A sound strategic parasite-control (SPC) program results in a high return on investment. Research has proven this time and time again. Here are 10 reasons that SPC should be a part of every cattle operation. 

1. Minimize economic losses. Research has shown that parasites are economically important at even lower levels than previously believed. Most of the problem is subclinical parasitism, responsible for up to 75% of the beef industry’s monetary losses.1

2. Protect immune system. Deworming improves animal health by removing the complex challenge to the immune system that parasites present. In fact, even a small number of worms can suppress the immune system, and animals with parasites don’t respond as well to vaccine antigens.2

3. Strong defense against liver flukes. Since very few liver flukes can have a detrimental effect on cattle gains, pregnancy rates and weaning weights, treatment is considered economically worthwhile -- and is less expensive than testing. Net returns from liver fluke control can average $15 to $31 per cow.3 In the feedlot, liver fluke-free cattle have demonstrated a 21-lb. advantage over those with liver flukes.4

Liver flukes. Photo by: Merial

4. Avoid spring infestations. Deworming before cattle are turned out on spring pastures is crucial in all parts of the country. In northern states, even in the harsh climate, parasite larvae can survive all winter underneath the snow. In the South, liver fluke transmission reaches a peak in the spring.

5. Increase fertility in heifers. Studies have shown that heifers treated for both worms and liver flukes had higher weight gains and a 15% higher pregnancy rate than untreated controls.5 In another study, flukes caused a 39-day delay in onset of puberty in heifers.6

6. Maximize appetite and weight gain. Worm burdens suppress appetites and slow growth. Some parasites, such as Ostertagia ostertagi, brown stomach worms, secrete a “substance” that negatively affects the appetite of the host. Not only will the calves eat less, they can use less of what they eat because Ostertagia destroy the gastric gland that impairs digestion.7

7. Maximize milk production. A study specific to dairy cows showed that even in small numbers liver fluke infestations have resulted in 5% drops in milk yields.8 In a North Dakota study, cows treated with ivermectin weaned calves a mean of 15.5 lbs. heavier than controls, suggesting increased milk production.9

8. Protect calves against early infestations. Both larvae and adult worms are emerging from winter
inhibition in the spring, and this is also when calves begin to graze. Research has shown that calves that were not dewormed early in life never caught up with those that were.10 Calves are very vulnerable, and early protection can prevent loss of growth and lifetime productivity.


Brown stomach worm, Ostertagia ostertagi. Photo by: Merial.

9. Prevent parasite buildup on pastures. Early season treatment prevents buildup of larvae on pastures, which would otherwise create a cycle of increasing infection. A cow-calf pair adds about 3 tons of manure to a pasture, and about 51 million total parasite eggs in a grazing season.11

10. Protect from horn flies. In the United States, annual losses from horn flies are estimated at $700 million.12 Weight gain in horn fly infested cattle may be reduced by up to 0.5 lbs. per day. In one study, uninfested heifers gained an average of 20 lbs. more than infested heifers over a 79-day period. Horn flies can also reduce milk production by 10-20%.13

This practice tip provided by Merial Limited.


1 Ciordia HC, et al:  Effect of ivermectin on performance of beef cattle on Georgia pastures. Am J Vet Res 2455-2457, 1984.

2 Bopp S: The importance of controlling parasites. Drovers 58-60, March 2005.

3 Kidwell B: Gadzooks! Liver flukes. Progressive Farmer, Oct 2001, accessed 20DEC2005 at www.findarticles.com/p/articles/mi_qa3832/is_200110/ai_n8955965.

4 Hicks RB, et al: The effect of liver flukes on the performance of feedlot steers. Animal Science Research Report 337-339, 1987.

5 Loyacano AF, et al: Effect of gastro-intestinal nematode and liver fluke infections on weight gain and reproductive performance of beef heifers. Vet Parasitol 107:227-234, 2002.

6 Lopez-Diaz M., et al: Puberty and serum concentrations of ovarian steroids during prepubertal period in friesian heifers artificially infected with Fasciola hepatica. Therio 50:587-593, 1998.

7  Parasites vs. the immune system. Drovers  28-29, March 2001.

8 O’Brien D: Keep Liver Fluke at Bay. Irish Agriculture and Food Development Authority, Sept 2002. Available at wwww.teagasc.ie/publications/2002/liverfluke.htm.

9 Wohlgemuth K, Melancon JJ: Relationship between weaning weights of North Dakota beef calves and treatment of their dams with Ivermectin. Agri-practice – Parasitology 23-26, Jan-Feb 1998.

10 Getting the bugs out. Drovers 28-30, March 2002.

11 Stromberg B:. Strategic Deworming: A Northern Cow/Calf Perspective. Merial Symposium, 2005.

12 Hawkins J: The Horn Fly (Haematobia irritans). Merial Veterinary Bulletin 3, Feb 2001.

13 Powell PK
: Horn Fly Biology and Management.West VirginiaUniversity Extension Service, Feb 1995. Available at www.caf.wvu.edu/~forage/10623.htm.