Steve Love, State Coordinator – Internal Parasites, IandD NSW Primary Industries, Armidale, comments on the liver fluke situation:
For those with liver fluke on their farms, now is the time for the most important liver fluke drench of the year.
With the past good seasons, liver fluke (Fasciola hepatica) numbers have built up. There have already been reports of liver fluke disease in sheep and other livestock, some with low liver fluke egg counts (typically less than 100 eggs per gram (epg) of faeces) and some with spectacularly high counts e.g. over 2000 fluke epg.
About now (May) it is getting too cold (below 10 degrees over night) for liver fluke eggs to develop, so now is a good time to strategically drench sheep with a highly efficient flukicide (ie triclabendazole-based) to clean sheep out.
Cattle producers also have the option of Nitromec(R) (ivermectin+clorsulon+nitroxynil), another highly efficient flukicide.
Remember however that, just like barber's pole worm (Haemonchus) in sheep, the infective stages of fluke (metacercariae),which were produced in summer and autumn, will survive over winter, especially if moisture is present. So, as with barber's pole worm, it may be too cold for the eggs over winter, but the infective stages produced in autumn will survive, albeit in declining numbers, through to spring. In the case of fluke, these infective larval stages are found in wetter areas on a farm, where the intermediate host snails are found.
Come spring, sheep could still have some liver fluke in them, even if they were treated with a highly efficient flukicide in May. Firstly, even the best flukicides do not kill every single fluke. In Australia 'effective' in the case of flukicides is defined as 90% kill or better. (The yardstick for broad-spectrum drenches is better than a 95% kill of target roundworms). Also, some of the very young fluke (harder to kill) will be left behind after the May drench. By Spring these will be adult flukes, living in the bile ducts, munching away, producing eggs, causing protein and blood loss, leading to clinical signs similar to haemonchosis ('bottlejaw', anaemia), but with the possible addition of jaundice as well.
The are two other reasons the sheep may have some fluke come Spring. One is that they picked up some metacercariae over winter from pasture. The other is that you have resistance to the flukicide you used.
Yes, there are cases of liver fluke resistant to triclabendazole and to closantel, although as yet we really do not have a good handle on how common this is.
If you want to check how well your triclabendazole drench worked, do a fluke egg count 28 days after they were drenched (and preferably a count on the day of drenching as well). The reason for waiting 28 days is that it can take this long for all the eggs left inside the liver (the bile ducts) to clear from the system.