Poultry producers should take a second look at their control strategies for coccidiosis following reports that the parasite Eimeria mivati is indeed a distinct species found in about 35 percent of broiler flocks, says Steve Fitz-Coy, PhD, of Schering-Plough Animal Health technical services.

This coccidian species has been controversial among poultry pathologists. Some believe it to be a variant of E. acervulina or a mixture of E. acervulina and E. mitis, but careful study of E. mivati by Fitz-Coy, a parasitologist, indicates otherwise.

In a study with broilers, Fitz-Coy immunized one group of birds with E. acervulina and a second group with a strain he identified as E. mivati. A third group served as controls, was coccidian-free and received anticoccidials for two weeks.

The birds were then inoculated with either E. acervulina or E. mivati 3 times weekly for 3 weeks. At 28 days of age, they were randomly challenged by gavage with 500,000 sporulated oocysts of either E. acervulina or E. mivati. Unimmunized control birds were also challenged with one or the other Eimeria species.

Six days after challenge, the birds were evaluated for weight, gross lesions (Figure 1) and microscopic evidence of parasite burdens, says Fitz-Coy, who presented these findings last year at the Poultry Science Association annual meeting.

Pathogenicity

Birds challenged with the same species they were immunized with had protection, but birds challenged with the other species—a heterologous species—had no protection, Fitz-Coy says.

"E. mivati was more pathogenic than E. acervulina because it caused greater growth suppression, more gross lesions (see Figure 1) and more microscopic parasitism than E. acervulina. In addition, the only mortality—18%—occurred in the controls that were challenged with E. mivati (Table 1), says Fitz-Coy. He notes that it has been reported that E. acervulina seldom causes death and that he has never conducted a challenge study with E. acervulina that resulted in deaths.

Birds immunized with E. acervulina and then challenged with E. mivati did not experience mortality, but had decreased weight and a moderately severe level of parasitism, he says.

In additional studies conducted by Fitz-Coy using coccidia-naïve chickens inoculated with semi-purified inocula, E. mivati produced mortality that ranged from 20% to 50%.

Fitz-Coy has also sent samples of E. mivati for independent polymerase chain reaction (PCR) testing, which was set up to find well-known pathogenic species of Eimeria such as E. acervulina, but not E. mivati. The test yielded negative results-it couldn't identify the specimens he sent, further indicating that E. mivati is a distinct species, the researcher notes.

Incidence

When E. mivati was first identified by Dr. S. Allen Edgar in 1959 during a persistent coccidiosis outbreak on poultry farms in Florida, Edgar reported that the incidence was as high as 50%.

Since 2003 to present, Fitz-Coy has conducted necropsy sessions and tested litter samples from chicken houses in numerous states, looking for E. mivati oocysts as Edgar first described them: smaller than E. acervulina and broadly oval (Figure 2). Based on this work, he estimates the incidence of E. mivati to be around 35%; that's a lower incidence compared to E. acervulina, but E. mivati appears to be more pathogenic and can still cause losses for producers, Fitz-Coy says (Figure 3).

Figure 2. An E. Mivati oocyst is at the end of the arrow, slightly to the right. E. Mivati oocysts are broadly oval and slightly smaller than E. Acervuliner oocysts

Figure 3. Studies to date indicate that E. mivati in the United States is lower in incidence than E. acervulina, but more pathogenic..

So far, Fitz-Coy has found E. mivati in poultry house samples from Georgia, South Carolina, North Carolina, Virginia, Delaware, Maryland, Missouri, Louisiana, Pennsylvania, California, Texas and Arkansas. There are several poultry-producing states that he's not yet obtained samples from, such as Kentucky and Ohio.

"E. mivati oocysts appear to be in great abundance and are found many times throughout the entire small intestine. E. mivati is present in poultry houses, it is not E. acervulina, it is pathogenic, prevalent - and it can kill chickens," Fitz-Coy says.

Spring 2008

Regresar a North American Edition (#1)