Currently many antiparasitic drugs are available for control and prevention of helminthosis in animals. Closantel is one of such widely used antiparasitic drugs for control and prevention of Fasciolla hepatica, Haemonchus contortus and Oestrous ovis infestation¹,². Closantel is an important member of halogenated salicylanilides class which seems to work through uncoupling of the oxidative phosphorylation in cell mitochondria, disturbing the process of ATP generation in parasites³. It also interrupts ions channels across parasite cell membrane and inhibits chitinase like enzymes which are essential for survival of parasites. Following oral administration closantel readily get distributed in blood and has strong binding affinity for plasma albumins¹,³. No specific antidotes available for closantel which worsens the toxicity conditions. There are few published reports of closantel toxicity in sheep⁴,⁵,⁶-⁷, goat⁴,⁵,⁸, dog⁹ and humans¹⁰. Present investigation revealed various clinical signs and histopathological changes in vital organs induced by accidental administration of closantel in overdose leading to morbidity and mortality in camels.

No appreciable gross lesions were detected in all three necropsied camels. However, microscopically, severe lesions were noted in cerebrum, cerebellum, optic nerve, retina while moderate to mild lesions were noted in liver, kidney and lungs.

Microscopically, white matter and gray matter of cerebellum and cerebrum showed marked spongy degeneration. Lesions were more severe in white matter as compared to gray matter. Spongy degeneration characterized by presence of sharply demarcated, variable sized, occasionally coalescing, clear vacuoles/cyst like spaces compressing neuropil (Fig. 1). Most if not all robin virchow space (perivascular space) were markedly expanded by clear spaces indicative of edema. Gray matter of cerebellum also showed similar changes. Clear vacuoles/cyst like spaces were evident in granular cell layer, Purkinje cell layer and molecular cells of gray matter as well as white matter of cerebellum. Rarely, Purkinje neurons showed either clear, sharply demarcated vacuole in soma or neuronal necrosis (eosinophilic neurons) (Fig. 2). Although no special staining performed on brain, lesions were due to intramyelinic edema, myelin splitting and vacuolation. Myelin of the optic nerve also underwent intramyelinic edema, myelin splitting and vacuolation however axons were hardly affected (Fig. 3, 4). Most of the Robin- Virchow space were markedly expanded by clear spaces. Retina was completely effaced by clear vacuoles/cyst like spaces found diffusely above pigmented/choroid layer (Fig. 5). Cornea showed edema and ulcer formation. Corneal stratified epithelium showed lysis/separation of upper epithelial cells with basal layer due to formation of coalescing, clear vacuoles, forming ulcer in cornea. Descemet‘s membrane was also separated from overlying corneal stroma by clear edematous fluid (Fig. 6). Collagen bundles of corneal stroma were also separated due to edema.

In liver, most hepatocytes showed variable sized, clear, one or occasionally more than one vacuoles in cytoplasm (Fig. 7). However, nature of cytoplasmic vacuoles (fat, glycogen or water) was not confirmed by special stains. Occasional mononuclear cellular infiltration was also evident. There was minimal increase in peri-lobular fibrous tissue. Diffusely, within the cortex and medulla of kidney, most tubules were separated by large multiple clear vacuoles or cystic spaces (interstitial edema) (Fig. 8). Tubular epithelium showed various changes viz., degeneration, necrosis, sloughing of epithelium and obliteration of tubular lumen. Many glomeruli were segmentally or globally compressed.

Among various anthelmintics used in veterinary medicine, probably salicylamides have longest half-life¹². After oral administration of closantel, peak plasma concentrations of this drug are reached within 24 to 48 hours. The long half-life of closantel is due to extensive binding of closantel to plasma proteins¹³. Closantel interfere with the proton gradient in the parasite‘s mitochondria that in turn inhibits the generation of ATP by the parasite¹⁴. These unique pharmacokinetic characteristics of closantel including rapid parenteral absorption and long half-life appear to play important roles in its efficacy and toxicity.

In cattle and sheep for prevention and control of Fasciola hepatica and Haemonchus contortus infestation closantel is widely used¹,². The drug is available for oral as well as parenteral administration. Clinical signs noted in intoxicated camels were of incoordination, muscle tremors, visual disturbances, blindness, anorexia and depression. Similar findings were also reported by earlier investigators⁴,⁵,⁶,⁷,⁸-⁹.

Histopathological examination of closantel intoxicated animals revealed marked spongy degeneration of brain, intramyelinic edema, myelin splitting and vacuolation in optic nerve, corneal edema, hepatocellular vacuolation and nephropathy. Earlier investigators⁴,⁵,⁶,⁷,⁸-⁹ had also reported similar lesions in brain, optic nerve and retina with variable severity. Some investigators suggests different mechanism for closantel induced neuropathy viz. either direct effect of closantel on axons⁶, possible nerve compression in optical channel because of edema or combination of both above mechanisms¹⁵. Severe retinopathy was observed in present investigation. As suggested by earlier investigator⁴ this kind of changes in retina might be due to degeneration in retinal layers because of active ingredient present in the compound. Hepatotoxicity (hepatocellular vacuolation and degeneration) of closantel has been recorded in goat⁸ and dog⁹, respectively. Lesions in kidney like interstitial edema, tubular degeneration and glomeruli compression were probably first time documented. When we compared the severity of histopathology lesions in goat¹⁶ and sheep¹⁷ in which toxic dose of closantel is estimated to be 8 to 10 times and 3 to 6 times more than the recommended dose, respectively, camel seems to be more susceptible for closantel intoxication.

Spongy degeneration of brain in animals need to be differentiated from some other compound producing similar lesions when history and circumstantial evidence were inadequate. Condition like hyperammonaemia¹⁸,¹⁹, Senecio jacobaea poisoning¹⁸ (a plant which contains pyrrolozidine alkaloids), hexachlorophene intoxication²⁰, 6-Aminonicotinamide intoxication²¹, bromethalin intoxication²² and Helichrysum argyrosphaerum poisoning²³,²⁴ produced similar spongy degeneration in brain.

In conclusion, closantel produced spongy degeneration of brain, optic neuropathy, retinal degeneration, corneal edema, hepatotoxicity and nephrotoxicity when administered in higher dose than recommended. To prevent closantel toxicity, closantel dose should be calculated on basis of body weight and concentration of drug in formulation.

Authors are thankful to the Dean and Principal, College of Veterinary Science and Animal Husbandry, Sardarkrushinagar Dantiwada Agricultural University, Sardarkrushinagar, Gujarat, India for providing the necessary facilities to carry out this work.

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