nothing dramatic is happening, the losses often go unnoticed until production records reveal the gap.
The Nervous Form
A less common but alarming presentation is nervous coccidiosis, seen primarily in young cattle. These animals develop involuntary eye movements, abnormal eye positioning, and episodes where they go down and contort their heads and necks. Between episodes, they may appear nearly normal. The exact mechanism behind these neurological signs isn’t fully understood, but the clinical picture is distinctive enough that experienced veterinarians can often identify it on sight. This form can be fatal if untreated.
Which Animals Are Most Vulnerable
Calves between three weeks and six months old are the classic targets, since they haven’t yet built immunity from prior exposure. But coccidiosis is not exclusively a calf disease. Adult cattle can develop clinical illness, particularly when stressed by transport, diet changes, weather extremes, or concurrent disease. Outbreaks tend to cluster around events that combine stress with environmental contamination: moving calves into group housing, weaning, or introducing new animals to contaminated pens.
Overcrowding is a major amplifier. Higher stocking densities mean more manure in a smaller area, which means more oocysts concentrated where animals eat and drink. Moist, warm conditions accelerate oocyst sporulation, so muddy lots, poorly drained pens, and humid barns are ideal breeding grounds for the parasite.
How It’s Diagnosed
Diagnosis usually starts with clinical signs: bloody diarrhea in young or stressed cattle, especially in a group setting. A veterinarian can confirm the diagnosis by examining a fecal sample under a microscope to identify and count oocysts. However, interpreting those counts requires some nuance. Low oocyst numbers are common in healthy cattle because most animals carry some level of Eimeria without clinical disease. The count needs to be considered alongside the animal’s symptoms, age, and the species of Eimeria identified. Finding large numbers of E. bovis or E. zuernii in a calf with bloody diarrhea is a straightforward diagnosis. Finding moderate numbers in an otherwise healthy animal is not.
Postmortem examination, when available, reveals characteristic damage: thickened, reddened intestinal walls, loss of the gut lining surface, and visible parasite stages embedded in the tissue of the cecum and colon.
Treatment and Prevention
Treatment focuses on stopping the parasite’s reproduction and supporting the animal through recovery. Amprolium is the primary therapeutic drug, given through feed at a treatment dose for five days. For prevention in herds where coccidiosis is an anticipated risk, amprolium can be fed at a lower dose for 21 days during the exposure period.
Several feed additives are approved for ongoing prevention and control. Monensin and lasalocid are ionophores commonly included in feed for growing beef cattle, and both specifically target E. bovis and E. zuernii. Decoquinate is another option, fed for at least 28 days during periods of expected exposure, and it can be used in milk replacer for young calves. These preventive drugs don’t eliminate the parasite entirely. Instead, they keep parasite numbers low enough that calves can gradually build natural immunity without experiencing clinical disease.
Supportive care matters too. Dehydrated animals need fluids. Severely affected cattle may need additional nutritional support while their intestinal lining heals, a process that can take weeks even after the active infection clears.
Reducing Risk Through Management
Because coccidiosis spreads through contaminated environments, management changes are just as important as medication. The two most impactful steps are reducing environmental moisture and decreasing stocking density. Keeping pens well drained, bedding dry, and water troughs clean cuts down on the warm, moist conditions oocysts need to become infectious.
Separating age groups helps protect the youngest, most vulnerable animals from heavy environmental contamination shed by older cattle. Feeding and watering equipment raised off the ground reduces fecal contamination of feed. Cleaning and drying pens between groups of calves breaks the cycle of oocyst buildup. None of these measures alone eliminates the risk, but in combination with strategic use of preventive feed additives, they can keep clinical outbreaks rare.
Economic Impact
The financial toll of coccidiosis is real but easy to underestimate because so much of it comes from subclinical disease. In Estonia, dairy farmers lost an estimated $6.23 per calf per year from calf mortality attributed to Eimeria alone, and that figure doesn’t account for the slower growth and reduced feed efficiency in surviving animals that were subclinically infected. In Mexico, annual losses from bovine Eimeria species were estimated at $23.78 million across the national cattle herd. These numbers underscore that coccidiosis isn’t just a problem when calves start dying. The quiet drag on performance in subclinically infected herds often represents the larger share of total economic loss.