Gastrointestinal Parasites of Ruminants: Introduction

Clinical Findings and Diagnosis

Treatment

General Control Measures

Cattle—Special Considerations

Sheep—Special Considerations

Clinical Findings and Diagnosis:

The clinical signs associated with GI parasitisms are shared by many diseases and conditions; however, presumptive diagnosis based on signs, grazing history, and season is often justified. Infection usually can be confirmed by demonstrating nematode eggs or tapeworm segments on fecal examination. However, in clinical evaluation of fecal examinations, 2 points should be remembered: an egg per gram of feces (EPG) count is not always an accurate indication of the number of adult worms present, and specific identification of eggs is impractical except in specialized laboratories. EPG counts can be negative or deceptively low in the presence of large numbers of immature worms; even when many adult parasites are present, the count can be low if egg production has been suppressed by immune reaction or previous anthelmintic treatment. Variations in the egg-producing capability of different worms (significantly lower for Trichostrongylus , Ostertagia , and Nematodirus than for Haemonchus ) also may distort the true picture. The ova of Nematodirus , Bunostomum , Strongyloides , and Trichuris are distinctive, but reliable differentiation of the more common species of ruminant nematode ova is difficult. Fecal cultures can produce distinctive third-stage larvae if differentiation is important premortem.

The advent of safe and effective broad-spectrum anthelmintics has largely reduced the need for differentiating the genera and species of these parasites. In areas where Ostertagia spp predominate, the analysis of sera for increased plasma pepsinogen levels is a useful diagnostic aid. Generally, tyrosine levels >3 IU reflecting pepsinogen activity are associated with clinical signs. Problems of interpretation may arise in immune animals under challenge, in which there are no clinical signs but the pepsinogen levels may be increased because of a hypersensitivity-type reaction in the abomasal mucosa. Where Haemonchus spp predominate, a PCV estimate provides a quick guide to the degree of anemia. In some countries, serologic diagnosis (ELISA) of important species such as Ostertagia and Cooperia infections in cattle is used. As yet, there is insufficient information on the correlation between serologic titers and parasite load.

In many management situations, high levels of infection can be expected, particularly after favorable temperatures and rainfall conditions in certain seasons. “Diagnostic drenching” is recommended when eggs are few or absent, yet history and signs suggest infections. A clinical response to a broad-spectrum anthelmintic permits a retrospective diagnosis, but the animals should be placed on “clean” pastures after treatment to avoid reinfection.

Necropsy is the most direct method to identify and quantitate GI parasitisms. Routine postmortem examinations can provide valuable parasitologic data about the status of the rest of the herd or flock.

On necropsy, Haemonchus , Bunostomum , Oesophagostomum , Trichuris , and Chabertia adults (or advanced immature worms) can be seen easily. Ostertagia , Trichostrongylus , Cooperia , and Nematodirus are difficult to see except by their movement in fluid digesta, and clinically important infections are easily overlooked with these genera. The total contents and all washings should be combined to a known volume, and a worm count done to evaluate the severity of the infection. Measured samples of GI contents and scrapings of the mucosa should be examined microscopically under low power. These smaller nematodes can be stained (5 min) with a strong iodine solution. After the background digesta and tissue are decolorized with 5% sodium thiosulfate, small nematodes are easily seen. The significance of the numbers of worms present varies according to species of worms and host species. For example, only 100 Haemonchus are of clinical significance in lambs, whereas 5,000-10,000 Ostertagia are probably required to be clinically significant. If the animals have been diarrheic for a few days, worms may have been shed, and the type and severity of gross lesions may also be of considerable diagnostic value.

Multiple causes should be considered in evaluating clinical, laboratory, and necropsy findings. Mixed parasite infections are the rule.

Diagnosis of ostertagiasis in cattle during the period of larval inhibition presents technical problems, particularly for the feedlot industry in the USA. Fecal egg counts and plasma pepsinogen analysis do not provide useful information. Predisposing factors for inhibition of larvae include age and geographic source of cattle, time of year or season of arrival, previous grazing history and management, weather conditions prevailing during the last grazing period, and prevalence of Ostertagia ostertagi in the source region.

Information on such factors usually is not available for feedlot cattle. If cattle have arrived after spring grazing in the south of the USA or fall grazing in the north, they could have heavy burdens of inhibited larvae. Lighter calves from areas where prevalence of parasites is high may also have such a problem. It is becoming more widely accepted that a significant cause of clinical disease or feed efficiency problems in feedlot cattle is parasitism, possibly ostertagiasis. When cattle are received from a suspect area and at a suspect time of year, it may be advisable to treat the new arrivals promptly with an anthelmintic effective against inhibited larvae.

Treatment:

Effective worm control cannot always be achieved by drugs alone; however, anthelmintics play an important role. (See also Anthelmintics: Introduction.) They may be used to reduce pasture contamination, particularly at times when seeding of the pasture with parasite eggs is a prerequisite for the development of an infective challenge necessary to cause clinical parasitism. Coordination with other methods of control, such as alternate grazing of different host species, integrated rotational grazing of different age groups within a single host species (including creep grazing), and alternation of grazing and cropping, are other management techniques that can provide safe pasture and give economic advantage when combined with anthelmintic treatment.

The “ideal” anthelmintic should be safe, highly effective against adults and immature stages (including hypobiotic larvae) of the important worms, available in convenient formulations, economical, and compatible with other commonly used compounds. Several drugs satisfy all or most of these requirements. Thiabendazole was the forerunner of the modern anthelmintics and set a new standard in efficacy and safety. Despite ineffectiveness against hypobiotic Ostertagia larvae in cattle and 1 or 2 other worm species, it is still widely used. After thiabendazole and mebendazole, other benzimidazoles (such as fenbendazole, oxfendazole, and albendazole) and the probenzimidazoles (thiophanate, febantel, and netobimin) were developed; these compounds are effective against most of the major GI parasites of ruminants and have varying levels of activity against hypobiotic larvae. Levamisole, morantel, and pyrantel also are highly effective, safe, wide-spectrum anthelmintics but have little activity against hypobiotic larvae in cattle. Avermectins and milbemycins are highly effective against adults and larval stages, including hypobiotic larvae of all the common GI nematodes of ruminants, and some of the important ectoparasites. Avermectins and milbemycins may persist in some ruminant species for some time after single subcutaneous or topical administration and may confer protection against reinfection during this period. Moxidectin is also persistent following oral administration. Some narrow-spectrum anthelmintics, such as the salicylanilides, closantel, and rafoxanide, have excellent activity against Haemonchus contortus in sheep and also remain in the host for a long time, which confers considerable prophylactic activity after administration.

Routes of administration other than drenching or injection (eg, incorporating into feed, drinking water, and mineral or energy blocks) are used to reduce labor costs and may be useful under drylot conditions or when grazing animals are being given supplemental feed. Another advantage of these “in-feed” routes is that continuous low-level administration of a drug can be achieved and pasture contamination reduced during periods that are optimal for free-living development of the parasites. Disadvantages include erratic consumption of anthelmintic, tissue residues (requiring observance of recommended withdrawal periods), and possible encouragement of drug resistance by continuous exposure. Another labor-saving route of administration is the “pour-on” topical treatment, used for some of the organophosphates (eg, trichlorfon), levamisole, and avermectins. A number of bolus preparations (eg, morantel, levamisole, ivermectin, or benzimidazoles) release drug in a sustained fashion or in pulses at intervals approximately equal to the prepatent periods of the most important GI parasites. The boluses used in cattle have been designed to give entire season pasture control in temperate areas if administered at turn-out to set-stocked herds. Boluses are also available that provide treatment and subsequent prophylaxis of animals already exposed to contaminated pasture and harboring parasites. Boluses in sheep may be used to reduce the periparturient rise in fecal egg output and thus the pasture contamination responsible for disease in their offspring later in the grazing season.

Niclosamide, morantel, praziquantel, and the newer benzimidazoles (albendazole, fenbendazole, and oxfendazole) are effective against tapeworms ( Moniezia spp ) in cattle and sheep. Treatment of Thysanosoma actinioides has presented problems, but niclosamide has been reported to be effective at 250 mg/kg. Additionally, bithionol (200 mg/kg) has been used.

When treating clinically affected animals, the following should be considered: 1) providing adequate nutrition, 2) treating all animals in the group, as a preventive measure and to reduce further pasture contamination, and 3) moving stock to “clean” pastures to minimize reinfection. The definition of safe pastures varies in different climates and depends on local knowledge of the seasonal mortality of infective larvae. Some authorities have suggested treating only the most severely affected animals in a flock or herd. This can be achieved by assessing the severity of anemia by observation of the color of the sclera of the eye for haemonchosis in sheep, or the severity of diarrhea or by quantitative fecal egg counts for parasitic gastroenteritis in sheep or cattle. The rationale for this strategy is based on the knowledge that a very large proportion of the parasite egg output (and thus pasture contamination) is associated with a relatively small proportion of the host animal population. Treatment of only these animals significantly reduces pasture contamination and reduces the overall selection pressure exerted by the use of an anthelmintic. Concerns also exist with respect to treatment and movement of stock to clean pasture. If any parasites with resistance genes survive the treatment, then the “clean pasture” will become seeded with a wholly resistant population.

Finally, development of drug resistance by populations of Haemonchus contortus , Trichostrongylus spp , and Ostertagia spp in sheep and goats to benzimidazoles, levamisole, and avermectins/milbemycins has been demonstrated. While such resistance is currently a problem only in certain areas, it should be considered when response to therapy is suboptimal and other factors can be excluded, eg, improper dosage, rapid reinfection, poor nutrition, or some disease state other than parasitism. Drug resistance in parasites of cattle has been demonstrated; overuse and otherwise indiscriminate treatment should be avoided.

General Control Measures:

“Control” generally implies the suppression of parasite burdens in the host below that level at which economic loss occurs. To do this effectively requires a comprehensive knowledge of the epidemiologic and ecologic factors that govern pasture larval populations and the role of host immunity to infection.

The goals of control are as follows: 1) prevent heavy exposure in susceptible hosts (recovery from heavy infection is always slow), 2) reduce overall levels of pasture contamination, 3) minimize the effects of parasite burdens, and 4) encourage the development of immunity in the animals (less important in fattening animals than in those that are to be kept for breeding purposes).

Strategic use of anthelmintics is designed to reduce worm burdens and, thereby, the contamination of pastures. Timing of administration is based on knowledge of the seasonal changes in infection and the regional epidemiology of the various helminthoses. Prompt recognition of circumstances likely to favor development of parasitic disease, eg, weather, grazing behavior, and loss of weight and condition, is essential.

For example, in the UK, where the pattern of disease caused by Nematodirus battus infection in sheep is clearly defined, strategic treatments with 2 or 3 doses of anthelmintic at 2- to 3-wk intervals, beginning just before the disease characteristically appears, are recommended. Similarly, in the northern USA, Canada, or western Europe, pasture levels of Ostertagia and other parasites increase substantially after mid July, ie, the general pattern of infectivity is minimal in spring but increases rapidly to peak levels in late summer and early fall. Current practices in these areas indicate the effectiveness of 2 or more anthelmintic treatments (usually at intervals of 3-5 wk) given when cattle first go to grass in spring. Single treatments with subsequent transfer of animals to safe pasture and treatment associated with delayed spring turn-out also have been effective.

In other countries of either cool or warm temperate climate, similar controls may be used if the seasonal pattern of the disease is known, but in most regions a tactical use of anthelmintics is used, eg, during warm, moist conditions.

Cattle—Special Considerations:

Worm problems are seen most frequently in young beef cattle from time of weaning and several months thereafter, and in segregated groups of dairy calves during the first season at grass. Immunity to GI nematodes is acquired slowly; 2 grazing seasons may be required before a significant level is attained. In endemic areas, cows may continue to harbor low burdens, which may be the cause of suboptimal production. GI parasitism in young stock may be controlled by use of broad-spectrum anthelmintics in conjunction with pasture management to limit reinfection; the latter includes a move to “clean” pastures (eg, grass conservation areas or silage or hay aftermath) or alternate grazing with other host species, or integrated rotational grazing in which susceptible calves are followed by immune adults. Alternate grazing with other host species may be ineffective in areas where parasite species (eg, Nematodirus ) infect both hosts; simple pasture rotation is not effective because the bovine fecal mass can protect larvae from adverse environmental conditions for several months, possibly causing reinfection in rotating calves at a later date.

In beef herds, anthelmintic treatment at weaning is of value, particularly if the young cattle are to be retained, eg, as replacement heifer stock or as steers to be fed. Cattle finished on grass should receive treatment at weaning and at intervals during the next 12 mo and, if possible, should be moved to safe pastures.

When cattle cannot be moved readily to other pastures, strategic treatments may be given to limit contamination of pastures and rapid reinfection. Alternatively, rumen boluses may be used in countries where they are approved. In warm temperate regions of the world, such as Australia and New Zealand, the southern USA, and the large cattle-raising regions of southern Brazil, Uruguay, and Argentina, young cattle may be given 2 or more treatments from late summer and into fall for prevention of large increases in pasture contamination and infection during winter and spring. Two or three strategic treatments, administered with a short interval, from the time of weaning in such regions could be just as effective as spring treatments in cool temperate regions. However, survival of infective larvae on pasture from the time of fall weaning in warm temperate regions is most often persistent, and longer intervals between treatments (eg, at weaning, during winter, and in late spring) may be more applicable. In many areas, anthelmintics are simply given at regular intervals after weaning. Intervals between treatments must necessarily vary according to the local epidemiology and the prophylaxis conferred by the persistence of the anthelmintic. When Type II ostertagiasis is a problem, treatment with an anthelmintic effective against hypobiotic larvae is recommended before the expected time of outbreak.

Sheep—Special Considerations:

A special strategic treatment is required in most regions to counter the postparturient relaxation of immunity (periparturient rise, etc) seen in ewes. The precise timing of such treatment varies between regions and for different species of parasites, but in general, treatment within the month before and again within the month after parturition appears desirable and may confer a production benefit on the ewe. Unfortunately, the periparturient rise may last for up to 8 wk in some flocks and 2 treatments with most anthelmintics are not effective in reducing pasture contamination sufficiently to ensure “safe” grazing for offspring later in the season. Bolus preparations containing albendazole or ivermectin are available in some countries and are more effective for this purpose. Furthermore, moxidectin has sufficient persistence in sheep to confer an epidemiologic benefit of treatment for the most important parasitic species. A treatment 2 wk before breeding, as part of a “flushing” program, is another strategic application of anthelmintics. Supportive management after treatment includes movement of sheep from contaminated pastures to cattle pastures, grass conservation areas, root crops, or pasture not grazed by sheep for several months. The latter period varies according to the seasonal pattern of larval mortality in different countries and may be as long as 1 yr in some temperate countries.

Sheep are more consistently susceptible to the adverse effects of worms than other livestock, and clinical disease is more common. Immunity to the parasites is acquired slowly and is generally incomplete. Frequent treatments may be required, particularly during the first year of life.