27 January 2023

Parasites of sheep and resistance to drenches – a New Zealand perspective

Dave Leathwick, AgResearch Grasslands, Tennent Drive, Palmerston North, New Zealand, recently presented his research parasites of sheep and resistance to drenches at the Teagasc National Sheep Conference 2023.

Anthelmintic (drench) resistance in nematode parasites is an issue facing farmers of grazing livestock (sheep, goats, cattle, deer, and horses) all over the world. It is unlikely that any country is immune to its advances. Resistance was first detected in the 1970s and 1980s not long after new modern drenches became available. Initially regarded as a curiosity and of little importance, resistance has continued to grow in prevalence and severity ever since. Today, in many countries, drench resistance on farms has become the norm and it is now rare to find farms where all drenches work against all the common parasite species. This is certainly the case in New Zealand where recent data shows that simultaneous resistance to all 3 of the older classes of broad-spectrum drenches, the benzimidazoles (e.g., oxfendazole), the imidazothiazole (levamisole), and macrocyclic lactones (e.g., ivermectin) is now present on about a third of sheep farms. Also, resistance in cattle is not far behind with resistance to all three of these drench classes now confirmed on a small number of properties.

So how did this situation become so bad without anyone doing anything about it? In reality, much has been done. Decades of research around the world has led to a reasonably good understanding of the factors which result in resistance developing (i.e., we know how to prevent it). There are also some good examples of where changes have been made to farming operations to minimise the development of resistance, and these have worked. Commercial farms exist today, on which all drenches still work as well as the day they first came to market. Clearly then, it is possible to farm without selecting for drench resistance, at least in some situations.

So how did we get into this mess? In New Zealand, at least, there are likely a small number of factors which have contributed to this widespread prevalence of severe drench resistance.

1. Farmers think they will ‘see’ resistance
2. A reluctance to spend money because of a lack of understanding of the ultimate cost of drench
3. Conflicting messages from different interest groups – in the presence of confusion just keep doing what worked in the past.
4. Aversion to risk.
5. Doing what you know how to do is always easier than learning to do something new.

Interestingly, none of these factors relate to not knowing what to do, they relate to making the decision to do something. They are about motivation and behaviour change.

Minimising resistance development

The sad part about the high levels of resistance we are seeing today is that science, in many countries over many years, has developed an understanding of how resistance is selected (we know most of the risky practices) and therefore how to delay or prevent resistance from becoming an issue. In New Zealand at least, these ‘resistance-delaying’ practices have been tested on real farms and shown to work; we still have some large, profitable, commercial farms with little or no resistance to any drenches.

1. Test that what you are using works

The first and most obvious practice that farmers should adopt to minimising resistance issues is to test the effectiveness of the drenches they are using and to do so on a regular basis (at least once every year). If you detect an issue early (as it is developing) you can do something about it reasonably easily. If you wait until the problem is advanced and resistant worms dominate on your farm, then fixing it becomes more difficult and costly.

Most farmers that I have talked to over many years think that everything is OK if their animals look to be in good condition. In reality, the subtle production losses will be considerable before the animals begin to look ‘wormy’ despite drenching. New Zealand trials showed that the immediate cost of using a drench that is not fully effective can be 10-15% of carcass value at slaughter. Another study showed that the amount of grass necessary to get lambs to target sale weight increased by 30% over a three-year period as drench efficacy declined due to resistance. In all these studies, the losses occurred before the farmers were able to ‘see’ that the lambs weren’t ‘doing’ as well as expected. For most farmers the cost of testing drench efficacy is miniscule compared to the potential losses of compromised growth rate. Unfortunately, many New Zealand farmers discover they have drench resistance by the presence of dead animals in the paddock.

2. Too much drench

There are a number of well recognized practices which have been shown clearly to select for resistant worms. These include excessive use of drenches (simply too many treatments), use of long-acting products (especially those with declining profiles of activity), and treatment coinciding with a shift to low contamination (clean) pastures such as hay or silage aftermath or new crops. Added to this is a failure to prevent importation of resistant worms with stock brought onto the farm. This latter has, I believe, become more important in New Zealand as multiple resistant worms have become more prevalent across the industry.

All these highly selective practices, except the last, have a common theme. They all give a large advantage to resistant worms while minimising the retention of susceptible worms on the farm. In scientific terms this pool of susceptible and unselected worms is known as ‘refugia’ – simply a refuge of susceptible worms. Very simply, if you have no drench-susceptible worms on your farm, then all you have is drench resistant ones (which obviously is not what you want). So, we have to think about (devise and implement) ways of retaining worms that we can kill with drenches – this we call ‘maintaining refugia’.

3. Retaining susceptible worms

Many people say that retaining susceptible worms (‘maintaining refugia’) is complicated and difficult. From my experience on commercial farms in New Zealand this is far from true. There is nearly always a way of maintaining some untreated animals in a mix or rotation with drenched animals (lambs) that will fit with the farmer’s operation. Some examples of such practices include,

• Running untreated ‘tail-end’ 2-tooth ewes (12-24 months of age) amongst drenched lambs on better pastures. The ewes benefit from the better pastures while cycling some susceptible worms to dilute any resistant worms passed by the drenched lambs.
• A farmer wanting to drench lambs onto pastures previously grazed for months by bulls (and therefore ‘clean’ of sheep worms) lambed some of his hoggets (12-month-old ewes) amongst the bulls prior to the lambs going on. Again, the hoggets benefit from the high-quality pastures and when the lambs are drenched onto these pastures after weaning there is a pool of unselected worm larvae to dilute any resistant eggs passed by the lambs.
• Rotational grazing of untreated adult ewes behind drenched Trial work we did showed that not only did the ewes cycle most of the same worm species as the lambs (but without exposure to drench) but that they reduced the number of worm larvae to which the lambs were exposed. Further, pasture quality was improved, and lamb growth rates were significantly higher when they were rotated with ewes compared to when they were grazed alone.

All these examples have demonstrated that ‘refugia’ can be included in a farming operation without compromising productivity (in fact often with increased profitability).

There are two approaches to leaving some animals undrenched to maintain refugia i.e., to focus on the ewes or on the lambs. By far the easiest is to minimise drenching of adult ewes and to make sure they are integrated / rotated in some way with the drenched lambs. Most New Zealand farmers who are managing resistance focus on stopping (or greatly reducing) any drenching of their ewes. Ensuring quantity and quality of feed, and hence good body condition, seems to be the key to making progress. Integrating cattle into rotations reduces the number of parasite larvae that ewes (and lambs) are exposed to. Adult ewes carry most of the same parasite species as lambs (the obvious exception is Nematodirus) and mostly they function as net removers of infective larvae i.e., they remove more larvae from the pasture than they put down. The immune system of the ewe reduces the percentage of parasite eggs which develop into infective larvae. So, rotating undrenched ewes with lambs has many benefits (including improved pasture quality).

Some farmers choose to leave some lambs undrenched but this is much less popular and is more difficult to manage without compromising growth rates. But this approach does fit some farming systems and it will work.

Close the borders

As resistance has become more common on farms, and therefore in livestock, so too has the risk of importing resistant worms increased. Today, any animal moved onto your farm comes with the very real possibility of carrying resistant worms (possibly of a type you don’t already have). In New Zealand today, importing resistant worms is possibly the most common way in which farmers acquire a significant resistance issue. The message about effective quarantine procedures has never been more relevant or important. All animals coming onto any farm (unless the source farm is known to be resistance free) should be treated with a new-active drench and if possible, held off pasture for at least 24 hours. To be sure, check that the quarantine treatment worked by egg counts 7-14 days after treatment. If the quarantine treatment wasn’t effective, the sooner you know, the better.

Take home messages

• Regularly test the effectiveness of the drenches you are using – by the time you ‘see’ a problem, it will have cost you plenty
• Don’t rely solely on drench – a drenching programme is not a parasite control plan
• ‘Think’ about how to retain some unselected worms on your farm – Refugia is the key, and it may be easier than you thought
• Don’t import someone else’s bad worms onto your farm – always apply an effective quarantine procedure.