Anthelmintic resistance in cattle and sheep

Liver fluke can cause disease in both sheep and cattle but the consequences are much more serious for sheep

Not treating the parasites results in severe disease. File Picture. 

In general, sheep of all ages, not just lambs, are often treated with anthelmintics regularly and so the selection pressure for resistance is quite high each year.

In addition, the consequences of treatment failure due to anthelmintic resistance can be severe; in regions where the blood-sucking stomach worm, Haemonchus contortus, predominates, deaths through anaemia can be seen in both ewes and lambs.

A more cosmopolitan parasite, the liver fluke, can cause disease in both sheep and cattle, but the consequences of triclabendazole resistance are most keenly felt by the sheep industry as there are essentially no alternative treatments for acute, often fatal, disease, caused by juvenile fluke destroying the liver.

What is the current situation in cattle in Ireland?

The liver fluke is the same parasite whether it is in a sheep, a cow or any other mammal, thus resistance can result in ineffective treatments in cattle, but the consequences are typically less serious than in sheep and there are other flukicides that can kill adult liver fluke, which are the principal cause of losses in cattle.

The other worm in which resistance has been identified is a small intestinal worm with the scientific name of Cooperia oncophora or gutworm.

Cattle at pasture develop quite a solid immunity to this parasite within a few months, so infection is mostly seen in cattle in their first full grazing season. 

It is not a particularly harmful species in its own right, but in conjunction with the common stomach worm, Ostertagia ostertagi, it contributes to the disease of parasitic gastroenteritis, commonly known as worm scours, in young stock.

Because neither of the cattle parasites in which resistance has been confirmed in Ireland causes serious losses, there is an understandable level of complacency regarding the management and mitigation of anthelmintic resistance on dairy and beef farms.

But we should really be using these examples as an early warning system for something that could be much more serious, the obvious examples being anthelmintic resistance in the brown stomach worm, Ostertagia ostertagi, referred to above, or the lungworm, Dictyocaulus viviparus, the cause of hoose.

Inability to treat either of these parasites could result in severe disease, production losses and poor welfare.

Control of parasites 

It is pivotal in the control of parasites that farmers and vets know which parasites are present, what impact they are making and how well they are being controlled.

In addition to monitoring clinical signs and performance, particularly growth rates at pasture, there are a number of samples and tests that can be performed to get more precise diagnoses.

A common test involves collecting fresh dung samples and examining them for worm eggs or larvae. 

This can not only tell you which parasites are present, but can also be used to assess the efficacy of wormer treatments.

The sampling is quite easy as are some of the laboratory procedures, but the interpretation of the results requires expertise, and veterinary advice should be taken.

The Animal Health Ireland (AHI) website contains much useful advice on the control of parasites, effective use of anthelmintics, and resistance management, but some key elements can be reduced to three simple things to remember and do The ABC of responsible anthelmintic use:

• Accurate dosing; 

Weigh scale or weigh band to use on each animal. Calibrate dosing equipment before each use Beware of high frequency (every four weeks) worm treatments over long periods (six months or more) Choose the right wormer and integrate treatments with other management practices to help reduce the risk and impact of parasites on your farm.

• There are of course lots of other things that can be done to help manage anthelmintic resistance, but, arguably, if you don’t get these three basics right, then other efforts may be wasted.

• The emphasis on individual weighing of cattle is quite deliberate: there is only one way to accurately weigh an animal: scales. Several studies have shown that neither experienced veterinary practitioners nor farmers can consistently and accurately estimate weight by eye, with the average discrepancy being at least 10% (50kg in a 500kg animal) and frequently more.

Treatment 

As virtually all veterinary treatments are dosed as a certain amount of the product given per kg of live weight, then without weigh scales under-dosing or overdosing is inevitable, and this can have numerous untoward consequences, including selection for anthelmintic resistance.

Of course, regular weighing of youngstock is also a crucial management tool to ensure that growth targets are reached and can also indicate the need for wormer treatment in under-performing stock.

Understanding anthelmintic resistance and incorporating these routines into your stock management will help ensure that the wormers we have in cattle remain effective for as long as possible.

• Andy Forbes, former Chair of Animal Health Ireland Parasite Control Technical Working Group