H. D. Chapman

Biochemical, genetic and applied aspects of drug resistance in Eimeria parasites of the fowl

Anticoccidial drugs are widely used for the control of coccidiosis in the fowl which has inevitably led to the development of drug resistance. Resistance has developed to all of the compounds that have been introduced and if chemotherapy is to remain the principal method of control of coccidiosis, it will be important to continue the search for new anticoccidial agents. Knowledge of biochemical pathways present in the Eimeria parasite and how they differ from those of the host might help identify novel targets for inhibition. Studies of the mode of action of drugs are required if the biochemical mechanisms of resistance are to be understood. Information on the genetic origins of resistance, the stability of resistance and the factors involved in the spread of resistance throughout parasite populations is required. Since there are no methods at present to prevent resistance, more attention should be given to developing strategies for preserving the efficacy of anticoccidial drugs.

Forty years of monensin for the control of coccidiosis in poultry

In July 1971, the polyether ionophorous antibiotic monensin was introduced in the United States for the control of coccidiosis in poultry. At that time, prospects for new anticoccidial agents were not good. Amprolium had enjoyed several years of use, but many other compounds had been abandoned as resistance to them developed. After the introduction of monensin, most commercial broilers were medicated with the drug and it is still widely used for this purpose today. Apart from in poultry, monensin is also used to control coccidiosis in game birds, sheep, and cattle. Indeed, more animals have been medicated with ionophores, such as monensin, for control of disease than any other medicinal agents in the history of veterinary medicine. In this review, we discuss the discovery, mode of action, and efficacy of monensin, together with matters of importance to the poultry industry such as commercial use, drug resistance, toxicity, pharmacology and residues, host immunity to coccidiosis, and effects in other avian species.

Milestones in avian coccidiosis research: A review

This article describes some of the milestones in research concerned with protozoan parasites of the genus Eimeria that infect birds and cause the disease coccidiosis. The time period covered is from 1891, when oocysts were first found in the ceca of diseased chickens, to the present. Progress in our understanding has lagged behind that of other protozoan parasites such as Toxoplasma and Plasmodium despite the enormous importance of Eimeria to animal livestock production. Nevertheless, applied research by universities, government agencies, and private industry has resulted in the successful development of methods of control, research that continues today. The topics covered and the references provided are selective and include life cycles and biology, pathology, ultrastructure, biochemistry, immunity, genetics, host cell invasion, species identification, taxonomy, chemotherapy, vaccination, and literature concerned with avian coccidiosis. This review is primarily concerned with the avian species of Eimeria that infect poultry, but some important advances, principally in immunology, have been made using species that infect rodents and rabbits. These are included where appropriate.

Practical use of vaccines for the control of coccidiosis in the chicken

Immunological control is recognised as the only major practical alternative to chemotherapy for the control of coccidiosis. Initial hopes that it would be possible to develop a genetically engineered vaccine against Eimeria species have not yet been realised, and there is therefore renewed interest in the development and application of vaccines based upon live parasites. Recognition that existing drugs are not as effective as in the past, that few new drugs are currently under investigation, and that the inclusion of drugs in the feed may be restricted in the future has focused attention on the need for new vaccines. Although considerable research has been undertaken on the basic mechanisms involved in the acquisition of immunity, very little attention has been given to the practical aspects of vaccination. In order for an anticoccidial drug to be successful, it is necessary to generate considerable information on efficacy and safety. With few exceptions, little information is available to support the use of coccidiosis vaccines. Hopefully, the introduction of new vaccines and improved application methods will encourage manufacturers to generate more information concerning their products. In future, coccidiosis should be controlled by adopting an integrated approach where drugs and vaccines are both used to prevent this disease. In addition to immunisation, a possible advantage to the application of certain vaccines is that their use could repopulate a poultry house with drug-sensitive organisms. Theoretical rotation programmes, in which the use of drugs is alternated with that of vaccines, are described.

Resistance to anticoccidial drugs in fowl

Resistance has been encountered wherever drugs have been used extensively for the control of parasitic infections. The poultry industry is dependent upon drugs for the control of coccidiosis, a major disease of chickens caused by protozoan parasites of the genus Eimeria. In modern poultry production, drugs are used prophylactically for the prevention of coccidiosis by including them in the diet. This has inevitably led to the development of resistance. We have been fortunate in that new drugs have become available to replace those to which resistance has developed, but this situation is unlikely to continue. The problem of drug resistance, discussed here by David Chapman, has provided impetus for the development of new approaches (such as vaccination) for the control of coccidiosis.

Anticoccidial drugs and their effects upon the development of immunity to Eimeria infections in poultry

The long-held view that anticoccidial drugs, to a greater or lesser extent, interfere with the development of immunity to Eimeria species may no longer be correct because few drugs, if any, are capable of preventing some degree of parasite multiplication. Acquisition of immunity is, therefore, a real possibility, providing sufficient parasites are present in the environment. Immunity is an important consideration during the rearing of replacement layers, broiler breeders, turkeys, and gamebirds, but little research has been carried out on the effects of drugs on immunity development in these categories of stock. In recent years, there has been a change in the perception of the significance of immunity in broilers, and some broiler producers have taken this into consideration when designing drug programs for use in the field. Studies in floor pens and the field indicate that broilers given various drug programs can develop immunity when exposed to natural infections but that this process takes time: solid immunity not developing until birds are 6 to 7 weeks of age. This suggests that drugs should not be withdrawn from the feed prematurely since birds may not have had time to develop adequate immunity.

World Association for the Advancement of Veterinary Parasitology (WAAVP) guidelines for evaluating the effectiveness of anthelmintics in chickens and turkeys

These guidelines have been prepared to assist in the planning, operation and interpretation of studies designed to assess the effectiveness of drugs against helminth parasites of chickens and turkeys. They are the first to be compiled under the auspices of the World Association for the Advancement of Veterinary Parasitology (WAAVP) for these parasites. The advantages and disadvantages of the widely used critical and controlled tests are discussed. Information is provided on the selection of animals for experiments, animal housing, feed, dose determination studies, confirmatory and field trials, record keeping and necropsy procedures. This document should help investigators and those involved in product approval and registration in conducting and evaluating studies concerned with determining the effectiveness and safety of anthelmintic drugs.

Drug resistance in avian coccidia (a review)

Drug resistance is now recognised as a major cause of the failure of drugs to control coccidiosis in the fowl. In this article, biological, biochemical and genetic aspects of resistance in Eimeria are reviewed and some of the problems that may limit progress in understanding the nature of resistance in coccidia are discussed.

Coccidiosis in the turkey

Coccidiosis is a widespread disease of considerable economic importance to the turkey industry, as is evident by the fact that many millions of dollars are spent annually on its control. Despite this, in recent years little research has been conducted on the causative organisms—protozoan parasites of the genus Eimeria. This is unfortunate because current methods of control are not without problems. It is possible, therefore, that in the future optimal productivity in turkey production will be difficult to achieve. There has been no recent comprehensive literature review of coccidiosis in the turkey. In the present article our knowledge of the biology of the species of Eimeria that infect the turkey is critically reviewed. Other topics considered include host specificity, host resistance, acquired immunity, in vitro cultivation, pathology, pathogenicity, clinical signs, diagnosis, epidemiology, prevalence, significance, chemotherapy, drug resistance, drug toxicity, and vaccination. The review is intended to provide a baseline for future studies of this important disease.

Serum ovotransferrin as a biomarker of inflammatory diseases in chickens

Infectious and metabolic disorders are common in poultry and cause stress, poor performance, and mortality that results in considerable economic loss. Identifying the nature of stress in chickens will assist the development of appropriate measures to improve health and welfare. Acute phase proteins are hepatic proteins, the blood concentrations of which change significantly in the event of many health problems including inflammation and physical injuries. Thus, acute phase proteins are used as nonspecific diagnostic markers for various health disorders. Our previous studies showed that serum ovotransferrin (OVT) is an acute phase protein in chickens. Therefore, in the present study, we investigated whether OVT concentration can be a marker of physiological stress using sera from chickens with different infectious and metabolic disorders. A competitive enzyme immunoassay was developed to measure serum OVT concentrations. The results show that with experimentally induced pulmonary hypertension syndrome and tibial dyschondroplasia, there were no significant changes in OVT levels compared with matched controls. In contrast, when chickens were infected with microbes such as the bacterium Escherichia coli, or protozoan parasites such as Eimeria maxima and Eimeria tenella, there was a significant increase in the levels of OVT in the serum. Chickens with spontaneous autoimmune vitiligo also showed a significant increase in blood OVT levels. These studies suggest that blood OVT concentration is modulated under inflammatory and microbial stress and can therefore be used as a diagnostic marker of infection and inflammation in chickens.