Course: Livestock pests | Last Updates: 12th October 2015
Farm control of internal parasites in cattle has changed little over the past 30 years, but a rise in the frequency of drug-resistant worm strains means a new approach is urgently needed. The sheep industry has already suffered from high levels of anthelmintic resistance, and unless cattle producers act now, they could face similar challenges. Now is the ideal time to adopt a more holistic, sustainable approach to worm control in cattle.
Which worms are the most problematic?
There are two main types of roundworm affecting the gut in UK cattle: Ostertagia ostertagi and Cooperia species. Ostertagia ostertagi is associated with the greatest clinical damage, affecting the abomasum, while Cooperia resides in the small intestine and is usually less pathogenic. Previous research indicates Cooperia is also the most likely species to develop anthelmintic resistance, so it could become increasingly important in the future.
Both types of roundworm cause diarrhoea, weight loss and reduced productivity. They can be diagnosed by carrying out faecal egg counts, or in the case of Ostertagi, a pepsinogen blood test.
A potentially more devastating parasite is lungworm, or Dictyocaulus viviparus, which causes parasitic bronchitis – now recognised as the most common respiratory disease of adult cattle in the UK. The parasite can cause severe damage to the lungs, resulting in coughing and respiratory distress, and can prove fatal within two or three weeks of infection. Larvae can be identified in dung via a Baermann test – faecal egg counts are not so informative in this case.
How does infection occur?
Both roundworms and lungworms prefer warm, moist conditions, and with changing weather patterns, outbreaks are now occurring almost nationwide.
Roundworm eggs are passed in the faeces, developing to the infective stage in two to three weeks at the height of summer. In moist conditions, the larvae migrate from dung onto pasture and are ingested by grazing cattle. They then develop into adult worms, over a period of about three weeks, which go on to lay more eggs.
Lungworms develop slightly differently, travelling from the intestines via the lymph and blood to mature in the respiratory system (see diagram). Under optimal conditions, infective larvae can develop in faeces in as short as five days – significantly faster than the roundworm life cycle.
Young calves in their first grazing season are most at risk of contracting large burdens of worms, as immunity tends to develop after some exposure to the parasites. Pasture infection levels tend to increase as the weather gets warmer, usually reaching a peak from July onwards.
How can you control the parasites?
Traditionally, farmers wormed all their stock at the start of the season to prevent eggs contaminating pasture later in the grazing season. While this proved to be highly effective in controlling roundworms, it has resulted in the development of anthelmintic-resistant strains, which are harder to control. Continuing this practice will therefore increase the proportions of resistant parasites in the future.
There are three different types of broad spectrum anthelmintic – white drenches (Group 1, or BZ types), yellow drenches (Group 2 or LV types), and clear drenches (Group 3 or ML types). Each acts in a different way, with Group 3 having the longest persistence and greatest market share.
They are typically applied orally, as a drench or bolus; or as a pour-on or subcutaneous injection. Problems arise when dosing is incorrect – it is essential that animals are weighed, and the correct dose measured and administered according to manufacturers’ guidelines. Pour-on wormers can be particularly vulnerable to misuse, as they can be licked off by other stock, or washed off in wet weather. Under-dosing not only limits the efficacy of the treatment, it also increases the likelihood of resistant worms.
Why does resistance develop?
The theory of natural selection means any parasites that survive a dose of wormer are likely to produce offspring that have resistance to that product. The more the same wormer is used, the greater the frequency of resistant worms in a population. Without appropriate quarantine measures, these worms will be transferred among populations through animal movements.
Many producers believe they are alternating different products, but are simply changing brands. Many anthelmintics with the same active ingredients, and within the same group type, are marketed under different brand names. So it is essential farmers choose a wormer that they know what the active ingredient is.
How can you control parasites without developing resistance?
To avoid problems in future, farmers need to adopt a more holistic approach to worm control, using more targeted treatments in conjunction with pasture rotation. It is critical to understand the type of parasite you want to control, its life cycle, and the risk levels of different pastures and cattle groups. Of course, individual farms will face different parasite risks, so it is important to draw up a control strategy with your vet.
Youngstock are most at risk of high worm burdens, but do require some exposure to worms to build up immunity. Older cattle are less likely to suffer, and, in some cases, can act as “hoovers”, cleaning up contaminated pasture. Ideally, graze youngstock on cleaner pasture with no recent history of disease outbreaks, and older stock on more contaminated leys.
Try to target anthelmintic treatments to animals that most need it – use faecal egg counts or the other tests mentioned above. Rotate the type of wormer, and always ensure dosing levels and techniques are correct. Where lungworm is a risk, vaccination of all young or new stock twice before turnout each year is the safest form of control.
To limit drug-resistant parasites, it is important to retain a sub-population of drug-susceptible worms. This is obtained by reducing reliance on anthelmintics. Avoid blanket-worming all stock and moving them to clean pasture, where the progeny of any worms surviving treatment will make up the majority of the next generation. Keep animals on the same pasture after treatment to reduce contamination, while also limiting the risk of drug-resistant worms. Testing efficacy of the drugs with your vet, using before- and after-treatment dung samples, is also helpful.
In a recent trial, four out of five herds were found to carry worms that showed signs of resistance to a common broad-spectrum anthelmintic. By adopting more sustainable control strategies, the industry can protect against further development of this worrying trend.
- Identify risk factors and draw up a parasite control strategy with your vet
- Ensure accurate dosing levels and best practice administration techniques
- Always check the active ingredient when rotating wormer products.
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