Cattle Parasites

Parasites are one of those farm problems that hide in plain sight. Unlike a cow with a visible injury or an obvious respiratory infection, a heavily parasitized animal often just looks like a somewhat unthrifty cow - rough coat, slower gains than her pasture mates, maybe a little dull in the eye. The performance losses are real but they're incremental, and without something to compare against it's easy to assume the cattle are just doing okay rather than recognizing that they're running well below where they should be.

Every cattle producer dealing with internal parasites is essentially dealing with an arms race they can't win outright. Stomach worms, lung parasites, and liver flukes are facts of life for cattle on pasture in most of the United States, and the goal of a parasite management program is not elimination - it's control. You want to keep the parasite burden below the threshold where it causes measurable performance loss, and you want to do that in a way that doesn't create drug-resistant populations that will be harder to control in the future.

Resistance is the elephant in the room with cattle parasite management. Decades of treating cattle on a rigid calendar schedule - spring and fall, everybody gets dewormed - has created populations of stomach worms in many areas of the country that are partially or fully resistant to the most commonly used dewormers. This is not a theoretical problem. Fecal egg count reductions of less than 95 percent after treatment indicate resistance, and some producers are finding counts in the 60 to 70 percent range, meaning almost a third of the worm population is surviving treatment. Changing how and when you use dewormers is the most important thing the cattle industry can do to slow the progression of resistance, and it starts with individual producers making smarter treatment decisions.

External parasites - flies, lice, ticks, and grubs - cause a different but equally real category of problems. Horn flies alone have been documented to reduce beef cattle gains by 10 to 20 pounds per animal per season, which adds up to real money across even a small herd. Face flies spread pinkeye. Ticks spread anaplasmosis and other diseases. Cattle grubs, which are the larvae of heel flies, tunnel through tissue and cause welfare problems as well as hide and meat damage. A good external parasite program addresses these in sequence through the season.

The foundation of a sound internal parasite program is knowing what you're dealing with before you treat. Fecal egg counts are the tool that makes this possible - a small fresh manure sample processed by a veterinary laboratory tells you how many parasite eggs per gram are present, and when done before and after treatment, it tells you whether your dewormer is working. This is not an expensive test and it provides information that changes treatment decisions in ways that prevent wasted money and unnecessary drug use. If you're currently treating cattle on a calendar schedule without ever checking whether the treatment actually worked, fecal testing is the single most useful upgrade you can make to your parasite program.

The major internal parasite classes you'll deal with are gastrointestinal nematodes (stomach worms, primarily Ostertagia and Cooperia species), lungworms (Dictyocaulus viviparus), and liver flukes (Fasciola hepatica and Fascioloides magna). In most of the humid regions of North America, gastrointestinal nematodes are the dominant concern. Youngstock - calves and yearlings - are considerably more susceptible to stomach worm burdens than adult cattle, and animals under two years old carry the heaviest burdens and suffer the most significant performance losses from them. Mature cows develop significant immunity over time and typically don't need the same aggressive treatment schedule as young cattle.

There are three main drug classes used for cattle deworming, and using them strategically rather than rotating them indiscriminately is the key to resistance management. The benzimidazoles (fenbendazole and albendazole) have been widely used for decades and resistance to them is common in some regions. The macrocyclic lactones (ivermectin and doramectin) are highly effective pour-ons and injectables that also cover external parasites. The imidazothiazoles (levamisole) represent a third class with a different mechanism of action. Using multiple drug classes in a single treatment - what some call combination therapy - may slow resistance development and can improve efficacy in situations where resistance to individual products is documented.

Liver fluke management requires specific flukicides because the standard dewormers are not effective against liver flukes. Fluke infections are most common in areas with wet pastures, standing water, or poorly drained lowlands where the aquatic snail intermediate host can survive. Not every cattle producer needs to worry about liver flukes - if your land is well-drained and historically free of snail habitat, your risk is low. But in fluke-prone areas, this parasite causes significant liver damage that reduces feed efficiency, impairs immune function, and in heavy infections causes mortality, particularly in young cattle.

For external parasites, a comprehensive seasonal program covers several phases. Spring horn fly emergence is the trigger to apply insecticide ear tags or begin pour-on fly control. Horn flies spend nearly all their time on the animal's back, shoulders, and sides and are easy to count - when you see more than 200 flies on a single animal, that's above economic threshold and fly control is warranted. Lice are a winter and early spring problem because they thrive in the dense winter coat and die back as cattle shed in spring. Heavy lice infestations cause intense rubbing and hair loss and can be significant welfare and production issues in cattle that are already nutritionally stressed in winter.

Daily parasite management is mostly built into the habit of observant cattle watching. When you're doing your routine walks, you should be noting body condition on youngstock in particular, looking for the telltale signs of a heavy worm burden - rough or staring coat, poor condition despite adequate feed, submandibular edema (swelling under the jaw, sometimes called bottle jaw) which indicates low protein from chronic parasite drain. Not every rough-coated calf has worms, but a group of calves that are consistently behind where they should be in late summer is worth a fecal check.

Fly counts during summer are worth a quick mental note every time you're in the pasture. Horn flies cluster on the animal's back and sides, and the threshold that matters practically is around 200 flies per animal. A quick scan tells you whether your ear tag or pour-on program is holding. Tags lose effectiveness as the season progresses and the chemical load they carry depletes - don't assume tags that worked in June are still working in September.

Rotational grazing is one of the most powerful non-chemical tools for internal parasite management. Moving cattle to fresh pasture before the larvae they've deposited through feces have had time to develop to infective stage, and resting grazed pastures long enough for larvae to die off before regrazing, reduces the pasture larval challenge significantly. This doesn't eliminate the need for deworming, but it can meaningfully reduce the frequency at which treatment is needed and the overall parasite pressure the herd faces.

Ostertagiasis - heavy stomach worm infection - is the most economically important internal parasite disease in beef cattle in North America. Type I ostertagiasis occurs in young growing cattle in summer when larval challenge on pasture is high, causing diarrhea, weight loss, and poor condition. Type II ostertagiasis is a late winter or early spring disease in animals that accumulated large numbers of inhibited larvae in the stomach lining during fall, which then emerge simultaneously and cause severe damage to the stomach wall. Type II can hit adult cattle that appear to have been doing reasonably well and cause rapid severe illness. Your vet may recommend strategic deworming in late fall specifically to kill inhibited larvae before they emerge.

Anaplasmosis is a tick-transmitted disease that causes severe anemia in adult cattle, particularly animals over two years of age. Young calves can get infected and remain carriers without showing clinical signs, which is one of the reasons the disease spreads within herds. Clinical anaplasmosis presents as sudden weakness, labored breathing, pale mucous membranes, and sometimes yellow discoloration of the eyes from jaundice. It can kill quickly. In tick-endemic areas, anaplasmosis prevention through tick control and in some situations through chlortetracycline in the mineral are important components of herd health management.

Pinkeye is the most common infectious eye disease in cattle and face flies are the primary vector. Face flies feed on secretions around the eyes and nose and physically spread the bacteria Moraxella bovis from animal to animal. Pinkeye causes painful corneal ulcers, temporary or permanent blindness in affected eyes, and significant production losses from the pain and visual impairment. Face fly control reduces pinkeye incidence significantly. Affected animals need prompt treatment - antibiotic ointment or injection is effective early, but advanced cases with severe corneal damage may require veterinary intervention including eye patches and long-acting antibiotics.

Breed differences in internal parasite resistance are real and are increasingly recognized as a factor worth considering in cattle selection. Brahman-influenced breeds and cattle with significant Bos indicus genetics generally show better acquired immunity to gastrointestinal nematodes than straight European Bos taurus breeds. Studies in warm, humid regions of the South have documented meaningfully lower fecal egg counts in Brahman crosses compared to straightbred Angus and Hereford cattle under the same conditions. This resistance advantage has contributed to the popularity of Brahman crosses for humid subtropical production systems where parasite pressure is highest.

Within the British beef breeds, there is individual animal variation in parasite resistance that is heritable enough to be worth selecting for. Animals that consistently carry low worm burdens while their pasture mates are accumulating high burdens are what researchers call refugia animals, and identifying them through fecal testing over multiple seasons is possible on individual farms. Breeding bulls with tested low parasite susceptibility is an emerging selection criterion in some purebred programs.

Dairy breeds in intensive management are typically dewormed on a more regular schedule than pasture-based beef cattle because of the continuous close confinement that allows larval buildup in housing areas. The principles of resistance management apply equally to dairy operations, but the treatment logistics are different when animals are not on open pasture. Dairy cattle are also more routinely tested for Johne's disease, which can be confused with parasite burden because both cause chronic scours and weight loss - distinguishing between the two requires testing.

Miniature cattle breeds have the same parasite susceptibilities as full-size cattle. Their smaller body weight means the appropriate dose of dewormer needs to be calculated carefully to avoid underdosing, which is one of the drivers of resistance development. Always dose by accurate body weight for small cattle, and don't estimate low.

Treating all cattle on a fixed calendar schedule regardless of actual parasite burden is the management practice most responsible for driving dewormer resistance on small farms. When you deworm every animal in the herd twice a year whether they need it or not, you're subjecting every worm population on the farm to constant drug pressure. The worms that survive that pressure - the naturally more resistant individuals - reproduce and pass on their resistance. Over years and decades, the population shifts toward resistance. The better approach is targeted selective treatment - identifying animals with high burdens through fecal testing or clinical signs, treating those animals, and leaving the lower-burden animals untreated so that a refugia population of non-resistant worms remains to dilute the resistant population.

Underdosing is one of the most effective ways to create resistant parasites. When you give an insufficient dose, you kill the most susceptible worms and leave the more resistant ones alive. Those survivors breed and pass on their resistance advantage. Underdosing happens for several reasons - body weight is estimated too low, the product is given at the wrong anatomical site (subcutaneous instead of intramuscular, for example), or pour-on products are applied on wet or dirty animals where absorption is compromised. Always weigh cattle or use a weight tape before treatment and dose to the heaviest animal in the group when you're using shared pasture management.

Using the same drug class repeatedly over years without any assessment of whether it's still working creates a resistance problem that eventually becomes unmanageable. If you've been using ivermectin pour-on every year for the last decade, there is a reasonable chance that your worm population has shifted toward resistance to it. A fecal egg count reduction test - fecal check before treatment, fecal check two weeks after - tells you definitively whether the product is working at 95 percent efficacy or better. This is not an expensive test relative to the value of knowing whether your treatment program is doing anything.

Ignoring external parasites because they seem like a minor nuisance rather than a real production issue is a mistake with a calculable cost. The documented impact of horn flies on beef cattle gain is 10 to 20 pounds per animal per season - at current cattle prices, that's real money on even a five-animal farm. Face fly control that reduces pinkeye incidence prevents not just the veterinary cost of treatment but the production loss from an animal that can't see well enough to graze and compete for feed. Adding up the combined impact of all external parasites makes a compelling case for a systematic fly and external parasite program rather than treating it as optional.

Buying cattle without considering their parasite and health history introduces problems to farms that might otherwise be well-managed. Animals purchased from high-density sale barns or from farms with known resistance problems can bring resistant worm populations with them. Running a fecal egg count on newly purchased animals before mixing them into your herd, and treating with a combination of drug classes at arrival, is standard biosecurity practice for parasite management. Quarantine of new arrivals for a minimum of two weeks before commingling gives you time to assess health and manage the parasite risk they carry.

Not accounting for seasonal variation in parasite pressure leads to poor timing of interventions. The time to treat is when the burden is high and growing, which varies by climate and season. In most temperate regions, spring and early summer are the highest-risk periods for larval challenge on pasture as overwintered larvae become infective and the warm, moist conditions favor larval survival. Fall treatment before housing targets inhibited larvae in beef cattle. Getting the timing right based on your region's typical parasite transmission patterns, rather than just when it's convenient, makes treatments more effective and reduces the total number of treatments needed.