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Understanding Internal Parasites |
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Understanding Internal Parasites |
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Understanding Intestinal Parasites Article from Goat Rancher
April 2000 |
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Parasites of sheep and goats are often shared, in general, because both hosts make their living in a similar fashion. However, the two hosts are different in their dietary selection and ability to extract nutrients from forages. They are raised for different purposes by owners and are therefore managed under different circumstances in geographically diverse areas. There can be no one answer as to how to control parasites of either sheep or goats. However, there are several approaches that may be taken when one has a good idea of when and where parasites are being acquired and how parasites are surviving in the environment. There are few differences between sheep and goats as far as species of helminths they share, but the way in which the animals are maintained may make a considerable difference in the level of parasitism in each host. Whereas there are no differences between hosts, there may be breed and individual differences within the two species so that in local areas, one host species will have parasite problems, but not the other. |
CoccidiosisCoccidia appear to be host specific, not shared between sheep and goats. Depending on management, the pattern of disease varies from situation to situation. For instance, in Texas, dairy goat kids which are weaned at birth may develop coccidiosis when only a few weeks of age whereas Angora kids do not develop signs until six months of age, when they are weaned, shorn and crowded together. Coccidiosis is a disease of young or stressed individuals. If exposed to low numbers of oocysts early in life, the young develop immunity to disease but not to infection. Almost all sheep or goats will produce a few oocysts in their feces at all times but disease only occurs under intensive management or stress conditions such as weaning. The use of coccidiostats in feed is not always adequate to protect from overwhelming exposure and certainly by the time clinical sign occur, these drugs do little to alleviate signs of disease. If a young animal is exposed to oocysts and to a coccidiostat, immunity may not be expressed if the coccidiostat is too effective. If the coccidiostat prevents disease but not infection then the young animal will gain protective immunity against subsequent challenge. However, when some organisms survive the coccidiostat there will be a selection for drug resistance. Later when the parasite has become resistant to the coccidiostat the level of exposure will be increased and lambs or kids will succumb to the environmental exposure. Certainly the most important factor in preventing Coccidiosis in sheep or goats is environmental sanitation. Preventing kids or lambs from getting into feed bunks, keeping water troughs from overflowing and providing dry, clean bedding will lessen exposure to oocysts. At the same time lowering stress, especially that involved with weaning, will lessen the chance of disease. While it is important that the young are not exposed to excessive number of oocysts, it appears important that lambs and kids be exposed to some oocysts from an early age. Recent studies in immunizing lambs by giving them small numbers of oocysts either weekly or three times a week resulted in lambs that were resistant to massive exposure which caused 80% mortality in controls. The key to immunization appears that it be done prior to the stress of weaning. Certainly vaccination to Coccidia by selection of precocious strains or biotechnical manipulation may be possible some day but is not feasible at this time. Lambs and kids are also susceptible to cryptosporidiosis as are other young animals. Kidding or lambing in buildings will probably increase the chances of the young acquiring the infection from the environment. Cryptosporidium parvum is a Coccidia which is parasitic on the microvillous border of enterocytes and can build up astronomic numbers in a short time. The signs seen are those of diarrhea, usually beginning at 5 to 10 days of age. There is no practical treatment and most young will survive and gain resistance in a short time. Keeping the young animal warm, dry and hydrated are the most important criterian for survival. Common disinfectants are probably useless sin a barn, but good sanitation, the removal of contaminated bedding and, if possible, steam cleaning of contaminated areas can all aid in lessening exposure. When possible, parturition in the open dilutes the exposure the young have the first few days of life. |
Gastrointestinal nematodesIt is absolutely essential that practitioners know what kinds of parasites occur in their practice areas, just as they need to know the poisonous plants, mineral unbalances and common management practices. Not all worms that pass strongyle type eggs are equally virulent, and until one understands that there are regional and some local differences in the epidemiology and magnitude of the worm burden, rational control programs cannot be developed. Most of the gastrointestinal nematodes have the same basic life cycle. Eggs are passed in the feces of the host. A first stage larva develops within the egg, which then hatches. The first stage larva feeds on bacteria within the feces and molts to a second stage larva which also feeds within the fecal pellet. The second stage larvae then molts to the infective or third stage larva. The third stage larva leaves the feces only when the feces are moist: it then moves on to the forage in a film of moisture, such as dew, and becomes available to the grazing host. Within the host the larva molts to the fourth stage, then the adult and becomes and egg producer in approximately three weeks following ingestion. Desiccation, or extremes of heat or cold are detrimental to the development of eggs and larvae in the environment. The egg and the first two larval stages are especially vulnerable to desiccation and will not develop or survive if the fecal pellet dries too quickly. The third stage larva is much more resistant to drying because it is ensheathed in the cuticle of the second larval stage. Thus ensheathed, the larva cannot feed and must utilize energy obtained during the first two larval stages which feed o bacteria in the fecal pellet. The larva will live only as long as it has energy reserves. Energy is depleted in direct relationship to ambient temperature, so survival of the infective stage is lessened during the summer and prolonged during the winter. In arid areas, drying the fecal pellet will prevent infective larvae from leaving, and the pellet becomes a repository for larvae that are released only when soaked. In a moist environment, larvae are picked up from pasture daily. With rains in arid regions or following period of drought, tremendous numbers of larvae may be acquired in a fairly short time period. The numbers of larvae acquired with the advent of rains following drought may be considerably higher than those gained grazing on lush, well watered pastures as the animals are often grazing closer to fecal pats and to the ground. In the presence of moisture the ambient temperature determines the rate of development from the egg to an infective larva and also determines the duration of survival of larvae in pasture when freed from the fecal pat. The higher the temperature, the shorter the survival. Thirty days in a hot moist environment may kill infective larvae where over wintering under snow will not. Cold does not kill worms; many infective larvae can survive the winter but will not live long once the weather has warmed in the spring. One exception, Nematodirus, is especially adapted to surviving harsh environments because the larva remains in the egg through development and hatches the following spring. Most species of Nematodirus are of trivial importance, probably because they stimulate a strong early protective immune response, and lambs begin to develop resistance while nursing. However, one species, N. battus, can be devastating to lambs. This parasite evolved in the cool moist regions of Europe, such as England, the Netherlands and Norway, where lambs are not pastured on the same ground for two consecutive years because of sever losses. The parasite was discovered in Oregon and Prince Edward Island in 1986 where it apparently has established. It appears to have spread to some other areas with a similar climate in North America. Not only does the host have to be exposed to large numbers of larvae to suffer from parasitic disease, it is essential that the parasites are able to establish in the host. Factors that will determine if a larval nematode is able to survive within the host are host and worm related. It appears that there are several host factors which limit the numbers of parasites able to establish in the individual host. The mechanism(s) of natural resistance are unknown but appear to be heritable. Most hosts will also develop acquired immunity. The level of protection is also heritable and requires stimulation of the immune system before it functions. It is usually impossible to differentiate natural and acquired immunity, and both may be functioning at the same time. The immune system can prevent the establishment of worms, or may cause the expulsion of populations that have established within a host. Immune expulsion or "self cure" is a hypersensitivity reaction apparently associated with the intake of large numbers of infective larvae or rapidly growing forage such as occurs after a rain following dry weather. The phenomena of self cure may help negate the exposure of infective larvae that occurs after drought. Hyprobiosis is an important adaptive mechanism that enables nematodes to survive adverse conditions. Hypobiosis is a cessation of development of the worm within the host. Larvae in the early stages of development (early 4th stage ) become metabolically inactive; they no longer feed but remain within the host in an inactive state until more favorable conditions occur in the host or environment for their development and the subsequent survival of their offspring. The stimuli that induce hypobiosis are not well defined but the exposure of the free living larval stages to adverse weather conditions has been implicated There is also a component of host immunity in the induction of hypobiosis. Lambs that have had experience with a particular species of parasite will have hypobiotic larvae, where as in naive lambs the parasites develop at the normal rate. Larvae in a state of hypobiosis are not only able to evade unfavorable environmental conditions, they also are able to evade the host's immunological surveillance system by being metabolically inert. Later when environmental conditions are more favorable for larval transmission, hypobiotic larvae resume development. This usually coincides with rapid pasture growth, although larvae may resume development even though the host is not even in a pasture. Larval development following hypobiosis often occurs near the normal parturient season of the host. Thus, the worm has an adaptive mechanism to ensure there are sufficient numbers of offspring to infect the next generation of hosts. Resistance to infection is abrogated at the time of kidding or lambing and during early lactation. This periparturient relaxation of resistance results in the dams' inability to expel adult worm. As a result, there is a subsequent rise in fecal egg counts which leads to serious pasture contamination. The use of an anthelmintic at or near the time of lambing or kidding has proven to be the value in neutralizing the periparturient rise. If an anthelmintic which has an effect against arrested larvae is used during the winter before the larvae resume development, pasture contamination will be minimized, especially if many of the infective larvae have died due to desiccation in the pasture. |
Control ProgramsPrevention rather than cure, is the philosophy used in developing control programs against gastrointestinal nematodes It must be assumed that worms cannot be eradicated but can be limited to the extent threat they will not cause serious economic loss to the producer. A combination of treatment and management are necessary to achieve control. Several approaches to the use of anthelmintics are considered. Strategic Strategic treatments aimed at hypobiotic worms or timed early in the reproductive cycle have proved effective in aiding in the control of worm burdens during the transmission season. In dry climates, a single deworming may be sufficient to keep the level of parasitism below the economic threshold for the entire season. Treating in the winter, before parturition, will not only kill arrested larvae that would emerge but circumvents the effects of the periparturient rise of nematode egg production. However, strategic treatments select for althelminic resistance as the offspring of survivors will have only other survivors to mate with. Treatment before lambing and again after weaning, and moving to a clean pasture along with yearly anthelmintic rotation will serve the farmer well in most years. Treating lambs or kids as they are weaned and moving them to clean pastures is a strategic approach. Tactical Opportunistic. Treatment may also be given when the livestock are gathered for reasons such as shearing, docking and castration of lambs or as a part of the flushing process. This treatment may be strategic or tactical but is usually just opportunistic. These treatments give the host a temporary reprieve from the deleterious, effects of parasites and this may be sufficient to protect from disease. However, opportunistic treatment seldom affects the population of parasites in the environment so the effects are usually short lived and give the owner a false sense of security. Individual. Treatment of wormy individuals may prove to be a worthwhile endeavor, especially where resistance to anthelminties is widespread. Certain individuals in a flock will have much higher egg excretion than the mean. This over-distribution of the parasite population can he lessened by the selective treatment of wormy individuals or by the removal of these individuals from the flock as a selection criterion. This is a very time consuming approach and requires individual evaluation so it is probably not economical except in very small flocks. This approach does not put any selection pressure against the antheimintic(s) used as many of the larvae ingested will come from untreated hosts.. Identifying animals with low egg counts may be a way of identifying resistant animals in a flock. With the reported failure of many anthelmintics for sheep and goats, this may become an important selection criterion in the future. Suppressive. Suppressive anthelmintic treatments are given at regular intervals. To be completely effective, this must be done before the worms that are acquired since the last deworming become reproducing adults themselves. This interval is approximately three weeks. However, this method of parasite control is expensive and fails to utilize the host's defenses where they are applicable. Suppressive deworming is probably the most effective means of keeping parasite numbers lowered for a period of time. However, this method will invariably lead to anthelmintic resistance by the parasite faster than when other approaches are utilized. When large numbers of animals are confined to limited grazing, and either pasture rest, alternate grazing by other species of livestock, or tillage is impossible, suppressive deworming may have to be used. Salvage. Salvage (treatment to save lives, not control parasites) is a frequently used antheimintic strategy in small ruminants. This is treatment in the face of disease; the animals are frequently anemic,may have bottle jaw or diarrhea due to the effects of worms. Whatever the case, sheep or goats may be in desperate straits and even, if they have the genetic ability to resist worms, they will not at this time be able to resist their reintroduction. Although antheiminties may remove thousands of worms from each of the treated animals, the pastures from which they came have billions of larvae awaiting ingestion. Under these circumstances, treatments at two week intervals may have to be practiced until weather conditions are no longer favorable for transmission. Pasture Rotation. Over the years there have been advocates of pasture rotation schemes to aid in the control of parasitic disease. For the most part, pasture rotation schemes on improved pastures increase stocking density and tend to increase populations of parasites, but improved nutritional status of the host may overcome the deleterious effects of the increased numbers of parasites. Pasture rotation may decrease parasite numbers in deferred grazing systems where a pasture is rested for at least six months during the cool season or three months in the warm part of the years. Anything less than this is unlikely to effectively reduce larval populations in temperate climates. However, if the pasture were tilled and replanted, by the time re-growth had occurred, most of the infective larvae will have succumbed to the effects of radiation and desiccation. Studies comparing various deferred grazing systems in west Texas range lands have not shown significant differences among various management systems in the levels of parasites acquired by lambs. The exception is Nematodirus, which was found in increased numbers in lambs grazing high-density low-frequency grazing systems at stocking rates that were higher than that utilized in other management schemes. These studies were done during t he "typical" dry summers which are the norm in west Texas. In wet tropics, due to the rapid depletion of energy reserves by infective larvae, short rest periods (as few as 60 days) appeared to be adequate to control internal parasites in goats. In climates Where sheep remain housed until the pasture comes into production in May, early lambing and weaning before the ewes go onto pasture eliminates exposing the lambs and prevents the increased pasture contamination that these naive animals provide for late summer (August) disease. The procedure of removing lambs from contaminated pastures, drenching them and moving them to clean pastures works in all climates but is most important where intensive production is practiced. Alternate grazing of species of ruminants may be of value in controlling some species of parasites. When the range is shared by several foraging species, the competition for nutrition is usually intraspecific. Interspecific competition for preferred forage is of lesser importance because of feeding behavior. When sheep and goats are cohabiting brushy country such as in the Edwards Plateau of Texas, sheep tend to graze and goats browse bushy herbage. In these circumstances, sheep may suffer from severe parasitic disease while the goats are relatively unscathed. On the other hand, when goats are grazing the same -pastures as sheep and have little opportunity to browse, the same parasite species may devastate the goat population while the sheep are minimally affected. |
Antheimintic resistanceWhen maximum small ruminant production is desired, parasitic disease maybe an important limiting factor and the judicious use of antheimintics is essential, although drought, good nutrition, tilling soil, alternate species grazing, dung destroying insects etc. may all contribute to the demise of parasites. Because of the lack of effective approved anthelrnintics in many areas of the United States, veterinary practitioners are by necessity going to have to evaluate drugs that are approved in other countries for use in small ruminants. The effectiveness of these drugs is going to be variable and may differ from farm to farm. Populations of Haemonchus resistant to thiabendazole were reported within a few years of its introduction in the United States and these populations are still resistant. Side resistance to other benzimidazoles is widespread and increased dosage or prolonged administration is a necessity if these drugs are to be effective. Other antheimintics may be effective against benzimidazoie resistant strains of haemonchus. For instance, levamisole appeared to be effective on approximately 60% of ranches evaluated in Texas a few years ago, but resistance to this compound is increasing. Ivermectin resistance has been documented and there is evidence that the resistance may be widespread, but it is still less evident than benzimidazole or levamisole resistance. Antheimintics with different modes of activity than those to which the worms have become resistant should be used when resistance is encountered. Because of the resistance of parasites to antheimintics, it is imperative that small ruminants be evaluated following the use of antheimintics to determine if the drugs used are truly effective. The simplest method to measure efficacy is to determine the relative egg counts both before and after (7 to 10 days) the use of an antheimintic. If possible, some animals should remain as untreated controls to determine 'if other factors may be contributing to parasite loss. Evaluation of anthelmintics should be done yearly on sheep or goat farms. This is one of the most important services a veterinarian can give to his/her clients as part of a herd health management scheme. There seems to be a difference as to how anthelmintics are metabolized between the two species. With presently available antheimintics, a rule of thumb should be to dose a goat at a level 1.5 times higher than a sheep unless a goat dosage has been established for the product. Levamisole at 12 mg/kg is approaching the toxicity level for goats and should be only administered orally. None of the cattle injectable anthelmintics should be injected into sheep or goats. It is very likely that the use of subcutaneously administered ivermectin significantly speeded the selection of resistant worm. In Great Britain and New Zealand, anthelminitic resistance was first noted in goats and then, if they were grazed with sheep, the resistant worms infected the sheep. This may or may not be a valid observation for North America, but certainly ivermectin resistance was seen in goats earlier than in sheep. One of the popular strategies intended to prevent anthelmintic resistance from occurring is to switch anthelmintics each time used or at least quite frequently. The rationale for doing this is that the worms will be exposed to a different type of drug than they had previously encountered and will not become resistant to the new drug. This is a very persuasive theory but unfortunately false. Researchers have demonstrated that rapid (within grazing season) rotation of antheimintics leads to resistance to the compounds used in the rotation faster than if a product is used until no longer effective, then another drug is substituted. However, slow rotation (use for a grazing season then switch the next year) is more likely to slow down the development of resistance. The worm population encountered by grazing sheep or goats in any one year is really only one or two generations rather than the five to 12 generations of worms that can be theoretically generated each year. Putting pressure on a generation of worms whose ancestors survived last years' drug is more likely to be effective than using different anthelmintics on the same generation. The most common way to select for resistant worms is to remove the susceptibles. There is a direct relationship between the number of treatments and the onset of anthehmintic resistance. Owners buy resistant worms, then select for them. They do not intentionally buy resistant worms but purchase animals with resistant worms. Then, by pot effectively treating and quarantining these animals, allow establishment of resistance on their property. They further select by deworming too often. If they are not 100% effective in controlling parasites, the surviving worms have only other survivors to mate with and a small, population can become large rapidly as each female Haemonchus produces 5,000 to 6,000 eggs per day. Fewer dewormings usually means that some susceptible worms will be present to mate with and the advent of observable, resistance takes longer. Another factor that will speed up the onset of resistance is under dosing by administering antheimintics to the flock with the dose based on mean weights rather than on the heaviest animals in the flock. Under dosing per se will not select for resistance when a single, dominant gene is responsible for resistance but it may where several genes are involved or where heterozygote worms have a level of resistance that is does dependent. With the safety margins of the presently available anthelmintics, adequate dosage is essential in preventing the onset of resistance. Repeated treatment at or near the prepatent period will also select for resistance as the population will be constantly culled and there will be no susceptibles to mate with. Suppressive deworming has been very successful in selecting for anthelmintic resistance. Suppressive deworming also may prevent the development of immunological resistance and increase dependency on antheimintic treatment. |
Evaluation of egg countsThere is a correlation between the numbers of economically important adult nematodes and fecal egg counts in small ruminants. Many producers look for clinical signs such as bottle jaw, diarrhea or pale mucous membranes to decide that treatment is necessary or to evaluate the effectiveness of an antheimintic if the animals show improvement after treatment. Evaluation of a fecal egg count may preclude the animals showing clinical signs by timely treatment and is a better evaluation of the success (or lack there of) of anthelmintic therapy than is clinical response. Any method of consistently evaluating egg counts will suffice, i.e., using standard amount of feces and the number of eggs seen by flotation. The method used is not as important as the consistency of evaluation. |
SummaryPresent recommendations are to:
Other recommendations may be made depending on the management, climate and forage the flock are subjected to. Selection of livestock resistant to worms, zero grazing systems or other suggestions may be ideal for some flocks but not at all practical for others. The interactions among parasites, hosts and environment are complex so there are no simple answers to everyone's problems. |
