Gastrointestinal Nematode Parasites Of Sheep Research Articles

Management of anthelmintic resistance: Inheritance of resistance and selection with persistent drugs

Management of anthelmintic resistance: inheritance of resistance and selection with persistent drugs. International Journal for Parasitology 26: 993–1000. Resistance to the benzimidazole (BZ) anthelmintics is inherited as an incomplete dominant/ incomplete recessive trait and is now widespread in populations of gastrointestinal nematode parasites of sheep. Unlike benzimidazole resistance, which is common in Haemonchus contortus, Trichostrongylus colubriformis and Ostertagia circumcincta, resistance to levamisole is relatively rare in H. contortus, although common in the other 2 species. One explanation for the slow spread of resistance to levamisole in H. contortus is that it is inherited as an autosomal recessive trait, while in T. colubriformis levamisole resistance is inherited as a recessive sex-linked trait. With the introduction of the avermectin/milbemycin class resistance has developed to the relatively short-acting ivermectin, but this time it is inherited as a completely dominant trait. The potentially more serious situation of a persistent anthelmintic selecting a dominant resistance gene was investigated using a simulation model. Efficacy against incoming infective larvae (L3) was assumed to decline or remain high over the period of drug persistence (3 days to 4 weeks), thus allowing the estimation of the relative importance of selecting resistant L3s on the development of resistance in the worm population. These factors were also examined against a background of initial efficacy levels, against adults, and mode of inheritance. Persistence and initial efficacy were found to be far more important in determining the rate of selection for resistance than was selection of resistant L3 as drug efficacy declined.

Resistance to thiabendazole, fenbendazole and levamisole in Haemonchus and Trichostrongylus species in sheep on a Kenyan farm

Resistance to thiabendazole (TBZ), fenbendazole (FBZ) and levamisole (LVM) in naturally acquired gastrointestinal nematode parasites in sheep was investigated on a farm where anthelmintic resistance was suspected. This was measured by both the in vitro egg hatch assay, and reductions in faecal egg and worm counts in treated animals. In the egg hatch assay, nematode eggs were incubated in various concentrations of either TBZ or LVM. The level of resistance was expressed as the drug concentration inhibiting 50% of the eggs from hatching (LC50). The nematode population had LC50 values of 0.26 μg ml −1 TBZ and 3.12 μg ml −1 LVM. In the faecal egg and worm count reduction test, naturally infected sheep were treated with either TBZ (88 mg kg −1), FBZ (10 mgkg −1) or LVM (15 mg kg −1). Faecal egg and total worm counts from these sheep were then compared with counts from untreated sheep. TBZ, FBZ and LVM failed to reduce the faecal egg counts and total worm counts by more than 90%. Based on the identification of larvae from faecal cultures, the most predominant nematode species in the resistant population were Haemonchus (62%) and Trichostrongylus (28%). TBZ reduced faecal egg counts for both species by less than 90%. FBZ and LVM also reduced Haemonchus spp. eggs by less than 90%. Other nematode species numbers did not satisfy criteria for the determination of efficacy.

The detection of resistance to ivermectin by the faecal egg count reduction test

Abstract Extract Anthelmintic resistance in gastrointestinal nematode parasites of sheep and goats is usually detected in the field by means of the faecal egg count reduction (FECR) test (1) (2) (3) . However, a recent publication in this journal (4) has raised some doubts regarding the appropriateness of this procedure for detecting resistance to ivermectin by suggesting that treatment with this drug may affect the fecundity of surviving worms. Little evidence was provided to support this suggestion by, for example, counting eggs in uteri and, in my opinion, such concerns are neither justified by the results of this study, nor warranted by the weight of other existing evidence.

Simultaneous resistance to two or more broad-spectrum anthelmintics by gastrointestinal nematode parasites of sheep and goats

The detection, by means of faecal egg count reduction tests and larval cultures, of two cases of multiple anthelmintic resistance in goats and two in sheep, is reported. The former cases appeared to involve resistance to all three broad-spectrum drench groups (benzimidazole, levamisole, ivermectin) by mainly Ostertagia spp., the latter resistance to two of them (benzimidazole, levamisole) by Trichostrongylus spp.

The distribution and localization of the stage-specific GP31 antigen from infective Ostertagia circumcincta larvae

The 31,000 mol. wt glycoprotein (GP31) antigen of infective thirdstage (L3) Ostertagia circumcincta larvae was shown, by surface labelling experiments and immunofluorescent antibody staining of whole larvae and larval sections, to be distributed internally. When transverse sections of L3 O. circumcincta, taken from the anterior pharyngeal region, were further examined by electron microscopy, after immunogold staining with rabbit anti-GP31 antiserum, the GP31 antigen was found to be specifically located in ‘secretory organelles’ within the cells of the oesophageal glands. By in vitro culturing L3 O. circumcincta in medium supplemented with 35S-methionine and then analysing the excretory-secretory material released by the larvae, it was found that the GP31 molecule was one of the major components of the excretory-secretory complex. The purified GP31 molecule had no detectable proteolytic activity in protein degradation assays. On examination of Triton X-100 extracts of infective larvae from other nematode parasite species, a predominant antigen similar to GP31 was found in Trichostrongylus colubriformis and Haemonchus contortus, but in Toxocara canis a minor component corresponding in mol. wt to GP31 was also detected. Based on these results the possible role of GP31 as a candidate antigen for a broad spectrum molecular vaccine against gastrointestinal nematode parasites in sheep is discussed.

Spatial distribution on pasture of infective larvae of the gastro-intestinal nematode parasites of sheep

Boag B., Topham P.B. and Webster R. 1989. Spatial distribution on pasture of infective larvae of the gastro-intestinal nematode parasites of sheep. International Journal for Parasitology 19: 681–685. The horizontal distributions of infective larvae on pasture grazed by sheep have been investigated. Using Taylor's Power Law it was found that larvae had a more aggregated distribution in September than August, the Law index of aggregation being 1.97 and 1.89 for the 2 months, respectively. However, at each time the degree of aggregation remained fairly constant for a range of spacings between points from 5 to 30 m. These results suggest that Taylor's Power Law could be used as a basis for devising an efficient pasture sampling strategy. More data are required, however, to determine the extent to which aggregation of the larvae varies with time of the year.

Gastrointestinal nematode parasites and the stability and productivity of intensive ruminant grazing systems.

This paper uses mathematical models, describing the transmission dynamics of directly transmitted gastrointestinal nematode parasites of sheep and cattle, to examine the impact of these parasites on the stability and productivity of ruminant grazing systems. Current models of the ecology of grass growth under grazing, and the epidemiology of trichostrongylid nematode parasites of ruminants, are combined in a formulation that captures the general features of the plant - (ruminant) herbivore - parasite interaction. The simplest case, in which herbivore numbers are constant and not food limited (the norm for many agricultural systems) is considered in detail. The effect of gastrointestinal parasitism in reducing herbivore feeding rates is shown to act as a potential density-dependent constraint on the parasite's infection rate. The process is manifested in the model as a progressive linearization of the relation between herbivore feeding rate and plant density at the parasite equilibrium. This effect acts to stabilize the dynamics of the model grazing system and significantly affects its predictions about the impact of parasite control and the pattern of host productivity. Model predictions are discussed in the light of relevant field observations, and areas for future research are identified.

Pathophysiology of parasitic infections.

Parasites can have a wide range of pathophysiological effects on the host. This review describes those associated with some parasites of major importance in man and animals. Haemoprotozoan diseases such as trypanosomiasis and malaria are primarily associated with anaemia. Such anaemias have a complex aetiology involving various mechanisms responsible for red cell destruction as well as possible defects in red cell production. In addition to these haematological effects these diseases are associated with marked disturbances in heart function and the nervous, immune and urinary systems. The other major groups of parasitic diseases are those associated with the gastrointestinal tract. The most advanced studies have been conducted on the pathophysiology of gastrointestinal nematode parasites of sheep and have revealed significant effects on feed intake, gastrointestinal function, and protein and energy metabolism. Similar studies have yet to be conducted in other hosts and parasitic diseases. There is also a need to examine in greater detail the factors which can modulate pathophysiological responses by the host to parasitic infections.

Epidemiological studies on gastro-intestinal nematode parasites of sheep: the seasonal number of generations and succession of species

Monthly post mortem worm counts were carried out from June to September on grazing lambs, in association with faecal egg counts and pasture larval counts. Similar patterns were recorded by all three methods, and egg counts proved to be a reliable reflection of worm burden over this period. A succession of species was recorded, in particular Ostertagia spp and Nematodirus spp in June; Haemonchus contortus in July; Trichostrogylus virinus in August; T axei, T colubriformis and Cooperia curticei in September. A new explanation for this succession is given, based on the overwintering capabilities and freeliving development time of individual species.

Some stochastic extensions to a deterministic treatment of sheep parasite cycles

This article provides a stochastic version of a deterministic model for the life cycle of gastrointestinal nematode parasites of sheep. This cycle is conveniently divided into a part that involves the sheep and a part that involves the pasture. Each of these sections of the cycle is described by a compartmental model, and the two parts of the model are then integrated into a single stochastic model. The asymptotic behavior of the final model is found to be different from that of its deterministic analogue.

Models for The Distribution on Pasture of Infective Larvae of the Gastrointestinal Nematode Parasites of Sheep

Two models are proposed for the distribution on pasture of infective larvae of the gastrointestinal nematode parasites of sheep. These models were developed to include as many as possible of the known biological components. Procedures for estimating the parameters of the models are outlined and advantages of these models over earlier attempts to describe the distribution of infective larvae on pasture are briefly discussed.

A Note on the Estimation of Larval Concentrations on Pasture

In a recent paper by Tallis and Donald (1964)t [which will be referred to subsequently as (T.D.)] models were developed to describe the distribution on pasture of the infective larvae of gastrointestinal nematode parasites of sheep. It was pointed out that the distribution of faecal deposits, the number of eggs per deposit, and the developmental rate to the infective larval stage were three important components determining the total larval distribution. Moreover, formulae for the expected concentration of larvae on the pasture lead to straightforward methods of estimation which are free from large biases.