Advances in the control of diseases of egg-producing flocks
Scientists from industry and academia report progress in poultry health.
Dr. Hugo Medina of Sparboe Companies has reported previously on the serious problem of peritonitis and airsacculitis in flocks. This condition occurs extensively throughout the Midwest and appears sporadically in other areas of the U.S.
Peritonitis is most obvious in second cycle hens and may result in flock mortality of up to 10% over four weeks. Characteristic post-mortem lesions include purulent caseous deposits on viscera including the intestines, ovary, and liver. Involvement of air sacs and the oviduct (salpingitis) are frequently observed.
The predominant bacteria isolated from lesions is E. coli; however, Pasteurella spp., Salmonella spp., and Staphylococcus spp. may be isolated from affected birds. In previous presentations, Dr. Medina has indicated that when confronted with outbreaks, mortality declined following a combination of terminating routine dust removal using air blowers, chlorination of drinking water, and administration of antibiotics including neoterramycin. In discussion with colleagues, it is apparent that peritonitis occurs in flocks which are predisposed to infection by immunosuppression. This is, in turn, due to early exposure to IBD and possibly chicken anemia virus or Marek’s disease, subclinical mycotoxicosis, and the stress associated with molting.
A detailed epidemiologic survey will be conducted in the near future to ascertain the prevalence of the condition, risk factors, and other details which would quantify the extent and severity of infection.
Studies conducted at Iowa State University have shown that pathogenic E. coli possess large plasmids, apparently associated with virulence. A study on stored isolates applying modern molecular biological assays confirms that more virulent types of E. coli are now isolated from commercial flocks. This will require new approaches to diagnosis and control which will inevitably include reduction of environmental stress, manipulation of the immune system, and possibly the application of autogenous vaccines. Simply administering antibiotics is not cost-effective and will lead to drug resistance.
An intriguing result from molecular biological assays conducted at Iowa State University confirms that pathogenic coliforms possess a specific gene, which is infrequently encountered in commensal (non-pathogenic) E. coli. It is now possible to develop a polymerase chain reaction-based diagnostic procedure to differentiate between pathogenic and innocuous isolates based on the presence of the tia gene. This will be significant in performing epidemiologic investigations of colibacillosis including peritonitis.
Similar studies have shown the presence of the iss gene which is frequently associated with pathogenicity in E. coli isolates. This gene encodes for an 11-kDa protein. The gene can be detected using monoclonal antibody immunofluorescent microscopy. There is an obvious correlation between wild-type E. coli—which possess the gene—and modified mutant strains which are non-pathogenic.
Turkeys which are extremely sensitive to mycoplasmosis were used as biological sentinels to investigate the strains of Mycoplasma gallisepticum (MG) in commercial flocks. Isolates were subjected to polymorphic DNA analysis and gene-targeted sequencing. Isolates were identified as field MG but with one isolate each of F-strain, ts-11 and a ts-11-derived strain. In controlled protection studies both ts-11 and 6/85 strain live vaccines were effective against the ts-11-like isolate, but provided less protection against the field strains of MG compared to F-strain.
Trials conducted at Mississippi State University evaluated administration of 6/85 strain MG vaccine at 10 weeks of age with and without overlay of F-strain at either 22 or 44 weeks of age. Non-vaccinated, non-infected hens served as controls. No significant differences were noted in egg weight, shell quality, or internal quality including blood spots or protein inclusions in the albumen as a result of the combinations of vaccines evaluated.
A study conducted at the University of Delaware evaluated the diagnosis of low-pathogenicity AI following administration of live vaccines against IB and ND. Detection was impaired three days following vaccination applying real time polymerase chain reaction assay. After 7 days there was no effect of concurrent infection with respiratory viruses on isolation. Serologic tests to detect antibody including ELISA, HI and AGID were not affected adversely during the 7 to 14 day period following IB and ND vaccination. Although this presentation related to broilers, it is expected that similar results would be obtained with commercial laying flocks. The only possible problem that might be encountered in egg production is interference in diagnosis following periodic boosting of laying hens with live ND and IB vaccines.
Field studies have highlighted the epidemiologic deficiencies inherent in low-intensity surveillance systems for SE. Insensitivity associated with drag swab samples coupled with routine microbiological assay of variable accuracy may provide false negative results with respect to a complex. Structured sampling of manure at intervals during the life cycle of flocks and using additional sites such as fan louvers, egg belts, and samples of rodent droppings will provide a more accurate indication of the SE status of a flock or farm.
The proposed FDA sampling protocol which will be mandated for 2007 will displace the current 5-star UEP program and similar state systems and will approximate the ages and frequency currently required under the PA-EQAP. Progressive sampling coupled with PCR assay will clearly differentiate between complexes with and without SE infection. This distinction is absolute. It is axiomatic that farms and complexes with a high rodent population or with deficient biosecurity are at risk of infection or will yield persistent positive samples. Once SE is introduced to an in-line complex, eradication requires diligent and thorough, long-term decontamination together with effective vaccination of flocks using both live attenuated ST priming vaccines and an inactivated emulsion SE vaccine at the time of transfer.