How Much Deworming is Enough, and How Much is Too Much?

Posted: December 10, 2012

Internal parasites continue to plague the livestock industries. Economic costs due to parasitism vary with animal age, stage of growth, degree of exposure, and level of nutrition. Late fall or winter is a good time for dairy producers to strategize with their veterinarian and develop a parasite control program that fits with animal care and planning for crops and forages.

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 livestock groups varies greatly due to management, nutrition, genetics, and rates of exposure. 

When considering parasites in general, it may be a good time for livestock producers and veterinarians to re-think their parasite control programs.

Here in the NE regions of the US the major parasite time has just passed, and there is much less infectivity on pastures following several hard killing frosts. Many veterinarians and producers consider late fall a good time to give one final anthleminthic (de-worming) treatment to ensure that most/all arrested parasites are removed. While the health concerns related to the more typical parasites (stomach worms) may fall with cold weather, protozoan parasites such as coccida or cryptosporidium may be concentrated when animals of all ages and production classes are housed in larger groups in barns. In this case when animals become clinically affected the therapy required and the control measures employed are much different. The problem is still parasitism, but to be effective in controlling this type of parasite a new way of thinking has to be employed. When considering parasites in general, it may be a good time for livestock producers and veterinarians to re-think their parasite control programs. The cattle industry has been blessed over the past forty years by the introduction of many very effective pharmaceuticals which made parasite control much easier. With them high productivity and low parasite burdens could be accomplished via a syringe, bolus, drench or pour on. Most producers could maintain a herd with few parasite problems simply by reaching for a bottle and not actually developing a strategy or whole farm approach. A whole farm approach would include a plan which includes the life cycle of the parasite, the immunity or resistance of the animal, management of the environment/pasture/stall, and the long term concerns for development of resistance. While high risk for round/stomach parasite exposure is over for this year in the Mid-Atlantic States, it is actually a very good time to consider how to implement a better, more cost effective plan. Since a good plan includes more than just the animal and the drug, a longer look at what factors influence the development of resistant parasite strains should be considered. 

Heavily parasitized animals, primarily livestock on pasture, can be severely affected. Effective treatment in these cases results in dramatic and measurable gains in animal growth or performance. For animal health and welfare, it is our duty as animal caretakers to minimize disease and suffering due to parasites. 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. Diseased animals should be treated appropriately and this frequently is referred to as acute care or salvage therapy. Hopefully with a well designed control plan these sorts of cases will be fewer and fewer and farther and farther in between. 

In cases when parasitism is suspected but dramatic clinical signs are not easy to see, diagnostic efforts should be made to determine parasite load vs. the more classical response of simply reaching for an anthleminthic and treating every few months. Subtle parasitism may or may not be an economic concern, while use of anthelminthics when not justified can be a long term resistance and environmental issue.

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. Many common parasites are no longer controlled by levamisole or by some of the older forms of the benzimidazoles. The macrolide compounds (e.g., Ivermectin® and Moxidectin®) still seem to have good efficacy for most cattle classes and horses. However, these extremely powerful compounds are showing signs of extensive resistance in some regions of the world with sheep and goats. Resistance is emerging for Ascarid infestation (roundworm) in horses. In many cases it has become easier for producers and veterinarians, perhaps more convenient, to simply reach for a dewormer vs. actually create a parasite control program. If these compounds are not used wisely and only as needed, it is just a matter of time before resistance becomes a major problem for cattle.

Dairy producers are encouraged to discuss developing a parasite control program with their veterinarian. Since a truly effective program will involve how animals are housed or pastured and where manure is spread, the late fall or winter is a very good time to strategize how parasite control fits with animal care and planning for crops and forages. 

What are some of the elements necessary for an effective parasite program? 

  1. Review of animal groups to determine the level of risk for parasite exposure as animals change groups or  housing.
  2. Review of feeding strategy to ensure animals have minimal exposure to fecal oral pathogens.
  3. Management strategies to minimize infective parasite larvae accumulation on pasture (e.g., pasture rotation, making hay, sward height, and maintenance of some susceptible genes in parasite population).
  4. Plan to limit manure application on pastures that will be grazed in the same year. 
  5. Routine fecal egg counts (FEC) of the various animal production groups (in cattle this should include a method for fecal exams to actually get a number of eggs/gram). When quantified, the FEC can be used to judge level of parasite burden or how effective treatment strategies have been (e.g., McMasters test for Fecal Egg Count).
  6. Producers in conjunction with their veterinarian should agree at some low background parasite burden that is acceptable and only treat animals or groups when necessary. 
  7. Veterinarians should work with producers to develop an effective product/treatment strategy to minimize the development of resistance. A system to periodically screen for the emergence of resistant parasites should be implemented.  


Reference: Ballweber, LS, editor, Ruminant Parasitology, Veterinary Clinics of North America, November 2006, Vol 22, No 3