Gastrointestinal parasites are responsible for causing various damages and alterations in the
bodies of horses. The abusive use of antiparasitic drugs, with the aim of preventing parasites
and treating animals contributes to increased chemical selection pressure for resistant parasites,
reducing the anthelmintic efficacy and increasing the parasite load, which can lead to animal
death. Thus, the objective of this research was to evaluate the anthelmintic efficacy of injectable
ivermectin 1% and doramectin 1% administered orally and intramuscularly in naturally infected
horses, as well as the possible kidney, liver, and muscle damage caused by these products, and
the cost-benefit of these drugs compared to commercial products intended for equines. To this
end, 60 horses, males and females, of different ages, from two rural properties in the
microregion of Araguaína, Tocantins, were used, establishing two groups of 30 animals. On
each property, the animals were divided into three subgroups, as follows: G1=control group
(n=10), which did not receive treatment; G2=animals treated with ivermectin or doramectin at
a dose of 0.2mg/kg intramuscularly (n=10); G3=animals treated with ivermectin or doramectin
at a dose of 0.2mg/kg orally (n=10). For parasitological evaluation, fecal samples were collected
directly from the rectal ampoule 30 days before the start of the experiment to confirm
parasitism, on the day of treatment before administration of the products (D0), and on days 14
(D+14) and 28 (D+28) post-treatment. In all samples, eggs per gram of feces (EPG) and stool
culture were performed. To test the effectiveness of anthelmintics, the Fecal Egg Count
Reduction Test (FECRT) proposed by Coles et al. (1992) was utilized. To identify a possible
acute effect of the products used, serum concentrations of aspartate aminotransferase (AST),
Gamma GT (GGT), creatine kinase (CK), lactate dehydrogenase (LDH), urea (UR) and
creatinine (CREAT) were determined on D0, and at 6, 24, and 48 hours, and 14 days post-
treatment. Thermographic analysis was carried out to evaluate the intramuscular application
site at 6, 24, and 48 hours after application as well as at 7, 14, 21, and 28 days after treatment.
For the cost-benefit analysis, the value per mL when obtaining the products was used and
calculated for 600kg, this being the highest live weight indicated by commercial oral products
suitable for the species. This value was then compared with the average market price, from
quotations obtained in three different stores in the city of Araguaína. The stool culture
demonstrated that the parasitism of the animals was exclusively caused by cyathostomins. The
EPG on D0 of all groups studied, for both ivermectin and doramectin, showed no statistical
difference (p>0.05), demonstrating homogeneity between them. On D+14 there was a
significant difference in G2 and G3 in relation to G1 for ivermectin and only in G3 for
doramectin. In the FECRT, regardless of the active ingredient and route of administration, the
minimum percentage required (95%) was not reached. For biochemical variables, although
statistically significant alterations were detected, it cannot be stated that these were due to the
use of the products, since the control group also presented these alterations, and no pattern of
occurrence was observed. The values found for the variables CK, AST, UR, and CREAT were
within the established reference range. In the thermographic evaluation carried out on animals
in G1 and G2, no alterations were observed in the image pattern or temperature values. In the
cost-benefit analysis, the costs of the products used were significantly lower than existing oral
pastes, however, the ineffectiveness of the products makes their use unfeasible. It is concluded
that there is parasitic resistance to the active principles used, and, thus, it is necessary to adopt
new strategies to control helminths in horses in the North of Tocantins. Furthermore, even after
the use of injectable ivermectin 1% or doramectin 1%, the health and well-being of horses can
be compromised by intestinal parasites.
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