Livestock producers in the United States continue to face extreme economic

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1 Vol. 23, No. 4 April 2001 S43 comments/questions to compendium@medimedia.com CE Article #8 (1.5 contact hours) Refereed Peer Review KEY FACTS Profitable bovine parasite control involves more than simply prescribing or applying a parasiticide. Producers are encouraged to read and follow label instructions before administering parasite control agents. Bovine Parasite Control Fort Dodge Animal Health Amarillo, Texas Bill C. Clymer, PhD ABSTRACT: Several factors should be considered when designing internal and external parasite control programs. This article discusses concepts of parasite management and the economic importance of those concepts. Little attention is devoted to specific life cycles and the biology of specific pests. New and different compounds for control as well as the importance of proper identification of the pests are outlined. The importance of fecal egg counts and the need for properly understanding product labels are addressed. Livestock producers in the United States continue to face extreme economic challenges as the high cost of production exceeds the commercial value of the products produced. The industry survivors will be producers who can adapt to changing trends and have management skills that are on the leading edge in both production and profit. Economic conditions call for veterinary decisions that have a proven cost benefit. Producers are willing to spend money on a particular management practice if they will receive a reasonable return on the investment. Practitioners should differentiate between making management decisions based on habit or tradition and making them based on the potential to improve profitability. This article discusses the important aspects of internal and external parasite control, with an emphasis on what practices will not only control the pests but also result in economic gain. It is important to remember that cattle are dewormed for only one reason to make money. The goal of veterinarians should to be to maintain the health of the animals while returning a reasonable profit. 1 IMPORTANCE OF PARASITE CONTROL The amount of actual parasite control producers engage in varies greatly in different regions of the country. Each segment of the cattle industry has its own unique problems, and many types of parasites occur in the different production systems. Surveys conducted over the past few years indicate that control efforts range from none to upwards of 85% of producers using some method of parasite control during the year. 2,3 In many parts of the country, the single biggest problem regarding parasite infestation is failure of producers to recognize and treat the problem. In a joint survey 4 conducted in 1993 by the U.S. Department of Agriculture Animal and Plant Health Inspection Service and the Federal Veterinary Services (FVS), producers were asked to rank what they considered to be the single most important economic problem in the cattle business. External parasites were rated as the biggest problem (79%) and internal parasites were rated second (67%). 4 Why were those two items ranked higher than reproduction, nutrition, and disease? Perhaps it is because more advertisements and education-

2 S44 Food Animal Compendium April 2001 al programs are available for parasite control. In addition, producers can see many of the external parasites, leading them to think more about this problem. In my opinion, internal parasites were ranked second but actually cause more economic damage than external parasites in most parts of the country. What is really the most important problem affecting profitability is debatable, but nutrition and reproduction are most likely the top-ranked items. 5 Because parasites can greatly impact both reproduction and nutrition, this brings their importance back to the forefront. After it has been established that a parasite problem exists, producers need to decide whether parasite control will be economically feasible. If the producer elects to use some type of control, then the method and timing of the treatment application should be determined. This decision should be based on research and not on advertisements. INTERNAL PARASITE INFESTATIONS Geographic region, management system, and the age and class of cattle significantly determine the type and severity of both internal and external parasites present in a specific operation. Stocker animals and replacement heifers are most likely to experience production losses and clinical disease as a result of internal parasite infestations. 6 These animals may continue to have high numbers of parasites until a certain amount of acquired immunity is attained at 15 to 18 months of age. 7 This particular age and class of animal may harbor extremely heavy worm burdens of up to one million per animal. 8 Although animals exhibiting clinical disease should call attention to the problem, subclinical signs often go unnoticed and can be damaging to the profitability of the operation. 9 Determining Parasite Burden These same animals also generally have high fecal egg counts or eggs per gram of feces (EPG). A 650-lb stocker with a count of 250 EPG may be contaminating the pasture with over one million eggs per day. 8 A cow and calf with light to moderate infections may place in excess of 50 million eggs on a pasture during a normal grazing season. If 30% of those eggs survive and produce viable larvae, 15 million larvae are put back on the pasture to reinfect the same or other animals. Fecal egg count is the technique most commonly used to determine relative numbers of parasites and potential parasite transmission. The usefulness of any assay depends on its accuracy. Proper use of the EPG value requires both knowledge of the biology of the parasite infestation in the sampling locale and experience with levels that are indicative of economic impact in a given situation. 10 Two distinct lines of thought exist among parasitologists concerning the use of EPG counts to determine parasite infections. Any EPG count indicates that at least one female worm is present and laying eggs. 10 These counts can be used to determine potential parasite transmission levels in groups of animals but not in individual animals. Therefore, EPG should not be used to measure production losses. A sample may be submitted that has very few to no eggs present. Later samples may indicate heavily infested animals with clinical signs caused by thousands of inhibited larvae (e.g., Ostertagia ostertagi) in the abomasum that later mature to adult worms. There may be high numbers of eggs present even though the particular genus (e.g., Cooperia species) is not known to cause economic losses to the specific age and class of animal being sampled. It is important to have larval cultures conducted on composite samples in order to determine what genera of worms are present. Few scientists are qualified to identify the parasites by looking at the eggs. In a careful study in which eight geometric parameters were subjected to statistical analysis by stepwise discriminant analysis, the success rate in identifying eggs from the five major genera ranged from 49% to 76%. 11 However, some genera such as Nematodirus are easily identified. Geography and Climate Differences in geographic regions also play a major role in determining the potential survival and infection potential of internal parasites in grazing animals. The most common nematode parasite in both warm and cool temperate regions is the brown stomach worm (O. ostertagi). This prolific parasite enters the lining of the digestive tract and can remain in a quiescent state for long periods. This usually occurs in northern climates during the winter months and in southern regions during the hot, dry summer months. 12 In many southern states, climatic conditions during the fall and winter grazing season provide an ideal situation for heavy parasite transmission to occur. Some of the practices implemented as a result of the winters in the northern states and in some dairies actually help keep parasite levels low. When animals are removed from pastures, infection levels will be lower because confinement precludes nematode transmission. 13 This does not mean, however, that the animals will not have internal parasites while they are in confinement. Animals can harbor infections for several months and can easily become reinfected if exposed to any grazing areas. In dairies, it is often thought that because of such management practices as total confinement, deworming

3 Compendium April 2001 Food Animal S45 will have no benefits. Although there may be some animals that have no parasite infestation, internal parasite infestations will occur in animals purchased from outside sources or those with access to grazing areas. This is especially true in replacement heifers. Replacement Heifers and Stocker Calves In a study 14 conducted on 13 Maine dairy farms over the course of a year, 262 of 263 dairy animals were found to be positive for nematode infections. During a research study conducted in Arkansas, 15 mature dairy cows in an untreated group had 1500 to 92,709 worms per animal. 15 In a recent Canadian trial, 16 dairy replacement heifers were divided into three groups at spring turnout. Two groups were treated with an endectocide, one at 3 and 10 weeks and one at turnout and 10 weeks; the third group remained untreated. The two treated groups gained 77.7 and 73.2 kg body weight, respectively, while untreated animals gained 57.9 kg over the 143-day grazing period. The untreated group also required more supplemental feed while on pasture. 16 In a milk production study, 17 internal parasite treatment yielded 3.5% to 5.8% increase over controls as measured over the entire posttreatment lactation. Producers should be reminded that a treatment at turnout will not provide adequate protection for the duration of the grazing season. In some areas, several treatments may be required during the year to effectively protect the susceptible animals from damaging infections. One primary objective is to prevent younger animals from adding to the pasture contamination. 18 Suppressive deworming, implemented early in the grazing season, is ultimately more effective than the same number of treatments administered after clinical signs are present. With the current price of stocker and weaned calves, not many additional pounds are necessary to pay for the cost of one or more treatments to control parasites. Numerous trials have been conducted to determine both efficacy and weight gain in stocker cattle. In a study 19 conducted in the southeastern United States comparing several endectocides, there was as much as a 57-lb difference between the untreated controls and animals treated with one of the products after a 112-day grazing period. 19 Mature Cows The decision to treat mature animals is more difficult. Cows normally have small numbers of gastrointestinal parasites and low egg counts. However, cows are a significant source of infection for younger animals and milk production may be affected if egg count numbers are high. If only one treatment per year is applied, it is normally administered in the fall. If two treatments are given, the second usually takes place at turnout in the spring. Deworming at branding is also becoming more popular. It is difficult to make determinations on the proper time of treatment unless all the parameters (e.g., rainfall, type of grazing program, geographic location) are known. In areas of moderate to heavy parasite burdens, deworming cows and calves will often improve the weights of the cow and her calf and also increase conception rates. Anthelmintic treatment of beef cows contributed to weaning weight advantages of up to 49.5 lb, with calf weight gains being positive in 26 of 27 studies. 20 In the same series of trials, conception rates improved in 25 of 33 studies. 20 EXTERNAL PARASITES External parasites (e.g., flies, mites, lice) can also have a marked effect on animals in confinement. Recent data indicate that the cattle tail itch mite (Chorioptes bovis) may cause a bigger reduction in milk production in confined dairy cows than was originally thought. 21 The major summer pests include the horn fly (Haematobia irritans), face fly (Musca autumnalis), and heel fly or cattle grub (Hypoderma bovis and Hypoderma lineatum). The major winter pests are four species of lice and the larval stage (warble) of the cattle grub. The list of other pests is extensive and includes stable flies, houseflies, ticks, mites, tabanids, and mosquitoes. 22 As is true of the internal parasites, the geographic location and time of year will dictate many of the problems with external parasites. The damage caused by arthropods on livestock can be divided into three general categories: (1) direct damage such as blood feeding, tick paralysis, damage to byproducts (e.g., leather), and altered behavior; (2) indirect damage such as disease transmission, injury, and stress-related pathogens; and (3) secondary infections and associated effects (e.g., flies associated with animal waste). 22 Flies Although flies make up a large percentage of cattle pests, the problems they cause vary with the type and location of the specific operation. In general, horn flies and face flies are pests of range cattle whereas houseflies and stable flies are pests of confined animals. Horn Flies Horn flies are obligate bloodsucking parasites responsible for the largest economic loss of any of the external parasites. 22 Both males and females spend most of their adult life on the host. Under ideal conditions, the life cycle may be as short as 2 weeks from egg to egg-laying

4 S46 Food Animal Compendium April 2001 adult. In the southern states, horn flies may reproduce year-round, and in the northern states they may overwinter in the pupal stage beneath a fecal pat. Cattle infested with horn flies spend most of their time trying to avoid flies, resulting in significant weight loss and reduced milk production. Numerous weight gain studies have shown significant differences in treated and untreated animals over the season. Haufe 23 found that the average daily gain of steers on good pasture increased 0.41 lb/day when horn flies were controlled. A 3.3% to 6.7% depression in weaning weight in calves is an indirect effect of decreased milk production caused by horn flies. 24 The exact economic threshold is often debated, but most researchers feel that somewhere between 100 and 200 flies per animal is the level at which economic loss may occur. There are several methods currently used for horn fly control; but due to the high migration of flies, no product or procedure will afford season-long control. Pyrethroid ear tags were first introduced in the mid-1970s with great success. However, within a few years, resistance was observed and has since spread through much of the United States. 25 Several new tags have been developed that provide good control. If ear tags are to be used, however, a rotational program should be implemented and the tags should be removed as soon as their efficacy starts to decrease. Treatment with a different class of chemicals (e.g., macrocyclic lactone pour-on products) often will help extend the time before resistance develops to the active ingredient in ear tags. An advantage of the endectocides is their ability to control both internal and external parasites. Good control of horn flies is usually obtained with the pour-ons, but they are generally not recommended for this pest alone because of their cost. If one herd in a small pasture was treated, control may only last for 7 to 10 days. If all the herds in the area were treated, effective control may last 5 to 6 weeks. Face Flies Although face flies are not blood feeders, they have a life cycle similar to the horn fly. They feed on various secretions such as tears, saliva, nasal mucus, wound exudate, perspiration, and manure and bedding that adhere to the haircoat. They congregate around the eyes of livestock, causing the animals to take the same type of evasive action that they do with horn and stable flies. As a vector for the transmission of Moraxella bovis, the face fly has been linked to severe outbreaks of infective keratoconjunctivitis. If M. bovis is not present, it appears that moderate infestations of face flies do not affect average daily gain. 26 Face flies are more restrictive in habitat than horn flies and are also more prevalent along waterways, in areas with an average annual rainfall of 30 inches or more, and on irrigated pastures. 26 Face flies are more prevalent in the eastern states. They overwinter as adults in farm buildings, houses, trees, and other protected areas. As the weather becomes warmer in the spring, the flies leave the sheltered areas and return to the cattle. The various methods of control that work for horn flies have been used with limited success for face fly control. 22 Insecticides must be applied daily because the flies spend considerable time off the host. Because flies congregate on the face, the insecticide must be on that area to be effective. Face flies are as attracted to calves as they are to mature animals. If dust bags or oilers are used, they should be positioned low enough to treat the face of both cows and calves. Often it is mandatory to force the animals to use the self-application devices to achieve proper control. Stable Flies and Houseflies The stable fly (Stomoxys calcitrans) is a blood-sucking parasite similar to the horn fly but about twice as large. They may breed in almost any decaying organic material, which they actually prefer over plain cow manure. If there is enough moisture to wet the material, spilled feed mixed with dirt and manure can become an ideal breeding area in feedlots or dairies. Stable flies may also breed in wet organic matter around lakes and ponds and attack cattle pastured near such breeding areas. They are strong fliers and have been reported to move over 135 miles by accompanying a strong weather front. 27 The frequent use of large round bales of hay, which provide excellent breeding sites, has significantly increased stable fly numbers in many areas. Control of stable flies is difficult due to their habit of feeding on the lower front legs and then going elsewhere to digest their blood meal. Pens should be cleaned to decrease fly breeding. Sprays provide only temporary control because the insecticide is rapidly washed off in water or wet grass. The treatment of fly resting areas and space sprays for adult knockdown is often used in animal confinement facilities. Some of the newer-generation synthetic pyrethroids show promise as residual sprays. This method is effective, but the product is rapidly covered with dust, which reduces its effectiveness. Parasitic wasps (e.g., parasitoids) are currently being used as one of the tools in effective stable and housefly control programs. Little information is available for their effectiveness in controlling face and horn flies. Regular releases of these parasitoids during the fly sea-

5 S48 Food Animal Compendium April 2001 son are generally necessary to have an impact on serious infestations. Like all control procedures, this approach should be used in conjunction with proper sanitation, spot treatment with pesticides, and general management. Care should be taken when selecting a supplier to ensure a good-quality product. Prices may vary among suppliers, but a less expensive product may not be the best purchase. Houseflies (Musca domestica), which are generally considered to be more problematic in humans, have habits similar to stable flies; therefore, control measures are similar. They do not appear to migrate as great a distance as do stable flies, probably limiting their range to 2 to 3 miles. Baits are effective, but pockets of resistance have been reported. If baits are used, they should be placed in areas not accessible to pets and children. The parasitic wasps can also be used as a tool in the control of this pest. Other Flying Pests Other fly (Diptera) pests, which are especially prevalent when animals are pastured near waterways that experience flooding, are blackflies (Simuliidae), mosquitoes (Culicidae), and deer- and horseflies (Tabanidae). 25 These insects are strong fliers and intermittent blood feeders that feed on most warm-blooded animals. Their life cycle makes them extremely difficult to control. The immature stages are aquatic or semiaquatic and develop in or near streams, ponds, and various water sources. Lice Other major external pests of cattle include lice, grubs, ticks, and mites. There are four different species of bloodsucking cattle lice in the United States (longnosed cattle louse [Linognathus vituli], short-nosed cattle louse [Haematopinus eurysternus], little blue louse [Solenopotes capillatus], and cattle tail louse [Haematopinus quadripertusus] ) and one species that feeds on the haircoat, skin, and skin exudates. The chewing or biting louse is commonly called the little red louse (Bovicola bovis) 26 and can reproduce rapidly with as many as 9 of 10 eggs hatching to be reproductive females. Males can be found but are not necessary for reproduction. The life cycle of all five species is similar. All are host specific and spend their entire life on cattle. The cattle tail louse is normally found only in the southern tip of Florida, southern Gulf Coast, and the Caribbean. It is also the only species that seems to be more prevalent in the warmer months of the year. The eggs, which are glued to the hairs of the host, will usually hatch in 10 to 14 days but may take longer depending on weather conditions. 28 The long life cycle of the louse increases the difficulty of control. Immature lice (nymphs) have the same feeding habits as the adults. There are numerous products labeled for lice control. Their efficacies vary, but no products currently on the market provide season-long control with one treatment. 22,25,26 A reason for breaks after the initial treatment is reexposure and subsequent reinfestation. Timing of application for lice control is important and may dictate the success rate of the treatment. All major manufacturers generally guarantee their products. However, many companies now offer specific lice guarantees. These guarantees do not ensure that retreatment will not be required and should be viewed accordingly. Cattle Grubs Two species of cattle grubs also belong to the Diptera family: the common or southern cattle grub (H. lineatum) and the northern cattle grub (H. bovis). The adult grub is referred to as a heel fly and causes cattle to gad (become frenzied and run wildly) when oviposition is occurring. The eggs are laid on the underside or lower legs of the animal. Yearling cattle and animals younger than 3 years of age usually have the highest numbers of grubs, probably due to individual animal resistance or tolerance in older animals. 25,26,29 The common cattle grub usually attacks cattle in the late spring or early summer and is the species most frequently observed. The young grubs hatch from the egg and bore into the skin at the hair follicle. The grub spends approximately 237 days migrating from the site of entry to the animal s back. 30,31 During this migratory phase, the grub is about the size of a grain of rice and is in the first of three instars. On entering the back, two moltings (instars) occur as the grub goes through the warble stage, which lasts approximately 60 days. 32,33 While in the back, the grub is difficult to kill because it is essentially walled off from the rest of the body and little exchange can take place. Natural mortality is normally high, ranging from 50% to 100%. When the grub reaches larval maturity, it emerges from the warble and drops to the ground to pupate and start the cycle over again. The northern cattle grub has a life cycle similar to the common species. However, instead of migrating to the subesophageal region and then to the back, the northern grub spends about the same amount of time in the animal but goes directly to the back. Adult activity may occur later in the year (even in September or October). Ideal treatment time for either species is soon after adult activity stops. The numbers of grubs found in cattle has decreased over the last few years because of the widespread use of systemic products, especially the

6 Compendium April 2001 Food Animal S49 macrocyclic lactones. Some of these products no longer have a treatment cut-off date on the label, primarily because of infrequency of reaction problems. Most feedyards treat all cattle on arrival with almost no known reactions. Although there have been some fears of resistance with cattle grubs due to an occasional failure, there have been no confirmed resistance cases to date. In many instances, the control failure actually resulted from fly strikes after the treatment had been applied. Mites Mites are also important cattle parasites. From the late 1960s to the late 1970s, the common scab mite (Psoroptes ovis) caused enormous financial losses to the cattle industry. In addition to causing itching and irritation, the scabies mite is detrimental to weight gain, milk production, and feed conversion. Heavily infested animals may experience more than a 56% decrease in average daily gain. 34 The mite was and still is a reportable pest that requires quarantine. Scabies mites are active but do not burrow like the sarcoptic or demodectic mite. Animal-to-animal contact spreads scabies mites. Scabies infestations, when first observed, resemble lice infestation. Small patches of haircoat may be missing, and the animal will lick and rub these areas, leading to severe abscesses, bruises, and cuts. Affected areas enlarge rapidly and can quickly spread over the entire body. In severe infestations, the skin thickness may double, resulting in an elephant hide appearance. Scabies was originally thought to be a cold-season problem, but severe infestations can occur during the warmer months if the animal has a long haircoat or cannot groom itself. Animal grooming is responsible for a vast majority of psoroptic mite control. In pasture situations, only about 20% of the animals develop severe lesions, although a much higher percentage will be positive for mites. Only a limited number of products are currently approved by the FVS for control. Two products (coumaphos and phosmet) are approved for use in a dip vat but are seldom used. These products require treatment twice in a 10- to 14-day interval. Another mite that is occasionally a problem in beef cattle but appears frequently in confined dairy animals is the cattle tail itch mite (Chorioptes bovis). This mite, which feeds on skin debris instead of piercing the skin, is most often found on older, heavily producing cows. Recent data 21 indicate that the cattle tail itch mite may cause more loss in production than was originally thought. Demodectic mites are occasionally found on dairy animals but little data are available on their economic significance. Sarcoptic mites are rarely seen in the United States and are a reportable disease. Ticks Ticks may be present on animals at any time of year. When they are observed on incoming animals, control may be achieved by spraying or dipping with an approved insecticide. Self-treatment devices are not as effective for tick control as they are for horn flies and lice. Pour-ons, spot-ons, and injectables generally reduce tick numbers but do not provide complete control. If native populations of ticks exist, animals continually become reinfested. The fire ant problem in the southeastern United States has greatly reduced the numbers of ticks in those areas. Not only can ticks cause excessive blood loss, but many species have also been identified as important disease vectors. Ticks cause large financial losses to the cattle industry but also impact the use of recreational areas. ROLE OF PARASITICIDES The identity of nematode genera is important in a control program. The currently registered products list species and life stages that are controlled by the use of the respective product. Many of the species listed are limited in distribution and may not cause economic losses in the particular age and class of cattle being treated. Some nematodes, even though present in high numbers, cause little if any economic damage. Some products kill the worms that are present in the digestive tract at the time of treatment, some control the external parasites, and others have persistence of several weeks. The introduction of avermectins into the industry in the mid-1980s significantly increased efficiency in the control of both internal and external parasites. These agents also provide variable persistence against many internal parasites. The ivermectin molecule dominated the market for several years, and then two avermectins (Dectomax [Pfizer Animal Health, Exton, PA] and Ivomec Eprinex [Merial, Iselin, NJ]) were introduced. In 1997, the first milbemycin product (Cydectin [Fort Dodge Animal Health, Overland Park, KS]) for cattle entered the market. Ivomec Injectable (Pfizer Animal Health) and Cydectin Pour-On (Fort Dodge Animal Health) also are approved for use on psoroptes mites. Since then, numerous branded avermectins have become available. The competition has decreased the price and made the macrocyclic lactones much more affordable. The use of these products is also becoming more sophisticated. Although they are all good products, some offer certain advantages. Practitioners should familiarize themselves with each of the products and make choices based on product characteristics rather than on cost alone. The macrocyclic lactones, including the avermectins (ivermectin and doramectin) and the milbemycins

7 S50 Food Animal Compendium April 2001 (moxidectin), provide some external parasite control. With the extended control period added to the prepatent period, these products are able not only to clean out the worms that are present at the time of treatment but also to prevent the animals from becoming reinfected over a certain period. This will ultimately decrease pasture contamination, thereby causing lighter infestations for a period of time after treatment. This will decrease the number of larvae in the pasture over time. 10 This new technology enables better management of pastures and decreases the reinfection rate of cattle. Although all the macrocyclic lactones have some activity against the various species of Cooperia, these genera of nematodes appear to be one of the weaker links. Cooperia species, as a group, are of less economic significance than abomasal nematodes in cattle other than young calves, and then only for a few weeks. Prior to the discovery of the endectocides, most internal parasite control was accomplished using white dewormers, which include the majority of the products (i.e., benzimidazoles) that came on the market before Many of these agents still do an effective job and are cost-effective. They do not, however, offer the broad-spectrum benefits that the macrocyclic lactones do. SUMMARY Producers view internal and external parasites as profit-limiting problems to the cattle industry, and sufficient data exist that show economic returns from many of the control procedures. 6,19,20,22 24,34,35 Geographic and climatologic variations across the United States require different control programs for each specific operation and location. The different types of cattle operations also dictate different and often special types of control. With nematodes, younger animals and replacement heifers are usually the most severely affected. However, all ages and classes of cattle may have damaging infections or infestations of parasites. Proper identification of the specific parasite and an evaluation of its importance are also necessary for proper treatment. There are numerous labeled products to aid in the control of both internal and external parasites. Care should be taken to choose the control procedure and product best suited for the specific use. Unfortunately, parasite control is often neglected during times of economic difficulties. When profit margins are low, the need for economic control of parasites is even more important. Proper safety and use of pesticides are a must. Producers should be encouraged to read and follow label directions when they administer products for parasite control. Specific withdrawal periods should be followed. REFERENCES 1. Herd RP: Treatment of dairy cattle with subclinical intestinal parasitic infections, in Mettrick DF, Dessser SS (eds): Parasites Their World and Ours. 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Vet Rec 105: , Snyder DE: Experimental design and critical evaluation of anthelmintic production trials with stocker cattle. Compend Contin Educ Pract Vet 15(5): , Williams JC, Bilkovich FR: Development and survival of infective larvae of the cattle nematode, Ostertagia ostertagia. J Parasitol 57: , Gasbarre LC, Leighton EA, Bryant D: Reliability of a single fecal egg per gram determination as a measure of individual and herd values for trichostrongyle nematodes of cattle. Am J Vet Res 57: , Georgi JR, McCulloch CE: Diagnostic morphometry: Identification of helminth eggs by discriminant analysis of morphometric data. Proc Helminthol Soc Wash 56:44 57, Prichard RK: Anthelmintics and control. Vet Parasitol 27: , Herd RP, Riedel RM, Heider LE: Identification and epidemiologic significance of nematodes in a dairy barn. JAVMA 176: , Yazwinski TA, Gibbs HC: Survey of helminth infections in Maine dairy cattle. Am J Vet Res 36: , Yazwinski TA, Tucker C, Copeland S, et al: Dose confirmation of moxidectin pour-on against natural nematode infections in lactating dairy cows. Vet Parasitol 86: , Elsner J, Villeneuve A, Descoteaux L, et al: Evaluation of a strategic deworming program in dairy heifers in Quebec based on the used of moxidectin, an endectocide with a long persistency. Can Vet J 42:38 44, Murphy AW: The effect of treatment with moxidectin, a long lasting endectocide, on milk production in lactating dairy cows. Proc 20th World Assoc Buiatr Cong: , Michel JF: The epidemiology and control of some nematode infections in grazing animals. Adv Vet Parasitol 7: , Williams JC, Loyacano AF, DeRosa A, et al: A comparison of persistent anthelmintic efficacy of topical formulations of doramectin, ivermectin, eprinomectin, and moxidectin against naturally acquired nematode infections of beef calves. Vet Parasitol 85: , Reinemeyer CR: The effects of anthelmintic treatment of beef cows on parasitologic and performance parameters. Compend Contin Educ Pract Vet 14(5): , 1992.

8 Compendium April 2001 Food Animal S Leslie K, Duffield T, Ten Hag J: Mange can rob you of milk. Hoard s Dairyman January 25, 2000, p Drummond RO, George JE, Kunz SE: Control of Arthropod Pests of Livestock: A Review of Technology. Boca Raton, FL, CRC Press, 1988, pp Haufe WO: Growth of range cattle protected from horn flies (Haematobia irritans) by ear tags impregnated with fenvalerate. Can J Anim Sci 62: , Campbell JB: Effect of horn fly control on cows as expressed by increased weaning weight of calves. J Econ Entomol 69: , Bay DE, Harris RL: Introduction to Veterinary Entomology. Bryan, TX, Stone Fly Publications, 1988, pp Williams RE, Hall RD, Broce AB, et al: Livestock Entomology. New York, John Wiley & Sons, 1985, pp Hogsette JA, Ruff JP: Stable fly (Diptera: Muscidae) migration in northwest Florida. Environ Entomol J 14:17 175, Colwell DD: Life history parameters of the sucking louse Linognathus vituli. Abstr Entomol Soc Canada/Entomol Soc Saskatchewan Joint Annu Meet:45, Colwell DD: Persistence of cattle grubs (Diptera: Oestridae) on a Canadian ranch with long-term, continuous control. Vet Parasitol 94: , Scholl PJ, Guillot FS, Wang GT: Moxidectin: Systemic activity against common cattle grubs (Hypoderma lineatum) (Diptera: Oestridae) and trichostrongyle nematodes in cattle. Vet Parasitol 41: , Scholl PJ: Biology and control of cattle grubs. Annu Rev Entomol 39:53 70, Pruitt JH, Kunz SE: Warble stage development of third instars of Hypoderma lineatum (Diptera: Osetridae). J Med Entomol 33(2): , Scholl PJ, Schwinghammer KA, Chamberlain WF: Technique for age-grading late third-instar Hypoderma lineatum (Diptera: Oestridae). J Med Entomol 26: , Fisher WF, Wright FC: Effects of the sheep scab mite on cumulative weight gains in cattle. J Econ Entomol 74: , Schmidtmann ET, Berkebile D, Miller RW, Douglass LW: The face fly (Diptera: Muscidae): Effect on Holstein milk secretion. J Econ Entomol 77: , About the Author Dr. Clymer is a livestock parasitologist with Fort Dodge Animal Health, Amarillo, Texas. For the past 30 years, he has been involved in private research and consulting and has helped develop many veterinary products being used today. ARTICLE #8 CE TEST The article you have read qualifies for 1.5 contact hours of Continuing Education Credit from the Auburn University College of Veterinary Medicine. Choose the one best answer to each of the following questions; then mark your answers on the test form inserted in Compendium. 1. According to a recent cattle producer survey, what were considered to be the two most important profit limiting items in cattle production? a. reproduction and nutrition b. internal and external parasite control c. hay and grain prices d. diseases and government intervention 2. What age and class of cattle are most likely to experience production losses and clinical disease as a result of internal parasites? a. newborn calves and older cows b. herd bulls and replacement heifers c. feedlot steers and heifers d. stockers and replacement heifers 3. At what age do most bovines reach an acquired immunity level to internal parasites? a. 12 to 15 months c. 6 to 8 months b. 24 months d. 15 to 18 months 4. In what season do brown stomach worms often become inhibited in the northern states? a. spring c. fall b. summer d. winter 5. Which nematode parasite is considered to be the most common and also most important economically in both the warm and cool temperate regions? a. lungworm c. threadneck worm b. brown stomach worm d. barber pole worm 6. What unique characteristic do all the macrocyclic lactones have in common that other parasiticides do not possess? a. broad spectrum of activity b. numerous formulations c. alcohol formulation d. activity against internal and external parasites 7. What is considered to be the most economically important external parasite of cattle? a. stable fly c. common cattle grub b. horn fly d. Texas cattle fever tick 8. Which of the following is not one of the three general categories of damage caused by arthropods (external parasites)? a. secondary c. direct b. indirect d. primary 9. What is the common name for the cattle louse B. bovis? a. long-nosed louse c. little blue louse b. flat-nosed louse d. little red louse 10. How many days are normally required from the time a common cattle grub enters the host s body until it is ready to drop to the ground and pupate? a. 175 to 185 c. 350 to 360 b. 290 to 300 d. 240 to 250