Comparative plasma disposition, bioavailability and efficacy ofivermectin following oral and pour-on administrations in horses

Cengiz Gokbulut a,*, Veli Y. Cirak b, Bayram Senlik b, Dilek Aksit a, Murat Durmaz c,Quintin A. McKellar d

a Department of Pharmacology and Toxicology, Faculty of Veterinary Medicine, Adnan Menderes University, Isikli Koyu, 09100 Aydin, Turkeyb Department of Parasitology, Faculty of Veterinary Medicine, Uludag University, Bursa, Turkeyc General Directory of Agricultural Farms (TIGEM), Karacabey Agricultural Farm, Bursa, Turkeyd The Royal Veterinary College, Hawkshead Lane, North Mymms, Hatfield, Hertfordshire AL9 7TA, UK

Veterinary Parasitology 170 (2010) 120–126

A R T I C L E I N F O

Article history:

Received 4 November 2009

Received in revised form 18 January 2010

Accepted 26 January 2010

Keywords:

Horse

Anthelmintic

Ivermectin

Pharmacokinetics

Efficacy

Pour-on

Per os

Haircoat

A B S T R A C T

Pour-on formulations of endectocides decrease the risk of injury for both user and animal,

and are particularly convenient for animal owners who can apply the product. This study

was designed to investigate the plasma disposition and efficacy of ivermectin (IVM)

following pour-on, per os and intravenous administrations. Eighteen female horses

weighing 510–610 kg were used in this study. The animals were allocated into three

groups (per os, pour-on and intravenous groups). The equine paste, bovine pour-on and

bovine injectable formulations of IVM were administered orally, topically and

intravenously at the dose rates of 0.2, 0.5 and 0.2 mg/kg bodyweight, respectively.

Heparinized blood samples and hair samples were collected at various times between 1 h

and 40 days. The samples were analysed by high performance liquid chromatography with

fluorescence detector. Faecal strongyle egg counts (EPG) were performed by a modified

McMaster’s technique before and at weekly intervals during 10 weeks after treatment. The

results indicated that the plasma concentration and systemic availability of IVM was lower

but the plasma persistence was prolonged after pour-on administration compared with

per os route. IVM (paste) reduced the EPG by >95% for 10 weeks, whereas the reduction in

pour-on group varied from 82 to 97%. EPG reduction in pour-on group was lower than that

of per os group. Degradation on the application site, cutaneous biotransformation, binding

of IVM to the haircoat and/or sebum are probably responsible for the relatively lower

bioavailability of IVM in horses after pour-on administration. In conclusion, the poor

plasma availability observed after pour-on administration could result in subtherapeutic

plasma concentrations, which may promote the development of drug resistance in

parasites.

� 2010 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Veterinary Parasitology

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1. Introduction

Ivermectin (IVM) was the first macrocyclic lactoneanthelmintic, introduced as a veterinary antiparasitic

* Corresponding author. Tel.: +90 256 247 03 40;

fax: +90 256 247 07 00.

E-mail address: cengizgokbulut@yahoo.com (C. Gokbulut).

0304-4017/$ – see front matter � 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.vetpar.2010.01.041

agent in France in 1981. The pharmacokinetic behaviourof IVM has been investigated more extensively than that ofthe other members of the endectocides and IVM is so farthe most widely used endectocide across animal species.IVM is currently marketed as injectable, pour-on (cattle)and oral (sheep, goats, horses) formulations to controlendo- and ectoparasites in livestock worldwide.

Ivermectin was first marketed for horses as a micellarformulation containing 20 mg of IVM per ml of sterile

C. Gokbulut et al. / Veterinary Parasitology 170 (2010) 120–126 121

aqueous solution (2%, w/v) for intramuscular injection.After parenteral administration, rare adverse reactionssuch as, Clostridia spp. infections and anaphylaxis wereobserved and these undesirable effects were responsiblefor the withdrawal of the parenteral preparation of IVM foruse in the horse in 1984 (Randi, 1984; Campbell et al.,1989). An oral paste formulation of IVM (1.87%) in titaniumdioxide and propylene glycol is available in graduateddelivery syringes of which each part is designed toadminister sufficient IVM (200 mg/kg) for 100 kg of bodyweight. A liquid formulation of IVM is also marketed foradministration by nasogastric intubation in some coun-tries.

Pour-on formulations of anthelmintics decrease the riskof injury for both user and animal and are particularlyconvenient for farmers who can apply the product easily(Hennessy, 1997). On the other hand, it has been reportedthat endectocides occasionally cause local irritation,swelling and/or pain at the injection site in differentanimal species following subcutaneous administration(Geurden et al., 2003; Mavrogianni et al., 2004; Gokbulutet al., 2008). For these reasons, pour-on administration haswidely replaced injectable routes in cattle farming practiceand has been routinely used to treat many animalsworldwide. Thus, this study was designed to determinethe potential of pour-on administration as an alternativetreatment route in horses. The aim of the study was toinvestigate the plasma disposition, availability and efficacyof IVM in horses after pour-on and per os administrations.Moreover, the hair depletion of IVM at the application andfar from the application sites was also examined followingpour-on administration.

2. Materials and methods

2.1. Experimental animals

Eighteen female horses weighing 510–610 kg wereused in this study. The animals were allocated into threegroups (per os, pour-on and intravenous groups) of six suchthat the pre-treatment mean egg count and mean weightof animals in each group was similar. They were housedand fed with wheat straw and barley. Water was suppliedad libitum. This study was approved by Animal EthicCommittee of Uludag University.

2.2. Treatments and sampling

The paste formulation of IVM for horses (Ivomec1

paste, 1.87% w/v, Merial, Germany) was administeredorally as a single bolus dose on the back of the tongue ofeach in the per os group at 0.2 mg/kg bodyweight (BW). Theextra label administration of pour-on and injectableformulations of IVM licensed for cattle was used in horsesin the present study. The pour-on formulation of IVM(Ivomec1 pour-on, 0.5% w/v, Merial, Germany) wasadministered topically at the dose rate of 0.5 mg/kg BWalong the midline of the back of horses. The injectableformulation of IVM (Ivomec1 injectable, 0.1% w/v, Merial,Germany) was administered intravenously at a dose rate of0.2 mg/kg BW. Heparinized blood samples were collected

by jugular venipuncture prior to drug administration thenat 1, 2, 4, 8, 12, 16, 24, 32, 48, 72, 96 and 120 h and 6, 8, 10,12,15, 20, 25, 30, 35 and 40 days.

Hair samples (>0.01 g) were also collected from bothapplication site and far from the application site (from thechest of each animal) with tweezers throughout the bloodsampling period in order to determine haircoat depletionof IVM. To prevent crosscontamination, gloves were usedand the tweezers were washed with methanol after eachhair sampling. Blood samples were centrifuged at 2000� g

for 30 min and plasma was transferred to plastic tubes. Allthe plasma and hair samples were stored at �20 8C untilthe estimation of drug concentration.

2.3. Analytical procedure

A stock solution (100 mg/ml) of pure standard of IVM(Sigma, St. Louis, MO, USA) was prepared using acetonitrile(Sigma, St. Louis, MO, USA) as the solvent. This was dilutedto give 5, 10, 100, 200 and 500 ng/ml and 0.5, 1, 5, 10 and50 mg/ml standard solutions for plasma and hair samples,respectively for calibration as standard curves and to addto drug-free plasma and hair samples to determine therecovery.

The plasma concentrations of IVM were analysed byhigh performance liquid chromatography (HPLC) withfluorescence detection following solid phase extraction(SPE) procedure according to a previously describedmethod Demontigny et al. (1990) and Alvinerie et al.(1995) and the hair samples were analysed by HPLC with aliquid–liquid phase extraction procedure adapted fromthat described by Scott and McKellar (1992).

Briefly, drug-free plasma samples (0.5 ml) were spikedwith IVM standard to reach the following final concentra-tions: 0.25, 0.5, 1, 10, 25 and 50 ng/ml. The plasma samples(spiked and experimental) were combined with 50 ml ofinternal standard (moxidectin, 250 ng/ml) and then mixedwith 0.5-ml acetonitrile. After mixing for 5 min, thesolvent–sample mixture was centrifuged at 10 000� g

for 10 min. The supernatant was transferred to a C18 SPEcartridge (500 mg/6 ml, AccuBOND, Agilent, Waldron,Germany) previously conditioned with 2 ml methanoland 2 ml deionised water. The cartridge was washed with2 ml of water/methanol (3:1) and dried under vacuum for1 h. The analytes were eluted with 3 ml of methanol andconcentrated to dryness at 45 8C in a sample concentrator(Maxi-dry plus, Heto Lab. Equipment, Denmark). Thereconstitution was done using 100 ml of a solution of N-methylimidazole in acetonitrile (1:1). Derivatization wasinitiated by adding 150 ml trifluoroacetic anhydridesolution in acetonitrile (1:2). After completion of thereaction, an aliquot (50 ml) of this solution was injecteddirectly into the chromatograph.

The mobile phase consisted of acetonitrile andmethanol (66:34, v/v) and was delivered (1100 SeriesQuatPump, Agilent, Waldron, Germany) at a flow rate of1.3 ml/min. A nucleosil C18 analytical column (Luna,3 mm, 150 mm� 4.6 mm, Phenomenex, Macclesfield,Cheshire, UK) with nucleosil C18 guard column (Phenom-enex, Macclesfield, Cheshire, UK) was used for theanalysis of the molecules. Fluorescence detection (1100

C. Gokbulut et al. / Veterinary Parasitology 170 (2010) 120–126122

Series, Agilent, Waldron, Germany) was at an excitationwavelength of 365 nm and an emission wavelength of475 nm.

The analytical methods used for IVM in plasma and hairsamples were validated prior to the start of the studies. Theanalyte was identified with the retention times of purereference standard. Recoveries of the molecule understudy were measured by the comparison of the peak areasfrom spiked plasma and hair samples with the areasresulting from direct injections of standards prepared inacetonitrile. The inter- and intra-assay precision of theextraction and chromatography procedures were evalu-ated by processing replicate aliquots of drug-free horseplasma and hair samples containing known amounts of thedrugs on different days.

Calibration graph for IVM was prepared (linear range0.1–50 ng/ml). The slope of the lines between peak areasand drug concentration was determined by least squareslinear regression and correlation coefficient (r) andcoefficient of variations (CV) calculated. Linearity wasestablished to determine the IVM concentration/detectorresponse relationship. The detection limit of the IVM wasestablished with HPLC analysis of blank plasma fortifiedwith the standard, measuring the baseline noise at theretention time of the peak. The mean baseline noise at thepeak retention time plus three standard deviations wasdefined as the detection limit (LOD). The mean baselinenoise plus six standard deviations was defined as the limitof quantification (LOQ).

2.4. Parasitological analysis

To determine the efficacy of IVM against strongyles ineach treatment group, faecal samples were collected atweekly intervals during 10 weeks after treatment.Strongyle egg counts per gram of faeces (EPG) weredetermined by a modified McMaster’s technique with asensitivity of 50 EPG (MAFF, 1986). Pooled group faecalsamples were incubated at 27 8C for 7 days for larvalidentification (MAFF, 1986). Arithmetic group mean EPGswere calculated before and after the treatment for eachsampling date. The percentage faecal egg count reduction(FECR) was calculated according to the formula:

FECRð%Þ ¼mean EPG ðbefore treatmentÞ �mean EPG

ðafter treatmentÞmean EPG ðbefore treatmentÞ � 100

2.5. Pharmacokinetic and statistical analysis of data

The plasma concentration vs. time curves obtained aftereach treatment in individual animals were fitted with theWinNonlin software program (Version 5.2, PharsightCorporation, Mountain View, CA, USA). Pharmacokineticparameters for each animal were analysed using non-compartmental model analysis for per os, pour-on andintravenous administration. The maximum plasma con-centration (Cmax) and time to reach maximum concentra-tion (tmax) were obtained from the plotted concentration–time curve of each drug in each animal. The trapezoidal

rule was used to calculate the area under the plasmaconcentration–time curve (AUC):

AUC0!1 ¼Xn

i¼1

Ci þ Ci�1

2� ðti � ti�1Þ þ

Cn

lz

where C represents the plasma concentration, i� 1 and i

are adjacent data point times. The area under the firstmovement curve (AUMC) was calculated using theequation:

AUMC0!1 ¼Xn

i¼1

Citi þ Ci�1ti�1

2� ðti � ti�1Þ þ

Cn

l2z

þ tnCn

lz

!

Thus, the mean residence time (MRT) was calculated as:

MRT0!1 ¼AUMC0!1AUC0!1

Terminal half-life (t1/2lz) was calculated as:

t1=2lz ¼�lnð2Þ

lz

where lz represent the first order rate constant associatedwith the terminal (log linear) portion of the curve. Themean absorption time (MAT) was calculated by thefollowing equations:

MAT ¼MRT per os or pour-on

MRTIV

The fraction of dose absorbed (i.e., F) was calculated by useof mean AUCs calculated for each route of administrationby use of the following equation:

F ¼ AUC per os or pour-on

AUCIV

� �� 100

The pharmacokinetic parameters are reported asmean� SD. The parameters were statistically compared byone-way analysis of variance (ANOVA). Data were analysedby use of the Anderson-Darling test to determine normality.All statistical analyses were performed by using MINITAB forWindows (release 12.1, Minitab Inc., State College, PA, USA).Mean values were considered significantly different atP< 0.05.

3. Results

Clinically no adverse reactions were observed in any ofthe horses treated with different IVM formulations. Theanalytical procedures and HPLC analysis of IVM werevalidated. The linear regression lines for IVM in the rangebetween 0.25 and 100 ng/ml showed correlation coeffi-cients of 0.9987. The mean recovery of IVM from plasmawas 83.57� 4.12%. The detection limit of the analyticaltechnique was 0.05 ng/ml; the quantification limit was0.18 ng/ml. The inter- and intra-assay precisions of theanalytical procedure obtained after HPLC analysis of spikedstandards of IVM (0.25–100 ng/ml) showed a CV of 2.32% and5.14%, respectively.

The horses in the pour-on group were observed for thefirst day after drug administration, no licking behaviourwas observed. Unlike cattle, self-licking behaviour is notusual in horses.

Table 1

Mean (�SD) pharmacokinetic parameters of ivermectin (IVM) following per os (0.2 mg/kg), pour-on (0.5 mg/kg) and intravenous (0.2 mg/kg) administration to

horses (n = 6).

Parameters Per os Pour-on Intravenous

t1/2lz (day) 6.53� 0.92 7.18� 0.60§ 5.78� 0.75

tmax (day) 0.17� 0.09¥ 4.33� 1.21 –

Cmax (ng/ml) 61.28� 10.73¥ 4.29� 1.54 –

C0 (ng/ml) – – 236.69� 61.64

AUC (ng day/ml) 164.96� 30.07¥* 59.93� 24.77§ 342.77� 93.28

Cl (L/day/kg) – – 0.63� 0.21

AUMC (ng d2/ml) 1241.51� 407.58¥* 726.83� 351.20§ 2197.13� 942.94

MRT (day) 7.34� 1.30¥ 11.80� 1.24§ 6.13� 1.35

MAT (day) 1.21� 0.57¥ 5.67� 2.17 –

Vdss (L/kg) – – 3.65� 0.59

Ftot (%) 48.13 6.99 –

tmax: time to reach peak plasma concentration, Cmax: peak plasma concentration, AUC: area under the (zero moment) curve from time 0 to the last

detectable concentration, t1/2lz: terminal half-life, AUMC: area under the moment curve from time 0 to the last detectable concentration, MRT: mean

residence time; MAT, mean absorption time; Cl, clearance of drug; Vdss, volume of distribution at steady-state; Ftot, bioavailability.¥The kinetic parameters of IVM in the per os group are significantly different (P< 0.001) from the pour-on group.*The kinetic parameters of IVM in the per os group are significantly different (P< 0.01) from the intravenous group.§The kinetic parameters of IVM in the pour-on group are significantly different (P< 0.01) from the intravenous group.

C. Gokbulut et al. / Veterinary Parasitology 170 (2010) 120–126 123

Pharmacokinetic parameters of IVM following per os

(0.2 mg/kg), pour-on (0.5 mg/kg) and intravenous (0.2 mg/kg) administration are given in Table 1 with the semi-logarithmic plot of the mean plasma concentration vs. timecurves shown in Fig. 1. In addition, haircoat depletion ofIVM from both application site and far from the applicationsite is given in Fig. 2.

Table 2 shows the mean egg counts and percentage eggcount reductions at the sampling times. All horses shedstrongyle eggs on the day of treatment. The mean strongyleegg counts ranged from 958 to 1042 and were notsignificantly different among the treatment groups. FECRwas 100% at all but 1 week (week 10: 99%) in the IVM(injectable) group. IVM (paste) reduced the strongyle eggcounts by 100% for up to 7 weeks and it remained at a highlevel (95–99%) until the end of the sampling time. Incontrast, IVM (pour-on) application resulted in a meanFECR of 82–97% throughout the entire study period. Larvae

Fig. 1. Semi-logarithmic plot of the mean plasma concentration vs. time

curves of ivermectin (IVM) following per os (0.2 mg/kg), pour-on (0.5 mg/

kg) and intravenous (0.2 mg/kg) administration to horses (n = 6).

of small strongyles (cyathostomes) were isolated fromfaecal cultures exclusively.

4. Discussion

The aqueous solubility of an active molecule mayaffect its bioavailability, which depends on the rate andextent of absorption of the drug from the site ofapplication into the bloodstream (Baggot and McKellar,1994). Although the antiparasitic spectrum and efficacyof the different endectocide molecules are similar,different physicochemical properties of endectocidesmay account for differences in kinetic behaviour,potency and persistence of their antiparasitic activity(Lifschitz et al., 2004) and in compatibility withexcipients for formulation. The vehicle in the pharma-ceutical formulations of endectocide may play animportant role in their absorption from the site of

Fig. 2. Semi-logarithmic plot of the mean hair concentrations at the

application site and far away from the application site of ivermectin (IVM)

following pour-on (0.5 mg/kg) administration to horses (n = 6).

Table 2

Range, median and mean strongyle egg counts (EPG� SD), and percentage faecal egg count reduction (FECR, %) in groups of six animals each treated with

ivermectin (IVM) paste (0.2 mg/kg BW, oral), IVM injectable (0.2 mg/kg BW, intravenous) or IVM pour-on (0.5 mg/kg BW, pour-on).

Week

0 1 2 3 4 5 6 7 8 9 10

IVM (paste)

Range (EPG) 50–3000 0 0 0 0 0 0 0 0–50 0–50 0–300

Median 825 0 0 0 0 0 0 0 0 0 0

EPG (�SD) 1042� 1073 0� 0 0� 0 0� 0 0� 0 0� 0 0� 0 0� 0 8� 20 8� 20 50� 122

FECR (%) 100 100 100 100 100 100 100 99 99 95

IVM (intravenous)

Range (EPG) 50–2900 0 0 0 0 0 0 0 0 0 0–50

Median 775 0 0 0 0 0 0 0 0 0 0

EPG (�SD) 1033� 1015 0� 0 0� 0 0� 0 0� 0 0� 0 0� 0 0� 0 0� 0 0� 0 8� 20

FECR (%) 100 100 100 100 100 100 100 100 100 99

IVM (pour-on)

Range (EPG) 50–2750 0–50 0–100 0–150 0–150 50–150 50–150 50–200 50–150 50–100 50–650

Median 725 50 50 50 50 75 50 50 50 50 50

EPG (�SD) 958� 1008 33� 26 50� 32 58� 49 58� 49 83� 41 67� 41 75� 61 67� 41 58� 20 175� 240

FECR (%) 97 95 94 94 91 93 92 93 94 82

C. Gokbulut et al. / Veterinary Parasitology 170 (2010) 120–126124

administration and bioavailability (Lo et al., 1985;Lanusse et al., 1997; Lifschitz et al., 2004).

The pharmacokinetic behaviour of IVM has beeninvestigated more extensively than that of the othermembers of the endectocides as IVM is so far the mostwidely used endectocide across animal species. Thepharmacokinetic behaviours of endectocides are signifi-cantly affected by route of administration, formulation ofthe drug, and interspecies and inter-individual variation(McKellar and Benchaoui, 1996). The pharmacokineticresults of the present study obtained after per os

administration differ substantially from those reportedpreviously in horses (Gokbulut et al., 2001) at the samedose rate. In this study, larger AUC (164.96 ng days/ml)and longer MRT (7.34 days) of IVM reflect longer drugpersistence and greater absorption from the gastrointest-inal tract compared to the previous study (AUC:46.1 ng days/ml, MRT: 2.3 days—Gokbulut et al., 2001).Differences between the studies may be associated withthe different diet of horses. Since in the present study, thehorses were kept indoors and fed wheat straw and barleywhereas in the previous study, the horses were yarded forthe period during and immediately (4 h) after drugadministration and were then returned to a grass paddock(Gokbulut et al., 2001). The previous studies indicated thathigh level of feed intake or grass can decrease IVMabsorption and bioavailability after per os administration(Taylor et al., 1992; Gokbulut et al., 2007). Moreover thevalues of Cmax, AUC and MRT of the present study aresubstantially higher, larger and longer, respectivelycompared to those previously reported for IVM in horsesafter per os administration (Perez et al., 1999, 2003). Thesedifferences may in part be due to methodology althoughthey would appear to be too large to be wholly attributedto only this. The most likely factor affecting thepharmacokinetics of ivermectin in the present studywas the breed and size of the animals used. Perez et al.(1999, 2003) used Chilean criollo horses weighing 380–

496 kg whereas a mixed group of thoroughbreds weighingbetween 510 and 610 kg were administered IVM in thepresent study.

Although IVM was administered at a higher dose via thetopical route (0.5 mg/kg) compared with oral and intra-venous administrations (0.2 mg/kg), the terminal half-lifewas significantly longer after the pour-on route comparedwith intravenous administration. In addition, MRT andMAT of IVM were significantly longer for pour-oncompared to oral administration. These results indicatethat ivermectin absorption from the skin was the rate-limiting kinetic process for pour-on administration.Similarly, it has been shown that absorption was therate-limiting process in the kinetics for pour-on formula-tions and the terminal half-life represents the half-life ofabsorption of IVM and DRM in cattle, this is termed theflip-flop effect in which the disposition of the drug from thebody is controlled by the absorption process (Gayrard etal., 1999).

Peak plasma concentration was about 14 times higherand AUC was about 3 times larger in the oral groupcompared with the pour-on group. The bioavailability ofIVM for pour-on administration was almost 7 times lowerthan for oral administration. Previous studies alsoindicated that lower availability was achieved in goatsafter pour-on administration of IVM and eprinomectin(Gokbulut et al., 2009; Alvinerie et al., 1995; Scott andMcKellar, 1992). It is likely that IVM binds physically tothe haircoat at the application site further preventingabsorption or that IVM was trapped in the skin orprecipitated at the skin surface of the application siteand very slowly released from there after pour-onadministration.

Relatively high concentrations and slow degradation ofIVM in hair samples collected from the application site wasobserved in the present study. In addition, detectableconcentrations were observed in the hair samples col-lected far away from the application site. These results

C. Gokbulut et al. / Veterinary Parasitology 170 (2010) 120–126 125

suggest that IVM was available at the skin surface for a longperiod of time after pour-on administration and thisprobably provides longer persistence of efficacy forectoparasites in horses.

The plasma concentrations and systemic availabilityof IVM in horses were lower compared with non-lickingcattle (Laffont et al., 2001) administered IVM at the samedose rate. The origin of this difference is also unclear.The difference between two species may relate tohistological and physiological variations in structure ofskin and haircoat since it was previously reported thatthe comparative number of cell layers, epidermal andcorneum thickness, and blood flow from different partsof the skin in the cow were different compared with thatof horses (Riviere and Papich, 2001). The epidermis andstratum corneum of the horse were thinner than in thecow which is counterintuitive considering the kineticresults. However, the blood flow to skin on the back ofhorses was slower than in cows and this could be relatedwith the lower plasma availability of IVM in horse. Hairfollicle density and/or length of haircoat and thesecretion of sweat and sebaceous glands are importantvariables for drug absorption from skin between cattleand horses (Monteiro-Riviere et al., 2008). Sebummay serve as a vehicle for endectocides that are solublein it.

IVM as a paste formulation is highly effective inremoving gastrointestinal nematodes including stron-gyles (Klei et al., 2001; Cirak et al., 2005; Rehbein et al.,2007). In the present study, FECR values determined afterpaste administration are in line with previous reports.However, there are no data available concerning theantiparasitic activity of pour-on IVM in horse. Eprino-mectin another macrocyclic lactone, has been tested forits effectiveness against psoroptic mange after pour-onapplication in horses (Ural et al., 2008). Eprinomectin wasfound to be effective and safe after this application route.In the present study, also no side effects were seen afterpour-on administration of IVM whereas the FECR waslower when compared to paste formulation (see Table 2).Although the FECR was above 90% for 9 weeks after pour-on application, the continued egg output in treated horseswould contribute to pasture contamination by parasiteeggs which increase the risk of re-infestation of theanimals.

5. Conclusion

The present study indicates that the plasma availabilityof IVM after pour-on administration is much lower thanoral administration. Also the reduction in EPG after pour-on application is lower than after paste formulation.Although the longer persistence of IVM on the haircoat atthe application site and far from the application site afterpour-on administration may prolong the efficacy againstectoparasites especially for non-blood-sucking parasitessuch as biting lice, the poor plasma availability could resultin subtherapeutic plasma concentrations, which mayincrease the risk of drug resistance in internal parasites(e.g. strongyles or Parascaris equorum) after pour-onadministration.

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