Isoenzyme analysis of Haemonchus contortus resistant or susceptible to ivermectin

The Identification of Haemonchus Species and Diagnosis of Haemonchosis

Diagnosis is often equated with identification or detection when discussing parasitic diseases. Unfortunately, these are not necessarily mutually exclusive activities; diseases and infections are generally diagnosed and organisms are identified. Diagnosis is commonly predicated upon some clinical signs; in an effort to determine the causative agent, identification of genera and species is subsequently performed. Both identification and diagnosis play critical roles in managing an infection, and involve the interplay of direct and indirect methods of detection, particularly in light of the complex and expanding problem of drug-resistance in parasites. Accurate and authoritative identification that is cost- and time-effective, based on structural and molecular attributes of specimens, provides a foundation for defining parasite diversity and changing patterns of geographical distribution, host association and emergence of disease. Most techniques developed thus far have been grounded in assumptions based on strict host associations between Haemonchus contortus and small ruminants, that is, sheep and goats, and between Haemonchus placei and bovids. Current research and increasing empirical evidence of natural infections in the field demonstrates that this assumption misrepresents the host associations for these species of Haemonchus. Furthermore, the capacity of H. contortus to utilize a considerably broad spectrum of ungulate hosts is reflected in our understanding of the role of anthropogenic forcing, the 'breakdown' of ecological isolation, global introduction and host switching as determinants of distribution. Nuanced insights about distribution, host association and epidemiology have emerged over the past 30years, coincidently with the development of increasingly robust means for parasite identification. In this review and for the sake of argument, we would like to delineate the diagnosis of haemonchosis from the identification of the specific pathogen. As a foundation for exploring host and parasite biology, we will examine the evolution of methods for distinguishing H. contortus from other common gastrointestinal nematodes of agriculturally significant and free-ranging wild ruminants using morphological, molecular and/or immunological methods for studies at the species and genus levels.

Anthelmintic resistance and alternative control methods

Anthelmintic resistance has been a problem almost from the first use of the drugs introduced during the past 50 years. Evaluation of anthelmintics on a specific farm is essential to determine which drugs may be used in controlling parasite numbers. Treatment of livestock during the season in which parasite transmission is unfavorable lessens pasture contamination. Targeting treatment places less pressure on susceptible worms, diluting the resistant parasite population. Pasture management and using safe pastures for animals at highest risk lower pasture exposure. Selection for individual animals resistant to the effects of parasites lessens the need for use of anthelmintics. Alternatives to anthelmintics include cryptic antigen vaccines, copper wires, and biologic compounds.

Ivermectin resistant and susceptible third-stage larvae of Haemonchus contortus: cholinesterase and phosphatase activities

Cholinesterase and acid phosphatase (AP), but not alkaline phosphatase activities, were detected in cytosolic and membrane-bound fractions of ivermectin resistant and susceptible Haemonchus contortus infective-stage larvae. Some differences in acetylcholinesterase activity of cytosolic fractions and in the AP activity of these fractions as well as in the response to AP inhibitors by membrane-bound fractions were detected. Data are discussed.

Multilocus enzyme electrophoresis: a valuable technique for providing answers to problems in parasite systematics

The aim of this review is to highlight the effectiveness of the technique of multilocus enzyme electrophoresis in answering questions relating to the systematics of parasites and to highlight errors in the way the technique has been used and the results interpreted. We have approached this topic by answering specific questions that we have been asked by colleagues and students not necessarily familiar with the technique, the method of data analysis and its application. Although the technique has been applied to provide answers for taxonomic and population genetics studies, it remains under-utilised, perhaps because of recent advances in newer molecular technology. Rather than not acknowledge or dismiss the value of more traditional technology, we suggest that researchers examine problems in the systematics of parasites by the comparison of data derived from morphological, biochemical and molecular techniques.

Genetic variation in parasitic nematodes and its implications

An absolute pre-requisite for a genetic response to a selective pressure is genetic variation within the population under selection. Helminth populations are clearly able to respond to selective pressures and must, therefore, be genetically heterogeneous. While not quite tautological, this is at best indirect evidence for the existence of genetic variation but there are few examples of well documented helminth phenotypic variation with a proven genetic basis. Isozyme analysis has provided more direct evidence for variation but attempts to link this variation to responses to selection or to identify the forces maintaining that variation have been largely unsuccessful. Thus there is a clear need for new techniques. The recent application of PCR and direct sequencing technology to the study of helminth genetics has allowed the genotypes of individual worms to be determined and the first direct measurements of allele frequencies to be made in this group of organisms. In addition, the application of genetic and molecular data from Caenorhabditis elegans is a potentially rich source of new markers. These techniques do not require that the genetic basis of the phenotype in question be known since a large number of loci can be examined and selection detected through changes in the frequency of anonymous linked marker loci. Phenotypes with complex genetic bases can, therefore, be analysed. I have applied these techniques to the study of anthelmintic resistance genetics and others have applied them to the genetics of inhibited development in Ostertagia. Other phenotypes that are of great interest are the potential for selection of resistance to vaccination and the use of genetically resistant hosts. The ease with which helminths have countered all classes of anthelminitics and the apparently high levels of polymorphism in helminth populations suggest that immunological control methods may also prove to be vulnerable to the adaptive capabilities of the parasite. Evidence from a mouse-helminth model system has already provided evidence that worms can meet the challenge.

Ivermectin resistance

In this review of ivermectin resistance, Wesley Shoop discusses the definition of resistance, catalogs all known cases of ivermectin resistance, argues that overmectins and milbemycins belong in the same action family, discusses the possibility of resistance in the filariae, and suggests that detection of ivermectin resistance is the area where future research is most needed.