Although we have known the importance of internal parasites in cattle for many years, we still face an endless battle to control
these organisms. Parasites are the ultimate "survivor". Parasites realize that their survival is dependent on not killing
their host. As a result, they have adapted as their hosts (cattle) are exposed to different management practices (including
parasiticide products and pasture management practices). There are numerous factors that influence the incidence and severity
of the species-specific infections that occur in cattle including: region of country, temperature, humidity, moisture, amount
of sunlight/shade, snow cover, type of forage, soil type, inherent genetic resistance of host, presence of coprophagous beetles,
season of year, historical parasiticide use and many others.1 In addition, there appears to be significant variation between individual farms within the same geographic region, so all
control programs must be designed for the individual farm based on their exposure and current management practices.
Common Nematodes in Cattle
Haemonchus contortus and Haemonchus placei ("Barber Pole" worm) live in abomasums and are voracious blood consumers. They have some degree of seasonal inhibition and
periparturient rise is common.
Ostertagia ostertagi (brown or medium stomach worm) is recognized as the most economically important nematode of cattle. This parasite is best
known for its ability to arrest and avoid detection by host's immune system when environment becomes unfavorable for survival.
Trichostrongylus axei (small or minute stomach worm) has limited ability to arrest.
Cooperia oncophora, C. punctata, and C. pectinata live in small intestine and are typically concerns in animals less than 3 years of age.
Nematodirus helvetianus (thread-necked worm) develop larval stages one through three within the egg, and it is known as dose-limiting nematode of
macrocyclic lactones.
Unfortunately most of these nematode eggs are difficult to distinguish during routine fecal flotations (except Nematodirus).
Economic Impact
Based on information presented by Lawrence and Ibarburu using information available in 2005 when comparing four other pharmaceutical
technologies, the expected impact on the breakeven selling price of eliminating dewormers was 34.3% in cow-calf segment (representing
an added cost of $165.47/head), 2.7% in the stocker cattle industry (representing an added cost of $20.77/head), 2.11% in
feedlot industry (representing an added cost of $22.16/head).
Additionally, improvements in average daily gain in stocker calves3 , weaning weights of calves4 , and improved pregnancy rates5 have all been documented with the use of chemical agents to control internal parasites. However, probably the single most
economically important direct physiologic response is a loss of appetite by the infected host.6
