Development of immunity against coccidiosis in turkeys varies markedly with the type of control program and appears to start earliest in birds that are vaccinated against the disease, reported Dr. John Radu, a technical service veterinarian with Intervet/Schering-Plough Animal Health.

Early immunity is desirable because it is less likely to interfere with performance. With in-feed anticoccidials, immunity against coccidiosis develops later into the production cycle, when birds experience a major growth spurt and consume more feed, he said.

To reach this conclusion, Radu reviewed the coccidial-shedding patterns of commercial turkey flocks in the US. Determining the shedding rate of coccidial oocysts is a good indicator of immunity development. It is also helpful for diagnosing coccidiosis, which tends to present as subclinical disease with no overt signs of infection.

Exposure to infective oocysts, he explained, occurs either when poults pick at litter or receive a controlled dose of live oocysts administered via live vaccine at 1 day of age. Seven species of Eimeria affect turkeys, and four of them are considered significant: Eimeria adenoids, E. gallopavonis, E. meleagrimitis and E. dispersa.

For the oocyst analysis, the lead investigator, Dr. Greg Mathis of Southern Poultry Research in Athens, Georgia, collected feces sequentially throughout the first 13 weeks of life and then counted the number of oocysts per gram.

Included in the analysis were flocks that received either diclazuril, the chemical anticoccidial; a combination of ionophore anticoccidials; or a live coccidiosis vaccine, which is becoming a more common method of coccidiosis control in US flocks. Vaccinated birds received Coccivac-T, a live-oocyst vaccine administered at day 1 that contains a controlled dose of the four Eimeria species that cause coccidiosis in turkeys.

Flocks receiving diclazuril developed immunity at 50 days of age (about 7 weeks of age). Birds on ionophore anticoccidials developed immunity earlier — by day 35 (5 weeks of age) — but did not appear to have complete immunity to E. meleagrimitis. Immunity against this coccidial species did not occur until later in the sampling period, from 56 to 84 days of age (8 to 12 weeks). Radu said it’s possible that E. meleagrimitis was less competitive with other coccidial species and cycled enough to initiate immunity only after the other species declined, he said.

Birds that received Coccivac-T produced an oocyst-shedding pattern similar to that seen in birds treated with ionophores, but immunity occurred 1 week earlier, at 28 days or 4 weeks of age (see Table 1); in addition, oocyst counts were not as high as some of the counts found in birds that received ionophores, and immunity to E. meleagrimitis appeared to be more complete. There was no late cycling in the 8-to-12-week window, Radu said.

The results not only demonstrated that coccidiosis is present on turkey farms, they showed that different coccidiosis-control programs in commercial turkey flocks produce markedly different coccidiosisimmunity patterns, Radu said.

“With vaccination,” Radu said in an interview following his presentation, “oocyst shedding occurs earlier, which allows immunity to develop at a time when the birds aren’t growing so fast. Overall performance is not affected much.

“With other coccidial-control methods such as ionophores or chemicals, we definitely see a pattern of oocyst shedding later in life. Immunity development is delayed and, when it does occur, it coincides with that spike in bird growth,” he said.

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