Eimeria species: studies using rRNA and rDNA probes

Advances in diagnosis of protozoan diseases

Microscopy still remains the gold standard procedure for the diagnosis of many protozoan infections in animals, but the specific identification requires skilled and experienced personnel. Immunoassays, detecting antibodies or specific protozoan antigens, have been developed but often lack sensitivity and specificity due to close relationship between many protozoa. Recent research has focussed almost exclusively on molecular based techniques for the identification and quantification of parasite DNA in samples. Opinion differ on most appropriate targets to use and there are very few diagnostic kits available making comparison between laboratories difficult. Future research needs to focus on robust, cheap field diagnostic assays.

Control of poultry coccidiosis: changing trends

Coccidiosis is the most important protozoan disease affecting the poultry industry worldwide. Control of poultry coccidiosis is presently based on managerial skills and the use of prophylactic coccidiostatic drugs. With the emergence of drug resistant Eimeria strains, emphasis has been laid on development and use of safer vaccines; some of them have been commercialized successfully. The present review deals with the various factors responsible for the development of clinical coccidiosis in poultry as well as an overview of the currently available inducers and boosters of immunity against coccidiosis. There are three groups of vaccines currently available against coccidiosis which can be distinguished on the basis of characteristics of the Eimeria species included in the respective products, viz. vaccines based on live virulent strains, vaccines based on live attenuated strains, and vaccines based on live strains that are relatively tolerant to the ionophore compounds. The latter vaccine combines the early chemotherapeutic effect of ionophores with the late prophylactic effect of vaccination. Although in the near future more varieties of oocyst based live vaccines are expected, identification of selective coccidian-specific immunoprotective molecules is likely to get more attention to facilitate the sustainable control of poultry coccidiosis.

Biotechnological advances in the diagnosis of avian coccidiosis and the analysis of genetic variation in Eimeria

Coccidiosis is an intestinal disease of chickens caused by various species of protozoan parasites within the genus Eimeria. This disease has a major economic impact to growers and to the poultry industry world-wide. The diagnosis and genetic characterization of the different species of Eimeria are central to the prevention, surveillance and control of coccidiosis, particularly now given the major problems with wide-spread resistance of Eimeria species against anticoccidial drugs (coccidiostats) and the residue problems associated with these compounds. While traditional methods have had major limitations in the specific diagnosis of coccidiosis, there have been significant advances in the development of molecular-diagnostic tools. The present article provides a background on coccidiosis, reviews the main molecular methods which have been used and describes recent advances in the establishment of polymerase chain reaction (PCR)-coupled electrophoretic approaches for the specific diagnosis of coccidiosis as well as the genetic characterization of species of Eimeria. These biotechnological advances are considered to represent a significant step toward the improved prevention and control of this important disease of poultry.

Inter- and intra-strain variation and PCR detection of the internal transcribed spacer 1 (ITS-1) sequences of Australian isolates of Eimeria species from chickens

The objective of this study was to confirm the presence of seven species of Eimeria involved in chicken coccidiosis in Australia by comparing internal transcribed spacer 1 (ITS-1) sequences, ITS-1 polymerase chain reaction (PCR) methods and to apply phylogenetic analysis to assess evolutionary relationships of Australian isolates. Twenty-two distinct ITS-1 regions of 15 Australian Eimeria isolates were sequenced, and analysed using maximum parsimony, distance and maximum likelihood methods. Poor bootstrap support, resulting from high ITS-1 sequence heterogeneity between all species groups, resulted in polychotomy of the Eimeria species in all three trees generated by these analyses. Percentage identity analyses revealed two distant ITS-1 lineages in both E. mitis and E. maxima at the same levels that separate the two species E. tenella and E. necatrix. One E. maxima lineage consisted of Australian isolates, the other American isolates, with one European sequence (originating from the same isolate) in each lineage. One Australian E. praecox sequence was only distantly related (33% variation) to three E. praecox sequences from Australian and European isolates. Short and long ITS-1 variants were isolated from both E. tenella (cloned line) and E. necatrix isolates with deletions (106 and 73 bp, respectively) in the short variants within the 3' region of the ITS-1 sequence. ITS-1 sequences of strains of both E. brunetti and E. acervulina species varied the least. Apart from E. maxima, all of the ITS-1 sequences of the six remaining individual species clustered to the exclusion of other species in all phylogenetic trees. Published ITS-1 tests for E. necatrix, E. acervulina, E. brunetti and E. tenella, combined with three new tests for E. mitis, E. praecox and Australian E. maxima amplified all respective Australian isolates specifically in a nested format using conserved ITS-1 PCR products as template to improve the sensitivity. All PCR tests were confirmed against a collection of 24 Australian chicken Eimeria isolates and contaminating species were detected in some instances. In conclusion, once the genetic variation between species and strains is determined, the ITS-1 is a good target for the development of species-specific assays, but the ITS-1 sequences alone do not seem suitable for the confirmation of phylogenetic inferences for these species. This study reports the first attempt at the analysis of the phylogeny and sequence comparison of the Eimeria species involved in chicken coccidiosis in Australia.

Recent advances in biology and immunobiology of Eimeria species and in diagnosis and control of infection with these coccidian parasites of poultry

Avian coccidiosis, an intestinal disease caused by protozoan parasites of the genus Eimeria, occurs worldwide. It is considered to be one of the most economically important diseases of domestic poultry. For many years, prophylactic use of anticoccidial feed additives has been the primary means of controlling coccidiosis in the broiler industry and has played a major role in the growth of this industry, which now can produce about 7.6 billion chickens annually. However, development of anticoccidial resistance has threatened the economic stability of the broiler industry. Although there has been little effort by the pharmaceutical industry to develop new anticoccidials, the mounting problem of drug resistance of Eimeria species has prompted major research efforts to seek alternative means of control through increased knowledge of parasite biology, host response, and nutritional modulation. As a consequence, important advancements have been made, particularly in defining parasite antigens that have potential use in vaccines, defining the Eimeria genome, understanding the immunology of coccidial infections, and the practical applications of live vaccines. This review describes the progress in these areas, most of which has occurred within the past 10 to 15 years.

Application of molecular biology in veterinary parasitology

The number of applications of molecular biology in veterinary parasitology is increasing rapidly. The techniques used with eukaryotic cells are generally applicable to the study of parasites and their hosts. The polymerase chain reaction is particularly important for identification and diagnosis of parasites, as well as for many other applications. With species and type specific probes or primers, sensitivities and specificities unheard of with conventional techniques can be achieved. The accumulation of more information on the DNA sequences of parasites will reveal many more unique sequences which can be used for identification, diagnosis, molecular epidemiology, vaccine development and for studying the evolutionary biology and the physiology of parasites and the host-parasite relationship. Similarly, the completion of genome projects on host organisms will greatly assist efforts to select for hosts that are genetically resistant to parasite infection. The study of the molecular biology of antiparasitic drug receptors, potential targets for chemotherapy, and the molecular genetics of drug resistance will allow molecular screens to be used with combinatorial chemistry in the search for new antiparasitic drugs, improvements to existing chemotherapeutic families and better diagnosis and monitoring of drug resistance. While there is a proliferation of molecular biology techniques, the availability of simple kits and of automated techniques and services for sequencing, library construction and oligonucleotide synthesis and other procedures is making it easier for non-specialists to apply many of the common techniques of molecular biology. Molecular biology and the benefits from its application are relevant for veterinary parasitologists in developing countries as well as developed countries and we should introduce aspects of molecular biology to the teaching and training of veterinary parasitologists.

Research on avian coccidia: an update

Despite the availability of many anticoccidial drugs, infections caused by species of Eimeria continue to be a source of significant economic loss to the poultry industry. After two decades in which the use world wide of ionophorous antibiotics gave unparalleled control of coccidiosis, drug resistance is once again tipping the balance in favour of the parasites. The realization that even the most spectacularly successful drugs might, after all, have a finite life if not used conservatively, has focused attention on ways in which the life span of drugs can be prolonged. Many drugs with different (if unknown) modes of action are available, and a variety of shuttle and rotation programmes can be considered. In view of the limitations of chemotherapy, particularly for the rearing of replacement flocks, there is considerable interest in the development of vaccines. Prospects for the introduction of live vaccines based on attenuated parasites are now very good, but the availability in the future of genetically engineered vaccines is more uncertain as little is known about the parasite molecules that stimulate protective immunity and, even if isolated, how they can be administered to the host so that it responds in the immunologically correct manner. Current research on Eimeria spp. in the chicken is broadly representative of that being done on other coccidia. Many lines of investigation are not connected with the development of new drugs or vaccination per se (and therefore have no obvious practical applications), but they are providing new insights into the biological complexity of the organisms and the ways in which they interact with their hosts. It remains possible, however, that a more detailed understanding and analysis of the molecules that are essential in the maintenance of the parasitic life style can be exploited in the future to provide alternative targets for chemical or immunological attack. The research topics considered in this review are arbitrarily grouped as studies on: (1) the basic biology of parasites, including aspects of the life cycle, and structure and function of the apical organelles; (2) the molecular biology of the parasites, including analyses of the number and structure of chromosomes, characterization of DNA sequences, and an account of the viral RNA that has been found in some species of Eimeria; and (3) control of coccidiosis, encompassing first immunity and the development of vaccines, and secondly, chemotherapy.

Babesia bovis, Babesia bigemina, Babesia canis, Babesia microti and Babesia rodhaini: comparison of ribosomal RNA gene organization

The three ribosomal DNA (rDNA) units have been cloned from an Australian isolate of Babesia bigemina. The organization of the units is very similar to that reported for a Mexican isolate of B. bigemina. In Babesia canis four rDNA units have been identified. Both Babesia rodhaini and Babesia microti contain two different rDNA units. A small number of different rDNA units appears to be a common feature of this group of Protozoa. Restriction enzyme analysis of the rDNA units form these species and B. bovis suggests that the genus Babesia as currently defined does indeed include two distinct groups of organisms namely, B. bovis, B. bigemina and B. canis and B. rodhaini and B. microti.

Changes in the messenger RNA population during sporulation of Eimeria maxima

Messenger RNA has been extracted from oocysts of Eimeria maxima. Using the techniques of in vitro translation and SDS-polyacrylamide gel electrophoresis, we have been able to show that major changes in gene transcription occur during sporulation. Following an overall reduction in the abundance of many mRNAs, several genes identified as the result of an increase in the abundance of their transcripts, are highly expressed during the latter stages of sporulation. A study of two genes whose transcription is down-regulated has provided evidence which shows that both single copy and repetitive sequences are regulated during sporulation of the oocyst.

Development of a genetically engineered vaccine against poultry coccidiosis

Coccidiosis is caused by infection with Eimeria spp. The disease is responsible for major economic loss to the poultry industry unless infections are controlled by anticoccidial drugs. John Ellis and Fiona Tomley discuss recent research on the characterization and cloning of antigens from Eimeria spp and advances towards the development of genetically engineered vaccines against poultry coccidiosis.