Can Parasite Control be More Effective and Reduce Resistance at the Same Time?

May 31, 2013 05:30 AM
 
Can Parasite Control be More Effective and Reduce Resistance at the Same Time?

By Dave Wolfgang, Field Studies Director, Penn State College of Agriculture Sciences

Dairy producers are encouraged to design a parasite control program with their veterinarian. This article presents some elements that might be considered in crafting a more effective program.

Internal parasites continue to be one of the largest problems that plague the livestock industries. Economic costs due to parasitism vary with animal age, stage of growth, degree of exposure, and level of nutrition. Various estimates have put the cost of internal parasites at nearly $2 billion per year. While parasites are ubiquitous, their impact on various dairy cattle groups varies greatly due to management, nutrition, genetics, and rates of exposure.


A good plan will help keep therapy effective, while at the same time reducing the risk of development of resistant parasite strains.


In the NE regions of the US, the major time for cattle to acquire new infections is just ahead. Therefore, it is a good time for producers and veterinarians to consider how to make parasite control programs not only effective, but to design control programs to minimize resistance as well. The cattle industry has been blessed over the past 40 years by the introduction of many very effective pharmaceuticals that made parasite control much easier. With these products high productivity and low parasite burdens could be accomplished via a number of convenient treatment options. It became easy for many people to consider parasite control as just a treatment issue. This allowed many producers and veterinarians to spend relatively little time developing a strategy or whole farm approach. A whole farm approach would include the life cycle of the parasite, the immunity or resistance of an animal or animal groups, management of the environment (pasture), and the long term concerns for development of resistance. A good plan will help keep therapy effective, while at the same time reducing the risk of development of resistant parasite strains. Resistant strains of parasites are a major concern in South Africa, New Zealand, and Australia, and they are an emerging problem in the southern US. Serious and hard to manage resistance problems have been seen in sheep, goats, horses, and cattle. Utilizing a more carefully thought out strategy will prevent or greatly delay this problem from affecting the dairy industry.

Heavily parasitized animals, primarily young cattle on pasture, can have severe health impacts. Broad spectrum anthelminthics have been a boon to animal health and productivity through effective removal of parasites. However, the indiscriminate use and improper dosing of these broad spectrum compounds has created emerging problems with resistant parasite strains. Strategic worming programs do not advocate zero care and allowing animals to suffer slowly or lose productivity gains simply to reduce the risk of developing resistance. For animal health, welfare, and profitability, animal caretakers have several legitimate reasons to minimize disease and suffering by parasites. Severely infested animals or groups need to be treated appropriately. Hopefully with a well designed control plan these sorts of cases will be few and far in between.

Much has been written in the last few years about the need to preserve some susceptible strains in the environment (refugia). No treatment strategy is 100% effective and a few parasites always remain. The goal of parasite control programs should not be 100% elimination of all parasites, rather it should be cost effective control that promotes healthy animals. Treatment strategies that do not match the life cycle of the parasite with the burden of parasites in the animal or on the pasture can actually select for more resistance genes. Conversely, just a few adjustments in treatment strategy can allow a mixing of good animal productivity along with the preservation of some susceptible strains. Allowing a greater proportion of the parasite population to remain susceptible dilutes the resistance genes (click here for a very reader-friendly summary from the FDA Center for Veterinary Medicine).

Dairy producers are encouraged to design a parasite control program with their veterinarian. Here are some elements that might be considered in crafting a more effective parasite program.

  1. Routinely use fecal egg counts (FEC; e.g., McMasters test for Fecal Egg Count) to determine which, if any, groups of animals are actually infected. Some groups may have little or no parasite load and do not need to be treated. A few heavy shedders can put most of the eggs into the environment.
  2. When it is determined that a group of animals needs to be treated, treat only one half of the group at a time. Treat the second half of the group 1 to 2 weeks later. This strategy allows more susceptible genes to remain in the parasite population.
  3. Management strategies can be employed to minimize infective parasite larvae accumulation on pasture (e.g., pasture rotation, making hay, maintaining a grass height over 1 inch, and providing plenty of alternate feeds when grass becomes limited).
  4. Do not treat and then move onto a ‘clean’ pasture. This strategy promotes more or only resistant parasite strains on the new pasture. Treat and allow animals to remain on the same pasture for a while before rotating pastures.
  5. Make sure that animals are dosed at rates appropriate for size. Correct mg/kg matters. Under dosing leads to resistance. Over dosing can lead to toxicity or residue concerns.
  6. Limit manure application on pastures that will be grazed in the same year.
  7. Veterinarians should work with producers to monitor effectiveness of product and treatment strategies. The FEC (on individual or pooled fecal samples) can be used as an approximation method to monitor effectiveness or the emergence of resistance. A therapeutic intervention is applied and a second FEC is done on the same population a minimum of 7 days later, but probably better in the 10 to 14 day range. If the treatment was effective, a 90% reduction in FEC should be expected. If this strategy is used to monitor effectiveness or resistance it is important that a consistent method of determining eggs per gram is used. Variations in methodology greatly reduce the validity when interpreting this information.


References:

Ballweber, LS, editor, Ruminant Parasitology, Veterinary Clinics of North America, November 2006, Vol 22, No 3
Godden, S and SM McGuirk, editors, Dairy Heifer Management, Veterinary Clinics of North America, March 2008, Vol 24, No 1
http://www.fda.gov/downloads/AnimalVeterinary/ResourcesforYou/UCM347442.pdf

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