Last month we took a look at the two new sheep drenches that have hit the market this year, Zolvix (Novartis) and Startect (Pfizer).
Both are significantly more expensive than existing combination drenches or endectocides. However they are now the products of choice for quarantine drenching, as triple combination resistance, while not commonplace, has certainly been diagnosed on a number of North Island breeding properties, and the number with undiagnosed resistance is likely to be much higher.
Zolvix
Zolvix (monepantel) was the pick of the bunch from over 600 compounds from a drug family now known as the Amino Acetylnitrile Derivatives (AADs). In a first for drench chemicals, the scientists at Novartis have done a whole lot of work to determine exactly how the monepantel molecule exerts its effect on worms. They have shown that:
- Monepantel works in a completely different way from any of the other drench chemicals
- It works at a site in the cell wall that is unique to parasitic nematodes, so the drug has minimal impact on non target species (e.g. earthworms) and is very safe for sheep, even if inadvertently overdosed
Other things to know about Zolvix:
- It is only a single active compound, not a combination. As such it is a poor choice for regular, routine drenching; for example the monthly drenching of lambs
- Monepantel is not ovicidal (does not kill worm eggs)
Thus when Zolvix is used as a quarantine drench, although all existing adult worms are likely to be killed, the eggs they have already laid will still be hatchable, and may take up to three days to pass out of the animal, though the majority will probably be gone by 24 hours. This makes it very important to have a properly thought-out quarantine procedure on farms where large numbers of lambs are purchased each year. Talk to us if you'd like help with this.
Startect
Startect (derquantel/abamectin combination) is the latest offering from Pfizer. The derquantel molecule has been around for some years but had not gained favour in early testing, until Pfizer looked at it again and realised it had potential, especially in combination.
Derquantel works by interfering with nerve transmission in the worm, causing flaccid paralysis and death. Again, the mode of action is unique. It doesn't have the same safety profile as monepantel, and is highly toxic to horses.
But the fact that derquantel has been combined with abamectin gives it a much wider range of uses than Zolvix, because of the resistance-delaying properties of combinations.
We know that the effective life of combinations is greatly prolonged where there is minimal or no resistance to the individual actives. So at the beginning, this will always be true for the derquantel component, but may not be so for abamectin.
On farms where abamectin is failing badly, especially against Ostertagia, Startect is not a good choice for routine drenching, but where abamectin is just starting to fail (very commonly seen in our local testing), using it in combination with derquantel is likely to be highly protective.
Modelling work by Dave Leathwick at AgResearch has recently predicted a period of more than 20 years' good protection of abamectin by derquantel where there was already moderate resistance to abamectin.
But while the profitability of sheep farming remains depressed and the price of Startect is around three times that of a triple combination, I can't see it being widely used for routine drenching, despite its theoretical advantages.
However, as already mentioned, either of these new drenches represents a more logical choice for quarantine drenching given the increasing prevalence of resistance to triple combinations.
Next month - exit drenching
