Haemonchus contortus resistance to ivermectin and netobimin in Brazilian sheep

Anthelmintic efficacy of trichlorfon and blood parameters of young lambs infected with Haemonchus contortus

In this study we evaluated the efficacy of trichlorfon against Haemonchus contortus, monitoring its influence on blood parameters and plasma enzymes of lambs with haemonchosis. A lamb group was orally treated with trichlorfon at 100 mg kg^-1 while the other group was untreated. Split-plot design analysis was performed with the lamb groups defined as plots while the subplots were the four periods (weeks) of collection. The trichlorfon treatment promoted a significant and effective reduction of fecal egg counts after one week, with efficacies > 99%. After 21 days of treatment, detected blood parameters and serum levels of plasma enzymes were normal. Additionally, serum albumin and urea concentrations increased to normal values, which were not observed in untreated lambs. The treatment with this organophosphate, using a correct oral administration, may represent an effective therapeutic alternative for sheep infected with multi resistant strain of H. contortus.

Overview of anthelmintic resistance of gastrointestinal nematodes of small ruminants in Brazil

Abstract Frequent and inappropriate use of all classes of antiparasitic drugs in small ruminants has led to failures in their effectiveness, culminating in a global problem of anthelmintic resistance. Brazil stands out as one of the world’s leaders in publications about anthelmintic resistance, and for having the most numerous reports of this resistance in small ruminants in the Americas. These studies have involved mainly the fecal egg count reduction test (FECRT) and its correlation with field management practices. In vivoeffectiveness testing is conducted in areas where livestock is of greater economic significance, e.g., in the South (sheep) and Northeast (goats), or is important for research and economic centers, such as the Southeast (sheep). The most widely studied species is sheep, for which the widest range of drugs is also evaluated. Despite significant advances achieved in molecular research, laboratory analyses should include knowledge about the reality in the field so that they can become feasible for the producer. Moreover, molecular studies can be underpinned by the analysis of field studies, such as the maintenance of antiparasitic effectiveness over time and the mechanisms involved in this process.

Research and implementation of novel approaches for the control of nematode parasites in Latin America and the Caribbean: is there sufficient incentive for a greater extension effort?

The widespread presence of anthelmintic resistant gastrointestinal parasitic nematodes in outdoor ruminant production systems has driven the need to identify and develop novel approaches for the control of helminths with the intention to reduce the dependence on commercial anthelmintic drugs. This paper identifies what has been done in Latin America (LA) in terms of estimating the prevalence of anthelmintic resistance (AR) in ruminant production systems and the application of different novel approaches for the control of helminths in those systems, including research and extension activities. Firstly, the paucity of knowledge of AR is discussed in the context of different countries, as well as, the available economic resources for research, the technical infrastructure available and the practical difficulties of the production systems. It is then proposed that the search for novel approaches is not only driven by AR but also by the need for techniques that are feasible for application by resource-poor farmers in non-commercial subsistence farming systems. However, the commercial benefits of these approaches are often limited and so are funding inputs in most countries. The workers participating in the research into different novel approaches are identified as well as the different methods being studied in the different areas of LA according to their published results. In addition, the difficulties experienced during extension efforts to reach farmers and help them to adopt novel approaches for the control of parasitic nematodes in LA are discussed. The role of regulatory authorities in these countries is discussed as some methods of control might need an official confirmation of their efficacy as well as authorization prior to application as they may affect animal products (i.e. residues) and/or impose a hazard for animal welfare. The role of the pharmaceutical companies is also discussed.

Anthelmintic action of eprinomectin in lactating Anglo-Nubian goats in Brazil

Eprinomectin is the only avermectin approved for use to control gastrointestinal nematodes in lactating cows. Some studies in the USA and Europe have also demonstrated that this drug is highly effective in small ruminants. The aim of the present work was to evaluate the anthelmintic efficacy of pour-on eprinomectin in Anglo-Nubian goats at the end of lactation. Twenty-four goats were used, naturally infected with gastrointestinal nematodes, and divided into two groups: control and treated with eprinomectin (Eprinex, Merial, pour-on 0.5%) at a dose of 850 microg/kg. Counts were made of the eggs per gram (EPG) of feces, along with coprocultures, on days -7, 0, 4, 8, 11, 15, 18, 22, 25 and 29. The milk production of each group was recorded throughout the experiment. The coprocultures detected 98% Haemonchus contortus and 2% Oesophagostomum. There was no statistically significant difference (P > 0.05) in daily milk output between the two groups. Eprinomectin at the tested dosage was not effective (P > 0.05) in reducing the EPG. Positive results would serve as basis for use of an avermectin without residues in dairy goats, providing a scientific basis for greater food safety.

Anthelmintic treatment and the suppression of egg production in gastro-intestinal nematodes of sheep and cattle: fact or fallacy?

Evidence for a temporary suppression of egg production in gastro-intestinal nematodes of sheep and cattle following anthelmintic treatment is reviewed and the possible relevance of this phenomenon to the appropriate sampling time in the faecal egg count reduction test is discussed. The results of this review suggest that if such suppressive effects do occur in sheep nematodes, then they are unlikely to be of much practical significance and that little benefit would be derived from extending the post-treatment sampling interval beyond 10 days. For cattle nematodes, however, the issues are less readily obvious but indicate, at least when testing for resistance to persistent anthelmintics, that adoption of a longer post-treatment interval might be advisable.

EFEITO DE CARICA PAPAYA L. (CARICACEAE) E MUSA PARADISIACA LINN. (MUSACEAE) SOBRE O DESENVOLVIMENTO DE OVOS DE NEMATÓDEOS GASTRINTESTINAIS DE OVINOS

RESUMO O presente trabalho avalia in vitro a ação do extrato aquoso e de óleo de sementes maduras de Carica papaya e extrato etanólico e látex puro de Musa paradisiaca sobre o desenvolvimento de ovos de nematódeos gastrintestinais de ovinos. Os testes foram realizados incubando-se 200 µL dos extratos e igual volume da suspensão de ovos, sendo realizadas leituras após 48 e 72h. Os resultados obtidos evidenciaram uma baixa atividade anti-helmíntica dos produtos testados.

Moxidectin: persistence and efficacy against drug-resistant Ostertagia circumcincta

In order to determine whether the efficacy of moxidectin against Ostertagia circumcincta is enhanced by its persistency, therapeutic efficacy was compared at intervals after treatment and with that of ivermectin, a closely related but more transient endectocide. Groups of 7-month-old New Zealand Romney lambs were infected with a strain of O. circumcincta known to be resistant to moxidectin. At patency of the infections, groups of lambs were treated with either moxidectin or ivermectin at the manufacturer's recommended dosages, or left untreated. At 3, 6 and 10 days post-treatment, faecal egg count was measured and groups of lambs were slaughtered for estimation of adult worm burden. Drug-resistant worm burdens were significantly reduced in those animals treated with moxidectin but not in those treated with ivermectin. No effect of time of slaughter on worm burden was observed with either drug, demonstrating that the higher therapeutic efficacy of moxidectin against this parasite was not due to an increased period of drug exposure. Faecal egg counts in the moxidectin treated animals increased with time after treatment indicating a temporary suppression of egg output by surviving worms. The implications of these findings on selection for anthelmintic resistance are discussed.

In vitro screening of Haemonchus contortus third stage larvae for ivermectin resistance

The indiscriminate repeated use of ivermectin has resulted in the emergence of ivermectin resistant field strains of Haemonchus contortus in sheep in South Africa. There is a need for a rapid and cost effective in vitro test to detect such strains. Infective H contortus larvae known to be resistant to ivermectin, were incubated in vitro in a diluted concentration of the drug. Subsequently their ability to migrate was compared with that of larvae of isolates known to be susceptible to ivermectin. The criterion used to determine resistance was the ability of the larvae to migrate out of gelled agar after exposure to ivermectin using diluted propylene glycol as the control compound. The results obtained by this technique confirmed the results obtained with critical controlled tests; significant differences between the ivermectin sensitive and resistant isolates were demonstrated, thereby confirming that this in vitro technique may be used as a screening method to identify ivermectin resistant isolates of H. contortus.

Moxidectin against ivermectin-resistant nematodes--a global view

Macrocyclic lactone endectocides include two chemically distinct compounds moxidectin, a milbemycin, and ivermectin, an avermectin. The significance of the chemical differences between these compounds in relation to nematode resistance remains to be established. Reported studies indicate that moxidectin at the recommended dose rate of 0.2 mg/kg controls identified strains of nematodes, isolated from sheep and goats, with demonstrated resistance to ivermectin. This reflects the significantly greater potency of moxidectin against the 3 genera of nematodes most commonly involved in anthelmintic resistance, Haemonchus, Ostertagia and Trichostrongylus. Moxidectin, in recommended strategic treatment programmes, should reduce the risk of further development of resistance to the macrocylic lactone endectocides.

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.