INTRODUCTION

Internal parasites in horses must be controlled with safe, innocuous and efficient formulations to maintain their productive performance. Parasitic resistance is a global problem, the parasites that affect horses are resistant to the main commercial chemicals used for their control (^{García et al., 2013}). To increase the efficacy of drugs and reduce the number of parasites, alternatives should be sought (^{Kaplan, 2002}; ^{Matthews, 2014}); one proposal is to combine active principles such as closantel, combined with fenbendazole.

Closantel belongs to the group of salicylanides, with the chemical formula C₂₂H₁₄C₁₂I₂N₂20₂ and a molecular weight of 663.07 g/mol, its LD₅₀ is 50 mg/kg IM via. It is administered at doses of 10 to 40 mg/kg to control Strongylus edentatus, Strongylus vulgaris, Triodontophorus spp, Gasterophilus intestinalis, and Anaplocephala perfoliata (^{Gokbulut and McKellar, 2018}). In horses, the suggested dose is 10 mg/Kg orally with 40 days of withdrawal time (^{Guerrero et al., 1983}; ^{Guerrero, 1984}). In the bloodstream, closantel binds to albumin and reaches its maximum concentration at 40 hours, its half-life is three weeks with oral bioavailability of 50%. The compound blocks oxidative phosphorylation at the mitochondrial level, preventing the availability of energy, causing the death of parasites (^{Hennessy and Ali, 1997}).

Fenbendazole is a white powder with the chemical name 5- (phenylthio) -1H- benzimidazol-2-yl carbonic acid methyl ester, molecular weight 299.34 g/mol and its formula is C₁₅H₁₃N₃0₂S (^{Hennessy and Ali, 1997}). It is used to control parasites in cattle, horses, pigs, dogs and cats. In horses it is indicated for large and small Strongylus spp and Oxyuris equi, it is recommended at a dose of 10 mg/kg orally for horses (^{Guerrero et al., 1983}; Guerrero, 1984). The LD₅₀ in horses is given when it exceeds 10 g/kg administered orally, it has no negative reports; however, hypersensitivity reactions may occur.

The objective of the present investigation was to know the effect and safety of closantel, combined with fenbendazole, at a dose of 10 mg/kg on the eggs of gastrointestinal parasites in horse feces.

MATERIAL AND METHODS

RESULTS

In the investigation 102 horses were sampled; 53 animals tested positive for Strongylus Spp., and two horses of the 53 also presented Parascaris Spp. eggs, the prevalence was 51.9%. The presence of eggs in feces decreased due to the effect of the treatment (P≤0.05); in males, the efficacy was 98.87, 83.56 and 91.68% and in females 99.23, 93.11 and 87.90%, at 15, 30 and 45 days (table 1), respectively. Furthermore, the amount of eggs in fecal feces of females was higher compared to males (P≤0.05). The Irapuato horses presented more eggs (25.52) in the feces (P≤0.05), compared with the Pénjamo horses (10.58) (Table 1). The age of the horses did not show any relationship with the amount of eggs found in the feces (data not shown).

In the physiological variables of the horses there were no changes due to the effect of the drug at 90 minutes after application (Table 2).

DISCUSSION

The control of gastrointestinal parasites in horses is a common activity integrated into the management calendar, but a diagnosis of the type and quantity of parasites in the locality or the place where the productive unit is located is not obtained (^{García et al., 2013}). ^{Bedoya et al. (2011}) reported prevalence of 92% and ^{Cala-Delgado et al. (2016)} of 71%, high percentages compared to the results obtained in the study of 51.9%, but they are percentages close to those reported by ^{Aromaa et al. (2018}) of 57.6% in different breeds and types of equines. The variation in the prevalence and quantity of parasites found can be caused by the zootechnical purpose, the locality, type of feeding, sanitary management of the animals and the diversity of the parasites in the study areas. In the investigations the most reported genera are Trichostrongylus, Trichonema spp. and Strongylus Spp (^{Cala-Delgado et al. (2016)}. However, in the study carried out, all positive horses presented Strongylus Spp eggs, and only two horses also presented Parascaris Spp.

The study only has the purpose of knowing the effect of pasta on the number of eggs and the species of parasites were not considered, but two were identified.

In horses, the control of gastrointestinal parasites is carried out on a daily basis; ^{Buzatuet al. (2015)} determined prevalence greater than 50 parasite eggs per gram of feces in 137 horses out of 195 sampled. At 10 weeks after treatment, they determined that the horses were only protected for less than three months. ^{Sanna et al. (2016}) used macrocyclic lactones and benzimidazoles to evaluate their effect from day 0 to 150 days after their application. The results obtained to eliminate total parasite eggs were 99.7% and at 14 days 99%. Similar results show the study presented with the proposed combination at the dose indicated by the manufacturer.

The active principle used can determine the efficiency of treatment and protection time in horses. The treatment of choice are paste drugs with ivermectin, pyrantel pamoate, moxidectin with praziquantel; or its ivermectin-praziquantel combination. ^{Caffe et al.(2018)} evaluated ivermectin at a dose of 0.2% mg/kg as the treatment of choice with efficacy to reduce the fecal egg count of 99.8% in mares and 100% in fillies at 15 days, results similar to those found in the present study combining closantel-fenbendazole. In no horse was the age effect in the samplings on the amount of eggs in feces; the youngest horses were 1.5 to 2 years old and the older ones about 30 years old.

In pack-horses or draft horses the parasite incidence is higher, compared to animals used for sports, entertainment or therapy. ^{Tedla and Abichu (2018}) determined 72.2% incidence in pack animals, percentages higher than those found in our study, due to the work environment and type of care they receive. In the stables, canvases and police groups the animals have periodic care, this means that they have preventive medicine programs generally established but horses destined for field work do not receive the same care, their metabolic wear is greater due to their general living conditions. Therefore, it is common to find results that exceed those reported in the present study, due to poor management, working conditions and nutritional status; as well as the grazing carried out in pastures or workplaces, age of the hosts, and even race and sex can affect the results of studies (^{Caffe et al., 2018}).

Horse sex can be a predisposing factor in describing and reporting results in equid investigations. ^{Bedoya et al., (2011}) report that the horse sex has no effect on the incidence of parasites; but our results indicate that females have 48% more parasite eggs per gram of feces, compared to males. Evident findings that may suggest that the number of parasite eggs may be related to the equine sex.

CONCLUSION

The efficacy found in the study confirms that closantel combined with fenbendazole at 10 mg per kg of each drug is 99.05% at 15 days, and its administration is safe for horses.