Range cattle are affected by external and internal parasites. External parasites include flies, lice, mites, cattle grubs, and ticks. Most of the economically important internal parasites are gastrointestinal nematodes (worms). The economic impact of parasitism cannot be easily determined because many external parasite species are very mobile. This makes it difficult to compare treated and untreated cattle because of reinfestation of the treated animals. Also, fecal egg counts cannot be relied upon to give a true indication of the level of internal parasitism. An animal stressed by parasites may also be stressed by other factors such as inadequate nutrition and inclement weather (heat or cold stress). The impact of only one stressor may not cause a decrease in animal performance, but the addition of a second stressor is additive and may result in reduced animal performance.

External Parasites

Horn Fly

The most common external parasite of range cattle is the horn fly. It is a small, obligate parasitic fly that feeds on cattle blood. Females deposit eggs in fresh cow manure where larval development occurs. In the south, the fly may breed throughout the year, while in the north, it overwinters in the pupal form in or near manure pats and starts emerging as an adult in late April and May. During the summer, the life cycle can be completed in two weeks or less. Left untreated, this results in very high horn fly numbers on cows.

Numerous trials have evaluated the economic affect of horn flies on cattle production. Several research trials conducted from the late 1970s to early 1990s looked at the effect of horn flies on cows and resulting calf weaning weights. Data from these trials indicate a general reduction in weaning weights of 10-20 lb per calf when horn fly numbers exceeded 200 flies per cow. Other studies noted similar losses in horn fly infested yearlings.

Face Fly

Like the horn fly, the face fly also deposits eggs in cow manure. But, the face fly requires about three weeks in the summer to complete its life cycle, and it over winters as an adult. This fly feeds on mucous around the eyes and nose. The face fly resembles the housefly in size and color, but has different mouthparts. The mouthparts are used in a rasping-type of feeding action that irritates the eyes of cattle, causing tearing which attracts more face flies. The feeding action causes abrasions on the ocular tissue (cornea and sclera) of cattle which provides an avenue for the transmission of Moraxilla bovis, the causative agent of pinkeye.

Cattle with a high infestation of face flies will bunch together. Consequently, grazing time is reduced. Face flies, unlike the horn fly which infests calves only when numbers on cows are high, are at least equally attracted to calves. Therefore, control efforts must be directed at both cows and calves, but horn fly control can be achieved by treating only the cows. The main economic loss caused by the face fly is the transmission of pinkeye.

Stable Fly and Others

In recent years, stable flies, which are generally considered a pest of confined cattle, have become a pest of grazing cattle as well. Nebraska data indicates the economic impact of stable flies on grazing yearling cattle is similar to that of feedlot cattle (about 0.44 lb/day/calf).

While mosquitoes, biting gnats, black flies, and deer and horse flies are only sporadic pests in the north, they are serious cattle pests in the southeastern part of the U.S. Summarized economic data for mosquito attacks on Hereford, Brahman, and H & B crossbreds indicate injury thresholds are 27, 50, and 33 blood-fed mosquitoes per 0.09m2 resting near cattle. These population levels caused weight gain losses of 0.1 lb/day/steer. Mosquito populations of this magnitude are common in the southeast.

Horse flies can bother cattle enough to depress gains. Hereford heifers protected from six species of horse flies gained 1.98 lb/head/day more than unprotected animals, and the protected cattle were 16.9% more feed efficient. In a 38-day trial with 0-9 horse flies/cow/day, cattle weight gains were reduced 0.64 lb/head/day.

The economic effect of biting gnats has not been determined. Biting gnats are known to transmit blue tongue virus and probably epizootic hemorrhagic disease (EHD) which is a viral pathogen closely related to the blue tongue virus. Although these diseases primarily affect white tail deer and sheep, it has caused abortions in naïve (non-exposed) first-calf heifers.

Also, there is no data that relates number of black flies to economic losses for cattle. Black flies inject a vasoactive histamine when feeding which causes a toxic reaction. Black flies have been known to cause death from anaphylaxis or asphyxiation under outbreak conditions.

Grubs

There are two species of cattle grubs (heel flies) in the U.S., the Northern and the Common cattle grub. Heel flies deposit eggs on hairs of cattle, usually on the hind legs. The larvae hatches from the egg and bores into the skin at a hair follicle site. The grub spends the next 7-8 months as an internal parasite migrating through the subcutaneous tissues of the animal. Eventually, it arrives at the back of the animal where it cuts a breathing hole and finishes its larval development. The walled off larval site is termed a warble. When larval development is completed, the grub emerges from the back through the breathing hole and falls to the ground where it pupates in a protected area. As temperatures warm, the adult heel fly emerges, mating occurs, and reinfestation of cattle takes place.

Economic losses probably occur due to cattle fleeing from heel flies. Cattle run from the fly attack with their tails curled over their backs (gadding). Weight gain depressions on grub-infested feedlot cattle have been reported at 0.12 lb/head/day. The impact of grubs on yearling cattle maintained on a maintenance ration was similar to the feedlot losses while grubs were in the back, but some compensatory gains occurred after the grubs had emerged, and the wounds in the back had healed.

Probably the greatest economic effect of cattle grubs is evident at slaughter. Packers generally reduce the price of the animal about $5.00 per hundredweight if more than five grubs per animal are present. The grub holes in the hide reduce its value. The grubby area on the carcass must be trimmed which increases labor costs and decreases the value of the meat in the loin area.

Lice

Cattle lice may be underestimated in terms of economic losses to cattle. Studies indicate a fairly high louse infestation rate is required to affect animal performance. When cattle were fed lower levels of nutrition, the reduction in weight gain was greater. One study indicated louse populations declined on cattle fed a high nutrition diet, but remained fairly constant on cattle fed lower nutrition levels.

Cattle lice may not only affect weight-gain performance but also may affect the physiological well being of the animals. Several studies implicate cattle lice in lowering red blood cell numbers and hemoglobin ratios in blood which may cause anemia. Anemic animals are more susceptible to disease, particularly respiratory diseases that are prevalent in the winter when lice populations are at their highest level.

Prevention/Control of External Parasites

Population control of most external parasites can be achieved. Horn flies and face flies may be controlled with forced use of dust bags and oilers. Insecticide impregnated ear tags may also provide control, but retagging may be required to provide seasonal control. Feed additives (insecticides incorporated into feed or mineral) will control immature flies developing in manure. These insecticides pass through the digestive tract relatively undisturbed and are available in the manure to control fly larvae. Boluses are also available which are retained in the animal’s reticulum where the insecticide is slowly released and is then available in the manure. This approach to control works best on isolated herds. Migrating flies from close neighboring herds may cause failure in this methodology. The animals must consume the insecticide at least every 24 hours for it to be effective.

No efficient and effective control methods are available for stable flies on grazing cattle or the aquatic complex of mosquitoes, biting gnats, horse flies, and black flies. Insecticide treatment will provide temporary relief; however, it would have to be repeated too often to be practical for seasonal control.

Systemic insecticides are very effective for grub control, and the new ones (endectocides) are broad-spectrum, controlling both internal and external parasites. Cattle are generally treated for grubs in the fall. This reduces lice numbers but may not provide lice control throughout the winter. However, a second treatment in January should provide lice control for the rest of the winter.

Internal Parasites

The most common internal parasites of cattle are the round-worms (Nematodes) of the family Trichostrongylidae which includes Ostertagia, Trichostrongylus, Cooperia, Haemonchus, and Nematodirus. These worms have both a free living and a parasitic stage in their development. All of these species have similar life cycles but differ in infestation site, epidemiological factors, pathogenicity, and economic importance.

Adult roundworms reside in the abomasum and produce eggs that are passed from the animal in the feces. The eggs hatch in the manure and develop into the infective parasitic stage. The infective larvae move to grass and are ingested by cattle. This migration requires moisture. They also migrate vertically on the grass. Ingested larvae burrow into the lining of the abomasum to develop. When the larval development is complete, the worms emerge from the stomach lining and attach to the stomach wall where they feed on blood. After about three weeks, the female worms reach maturity and start laying eggs. A female may lay as many as 10,000 eggs per day.

Clinical signs of a severe worm infection, which are easy to detect, include diarrhea, loss of appetite, an unkempt appearance, and obvious weakness. Subclinical infections require fecal examinations for detection. The fecal egg count diagnosis method is often criticized as being inadequate. Worms may be present in the animal, but because of arrested development, may not be depositing eggs. Worms undergoing arrested development stop maturing within the host when environmental conditions are unfavorable for hatching survival or development. Nevertheless, fecal egg counts indicate to some extent what worms are present and suggests the level of worm infection.

Economic Injury Level

All parasites by definition are of some economic importance, but the difficulty in a treatment decision process is in determining the economic injury level (EIL). The EIL is the point at which costs of prevention or control of a parasite equals the value of the improved performance of the host (weight gain, milk production, reproduction rate) in the absence of the parasite. The EIL usually depends on the economic threshold (ET) of the number of parasites required to create the EIL. Knowledge of these factors is critical for establishment of an integrated pest management approach for parasite control. Since these factors aren’t known for most parasites, treatment decisions are simply based on the presence or absence of the parasite. Most research reports indicate animal performance improvement (weight gains, increased calving percentage, and calf survival) as a result of deworming. Economic thresholds for internal parasites are difficult to determine because they are subject to changes dependent on complex interactions and relationships of the host, parasite, environment, and management practices of the producer.

Prevention/Control of Internal Parasites

Many available anthelmintics provide acceptable control of internal parasites. However, control programs should be based on parasite life cycles, larval ecology, and the epidemiology of infections. An integrated parasite management program would limit contact between the parasite and the host by the use of anthelmintics, pasture management, and the host’s ability to acquire immunity. This approach would reduce the numbers of larvae on pasture at critical times, treat cattle when large numbers of infective larvae were probable, and remove cattle from heavily contaminated pastures. Local practicing veterinarians or university parasitologists can best make recommendations on what anthelmintics to use and when to use them. These individuals would be familiar with the parasite species prevalent in the region, the life cycles, the influence of climatic conditions on the life cycles, pasture and cattle management practices, and product efficacy for the region and state.

Conclusion

While it is difficult to establish economic thresholds for parasites, it is clear that parasites do negatively impact cattle performance. Various prevention/control methods and programs can be incorporated into the cattle management program to avoid production losses. The best prevention/control programs take into account the parasite life cycles, region, climatic conditions, management limitations, and product efficacy.

References available upon request.