"The message is that four of the five ‘mectins’ are toxic to dung beetles, and moxidectin is the dung-beetle-friendly mectin. This is the wormer present in Equest. Equest-Plus contains Praziquantel which kills tape worm. We do not know how Praziquantel affects dung beetles. The Adelaide Uni Vet at Roseworthy (Michael Cathcart) assures me that once a year for tapeworm worm is quite sufficient.
Also only 1 or 2 horses per 10 or so are what is termed ‘shedders’. They carry the highest worm burdens and are largely responsible for the infective stages on the pastures and for the worm burdens in a mob of horses. The others, due to challenge from their relatively small worm burdens, will retain an acceptable level of natural resistance. It is most likely that these animals never need to be treated.
The best practice is to identify the shedders and treat them only. This will remove the worms from the shedders leaving the majority of the mob with low but acceptable worm burdens. These low worm burdens will provide the immunological stimulus that will boost natural immunity in the horses and so control worms naturally. This has the added, but very important, benefit of reducing very substantially the evolutionary pressure that selects for resistance to chemicals amongst parasitic worms. We must preserve our chemical for the future by stopping the development of resistance.
Possibly the worst thing is to treat the whole mob regularly as promoted by the manufacturers of worm pastes. When this occurs, worm burdens are very low or absent and natural immunity wanes in the whole mob, which then becomes susceptible to severe infestation (due to the lack of natural immunity), and so the cycle of chemical dependence perpetuates itself.
Of course, one of the main impediments to this is “ how do we identify shedders?”.
Faecal worm counts are the obvious best way, but at present the Postmaster General will not allow worm infected dung to be sent to Roseworthy by post, and so it needs to be delivered, fresh. Quite a problem for most horse owners.
The other, cheaper, way to id shedders is to assume that those in poor condition are the shedders: probably not far from the truth. So these can be treated and their ‘condition’ will probably improve, suggesting a link between apparent condition and worm burdens. "
- Dr Bernard Doube
if you would like more information about this, email Dr Doube at: email@example.com