One aspect of internal parasite infections that is often overlooked is their impact on the nutritional status of the animal. Internal parasites negatively impact feed intake in cattle. While this has an obvious impact on nutritional status of the animal, the impacts of internal parasites on nutrient absorption and nutrient requirements of the animal are seldom discussed. The effects of internal parasite infections on the nutritional status of the animal are important because they impact every aspect of biology. Without a proper supply of nutrients, the ability to fight off infection, grow, and reproduce are compromised.

Internal Parasites and Feed Intake
The largest impact that internal parasites have on beef cattle production is a reduction in feed intake. Cattle with relatively low parasite burdens (324 total slaughter worm counts) have been shown to have depressions in feed intake of up to 3.2%, while cattle with high parasite burdens (11,164 total slaughter worm counts) have been shown to have depressions in feed intake of up to 7.8%. Less feed intake means that the animals are consuming less energy, protein, vitamins and minerals, all of which play a vital role in animal growth, reproduction, and immune function.

While parasites negatively impact the immune response independent of their effect on nutritional status, their impacts on feed intake, nutrient absorption, and energy status of the animal further impair its ability to respond to bacterial and viral infections. The depression in feed intake is especially detrimental to recently weaned or newly received calves that typically have low intakes without the added stress of an internal parasite challenge. Because of the stress these cattle experience, they are susceptible to infection by a myriad of bacteria and viruses. The immune system of these stressed animals is already compromised because of the immunosuppressive effects of the hormones that are released in response to stressors. The presence of internal parasites will exacerbate that problem because the animal now must mount an immune response to the parasites, and with the concurrent depression in feed intake it has less energy, protein, vitamins and minerals to do so. Several studies have demonstrated the benefits of proper deworming on the health status of the animal. For example, Reinhardt et al. compared the effects of on-arrival feedlot deworming treatments on the health and performance of sale barn-purchased heifers.

Effects of Internal Parasites on the Gastrointestinal Tract
In addition to a reduction in feed intake, parasites also have a detrimental impact on the function and integrity of the gastrointestinal tract. These effects include an increase in the pH of the abomasum (gastric stomach) of ruminants. This increase in pH may be due to several factors, including inflammation that occurs in response to the presence of parasites. Pathological changes in the small intestine include an increase in the density of mucus secreting goblet cells 60 days after experimental infection with Cooperia punctata. Other observations from that study included thickening of the intestinal wall and enlarged mesenteric lymph nodes. These changes are indicative of a severe immune response and a shift from absorptive cells (enterocytes) to mucus-secreting goblet cells and provide an explanation for the reduction in the absorptive function of the small intestine in response to parasite infections. Some parasites also inhabit the cecum and proximal large intestine, and their presence can be associated with damage and inflammation in those areas as well.

Effects of Internal Parasites on Energy Status
The biggest impact of internal parasites on the energy status of an animal is due to the depression in feed intake. The reduction in feed intake means a reduction in intake of calories (i.e. energy) for maintenance of body weight, growth, and reproduction. The metabolic costs of internal parasite infections are somewhat more difficult to measure yet are a consequence of altered protein metabolism (discussed below) and the metabolic cost of immunity. Several pro-inflammatory cytokines are up regulated in cattle following internal parasite infection, which increases the energetic needs of the animals.

Effects of Internal Parasites on Protein Status
Internal parasites impact protein status of the animal several ways, and the presence of internal parasites in ruminants such as sheep has been estimated to cost 17 g/day of metabolizable protein. This loss of protein is from increased nitrogen loss in the small intestine from endogenous sources and increased urinary nitrogen excretion. Additionally, parasites that infect the abomasum (i.e. the gastric stomach), such as Ostertagia ostertagi (brown stomach worm), Haemonchus placei (barber pole worm), and Trichostrongylus axei (small stomach worm) can damage the parietal cells which produce hydrochloric acid in the stomach. This acid is necessary to reduce the pH of the abomasum in order to activate the enzyme pepsinogen which is a crucial enzyme in the breakdown of protein. The failure to activate pepsinogen will negatively impact protein digestion.

Effects of Parasites on Vitamin and Mineral Status                                                                                         The specific effects of internal parasites on the vitamin and mineral status of animals are poorly understood. However, a reduction in intake would decrease the supply of both vitamins and minerals to the animal. Moreover, the damage caused by parasites to the gastrointestinal tract would limit the absorption of the nutrients that are consumed. This concept is supported by the observation that the absorption and retention of both calcium and phosphorus is compromised in the animals with internal parasite infections. Additionally, abomasal parasites impact copper absorption because its solubility is dependent on the low pH of the abomasum.

Conclusion
In summary, internal parasites impact the nutritional status of the animal in three ways: they decrease feed intake, they decrease nutrient absorption, and they increase nutrient requirements of the animal. Through these mechanisms, parasites impact the energy, protein, vitamin, and mineral status of the animal, thereby impacting every aspect of biology of the animal that is relevant to production. The reduction in nutrient intake and absorption is especially detrimental in high-stress cattle because the poor nutritional status of an animal infected with parasites contributes to their inability to respond to a microbial disease challenge. However, the negative effects of internal parasites are not limited to high-stress cattle, and other classes of infected cattle will also have fewer nutrients available for growth and reproduction.