Internal parasites can negatively impact milk production, especially during the first 90 days of lactation. During this critical period, the fresh cow is under a great deal of stress and her immune system is at its weakest point. A University of Wisconsin study showed the difference in milk production between infected versus non-infected dairy cows during the first 90 days of lactation was 6.47 lb/day. Parasitism can also slow growth of replacement dairy heifers which can affect future lactation performance.

Dairy producers are concerned about the cost of production. Losses, especially those caused by preventable diseases, are a concern. Knowing how to reduce or prevent these losses from occurring is very valuable to the efficiency of an operation since losses caused by disease are usually cumulative over a period of time. Profitability can easily be determined by subtracting the cost of prevention from the potential losses incurred by the disease.

Foremost in the economic analysis is the ability to detect and evaluate losses as they occur. The measurement of the actual loss or losses involved, however, is often difficult because either too many economic factors are involved or the proper parameters for measuring the economics of the losses are lacking or overlooked. A need exists for specific and adequate sensitive tests to detect the existence of a known subclinical disease, as well as to measure the adverse effects of subclinical disease.

Risk Factors and Production Losses
With internal parasites, it is well established that even a few parasites present during early lactation become a detriment to achieving production potential. Parasitized cattle are harmed, not only by the parasites themselves, but also by the indirect damage the parasites cause to the immune system. A recent feedlot study showed dewormed cattle had significantly fewer (unrelated) health problems compared to non-dewormed cattle. Grazing cattle have the greatest risk since their exposure to parasites is higher than cattle housed on dirt lots or in a confined facility.

Deworming studies conducted in the U.S. and Canada have demonstrated lactating dairy cows may lose from 100 to 1,200 lb of milk per lactation due to internal parasites. The greatest responses came from high-producing herds with some exposure to internal parasites dewormed at freshening and again six to eight weeks later. These studies showed that by removing parasites during the period of greatest stress during early lactation, production losses due to internal parasites could be prevented.
Deworming replacement heifers to prevent parasite infections provides the dairy producer one of the best tools for raising healthy heifers. Replacement heifer deworming trials conducted in Minnesota, Wisconsin, and Vermont demonstrated dewormed heifers reached breeding size 30 to 60 days sooner than non-dewormed heifers.

It now also appears that parasites can adversely affect breeding efficiency. Deworming studies conducted with adult brood cows in Minnesota, Georgia, and Florida have demonstrated parasites can have a detrimental effect on breeding efficiency. In these studies, dewormed cattle had 9% to 22% higher breeding efficiency when compared to non-dewormed cattle. The process whereby a 1,600 lb highly efficient dairy cow can be harmed by a few tiny parasitic worms is complicated.

Changes in the abomasum which cause an increase in abomasal pH is the underlining basis for parasite damage. Basically, the consequences of damage caused by even a small number of parasites result in:

• Depression of dry matter digestibility, including dietary crude protein.

• Delayed passage of digested materials through the gastrointestinal tract, which then reduces dry matter intake.

In addition to production losses, high numbers of parasites may also cause the following problems: diarrhea, anorexia, weight loss, hemoconcentration (decrease of the fluid content of the blood), dehydration, hypoproteinemia (low blood protein), and eventually death.

Control Strategies
The economics of parasitism not only involve the development of parasites under pasture and confined systems of management, but also involve the prevalence of parasitism in these systems. Knowing whether parasites are present on the operation is the first step to establishing a control strategy.

Several methods can be used to determine whether parasite control is necessary in dairy herds. Determining how much exposure the animals have or have had to a parasite-contaminated environment is the first step. The easiest way to determine this is to know how much time these animals have spent in confinement on concrete. Parasite contamination on concrete is usually very low except where bedding and manure build-up occur. Parasite transmission in dairy herds predominately occurs on pasture, exercise lots, and dirt lots.

The "old fashion" fecal exam is still the best method to determine parasite exposure. Simply running a "fecal check," however, is not the total answer. First, a producer must find the proper place to have the fecal check conducted. Many veterinary clinics and diagnostic labs use a fecal technique which is not applicable for dairy cows. Most of these labs use a fecal technique which was developed for small animals or sheep where only small amounts of fecal matter are shed daily and parasite eggs are easily found.
In dairy cows, where a large volume of fecal material is excreted daily, the diagnostic technique must be sensitive. A false negative sample provides a false sense of security, allowing unnecessary production losses to occur. The only test that has been proven effective with dairy cows is the Wisconsin fecal flotation technique that uses a super saturation sugar solution to float the eggs out of the fecal debris. Pro-ducers should check with their veterinarian concerning the technique they employ.
Once parasite presence is established, a control strategy can be implemented. Three steps are necessary for successful prevention of parasitism:

1. Select the Correct Product
   A deworming product should be highly efficacious with a 98% efficacy against all important internal parasites (including lungworms) and all stages of the parasite within the animal. This strategy is important because late fall deworming should remove all parasites in the animal at the time of treatment so that the cattle remain relatively parasite-free until the following spring.

2. Select the Correct Treatment Time
   The best dewormer used at the wrong time is a wasted resource. Treatment can be given on a herd basis, an individual basis, or a combination thereof.
   Herd Treatment--This treatment regime should be initiated in late fall with a follow-up deworming given four to six weeks into spring grazing.
   Individual or Group Treatment--Treat individual cows or use a feed through dewormer every three weeks in the prefresh group (three weeks prepartum). Ideally, deworming should be repeated at six weeks postpartum.
   Combination Treatment--Cows are dewormed in the fall and a follow-up deworming is given when the cow freshens.

3. Maintenance Treatment Program
   The economic benefits from strategic deworming improve year after year because as parasite contamination is reduced in the cows' environment, parasite control is easier to achieve.

Conclusion
For animals to remain an economical food supply source, efforts to increase efficiency must continue. Deworming dairy cattle is a venture beyond treating clinical disease; the treatment of parasitism should be aimed first at the elimination of the threat of economic loss and later at the prevention or elimination of the parasites.