"And once you've removed the protective effects of the subtherapeutic antibiotics, necrotic enteritis plays a bigger role."
DR. CHARLIE BROUSSARD

The world's first vaccine for managing necrotic enteritis in broilers is designed to combat a toxin produced by Clostridium perfringens type A—the toxin that is associated with development of the disease.

Officially known as Clostridium perfringens type A toxoid, the conditionally approved vaccine employs an inactivated fragment of the toxin, called a toxoid, to elicit immunity to the toxin. The hens then pass on that immunity on to their progeny.

The vaccine, developed by Schering-Plough Animal Health, got high marks in laboratory studies and in limited field trials. But how would this NE toxoid vaccine perform in real-world conditions?

To find that out, Schering-Plough put together a series of trials, vaccinating 1.5 million hens and generating data on more than 21 million of their progeny.

At the Orlando symposium, Dr. Charlie Broussard, worldwide technical service veterinarian for the company's poultry business unit, reported on trials aimed at evaluating the NE toxoid's efficacy in broilers that received no antibiotics.

Antibiotic-free birds face increased risk of NE

Broussard noted that this arm of the study was especially important because more and more broiler producers are bending to consumer and regulatory pressures by pulling antibiotics and growth promoters from their birds' rations.

"And once you've removed the protective effects of the subtherapeutic antibiotics, necrotic enteritis plays a bigger role," he said.

Broussard also stressed that while producers are well aware of how many birds NE can kill, they tend to be less familiar with the devastating effects that subclinical disease can have on the performance of their flocks. In one recent survey, U.S. veterinarians who were polled said they believe losses from subclinical NE may total as much as 5 cents per bird. "So there's a lot of performance loss going on in addition to what most people think of as necrotic enteritis—dead birds," Broussard said.

The company chosen for the site of the antibiotic-free trial had a history of NE, especially during cold and rainy weather. The trial began in February 2005; about 80,000 pullets were vaccinated via intramuscular injection and were given a booster vaccination 8 weeks later.

The vaccinated pullets were then shipped to 7 breeder farms. "The pullets were vaccinated over time," Broussard explained, "so we had breeder ages all the way from young to the old in the trial." Eggs were collected 4 times each day at the farms and the chicks hatched.

Before he discussed the results of the trial on the progeny from the vaccinated hens, Broussard emphasized an important point. He said that for evaluation purposes, progeny flocks in the trials were divided into only two categories:

1. Flocks in which all the birds came from vaccinated hens
2. Flocks in which less than all the birds came from vaccinated hens

Broussard emphasized an important point. In some cases, he pointed out, the flocks categorized as nonvaccinated progeny contained a sizable number of birds whose hen parent had been vaccinated. In total, the trial included about 1.3 million birds in the all-vaccinated group and about 4.6 million in the standard group. Bird size of the progeny ranged from about 4.5 to 5 pounds (2.04 to 2.27 kg).

One other caveat: During the trial, when any flock exhibited NE-related mortality at a rate of 4 birds per 1,000 or higher, they were treated with the antibiotic sulfadimathoxine. "So you need to factor in that any time mortality really started going out of whack for these birds in the standard category, they were getting some treatment for necrotic enteritis," Broussard explained.

As to results of the trial, Broussard first compared the performance of all vaccinated progeny versus standard birds early in the trial, at days 8 to 14 (Figure 1).

"Even at this point, we were already seeing a difference of approximately a 0.25% in mortality—0.63% for the all-vaccinated progeny and 0.88% for the standard birds," he said.

During the next evaluation period, 15-21 days, when mortality tends to be on the rise, mortality rates were .89% for the all-vaccinated progeny and 1.27% for the controls (Figure 2).

Historically, for companies raising broilers, the next period of growth—days 22-28—would be a high-mortality time for NE, and that was indeed the case for the company that hosted the trial. Even during this difficult time, however, vaccinated progeny continued to do well, with 47% reduced mortality, .98% versus 1.45% in standard birds, Broussard said (Figure 3).

What particularly caught the eye of the host company, Broussard said, was when the researchers carrying out the study broke out mortality data for only the colder, wetter months.

"When we looked at only that particular time, when the company had told us that they—like a lot of companies—had experienced their main NE challenge, we found that progeny from the vaccinated hens had 72% less chance of mortality than progeny in the standard group," Broussard said, adding that the difference takes into account that some of the birds in the standard group had come from vaccinated hens.

Broussard said the host company was impressed by the results of the trials. "So much so that they recently decided to vaccinate all their breeder flocks with the NE toxoid," he reported.

Editor's note: The poultry company involved with the NE toxoid trial agreed to share this data on the condition that the company's name and location not be reported.

Spring 2008

Regresar a North American Edition (#1)