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Biomarkers are specific molecular, histological, or physiological characteristics of normal or pathogenic biological processes and are promising in the diagnosis of gastrointestinal nematodes (GINs). Although some biomarkers have been validated for infection by Ostertagia sp. in cattle raised in temperate regions, there is a lack of information for tropical regions. The aim of this project was to assess potential biomarkers and validate the most promising. In the first study, 36 bovines (Nelore breed) naturally infected by GINs were distributed into two groups: infected (not treated with anthelmintic) and treated (treated with fenbendazole on days 0, 7, 14, 21, 28, 42, and 56). The variables of interest were live weight, fecal egg count, hemogram, serum biochemical markers, phosphorus, gastrin, and pepsinogen. In the second step, pepsinogen was assessed in cattle of the Nelore breed distributed among three groups: infected (not treated with anthelmintic), MOX (treated with moxidectin), and IVM + BZD (treated with ivermectin + albendazole). In the first study, no difference between groups was found for weight, albumin, hematocrit (corpuscular volume [CV]), erythrocytes, or hemoglobin. Negative correlations were found between pepsinogen and both CV and albumin, and albumin was negatively correlated with the percentage of Haemonchus sp. in the fecal culture. Among the biomarkers, only pepsinogen differentiated treated and infected (beginning with the 28th day of the study). In the second study, a reduction in pepsinogen was found after anthelmintic treatment. Therefore, pepsinogen is a promising biomarker of worms in cattle naturally infected by the genera Haemonchus and Cooperia in tropical areas., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Fasciolosis is an important zoonotic disease caused by the trematode Fasciola hepatica, and it causes great losses in bovine production. The anthelmintic resistance is a major problem in the control of fasciolosis. In this study, the F. hepatica egg development and hatching test (EDHT) was used for the evaluation of the ovicidal activity of commercial drugs, commonly used for treating infected cattle, which reflects F. hepatica anthelminthic resistance in infected bovines, according to recent literature. Bile samples from F. hepatica naturally parasitized cattle were obtained from slaughterhouses in the cities of Lages and Otacílio Costa, Santa Catarina State, Brazil. The bile was washed, the eggs were recovered, quantified, and distributed in universal collectors, with a minimum of 1,000 eggs per vial. Four commercial drugs were used in this study, containing albendazole sulfoxide (ABDZ), closantel (CSTL), nitroxynil (NTXL), and triclabendazole with fenbendazole (TBZF). The drugs were diluted according to the manufacturer instructions. All drugs, and the respective control, were tested in triplicates, with the quantity of recovered eggs determining the number of drugs to be tested. The vials were incubated for 28 days at 27 °C, and the eggs were classified according to their degree of development under a stereomicroscope. In total, 121 egg samples were analyzed. Two samples were identified as resistant to TBZF. Undetermined resistance/susceptibility has been found in two isolates treated with ABDZ, one treated with NTXL and six treated with TBZF. CSTL did not present ovicidal activity and cannot be used in EDHT. This is the first time that commercial drugs were used in F. hepatica EDHT., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Helminth infections are detrimental to the overall health of dogs; therefore, this study aimed to identify antiparasitic-resistant helminths and evaluate the infection rate and risk factors for parasitism in canines. For this purpose, a parasitological evaluation of 38 randomly selected animals was performed, followed by the evaluation of the anthelminthic efficacy of three drugs: pyrantel pamoate with praziquantel (Canex Composto®), fenbendazole (Fenzol Pet®), and milbemycin oxime with praziquantel (Milbemax C®). Among the evaluated animals, 22/38 (57.89%) tested negative and 16/38 (42.71%) tested positive for Ancylostoma caninum infection. Evaluation of the efficacy of antiparasitic drugs showed that 12/16 (75%) dogs were infected with helminths that were susceptible to pyrantel pamoate with praziquantel. Among those for which pyrantel pamoate with praziquantel was not effective, 3/4 (75%) were susceptible to fenbendazole, while the remaining case resistant to both pyrantel pamoate with praziquantel and fenbendazole was sensitive to milbemycin oxime with praziquantel (100%). The odds ratio of infection in dogs inhabiting environments containing soil or grass was 6.67 times higher than that in dogs inhabiting impermeable environments. Mixed-breed dogs (SRD) were 6.54 times more likely to be infected compared to purebred dogs. A. caninum resistant to pyrantel pamoate with praziquantel (4/16, 25%) and fenbendazole (1/4, 25%) were detected. The results of this study demonstrated the importance of coproparasitological monitoring by professionals before and after treatments to assess antiparasitic drug effectiveness, ensure animal health and welfare, and minimize animal exposure to risk factors., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Pulmonary capillariasis is a parasitic disease caused by the nematode Eucoleus aerophilus which affects wild and domestic carnivores. Currently, there are no anthelmintics approved for use in the treatment of dogs infected with E. aerophilus. The use of several anthelmintics has been reported in a few case reports and field efficacy studies in cats; much less is known on the treatment of dogs infected with E. aerophilus. The paper describes a case of a 4-month-old, mixed breed intact male referred to the Veterinary Teaching Hospital (VTH) of the Department of Veterinary Medical Science of the University of Bologna for a routine vaccination and tested positive for E. aerophilus. The dog has not been responding to three different administered treatments, such as moxidectin, fenbendazole, and milbemycin oxime. Eighteen months after the first fecal examination, owner has brought in the dog for a routine visit; a coprological examination was requested and performed resulting negative for parasites. Veterinary practitioners, parasitologists, diagnostic laboratories, and dog owners need to be aware of the increased danger of possible treatment failure when attempting to control parasitic infections for which there are no approved anthelmintics with established efficacies available for use.

Anthelmintic resistance in equine cyathostomin parasites is widespread. A surveillance-based parasite control program using fecal egg counts (FECs) and fecal egg count reduction tests (FECRTs) to decrease anthelmintic use and monitor treatment efficacy is recommended. The purpose of this study was to examine shifts in equine parasite control program management practices via a short course presented by the Penn State Extension, and to highlight how data collected from these programs is useful for monitoring anthelmintic efficacy on a large scale. Horse owners were enrolled after participating in a short course and filled out questionnaire surveys about their parasite management programs pre and post study, horse information, and farm information. FECs were performed at three time points, and horses above a 300 strongyle eggs per gram cut-off were treated with pyrantel pamoate, fenbendazole, or ivermectin. Two weeks post-treatment, FECRTs were performed to determine treatment efficacy, which included 29 farms with 513 individual treatments. Prior to the study, only 30.6% of farms used FECs, but after the study, 97.3% of farms said they would use FECs in the future. Horses were given an average of 4.1 anthelmintic treatments per year before the study, and post study 89.2% of farms were able to reduce the number of anthelmintic treatments used. Fenbendazole was effective on zero farms, pyrantel pamoate on 7.4% of farms, and ivermectin on 92.9% of farms. This outreach project helped generate information about anthelmintic efficacy levels, causing a shift in practices on participating farms, and collected useful anthelmintic resistance data.

Peritoneal larval cestodiasis caused by Mesocestoides spp. is a rare infection in dogs. A 6-year-old female dog was presented for veterinary care with urinary incontinence which started 1 year earlier. After performing hematology, ultrasound, and computerized tomography, an exploratory laparotomy revealed canine peritoneal larval cestodiasis (CPLC) with the presence of Mesocestoides vogae (syn. Mesocestoides corti) tetrathyridia confirmed by morphological identification and PCR and DNA sequencing. Parasitic cysts were found around the urinary bladder and appeared to inhibit its normal function. An initial treatment with 5 mg/kg praziquantel subcutaneously every 2 weeks for four treatments failed to alleviate the clinical signs, and only treatment with fenbendazole at 100 mg/kg P.O. twice daily for 28 days was associated with the disappearance of ascites and regaining of urinary control. This is the first report of CPLC associated with urinary incontinence in dogs and the first description of this cyclophyllidean cestode in dogs in Israel.

The activity of three anthelmintics (fenbendazole-FBZ; oxibendazole-OBZ; and pyrantel pamoate-PRT) was ascertained against the ascarid Parascaris equorum in horse foals on eight farms in Central Kentucky (2009-2010) in field tests. A total of 316 foals were treated, and 168 (53.2%) were passing ascarid eggs on the day of treatment. Evaluation of drug efficacy was determined qualitatively by comparing the number of foals passing ascarid eggs in their feces before and after treatment. The main purpose was to obtain data on current activity of these compounds against ascarids. Additionally, the objective was to compare these findings with those from earlier data on the efficacy of these three compounds on nematodes in foals in this geographical area. Efficacies (average) for the foals ranged for FBZ (10 mg/kg) from 50% to 100% (80%), for OBZ (10 mg/kg) from 75% to 100% (97%), and for PRT at 1× (6.6 mg base/kg) from 0% to 71% (2%) and at 2× (13.2 mg base/kg) 0% to 0% (0%). Although the efficacy varied among the drugs, combined data for all farms indicated a significant reduction of ascarid infections for FBZ (p < 0.0001) and OBZ (p < 0.0001) but not for PRT (p = 0.0953).

An anthelmintic efficacy trial was conducted in sheep harbouring anthelmintic-resistant worms in Argentina. Seventy lambs were selected from a flock that had been grazed on pastures infected with trichostrongyles previously shown to be resistant to the main anthelmintic groups. Lambs were allocated to comparable groups of ten animals each and treated with trichlorphon (50 mg/kg body weight (b.w.) orally); naphthalophos (50 mg/kg b.w. orally); ivermectin (0.2 mg/kg b.w. subcutaneously); fenbendazole (5 mg/kg b.w. orally); levamisole (8 mg/kg b.w. subcutaneously) and closantel (10 mg/kg b.w. orally). There was also an untreated group. The dose selection was based on manufacturer's recommendations.Faecal samples were collected 0 and 10 days post treatment to estimate efficacy (faecal egg count reduction). Six animals from each group were necropsied at day 10 for enumeration/identification of worms from the abomasum, small and large intestines to determine the absolute efficacy of each agent (controlled efficacy test). Trichlorphon and naphthalophos were effective (> 99 %) against Haemonchus contortus (p < 0.05).Naphthalophos also showed efficacy against Trichostrongylus axei (99.3 %), Teladorsagia circumcincta (97.8 %), Trichostrongylus colubriformis (99.2 %), Cooperia punctata/curticei/pectinata (90.4 %), Nematodirus spathiger (89.2 %) and Oesophagostomum venulosum/columbianum (93.7 %). Fenbendazole and levamisole showed efficacy (> 95 %) against all nematodes except T. colubriformis. The efficacy of ivermectin was low against H. contortus (23 %) and Cooperia spp. (46.3 %). Closantel showed low efficacy against T. axei (64.4 %), H. contortus (80.6 %) and T. colubriformis (59.5 %).When anthelmintic resistance is widespread, trichlorphon treatment is appropriate if H. contortus is present; however, naphthalophos represents an effective therapeutic alternative for incorporation into worm control programmes.

The objective of the present study was to evaluate the efficacy and safety of the antiparasitic spot-on formulation containing imidacloprid 10%/moxidectin 1% (Advocate, Bayer) in the treatment of natural feline infection with the lungworm Aelurostrongylus abstrusus (Nematoda, Strongylida). The efficacy of Advocate administered once was tested in comparison to a control oral formulation containing fenbendazole 18.75% (Panacur Intervet) administered over three consecutive days based on larvae per gramme of faeces (LPG), measured on days 28 +/- 2 following treatment and compared to counts on days -6 to -2. In total 24 cats treated either with Advocate (n = 12) or with Panacur (n = 12) were included. Mean LPG postbaseline (days 28 +/- 2) were low in both treatment groups, i.e., 0 LPG for Advocate and 1.3 LPG for Panacur. Reduction of post-baseline larval counts showed Advocate (100% reduction) to be superior in efficacy compared to the control product (99.29% reduction). No treated animals showed adverse events. This trial demonstrated that both Advocate spot-on formulation and Panacur oral paste are safe and effective in the treatment of aelurostrongylosis in cats. Future practical perspectives in feline medicine and the major advantages of the spot-on product compared to the oral paste are discussed.

The objective of the present study was to evaluate the efficacy and safety of the antiparasitic spot-on formulation containing emodepside 2.1%/praziquantel 8.6% (Profender, Bayer) in the treatment of natural feline infection with the lungworm Aelurostrongylus abstrusus (Nematoda, Strongylida). Efficacy of Profender given once at the licensed dose was tested in comparison to a control oral formulation containing fenbendazole 18.75% (Panacur, Intervet) given over three consecutive days at the licensed dose. Efficacy assessment was based on larvae per gramme of faeces (LPG) counts, measured on days 28 +/- 2 following treatment and compared to counts on days -6 to -2. In total 24 cats treated either with Profender (n = 12) or with Panacur (n = 12) were included in the assessment of efficacy and safety. Mean LPG post-baseline counts (days 28 +/- 2) were 1.3 LPG for both Profender and Panacur, demonstrating similar efficacy of 99.38% for Profender and 99.29% for the control product. No treated animals showed adverse events. This trial demonstrated that both Profender spot-on formulation and oral paste Panacur are safe and effective in the treatment of aelurotrongylosis in cats. Future practical perspectives in feline medicine and the major advantages of the spot-on product compared to the oral paste are discussed.

Horse foals on five farms in Central Kentucky were used in field studies in 2007 evaluating activity of paste formulations of four compounds (fenbendazole-FBZ, ivermectin-IVM, oxibendazole-OBZ, and pyrantel pamoate-PRT) against internal parasites with emphasis on ascarids (Parascaris equorum). It has been well established the last few years that there is widespread resistance of P. equorum to ivermectin. The main purpose of the present research was to obtain current data on ascaridicidal activity of FBZ, OBZ, and PRT; also, to acquire further information on ascarid resistance to ivermectin. Additionally, data were documented on drug activity on small strongyles. Detection of ascarid and strongyle eggs in feces of foals was by a qualitative method (presence or absence) or a quantitative method (eggs per gram of feces). Strongyle eggs all were assumed to be from small strongyles. This is based on fecal cultures from horses on one farm and historic records from horses in this area on excellent deworming programs. A girth tape was used to estimate the body weight of each foal so that the appropriate dose rate of each drug could be given. Many of the foals were used in more than one cycle of treatments. Efficacy of the drugs, administered intraorally, was determined by calculating the average percentage reduction (% red.) of the number of foals passing eggs after vs. before treatment: (1) FBZ at 10 mg/kg was tested on four farms; 76 foals were examined, 50 with ascarid eggs (84% red.) and 62 with strongyle eggs (0% red.); (2) IVM at 200 microg/kg was tested on three farms; 58 foals were examined, 18 with ascarid eggs (0% red.) and 48 with strongyle eggs (100% red.); (3) OBZ at 10 mg/kg was tested on three farms; 181 foals were examined, 78 with ascarid eggs (94% red.) and 79 with strongyle eggs (0% red.); (4) PRT was tested on two farms, one farm at 1x (6.6 mg base/kg); 42 were foals examined, 16 with ascarid eggs (0% red.) and 33 with strongyle eggs (12% red.) and one farm at 2x (13.2 mg base/kg); 18 foals were examined, 13 with ascarid eggs (23% red.) and 15 with strongyle eggs (27% red.).

Thirty-six tortoises (Testudo hermanni) with naturally acquired oxyurids infections were used to assess the anthelmintic efficacy of oxfendazole (Dolthene; Merial) and fenbendazole (Panacur; Hoechst Roussel Vet). Animals were randomly assigned to three groups (A, B, and C) based on sex and weight. Animals in group A (seven males and six females) were orally treated with oxfendazole at dose rate of 66 mg/kg, group B animals (nine males and eight females) were orally treated with fenbendazole at dose rate of 100 mg/kg, and group C animals (three males and three females) were not treated and served as controls. All animals were individually stabled in plexiglas boxes under controlled conditions of temperature, humidity, and light beginning 7 days pretreatment and continuing for the duration of the trial. Individual tortoises feces were examined daily by the McMaster technique and drugs efficacy was assessed by the fecal eggs count reduction (FECR) test. Both drugs showed 100% of FECR. However, oxfendazole reached this level 12 days after treatment, whereas 31 days after treatment were necessary to obtain the same stable result with fenbendazole. The two drugs were well tolerated by all the animals and no adverse reactions were observed after treatment.

Using routine coprological methods, 1161 faecal samples from animal shelters located in Madrid (Spain) were analysed, showing a 28% prevalence for different intestinal parasites: Giardia duodenalis (7%), Cystoisopora spp. (3.8%), Toxocara canis (7.8%), Toxascaris leonina (6.3%), Ancylostomidae (4%), Trichuris vulpis (3.3%), Taenidae (2.9%) and Dipylidium caninum (0.9%). The therapeutic efficacies of mebendazole at a dose of 22 mg/kg once daily for 3 days, fenbendazole at a dose of 50 mg/kg once daily for 3 days and a drug combination of febantel-pyrantel-praziquantel at a dose of 15-5-5 mg/kg once were valuated and compared by collecting faecal samples on days 9 and 16 post-treatment from naturally infected dogs in field-trial conditions. From the infected dogs (321 dogs), 150 animals were selected for the study. Distribution randomly divided the animals into three study groups of ten dogs per parasite and per treatment group: group A, mebendazole; group B, fenbendazole and group C, febantel-pyrantel-praziquantel. The therapeutic efficacy against ascarids and ancylostomids (days 9-16) was very high (75-100%) for the three groups: for T. canis, 100% in group A, 80-100% in group B, 97-100% in group C; for T. leonina, 98-100% in group A, 100% in group B, 92-94% in group C and for ancylostomids, 100% in group A, 99-100% in group B, 90-100% in group C. On the other hand, the highest efficacy against Taenidae infections was in group B (90-100%), followed by groups C (73-91%) and A (70-90%).

The efficacy of five daily fenbendazole (FBZ) treatments was tested against benzimidazole-resistant cyathostomins in naturally infected horses (n=13). Horses were treated with pyrantel embonate (PYR) to remove adult strongyles followed, 7 days later, by a 5-day course of FBZ. The PYR treatment produced an average faecal egg count reduction of 98%. All samples were negative by faecal egg count 7 days after the start of the FBZ treatment. Positive egg counts were observed from 28 days after the start of FBZ treatment and all horses displayed positive faecal egg counts by 77 days after treatment. Strongyle eggs were harvested from the faeces of the horses prior to treatment and then weekly from 42 to 70 days post-treatment. DNA was obtained from eggs in groups of ten. A PCR-ELISA, based on species-specific differences in intergenic DNA sequences, was used to identify the presence of six cyathostomin species. In pre-treatment samples, Cyathostomum catinatum was detected in nine out of the 13 horses and Cylicostephanus longibursatus, Cylicostephanus goldi and Cylicocyclus nassatus, were found in samples from eight animals. Cylicocyclus ashworthi and Cylicocyclus insigne were not detected pre-treatment. After anthelmintic treatment, C. catinatum and C. longibursatus were most frequently detected, followed by C. nassatus, C. goldi and C. ashworthi. C. insigne was detected at only one time point in a sample from a single horse.

Fenbendazole (FBZ) suspension was administered intraorally at the dose rate of 7.0-10.3 mg/kg once a day for 5 consecutive days to 58 thoroughbred yearlings on two farms in central Kentucky in April, 2002. The average dose rates of drug given to groups of colts and fillies on each farm were 7.8-8.5 mg/kg. Only 3 of the yearlings had negative counts of strongyle eggs per gram of feces (EPGs) after treatment which was at 8.4, 8.7, or 9.4 mg/kg; the pretreatment EPG counts were low (10-30). Reduction of EPG values at the highest dose rates was 0% (at 9.5 mg/kg) and 78% (at 10.3 mg/kg). This study was repeated in April, 2003 in 38 thoroughbred yearlings on one of the two same farms used in the 2002 research, but all horses were treated at the same dose rate (10 mg/kg) of FBZ paste once daily for 5 consecutive days. Only 1 of these yearlings had a negative EPG count after treatment, but this value was also negative before treatment. Reductions of EPG counts after treatment ranged from 0% to 85% (mean =22%) for the colts and from 0 to 63% (mean =14%) for the fillies. Examination of cultures of fecal samples from these yearlings revealed that only small strongyle larvae were present. There was obvious FBZ-resistance of the small strongyles in yearlings on both farms at the dose rates used.

The nematode Teladorsagia circumcincta is a polymorphic species, morphs T. c. trifurcata and T. c. circumcincta being the most frequent. Among other traits, T. c. trifurcata has lower infectivity than T. c. circumcincta. Anthelmintic treatments that regularly remove populations should favour the more infective T. c. circumcincta. The present investigation was intended to test the respective capabilities of the two morphs to withstand anthelmintic pressure. The nematode strain was susceptible to imidazothiazole and partly resistant to benzimidazole at the onset of experiment. A 2-year grazing season study was undertaken under four levels of anthelmintic pressure [from untreated controls to frequently treated with a poorly efficient benzimidazole (fenbendazole) or an efficient imidazothiazole (levamisole)]. The strain previously maintained in laboratory conditions regained its natural, original proportion of T. c. trifurcata during the first grazing season. The two morphs were equally resistant to fenbendazole; and the proportion of resistant nematodes did not vary between the two morphs. During the second grazing season, the treatments indirectly depleted the proportions of T. c. trifurcata, probably due to the poor infectivity of this morph.

The objectives of the present study were, first, to establish a methodology for evaluation of the permeability in vitro of hydatid cysts to different drugs and, second, to compare the permeability to albendazole sulfoxide of cysts from untreated animals, cysts from animals treated with 50 mg/kg netobimin for 5 days, and cysts from animals treated with 50 mg/kg netobimin plus 1.1 mg/kg fenbendazole for 5 days. The drug flow follows the Fick law, i.e., the uptake occurs by simple diffusion. We calculated the permeability constant of the cyst membrane by taking into account the disappearance velocity constant, the cyst area, and the incubation solution volume. The permeability value obtained for albendazole sulfoxide was 8.06+/-2.30 x 10(-6) cm s(-1) in cysts from untreated animals, 5.56+/-2.53 x l0(-6) cm s(-1) in cysts from animals treated with netobimin, and 7.05+/-3.04 x 10(-6) cm s(-1) in cysts from animals treated with netobimin +/- fenbendazole. These permeability values show significant differences (P < 0.05).

Three different benzimidazole derivatives, albendazole [methyl-5-(propylthio)-2-benzimidazolcarbamate], mebendazole (methyl-5-benzoyl-2-benzimidazolcarbamatic acid methyl ester), and fenbendazole [methyl-5-(phenylthio)-2-benzimidazolcarbamate] were tested in vivo against Glugea anomala parasitizing the connective tissue of sticklebacks (Gasterosteus aculeatus). Naturally infected sticklebacks were incubated in aerated plastic aquaria (10 1) at 22 degrees C in water containing 0, 1, 5, 10, or 50 micrograms of either albendazole, mebendazole or febendazole for 2 or 6 h. For intermittent treatment, 2 micrograms substance was administered three times for 6 h at intervals of 36 h. At the ultrastructural level, at all developmental stages of G. anomala there were no significant differences in the kind of damage caused by either albendazole, mebendazole, or febendazole. Starting with a dose of 1 microgram/ml for 2 h, each of the drugs irreversibly damaged uni- and multinucleate meronts, sporogonial plasmodia, and sporoblasts. Disorganized spores were also observed. Treatment with higher doses (10 micrograms/ml, 2 or 6 h) caused malformations of the merogonic and the sporogonic stages, a significant reduction in the number of ribosomes, and disruptions of the nuclear membranes. The first recognizable treatment effect was an enlargement of the smooth endoplasmic reticulum. In the sporogonial plasmodia, the membranes of the sporophorous vesicle envelopes were lumpy or even completely destroyed. After incubation with the highest dose (50 micrograms/ml, 6 h), microtubules were apparent within the karyoplasm of the uninucleate meronts. After interval treatment, all forms of damage were intensified, especially in the mature spores. When treatment was done three times at low doses (3 x 2 micrograms/ml, 6 h, 36-h intervals), spore infectivity was drastically lowered. Therefore, it seems likely that an intermittent regimen of medicinal baths can be successfully applied against susceptible Microsporidia in fish.

Eggs with filaments were detected in the faeces of Macaca mulatta. On the basis of morphological indications and reports from the literature, this infection was most probably caused by the genus Ogmocotyle. Egg output and the onset of diarrhoea were arrested by treatment with 20 mg fenbendazole/kg body weight for 5 days in the form of medicated feed. Follow-up studies over 9 months showed neither new egg output nor the occurrence of clinical symptoms.

The effects of five nematocidal drugs (levamisole HCl, metrifonate, fenbendazole, mebendazole, and ivermectin) on the nematode Anguillicola crassus, pathogenic in eels and recently introduced in Europe, were tested under in vivo conditions. The resulting tissue alterations were studied by means of light and electron microscopy. It was found that levamisole and metrifonate were most effective in a freshwater bath (1 mg/l) for 24 h. The 50% lethal dose of levamisole was 250 mg/l per 24 hours, whereas that of metrifonate was only 10 mg/l per 2 hours. Morphological studies of the nematodes showed that there was no drug-specific reaction. In general, the hypodermis, the cytoplasmic portions of the muscle cells, and the intestinal wall were most intensively damaged, leading to an irreversible vacuolation and finally to death.