Parasites in Dairy Cattle
Parasites in Dairy Cattle – Prevention and Treatment
All cattle are affected by biting flies unless they are properly controlled. Other external parasites, including lice, mites, and ticks, also reduce animal comfort and productivity.
Lice and mites cause severe skin irritation and itching, leading cattle to rub against fences and equipment, resulting in damage and production losses. Because treatments generally do not kill parasite eggs, retreatment about three weeks later is usually necessary.
Internal parasites include roundworms, tapeworms, and liver flukes. Roundworms typically have the greatest economic impact, particularly in grazing cattle.
External Parasites
Flies
Flies are among the most significant pests affecting dairy cattle.
Horn flies, stable flies, and horse/deer flies are blood feeders that are most active during warm months. Face flies are also common and can spread disease.
Mange mites burrow into the skin and hair follicles, causing severe irritation that may reduce milk production and growth in replacement heifers.
Understanding Fly Life Cycles
Fly control is most effective when it interrupts the insect’s life cycle.
All flies progress through four stages:
- Egg
- Larva (maggot)
- Pupa
- Adult
The three most important dairy pests are:
- Stable flies
- Horn flies
- Face flies
Stable flies develop in decomposing organic matter such as soiled bedding and spilled feed around confined cattle.
Horn flies and face flies develop in fresh cattle manure and are particularly troublesome in grazing herds.
Together, these flies can reduce milk production by 10% to 30% on organic dairies. They also reduce pasture intake and spread diseases such as pinkeye and other bacterial infections.
Integrated Fly Management
The most effective fly control programs use Integrated Pest Management (IPM) to maximize control while minimizing pesticide use and protecting beneficial insects.
Sanitation Is the Foundation
Good sanitation is the most effective and economical fly-control practice.
Poor sanitation cannot be overcome with pesticides alone.
Since the fly life cycle typically lasts 10 to 21 days, cleaning facilities weekly can interrupt reproduction.
Focus on removing breeding material such as:
- Manure
- Spilled silage
- Wet hay
- Feed debris
- Wet grain
Pay particular attention to:
- Calf hutches
- Holding pens
- Loafing sheds
- Walkways
- Milking parlors
- Stalls
Using sawdust bedding can also reduce fly development.
Manure should either be spread thinly to dry or stacked and covered with black plastic to prevent egg laying and kill developing larvae.
Additional Fly Control Options
Successful fly control combines sanitation with several management tools.
These include:
- Residual sprays
- Fly baits
- Space sprays
- Larvicides
Beginning control early is far easier and less expensive than trying to reduce large fly populations later in the season.
Stable Flies
Stable flies were once considered mainly a confinement problem because they breed in decomposing organic matter.
Today they are increasingly found on pasture, often around large round-bale feeding sites.
Research suggests that 95% of stable flies develop in less than 5% of the available habitat.
Cleaning around feed bunks, hay residue, wet areas, and feeding sites can dramatically reduce populations.
Horn Fly Traps and Vacuums
Mechanical control tools can effectively reduce horn fly numbers.
Researchers at the University of Minnesota West Central Research and Outreach Center (WCROC) evaluated both the Bruce Trap and the Spalding Cow-Vac™ on organic dairies.
Bruce Trap
The Bruce Trap is a walk-through box fitted with one-way screened baffles.
As cattle pass through, fabric brushes dislodge flies, which move toward the light, become trapped, and die.
Cow-Vac™
The Cow-Vac is installed at the entrance or exit of a milking parlor or barn.
As cows walk through, air blows flies off the animal into a vacuum collection system.
During a 2015 study involving eight Minnesota organic dairies, researchers found that:
- Horn fly numbers declined 44% where Cow-Vac units were installed.
- Stable fly and face fly numbers showed little change.
Lice
Lice are primarily a winter problem.
Although small populations survive summer, lice reproduce rapidly during cold weather when cattle have heavy winter hair coats.
Both chewing and sucking lice cause:
- Severe itching
- Hair loss
- Reduced milk production
Common signs include excessive rubbing and hair loss along the neck, shoulders, and back.
Because systemic dewormers control only sucking lice, pour-on insecticides are often needed to eliminate all species.
All cattle should be treated at the same time.
Most products require retreatment approximately three weeks later because eggs survive the initial treatment.
Mites
The primary mange mite affecting dairy cattle is Chorioptes bovis.
Infestations cause:
- Intense itching
- Hair loss
- Thickened skin
- Reduced milk production
Lesions most commonly occur on:
- Udder
- Tail head
- Legs
Treatment usually involves systemic acaricides or lime-sulfur sprays.
Ticks
Ticks mainly affect cattle on pasture.
The most serious species include:
- Cattle fever ticks
- Asian longhorned ticks
These parasites reduce milk production, damage hides, cause anemia, and transmit diseases such as:
- Babesiosis
- Anaplasmosis
- Theileriosis
Other species, including deer ticks and American dog ticks, are generally less damaging but still warrant control.
Organic producers may use essential-oil sprays containing rosemary, peppermint, cinnamon, lemongrass, garlic, or soybean oil.
Conventional options include insecticide ear tags, sprays, pour-ons, and dust formulations.
Internal Parasites
Roundworms
Roundworms are the most economically important internal parasite in cattle.
Young cattle with heavy infestations often grow more slowly.
Pastured cattle face the greatest risk because larvae develop on forage after eggs hatch in manure.
Heifers raised entirely in confinement rarely develop significant worm burdens.
Grazing Management
Rotational grazing can greatly reduce parasite exposure.
If cattle are moved before larvae become infective—generally within 21 days—worm burdens remain very low.
Longer grazing periods increase reinfection risk.
Deworming Programs
Strategic deworming depends on age and management.
Common treatment times include:
- 4 to 6 months of age.
- Before breeding.
- Three to four weeks before calving.
- At calving.
Always select products approved for lactating dairy cattle when treating milking animals.
Veterinary guidance is essential because some products require extended milk-withdrawal periods.
Monitoring Parasites
Fecal samples do not always reflect actual worm burdens.
Recently acquired worms may not yet be producing eggs, even though they are already affecting the animal.
Many veterinarians recommend fecal egg count reduction tests to evaluate whether dewormers remain effective and to monitor for drug resistance.
Liver Flukes
Liver flukes are another concern for grazing cattle.
The two primary species are:
- Fasciola hepatica (cattle liver fluke)
- Fascioloides magna (deer liver fluke)
Most infections cause chronic disease rather than sudden death.
Signs include:
- Poor weight gain
- Weight loss
- Reduced overall health
- Increased culling
Because liver damage affects nutrient metabolism, growth, reproduction, and overall performance also decline.
Treating Liver Flukes
The fluke life cycle requires freshwater snails as an intermediate host before infecting grazing animals.
Only a few products are available in the United States.
Albendazole (Valbazen) remains the standard treatment for mature cattle liver flukes, although it is less effective against deer flukes.
Treatment timing is critical because flukes must reach a susceptible stage before treatment.
Work closely with your veterinarian to determine the proper product, dosage, timing, and pregnancy precautions for your operation.
May 2026
By Heather Smith Thomas
