605 West 5th Street

Morris, MN 56267 US



Bovine Parasites

Internal Parasites

Most cattle have some worms or gastro-intestinal nematodes in their digestive tract. Consider economics and your type of cattle management system when developing a control strategy for your herd.

The most economically important gastro-intestinal nematodes are small, thread-like worms that are located in either the abomasum or intestinal tract of cattle. The life cycles of the major species are similar. Economically important in the Midwest are Ostertagia, Haemonchus, and Trichostrongyles.

Life Cycle
Worms are primarily a disease of pastured cattle. However, almost all cattle carry a few worms and can serve as a source of pasture contamination and infection of herd mates, especially animals younger than two years of age. Adult cattle are much more resistant to heavy infection than young animals.

Adult worms in the stomach or intestinal tract lay millions of microscopic-sized eggs that are passed in the feces. Under favorable pasture conditions, these eggs hatch and the worms mature through three larval stages. Stage three larvae are ingested, invade the wall of the digestive tract, and become fourth stage larvae, which eventually mature into adult worms. In some worm species, especially Ostertagia, which is the most important nematode of cattle, the larvae may lie in the lining of the digestive tract in an arrested state for long periods of time before becoming mature adults. In northern states this generally occurs during autumn and winter. It is a mechanism that nature uses to ensure the survival of the parasite.

Arrested stage four larvae do not mature until near calving, when they rapidly develop into egg-laying adults and heavily contaminate spring pastures. Heifers may develop clinical disease, including diarrhea, loss of weight and anemia. This phenomenon is referred to as Type II Ostertagiasis. Young grazing calves are very susceptible to worm infection and may become heavily infected. This delayed maturation of stage four larvae ensures heavy pasture contamination in the spring and is a major factor that must be considered when developing worm control strategies.

Under ideal conditions, worm eggs shed on pasture will mature to the stage three larvae in as little as three weeks. Rapid development and maturation of the larvae occurs in warm, moist conditions. Thick, dense pasture growth is conducive to this rapid development. In northern states, many larvae can survive winter as eggs, which hatch in the spring and become infective larvae.

Heavily parasitized calves are unhealthy in appearance and usually have a dry, dull hair coat. Growth and feed efficiency are significantly reduced. Anemia, diarrhea, weight loss and death may occur when infection is severe. Subtle losses are possible, usually in the form of reduced weaning weight, with reductions of 40 pounds or more being common. Moderate levels of infection may cause reduced milk production in dairy herds. Nutritional requirements of developing heifers become difficult to meet and so-called nutritional infertility may be the long-term result of heavy worm infection in heifers. In northern states, symptoms of parasitism are usually most evident in late summer and early fall.

There is a tendency to regard all poor-doers as wormy cattle. Such problems can also be due to infectious diseases, management, nutritional problems, or toxins. Accurate diagnosis is important to avoid costly, unnecessary deworming of cattle. Veterinarians can microscopically analyze fecal samples for parasites using a procedure called fecal flotation. On a herd basis, fecal examinations of suspected wormy cattle are helpful, but do not reliably indicate the degree of parasitism of individual animals because egg production of adult worms is extremely variable. Careful examination of the herd, multiple fecal examinations, ration analysis, blood tests, and post mortem examination of any dead cattle all help determine the level of worm infection in a herd.

The economic effect of low levels of parasitism in cattle is difficult to determine and researchers disagree about the importance of it. Eggs passed by adult cows can contaminate pastures, but the level of pasture contamination caused by calves and yearlings is more significant. Weather and pasture conditions can affect the risk of parasitism. All producers should develop a worm control program that fits their management situation. Management practices and strategic use of anthelmintics (dewormers) are equally important for parasite control.

Feedlot Cattle
Most calves and yearlings coming into feedlots are of uncertain background. The worm burden is thus unknown. Because most replacements have been pastured, significant infection is likely. This infection can reduce gain and feed efficiency. Parasitism increases the risk of feedlot diseases such as shipping fever. Heavily parasitized animals do not respond well to treatment for respiratory and other feedlot diseases. Response to vaccines may be poor. Even though the level of worm infection decreases in the feedlot, it is generally cost effective to treat feedlot cattle with an anthlemintic upon arrival. Pour-ons are widely used but dewormers can also be administered by injection or by oral administration of boluses, drenches, gels or paste products. A variety of safe, effective anthelmintics are available. Treating calves on arrival for worms is not an undue stress.

Beef Cow/Calf Herds
It may not be necessary to deworm every adult cow each year in commercial cow/calf enterprises in the upper Midwest. However, even lightly parasitized cows can serve as a significant source of infection for nursing calves. Herds with ample pasture and good grazing conditions are less likely to have significant parasite problems than when drought or over-grazing occurs. Cow herd deworming recommendations should be based on a total herd health and production medicine program. Most veterinarians recommend deworming adult cows in the fall when pre-conditioning calves and pregnancy testing occurs. Replacement heifers often have significant levels of parasites and serve as a source of infection for the next year’s calf crop. If they have been pastured after deworming in the fall, heifers should be dewormed in the spring four to five weeks prior to breeding. In herds allowed to overgraze pastures, more frequent deworming is advisable. Cattle do not pick up significant levels of worms when grazing cornstalks.

Some confusion exists about pasture rotation and worm control. Rapid pasture rotation (every few weeks) is good pasture and nutritional management, but does not provide parasite control. In northern latitudes, deworming in mid-summer when calves are moved to a new pasture is often cost effective. This removes worms that are in the animals and allows them to graze relatively worm-free pastures.

Continuous use of low-level anthelmintics in creep feed or similar formats may help reduce parasite infection, but should not replace individual treatment when calves are weaned and pre-conditioned.

Dairy Cattle
Adult dairy cows are resistant to infection with nematodes. Cows kept in dry lot or confinement conditions do not pick up worms and do not need routine deworming. When cows graze pasture, the level of infection should be monitored. Infection levels rarely become significant, however.

Calves pastured during the spring and summer are highly susceptible to parasites. Ideally, winter and spring born heifers should be dewormed three and six weeks after turnout to pasture in the spring. This gets rid of larvae that have over-wintered on pasture and are being picked up by the calves. It is a good idea to do a third deworming in mid-summer or early fall when heifers are moved to a different pasture. Yearling heifers should be dewormed in the spring and again several weeks prior to breeding. Move heifers to fresh, lush pastures prior to breeding and deworm at the same time to avoid heavy pasture contamination. Heifers held and bred in dry-lot situations do not need to be dewormed as frequently, but should be dewormed when moved from pasture to dry lot. Deworm heifers about one month prior to calving, regardless of housing and management systems.

Lungworm infection can cause a persistent dry, husky cough and high susceptibility to pneumonia and other pulmonary diseases. Death can occur if infection is severe. Infection usually occurs in areas where pasture conditions are wet or swampy on a continuous basis. Lungworm infection is endemic in some areas and rarely seen in others. Several anthelmintics are highly effective against lungworms. Feedlot operators who purchase cattle of unknown background should use an anthelmintic that is effective against lungworms.

Liver Flukes
Cattle living in wet areas with alkaline soils may develop liver fluke infections. Liver flukes are transmitted when:

  • Infected cattle, deer, and rabbits pass eggs in manure and drop the manure in water
  • Eggs hatch in water and larvae develop in snails
  • Cattle swallow cysts on grass or hay

Clinical signs of digestive inefficiency are evident in young cattle with acute liver disease and in older cattle with chronic liver disease. Fluke infected cattle show signs similar to those with malnutrition and stomach worms.

Several highly effective dewormers are available. Each anthelmintic has advantages and disadvantages. Efficacy against gastro-intestinal nematodes, lungworms, and external parasites such as grubs and lice should be considered when selecting an anthelmintic. It is generally advisable to administer these individually, rather than in feed, since dosage is critical. Under-dosing will result in poor effectiveness. Follow label directions for dosage, route of administration, slaughter withdrawal and milk withhold carefully. Injectable products should only be given in the neck or shoulder region in order to avoid injection site lesions. Use clean, sharp needles. Oral dosing with anthelmintics in the liquid or bolus form should be done carefully to avoid trauma to the throat region of calves.

Coccidia causes an intestinal disease of young cattle, usually three weeks to six months old, but can affect cattle up to two years old. They are transmitted when:

  • Infected cattle pass oocysts in manure
  • Rain washes the oocysts from the manure
  • The oocysts develop under moist and moderate temperature conditions
  • Oocysts on moist ground are swallowed by cattle

Transmission is most common during rainy times in spring and fall. The diarrhea caused by coccidia may be confused with the diarrhea caused by stomach worms, bacteria, and viruses.

Dewormers are ineffective against coccidia. Effective drugs are amprolium, decoquinate, lasalocid and sulfonamides.

External Parasites

Common external parasites include horn flies, lice, and grubs.

Horn flies
Horn flies reproduce in fresh cattle manure from early spring to late fall. Horn fly populations usually peak in late spring and again in late summer or early fall. Hot, dry conditions may naturally reduce horn fly numbers during mid-summer. Thousands of flies may infest a single animal, causing extreme nervousness and energy loss. Horn flies suck blood, irritate and annoy, reduce weight gains, and cause weight losses. The annoyance and irritation interfere with feeding and resting.

Treatment is economically justified when horn fly populations reach 250 per head. To control them satisfactorily throughout the season, use self-treatment insecticides or routinely apply spray, pour-on, spot-on, or dust chemicals.

Used properly, self-treatment devices are more effective than hand application in controlling horn flies and lice. Such devices include oil back rubbers, dust bags and tubes, liquid wicks, and impregnated ear tags. Insecticide-impregnated ear tags control horn flies well for two to five months.


Biting lice and blood-sucking lice are transmitted between cattle by contact, especially in the fall, winter, and spring when egg production increases in cool weather. Because cattle tend to bunch up more in cold weather, uncontrolled lice spread easily from animal to animal and quickly infest an entire herd.

Clinical signs are dry, scaly skin, hair loss, and itching exhibited by biting, rubbing, and scratching. Lice bites and allergies to lice cause the itching. The allergic reaction may persist after the lice are gone. These signs may be confused with malnutrition and allergies caused by horn flies, mosquitoes and gnats.

Although chemicals do not harm lice eggs, cattle can be treated effectively by administering a spray, dust, pour-on, spot-on, or injection. Injection does not work for biting lice.

Cattle grubs (warbles, wolves) are larvae of heel flies, which lay eggs on the hair of the lower legs of cattle in late winter and spring. Grubs appear in the backs of cattle in winter. The migratory damage by the grubs causes weight loss and reduced weight gains and milk production.

To control grubs, administer systemic organophosphate insecticides to cattle no later than three months before grubs appear in the back. Use pour-on, spot-on, spray, or injection methods to kill migrating grubs before they reach the esophagus. If cattle are not treated for grubs in the summer, the systemic organophosphate insecticides and ivermectins used in the fall and winter for control of lice, horn flies, and internal parasites may cause reactions in the esophagus if many grubs are present.

Office Hours

Our Regular Schedule


8:00 am

5:00 pm


8:00 am

5:00 pm


8:00 am

5:00 pm


8:00 am

5:00 pm


8:00 am

5:00 pm


8:00 am

12:00 pm





Find us on the map