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Getting worms to squirm

THE North Coast Livestock Health and Pest Authority has been working closely with beef producers in the region about how to maximise productivity by controlling internal parasites and avoiding the development of resistance.

In Australia, and especially on the North Coast of NSW, resistance has arisen to all the major families of broad spectrum anthelmintics including benzimidazoles (or BZs); closantel; levamisoles and "mectins" which include ivermectin, doramectin and moxidectin.

Because there's currently no effective alternative to the chemical control of parasitic helminths, resistance to treatments threatens both agricultural income and animal welfare of our regional beef producers, and we need to prevent resistance from developing.

In fact, worms have become public enemy No 1 in the region, accounting for more cattle deaths than any other single issue, and the developing drench resistance is not assisting this situation.

Over the last several thousand years our relationship with animals and the way we manage them has changed dramatically, which in turn means that our relationship with parasites has had to change dramatically as well.

Think of the great migrations you see in Africa, where millions of wildebeest - which are really very similar anatomically and physiologically to our domestic beef cattle - are constantly on the move.

As a mass unit they eat the grass in front of them, poo on the plain behind them, move on, and don't come back until the following season, by which time, their manure has long since dried up and been carried away by the wind and dung beetles, and any parasite eggs have long since died.

We keep our cattle fenced in, which means their dung doesn't get the chance to be cleaned away by natural systems before those same cattle are grazing on grass that has had ample opportunity to be infected by parasite eggs.

Treating cattle against worms is simple in principle, but the way the various chemicals work can actually be quite complex and the downside to this is the development of resistance in worm populations to the popular chemicals available to combat the problem.

We have some great weapons to use against them - anthelmintic drenches - which have worked that effectively up until now, but many users have become complacent in the way they use them.

But the worms have definitely got the numbers on their side and, like any enemy, over time they've been learning how to avoid our attacks.

By developing resistance, they are learning how to avoid our weapons and, alarmingly, we don't have any secret weapon up our sleeves to fall back once resistance develops.

That means the strategy needs to change.

We need to maintain the efficacy of our drenches, and minimise practices that encourage the emergence and spread of resistance.

We should not use and reuse the same drench repeatedly with our herd.

There's a family of drenches called "mectins", which include anything that ends in the suffix "-ectin".

These drenches are cheap, so they tend to be the first reached for. But farmers need to be aware that overuse of these will, in the long run, give the worms the upper hand.

A far better approach is to use combination drenches - each animal gets treated with two or more drench families at once.

Combination drenches are one of the best ways of preventing the development of resistance. Farmers may also find that because the worms are getting hit from two different directions, their animals just seem to respond better to drenching.

This approach becomes less effective after resistance to one component of the mixture develops, though, so it's important to begin combination drenching before resistance to any chemical group develops.

Farmers should also understand the difference between preventative drenching and curative drenching. Curative drenching is when the farmer recognises a worm problem and then brings the affected animals in for treatment.

But there will almost inevitably be a much greater number of "subclinical" animals beneath the surface that are costing you money.

Preventative drenching is a better option and all weaned cattle less than 20 months old on the North Coast should be given a preventative drench every three to four months.

When introducing new animals to a farm or herd, quarantine treatments should be applied to the incoming animals before they are introduced to the farm or herd.

Finally, by selecting for genetic immunity in the host - by careful selection of bulls and replacement heifers, the impact of parasites can also be minimised.

Topics:  animal health lhpa livestock