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Field experience shows perils of late coccidiosis challenge
Real-world experience is corroborating research indicating that a coccidiosis challenge late during the production cycle takes a far more serious toll on broilers and the producer’s wallet than an early coccidiosis challenge.

According to research by nutritionist Robert Teeter, a late coccidiosis challenge has a major negative impact on performance and flock profitability, while a challenge early in the production cycle has a minor negative effect (see accompanying article).

Now field data from a commercial poultry farm is proving the research true, said Dr. Linnea Newman, a consulting veterinarian with Intervet/Schering-Plough Animal Health.

The veterinarian described a trial conducted by a broiler producer in Ontario who implemented coccidiosis vaccination on his farm because he wanted to develop an antibiotic-free line of birds; he also wanted to see if vaccination renewed coccidial-oocyst sensitivity to in-feed anticoccidials.

The trial involved five flocks raised to ages 35 to 36 days. The producer collected oocyst samples and took daily weights on in-house scales. Anticoccidial control in the flocks consisted of the following:
  • Flock 1: Nicarbazin-narasin
  • Flock 2: Coccidiosis vaccine, no antibiotics
  • Flock 3: Coccidiosis vaccine, no antibiotics
  • Flock 4: Coccidiosis vaccine, no antibiotics
  • Flock 5: Return to narasin anticoccidials
Birds that were vaccinated received a live-oocyst vaccine at 1 day of age in the hatchery via spray cabinet. The vaccine initiates immunity to coccidiosis during the first weeks of the bird’s life, then provides lifelong protection against the disease.

Extremely high oocyst counts

The first flock, which received in-feed anticoccidials, had extremely high coccidial-oocyst counts during the later part of the production cycle but no clinical signs, which is typical of subclinical coccidiosis.

They had been gaining weight, as expected  according to the Ross standard, but weight then fell off significantly — by 292 grams; during the last 3 days, they had zero weight gain.

Dr. Teeter’s research has shown that birds near market age with subclinical coccidiosis, represented by coccidial lesion scores of only 2, have an average daily gain approaching zero — and that’s exactly what happened in the Canadian field trial, Newman said.

In Flock 2, which received no in-feed anticoccidials but was vaccinated against coccidiosis, oocyst counts weren’t taken but weight gain improved. Flock 3, which also received the vaccine, still had high oocyst counts, but peak shedding was shifting to a time earlier in the production cycle; these birds also had improved weight gain.

High oocyst counts, Newman explained, are the result of carryover from the previous flock that was on anticoccidials. Anticoccidials allow leakage of resistant coccidial oocysts, which are still in the house and hard to destroy, Newman said. When birds instead receive a coccidiosis vaccine, the house is gradually seeded with coccidial oocysts that have never been exposed to anticoccidials and are still sensitive to anticoccidials, but it takes a few cycles, which is reflected in the Canadian trial and by experience at Wayne Farms in the US (see article in Intestinal Health/North America, issue 4, page 11).

By Flock 4 and the third cycle of vaccination in the Canadian trial, “we’ve fallen into a complete vaccination pattern,” Newman continued. Oocyst cycling was earlier and the oocyst counts lower. Weight improved 240 grams compared to Flock 1 that received in-feed anticoccidials.

Astounding weight improvement

For Flock 5, the producer returned to the use of anticoccidials, which were once again effective. The difference in weight gain between the first flock and the fifth was an astounding 452 grams (0.996 pound). “That’s huge. Dr. Teeter’s research was correct. There’s a lot to be lost if you have coccidiosis late in the production cycle,” Newman said (see Figure 1).

“It’s those last 2 weeks of the flock where most of our feed is going to be consumed and it’s when most of the muscle mass is being put on. That’s the money part of the bird,” she added.

The field trial results also bolster Teeter’s finding that for every increase in the coccidiosis lesion score, average daily weight decreases by 1.5% of bodyweight, Newman added.

She also presented recent data from another Canadian producer who has used a coccidiosis vaccine continuously. “This further demonstrated that once we establish the immunity pattern, stop the carryover effect and seed the house with vaccine strains, coccidiosis tends to become very much more predictable, lower, consistent and that’s the goal,” the veterinarian added.

Late subclinical coccidiosis, she said, can have a measurable, negative impact on performance. “Dr. Teeter’s mathematical models really seem to do a good job of offering a measure of economic impact; they seem to be real under field conditions.”

Vaccination reduces challenge

Vaccination, Newman continued, “offers an opportunity to manage your coccidiosis challenge. Establishment of an early, consistent immunity pattern is going to eliminate carryover and stop that cycle of resistance-building. It’s going to reduce the overall coccidiosis challenge and will give you the opportunity to renew sensitivity to in-feed anticoccidials.”

Vaccination, which can be used for broilers at any slaughter weight, is a process with a long-term goal, she emphasized.


The oocyst shedding that occurs with in-feed anticoccidials tends to peak at about 4 weeks of age, close to market age. If slaughter age were at 63 days, as it was 20 years ago, then an oocyst-shedding peak at 28 to 35 days wouldn’t matter. “At this moment, though, our coccidiosis peak and our slaughter age are directly on top of each other, and our peaks are increasing because we’ve lost sensitivity to the anticoccidials,” she said.

Another factor to consider is that cleanout and disinfection aren’t effective against coccidia. The overall numbers in the house can be reduced if the litter is removed, but generally it’s not possible to kill coccidial oocysts, Newman said.

“Know what your coccidiosis challenge looks like. If oocyst counts are high, you won’t get maximum performance from your birds. Keep in mind that our flocks aren’t breaking with coccidiosis and this isn’t complete resistance. It’s just high oocyst counts, reflecting subclinical coccidiosis. They’re minor lesions, but as Dr. Teeter’s research and field experience demonstrate, they can have major consequences,” she said.

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