Cows carry a wide range of parasites, both internally and on their skin. In fact, parasitic infections are so common in cattle that they cost beef producers billions of dollars worldwide every year. Nearly every cow will encounter parasites during its lifetime, and the specific types depend on climate, grazing conditions, and how well the herd is managed.
Internal Parasites: Worms and More
The most important internal parasite in cattle is the brown stomach worm, a tiny half-inch worm that lives on the lining of the cow’s true stomach (the abomasum). Its larvae burrow into the stomach’s acid-producing glands, destroying them and replacing healthy tissue with mucus-producing cells. This raises the stomach’s pH, which impairs digestion and reduces the cow’s ability to absorb protein from its food. A cow can be heavily infected and show no obvious signs for weeks.
Several other worm species target different parts of the digestive tract. The barberpole worm attaches to the stomach lining and feeds on blood, sometimes causing serious anemia. Hookworms do the same in the small intestine. Whipworms and twisted wireworms disrupt intestinal function further down the gut. Coccidia, a single-celled parasite rather than a worm, is especially common in young calves and causes severe, sometimes bloody diarrhea that can be fatal if untreated.
Lungworms settle in the respiratory tract, where they cause inflammation, mucus buildup, and in severe cases, damage to lung tissue that makes breathing difficult. Liver flukes are another concern, particularly in wet regions. These flat parasites require freshwater snails to complete their life cycle. Snails release larval flukes that attach to aquatic vegetation, and cattle become infected when they graze near ponds, streams, or flooded pastures. Liver flukes are found in more than 70 countries where cattle or sheep are raised.
External Parasites
Ticks, lice, and mites are the major external parasites on cattle. Ticks are considered the most damaging. Beyond the direct irritation and blood loss they cause, ticks transmit several serious blood-borne diseases that can lead to fever, severe anemia, and death. A heavy tick burden alone can weaken a cow enough to reduce both milk and meat production significantly.
Lice infestations are extremely common, with reported prevalence ranging from about 2% to 94% of cattle depending on the management system. Lice damage the hide, which also affects the leather and tanning industries. Mange mites burrow into or feed on the skin surface, causing intense itching, hair loss, thickened and crusty skin, and in large numbers, anemia and even death. Flies, including horn flies and face flies, are a constant seasonal nuisance that reduces weight gain simply by keeping cattle agitated and off feed.
Signs of a Heavy Parasite Load
A lightly infected cow often looks perfectly normal. As the parasite burden increases, the signs become more visible. Classic indicators include a rough, dry coat that lacks its usual sheen, persistent diarrhea, weight loss despite adequate feed, and a pot-bellied appearance in younger animals. Pale gums and the inside of the eyelids signal anemia from blood-feeding parasites like the barberpole worm or hookworm.
One distinctive sign is “bottle jaw,” a soft swelling under the lower jaw caused by fluid accumulation when protein levels in the blood drop too low. Heavily parasitized cattle may stop grazing, stop chewing their cud, and become too weak to stand. The diarrhea from worms can look similar to diarrhea caused by coccidia, bacteria, or viruses, so a fecal test is usually needed to tell the difference.
How Parasites Are Detected
Veterinarians typically use a fecal egg count, examining a small manure sample under a microscope to count parasite eggs. For adult cows, fewer than five eggs in a standard three-gram sample is considered low, five to twenty is moderate, and anything above twenty is high enough to warrant treatment. This simple test helps ranchers avoid unnecessary deworming, which matters because overuse of deworming drugs has created a serious resistance problem.
Drug Resistance Is Growing
Deworming medications have been a cornerstone of cattle parasite management for decades, but parasites are rapidly evolving to survive them. A 2025 study of beef cattle farms in Georgia found that 100% of the farms tested had worms resistant to at least one common deworming drug in the avermectin class. On 75% of those farms, resistance was confirmed across multiple worm species. The barberpole worm showed resistance on 75% of farms, and the brown stomach worm on 40%.
This means that on many operations, the standard dewormer a rancher has relied on for years may no longer be fully effective. Fecal egg counts taken before and after treatment are the only reliable way to know whether a dewormer is still working on a given farm.
Pasture Management as Prevention
Because most cattle pick up parasites while grazing, how pastures are managed plays a major role in infection rates. Rotational grazing, where cattle are moved between paddocks on a schedule, can significantly reduce parasite exposure. Research on tick control found that resting pastures for 45 days between grazing periods dramatically reduced tick infestations compared to 30-day rest periods.
The reason comes down to timing and exposure. With shorter rest periods, parasite larvae in the grass are still young and viable when cattle return. With 45 days of rest, larvae have aged past their peak infectivity and used up their energy reserves. Taller grass before grazing also matters: paddocks grazed on 30-day rotations retained more leaf cover after the cattle left, which shielded tick eggs and larvae from sun and heat. The paddocks rested for 45 days had less ground cover after grazing, exposing parasites to drying conditions that killed them off.
The Economic Toll
Parasite infections reduce weight gain, lower milk production, damage hides, and in severe cases kill animals outright. Globally, beef cattle producers lose billions of dollars to parasites every year. Brazil, the world’s second-largest beef producer, provides a stark example: combined annual losses from gastrointestinal worms, liver flukes, ticks, and flies total roughly $14 billion. Gastrointestinal worms alone account for over $7 billion of that figure, with ticks adding another $3.2 billion.
Can Cattle Parasites Affect Humans?
Most cattle parasites cannot infect people, but one notable exception is the beef tapeworm. Humans become infected by eating raw or undercooked beef containing tapeworm larvae. The adult tapeworm can grow up to 10 meters (about 33 feet) long in the human intestine. Infections occur worldwide but are most common in Eastern Europe, Russia, eastern Africa, and Latin America. In the United States, cases are rare and mostly linked to areas where cattle and people are concentrated and sanitation is limited, such as around large feedlots.
Proper cooking eliminates the risk entirely. The beef tapeworm does not cause the more dangerous condition called cysticercosis, which is associated only with the pork tapeworm. So while a beef tapeworm infection is unpleasant, it is treatable and far less serious than its pork counterpart.