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"We think growers who protect their birds against clostridium may see other improvements too."
DR. MATILDE ALFONSO

Although some broiler producers are backing off ionophore antibiotics for controlling coccidiosis in broilers, a good number still rely heavily on the use of those agents—at least for now.

To find out how its new Clostridium perfringens type A toxoid would control necrotic enteritis (NE) in these types of operations, Schering-Plough Animal Health conducted a huge trial involving nearly 22 million broilers.

Dr. Matilde Alfonso, a technical services veterinarian for the company who was involved with the trial, explained that the design of the study was straightforward—compare the progeny of vaccinated hens with those of non-vaccinated hens. Developing such a pure comparison in real-world field conditions created some challenges, however.

"Most growers stock their barns with birds from different breeder sources, so that made it tough to find a 100% vaccinated flock," she explained.

That being the case, researchers decided that flocks with 70% or more of birds vaccinated would be evaluated as part of the vaccinated group. Flocks with less than 70% vaccinated birds served as controls.

In all, more than 750,000 hens were vaccinated. Vaccinated birds received a 0.5-ml subcutaneous injection of the NE toxoid at 10 weeks of age and a booster at 18 weeks. Researchers then followed 22 million of their progeny, along with controls, over a 6-month period.

All birds were fed a standard corn-soybean mix, with an in-feed ionophore. "No antibiotic growth promoters were used," Alfonso said, "and that's a key point" of the study because those compounds help to control necrotic enteritis.

There was considerable variation in the size of the birds included in the trial, ranging from 4 to 6 pounds (1.81 to 2.72 kg). There was variation, too, in the respective locations of the four farms that took part in the study, with operations scattered across a wide geographic and climatic range.

Figure 1. Trials showed an improvement in livability of 0.42% with significant reductions in feed conversion. Calorie conversion and adjusted calorie conversion also improved, as did standard cost.

Moving on to results of the trial, Alfonso emphasized that incidence of necrotic enteritis on these farms was historically low, seen in only 0.07% to 1.1% of flocks each month.

"The anticoccidials were keeping the lid on necrotic enteritis," she said. "The incidence was very sporadic."

So how did the progeny of the NE-toxoid vaccinated birds compare with progeny of non-vaccinated birds?

"We saw positive trends for the NE toxoid in each of the individual study complexes," Alfonso reported, adding that some trends for individual complexes, when evaluated individually, did not reach statistical significance.

That changed, however, when the data for the study sites were combined (Figure 1). Alfonso said that by pooling the results and thereby increasing the sample size, some of the differences in the two groups became statistically significant.

"We showed an improvement in livability of 0.42%. We also got a significant reduction in feed conversion," she said of the progeny from the vaccinated hens. "Both calorie conversion and adjusted calorie conversion improved, -13 and -15, respectively, as did standard cost, which went down by 0.001 dollar per pound of live weight."

Average weight and rates of daily gain also showed an increase, though those improvements were not statistically significant. "But the trend was positive, and that's what counts," she said.

Alfonso noted that overall, the progeny of NE-toxoid vaccinated hens showed better performance. "And we think that may be due to the vaccine controlling subclinical necrotic enteritis, which in turn may help reduce other health problems that tend to be associated with NE."

She added that she thinks growers who are seeing poor performance in their flocks might benefit from using the new NE toxoid.

"And we're not just talking about mortality here," she emphasized. "We think growers who protect their birds against clostridium may see other improvements too."

Spring 2008

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