- Email: [email protected]
Studies on the control of Toxocara canis in breeding kennels
veterinary parasitology ELSEVIER
Veterinary Parasitology 55 (1994) 87-92
Studies on the control of Toxocara canis in breeding kennels M.A. Fisher", D.E. Jacobs *,a, M.J. Hutchinson a, I.G.C. D i c k b aDepartment of Pathology and Infectious Diseases, The Royal Veterinal:v College (University of London), Boltons Park, PottersBar, EN6 INB, UK bBayer PLC, Bury St Edmunds, IP32 7/t11, UK
Accepted 2 November 1993
Abstract The control of Toxocara canis was investigated in naturally infected unweaned puppies. Anthelmintic treatments were administered to three litters of pups at 2, 4 and 6 weeks of age. When either a new combination anthelmintic containing febantel, pyrantel embonate and praziquantel or fenbendazole was used, the faecal egg output over the first 7 weeks of life was reduced by more than 80% and worm burdens by over 90%. In contrast, piperazine adipate had no appreciable effect on T. canis egg output, even though worm burdens were reduced by 86% by 7 weeks of age. In a further trial using three litters, the worm burden of pups treated with the combination anthelmintic was profiled before and after the 2 week dose and after the 4 week dose. Although worm numbers were substantially reduced by treatment, there was evidence of significant reinfection taking place throughout the control programme. It is concluded that more potent anthelmintics can provide longer term benefits by reducing the numbers of T. canis eggs shed into the environment, but that multiple dosing remains essential for this purpose. Keywords: Toxocara canis; Dog; Control methods-Nematoda
1. Introduction Toxocara canis infections in p u p p i e s are n o r m a l l y established prenatally (Sprent, 1958 ), although reinfection m a y subsequently occur via the d a m ' s milk (Burke and R o b e r s o n , 1985 ) or by ingestion o f e m b r y o n a t e d eggs f r o m the env i r o n m e n t . Multiple dosing is generally considered necessary to suppress infec-
*Corresponding author. 0304-4017/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD10304-4017 ( 93 ) 00620-E
88
M.A. Fisheret al, / VeterinaryParasitology55 (1994) 87-92
tion (Sprent and English, 1958; Herd, 1979; Barriga, 1991 ) and piperazine has been used for this purpose for many years. While this compound possesses useful activity against mature T. canis at commonly employed dose rates (Jacobs, 1987 ), it is less effective against developing T. canis larvae (Fisher et al., 1993). The practical advantages to be gained by exploiting the greater potency of newer anthelmintics have yet to be fully defined. This paper compares the reduction in faecal T. canis egg output attained when unweaned puppies at infected kennels were treated at 2 week intervals with piperazine, fenbendazole or a combination product containing febantel, pyrantel and praziquantel. It also examines the population dynamics of T. canis while using the combination anthelmintic in such a programme. 2. Materials and methods 2.1. Animals
Six greyhound bitches, three for Trial A and three for Trial B, were bred at kennels where T. canis is endemic, and provided a total of 44 naturally infected pups. These were kept with their dam for the duration of the trial. 2.2. Anthelmintics and dosage schedules
Piperazine adipate tablets (Head-to-Tail Veterinary Roundworm Tablets, Coopers-Pitman Moore, Crewe, UK) containing 450 mg active ingredient (a.i.) were used at a dose rate of one tablet per 4.5 kg body weight to the nearest quarter tablet (i.e. a target dose of 100 mg a.i. kg-1 ). Fenbendazole suspension (Panacur 10% Suspension, Hoechst, Milton Keynes, UK) was given at 1 ml kg -1 body weight to the nearest 0.5 ml (i.e. a target dose of 100 mg kg- ~). The combination anthelmintic (Drontal Plus, Bayer, Bury St. Edmunds, UK) is formulated as scored tablets containing 150 mg febantel, 144 mg pyrantel embonate and 50 mg praziquantel. Pups under 2 kg received a quarter tablet and those between 2 and 5 kg were given a half tablet. 2.3. Experimental design
Trial A was designed to monitor the output of T. canis eggs of pups dosed at 2, 4 and 6 weeks of age and to evaluate the reduction in worm numbers at the completion of the programme. At 2 weeks of age, pups were ranked according to body weight and allocated to three matched groups. Treatments were assigned to groups by lottery. One group was kept as an untreated control, one group received the combination anthelmintic while pups in the third group were given either piperazine (one litter) or fenbendazole (two litters). Overall, eight pups were dosed with the combination anthelmintic, three with piperazine and four with fenbendazole. Faecal egg counts were monitored at 2 weeks of age and at weekly intervals thereafter. A crude comparison of the faecal egg output of the various groups was made by plotting the egg counts against time and comparing the areas under
M.A. Fisher et al. / Veterinary Parasitology 55 (1994) 8 7-92
89
each curve. Post-mortem worm counts were performed after intravenous injection of an overdose of pentobarbitone sodium at 7 weeks of age. Trial B examined the population dynamics of T. canis in pups treated with the combination anthelmintic at 2 week intervals by providing 'snapshot' pictures of the parasite burden of the pups at various times during the first 5 weeks of life. To achieve this aim, post-mortem worm counts were made in randomly selected pups: ( 1 ) before the litter was dosed at 2 weeks of age (two pups were examined from each of two litters at 12-14 days of age); (2) 1 week after dosing at 2 weeks of age (one pup from each of two litters on Day 21, plus one pup that died unexpectedly owing to an intestinal intussusception at 19 days of age); (3) 1 week after the second dose ( 12 pups from three litters at 35-37 days of age). Patency was determined either by microscopic inspection of the worms or by examining faeces for T. canis eggs. The design of the experiment did not allow the inclusion of control animals for direct comparison but a data base exists of worm counts from untreated pups born at the same premises. 2.4. Parasitology Faecal samples were collected from the rectum of the pups using a small plastic spatula. The number of eggs in a weighed sample was estimated by a modification of the McMaster technique. The contents of the stomach and small intestine were washed over a 150/tm sieve and, in Trial B, an acid-pepsin digest of the lungs, liver and alimentary tract was washed over a 53 pm sieve. The residues were examined against a black background or under a dissecting microscope, as appropriate.
3. Results
All treatments were easy to administer and no side-effects were evident. It was considered impractical to divide tablets into units smaller than one-quarter. This reflects field usage but resulted in some variability in the actual dose received by the pups, on a milligram per kilogram body weight basis. Thus in Trial A, the mean doses administered of the nematocidal ingredients of the combination anthelmintic were 29.4 mg febantel (range 17.4-48.0 mg) and 9.8 mg pyrantel (range 5.8-16.0 mg) base per kilogram body weight. The corresponding figure for piperazine adipate was 95.1 (68.6-125.0) mg kg-t and for fenbendazole suspension 100.5 (88.2-120) mg kg -~. In Trial B, a mean of 28.8 mg febantel (range 20.737.5 mg) and 9.6 mg pyrantel (range 6.9-12.5 mg) base per kilogram body weight was given. No helminth other than T. canis was found in any pup.
90
M.A. Fisher et al. / Veterinary Parasitology 55 (1994.) 87-92
3.1. Trial A Faecal examination revealed that no pup in the three litters had a patent T. canis infection at 2 weeks of age (Table 1 ), but by 3 weeks all untreated controls were shedding eggs with an overall mean of 15 200 eggs per gram faeces (epg). The mean for all controls remained over 1000 epg for the remainder of the study. At 7 weeks, all untreated pups harboured T. canis (mean 47.8, range 17-68), most of which were mature adults. All pups treated with the combination anthelmintic had patent infections at 4 weeks, but the egg output was reduced by 84.7% over the period of the study when compared with controls. At 7 weeks, three of the eight pups had no T. canis while the remainder harboured between one and three worms (mean 1.1 ). The percentage reduction in worm count attributable to the control programme was therefore 98.1. Of the nine worms recovered, two were mature while the remainder were less than 40 mm long. Three of the four pups treated with fenbendazole were shedding T. canis eggs at 4 weeks, but overall the egg output was reduced by 88.2%. The worm counts at 7 weeks ranged from one to 12 (mean 4.3) giving an efficacy value of 93.0%. Twelve of the 17 worms recovered were mature. At the end of the study, the corresponding efficacy value for piperazine adipate was 86.2%, with five, six and 11 adult worms being recovered from the three treated pups. Despite this high overall reduction in worm numbers, there was no appreciable reduction in egg output during the course of the control programme. Table 1 Mean ( _+SD) Toxocara can is egg counts (thousands g-~ faeces) of pups treated at 2, 4 and 6 weeks of age and of untreated controls Treatment
Litters
No. of pups
Age (weeks)
2 Combination a Treated
1, 2, 3
8
0
Untreated
1, 2, 3
10
0
Fenbendazole Treated
2, 3
4
0
Untreated
2, 3
6
0
1
3
0
1
4
0
Piperazine Treated Untreated
aFebantel/pyrantel/praziquantel.
3
4
5
6
7
0.1 (0.3) 15.2 (33.0)
2.5 (2.4) 2.5 (3.4)
0.1 (0.3) 4.6 (8.6)
1.8 (5.2) 3.9 (4.0)
<0.l (<0.1) 7.9 (5.0)
<0.1 (<0.1) 2.6 (2.9)
1.7 (1.7) 2.8 (3.9)
<0.1 (0.2) 5.8 (11.1)
<0.1 (<0.1) 1.2 (1.5)
<0.1 (<0.1) 6.0 (5.1
26.4 (20.0) 34.1 (49.6)
26.3 (38.9) 1.9 (2.9)
5.8 (6.3) 2.8 (2.6)
10.3 (9.4) 7.9 (2.6)
1.0 (0.8 10.6 (3.7
M.A. Fisher et al. / Veterinary Parasitology 55 (1994) 87-92
91
3.2. Trial B
The puppies examined at 12-14 days of age prior to treatment harboured 32, 36, 68 and 72 worms. One week after the first dose of the combination anthelmintic, the numbers of worms recovered from individual pups were 5, 7 and 16. Most were immature, ranging in size from less than 5 to 45 mm, but there was one egg-laying female. The historical data base records three recent litters at these kennels in which untreated pups of 21-27 days of age were found to harbour between 49 and 349 worms (mean 158 ). One week after the second anthelmintic treatment, worm burdens varied from 0 to 6 (mean 2.8 ). Eleven of the 33 worms recovered were less than 16 mm long and were probably acquired after treatment. The remainder ranged from 16 to 76 mm and all were immature. In the historical record, a mean of 52 (range 12-138) worms had been found in eight litters examined at 29-30 days of age. Although the present study did not include untreated controls, a comparison of pre- and post-treatment worm counts suggests that a substantial reduction in worm numbers had taken place. This inference is supported by comparison of the results of Days 19-21 or Days 35-37 with the historical record.
4. Discussion The control of T. canis infections in breeding kennels has two major objectives: first, to protect pups from the harmful effects of their worm burden and second, to suppress faecal output of eggs (Jacobs and Fisher, 1993). Worm counts performed after the third treatment in Trial A indicate that all three anthelmintics achieved the first objective, with worm burdens reduced by 86.2-98.1%. These data, however, refer to a single time-point. The evaluation of a treatment programme also requires knowledge of what is happening during the course of the regime. 'Snapshot' profiles of the structure of the worm burden at various timepoints, as used in Trial B, provide a means of doing this. While treatment with the combination anthelmintic at 2 or 4 weeks of age did not eliminate T. canis completely, the worm population appears, nevertheless, to have been reduced substantially. Only one egg-laying worm was recovered from the 19 pups, suggesting that T. canis egg output would have been almost completely suppressed for the duration of this study. This result is consistent with the suppression of T. canis egg output observed in Trial A where the combination anthelmintic reduced faecal counts by 84.7% over the first 7 weeks of life. The corresponding figure for fenbendazole was 88.2%. In contrast, however, the use of piperazine at 2 week intervals gave no appreciable control of faecal egg output. The failure of piperazine to reduce faecal egg output probably reflects the fact that this compound at these dose rates has little activity against developing stages of T. canis. New egg-laying worms will, therefore, mature soon after each treatment. Fisher et al. ( 1993 ) showed that piperazine adipate administered at a dose rate of 150.0-187.5 mg a.i. kg-~ body weight reduced the numbers of third and
92
M.A. Fisher et al. / Veterinary Parasitology 55 (1994) 87-92
fourth stage worms in l-week-old pups by only 35.5%, while the corresponding figure for fenbendazole given at 50 mg kg-1 daily for 3 consecutive days was 94.0%. This distinction between piperazine and the newer broad-spectrum anthelmintics is of considerable practical significance. While pups treated with piperazine may be protected from the clinical effects of T. canis, they may, nevertheless, continue to pass substantial numbers of eggs and perpetuate the problem. The substitution of a more potent anthelmintic in a similar programme will provide the benefit of suppressing egg output more effectively, thereby reducing environmental contamination and the potential hazard for future litters and for man. The realisation that some anthelmintics have a more potent effect against the larval stages of T. canis than piperazine has provided an impetus for the re-examination of concepts underlying current dosing strategies. The optimal dosing interval, for example, could be longer than the traditional 2 weeks based on piperazine usage. In one recent study, the number of fenbendazole treatments needed to maintain faecal egg counts below 200 epg for the first 12 weeks of life varied from one to three (Fisher et al., 1993). The consistent finding of very small worms in Trial B of the present work indicates that significant re-infection was taking place throughout the suckling period. However potent the anthelmintic, therefore, repeat dosing will normally be a necessity for the satisfactory control of T. canis in breeding kennels. Further work is required to define optimum dosing intervals.
References Barriga, O.O., 1991. Rational control of canine toxocariasis by the veterinary practitioner. J. Am. Vet. Med. Assoc., 198; 216-221. Burke, T.M. and Roberson, E.L., 1985. Prenatal and lactational transmission of Toxocara canis and Ancylostoma caninum: experimental infection of the bitch at midpregnancy and at parturition. Int. J. Parasitol., 15: 485-490. Fisher, M.A., Jacobs, D.E., Hutchinson, M.J. and Abbott, E.M., 1993. Efficacy of fenbendazole and piperazine against developing stages of toxocara and toxascaris in dogs. Vet. Rec., 132: 473-475. Herd, R., 1979. Preventing visceral larva migrans. J. Am. Vet. Med. Assoc., 174; 780-782. Jacobs, D.E., 1987. Control of Toxocara canis in puppies: a comparison of screening techniques and evaluation of a dosing programme. J. Vet. Pharm. Therap., 10: 23-29. Jacobs, D.E. and Fisher, M.A., 1993. Recent developments in the chemotherapy of Toxocara canis infection in puppies and the prevention of toxocariasis. In: J.W. Lewis and R.M. Maizels (Editors), Toxocara and Toxocariasis: Clinical, Epidemiological and Molecular Perspectives. Institute of Biology, London, pp. 11 l - 116. Sprent, J.F.A., 1958. Observations on the development of Toxocara canis (Werner, 1782 ) in the dog. Parasitology, 48:184-208. Sprent, J.F.A. and English, P.B., 1958. The large roundworms of dogs and cats--a public health problem. Aust. Vet. J., 34: 161-171.
Veterinary Parasitology 55 (1994) 87-92
Studies on the control of Toxocara canis in breeding kennels M.A. Fisher", D.E. Jacobs *,a, M.J. Hutchinson a, I.G.C. D i c k b aDepartment of Pathology and Infectious Diseases, The Royal Veterinal:v College (University of London), Boltons Park, PottersBar, EN6 INB, UK bBayer PLC, Bury St Edmunds, IP32 7/t11, UK
Accepted 2 November 1993
Abstract The control of Toxocara canis was investigated in naturally infected unweaned puppies. Anthelmintic treatments were administered to three litters of pups at 2, 4 and 6 weeks of age. When either a new combination anthelmintic containing febantel, pyrantel embonate and praziquantel or fenbendazole was used, the faecal egg output over the first 7 weeks of life was reduced by more than 80% and worm burdens by over 90%. In contrast, piperazine adipate had no appreciable effect on T. canis egg output, even though worm burdens were reduced by 86% by 7 weeks of age. In a further trial using three litters, the worm burden of pups treated with the combination anthelmintic was profiled before and after the 2 week dose and after the 4 week dose. Although worm numbers were substantially reduced by treatment, there was evidence of significant reinfection taking place throughout the control programme. It is concluded that more potent anthelmintics can provide longer term benefits by reducing the numbers of T. canis eggs shed into the environment, but that multiple dosing remains essential for this purpose. Keywords: Toxocara canis; Dog; Control methods-Nematoda
1. Introduction Toxocara canis infections in p u p p i e s are n o r m a l l y established prenatally (Sprent, 1958 ), although reinfection m a y subsequently occur via the d a m ' s milk (Burke and R o b e r s o n , 1985 ) or by ingestion o f e m b r y o n a t e d eggs f r o m the env i r o n m e n t . Multiple dosing is generally considered necessary to suppress infec-
*Corresponding author. 0304-4017/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD10304-4017 ( 93 ) 00620-E
88
M.A. Fisheret al, / VeterinaryParasitology55 (1994) 87-92
tion (Sprent and English, 1958; Herd, 1979; Barriga, 1991 ) and piperazine has been used for this purpose for many years. While this compound possesses useful activity against mature T. canis at commonly employed dose rates (Jacobs, 1987 ), it is less effective against developing T. canis larvae (Fisher et al., 1993). The practical advantages to be gained by exploiting the greater potency of newer anthelmintics have yet to be fully defined. This paper compares the reduction in faecal T. canis egg output attained when unweaned puppies at infected kennels were treated at 2 week intervals with piperazine, fenbendazole or a combination product containing febantel, pyrantel and praziquantel. It also examines the population dynamics of T. canis while using the combination anthelmintic in such a programme. 2. Materials and methods 2.1. Animals
Six greyhound bitches, three for Trial A and three for Trial B, were bred at kennels where T. canis is endemic, and provided a total of 44 naturally infected pups. These were kept with their dam for the duration of the trial. 2.2. Anthelmintics and dosage schedules
Piperazine adipate tablets (Head-to-Tail Veterinary Roundworm Tablets, Coopers-Pitman Moore, Crewe, UK) containing 450 mg active ingredient (a.i.) were used at a dose rate of one tablet per 4.5 kg body weight to the nearest quarter tablet (i.e. a target dose of 100 mg a.i. kg-1 ). Fenbendazole suspension (Panacur 10% Suspension, Hoechst, Milton Keynes, UK) was given at 1 ml kg -1 body weight to the nearest 0.5 ml (i.e. a target dose of 100 mg kg- ~). The combination anthelmintic (Drontal Plus, Bayer, Bury St. Edmunds, UK) is formulated as scored tablets containing 150 mg febantel, 144 mg pyrantel embonate and 50 mg praziquantel. Pups under 2 kg received a quarter tablet and those between 2 and 5 kg were given a half tablet. 2.3. Experimental design
Trial A was designed to monitor the output of T. canis eggs of pups dosed at 2, 4 and 6 weeks of age and to evaluate the reduction in worm numbers at the completion of the programme. At 2 weeks of age, pups were ranked according to body weight and allocated to three matched groups. Treatments were assigned to groups by lottery. One group was kept as an untreated control, one group received the combination anthelmintic while pups in the third group were given either piperazine (one litter) or fenbendazole (two litters). Overall, eight pups were dosed with the combination anthelmintic, three with piperazine and four with fenbendazole. Faecal egg counts were monitored at 2 weeks of age and at weekly intervals thereafter. A crude comparison of the faecal egg output of the various groups was made by plotting the egg counts against time and comparing the areas under
M.A. Fisher et al. / Veterinary Parasitology 55 (1994) 8 7-92
89
each curve. Post-mortem worm counts were performed after intravenous injection of an overdose of pentobarbitone sodium at 7 weeks of age. Trial B examined the population dynamics of T. canis in pups treated with the combination anthelmintic at 2 week intervals by providing 'snapshot' pictures of the parasite burden of the pups at various times during the first 5 weeks of life. To achieve this aim, post-mortem worm counts were made in randomly selected pups: ( 1 ) before the litter was dosed at 2 weeks of age (two pups were examined from each of two litters at 12-14 days of age); (2) 1 week after dosing at 2 weeks of age (one pup from each of two litters on Day 21, plus one pup that died unexpectedly owing to an intestinal intussusception at 19 days of age); (3) 1 week after the second dose ( 12 pups from three litters at 35-37 days of age). Patency was determined either by microscopic inspection of the worms or by examining faeces for T. canis eggs. The design of the experiment did not allow the inclusion of control animals for direct comparison but a data base exists of worm counts from untreated pups born at the same premises. 2.4. Parasitology Faecal samples were collected from the rectum of the pups using a small plastic spatula. The number of eggs in a weighed sample was estimated by a modification of the McMaster technique. The contents of the stomach and small intestine were washed over a 150/tm sieve and, in Trial B, an acid-pepsin digest of the lungs, liver and alimentary tract was washed over a 53 pm sieve. The residues were examined against a black background or under a dissecting microscope, as appropriate.
3. Results
All treatments were easy to administer and no side-effects were evident. It was considered impractical to divide tablets into units smaller than one-quarter. This reflects field usage but resulted in some variability in the actual dose received by the pups, on a milligram per kilogram body weight basis. Thus in Trial A, the mean doses administered of the nematocidal ingredients of the combination anthelmintic were 29.4 mg febantel (range 17.4-48.0 mg) and 9.8 mg pyrantel (range 5.8-16.0 mg) base per kilogram body weight. The corresponding figure for piperazine adipate was 95.1 (68.6-125.0) mg kg-t and for fenbendazole suspension 100.5 (88.2-120) mg kg -~. In Trial B, a mean of 28.8 mg febantel (range 20.737.5 mg) and 9.6 mg pyrantel (range 6.9-12.5 mg) base per kilogram body weight was given. No helminth other than T. canis was found in any pup.
90
M.A. Fisher et al. / Veterinary Parasitology 55 (1994.) 87-92
3.1. Trial A Faecal examination revealed that no pup in the three litters had a patent T. canis infection at 2 weeks of age (Table 1 ), but by 3 weeks all untreated controls were shedding eggs with an overall mean of 15 200 eggs per gram faeces (epg). The mean for all controls remained over 1000 epg for the remainder of the study. At 7 weeks, all untreated pups harboured T. canis (mean 47.8, range 17-68), most of which were mature adults. All pups treated with the combination anthelmintic had patent infections at 4 weeks, but the egg output was reduced by 84.7% over the period of the study when compared with controls. At 7 weeks, three of the eight pups had no T. canis while the remainder harboured between one and three worms (mean 1.1 ). The percentage reduction in worm count attributable to the control programme was therefore 98.1. Of the nine worms recovered, two were mature while the remainder were less than 40 mm long. Three of the four pups treated with fenbendazole were shedding T. canis eggs at 4 weeks, but overall the egg output was reduced by 88.2%. The worm counts at 7 weeks ranged from one to 12 (mean 4.3) giving an efficacy value of 93.0%. Twelve of the 17 worms recovered were mature. At the end of the study, the corresponding efficacy value for piperazine adipate was 86.2%, with five, six and 11 adult worms being recovered from the three treated pups. Despite this high overall reduction in worm numbers, there was no appreciable reduction in egg output during the course of the control programme. Table 1 Mean ( _+SD) Toxocara can is egg counts (thousands g-~ faeces) of pups treated at 2, 4 and 6 weeks of age and of untreated controls Treatment
Litters
No. of pups
Age (weeks)
2 Combination a Treated
1, 2, 3
8
0
Untreated
1, 2, 3
10
0
Fenbendazole Treated
2, 3
4
0
Untreated
2, 3
6
0
1
3
0
1
4
0
Piperazine Treated Untreated
aFebantel/pyrantel/praziquantel.
3
4
5
6
7
0.1 (0.3) 15.2 (33.0)
2.5 (2.4) 2.5 (3.4)
0.1 (0.3) 4.6 (8.6)
1.8 (5.2) 3.9 (4.0)
<0.l (<0.1) 7.9 (5.0)
<0.1 (<0.1) 2.6 (2.9)
1.7 (1.7) 2.8 (3.9)
<0.1 (0.2) 5.8 (11.1)
<0.1 (<0.1) 1.2 (1.5)
<0.1 (<0.1) 6.0 (5.1
26.4 (20.0) 34.1 (49.6)
26.3 (38.9) 1.9 (2.9)
5.8 (6.3) 2.8 (2.6)
10.3 (9.4) 7.9 (2.6)
1.0 (0.8 10.6 (3.7
M.A. Fisher et al. / Veterinary Parasitology 55 (1994) 87-92
91
3.2. Trial B
The puppies examined at 12-14 days of age prior to treatment harboured 32, 36, 68 and 72 worms. One week after the first dose of the combination anthelmintic, the numbers of worms recovered from individual pups were 5, 7 and 16. Most were immature, ranging in size from less than 5 to 45 mm, but there was one egg-laying female. The historical data base records three recent litters at these kennels in which untreated pups of 21-27 days of age were found to harbour between 49 and 349 worms (mean 158 ). One week after the second anthelmintic treatment, worm burdens varied from 0 to 6 (mean 2.8 ). Eleven of the 33 worms recovered were less than 16 mm long and were probably acquired after treatment. The remainder ranged from 16 to 76 mm and all were immature. In the historical record, a mean of 52 (range 12-138) worms had been found in eight litters examined at 29-30 days of age. Although the present study did not include untreated controls, a comparison of pre- and post-treatment worm counts suggests that a substantial reduction in worm numbers had taken place. This inference is supported by comparison of the results of Days 19-21 or Days 35-37 with the historical record.
4. Discussion The control of T. canis infections in breeding kennels has two major objectives: first, to protect pups from the harmful effects of their worm burden and second, to suppress faecal output of eggs (Jacobs and Fisher, 1993). Worm counts performed after the third treatment in Trial A indicate that all three anthelmintics achieved the first objective, with worm burdens reduced by 86.2-98.1%. These data, however, refer to a single time-point. The evaluation of a treatment programme also requires knowledge of what is happening during the course of the regime. 'Snapshot' profiles of the structure of the worm burden at various timepoints, as used in Trial B, provide a means of doing this. While treatment with the combination anthelmintic at 2 or 4 weeks of age did not eliminate T. canis completely, the worm population appears, nevertheless, to have been reduced substantially. Only one egg-laying worm was recovered from the 19 pups, suggesting that T. canis egg output would have been almost completely suppressed for the duration of this study. This result is consistent with the suppression of T. canis egg output observed in Trial A where the combination anthelmintic reduced faecal counts by 84.7% over the first 7 weeks of life. The corresponding figure for fenbendazole was 88.2%. In contrast, however, the use of piperazine at 2 week intervals gave no appreciable control of faecal egg output. The failure of piperazine to reduce faecal egg output probably reflects the fact that this compound at these dose rates has little activity against developing stages of T. canis. New egg-laying worms will, therefore, mature soon after each treatment. Fisher et al. ( 1993 ) showed that piperazine adipate administered at a dose rate of 150.0-187.5 mg a.i. kg-~ body weight reduced the numbers of third and
92
M.A. Fisher et al. / Veterinary Parasitology 55 (1994) 87-92
fourth stage worms in l-week-old pups by only 35.5%, while the corresponding figure for fenbendazole given at 50 mg kg-1 daily for 3 consecutive days was 94.0%. This distinction between piperazine and the newer broad-spectrum anthelmintics is of considerable practical significance. While pups treated with piperazine may be protected from the clinical effects of T. canis, they may, nevertheless, continue to pass substantial numbers of eggs and perpetuate the problem. The substitution of a more potent anthelmintic in a similar programme will provide the benefit of suppressing egg output more effectively, thereby reducing environmental contamination and the potential hazard for future litters and for man. The realisation that some anthelmintics have a more potent effect against the larval stages of T. canis than piperazine has provided an impetus for the re-examination of concepts underlying current dosing strategies. The optimal dosing interval, for example, could be longer than the traditional 2 weeks based on piperazine usage. In one recent study, the number of fenbendazole treatments needed to maintain faecal egg counts below 200 epg for the first 12 weeks of life varied from one to three (Fisher et al., 1993). The consistent finding of very small worms in Trial B of the present work indicates that significant re-infection was taking place throughout the suckling period. However potent the anthelmintic, therefore, repeat dosing will normally be a necessity for the satisfactory control of T. canis in breeding kennels. Further work is required to define optimum dosing intervals.
References Barriga, O.O., 1991. Rational control of canine toxocariasis by the veterinary practitioner. J. Am. Vet. Med. Assoc., 198; 216-221. Burke, T.M. and Roberson, E.L., 1985. Prenatal and lactational transmission of Toxocara canis and Ancylostoma caninum: experimental infection of the bitch at midpregnancy and at parturition. Int. J. Parasitol., 15: 485-490. Fisher, M.A., Jacobs, D.E., Hutchinson, M.J. and Abbott, E.M., 1993. Efficacy of fenbendazole and piperazine against developing stages of toxocara and toxascaris in dogs. Vet. Rec., 132: 473-475. Herd, R., 1979. Preventing visceral larva migrans. J. Am. Vet. Med. Assoc., 174; 780-782. Jacobs, D.E., 1987. Control of Toxocara canis in puppies: a comparison of screening techniques and evaluation of a dosing programme. J. Vet. Pharm. Therap., 10: 23-29. Jacobs, D.E. and Fisher, M.A., 1993. Recent developments in the chemotherapy of Toxocara canis infection in puppies and the prevention of toxocariasis. In: J.W. Lewis and R.M. Maizels (Editors), Toxocara and Toxocariasis: Clinical, Epidemiological and Molecular Perspectives. Institute of Biology, London, pp. 11 l - 116. Sprent, J.F.A., 1958. Observations on the development of Toxocara canis (Werner, 1782 ) in the dog. Parasitology, 48:184-208. Sprent, J.F.A. and English, P.B., 1958. The large roundworms of dogs and cats--a public health problem. Aust. Vet. J., 34: 161-171.