Dr Stephen Love, State Coordinator - Internal parasites, I&D NSW Primary Industries, ArmidaleApril
For those with liver fluke on their farms, now is the time for the most important liver fluke drench of the year.
With above average seasons in many areas for the last 2 years or so, 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 epg of faeces) and some with spectacularly high counts e.g. over 2000 fluke epg in an alpaca.
About now (May) it is getting too cold (below 10°C overnight) for liver fluke eggs to develop, so now is a good time to strategically drench sheep with a highly efficient flukicide (i.e. Triclabendazole-based) to clean sheep out. Cattle producers also have the option of Nitromec® (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.
There are two other reasons the sheep may have some fluke come spring. One is that they picked up some Metacercariae over winter from pasture, and the other is resistance to the flukicide you used.
Yes, there are cases of liver fluke resistant to Triclabendazole and to Closantel, but 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.
In passing I should mention that Charles Sturt University at Wagga is trialling an immunodiagnostic test (an ELISA) that detects liver fluke antigen in faeces. They hope this will be useful as a fluke resistance test. If you are a cattle producer, and you are interested, contact your local Virbac rep. (Virbac is assisting CSU in this work.)