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Recent research, testing indicate the once elusive coccidial pathogen is a distinct species.

Fitz-Coy: 'E. mivati oocysts tend to be smaller and broadly ovoid"�'

For decades, Eimeria mivati "one of nine species of Eimeria known to cause coccidiosis in chickens "has been a source of controversy among poultry pathologists.

Some believed it to be a distinct Eimeria species that posed a threat to broilers, but others have been doubtful and consider it either a variant of E. acervulina or a mixture of the E. acervulina and E. mitis species.

However, recent studies, conducted by Dr. Steve Fitz-Coy of Schering- Plough Animal Health Corporation, as well as polymerase chain reaction testing, indicate that E. mivati is a unique type of Eimeria.

Studies also show that the prevalence of E. mivati, based on litter sampling, may be as high as 22% and that it causes disease and performance loss in chickens if not controlled.

Putting it in perspective

"For a long time, the issue of E. mivati was not a major concern because anticoccidials were effectively controlling the primary pathogenic species of Eimeria that threaten commercial chickens," says Dr. Rick Phillips, a veterinarian at Schering-Plough Animal Health Corporation.

"Any controversy about whether E. mivati is a distinct species was primarily one of academic interest, but it had little practical relevance," he says. Now, the situation is different, he says.

With the shift from in-feed anticoccidials to vaccination of chickens for coccidiosis control, E. mivati has to be considered. Anticoccidials control coccidiosis by killing parasites, but vaccines work by enabling chickens to build immunity that naturally fights off the disease, Phillips explains.

"Producers need to be sure that the coccidiosis vaccine they use protects against the major Eimeria species that cause disease in chickens, including E. mivati," he says.

Keep in mind, Phillips continues, that there is no cross-protection when it comes to Eimeria, he says."If chickens are immune to E. acervulina, they only have immunity against E. acervulina and, if exposed to E. mivati, they'll succumb to the new infection. "Immunity against one species doesn't protect against another," he says.

E. mivati history

E. mivati was first isolated in 1959 from a poultry farm in Zephyr Hill, Florida, by the late Dr. S. Allen Edgar, a world renowned poultry pathologist from Alabama's Auburn University. Prior to recognition of the parasite, the farm had experienced persistent and unusual outbreaks of coccidiosis.

For several years after his discovery, Edgar conducted extensive research to determine the characteristics of E. mivati and validated its differences from other chicken coccidia.

In 1964, E. mivati was added to the list of coccidia affecting chickens when Edgar published his findings.¹ Subsequently, several researchers reported finding E. mivati in other countries, including Canada, Great Britain, Germany and France.

By the 1970s, however, some researchers began to question the existence of E. mivati.

In 1973, P.L. Long of Houghton Poultry Research Station (now the Institute for Animal Health), Huntingdon, England, concluded that E. mivati was not sufficiently different from E. acervulina to be a distinct species and that it should be referred to as E. acervulina var. mivati.²

Fitz-Coy, a parasitologist who worked with Edgar, says that the most influential report questioning the existence of E. mivati appeared in 1983, after Dr. Martin W. Shirley and associates, also of Houghton Poultry Research Station, used electrophoresis to study a potential field isolate of E. mivati provided by Auburn University.³

"They concluded that it was probably a combination of E. acervulina and E. mitis and should be considered "nomina dubia" "in other words, its existence is doubtful," he says.

The conclusions of that report stuck and, long after it was published, many researchers continued to question the validity of E. mivati as a unique species, says Fitz-Coy, now a technical service representative for Schering- Plough Animal Health. In subsequent years, while anticoccidials were effectively controlling coccidiosis, E. mivati received little attention as a research infectious agent; it just wasn't much of a concern or it was lumped into an E. acervulina-like category.

In addition, says Fitz-Coy, E. mivati isn't as easy to work with as other coccidial species such as E. acervulina, E. maxima and E. tenella. The lesions of E. acervulina and E. mivati are similar and both "parasitize" some of the same regions of the intestines. Most field isolates are a composition of multiple species of Eimeria. "All these factors contribute to the difficulty in identifying E. mivati," he says.

There are differences, however. "Traditionally, identification of coccidia species is based on morphology, pathology, the prepatent period and cross-immunization evaluations. E. mivati oocysts tend to be smaller and are broadly ovoid compared with those of E. acervulina," he says.

Further research

Not widely known is that Fitz-Coy continued research with E. mivati, though his findings were not always published.

Between 1988 and 1990, when Fitz- Coy worked for the University of Maryland, Eastern Shore, he isolated three probable isolates of E. mivati. They were obtained from commercial broiler farms in the Delmarva region of the United States. All three had similarities to the E. mivati described by Edgar, his mentor.

When Fitz-Coy immunized chickens with E. acervulina and challenged them with the E. mivati he had isolated, the birds had no protection and developed coccidiosis. But when he immunized chickens with the E. mivati and challenged them with E. mivati, they had good protection.

PCR testing

According to Phillips, the most compelling evidence that E. mivati is a distinct species comes from recent PCR testing, a sensitive, state-of-the-art technique that enables identification of small DNA fragments.

Current PCR kits identify E. acervulina, E. maxima, E. necatrix, E. mitis, E. brunetti and E. tenella. When Fitz-Coy provided blind or unidentified samples of E. mivati and other species to a research investigator for PCR testing, the known species were identified, but the E. mivati samples could not be identified.

Fitz-Coy plans to publish his findings with details, and an E. mivati PCRprimer is currently being developed for rapid identification purposes.

Phillips: The current prevalence is probably underestimated"�'

Prevalence

For producers, the real significance of E. mivati, Phillips points out, is its prevalence in the field and its affect on flock performance.

In the early 1960s, Edgar had found a 50% incidence of E. mivati organisms in samples sent to Auburn from Florida.

Fitz-Coy says, "Based on my observations over the years, study of field isolates and routine necropsy evaluations of chickens from commercial broilers farms in the Atlantic, Southeast and West Coast, organisms that morphologically resemble those of E. mivati are seen. They can appear in great abundance and are found throughout the entire small intestine."

From 2001 to 2004, Fitz-Coy analyzed data from approximately 130 necropsy sessions in the United States, and found 24 were positive for E. mivati. In other words, 18% of the necropsy cases were positive for E. mivati. Between 2002 and 2004, when he tested 55 litter samples from major US broiler production areas, 12 were positive for E. mivati, yielding a 22% incidence, he says.

Phillips says, "The current prevalence is probably underestimated because E. mivati is under-recognized, and even a 20% incidence is pretty significant."

Impaired weight gain, mortality

Pathologic changes

Table 1. Growth rate and mortality in birds challenged with E. mivati.

Another way to demonstrate the pathogenicity of E. mivati is by examining the pathologic changes it causes in chickens. E. mivati oocysts, says Fitz- Coy, usually are found in intestines that have been scored with mucoid and/or watery enteritis. They are smaller oocysts than those of E. acervulina, and are broadly ovoid.

As far back a 1980, researchers Norton and Joyner wrote in Parasitology that they had found "clear distinctions" between the damage done by E. mivati and E. acervulina isolates.4 E. mivati produced scattered petechiae (red spots), but the intestinal lesions were not as prominent compared to those seen with E. acervulina. The manifestations of E. mivati were most numerous in the lower small intestine and proximal ceca.

In addition, the ratio of villus height to total mucosal thickness in the lower intestine was reduced with E. mivati, while similar changes due to E. acervulina were seen only in the anterior intestine, Norton and Joyner said.

Fitz-Coy has no doubt that "E. mivati is pathogenic to chickens, resulting in impaired feed utilization, impaired growth and, sometimes, mortality depending on the level of challenge."

Phillips agrees and says, "E. mivati is real. Be sure that the vaccine you are using to control coccidiosis protects against E. mivati. Coccivac-B has always contained E. mivati and, currently, is the only licensed commercial vaccine that protects against this Eimeria species.

"We didn't have to deal with E. mivati before because anticoccidials were controlling it, but it's safe to assume that E. mivati resistance to anticoccidials may be developing just as it has for other Eimeria species. On the plus side, we have an effective method of control," he says.

Further PCR research is being aggressively pursued, Phillips adds. "Since E. mivati can affect poultry performance, we want to make sure that any questions about its existence are resolved once and for all. There's also still a lot to learn about this Eimeria species," he concludes.

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