Monensin toxicosis in broiler chickens

Monensin poisoning outbreak in free-ranging and captive birds

Monensin poisoning is uncommon and has been rarely reported in birds. This work aimed to described clinical-pathological aspects of an outbreak of monensin poisoning in captive and free-ranging birds. Thirty-seven of 600 captive birds fed a diet containing 893.19 mg/kg of monensin died within 10 days (mortality 6.17%). There was no ionophore antibiotics on the feed label supplied to captive birds, which established an error in feed production. Necropsies were performed on twelve animals: Muscovy duck (Cairina moschata) (2/12), greater rhea (Rhea americana) (2/12), black-necked swan (Cygnus melancoryphus) (2/12), garganey (Anas querquedula) (1/12), ostrich (Struthio camelus) (1/12), and common pigeon (Columbus livia) (4/12). These four common pigeons were free-ranging birds and died after eating the same contaminated feed. Birds were mainly found dead, however in animals which clinical signs were observed (Columba livia, Rhea americana, Cairina moschata, Anas querquedula, and Struthio camelus), they included incoordination, inability to stand, and intense prostration, that ranged from 24 to 72 h until death. Grossly, five birds had focally extensive pale firm areas in the myocardium and two had in the skeletal muscles, one being concomitant lesions. Histologically, muscle necrosis and degeneration were observed in striated musculature (skeletal and/or heart) in all birds analyzed. Monensin poisoning outbreaks can affect free-ranging birds that are fed on external feeders, as well as captive birds, due to an error in the feed formulation.

Safety and efficacy of Monteban® G100 (narasin) for chickens for fattening

The feed additive Monteban® G100, containing the active substance narasin, an ionophore anticoccidial, is intended to control coccidiosis in chickens for fattening at a dose of 60-70 mg/kg complete feed. Narasin is produced by fermentation. Limited data on the taxonomic identification of the production strain did not allow the proper identification of strain NRRL 8092 as Streptomyces aureofaciens. The FEEDAP Panel cannot conclude on the absence of genetic determinants for antimicrobial resistance in Streptomyces spp. under assessment. Based on the available data set, the FEEDAP Panel cannot conclude on the safety of Monteban® G100 for chickens for fattening. The simultaneous use of Monteban® G100 and certain antibiotic drugs (e.g. tiamulin) is contraindicated. Narasin is not genotoxic. No indication of carcinogenicity or developmental toxicity was found at the doses tested in the mouse, rat and rabbit. The lowest no observed effect level (NOEL) identified in the oral toxicity studies was 0.5 mg/kg body weight (bw) per day for the neuropathy seen in a one-year dog study. The acceptable daily intake (ADI) derived from this NOEL is 0.005 mg narasin/kg bw applying a uncertainty factor of 100. Monteban® G100 is safe for the consumer. Maximum residue limits (MRLs) of 50 μg narasin/kg for all wet tissues ensure consumer safety. Monteban® G100 is irritatant to the eyes but not to the skin. It has the potential to induce skin sensitisation. Inhalation exposure would pose a risk to persons handling the additive. Narasin, when used as a feed additive for chickens for fattening at 70 mg/kg feed, is not expected to pose a risk to the environment. The risk for sediment compartment cannot be assessed. The FEEDAP Panel cannot conclude on the efficacy of Monteban® at the minimum applied dose of 60 mg narasin/kg complete feed for chickens for fattening.

Intoxicação acidental por monensina em ovinos no Estado do Rio de Janeiro

Descreve-se um surto de intoxicação por monensina em ovinos no Estado do Rio de Janeiro, no qual de 180 animais, oito morreram após serem alimentados com ração contendo o ionóforo. A enfermidade, de evolução variável, caracterizou-se clinicamente por apatia, arritmia cardíaca, mioglobinúria, incoordenação, incapacidade de se levantar, decúbito esternal; uma ovelha abortou. As lesões macroscópicas consistiram de áreas pálidas no miocárdio, hidroperitônio, hidrotórax e edema pulmonar. O exame histopatológico evidenciou alterações degenerativo-necróticas no coração e na musculatura esquelética. No miocárdio, as lesões eram mais marcadas e caracterizavam-se por necrose multifocal com substituição das miofibras por tecido conjuntivo fibroso e inflamação intersticial mononuclear. Adicionalmente, verificaram-se proliferação de células satélite e reação inflamatória mononuclear em músculos esqueléticos. Ao que tudo indica, a adição excessiva de monensina sódica, talvez associada à homogeneização inadequada da droga ao alimento, tenha determinado a ingestão de grande quantidade de monensina por parte dos animais.

Intoxicação por antibióticos ionóforos em animais

O uso terapêutico de antibióticos ionóforos em medicina veterinária difundiu-se muito nos últimos anos, com conseqüente aumento no risco de intoxicação em animais. Antibióticos ionóforos são usados como coccidiostáticos e como aditivo em alimentos para animais, com o propósito de estimular o desenvolvimento e o ganho de peso. Os ionóforos mais utilizados na alimentação de animais são a monensina, lasalocida, nasarina e salinomicina. Há uma grande variação na susceptibilidade dos efeitos tóxicos dos ionóforos de acordo com a espécie animal. A intoxicação pode ocorrer quando dosagens elevadas de ionóforos são adicionadas aos alimentos, ou quando ionóforos são incluídos inadvertidamente ou acidentalmente em dosagens não corretas para determinada espécie animal. Casos de intoxicação têm sido descritos em bovinos, ovinos, suínos, eqüinos, cães e aves. Para os eqüinos os ionóforos são extremamente tóxicos. São considerados seguros quando usados nas espécies-alvo, dentro das dosagens recomendadas pelo fabricante.

Histopathology of monensin-tiamulin myopathy in broiler chicks

Thirty-six 7-day-old broiler chicks were simultaneously given food containing monensin, and water containing tiamulin, both drugs being at normal levels of usage. Equal numbers of chicks on a basal diet and plain water served as the controls. Anorexia, depression, drowsiness, leg weakness and a decrease in body weight appeared on days 2 to 3 of administration in several treated chicks. These clinical signs and growth retardation were prevalent and severe on days 4 to 7, at which time some chicks became recumbent. From day 9, chicks showed gradual recovery from the clinical signs and growth retardation. Histopathologically, the neck and leg skeletal muscles examined were severely affected in treated chicks, but cardiac and pectoral muscles were intact. Besides hyalinisation and floccular change which appeared infrequently in early stage of the experiment, muscle fibres showing an enlargement of the nuclei and a distention of a pale to basophilic sarcoplasm, suggestive of partial myofibrillar lysis and subsequent reparative change, dominated all affected muscles. These degenerative and reparative changes were considered to be distinctive for monensin-tiamulin myopathy in chicks.

Interaction of ionophore and vitamin E in knockdown syndrome of turkeys

Monensin and vitamin E concentrations, as well as histopathology of skeletal muscles and myocardium, were evaluated in broad-breasted white turkeys kept in commercial facilities. Turkeys with knockdown syndrome had myopathy of skeletal muscles, but no lesions in the myocardium. Generally, concentration of monensin in serum was highest in turkeys diagnosed with knockdown syndrome given more than 90 mg/kg of monensin in the diet, followed by turkeys diagnosed with knockdown syndrome given <90 mg/kg of monensin in the diet, healthy turkeys fed a diet that contained <90 mg/kg of monensin, and finally healthy turkeys fed a diet free of monensin (not detectable). However, the concentration of monensin was highly variable within each group, and the median was lower than the average. Vitamin E concentrations in the livers varied from low-normal to below normal and were statistically higher in healthy turkeys fed a diet free of monensin than in the livers of birds from the 3 groups exposed to monensin. This suggests that the concentration of monensin in serum positively correlates to the severity of clinical signs and pathology and to the amount of monensin in the feed. Although the methodology developed to detect serum monensin concentrations is beneficial and accurate for case investigations, it is recommended that several samples from each flock be evaluated because of variation within a flock. The current study also suggests that monensin in the feed could induce lower concentrations of vitamin E in the liver of turkeys and can predispose the turkeys to knockdown syndrome.

Tiamulin and semduramicin: effects of simultaneous administration on performance and health of growing broiler chickens

The pleuromutilin antibiotic tiamulin (TIA) is known to produce a variety of negative interactive effects when it is administered in combination with several anticoccidial ionophores. A 35-d growth study was performed in cages to evaluate the compatibility of TIA when it was administered concurrently with the poly-ether ionophore anticoccidial semduramicin (SEM). Tiamulin and SEM, both alone and in combination, were administered to 10 replicates of female broilers arranged in a completely randomized block design. Tiamulin was administered in drinking water (250 mg of TIA/kg of water) from d 15 through 19 of the study, whereas SEM was incorporated in feed (25 mg/kg) from placement to the conclusion of the test. Water consumption was determined during the period of concurrent administration of the drugs and weekly measurements of feed intake and bird performance were recorded. In addition, hematocrit, blood cell counts, serum protein, albumin, glucose, uric acid, electrolytes, and activities of several enzymes were determined from blood samples taken at d 35. Results indicated that simultaneous administration of TIA and SEM during the third week of the trial reduced water and feed intake resulting in a temporary growth depression. Feed efficiency was transiently affected during the period of coadministration. However, during the fourth week of the test, negative effects in body weight were not observed for any treatment and feed conversion improved for birds concurrently receiving TIA + SEM. By the termination of the experiment, no adverse effects were observed in final performance for any treatment. Histopathological and hematological parameters were unaffected by treatment at d 35 of the test. These results demonstrated that simultaneous administration of TIA and SEM produced only temporary impairments of water and feed consumption that transiently influenced performance. Neither mortality nor long-term effects on performance variables occurred in broilers.

Veterinary pharmacovigilance. Part 3. Adverse effects of veterinary medicinal products in animals and on the environment

Like humans, animals may experience adverse effects when treated with medicinal products. These effects may be related to the pharmacological or toxicological properties of the substances used or they may arise because of hypersensitivity. Veterinary medicinal products may also possess the ability to harm the environment. This paper reviews the potential of veterinary medicinal products to cause adverse effects in animals and on the environment.

Biochemical background of toxic interaction between tiamulin and monensin

Tiamulin, a diterpene antibiotic, is used for treatment of pulmonary and gastrointestinal infections in swine and poultry. Combined administration of tiamulin and ionophores (e.g. monensin) to farm animals may lead to intoxication manifested in severe clinical symptoms. Tiamulin metabolite complex with cytochrome P450 has been suggested to be the basis of drug-interactions. However, the formation of metabolic intermediate complex is questionable. The effect of tiamulin-treatment on cytochrome P450 activities was investigated in rats. Ethylmorphine and aminopyrine N-demethylation activities as well as monensin metabolism (O-demethylation) increased in liver microsomes of tiamulin-treated (200 mg/kg) animals. CYP3A1 induction caused by tiamulin was confirmed by the results of Western blot analysis. To test metabolic intermediate complex formation as a result of tiamulin treatment, cytochrome P450 activities were also determined in the presence of potassium ferricyanide. The findings together with those of in vitro complex formation suggested that formation of metabolic intermediate complexes of tiamulin with cytochrome P450 could be excluded. On the other hand, the results of inhibition studies showed significant decrease of ethylmorphine or aminopyrine as well as monensin demethylation in the presence of tiamulin. Our results proved that tiamulin has dual effect on cytochromes P450. It is able to induce and directly inhibit CYP3A enzymes, which are predominantly responsible for monensin O-demethylation. The direct effect of tiamulin as an inhibitor might play a more important role in toxicity than its putative effect as a chemical inducer of CYP3A enzymes.

Macrolide antibiotics, drug interactions and microsomal enzymes: implications for veterinary medicine

The macrolide group of antibiotics includes natural members, pro-drugs and semi-synthetic derivatives, thus named because they are composed of a large aglycone ring (from 14 to 16 carbon atoms), to which are attached several sugars. Some of them are amino-sugars, containing a diethylamino, tertiary amine function. A number of antibiotics, including erythromycin, oleandomycin, triacetyl-oleandomycin (troleandomycin), carbomycin, spiramycin, tylosin, rosamicin, azithromycin, clarithromycin, dirithromycin and others, are members of this group. On a comparative basis, erythromycin and oleandomycin are similar, with the same basic 14-carbon lactone ring and side chain sugars. The remaining compounds contain a basic 15- or 16-carbon lactone ring and one or two side-chain sugars. Most of the macrolides are produced by Streptomyces spp bacteria. An exception is rosamicin, which is produced by Micromonospora. Clarithromycin and azithromycin are new semi-synthetic derivatives of erythromycin.

Feed Additives: Do They Add to Animal Welfare? An Evaluation

The welfare of farm animals is strongly influenced by the man-made environment. Welfare problems also arise from reduced homeostatic capacities in animals. Feed additives, used to promote growth or to prevent diseases can alter the animals' self-regulating capacities thus affecting their welfare. The EU regulates the use of these additives within specified groups of Directive 70/524/EEC. Although these feed additives can be regarded as prescription-free veterinary drugs, critical remarks on their desired and adverse effects have received little attention.

A survey of the available literature shows that about one-third of licensed feed additives alter adrenal function in vitro. Reports of the adverse effects of anticoccidial additives in vivo suggest they can be classified under three headings: (i) substances with a very narrow safety margin (the difference between the permitted dose and the dose with adverse effects) and often irreversible effects on growth and feed conversion; (ii) substances with a narrow safety margin and largely reversible effects; (iii) substances with an adequate safety margin. The growth promoters (including antibiotic growth promoters) can - on the basis of their adverse effects - be classified into two groups: (i) substances with a very narrow safety margin; and (ii) substances with an adequate safety margin.

On the one hand, animal welfare considerations require use of disease-preventing additives, but on the other hand, they also demand discontinuation of current practices. Judicious use of additives can add to animal welfare. However, their unlimited use to obscure defects in husbandry is detrimental to animal welfare. A major obstacle to the judicious use of feed additives, is the lack of published, unbiased information on their efficacy and safety for farm animals.

Does pulmonary hypertension syndrome (ascites) occur more frequently in broilers medicated with monensin?

The performance of broilers reared in floor pens and given monensin in the feed at 121 ppm was compared with that of birds given no drug. Feed intake and BW gain of medicated birds was significantly lower than that of unmedicated birds from 0 to 22 d of age. Feed intake and feed conversion of medicated birds was significantly reduced, compared with unmedicated birds, from 22 to 53 and 0 to 60 d of age. Total mortality, and mortality due to leg abnormalities from 22 to 53 and 0 to 60 d, was significantly lower in birds given monensin. There was no difference in the incidence of tibial dyschondroplasia (TD) by 60 d. No differences in mortality due to pulmonary hypertension syndrome (PHS) were observed for any age period. Birds removed from pens at 28 d that had received monensin had lower hematocrit and percentage saturation of hemoglobin with oxygen in the blood than unmedicated birds. No differences in these variables were found at 54 d. There were no differences in the right ventricle weight: total ventricular weight ratios or electrocardiogram lead II values at 28 or 54 d. The results indicate that PHS does not occur more frequently in broilers medicated with monensin.

Pharmacokinetic profile and tissue distribution of monensin in broiler chickens

1. The pharmacokinetics of monensin, including half-life, apparent volume of distribution, total body clearance, systemic bioavailability and tissue residues were determined in broiler chickens. The drug was given by intracrop and intravenous routes in a single dose of 40 mg/kg body weight. 2. Following intravenous injection the kinetic disposition of monensin followed a two compartments open model with absorption half life of 0.59 h, volume of distribution of 4.11 l/kg and total body clearance of 28.36 ml/kg/min. The highest serum concentrations of monensin were reached 0.5 h after intracrop dosage with an absorption half-life of 0.27 h and an elimination half life of 2.11 h. The systemic bioavailability was 65.1% after intracrop administration. Serum protein-binding tendency of monensin calculated in vitro was 22.8%. 3. Monensin concentrations in the serum and tissues of chickens after a single intracrop dose of pure monensin (40 mg/kg body weight) were higher than those after feeding a supplemented monensin premix (120 mg/kg) for 2 weeks. Monensin residues were detected in tested body tissues, collected 2, 4, 6 and 8 h after oral administration. The highest concentration was found in the liver. In addition, monensin residues were detected only in liver, kidney and fat 24 h after the last oral dose. No monensin residues could be detected in tissues after 48 h, except in liver which cleared completely by 72 h.

Potential of chemical regulation of food intake and body weight of broiler breeder chicks

1. Two experiments were performed to evaluate the potential of phenylpropanolamine hydrochloride and monensin sodium as appetite- and weight-control agents for Indian River broiler breeder chicks. 2. In experiment 1, a total of 300 day-old sexed broiler breeder chicks were individually weighed and placed in battery cages. They were randomly assigned to 5 dietary treatments, namely 0, 50, 100, 200 and 400 mg/kg of phenylpropanolamine hydrochloride added to a maize-soyabean meal basal diet. 3. In experiment 2, a total of 400 day-old sexed broiler breeder chicks were randomly assigned to 10 dietary treatments which were a combination of two concentrations of dietary crude protein (200 and 150 g/kg) and 5 different concentrations of added drugs in the diet, namely 0, 500 and 800 mg/kg of phenylpropanolamine hydrochloride and 200 and 300 mg/kg of monensin sodium. 4. Food consumption and body weight gain were significantly reduced by feeding diets containing the drugs but mortality was not significantly affected. Birds showed evidence of increased tolerance, with age, to phenylpropanolamine but not to monensin. 5. Monensin sodium, at high inclusion rates, was found to be a more potent and effective appetite- and growth-depressing agent for broiler breeder chicks than phenylpropanolamine and may have application in broiler breeder production using an ad libitum feeding programme.

Alteration of intracellular traffic by monensin; mechanism, specificity and relationship to toxicity

Monensin, a monovalent ion-selective ionophore, facilitates the transmembrane exchange of principally sodium ions for protons. The outer surface of the ionophore-ion complex is composed largely of nonpolar hydrocarbon, which imparts a high solubility to the complexes in nonpolar solvents. In biological systems, these complexes are freely soluble in the lipid components of membranes and, presumably, diffuse or shuttle through the membranes from one aqueous membrane interface to the other. The net effect for monensin is a trans-membrane exchange of sodium ions for protons. However, the interaction of an ionophore with biological membranes, and its ionophoric expression, is highly dependent on the biochemical configuration of the membrane itself. One apparent consequence of this exchange is the neutralization of acidic intracellular compartments such as the trans Golgi apparatus cisternae and associated elements, lysosomes, and certain endosomes. This is accompanied by a disruption of trans Golgi apparatus cisternae and of lysosome and acidic endosome function. At the same time, Golgi apparatus cisternae appear to swell, presumably due to osmotic uptake of water resulting from the inward movement of ions. Monensin effects on Golgi apparatus are observed in cells from a wide range of plant and animal species. The action of monensin is most often exerted on the trans half of the stacked cisternae, often near the point of exit of secretory vesicles at the trans face of the stacked cisternae, or, especially at low monensin concentrations or short exposure times, near the middle of the stacked cisternae. The effects of monensin are quite rapid in both animal and plant cells; i.e., changes in Golgi apparatus may be observed after only 2-5 min of exposure. It is implicit in these observations that the uptake of osmotically active cations is accompanied by a concomitant efflux of H+ and that a net influx of protons would be required to sustain the ionic exchange long enough to account for the swelling of cisternae observed in electron micrographs. In the Golgi apparatus, late processing events such as terminal glycosylation and proteolytic cleavages are most susceptible to inhibition by monensin. Yet, many incompletely processed molecules may still be secreted via yet poorly understood mechanisms that appear to bypass the Golgi apparatus. In endocytosis, monensin does not prevent internalization. However, intracellular degradation of internalized ligands may be prevented.(ABSTRACT TRUNCATED AT 400 WORDS)

Salinomycin poisoning in point-of-lay turkeys

Salinomycin poisoning occurred in a flock of 700 point-of-lay turkeys; 400 birds died over 7 days as a result of consuming feed contaminated with 50 ppm salinomycin. No gross lesions were detected. Histologic evidence of a myopathy was most readily detected in leg muscles of turkeys 5 to 7 d after ingesting salinomycin. Feeding trials were undertaken and individual susceptibility to the drug was found to vary greatly. In affected birds the plasma concentrations of creatine kinase (CK) and aspartate aminotransferase (AST) were found to be in the range of 500,000 to 2,500,000 IU/l and 9000 to 25,000 IU/l, respectively. The marked increase in the plasma activities of these enzymes preceded histological evidence of segmental muscle necrosis.

Effect of high levels of monensin during the growing period on subsequent breeding performance of adult broiler breeders

The breeding performance of broiler breeder chickens previously raised on high levels of monensin was evaluated. The birds were raised on concrete floors in an open-sided pullet brooding and growing house. At 21 weeks of age monensin feeding was stopped and birds were placed in the breeder house. A breeder ration formulated to contain 2,920 kcal metabolizable energy and 16% crude protein was fed to all the birds irrespective of the growing system used. A standard feeding allowance and lighting program as recommended by Indian River International was followed during the laying period. The parameters determined were body weight uniformity at 21 weeks of age, egg production, egg weight, shell weight, shell thickness, feed conversion (kilograms feed/dozen eggs), mortality, fertility, and hatchability. Assays for monensin residue in the breast muscle, liver, and abdominal fat were run at 52 weeks of age (31 weeks after monensin withdrawal). Birds grown on high levels of monensin were not as uniform in body weight at 21 weeks of age as the restricted controls. Only birds fed 400 ppm of monensin with a low protein diet during the growing period showed a significant reduction in egg production, shell weight, and shell thickness. There were no significant differences among the growing systems in mortality during lay, fertility, hatch of fertile eggs, and hatch of total eggs. Monensin was not detectable (less than .1 ppm) in the liver, abdominal fat, and breast muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

The potential of monensin for body weight control and ad libitum feeding of broiler breeders from day-old to sexual maturity

A total of 180 male and 360 female day-old Indian River broiler breeder chicks were randomly allocated to 18 floor pens in an open-sided pullet brooding and growing house. The birds were subjected to six growing systems that were a combination of three levels of monensin (100, 300, and 400 ppm) and two levels of starter crude protein (CP) (14 and 18% CP) in a randomized complete block design. The basal diets were formulated to be isocaloric, containing 2920 kcal ME/kg. Birds fed 100 ppm of monensin were feed restricted and served as controls, while birds fed 300 or 400 ppm of the drug were fed ad libitum from day-old to sexual maturity. The parameters determined were biweekly individual body weights, feed consumption, internal organ weights, bone and serum mineral composition, and mortality to 21 weeks of age. In general, birds fed high levels of monensin plus ad libitum feeding from day-old were more variable in body weight compared to the feed-restricted controls. In the females, high levels of monensin in the diet were associated with significant reductions in body weight, even with ad libitum feeding, to 21 weeks of age. Male birds fed 300 ppm of the drug were significantly heavier than the feed-restricted controls but were still within the range recommended at this age. High levels of monensin in the diet were associated with a significant reduction in cumulative ad libitum feed consumption compared to the feed-restricted controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced myotoxicity and involvement of both type I and II fibers in monensin-tiamulin toxicosis in pigs

Simultaneous administration of monensin and tiamulin to pigs resulted in enhanced myotoxicity. Skeletal muscles of tongue, diaphragm and legs were preferentially affected, whereas the masseter, longissimus thoracis and cardiac muscles, including the left auricle, were spared. Histochemical examination revealed an involvement of both type I and II fibers of skeletal muscles.

Suspected monensin toxicosis in feedlot cattle

Suspected monensin toxicosis was seen in feedlot cattle aged 6 to 9 months. Twenty cattle died following inclusion of monensin in the feed at 400g/tonne, which was 13 times the recommended level. The deaths occurred over 2 weeks. Clinical signs were inappetance, respiratory distress and sudden death. Post-mortem features were those of right-sided heart failure and included dependent subcutaneous oedema, ascites, hydrothorax, and periancinar hepatocyte congestion and necrosis. However, in contrast to previous reports no myocardial necrosis was found, but focal skeletal muscle necrosis was observed. Additional findings were marked pulmonary oedema accompanied by fibrin and erythrocyte exudation into alveoli and interlobular lymphatics. From these findings it appears that monensin, as well as affecting both cardiac and skeletal muscle, has a primary effect on lung vasculature.

Light and electron microscopic changes in cardiac and skeletal muscle of sheep with experimental monensin toxicosis

Monensin toxicosis was induced in lambs by either a single oral dose of 12 mg/kg or six daily doses of 8 mg/kg. Clinical signs of toxicosis consisted of depression, dyspnea, stiffness of gait, reluctance to move, and recumbency. Serum creatine phosphokinase activity was increased. Samples of skeletal and cardiac muscle were obtained over a six-day period and examined by light and electron microscopy. Light microscopic changes in cardiac and skeletal muscles consisted initially of vacuolation and intracellular edema of muscle cells followed by segmental necrosis. Interstitial fibrosis was present on days 5 and 6 postexposure. Muscle fiber necrosis was more severe in skeletal than cardiac muscles and most severe in sheep given 8 mg/kg of monensin daily. Macrophages were seen only in areas of severe necrosis. The earliest ultrastructural change was severe swelling of mitochondria. Secondary changes consisted of lipid accumulation and myofibrillar alterations. Myoblast proliferation was present as early as four days after initial exposure to monensin.

Subchronic toxicity of monensin in broiler chickens

One hundred ninety-two male broiler chicks were dosed with monensin at concentrations of 0, 121, or 242 mg/kg feed throughout the normal growing period (50 days). Body weight gain and feed efficiency were determined weekly, and cardiac muscle was examined grossly and histologically at the end of the experiment. Livers also were weighed and examined grossly. Feed intake was determined daily, allowing continuous monitoring of drug intake. No depressing effects of the drug on growth rate and efficiency were observed until after four weeks, and then were evident only in the chicks receiving the 242 mg/kg diet. Subepicardial hemorrhage and congestion occurred in 40% of the hearts from the chickens fed the high monensin dose and were nonexistent in the other treatments. There appeared to be an inverse relationship between monensin dose and liver weight. The paralytic effects previously reported from acute dosing experiments were not observed. The results show that the heart and probably the liver are sensitive indicators of monensin toxicity and that the subchronic toxic dose is less than 18 mg/kg body weight per day.