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Epidemiology and control of bovine ostertagiasis in South America
Veterinary Parasitology, 27 (1988) 59-65 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
59
Epidemiology and Control of Bovine Ostertagiasis in South America CARLOS M. ENTROCASSO
D~artamentodeProduccidnAnima~INTA-Balcarce,(7620) Ba&arce(A~entina)
ABSTRACT Entrocasso, C.M., 1988. Epidemiology and control of bovine ostertagiasis in South America. Vet. Parasitol., 27: 59-65. Gastrointestinal parasitism has been recognized by practitioners as the most common disease in beef cattle, mainly in weaning calves and fattening steers. Among the different genera, Ostertagia ostertagi is the predominant parasite in the temperate climate, in which the major beef and dairy cattle area of South America is situated. Outbreaks of Type I ostertagiasis are usually seen after weaning time (autumn-winter) when larvae counts are high and food availability is low. The development of the disease is rapidly established and 15-30 kg are lost in 30-50 days. Epidemiological studies have demonstrated a fast evolution of parasite eggs to larvae (L3) in summer (1 week or less), evolution being 30-45 days during winter. Inhibition O. ostertagi occurs during spring (September-December) and development resumes in late summer and early autumn. The production effect is seen as a significant reduction in body weight gain and occasionally clinical Type II ostertagiasis appears. A similar epidemiological pattern of inhibition of Ostertagia sp. has been recorded in Uruguay and temperate areas in southern Brazil.
INTRODUCTION
The most important cattle-raising area in South America is situated in the temperate zone of the continent. This includes southern Brazil, Uruguay and the central areas of Argentina and Chile. The estimated number of cattle in this area is 60 million. These cattle graze in a typical extensive production system where the animals are mainly on native grass all year round. CLIMATE CONDITIONS
The annual average rainfall of Argentina is 900-1500 mm in the east and 400-600 mm in the west, with an annual mean temperature of 10-20°C. Summer is usually warm and drier than the other periods. Autumn is wet and fresh and is the time of the year when weaning occurs. Winter is wet and cold with frequent frost and some snow in western and southern Argentina and Chile.
0304-4017/88/$03.50
© 1988 Elsevier Science Publishers B.V.
60 Calving occurs most commonly from the end of winter to spring. Spring is cool at the beginning, becoming warm later and quite wet. The grass production of native pasture is poor during winter, but is rich during spring. The dry and warm summers cause a drop in the availability and digestibility of grass. This situation reverses in the autumn. IMPORTANCE OF THE DISEASE In Argentina, the results of a survey made among practitioners of the temperate area showed that gastrointestinal parasitism has been recognized as a major problem in growing animals of 4-24 months of age. This information is coincident with data recorded during 15 years of diagnosis at the Animal Health Unit, INTA-Balcarce. SEASONALOCCURRENCE Outbreaks of the Type I parasitic disease usually occur after weaning, during the end of autumn and in winter when young animals have the highest requirements due to their age and the unfavourable weather conditions. Low grass production in these seasons forces calves to graze close to the ground and the dung pats, resulting in a high intake of infective larvae. This is coincident with the highest counts of number of eggs per gram (e.p.g.) and larvae on grass. The dominant genera are Ostertagia., Cooperia, Trichostrongylus and Haemonchus (Pinheiro, 1970; Entrocasso and Steffan, 1980; Rassier et al., 1980; Steffan et al., 1982; Eddi et al., 1985; Nari and Cardozo, 1986). Type II ostertagiasis occurs most frequently during the end of summer-autumn. EPIDEMIOLOGICALDATA The results of an epidemiological assay at INTA-Balcarce (Argentina) have demonstrated that the time needed for the parasite egg to develop into infective larvae is about 3-6 weeks in winter and about 1 week in summer. The persistence of some of these larvae could be as long as 18 months ( Steffan and Fiel, 1986). The dung pats are the natural reservoir and the importance of this long survival is given by the fact that many larvae may survive to the next autumn and be the base to build the infestation for the new generation of young animals. This is enhanced by certain management practices ( like having the same paddock for young calves in successive years). Data of several production trials performed in the area have shown that there is an increase in larval count on grass from autumn to winter, the period of time when the e.p.g, count increases and the effects of the disease are also
61 observed (Nesbit et al., 1970; Entrocasso and Steffan, 1980; Steffan et al., 1982; Eddi et al., 1985). The increase of the larval count on pasture may be due in part to a reduction in grass availability. However, it must also be recognized that in autumn the weather conditions (wet with mild temperatures) enhance the evolution and migration of larvae from dung pats to the pasture. Usually there is a great reduction in pasture larval count by the end of spring, depending on the weather conditions (hot and dry), with the count being very low or nil during summer. It was also found that the number of larvae necessary to reduce meat production in winter could be as few as 1000 L J k g dry herbage when the availability of grass is low and the animals are in maintenance or slowly reducing body weight ( Steffan et al., 1986). Recent data obtained in INTA-Balcarce and Anguil ( Suarez et al., 1985; Fiel et al., 1985a) showed that in cattle the inhibition period for Ostertagia is in spring, from September through January, as described in Brazil by Pinheiro et al. (1978) and Melo (1979) and in Uruguay by Nari and Cardozo (1986), and that there is a higher percentage in Argentina and Uruguay ( 70-90% ) than in Brazil ( 40-50% ). Inhibition in spring has also been recorded in southern U.S.A. by Williams et al. (1983), and by Anderson (1972) and Smeal et al. (1980) in southern Australia. Severe ostertagiasis Type II outbreaks have been recorded in animals brought from the northern part of the country to the south for fattening. These animals came from areas where there are low numbers of Ostertagia and they have not developed enough immunity against this parasite. Generally, the clinical Type II ostertagiasis is sporadic and usually seen from summer to autumn (February-May). However, a reduction in the pattern of bodyweight gains in untreated animals detected by production trials during this period is frequently observed (Fiel et al., 1985; Steffan et al., 1986; Entrocasso et al., unpublished) and may represent a 20-30 kg loss in meat production. EFFECTS ON ANIMALPRODUCTION In beef cattle, a mortality level of 1-2% is not uncommon (the highest data recorded is 42% ). This was observed in a farm with a high stocking rate and poor grass availability during the winter of 1977. Losses in meat production of 20-30% have been detected in the area by a group of farmers because although they invested in new cultivated pastures they did not pay enough attention to a parasite control program. Based on some authors (Pinheiro, 1970; Santos, 1973; Entrocasso and Steffan, 1980; Steffan et al., 1982; Mufiiz et al., 1985; A Nari, personal communication, 1986), the subclinical losses may easily reach 20% (20-40 kg) of the bodyweight gain in a short period of time (30-60 days) but if symptoms are clear may rise up to 30-40% (30-60 kg). The weight losses are generally not
62 recouped either after treatment, or following provision of better food or both. The price could be reduced nearly 20% less per kilogram of liveweight in groups of animals after suffering the disease. To date, only one assay has been carried out on milk production in Argentina (Biondani and Steffan, 1985 ), in which a significant loss of 200 kg of milk per year in non-treated groups against suppressing treatment groups was found. These observations were made on 387 cows in eight farms, comparing the production of each animal with its own previous production records. EFFECT ON ANIMALMETABOLISM Studies carried out at the Glasgow Veterinary School (Entrocasso et al., 1986a,b) in naturally infected growing cattle showed the effects of parasites on digestive physiology in two periods, pre-Type II and Type II ostertagiasis, compared with clean and other efficiently treated groups given a morantel sustained release bolus (Paratec, Pfizer).
Alteration in the digestibility In this trial, a pair feeding system was used during the winter period. The mean value during the pre-Type II phase for dry matter and gross energy apparent digestibility were significantly lower in the control calves than in either of the other two groups. The apparent digestibility for all the other feed fractions was also lower in the control group (but not significantly). During the Type II phase, both the organic matter and gross energy digestibility were significantly greater in the morantel bolus treated group than in both the clean and control calves. Otherwise, there were no differences in apparent digestibility of the other feed fractions. The overall means of the phases of dry matter, crude protein and gross energy were significantly lower in the control group than in the MSRB group.
Alteration of nitrogen balance The periodic output of faecal and urinary nitrogen varied, being higher in the control group. However, during the Type II phase, the nitrogen retention failure in control calves was dramatic, showing that ostertagiasis produced a severe gastropathy due to the loss of endogen protein accompanied by low feed intake and poor digestion.
Alteration in protein deposition Protein deposition was affected by the course of both the Type I and the Type II ostertagiasis in the control animals. The reduced leg muscle mass, as
63 shown from the circumference measurements at the symphysis pubis, and the lower lean tissue weights from the seventh to tenth forerib dissection, together with the eye muscle analyses for area, weight and crude protein concentration, confirmed the real occurrence of a lower protein deposition in the control animals. Similar results were obtained in Argentina by Garriz et al. (1986) in a production trial. TREATMENTS The most common drugs used for the control of gastrointestinal parasites are the benzimidazoles such as fenbendazole, oxfendazole and albendazole and now ivermectin. All of them are wide-spectrum drugs and highly efficient against adults and developing and arrested larvae of Ostertagia. Less used are levamisole, closantel and febantel. The great success of ivermectin is based on the high efficacy and longer action against gastrointestinal parasites and also scab (Psoroptes communis var. bovis), avoiding the use of double dipping which risks death of young calves during winter. CONTROL PROGRAMS Control programs for ostertagiasis are widely used among farmers, with the strategic treatments carried out at weaning time and at the end of winter; however, many farmers only treat the animals when signs of the disease appear and have achieved poor results with both treatment times. The concept of symptomatic treatment is far from being economically profitable due to the mistake of believing that money is well invested when clear signs of the disease are stopped by treatment. However, as mentioned previously, losses of 20-40 kg per animal could have occurred without symptoms. On the other hand, the animals often return to the same paddock after treatment, thus reducing the possibility of recovering. In order to decide on treatment, more and more practitioners and farmers are adopting the difference in bodyweight gains between a group of calves treated monthly against another group which represents the rest of the herd, having in consideration the treatment cost. The results obtained are better than using only the e.p.g, counts as an indicator because of the well-known fact of there being no relationship between those values and the number of parasites. In many farms with a history of outbreaks it was possible to achieve good endoparasitic control by integrating the use of'safe pasture' ( e.g. new pasture) and frequent anthelminthic treatments during autumn and winter. Obviously, strategic control of the disease has to be based on epidemiological data and finding the most economical alternative in each particular case. In conclusion, gastrointestinal parasitic disease is not caused by accidental contamination. It is the logical consequence of improper animal management
64
that allows young susceptible animals to accumulate enough larvae to reduce productivity.
REFERENCES Anderson, N., 1972. Ostertagiasis in beef cattle. Vict. Vet. Proc., 30: 36-38. Biondani, C. and Steffan, P., 1985. Efecto de las parasitosis gastrointestinales sobre la producci6n lactea en rodeos lecheros. 10 ° Congreso Panamericano de Veterinaria y Zootecnia, September 1985, Buenos Aires, p. 140. Eddi, C., Niec, R., McLouglin, R., Carcagno, R., Dughetti, R., Corbellini, C. and Benitz, A., 1985. Netobimin (Totabin-Sch.) efficacy in cattle in Argentina. Abstracts l l t h Conference of W.A.A.V.P., Rio de Janeiro, Brazil, p. 30. Entrocasso, C. and Steffan, P., 1980. Programa de control de pardsitos gastrointestinales en sistemas reales de producci6n de carne bovina. Memorias del III Congreso Argentino de Ciencias Veterinarias, Buenos Aires, pp. 170-181. Entrocasso, C., Parkins, J.J., Armour, J., Bairden, K. and McWilliams, P.N., 1986a. Metabolism and growth in housed calves given a morantel sustained released bolus and exposed to natural trichostrongyle infection. Res. Vet. Sci., 40: 65-75. Entrocasso, C., Parkins, J.J., Armour, J., Bairden, K. and McWilliams, P.N., 1986b. Production, parasitological and carcase evaluation studies in steers exposed to trichostrongyle infection and treated with a morantel bolus or fenbendazole in two consecutive grazing seasons. Res. Vet. Sci., 40: 76-85. Fiel, C., Steffan, P. and Biondani, C.A., 1985a. Efecto de la parasitosis gastrointestinal bovina sobre la ganancia de pseo de animales en crecimiento. 10 ° Congreso Panamericano de Veterinaria y Zootecnia, Septiembre 1985, Buenos Aires, p. 157. Fiel, C., Steffan, P. and Entrocasso, C., 1985b. Gastroenteritis verminoasa bovina: estudio del fen6meno de hipobiosis y su posible relaci6n con bajas ganancias de peso vivo estivales. 10 ° Congreso Panamericano de Veterinaria y Zootecnia, Septiembre 1985, Buenos Aires, p. 158. Garriz, C.A., Tardiolli de, G., Steffan, P., Fiel, C., Ambrustolo, R. and Biondani, C.A., 1986. Control de par~sitos, mds produccci6n y m~s rendimiento de carne. Informe Ganadero No. 100, p. 17. Melo, H.J.H., 1979. Importancio epidemiol6gica de hipobiose ou inibiqao de desenvolvimento dos nematoides gastrointestinais. Anais do I Semin~irio Nacional sobre Parasitoses dos Bovinos. Campo Grande, p. 59. Mufiiz, R.A., Ledesma, C. and Eddi, C., 1985. The efficacy of a program of morantel sustained released boluses for the control of parasitic gastroenteritis in cattle under different grazing systems in Argentina. 11th Conference W.A.A.V.P., 1985, Rio de Janeiro, Brazil. Nari, A. and Cardozo, H., 1986. Bases epidemiol6gicas para el control de nemato des gastrointestinales en rumiantes del Uruguay. XIV Jornadas Uruguayas de Buiatrfa, Paysandd, Uruguay. Nesbit, G.H., Rosa, W.A., Niec, R., Lucovich, R., San Martin, L. and Echenique, J.A., 1970. Evaluation of certain factors related to subclinical parasitism in cattle in Argentina. Am. J. Vet. Res., 31: 981-987. Pinheiro, A.C., 1970. Epizootiologfa da helmintose dos bovinos de Bag~ (Rio Grande do Sul, Brasil) Congresso Brasileiro de Medicina Veterinaria, 12, Porto Alegre, 1970. Anais, pp. 247-255. Pinheiro, A.C., Echevarria, F. and Branco, F.A., 1978. Esquema de tratamiento anti-helmintico em bezerro Heresford manejados em pastagen cultivada de invierno. Congresso Brasileiro de Medicina Veterinaria, 16, Salvador, 1978. Brasil. Anais, pp. 87-89. Rassier, D.S.S., Arnoni, J.V. and Fernandez, F.G., 1980. Ostertagia ostertagi (Stiles, 1892) em
65 bovinos na regiao encosta do sudeste do Rio Grande do Sul. 11 ° Seminario Brasileiro de Parasitologia Veterinaria, Fortaleza, 1980. Brasil, 294 pp. Santos, V., 1973. Avaliaqao dos prejuizos causados pelos helmintoses em bovinos de criaqao estensivos em zona rural da Depress~o Central. Rev. Centro Cienc. Rurais, 3: 61-70. Smeal, M.G., Fraser, G.C. and Robinson, G.G., 1980. Seasonal changes in the structure of nematode population of cattle in New South Wales in relation to inhibited larvae developemt. Aust. Vet. J., 56: 80-86. Steffan, P. and Fiel, C., 1986. Bioecolog/a de los nematodes gastrointestinales. Restlmenes del 12 ° Congreso de Producci6n Animal, Neuqu~n, 1986. p. 139. Steffan, P., Fiel, C., Entrocasso, C., Acufia, C. and Rojas Panelo, F., 1982. Parasitosis gastrointestinal de los bovinos: I. Estudio epidemiol6gico. II. Influencia sobre la ganancia de peso. Gaceta Veterinaria TXLIV, 369: 278-290. Steffan, P., Fiel, C., Entrocasso, C. and Chavarino, J., 1986. Gastroenteritis verminosa y sarna en bovinos: Metodologia de control. Res6menes del 12 ° Congreso Argentino de Producci6n Animal, Neuqudn, 1986, p. 138. Suarez, V., Ciminari, O.E., Bello, E.M., Busetti, M.R. and Bedotti, D.O., 1985. Primeras observaciones sobre parasitismo gastrointestinal en invernada de novillos cruzas Cebtl en la regidn sub-htlmeda pampeana (Argentina). 10 ° Congreso Panamericano de Veterinaria y Zootecnia, Septiembre 1985. Buenos Aires, p. 139. Williams, J.C., Knox, J.W., Baumann, B.A., Snider, T.C., Kimball, M.D. and Hoerner, T.J., 1983. Seasonal changes of gastrointestinal nematode populations in yearling beef cattle in Louisiana with emphasis on prevalence of infective strongyle larvae on pasture. Int. J. Parasitol., 13: 133-143.
59
Epidemiology and Control of Bovine Ostertagiasis in South America CARLOS M. ENTROCASSO
D~artamentodeProduccidnAnima~INTA-Balcarce,(7620) Ba&arce(A~entina)
ABSTRACT Entrocasso, C.M., 1988. Epidemiology and control of bovine ostertagiasis in South America. Vet. Parasitol., 27: 59-65. Gastrointestinal parasitism has been recognized by practitioners as the most common disease in beef cattle, mainly in weaning calves and fattening steers. Among the different genera, Ostertagia ostertagi is the predominant parasite in the temperate climate, in which the major beef and dairy cattle area of South America is situated. Outbreaks of Type I ostertagiasis are usually seen after weaning time (autumn-winter) when larvae counts are high and food availability is low. The development of the disease is rapidly established and 15-30 kg are lost in 30-50 days. Epidemiological studies have demonstrated a fast evolution of parasite eggs to larvae (L3) in summer (1 week or less), evolution being 30-45 days during winter. Inhibition O. ostertagi occurs during spring (September-December) and development resumes in late summer and early autumn. The production effect is seen as a significant reduction in body weight gain and occasionally clinical Type II ostertagiasis appears. A similar epidemiological pattern of inhibition of Ostertagia sp. has been recorded in Uruguay and temperate areas in southern Brazil.
INTRODUCTION
The most important cattle-raising area in South America is situated in the temperate zone of the continent. This includes southern Brazil, Uruguay and the central areas of Argentina and Chile. The estimated number of cattle in this area is 60 million. These cattle graze in a typical extensive production system where the animals are mainly on native grass all year round. CLIMATE CONDITIONS
The annual average rainfall of Argentina is 900-1500 mm in the east and 400-600 mm in the west, with an annual mean temperature of 10-20°C. Summer is usually warm and drier than the other periods. Autumn is wet and fresh and is the time of the year when weaning occurs. Winter is wet and cold with frequent frost and some snow in western and southern Argentina and Chile.
0304-4017/88/$03.50
© 1988 Elsevier Science Publishers B.V.
60 Calving occurs most commonly from the end of winter to spring. Spring is cool at the beginning, becoming warm later and quite wet. The grass production of native pasture is poor during winter, but is rich during spring. The dry and warm summers cause a drop in the availability and digestibility of grass. This situation reverses in the autumn. IMPORTANCE OF THE DISEASE In Argentina, the results of a survey made among practitioners of the temperate area showed that gastrointestinal parasitism has been recognized as a major problem in growing animals of 4-24 months of age. This information is coincident with data recorded during 15 years of diagnosis at the Animal Health Unit, INTA-Balcarce. SEASONALOCCURRENCE Outbreaks of the Type I parasitic disease usually occur after weaning, during the end of autumn and in winter when young animals have the highest requirements due to their age and the unfavourable weather conditions. Low grass production in these seasons forces calves to graze close to the ground and the dung pats, resulting in a high intake of infective larvae. This is coincident with the highest counts of number of eggs per gram (e.p.g.) and larvae on grass. The dominant genera are Ostertagia., Cooperia, Trichostrongylus and Haemonchus (Pinheiro, 1970; Entrocasso and Steffan, 1980; Rassier et al., 1980; Steffan et al., 1982; Eddi et al., 1985; Nari and Cardozo, 1986). Type II ostertagiasis occurs most frequently during the end of summer-autumn. EPIDEMIOLOGICALDATA The results of an epidemiological assay at INTA-Balcarce (Argentina) have demonstrated that the time needed for the parasite egg to develop into infective larvae is about 3-6 weeks in winter and about 1 week in summer. The persistence of some of these larvae could be as long as 18 months ( Steffan and Fiel, 1986). The dung pats are the natural reservoir and the importance of this long survival is given by the fact that many larvae may survive to the next autumn and be the base to build the infestation for the new generation of young animals. This is enhanced by certain management practices ( like having the same paddock for young calves in successive years). Data of several production trials performed in the area have shown that there is an increase in larval count on grass from autumn to winter, the period of time when the e.p.g, count increases and the effects of the disease are also
61 observed (Nesbit et al., 1970; Entrocasso and Steffan, 1980; Steffan et al., 1982; Eddi et al., 1985). The increase of the larval count on pasture may be due in part to a reduction in grass availability. However, it must also be recognized that in autumn the weather conditions (wet with mild temperatures) enhance the evolution and migration of larvae from dung pats to the pasture. Usually there is a great reduction in pasture larval count by the end of spring, depending on the weather conditions (hot and dry), with the count being very low or nil during summer. It was also found that the number of larvae necessary to reduce meat production in winter could be as few as 1000 L J k g dry herbage when the availability of grass is low and the animals are in maintenance or slowly reducing body weight ( Steffan et al., 1986). Recent data obtained in INTA-Balcarce and Anguil ( Suarez et al., 1985; Fiel et al., 1985a) showed that in cattle the inhibition period for Ostertagia is in spring, from September through January, as described in Brazil by Pinheiro et al. (1978) and Melo (1979) and in Uruguay by Nari and Cardozo (1986), and that there is a higher percentage in Argentina and Uruguay ( 70-90% ) than in Brazil ( 40-50% ). Inhibition in spring has also been recorded in southern U.S.A. by Williams et al. (1983), and by Anderson (1972) and Smeal et al. (1980) in southern Australia. Severe ostertagiasis Type II outbreaks have been recorded in animals brought from the northern part of the country to the south for fattening. These animals came from areas where there are low numbers of Ostertagia and they have not developed enough immunity against this parasite. Generally, the clinical Type II ostertagiasis is sporadic and usually seen from summer to autumn (February-May). However, a reduction in the pattern of bodyweight gains in untreated animals detected by production trials during this period is frequently observed (Fiel et al., 1985; Steffan et al., 1986; Entrocasso et al., unpublished) and may represent a 20-30 kg loss in meat production. EFFECTS ON ANIMALPRODUCTION In beef cattle, a mortality level of 1-2% is not uncommon (the highest data recorded is 42% ). This was observed in a farm with a high stocking rate and poor grass availability during the winter of 1977. Losses in meat production of 20-30% have been detected in the area by a group of farmers because although they invested in new cultivated pastures they did not pay enough attention to a parasite control program. Based on some authors (Pinheiro, 1970; Santos, 1973; Entrocasso and Steffan, 1980; Steffan et al., 1982; Mufiiz et al., 1985; A Nari, personal communication, 1986), the subclinical losses may easily reach 20% (20-40 kg) of the bodyweight gain in a short period of time (30-60 days) but if symptoms are clear may rise up to 30-40% (30-60 kg). The weight losses are generally not
62 recouped either after treatment, or following provision of better food or both. The price could be reduced nearly 20% less per kilogram of liveweight in groups of animals after suffering the disease. To date, only one assay has been carried out on milk production in Argentina (Biondani and Steffan, 1985 ), in which a significant loss of 200 kg of milk per year in non-treated groups against suppressing treatment groups was found. These observations were made on 387 cows in eight farms, comparing the production of each animal with its own previous production records. EFFECT ON ANIMALMETABOLISM Studies carried out at the Glasgow Veterinary School (Entrocasso et al., 1986a,b) in naturally infected growing cattle showed the effects of parasites on digestive physiology in two periods, pre-Type II and Type II ostertagiasis, compared with clean and other efficiently treated groups given a morantel sustained release bolus (Paratec, Pfizer).
Alteration in the digestibility In this trial, a pair feeding system was used during the winter period. The mean value during the pre-Type II phase for dry matter and gross energy apparent digestibility were significantly lower in the control calves than in either of the other two groups. The apparent digestibility for all the other feed fractions was also lower in the control group (but not significantly). During the Type II phase, both the organic matter and gross energy digestibility were significantly greater in the morantel bolus treated group than in both the clean and control calves. Otherwise, there were no differences in apparent digestibility of the other feed fractions. The overall means of the phases of dry matter, crude protein and gross energy were significantly lower in the control group than in the MSRB group.
Alteration of nitrogen balance The periodic output of faecal and urinary nitrogen varied, being higher in the control group. However, during the Type II phase, the nitrogen retention failure in control calves was dramatic, showing that ostertagiasis produced a severe gastropathy due to the loss of endogen protein accompanied by low feed intake and poor digestion.
Alteration in protein deposition Protein deposition was affected by the course of both the Type I and the Type II ostertagiasis in the control animals. The reduced leg muscle mass, as
63 shown from the circumference measurements at the symphysis pubis, and the lower lean tissue weights from the seventh to tenth forerib dissection, together with the eye muscle analyses for area, weight and crude protein concentration, confirmed the real occurrence of a lower protein deposition in the control animals. Similar results were obtained in Argentina by Garriz et al. (1986) in a production trial. TREATMENTS The most common drugs used for the control of gastrointestinal parasites are the benzimidazoles such as fenbendazole, oxfendazole and albendazole and now ivermectin. All of them are wide-spectrum drugs and highly efficient against adults and developing and arrested larvae of Ostertagia. Less used are levamisole, closantel and febantel. The great success of ivermectin is based on the high efficacy and longer action against gastrointestinal parasites and also scab (Psoroptes communis var. bovis), avoiding the use of double dipping which risks death of young calves during winter. CONTROL PROGRAMS Control programs for ostertagiasis are widely used among farmers, with the strategic treatments carried out at weaning time and at the end of winter; however, many farmers only treat the animals when signs of the disease appear and have achieved poor results with both treatment times. The concept of symptomatic treatment is far from being economically profitable due to the mistake of believing that money is well invested when clear signs of the disease are stopped by treatment. However, as mentioned previously, losses of 20-40 kg per animal could have occurred without symptoms. On the other hand, the animals often return to the same paddock after treatment, thus reducing the possibility of recovering. In order to decide on treatment, more and more practitioners and farmers are adopting the difference in bodyweight gains between a group of calves treated monthly against another group which represents the rest of the herd, having in consideration the treatment cost. The results obtained are better than using only the e.p.g, counts as an indicator because of the well-known fact of there being no relationship between those values and the number of parasites. In many farms with a history of outbreaks it was possible to achieve good endoparasitic control by integrating the use of'safe pasture' ( e.g. new pasture) and frequent anthelminthic treatments during autumn and winter. Obviously, strategic control of the disease has to be based on epidemiological data and finding the most economical alternative in each particular case. In conclusion, gastrointestinal parasitic disease is not caused by accidental contamination. It is the logical consequence of improper animal management
64
that allows young susceptible animals to accumulate enough larvae to reduce productivity.
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