An in vitro larval motility assay characterizes anthelmintic efficacy against Crenosoma vulpis, Angiostrongylus vasorum, and Aelurostrongylus abstrusus

OBJECTIVE

This study determined the in vitro efficacy of 6 common anthelmintics (eprinomectin, ivermectin, milbemycin oxime, moxidectin, selamectin, and fenbendazole) on motility (viability) of infectious third-stage larvae (L3) of Crenosoma vulpis, Angiostrongylus vasorum, and Aelurostrongylus abstrusus, which are important causes of canine and feline cardiopulmonary disease.

SAMPLES

First-stage larvae (L1) from C vulpis, An vasorum, and Ae abstrusus.

PROCEDURES

Naïve Limax maximus slugs were fed 1,000 to 2,000 L1 and held at 16 °C for at least 4 weeks to produce live L3. Approximately 50 to 100 L3/well were subsequently incubated in culture media alone or media containing 6 separate test anthelmintics at 4 concentrations, to bracket expected in vivo drug plasma levels in anthelmintic-treated dogs and cats. Drug effects on L3 motility (viability) were analyzed by multilevel logistic models, generating dose-response relationships. Experiments were completed 1-9/2019.

RESULTS

Drug concentration estimates corresponding to a 50% larval mortality rate identified that C vulpis was the most sensitive species to the anthelmintics tested. Ae abstrusus was most susceptible to moxidectin and selamectin, while An vasorum was insusceptible to all anthelmintics tested, except for selamectin at high drug concentrations.

CLINICAL RELEVANCE

The in vitro anthelmintic response to antiparasitic agents may guide and improve disease therapy and prevention. Considering the observed lack of efficacy against L3, monthly anthelmintic treatment for protection against An vasorum infection in dogs would primarily rely on the anthelmintic's adulticidal activity. Maximal preventive control for An vasorum would, therefore, require at least 1 treatment administered a minimum of 1 week after the end of the transmission season.

Massive hepatic trematodosis in 5 juvenile bald eagles

Hepatic trematodosis by opisthorchiid flukes has been reported sporadically in North American fish-eating raptors. Bald eagles (Haliaeetus leucocephalus) infected by these flukes often have various degrees of granulomatous cholangitis, pericholangitis, necrosis of adjacent hepatocytes, and subsequent hepatic fibrosis. Species identification has been complicated by the inability to dissect intact specimens from liver tissue. Between 2007 and 2018, 5 juvenile bald eagles with massive hepatic trematodosis were identified at autopsy. Histologically, flukes were non-spinous. Parasitologic identification revealed ventral suckers (80-93 µm diameter), and uteri containing golden, operculated eggs (~25.0 × 12.0 µm). An unfixed frozen liver sample of one eagle was analyzed by PCR and DNA sequencing targeting the large subunit rRNA, ITS region, and cox1 genes of the parasite. The fluke DNA sequences shared 99.6%, 98.4%, and 87.0% similarity, respectively, with Erschoviorchis anuiensis, a newly described opisthorchiid species infecting the liver and pancreas of fish-eating birds in Europe and Asia. Infection by E. anuiensis is highly pathogenic in several piscivorous bird species. The clinical significance of trematodosis in our 5 cases is uncertain because all birds had comorbidities.

Echinococcus multilocularis infection in a red fox (Vulpes vulpes) on Prince Edward Island, Canada

OBJECTIVE

Molecular identification of small cestodes, morphologically consistent with Echinococcus multilocularis, recovered at necropsy from the gastrointestinal tract contents of a red fox, was accomplished by PCR using published species-specific n ad1 primers and methods.

ANIMAL

Red fox (Vulpes vulpes).

PROCEDURE

Small cestodes recovered from intestinal contents of a red fox trapped on Prince Edward Island in December 2020 (frozen at -20°C before being processed for parasite recovery in June 2021) were morphologically identified. Species identity confirmation and haplotyping of the cestodes were done via PCR and DNA sequencing of the n ad1, nad2, and cob genes.

RESULTS

Small cestodes morphologically consistent with E. multilocularis were detected in the gastrointestinal tract contents of a red fox trapped near Montague, PEI. The species identity was confirmed via PCR. Haplotyping revealed that they were of the European E1 haplotype.

CONCLUSION

In Canada, E. multilocularis has been reported as far east as Québec, with most reports being in central and western provinces and territories. This is the first report of E. multilocularis infection in a canid host east of Ontario, Canada and illustrates the need for regular wildlife disease surveillance to enhance our understanding of emerging pathogens of veterinary and medical importance.

CLINICAL RELEVANCE

Echinococcus multilocularis is a highly pathogenic zoonotic cestode from the family Taeniidae that can cause alveolar echinococcosis (AE) when rodents, dogs, horses, pigs, non-human primates, or humans ingest its eggs. Alveolar echinococcosis is challenging to treat, and survival rates for untreated individuals are low.

First report of Angiostrongylus vasorum (French heartworm) in red foxes (Vulpes vulpes) on Prince Edward Island

OBJECTIVE

To identify first-stage nematode larvae (L1) recovered from a red fox scat sample and adult female worms recovered from 2 red fox lungs at necropsy, using published molecular methods to confirm a morphological diagnosis of Angiostrongylus vasorum (French heartworm).

ANIMAL

Red fox (Vulpes vulpes).

PROCEDURE

Nematode larvae recovered from a Baermann examination survey of wild canid scats (n = 101) conducted from January 2017 to August 2020, were identified by size and morphology and subjected to PCR and DNA sequencing of the small subunit (SSU) rRNA gene, the large subunit (LSU) rRNA gene, or the second internal transcribed spacer (ITS2). In addition, these techniques were applied to adult female worms recovered from the heart/lungs of 2 red foxes (obtained from PEI trappers and stored frozen at -20°C since December of 2018 and 2020).

RESULTS

Size and morphology of L1 recovered by Baermann examination from a wild canid scat sample (presumed to be red fox) collected near Montague, PEI and adult female worms recovered at necropsy from 2 red fox carcasses were identified as A. vasorum. Molecular analysis confirmed the larvae and adult worms were A. vasorum.

CONCLUSION

These findings indicated that A. vasorum has become endemic in the red fox population on PEI.

CLINICAL RELEVANCE

Angiostrongylus vasorum infection is potentially fatal in dogs. Veterinarians and regional diagnostic laboratories in the Maritime provinces should consider the possibility of A. vasorum infection in dogs with clinical signs of cardiopulmonary and/or central nervous system disease or bleeding disorders.

Endoparasites in dogs and cats diagnosed at the Veterinary Teaching Hospital (VTH) of the University of Prince Edward Island between 2000 and 2017. A large-scale retrospective study

Although many studies on the frequency of endoparasites in dogs and cats in Canada have been reported, seasonal and/or annual patterns are often not evaluated. The frequency and risk factors of endoparasite infections from fecal samples of cats and dogs submitted to the Veterinary Teaching Hospital of the Atlantic Veterinary College, University of Prince Edward Island-Canada were determined, using univariable and multivariable logistic regression analyses. Investigated predictors of endoparasitism available in the 2000-2017 database included sex, age, geographic origin and seasonality. A total of 15,016 dogs and 2,391 cats were evaluated for endoparasite status using specific diagnostic tests: direct smear, Baermann, and/or 33 % zinc sulfate solution in a standardized centrifugal flotation method. Overall, twelve and eight parasite genera were detected in dogs and cats, respectively. The overall proportional infection was 14.6 %, and the cat population showed a higher frequency of positivity to parasites compared to the dog population (P < 0.001). The most frequent genera recovered in the whole population (dogs and cats), were Giardia duodenalis (5.2 %), Cystoisospora spp. (3.3 %) and Toxocara spp. (3.2 %). Endoparasitism levels were diagnosed more in feces submitted from young, female intact dogs from PEI compared to the baselines of mature, sterilized male dogs from other provinces, respectively, and diagnoses occurred more often in autumn months than in winter months. There was no significant diagnostic trend across the years for the individual parasites models. The frequency of detected potentially zoonotic parasites in this study highlights the veterinary public health and One Health context of parasitic infections in pets. Although the presented results are not from a random sample and therefore frequency results should be interpreted with caution, the model relationship results may still be relevant. In addition, results are of value to estimate parasite impact and to assist researchers, veterinarians and pet-owners with suitable information to control parasites.

Pulmonary trematodosis (Pharyngostomoides sp.) in a juvenile raccoon (Procyon lotor)

A lethargic, wild, juvenile raccoon died and was subsequently presented for necropsy. The lungs had widely disseminated, finely granular, yellow-tan foci with moderate pulmonary, as well as, thoracic lymphadenopathy. Histopathology revealed numerous cross-sections of larval trematodes morphologically consistent with diplostomid mesocercariae and metacercariae as well as moderate alveolointerstitial pneumonia with a mixed population of eosinophils, macrophages, lymphocytes, and plasma cells. The presence of larval trematodes within lymphatics in multiple organs indicates a route of fluke migration. Live specimens of mesocercariae and metacercariae recovered from refrigerated lung tissue after 7 days were identified as Pharyngostomoides sp., an intestinal diplostomid parasite (trematode) of raccoons and other mammals that is similar in appearance and life cycle to Alaria spp.

Serologic detection of Angiostrongylus vasorum infection in dogs

Angiostrongylus vasorum, French Heartworm, is a metastrongylid nematode infecting the pulmonary arteries and right heart of wild and domestic canids in various regions of the world. Infection in dogs can result in fatal cardiopulmonary disease. A single endemic focus of A. vasorum in North America occurs in the southeastern portion of Newfoundland, Canada. Dogs are currently diagnosed by detection of first-stage larvae shed in feces using the Baermann technique or fecal flotation. However, these procedures may lack sensitivity due to intermittent fecal larval shedding. The potential for using detection of circulating worm antigen for diagnosis was investigated by developing a sandwich-ELISA using rabbit anti-whole adult worm antiserum. This test detected circulating antigen in sera from 22/24 Baermann positive dogs naturally infected with A. vasorum. Negative results (0/52) were obtained from sera collected from Baermann negative dogs from outside of the endemic region, and from sera (0/30) from dogs from non-endemic regions that were infected with Crenosoma vulpis, the fox lung worm. Receiver operating curve analysis gave a specificity of 100% and a sensitivity of 92% for the sandwich-ELISA at an optical density cut-off of 0.19. Subsequently, 239 dogs from Newfoundland displaying clinical signs of cardiopulmonary disease, were examined using both the Baermann fecal examination and the sandwich-ELISA. Larvae were detected in 10% (24/239) of these dogs by fecal examination, whereas the sandwich-ELISA detected circulating antigen of A. vasorum in serum from 18.8% (45/239) of the dogs. This suggests that fecal diagnostics may have missed approximately half of the A. vasorum infected dogs, and that the sandwich-ELISA may be a useful tool in the diagnosis of this parasite.

A new trichosomoidid from the skin of Sebastes spp. (Pisces) from British Columbia, Canada

A new species of trichosomoidid nematode, Huffinanela canadensis n. sp., is described from the skin of rockfish (Sebastes spp.) from the Pacific Ocean off the coast of Vancouver Island (Clayoquot Sound region), British Columbia, Canada, on the basis of the morphology of the adult worms and their eggs in the host's tissue and the biological characters. The species is characterized mainly by the shape and structure of the fully developed eggs (absence of surface envelope, surface with transverse ridges), by their small size (48-63 x 24-27 microm), and by the site of infection (skin). Besides Huffmanela huffmani, this is the second Huffmanela species in which adult worms are known in addition to the eggs from the host's tissues (most Huffmanela spp. have been described only from their conspicuous eggs occurring in various tissues of fishes). Adults of H. canadensis differ morphologically from those of H. huffmani, mainly in the structure of the male caudal end and in the distinctly elevated anterior vulvar lip of the female.

Susceptibility of rainbow trout Oncorhynchus mykiss, Atlantic salmon Salmo salar and coho salmon Oncorhynchus kisutch to experimental infection with sea lice Lepeophtheirus salmonis

Physiological, immunological and biochemical parameters of blood and mucus, as well as skin histology, were compared in 3 salmonid species (rainbow trout Oncorhynchus mykiss, Atlantic salmon Salmo salar and coho salmon O. kisutch) following experimental infection with sea lice Lepeophtheirus salmonis. The 3 salmonid species were cohabited in order to standardize initial infection conditions. Lice density was significantly reduced on coho salmon within 7 to 14 d, while lice persisted in higher numbers on rainbow trout and Atlantic salmon. Lice matured more slowly on coho salmon than on the other 2 species, and maturation was slightly slower on rainbow trout than on Atlantic salmon. Head kidney macrophages from infected Atlantic salmon had diminished respiratory burst and phagocytic capacity at 14 and 21 d post-infection (dpi), while infected rainbow trout macrophages had reduced respiratory burst and phagocytic capacities at 21 dpi, compared to controls. The slower development of lice, coupled with delayed suppression of immune parameters, suggests that rainbow trout are slightly more resistant to lice than Atlantic salmon. Infected rainbow trout and Atlantic salmon showed increases in mucus lysozyme activities at 1 dpi, which decreased over the rest of the study. Mucus lysozyme activities of infected rainbow trout, however, remained higher than controls over the entire period. Coho salmon lysozyme activities did not increase in infected fish until 21 dpi. Mucus alkaline phosphatase levels were also higher in infected Atlantic salmon compared to controls at 3 and 21 dpi. Low molecular weight (LMW) proteases increased in infected rainbow trout and Atlantic salmon between 14 and 21 dpi. Histological analysis of the outer epithelium revealed mucus cell hypertrophy in rainbow trout and Atlantic salmon following infection. Plasma cortisol, glucose, electrolyte and protein concentrations and hematocrit all remained within physiological limits for each species, with no differences occurring between infected and control fish. Our results demonstrate that significant differences in mucus biochemistry and numbers of L. salmonis occur between these species.

Dermal nematodosis in commercially captured rockfish (Sebastes spp) from coastal British Columbia, Canada

Members of an Ahousaht First Nations community of Vancouver Island, Canada have developed a market for wild-caught species of indigenous rockfish (Sebastes spp.). The fish, which are caught by hook and line, are transported live to fish markets in Vancouver. Recently, market place downgrading of the fish has occurred due to a syndrome referred to as "black-mould", characterized by linear black disfigurations, and erosions, of the skin. Samples from 14 wild-caught rockfish were examined to determine the cause. On the basis of sub-gross and histological examination, the black-mould syndrome was attributed to intraepithelial deposition of eggs from a trichuroid nematode of the genus Huffmanela, coupled with an inflammatory response. The eggs observed in the tissues of the rockfish differed in size and morphology from those described for other species of Huffmanela. This is the first report of Huffmanela spp. infection in rockfish, probably due to a previously undescribed species.

Effects of sea lice (Lepeophtheirus salmonis Kröyer, 1837) infestation on macrophage functions in Atlantic salmon (Salmo salar L.)

Experiments were conducted to determine the effects of sea lice, Lepeophtheirus salmonis, on non-specific defence mechanisms in Atlantic salmon, Salmo salar, by experimentally infesting hatchery-reared 1 and 2 year old post-smolts, S1 and S2, with laboratory grown infective copepodids at moderate to high infection intensities ranging from 15-285 lice per fish. The effects of sea lice-induced stress were investigated by measuring the blood levels of cortisol and glucose as indicators of primary and secondary stress responses, and by changes in macrophage respiratory burst activity and phagocytosis as indicators of tertiary stress responses as well as non-specific defence mechanisms. Fish were sampled prior to sea lice infestation at day 0 and at days 3, 7, 14 and 21 post-infestation. Sea lice were at copepodid stage at day 3, at chalimus stages at days 7 and 14, and at pre-adult stage at day 21. Blood levels of cortisol and glucose were found to be significantly increased at day 21 in fish-infested with the highest levels. Macrophage respiratory burst and phagocytic activities were found to be significantly decreased only at day 21. These results indicate that sea lice do not suppress host defence mechanisms during the earlier stages of infestation. They do have effects on the development of chronic stress and on the host non-specific defence mechanisms soon after the lice reach the pre-adult stage.

Lungworm (Crenosoma vulpis) infection in dogs on Prince Edward Island

Crenosoma vulpis is a nematode lungworm that is highly prevalent in the red fox population of Atlantic Canada. Dogs are susceptible to infection with clinical signs consisting primarily of a chronic cough. A recent report of C. vulpis infection in 3 dogs on Prince Edward Island prompted an investigation into the importance of this parasite as a cause of chronic respiratory disease in Island dogs. A general prevalence was determined through the necropsy of dogs euthanized at the local humane society. Lungs were removed and examined for parasites using a lung flush technique. Rectal feces was collected and examined for first-stage larvae using the Baermann technique and zinc sulfate centrifugal flotation. Ten of 310 dogs (3.2%) were positive with 0-35 worms (mean = 11.0 +/- 13.4) recovered. First-stage larvae of C. vulpis were recovered in the rectal feces of the one animal in which no worms were recovered on lung flush. A second survey was conducted examining fecal samples with the Baermann technique from afebrile dogs with presenting signs of chronic cough that had no history of recent anthelmintic treatment and showed no signs of cardiac disease, based on physical examination. Fifteen of 55 dogs examined (27.3%) were definitively diagnosed as C. vulpis-positive. All of the infected dogs were treated with fenbendazole (50 mg/kg body weight, p.o. q24 h for 3-7 days). Clinical signs resolved in all of the dogs and fecal samples were negative 2-4 weeks posttreatment. It was concluded that C. vulpis infection was a significant cause of upper respiratory disease in dogs on Prince Edward Island and should be considered in all dogs with presenting signs of chronic cough.

Eosinophilic bronchitis caused by Crenosoma vulpis infection in dogs

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Hematology and clinical pathology of experimental Fascioloides magna infection in cattle and guinea pigs

The hematologic and clinico-pathologic response to Fascioloides magna infection in cattle and guinea pigs was investigated. Twelve calves (six infected and six controls) were monitored for 26 weeks after inoculation with 1000 metacercariae. All calves remained healthy and there were no significant differences in weight gains between infected and control groups. Flukes (mean = 9.2, range 1-32) were recovered from the liver and abdominal cavity of all infected calves. The only significant response observed in the complete blood counts was an eosinophilia present in the infected calves extending from Weeks 2 to 26 post-infection. There were no significant differences in serum levels of aspartate aminotransferase and only minor increases in the levels of gamma-glutamyl transferase and sorbitol dehydrogenase. A total of 48 infected and 48 control guinea pigs from three separate experiments were monitored for 16 weeks after inoculation with 20 metacercariae of Fascioloides magna. Infected guinea pigs died between 7 and 114 days after infection, and flukes (mean = 2.5, range 0-13) were recovered from the liver, abdominal cavity, lungs, thoracic cavity, skeletal muscle and subcutaneous tissue. There were no differences in weight gains between infected and control guinea pigs. Complete blood counts showed increases in white blood cells, monocyte and neutrophil counts from between the third and fourteenth weeks post-infection; however, the differences were not consistently significant. Infected guinea pigs developed a significant eosinophilia and basophilia from 2 to 16 weeks post-infection. There were no significant changes in the serum levels of alanine aminotransferase or gamma-glutamyl transferase. There was an increase in the serum levels of aspartate aminotransferase beginning at 5 weeks post-infection. The response observed in the guinea pigs was similar to that reported in sheep, suggesting the suitability of the guinea pig as a model for Fascioloides magna infection in the sheep.

Hepatic and pulmonary pathology of experimental Fascioloides magna infection in guinea pigs

The guinea pig was used to study the pathology of Fascioloides magna, an important pathogen for sheep. Although flukes migrated freely through various tissues in infected guinea pigs, the most serious lesions occurred in the liver and lungs. The sequential development of lesions indicated that flukes first invaded the quadrate lobe of the liver and subsequently migrated to other liver lobes and tissues. Six weeks post-infection, there was a marked drop in the recovery of flukes from the liver along with a dramatic increase in pulmonary involvement. Much of the hepatic and pulmonary pathology in infected animals was secondary to extensive vascular lesions caused by migrating flukes. In the liver, vascular lesions predominantly involved the portal and hepatic veins. Thrombophlebitis and locally extensive necrosis, resembling infarction, were observed. Vascular lesions in the lungs occurred in the pulmonary arteries leading to thrombosis and haemorrhagic infarction. Discovery of a fluke in a pulmonary artery, along with the pattern of hepatic and pulmonary lesions, suggested that flukes probably used the cardiovascular system as a pathway for dissemination. Death in fluke-infested guinea pigs was most often associated with severe pulmonary lesions. The nature and distribution of fluke-induced lesions observed in this study demonstrate that the guinea pig is a suitable animal model for Fascioloides magna infection in sheep.

Efficacy of clorsulon against Fascioloides magna infection in sheep

In a study to evaluate the efficacy of clorsulon against Fascioloides magna infection in sheep, 12 ewes were inoculated orally with 100 metacercariae of F magna, and 6 were treated with clorsulon (15 mg/kg of body weight) 8 weeks after inoculation. The sheep were euthanatized 16 weeks after inoculation, flukes were recovered, and the liver and other tissues were subjectively scored for the severity of lesions (0 to 4+). The number of flukes recovered from the clorsulon-treated group (3.8 +/- 1.2 flukes) was significantly (P = 0.025) lower than the number of flukes recovered from the group of untreated controls (10.0 +/- 6.6 flukes). The severity of lesions was significantly (P = 0.004) reduced (45.9%) in the treated group (2.0 +/- 1.1), compared with that in the untreated controls (3.7 +/- 0.5). In the untreated group, 3 sheep died and 1 became moribund 14 to 16 weeks after inoculation. The data suggested that a single treatment with clorsulon at a dosage of 15 mg/kg 8 weeks after inoculation was not effective in preventing F magna infection in sheep, because the survival of only a few F magna is potentially fatal in sheep within 6 months after infection.

A natural infection of Fascioloides magna in a llama (Lama glama)

A young, female llama (Lama glama) was euthanized following the onset of hindleg paresis and paralysis. Live trematodes, identified as Fascioloides magna, were identified from the liver of this animal. This represents the first report of F. magna in a llama.

Activity of closantel against experimentally induced Fascioloides magna infection in sheep

The efficacy of closantel against experimentally induced Fascioloides magna infection in sheep was studied. In each of 3 experiments, closantel was administered 8 weeks after the sheep were given (oral inoculation) 100 metacercariae of F magna. In the 1st experiment, closantel was given orally to 5 groups of 6 sheep each at dosages of 0 (nontreated control), 5, 7.5, 10, and 15 mg/kg of body weight. In the 2nd and 3rd experiments, groups of 10 or 12 sheep were treated to confirm the efficacy of the previously determined optimal dosage of 15 mg/kg. An additional group of sheep (n = 10) was used in the 3rd experiment to evaluate the efficacy of closantel given IM at a dosage of 7.5 mg/kg. Closantel given orally at a dosage level of 15 mg/kg was highly effective (94.6% to 97.7%) in reducing F magna burdens. Also, pathologic scores associated with the F magna infection were reduced by 81.3% to 92.6% in sheep given this dosage of closantel. Efficacy of the IM administered dosage of 7.5 mg of drug/kg was equivalent to that of the 15 mg/kg oral dosage. Other than mild, transient lameness of the limbs which were injected with the drug (group 10), side effects were not observed.

Pathophysiologic effects of experimentally induced Fascioloides magna infection in sheep

The pathophysiologic responses of 13 sheep inoculated orally with 100 metacercariae of Fascioloides magna were monitored for 4 months after inoculation. There were no differences in weight gains between these and a number of noninoculated control sheep throughout the experiment. Complete blood cell counts showed an increase only in the absolute number of eosinophils. Serum preparations (2 times a week) from 7 inoculated and 7 noninoculated sheep did not identify any significant changes in alkaline phosphatase, aspartate aminotransferase, alanine aminotransferase, and gamma-glutamyl transferase activities. There were no changes in total bilirubin, BUN, creatinine, inorganic phosphorus, calcium, albumin, chloride, potassium, and sodium values. Four months after they were inoculated, all sheep were necropsied, and flukes were recovered. Gross lesions attributed to fluke migration were found in the liver and portal lymph nodes, diaphragm, lungs, kidneys, and spleen. Microscopically, liver lesions in inoculated sheep occurred in the portal areas, veins, and Glisson's capsule and were characterized by both active and chronic forms of inflammation. Abundant infiltrates of eosinophils and plasma cells often marked the portal areas. Endophlebitis, with or without thrombosis, was the predominate vascular lesion. The flukes recovered varied greatly in size (6.5 to 48.0-mm long) and demonstrated some sexual development, but none was sexually mature.

Efficacy of albendazole against Fascioloides magna in sheep

Sheep experimentally infected with Fascioloides magna were used in a controlled study to determine the flukicide activity of different doses of albendazole (methyl [5(propylthio)-1H-benzimidazol-2-yl] carbamate). Sheep, about 4 months old, were inoculated with 100 metacercariae of F magna and treated with the various doses of albendazole 10 weeks later. At 6 weeks after treatment, the sheep were necropsied, flukes were recovered and measured, and the pathologic changes were recorded. The sheep were given different doses of albendazole (5, 7.5 [single dose or double dose], 10, and 15 mg/kg) and were compared with both nontreated inoculated and nontreated noninoculated controls. There were up to 70% fewer worms recovered in the group given 2 doses of 7.5 mg/kg than there were in the nontreated controls. There was also a significant reduction in flukes recovered in the groups given the 7.5, 10, and 15 mg/kg doses. Because a single F magna fluke has the potential to kill a sheep, it is imperative to consider the number of sheep with no flukes after treatment. According to that criterion, 50% of the sheep were protected at dose rates of 7.5 (single dose, double dose), 10, and 15 mg/kg of body weight.