Worm resistance to sheep drenches
Paul Hughes, veterinarian from Taihape Veterinary Services, recently conducted a worm resistance survey of farms in the central North Island. The results showed higher than expected levels of resistance including an alarming amount of emerging resistance to the macrocyclic lactone (ML) family of drenches.
The survey of drench resistance in the Taihape area was initiated after lambs on two farms were found to be suffering from severe clinical parasitism because of unexpected resistance to drenches.
A total of 36 farms in the Taihape district were tested between April and August 2004. Sheep to be tested were randomly put into one of two equal-sized groups and given either half the recommended dose of an ML drench (ivermectin) or a full dose of a benzimidazole drench (oxfendazole). Faecal egg counts and larval cultures were carried out at the time of treatment and 7-10 days later.
This is the first time we have used the half-dose ivermectin technique rather than using the full dose, Paul Hughes says. The half-dose is more sensitive, gives earlier detection of resistance and will show if resistance is emerging. A full-dose test could also be used but once there is full-dose resistance, nematode management options are reduced. Our follow-up tests on farms which failed the half-dose test have shown that full-dose resistance is also common.
Paul Hughes says that results of the survey indicated resistance to oxfendazole was present on 43% of farms and emerging resistance to ivermectin on a surprising 33% of properties.
We expected resistance to the benzimidole drench, but we did not expect the high level of emerging ivermectin resistance. This is of tremendous concern as, until now, resistance to this group was believed to be rare, with only a few cases being recorded.
There are only three groups, or families, of broad-spectrum drenches benzimidazoles, the ML group and levamisoles. Although not included in this survey, the evidence we have suggests that levamisole drenches would give similar results.
Overall the survey shows that there is a lot of established and emerging resistance on farms much more than we expected. Of particular concern, it shows that emerging resistance to the ML drench ivermectin is very common on farms in the Taihape district. It is likely to be the same throughout New Zealand. If so, it sends a strong warning to all farmers that vigilance is needed if they are to maintain effective worm control. Carrying on doing what has been done in the past is not going to work in the longer term, especially with a third of farms showing ML resistance.
The solution:
Every farm is different, and therefore needs a close look at what could be a complex problem. Different parasite species which are becoming resistant to different drench groups at different stages, and specialist help from a veterinarian is recommended.
Paul Hughes starts with a faecal egg count reduction test plus, larval cultures, although not cheap, need to be carried out to sort out what worm types are present, and just what their individual resistance situation is. For example, the survey found that resistance to benzimidazole drenches involved six worm species, but ML resistance was almost entirely restricted to Ostertagia and Cooperia. The Cooperia resistance was a particular surprise as ML resistance in this species had not previously been recorded.
A whole-farm control strategy based on the results should then be implemented before lamb growth is seriously affected by parasite infections, and income is reduced. Once an initial picture is gained, testing can be carried out every one to two years.
Paul Hughes says that it is now common for parasites on farms to be resistant to more than one drench group. To counter this, two types of combination drenches are now available: doubles, using either a benzimidazole and levamisole mix or an ivermectin and levamisole mix, and triple drenches which include all three drench families. Like all drenches, combination drenches must be used in conjunction with a whole-farm parasite management plan to reduce the risk and effect of parasite drench resistance.
From our experience, these combination drenches appear to be working well.
We recommend that farmers should use combination drenches and the sooner they use them, the better off they will be, because combination drenches can also delay the onset of resistance, he said,
The survey of drench resistance in the Taihape area was initiated after lambs on two farms were found to be suffering from severe clinical parasitism because of unexpected resistance to drenches.
A total of 36 farms in the Taihape district were tested between April and August 2004. Sheep to be tested were randomly put into one of two equal-sized groups and given either half the recommended dose of an ML drench (ivermectin) or a full dose of a benzimidazole drench (oxfendazole). Faecal egg counts and larval cultures were carried out at the time of treatment and 7-10 days later.
This is the first time we have used the half-dose ivermectin technique rather than using the full dose, Paul Hughes says. The half-dose is more sensitive, gives earlier detection of resistance and will show if resistance is emerging. A full-dose test could also be used but once there is full-dose resistance, nematode management options are reduced. Our follow-up tests on farms which failed the half-dose test have shown that full-dose resistance is also common.
Paul Hughes says that results of the survey indicated resistance to oxfendazole was present on 43% of farms and emerging resistance to ivermectin on a surprising 33% of properties.
We expected resistance to the benzimidole drench, but we did not expect the high level of emerging ivermectin resistance. This is of tremendous concern as, until now, resistance to this group was believed to be rare, with only a few cases being recorded.
There are only three groups, or families, of broad-spectrum drenches benzimidazoles, the ML group and levamisoles. Although not included in this survey, the evidence we have suggests that levamisole drenches would give similar results.
Overall the survey shows that there is a lot of established and emerging resistance on farms much more than we expected. Of particular concern, it shows that emerging resistance to the ML drench ivermectin is very common on farms in the Taihape district. It is likely to be the same throughout New Zealand. If so, it sends a strong warning to all farmers that vigilance is needed if they are to maintain effective worm control. Carrying on doing what has been done in the past is not going to work in the longer term, especially with a third of farms showing ML resistance.
The solution:
Every farm is different, and therefore needs a close look at what could be a complex problem. Different parasite species which are becoming resistant to different drench groups at different stages, and specialist help from a veterinarian is recommended.
Paul Hughes starts with a faecal egg count reduction test plus, larval cultures, although not cheap, need to be carried out to sort out what worm types are present, and just what their individual resistance situation is. For example, the survey found that resistance to benzimidazole drenches involved six worm species, but ML resistance was almost entirely restricted to Ostertagia and Cooperia. The Cooperia resistance was a particular surprise as ML resistance in this species had not previously been recorded.
A whole-farm control strategy based on the results should then be implemented before lamb growth is seriously affected by parasite infections, and income is reduced. Once an initial picture is gained, testing can be carried out every one to two years.
Paul Hughes says that it is now common for parasites on farms to be resistant to more than one drench group. To counter this, two types of combination drenches are now available: doubles, using either a benzimidazole and levamisole mix or an ivermectin and levamisole mix, and triple drenches which include all three drench families. Like all drenches, combination drenches must be used in conjunction with a whole-farm parasite management plan to reduce the risk and effect of parasite drench resistance.
From our experience, these combination drenches appear to be working well.
We recommend that farmers should use combination drenches and the sooner they use them, the better off they will be, because combination drenches can also delay the onset of resistance, he said,