Controlling internal parasites in grazing cattle has a signiﬁcant positive return on investment; in most cases greater than any other management practice. However, there is increasing concern about resistance of cattle parasites to dewormers (anthelmintic resistance) and the ability of cattlemen to continue to have cost-eﬀective parasite control. Worldwide there is documented anthelmintic resistance to all commercially available products in all of the important cattle parasites. But the extent of the resistance varies from country to country and from ranch to ranch. Anthelmintic resistance is a very complex and serious issue. Cattlemen need a basic under- standing of parasite biology and control measures so they can work with their veterinarian to develop parasite control programs that balance the short-term economic beneﬁts of deworming with the long-term impact of anthelmintic use on resistance.
Adult parasites live in the gastrointestinal tract of cattle and lay eggs that are shed in manure. These eggs hatch and develop into infective larvae. These larvae crawl onto the grass and are ingested when cattle graze. The larvae then develop into adults in the gastrointestinal tract and lay more eggs.
Many factors, including rainfall, environmental temperatures, pasture type, grazing management, age and immune status of animals, previous product use and anthelmintic resistance patterns all determine the severity of parasite problems on an individual ranch in a given year. A unique parasite control program must be developed for each. The parasite load in the animal and on the pasture must both be considered.
Ostertagia ostertagi is an important cattle parasite in Louisiana. It can impact both young and mature animals. Ostertagia likes cool weather, and infective larvae do not survive well in the heat of Louisiana summers.
Cooperia sp. and Haemonchus placei are common parasites of calves. They are warm-season parasites, so large numbers build up in summer months. Cattle usually develop immunity to these parasites by the time they are yearlings, but adult cattle will continue to have low numbers that will contaminate pastures for calves.
Anthelmintic resistance is an inevitable consequence of the use of anthelmintics over time. Resistant parasites have genes that protect them from the eﬀects of the anthelmintic. The parasites may be re- sistant to one or multiple products at the same time. Ranches can also acquire anthelmintic-resistant parasites with herd additions.
Anthelmintic resistance is usually suspected when a deworming fails to give the expected production responses. But poor performance or clinical signs of parasitism that don’t improve following deworm- ing should not immediately be interpreted as a failure of the product. Other factors to check include:
Proper product storage
Proper product administration including dose and route
One of the key concepts in slowing down the development of resistance is the maintenance of refugia. Parasites in refugia do not have genes for anthelmintic resistance – they are still susceptible to anthelmintics. The more refugia in a population, the more the resistance genes in a population are diluted and the more eﬀective anthelmintics will be.
Parasites in refugia can be on pasture or in animals. When an entire group of cattle is dewormed, we eliminate refugia in the animals. The only parasites that survive the deworming are the few that are resistant. These resistant parasites then mate and multiply and soon take over. Eventually, there is failure of the dewormer to work as expected.
The overall level of infective larval contamination of pasture is inﬂuenced by pasture management practices and environmental pressures. These inﬂuences will impact larvae from both resistant parasites and refugia.
Examples of diﬀering levels of pasture contamination:
The quickest way to get widespread anthelmintic resistance is to deworm an entire group of cattle and then put them on a clean pasture. In this way we have no refugia left on pasture and we eliminate refugia in the animals. The only parasites left in the animals are resistant. When they reproduce, they will contaminate the pasture with an almost pure population of resistant parasites. There are no refugia on pasture to dilute the resistant worms.
Another common scenario for development of resistance is grazing stocker calves on permanent pastures combined with frequent use of dewormers. This is especially true of the macrocyclic lactones that may have a residual eﬀect for weeks. Initially there is a mix of refugia and resistant parasite larvae on the pasture, but as the calves graze day after day, the refugia larvae are killed by the residual product, and only the resistant parasites survive. Egg shedding is then only by resistant parasites, which eventually shifts the population to mostly resistant parasites both in the animals and on the pastures.
The above situation can be avoided by trying not to eliminate all parasites on a ranch. Cattlemen should work with their veterinarian to ﬁnd a balance between keeping overall parasite levels low enough to prevent economic losses while at the same time retaining some refugia to slow the progression of anthelmintic resistance.
With the emergence of resistance a “deworming schedule” is no longer appropriate.Designing a parasite control program must be done on each individual operation based on diagnostic testing results.There are two types of tests that should be performed: