Parasites in Dairy Cattle – Prevention and Treatment
Parasites in Dairy Cattle – Prevention and Treatment
All cattle are negatively impacted by biting flies (unless flies are controlled) and sometimes other external parasites like lice, mites or ticks. Lice and mites cause skin irritation and itching. This can reduce production or damage fences and equipment as the animals rub on any available surface. When treating for lice or mites, retreatment will usually be necessary approximately 3 weeks later due to louse or mite life cycles. The first treatment generally won’t kill the eggs and these hatch later.
Internal parasites that affect cattle include roundworms, tapeworms, and flukes. Roundworms often have the most impact on cattle and should be treated. All cattle will have some level of parasites, depending on their environment and how the cattle are managed. Cattle on pasture have more risk for internal parasites.
External Parasites
Flies, lice, and mites occur on dairy animals at different times of year. Horn flies, stable flies, horse/deer flies, are blood feeders and occur in the warmer months. Lice are generally found only in cooler months but are also blood feeders. Mites on dairy animals include mange mites, which burrow into the skin and into hair follicles or under scabs that form as a result of their burrowing. Mange mites cause a great deal of irritation to the host animal, and this irritation may result in lowered milk production or reduced growth in replacement heifers.
Flies are a major nuisance, especially for pastured cattle. Control of flies is generally aimed at breaking their life cycle, to prevent new batches. All flies pass through four life stages: egg, larva (maggot), pupa, and adult. The three most significant pest flies are the stable fly, horn fly, and face fly. Stable flies develop as maggots in decomposing organic matter, including soiled bedding and feed debris that accumulates where cattle are confined. Horn flies and face flies develop in fresh cattle manure, and are a serious problem in pastured herds. Horn flies, stable flies, and face flies on organic dairies can cause 10 to 30% reduction in milk production. These flies are irritating and can reduce pasture feed intake. Face flies can spread pinkeye, and stable flies may spread other diseases from one animal to another.
The most successful way to combat fly populations is through integrated pest management to maximize effectiveness of pest control while at the same time minimizing pesticide use which can be harmful for the environment and for beneficial insects that prey on flies.
Good sanitation/waste management is the basis of any control program. Not only is it the most effective way to rid a facility of flies, but also most economical. Pesticides cannot eradicate flies when sanitation efforts are poor. On average, a fly life cycle lasts from 10 to 21 days, so if you clean your barn weekly, you can break the life cycle. Each week, remove or spread out the materials where flies breed, including manure, spilled silage, moist hay, wet grain, etc. Areas to focus on include calf hutches, holding pens, loafing sheds, paths to milking parlors and stalls.
Using sawdust instead of other materials for bedding reduces fly development. Spread manure thinly outdoors, so fly eggs and larvae will be killed by drying, or stack this waste and cover with black plastic. The flies can’t get to it to lay eggs, and heat under the covering will kill any eggs or maggots already in it.
To be successful in controlling flies, implement a program that best fits your operation. Reliance on a single practice or pesticide product is not the best approach. It works better to combine routine sanitation with a variety of pesticide strategies, such as baits, residual sprays, space sprays, and larvicides when flies are a problem. Don’t wait for large fly populations to build up. It is much easier and less expensive to keep fly populations from increasing at the beginning of the season than to try to control them after they reach high levels.
Typically stable flies are more problem where cattle are confined (since they breed in rotting organic material), but the stable fly situation has changed significantly in the past 60 years. Earlier they were mainly a problem around barns and pens. Today, they have become a pasture problem as well, possibly because of large bales fed out on the pasture in feeders.
About 95% of the stable flies develop in less than 5% of the areas where there are cattle. If you clean up those areas, it makes a big difference. If you clean out from under and around feed troughs and debris from old hay/bedding, you have most of the problem solved. There will often be thousands of maggots in a small area. If you clean those up, drain the wet spots, etc. you won’t have very many flies.
Horn flies can sometimes be reduced with fly vacuums. The University of Minnesota West Central Research and Outreach Center (WCROC) dairy evaluated two methods (the Bruce Trap and Spalding Cow-Vac™) for controlling pasture flies. Bruce traps and the Cow-Vac are compatible with organic dairying, because a trap can be positioned at the entrance to a milking parlor, where cows come and go.
To combat horn flies, W. G. Bruce, a USDA entomologist, built a box with one-way fly-screen baffles on the otherwise transparent sides, and walked fly-infested cattle through it to remove and capture their flies. This simple design is known as the Bruce walk-thru fly trap, and different versions have been studied for horn fly control in various parts of the country. The fabric dislodges flies, which are drawn by light to the sides through baffles, where they are trapped and die.
The other fly vacuum, the Cow-Vac, can be placed at the entry or exit of a milk parlor or barn. As cows walk through, the Cow-Vac blows horn flies off their back, belly, face, flanks and legs into a vacuum system that collects them in a removable bag for disposal. A 220-volt electrical outlet is needed, that can be reached by the 10-foot power cord. At the WCROC dairy, it took about a week to get the cows conditioned to going through both fly traps.
During the summer of 2015, the researchers evaluated efficacy of the Cow-Vac in on-farm organic dairies to control horn flies, stable flies, and face flies. The study partnered with eight organic dairy farms in Minnesota, with herds ranging from 30 to 350 cows. The farms were divided into pairs by location and during June-July the Cow-Vac was set up on one farm and during August-September the Cow-Vac was sent to its paired farm. Farms were visited once a week to collect flies from the CowVac, and to count and record flies on cows.
Horn fly numbers on cows were reduced by 44% on the farm in the presence of a Cow-Vac compared to absence of a Cow-Vac. Stable fly and face fly numbers were similar on each farm whether the Cow-Vac was present or absent.
Lice on dairy cattle are primarily a winter issue, since they don’t reproduce much during warm weather. They spend their entire life on cattle and there are always a few lice that survive on the cattle through summer, but most of them die in hot weather or do not reproduce readily. They are dormant and not feeding. As temperatures drop, they become active again and their populations increase.
Lice populations dramatically increase during cold weather partly because they thrive in cool temperatures and the cattle have winter hair for them to hide in. Chewing and sucking lice both cause intense itching, hair loss, and potential milk production losses. Signs include rubbing, hair loss on the neck, shoulders, and back from all the rubbing. A problem in some herds is that systemic products and treatments that give good control on sucking lice don’t affect the chewing lice. A pour-on product will usually control the 4 common species of lice, but a systemic endectocide will only control the sucking lice species.
Treatment requires treating all animals in the herd (otherwise the untreated ones will pass lice to the treated animals), often with pour-on insecticides or, for organic operations, oil-based treatments that don’t contain an insecticide. Effective management involves treating in late fall/early winter (after weather turns cold) to prevent a buildup in lice populations on the cattle by mid-winter. Most treatments need to be repeated in about 3 weeks, to kill the lice that hatch afterward since the eggs are unaffected. If lice start becoming a problem again, another treatment may be needed by mid-winter.
Mites on dairy cattle, primarily Chorioptes bovis, cause contagious skin conditions (mange) characterized by intense itching, hair loss, scaly, thickened skin, and reduced milk production, peaking in late winter and early spring. Lesions typically appear on the udder, tail base, and legs. Treatment involves systemic or spray acaricides like eprinomectin or lime-sulfur.
Ticks on dairy cattle are usually only a problem on pastured cattle that come into contact with them. Some types of ticks are more dangerous than others. Cattle fever ticks and Asian longhorned ticks cause severe production losses, including reduced milk yields, hide damage, and anemia. They infest areas like the udder, tail-head, and brisket, spreading diseases such as babesiosis, anaplasmosis, and theileriosis. These ticks have been a threat to American agriculture for generations because they spread the disease bovine babesiosis, commonly called cattle fever, which is often fatal.
Often fatal, this disease caused enormous losses to the U.S. cattle industry in the late 1800s and early 1900s. The disease was eradicated from the U.S. by 1943, except for a permanent quarantine area along the Texas/Mexico border, where cattle fever ticks are still found.
Other ticks include Ixodes species. (deer ticks, and western blacklegged tick) and Dermacentor species (American dog tick). These are not as dangerous but can spread other diseases. Large numbers on cattle can be stressful and unhealthy. Any ticks found attached to cattle should be removed. Chemical control includes ear tags containing abamectin/piperonyl butoxide, and certain pour-on products and sprays. A pour-on containing diflubenzuron/permethrin (Clean-Up II; Lice-Ban) can be used as needed (follow label directions for dosage), but no more than once every 2 weeks. There are also some permethrin sprays ready-to-use or as a concentrate to be diluted in water.
For organic dairies, rosemary oil/peppermint oil spray can be used if applied at least 20 minutes prior to milking or after milking has been completed. Soybean oil/peppermint oil/cinnamon oil/lemongrass oil/garlic oil concentrate can also be used, if diluted. Dusting powders like zeta-cypermethrin/piperonyl butoxide dust (PYthon II) can be repeated as needed but not more than once every 3 days. Do not use dusts at the same time with insecticide ear tags containing zeta-cypermethrin or cypermethrin.
Internal Parasites
Large numbers of worms can impact health and growth; young cattle that are heavily infested don’t grow as well as they would otherwise. Dairy heifers may or may not have very many worms, depending on how they are raised and the environment they live in.
Cows or heifers on pasture are likely to pick up worms. Heifers raised in confinement systems, where manure is removed daily and they are never out on pasture rarely have many worms. Even if they get exposed and end up with a few worms and pass some worm eggs in their manure, if the manure is cleaned up every day, there is very little chance for the animals to be re-exposed.
The worm life cycle depends on the eggs hatching and the larvae moving onto forage plants to be eaten by a grazing animal. If the heifers are not on pasture, there is very little risk for re-exposure. Cattle on pasture are the ones to worry about but the risk will vary depending on how the pastures are managed and grazed. In an intensive rotational grazing system in which the animals are moved to new grass frequently, the worms may not be much problem.
If your rotation (and move to new pasture) is shorter than 21 days, you move the cattle away before those worms can re-infect them. Worm burdens will very low and almost non-existent. But if you have cattle in that same area of pasture after those worms become infective, the cattle will pick up worms
If your heifers go out on pasture where there are also mature cows (which are generally shedding worm eggs) and are not moved very often, they probably are exposed. Many dairy operations calve year-round, so the heifers are different ages. Producers just pick strategic times for deworming for cattle at pasture. It’s common to deworm when calves are 4 to 6 months of age. Heifers can be dewormed again pre-breeding. Heifers will usually be dewormed when moved into the prefresh group—maybe 3 to 4 weeks before calving—or dewormed at calving. In that situation, select a deworming drug that is approved for lactating animals.
It is always important to read labels and work with your herd health veterinarian to determine what is best to use. If you pick the wrong dewormer, you will be discarding the milk for a long time.
Taking fecal samples won’t always tell you how much worm burden the animals have. If cattle have recently picked up worm larvae, it takes a while before those worms mature in the animal and start laying eggs, so it may be a little while before you find worm eggs in a fecal sample. By that time the worms have been living within the host several weeks and have already done some damage. It’s also important to know if the dewormer you use is effective.
With grazing dairy herds, you might want to do fecal egg count reduction tests (checking egg counts at the time of worming and again later, to see if the dewormer was effective). Take a manure sample, count the eggs, and then deworm. Two weeks later another sample is taken to see if there is a significant drop in the egg count. This tells you whether the drug worked. In some regions, certain worms are becoming resistant to the deworming drugs.
Work with your veterinarian on a worm control program. Many veterinarians recommend using Eprinex or Cydectin for dry cows, for heifers due within 2 months, and for lactating cows –since these products have no milk withdrawal. You can use Ivomec or Bimectin for your other heifers and calves. Initial deworming of calves should occur 3-4 weeks after turnout to pasture. Milking cows should be dewormed only with products approved for lactating dairy cattle and milk withdrawal times should always be observed.
Liver flukes can be a problem for cattle on pasture. Most cattle in the U.S. can be exposed to two different types of liver flukes—the cattle fluke, Fasciola hepatica, and the deer fluke, Fascioloides magna. The cattle liver fluke is the one we see most, and the most widely distributed geographically.
The deer fluke can be very difficult to control in cattle. Both species of flukes can kill cattle, but it is unusual to see cattle liver fluke actually killing cattle. Most of the time they just cause chronic slowly-developing disease that reduces weight gain or causes weight loss and reduces overall animal health. Cows can become poor doers and eventually get culled. More serious is the fact that flukes damage the liver, which may set cattle up for other health issues and redwater disease.
Liver damage negatively affects nearly everything the body needs to do in converting nutrients into utilizable proteins, energy, vitamins, etc. Thus liver flukes can affect growth in young cattle, but this is usually a relatively slow-developing problem.
The fluke life cycle depends on snails as an intermediate host. The flukes start their life cycle in tiny freshwater snails but end up in the liver of a grazing animal (cattle, sheep, elk, deer) where they grow and mature in the liver and pass their eggs.
There are only a few products available in the U.S. for treating liver flukes in cattle. Triclabendazole is the first choice worldwide, because it kills immature flukes as well as the egg-laying adults, but it is not registered for use in cattle in the U.S—even though it can be used in humans.
The traditional treatment for cattle flukes is Albendazole (Valbazen), which works quite well to kill mature flukes in the liver. But it doesn’t work as well for deer flukes. Timing of treatment is very important. It will be different in the Southeast than in the Northwest.
Work with their veterinarian regarding products, dosage and timing, since you need to treat the cattle at a certain stage of the fluke life cycle. The flukes need to be at a certain level of maturity before they are susceptible to the drug. There are also some cautions about using Albendazole during the first trimester of pregnancy
May 2026
By Heather Smith Thomas
