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).
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.
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).
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
Currently, there are 3 broad-spectrum anthelmintic groups of parasiticides available for the control of nematodes in cattle in the United States. These groups include:
2. Imidazothiazoles (levamisole) and hydropyrimidines (pyrantel/morantel)
3. Macrocyclic lactones (avermectins and milbemycins)
Also, there are no new anthelmintics expected in the market in the near future. Therefore, it is imperative to maintain the efficacy of the existing anthelmintics; otherwise significant economic losses associated with both productivity and welfare may occur.
Anthelmintic resistance in sheep and goats is well documented. Additionally, anthelmintic resistance in cattle is being reported in many portions of the world including northern Europe, New Zealand, Argentina, and Brazil. When anthelmintic resistance is defined as "A genetic trait that allows a population of worms to become tolerant of an anthelmintic at a dose and presentation that was previously lethal" there appears to be resistance in North America.7
Based on research by Yazwinski and Tucker, decreasing drug efficacy is believed to be a result of (1) use of a product at epidemiologically inappropriate times, (2) administration of product at incorrect dosages, (3) nematode resistance, (4) animal level modifications of effective drug levels including rate of gut passage, rumen microbe bio-transformations, gut-level inflammation, blood protein binding capacity, depth of Ostertagia inhibition, self and allogrooming by animals treated with topical, and amount and position of animal fat deposits.1 Additionally, the presence of products with longer residual activity combined with higher treatment frequency may be accelerating the presence of parasites resistant to anthelmintics.8
The most common method of determining resistance of parasites to parasiticides at the farm level is the use of the fecal egg reduction count (FECR). This process requires fecal collection from a subset of animals on the farm. Since fecal egg per gram are not normally distributed within a herd, it is recommended that at least 20 animals within the herd be sampled to increase the likelihood that one animal with elevated egg per gram count will be included.9 Fecal samples should be collected from the rectum whenever possible to avoid contamination by soil nematodes or fly larvae. If samples cannot be examined within 2 hours, the samples should be stored at 4° C. The laboratory should be contacted prior to time of collection to insure any special procedures that should be taken regarding the samples. At the time of the initial sample, animals will be treated with the designated parasiticide.
Since cattle typically shed lower numbers of parasite eggs, it is recommended that these fecal samples be examined using the modified Wisconsin technique.10 Follow-up fecal samples are collected 14 days later. If macrocyclic lactone products are used, a second sample at 28 days post-treatment is recommended.10 Fecal egg reduction percentage is determined by the equation: FECR = 100(1-arithmetic mean of treated group fecal egg count/arithmetic mean of the control group).11 Fecal egg reduction counts that are less than 90% are strongly suggestive of anthelmintic resistance.11 Although a clinical benefit is most likely seen with 40 to 50% reduction in fecal egg counts, the remaining population of parasites will serve as reservoir for further pasture contamination of "resistant" populations of nematodes.12
Unfortunately, this population of "resistant" parasite will remain unless there are factors that render them less fit for survival.12 These "resistant" parasites will then mate with other "resistant" parasites insuring their survival by heavily contaminating the environment with the next generation of "super parasites".
Although the FECR is an excellent screening tool, many of the parasite eggs are very similar visually. Therefore, the identification of the worm by post mortem examination is required to know the exact species. Unfortunately, this is a very expensive and time consuming endeavor. Therefore additional techniques to determine parasite resistance in live cattle are still needed. Additionally, other control programs that include methods other than chemical control of parasites are needed.
Yazwinski TA and Tucker CA. "A sampling of factors relative to the epidemiology of gastrointestinal nematode parasites of cattle in the United States". Veterinary Clinics of North America. Food Animal Practice. 22, 3. November 2006. 501-527.
Lawrence JD and Ibarburu, MA. "Economic Analysis of Pharmaceutical Technologies in Modern Beef Production. 2005.
Ballwebber LR, Smith LL, Stuedemann JA, et al. "The effectiveness of a single treatment with doramectin or ivermectin in the control of gastrointestinal nematodes in grazing yearling stocker cattle. Veterinary Parasitology 1997; 72:53-68.
Stromberg BE, Vatthauer RJ, Schlotthauer JC, et al. Production response following strategic parasite control in a beef cow-calf. Veterinary Parasitology 1997; 68:315-322.
Stuedeman JA, Ciordia H, Myers GH, et al. Effect of a single strategically designed timed dose of fenbendazole on cow and calf performance. Veterinary Parasitology 1989; 34:77-86.
Stromberg BE and Gasbarre, LC. "Gastrointestinal nematode control programs with an emphasis on cattle." Veterinary Clinics of North America. Food Animal Practice. 22, 3. November 2006. 543-565.
Craig, TM. "Anthelmintic resistance and alternative control programs." Veterinary Clinics of North America. Food Animal Practice. 22, 3. November 2006. 567-581.
Fiel CA, Saumell CA, Steffan PE, Rodrigez EM. Resistance of Cooperia to ivermectin treatments in grazing cattle of the Humid Pampa, Argentina. Veterinary Parasitology 2001; 97: 211-217.
Ballwebber, LR. "Diagnostic methods for parasitic infections in livestock." Veterinary Clinics of North America. Food Animal Practice. 22, 3. November 2006. 695-705.
Coles GC, Jackson F, Pomroy WE, et al. The detection of anthelmintic resistance in nematodes of veterinary importance. Veterinary Parasitology 2006; 136:167-185.
Coles GC, Bauer C, Borgsteede FHM, et al. World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P) methods for the detection of anthelmintic resistance in nematodes of veterinary importance. Veterinary Parasitology 1992; 44:35-44.
Craig, TM. Anthelmintic resistance in cattle. Proceedings from American Veterinary Medical Association Annual Convention. 2008.
Wood IB, Amaral NK, Bairden K, et al. "World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P) second edition of guidelines for evaluating the efficacy of anthelmintics in ruminants (bovine, ovine, Caprine). Veterinary Parasitology 1995; 58:181-213.