By: Frank Wardynski, Michigan State University Extension
Using routine anthelmintics in beef cattle has long been a recommended and accepted management practice to improve animal performance and maintain animal health. For many beef producers, anthelmintic drugs commonly referred to as dewormers are used on a regular basis at the same time cattle are vaccinated in the cow herd at spring and fall processing. Dewormers are used to control parasitic nematodes that attach to the gastronintestinal tract of cattle and feast on blood and its constituents. New findings indicate that drug resistance is accelerating in cattle parasite which challenges the long term practice of routine drug treatment.
Internal parasite organisms have been shown to develop resistance to deworming drugs followed repeated use of the same product over time. Frequently exposing the entire parasite population on a farm to the same drug will select for drug resistant parasites. Over time these resistant organisms will dominant the parasite population resulting in a population that is highly resistant to that drug. Once the majority of parasite a population become drug resistant, the drug has little efficacy and may be no better than treating the animals with water.
New parasite control methods use selective and targeted use of drugs and not serial deworming of all animals at routine time points. This requires monitoring of parasite infection and an effective way of doing this is by performing fecal egg counts (FEC) on a sample of cattle. Producers should collect fecal samples from the cattle and utilize labs or veterinarians to perform a quantitative FEC. The results will be a number of fecal eggs per three-gram manure sample. Utilizing fecal egg counts to determine treatment thresholds will reduce the frequency of drug use. Once fecal egg counts have been determined, producers can decide if treatment is required. It has been determined that not all cattle need to be treated. Proper grazing management and some cattle having developed immunity to internal parasites and consequently have low numbers of internal parasites.
As mentioned previously, many cow-calf producers process cattle in the spring and fall with part of that processing to include routine deworming. Now producers are being encouraged to monitor herd infection using FEC. Table 1. Indicates egg count levels that establish a threshold FEC for treatment. Low egg counts indicate that cows don’t require treatment and allows producers to save dollars that would have been spent on dewormers. Reducing the use of dewormers not only saves money but also reduces the amount of chemical product used in beef production and reduces the chances of parasites developing resistance. Protocols have been developed to collect fecal samples and compare egg counts to determine need for treatment. In instances calling for treatment, fecal samples can be collected two weeks post treatment to determine product effectiveness. In cases that a given product is determined to be ineffective, switching product class usually is highly effective against internal parasites in beef cattle. Consult with your veterinarian if drug resistance is found to multiple drugs.
Mature cows are more likely to have developed immunity to internal parasites than younger animals. Each class of cattle should be sampled on a farm. Many farms have shown no need for treatment in the mature cows while calves and yearlings may have elevated FEC and require treatment. By treating just the susceptible calves you control infection and maintain a reservoir of untreated parasites in the mother. This population not exposed to drugs is called the refugia population and helps to dilute the resistance populations that inevitably grows on a farm with frequent drug use.
Adult nematodes living in the intestinal tract of cattle lay eggs that are excreted in the manure. First stage larva hatch from these eggs and then molt twice to become third stage larva. The third stage larva then migrate from the manure and onto forage. Cattle are infected with internal parasites by consuming these larvae that are found in the film of dew or moisture at high concentrations low in the pasture sward.
Grazing management is an important factor that influences internal parasite infestation. Some have wrongly identified rotational grazing as a practice that can reduce infestation. Rotational grazing alone actually enhances to opportunity for cattle to become infected. Eggs released in the manure only require a few days to hatch and molt into larva that can infect the animal. Once third stage larva are present in pasture they can survive many months waiting be consumed by cattle. Rotating back into paddocks one or even two months later is ideal to allow cattle become infected by consuming the larva and continuing the parasite life cycle. A critical point is forage height. Larva attach to forage at about four inches of height. Grazing above that level may help reduce infection and provide better nutrition to maintain strong immunity against infection.
The cattle at the Michigan State University Upper Peninsula Research and Extension Center in Chatham are part of the Red Angus herd on a grass finishing research program. These cattle are intensively managed in a rotational system. The key factor is that they are nearly always consuming forages over four inches. These cattle are not dewormed and have shown fecal egg counts that indicate treatment is not required. It’s not the rotational system but rather the fact that these cattle seldom overgraze forages to the length that allow cattle to become infected by consuming larva. Another critical factor to keeping FEC low is the simple fact that FEC are low. Cattle with low fecal egg counts are not contaminating pastures at nearly the same rate as cattle with high numbers.
New recommendations continue to come forward as more information is obtained. Today producers are encouraged to collect manure samples to determine FEC to determine if cattle should be dewormed and to evaluate the effectiveness of the deworming products we are using.
Assessment of three-gram fecal egg counts.
*From The Fecal Exam: A Missing Link in Food Animal Practice. D.H. Bliss and W.G. Kvasnicka. The compendium. April; 104-109, 1997.