Eimeria tenella: accumulation and retention of anticoccidial ionophores by extracellular sporozoites

Genetic selection of Eimeria parasites in the chicken for improvement of poultry health: implications for drug resistance and live vaccine development

AbstractApicomplexan parasites of the genus Eimeria are widespread in poultry flocks and can cause the intestinal disease coccidiosis. Early studies, concerned with intraspecific variation in oocyst morphology, indicated that phenotypic changes may be induced by selection experiments conducted in vivo. Genetic selection driven by targeted selection for specific phenotypes has contributed to our understanding of the phenomenon of drug resistance and the development of live attenuated vaccines. Our present knowledge regarding genetics of Eimeria is largely based upon the utilization of such selected strains as genetic markers. Practical advantages of working with Eimeria spp. in the chicken are discussed. The selection of drug resistant strains by serial propagation has provided useful information regarding the mechanisms of drug resistance and likely longevity of anticoccidial drugs when introduced in the field. Selection experiments to develop precocious strains of Eimeria and growth in chicken embryos have contributed to the development of safe and effective live attenuated vaccines for control of coccidiosis. Establishment of protocols for genetic complementation by transient or stable transfection of Eimeria is now supporting direct manipulation of parasite genotypes, creating opportunities to expand the range and value of live parasite vaccines. Procedures for developing drug resistant and precocious lines of Eimeria and/or genetic markers described here are likely to prove useful for researchers investigating the propensity for resistance development to novel compounds and the development of new attenuated vaccines. Such investigations can be helpful in providing a better understanding of biochemical and molecular aspects of the biology of these parasites.

Focused review: The role of drug combinations for the control of coccidiosis in commercially reared chickens

A survey of drug combinations employed by the poultry industry indicates that they have played an important role in the control of coccidiosis in chickens. The mode of action of their component drugs is described. Advantages that accrue from their use may include a reduction in potential toxicity, a broadening of their spectrum of activity against different species of Eimeria, activity against different stages of the life cycle, and improved efficacy due to synergism between component drugs. Integration of management procedures involving rotation of drug combinations with vaccination is desirable because this has been shown to result in a restoration of drug sensitivity where drug resistance is present and could contribute to the sustainable control of coccidiosis. Threats to the future use of the most widely used combinations, those that include ionophores, stem from the recent desire to eliminate antibiotics from poultry feeds.

In Vitro Assessment of Anticoccidials: Methods and Molecules

Simple Summary

Coccidiosis is a major problem in poultry production, leading to significant economic losses. Due to the outbreak of resistance to the available treatments, research is focusing on finding new molecules that work against the pathogen. Botanical compounds represent promising alternatives, but reliable in vitro tests are needed for their screening and to understand their mechanism of action. Research in vitro involves studies on the environmental phase of the parasite and studies on the endogenous development, which occurs inside the host cells and that requires cell cultures or in ovo models to be studied. This review aims to summarize the protocols that have been successfully applied so far, as well as to suggest potential cues to improve research on this field. Moreover, as the surge of botanicals as anticoccidial molecules is on the rise, the intent is to provide an overview of the methods to assess their effectiveness in vitro in comparison with conventional drugs.

Abstract

Avian coccidiosis is a disease causing considerable economic losses in the poultry industry. It is caused by Eimeria spp., protozoan parasites characterized by an exogenous–endogenous lifecycle. In vitro research on these pathogens is very complicated and lacks standardization. This review provides a description of the main in vitro protocols so far assessed focusing on the exogenous phase, with oocyst viability and sporulation assays, and on the endogenous phase, with invasion and developmental assays in cell cultures and in ovo. An overview of these in vitro applications to screen both old and new remedies and to understand the relative mode of action is also discussed.

Effect of anticoccidial monensin with oregano essential oil on broilers experimentally challenged with mixed Eimeria spp

Essential oil of oregano ( OEO: ) has proven to be a potential candidate for controlling chicken coccidiosis. The aim of the current study is to determine whether OEO and an approved anticoccidial, monensin sodium ( MON: ), as in-feed supplements could create a synergism when combined at low dosages. Day-old broiler chickens were separated into six equal groups with six replicate pens of 36 birds. One of the groups was given a basal diet and served as the control ( CNT: ). The remaining groups received the basal diet supplemented with 100 mg/kg MON, 50 mg/kg MON, 24 mg/kg OEO, 12 mg/kg OEO, or 50 mg/kg MON + 12 mg/kg OEO. All of the chickens were challenged with field-type mixed Eimeria species at 12 d of age. Following the infection (i.e., d 13 to 42), the greatest growth gains and lowest feed conversion ratio values were recorded for the group of birds fed 100 mg/kg MON (P < 0.05), whereas results for the CNT treatment were inferior. Dietary OEO supplementations could not support growth to a level comparable with the MON (100 mg/kg). The MON programs were more efficacious in reducing fecal oocyst numbers compared to CNT and OEO treatments (P < 0.05). Serum malondialdehyde and nitric oxide concentrations were decreased (P < 0.01), whereas superoxide dismutase (P < 0.05) and total antioxidant status (P < 0.01) were increased in response to dietary medication with MON and OEO. All MON and OEO treatments conferred intestinal health benefits to chickens by improving their morphological development and enzymatic activities. The results suggest that OEO supported the intestinal absorptive capacity and antioxidant defense system during Eimeria infection; however, it displayed little direct activity on the reproductive capacity of Eimeria This might be the reason for inferior compensatory growth potential of OEO compared to that MON following the challenge. Combination MON with OEO was not considered to show promise for controlling chicken coccidiosis because of the lack of a synergistic or additive effect.

A selective review of advances in coccidiosis research

Coccidiosis is a widespread and economically significant disease of livestock caused by protozoan parasites of the genus Eimeria. This disease is worldwide in occurrence and costs the animal agricultural industry many millions of dollars to control. In recent years, the modern tools of molecular biology, biochemistry, cell biology and immunology have been used to expand greatly our knowledge of these parasites and the disease they cause. Such studies are essential if we are to develop new means for the control of coccidiosis. In this chapter, selective aspects of the biology of these organisms, with emphasis on recent research in poultry, are reviewed. Topics considered include taxonomy, systematics, genetics, genomics, transcriptomics, proteomics, transfection, oocyst biogenesis, host cell invasion, immunobiology, diagnostics and control.

The long view: a selective review of 40 years of coccidiosis research

This selective review of 40 years of coccidiosis research is one of a number on important diseases of poultry to celebrate the 40th anniversary of the birth of Avian Pathology, the journal of the World Veterinary Poultry Association, and is written for the non-specialist. The intention is to provide a flavour of the field problems and intellectual challenges, with emphasis in the areas of immunology and vaccinology that drove research in the 1970s, and to reflect on research progress since.

Polyether ionophores: broad-spectrum and promising biologically active molecules for the control of drug-resistant bacteria and parasites

BACKGROUND

As multidrug-resistant (MDR) pathogens continue to emerge, there is a substantial amount of pressure to identify new drug candidates. Carboxyl polyethers, also referred to as polyether antibiotics, are a unique class of compounds with outstanding potency against a variety of critical infectious disease targets including protozoa, bacteria and viruses. The characteristics of these molecules that are of key interest are their selectivity and high potency against several MDR etiological agents.

OBJECTIVE

Although many studies have been published about carboxyl polyether antibiotics, there are no recent reviews of this class of drugs. The purpose of this review is to provide the reader with an overview of the spectrum of activity of polyether antibiotics, their mechanism of action, toxicity and potential as drug candidates to combat drug-resistant infectious diseases.

CONCLUSION

Polyether ionophores show a high degree of promise for the potential control of drug-resistant bacterial and parasitic infections. Despite the long history of use of this class of drugs, very limited medicinal chemistry and drug optimization studies have been reported, thus leaving the door open to these opportunities in the future. Scifinder and PubMed were the main search engines used to locate articles relevant to the topic presented in the present review. Keywords used in our search were specific names of each of the 88 compounds presented in the review as well as more general terms such as polyethers, ionophores, carboxylic polyethers and polyether antibiotics.

Anticoccidial vaccines for broiler chickens: pathways to success

The use of live vaccines, either attenuated or non-attenuated, for the control of coccidiosis due to Eimeria infections in broiler breeder or layer chickens is well established. Use in broilers, however, has been slow to gain acceptance. This has been partly for economic reasons, but also because of perceived adverse effects on early chick growth, particularly with non-attenuated vaccines, and concerns about timely onset of protective immunity in such short-lived birds. This review describes advances in understanding of epidemiological factors and recent improvements of administration methods that have helped to allay these fears and to make the use of anticoccidial vaccines in broilers technically achievable. Topics discussed include: (1) types of commercially available vaccine, (2) vaccines in development, (3) vaccination methods and equipment, (4) basis of vaccine efficacy and immunogenic variation of parasites, (5) key factors in the survival, sporulation and dissemination of vaccinal oocysts, (6) descriptions and significance of patterns of litter oocyst accumulation and occurrence of intestinal lesions in vaccinated flocks, (7) rotation of anticoccidial vaccination and chemotherapy to restore drug sensitivity to resistant wild-type coccidia, (8) combinations of anticoccidial vaccination and chemotherapy, (9) interactions between coccidiosis and clostridiosis in broilers and compatibilities of potential control methods, (10) published performance data for live anticoccidial vaccines in broilers, (11) possible further developments of live vaccines.

Mapping for B-cell epitopes in the GX3262 antigenic sequence derived from Eimeria tenella sporulated oocysts

Polypropylene pins were impregnated with synthetic overlapping heptapeptides based on the GX3262 Eimeria tenella antigenic sequence (Miller et al., 1989). Using these coated pins as the solid phase of an enzyme immunoassay (EIA), binding of sera from chickens and rabbits infected and immunised respectively with five different species of Eimeria were examined. Antibody reactions to the individual heptapeptides were then analysed by a number of criteria based on the amino acid sequence including hydropathy, chain flexibility and secondary structure.

High-resolution mapping of B-cell epitopes within an antigenic sequence from Eimeria tenella

Overlapping hexapeptides representing part of an Eimeria tenella antigenic sequence, shown to induce partial immunity to homologous challenge in chickens, were synthesized on polypropylene pins (Pepskan technique; Cambridge Research Biochemicals, Cambridge, United Kingdom). The binding to these hexapeptides of antibodies from chickens infected and rabbits immunized with five species of Eimeria was studied, using the coated pins as the solid phase of an enzyme-linked immunoassay. Antibody binding to most regions of the sequence was demonstrated, with peak areas of antigenicity correlating with the most hydrophilic regions. A particularly hydrophilic and antigenic area towards the N terminus of the sequence consists of a peptide motif repeated five times in the native antigen. Homologous antisera (chicken and rabbit anti-E. tenella antisera) differed in their pattern of reactivity from heterologous sera raised against other Eimeria species. While the former bound to fewer of the hexapeptides than the latter, they did so very strongly, indicating affinity maturation of the antibody response to E. tenella-specific sequences. No antibody reactivity to two regions of the sequence was detected. These regions occur in relatively hydrophilic areas and so are unlikely to be situated in transmembrane domains or in the interior of globular proteins. Synthetic peptides, as used in these experiments, make possible analysis of the fine specificity of immune responses and thus have a role to play in the development of novel vaccines for the control of coccidiosis.

Amplification of ionophore resistance in a field strain of Eimeria tenella by treating free sporozoites with monensin in vitro

Monensin resistance in a field strain of Eimeria tenella (FS139) was successfully amplified by an in vitro method. Sporozoites of parent FS139, designated as FS139(0), were treated with monensin at 5 micrograms ml-1 before inoculation into chickens. Oocysts developing from these drug-treated sporozoites were designated FS139(5). Sporozoites of FS139(5) were then treated with a higher level of monensin (25 micrograms ml-1), which produced another line called FS139(25). Sporozoites of FS139(25) received a monensin treatment of 100 micrograms ml-1 which yielded line FS139(100). After one passage in chickens, sporozoites of FS139(0), FS139(25) and FS139(100) were treated with either 0, 1, 25, or 100 micrograms ml-1 of monensin and inoculated into chicken primary kidney cell cultures to observe sporozoite invasion rates and development of first generation schizonts. Data on invasion rates and development of schizonts showed that FS139(100) was the most drug-resistant line while FS139(25) was the second most resistant line compared with the parent line FS139(0). These in vitro treatment techniques could be used to develop resistant coccidia for laboratory study of the physiological mechanisms of resistance.

Effect of monensin on ultrastructure and cellular invasion by the turkey coccidia Eimeria adenoeides and Eimeria meleagrimitis

Freshly excysted sporozoites (SZ) of the turkey coccidia Eimeria meleagrimitis and Eimeria adenoeides were incubated at 41 C in concentrations of monensin from .01 to 1.0 microgram/mL, washed free of the drug, and either processed for phase, fluorescence, and transmission electron microscopy or inoculated into cultures of turkey kidney cells. Phase microscopy indicated that after 1.5 h incubation in 1.0 micrograms/mL monensin, about 60% of the SZ of E. meleagrimitis had become notably rounded or displayed localized protrusions. These alterations were accompanied by ultrastructural abnormalities (in 90% of the SZ) including vacuoles in the cytoplasm, bulging and separation of plasma membrane layers, and dense bands in the refractile bodies that extended toward the periphery of the refractile body. Similar morphological and ultrastructural changes were observed in over half of the E. adenoeides SZ after 2 h incubation in 1.0 micrograms/mL monensin. Additionally, some specimens contained a pycnotic nucleus that was usually surrounded by a large vacuole. After 4 h incubation, almost all of the SZ displayed some degree of ultrastructural damage. Indirect fluorescent antibody labeling with parasite-specific monoclonal antibodies demonstrated clouds of antigen surrounding the monensin-treated but not the untreated SZ, suggesting an increase in permeability with incubation in monensin. With both E. meleagrimitis and E. adenoeides, the structural changes were reflected in a significant inhibition of cellular invasion. The inhibitory activity of monensin was concentration- and time-dependent in that the greatest inhibition of invasion was observed in SZ incubated for 4 h in 1.0 micrograms/mL of monensin; shorter incubation times or lower concentrations of monensin having less effect.

Ultrastructural studies of the effects of the ionophore lasalocid on Eimeria tenella in chickens

The ultrastructural development of Eimeria tenella was studied in experimentally infected chicks fed 90 ppm lasalocid, an ionophorous anticoccidial antibiotic. Drug treatment was timed to target-specific endogenous stages. At 6 h after infection, many sporozoites within the epithelium showed degradation as a result of drug action. Only a few intact sporozoites were seen. The drug caused outer-membrane blistering, large surface swellings and enlarged mitochondria in both first- and second-generation merozoites. No effect on the gamonts was discerned.

Studies on the stage of action of lasalocid against Eimeria tenella and Eimeria acervulina in the chicken

Broiler chickens in battery pens were either fed a diet containing 100 ppm lasalocid or no drug for 24 h prior to inoculation with sporulated oocysts of Eimeria tenella or Eimeria acervulina. Different groups of birds remained on medicated feed for 24, 48, 72, 96, 120 or 144 h after inoculation. Conversely, other groups started on an unmedicated diet, were given medicated feed at different times after oocyst inoculation. Starting lasalocid medication 24 h (E. tenella) or 48 h (E. acervulina) after inoculation reduced the lesions and improved the weight gain. There was no significant difference in performance of birds after withdrawal of the drug at 48 h (E. tenella) or 72 h (E. acervulina) and thereafter. Starting lasalocid medication at 96 or 120 h did not suppress but rather reduced oocyst production.

Effect of lasalocid on the development of Eimeria tenella in chicken embryos

Sporozoites of Eimeria tenella were treated with lasalocid in vitro and their viability tested by inoculating them into the allantoic cavity of 11-day-old chicken embryos. Concentrations of 0.1, 1.0 and 10.0 micrograms ml-1 reduced sporozoite viability, as judged by oocyst production. Injections into the embryos of 5, 50 or 500 micrograms of lasalocid 92-93 h after infection also reduced oocyst production, indicating activity against the later stages of the life cycle.

The interactions between drugs and the parasite surface

The interrelationships between drugs and parasite surfaces are considered under the headings of (a) effects on membrane transport, (b) drug uptake mechanisms and (c) effects on surface morphology and function: praziquantel is discussed under a separate heading. The range of chemotherapeutic compounds that cause permeability changes and concomitant morphological disruption is discussed in terms of mode of drug action. Interpretation of the available data renders it difficult to identify the primary mode of action in the drugs considered. Drug uptake mechanisms are known for relatively few compounds; drug resistance as a function of drug acquisition is discussed. The role of the parasite surface as a specific drug target is argued.

Effect of ionophorous anticoccidials on invasion and development of Eimeria: comparison of sensitive and resistant isolates and correlation with drug uptake

Prophylactic levels of three ionophorous antibiotics, monensin, salinomycin, and lasalocid, were administered to groups of chickens and turkeys. All three ionophores markedly inhibited invasion of cecal tissues by sporozoites of ionophore-sensitive (IS) Eimeria tenella. Monensin and salinomycin also reduced invasion in turkeys by sporozoites of E. adenoeides, but lasalocid only minimally inhibited invasion. Invasion of ceca of monensin-medicated chickens was significantly greater by sporozoites of ionophore-resistant (IR) E. tenella than of the IS isolate. Concomitant experiments showed significant differences in [14C]monensin accumulation among IS and IR isolates of E. tenella. The decreased uptake of monensin by the IR isolates appeared to be accompanied by a decrease in responsiveness to the activity of monensin as well as to two other ionophores, salinomycin and narasin in cell culture. The amount of monensin, salinomycin or narasin required to inhibit development of E. tenella by 50% was 20 to 40 times higher for the IR isolates than for the IS ones. Collectively, the data suggest that differences in ionophore accumulation by IS and IR isolates of E. tenella might reflect differences in membrane chemistry and that these differences are responsible for the expressions of resistance that were observed in these studies. This expression of resistance appears to be common to all ionophores tested.

Control of chicken coccidiosis

Coccidiosis could potentially cause enormous economic loss to the poultry industry, especially in the production of broiler chickens (see Box 1). Losses are currently minimized by chemotherapeutic treatment but the effectiveness of many drugs seems to be declining. In this article, Peter Long and Tom Jeffers discuss the future for coccidial chemotherapy, and the potential for immunological control methods.

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.

Gordon memorial lecture. Coccidiosis control: past, present and future

Hygiene, management, chemotherapy and immunity all play a part in the control of coccidiosis. Under the conditions of modern intensive poultry rearing, special reliance is placed upon chemotherapy and since the introduction of the sulphonamides in 1939 a sequence of different types of drugs has been developed. At present the field is dominated by the ionophore antibiotics which have a special mode of action against the extracellular phases of the parasitic life cycle. Drug resistance is a continuing problem which has limited the effective life of most types of drug, although it has been most significant for particular compounds. So far it has not severely affected the efficacy of the ionophores. Immunity is involved in effective prophylaxis and the strong protective immunity which is a feature of most coccidial infections offers promise of a vaccination system. However, much more research will be necessary to bring this promise to fruition. Studies on immune mechanisms, antigenicity, biochemistry and in vitro cultivation may all contribute to the development of methods for controlling a series of infections which are an important obstacle to the development of modern husbandry methods.

The action of polyether ionophorous antibiotics (monensin, salinomycin, lasalocid) on developmental stages of Eimeria tenella (Coccidia, Sporozoa) in vivo and in vitro: study by light and electron microscopy

The effect of three polyether antibiotics (monensin, salinomycin, lasalocid) on developmental stages of Eimeria tenella (Coccidia, Sporozoa) was studied in vivo and in vitro by means of light and electron microscopy. It was found that these three drugs act against free merozoites, which are destroyed by bursting of the cell border (i.e. pellicle), endoplasmic reticulum and internal organelles even after very short exposure times (20 min) in media containing 1 ppm, 10 ppm or 100 ppm of these drugs. Sporozoites, however, survived these drug concentrations during an exposure time of 30 min (this would be sufficient to penetrate host cells and start development). Intracellular stages, which were situated in a parasitophorous vacuole within an intact host cell, were not attacked, apparently because these drugs are almost incapable of penetrating host cells. On the other hand, parasites (such as differentiated schizonts, gamonts) located within degenerating host cells showed slight disintegration, which did not necessarily led to their death. From these results it becomes clear why these polyether antibiotics have to be fed daily. Doses of 70 ppm salinomycin, 125 ppm monensin and 125 ppm lasalocid were found to bring about an equivalent protective effect against an infection with 40,000 Eimeria tenella oocysts.

The chemotherapy of protozoal infections of veterinary importance

The use of drugs in domestic animals is dominated by considerations of cost-effectiveness and profitability. They are extensively used against coccidial infections of poultry where they are an important factor in intensive husbandry. Ionophore antibiotics, which dominate this field, may have applications in ruminants. Imidocarb is of therapeutic and prophylactic value against babesial infections anf there are new prospects for control of theileriasis. Effective drugs for the control of African trypanosomiasis are limited and attention is being given to alternative uses of available compounds.