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Ocular larva migrans and histopathological lesions in mice experimentally infected with Baylisascaris transfuga embryonated eggs
veterinary parasitology ELSEVIER
Veterinary Parasitology 61 (1996) 315-320
Ocular larva migrans and histopathological lesions in mice experimentally infected with Baylisascaris transfuga embryonated eggs R. Papini a,*, G. Renzoni a, S. LO Piccolo b, L. Casarosa a a Dipartimento di Patologia Animale, Profilassi e lgiene degli Alimenti. Viale delle Piagge 2, 56100 Pisa, Italy b Clinica Pediatrica, Via del Vespro 127, 90127 Palermo, Italy
Received 21 December 1994; accepted 20 March 1995
Abstract The ability of Baylisascaris transfuga larvae to cause ocular larva migrans (OLM) in mice was examined. Mice were given approximately 3500 infective eggs of B. transfuga per os. Their eyes were removed and examined either microscopically or histologically at various intervals post-infection. Larvae were recovered beginning 7 days after infection, Histologically, free larvae were observed in the posterior chamber and within the ocular membranes. Larval granulomas were present in the choroid with involvement of retinal layers. It was concluded that B. transfuga larvae have the ability to produce OLM in mice following oral infection and should be considered as possible agents of OLM in other animals and in human beings. Keywords: Baylisascaris transfuga; Mouse; Larva migrans; Pathology-Nematoda
1. Introduction Baylisascaris transfuga is a common intestinal roundworm of bears (Sprent, 1968). The prolonged migration of B. transfuga larvae in deep tissues o f mice, causing visceral larva migrans, has been reported (Sprent, 1951, 1952, 1953, 1955). When infective eggs of B. transfuga are ingested by mice, hatching occurs in the gastrointestinal tract. Most of the larvae remain in the intestinal wall, but many of them invade the liver, lungs and
* Corresponding author. 0304-4017/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD1 0304-4017(95)00825-X
316
R. Papini et al./ Veterinary Parasitology 61 (1996) 315-320
musculature, and a few reach the brain and other organs. Sprent (1964) suggested the likelihood of infection of human beings with this parasite. Ocular larva migrans (OLM) is an ocular disease resulting from invasion of the eyes by nematode larvae (Olson et al., 1970; Olson, 1976; Kazakos et al., 1984). The invasion of eyes by B. transfuga larvae has not been investigated previously. The purpose of the present study was to examine the ability of B. transfuga larvae to cause OLM in mice as an indication of its possible role in causing ocular disease in other animals and in human beings.
2. Materials and methods
B. transfuga adults were obtained from naturally infected captive brown bears (Ursus arctos arctos), living in the town park of Leghorn (Italy), after deworming with piperazine adipate (200 mg kg-1 body weight; Izovermina®; Izo, Brescia, Italy). Eggs were obtained by dissection of the uteri from freshly recovered adult females. The eggs were incubated for 30-35 days at room temperature (22-25°C) in Petri dishes containing 0.1 N sulphuric acid at a depth of 2 - 3 mm. After embryonation, infectivity of the eggs was tested by infecting pairs of mice per os. Eggs were stored at + 5°C until use. Fresh solution was added to avoid loss of medium due to evaporation. Different lots of eggs were pooled before use. Mice were infected, under light ether anaesthesia, by administration of eggs suspended in tap water through a stomach tube. According to the method of Olson et al. (1970), three samples of each egg suspension were examined microscopically. Only eggs containing larvae were counted. The mean of these samples was used to estimate the total number of eggs per millilitre of suspension. The suspensions were adjusted by removal or addition of tap water to deliver a dose of approximately 3500 B. transfuga eggs to each mouse in a volume of 0.2-0.5 ml. Seventy-eight laboratory albino mice, of both sexes, weighing 20-25 g each, were selected. Mice were given commercial pellet food and water ad libitum. Groups of six infected mice were killed at various intervals after infection as shown in Table 1. At necropsy, eyeballs were removed with small forceps and placed in saline solution. Left eyes were squashed between two glass slides and examined for larvae at 40 X magnification (Burren, 1971); right eyes were fixed in 10% formalin and later processed for histology using paraffin embedding. Sections of each eyeball were cut at 6 /xm thickness and stained with haematoxylin-eosin and Van Gieson.
3. Results
Results are shown in Table 1. Eleven (14%) of 78 infected mice harbored B.
transfuga larvae in their eyes, as determined by tissue-press examination and serial histological sections. The number of positive mice ranged from 1 to 2 per group. In these mice, the total number of larvae in the eyes ranged from 2 to 6. Tissue-press examination showed that many of the recovered larvae were alive and motile. Histologi-
R. Papini et al./Veterinary Parasitology 61 (1996) 315-320
317
Table 1 Number of mice harboring B. transfuga larvae in one or both eyes, and total number of larvae in their eyes Days p.i.
No. mice with larvae in eyes
Total no. larvae in eyes
1 2 3 5 7 8 10 13 14 15 19 20 27
0 0 0 0 1 2 2 2 1 2 1 0 0
0 0 0 0 2 3 6 5 3 4 3 0 0
Mice were orally infected with approximately 3500 B. transfuga infective eggs and killed (n = 6 per day) at the times shown.
cally, examination of serial sections showed that B. transfuga larvae were located in the choroid (five mice), in the posterior chamber (three mice) or in the extraocular musculature (three mice). Seldom larvae found in the posterior chamber of the eye (Fig. 1) or within the ocular membranes (Fig. 2) were free of inflammatory cells. This was believed to represent a recent localization or a rapid migration, staying ahead of white blood cells migration. Lesions in the choroid consisted of focal chorioiditis with larvae undergoing granulomatous encapsulation. Larval granulomas had a centrally located
Fig. 1. Histological section of ocular globe from mouse killed on Day 7 p.i. Sections of B. transfuga larva free in the posterior chamber. Haematoxylin-eosin, X 80.
318
R. Papini et a L / Veterinary Parasitology 61 (1996) 315-320
Fig. 2. Histological section of ocular globe from mouse killed on Day 7 p.i.B, transfuga larva with evident lateral alae within ocular membranes free of any inflammatory reaction. Haematoxylin-eosin, X 500.
larva surrounded mainly by eosinophils and a few plasma cells. F r e q u e n t l y g r a n u l o m a tous chorioiditis resulted in retinal i n v o l v e m e n t with elevation, disruption and necrosis o f the retinal layers (Figs. 3(a) and 3(b)). In m o r e a d v a n c e d reactions, larvae w e r e
(al
Ibl
I
Fig. 3. (a and b) Histological sections of ocular globe from mouse killed on Day 10 p.i. Incipient chorioretinal granuloma containing a B. transfuga larva surrounded by moderate amount of eosinophils with elevation, focal disruption and necrosis of retinal layers. Haematoxylin-eosin, (a) x 125, (b) x 500.
R. Papiniet al. / VeterinaryParasitology61 (1996)315-320
319
surrounded by fibrous connective tissue in which inflammatory cells (mainly eosinophils) were scattered. Based on their internal structure and staining characteristics, we can suppose that many of the larvae were viable at the time of fixation.
4. Discussion The results of our experiments show that B. transfuga infective larvae have the ability to produce OLM in mice, so that B. transfuga larvae can presumably be indicated as possible agents of OLM in animals and human beings. We found that the eyes of infected mice harbored B. transfuga larvae as early as 7 days post-infection (p.i.). There was no indication of an increase or decrease in the number of mice harboring larvae in their eyes as the time after infection increased. Viable larvae were recovered from the eyes of mice until 19 days after infection. The life span of B. transfuga larvae in the mouse eye is not known. Sprent (1953) showed that B. transfuga larvae may remain in tissues of mice in viable conditions for several months. Therefore, we can suppose that after B. transfuga larvae invade the eye many of them may remain viable therein for a considerable length of time. We found no report that B. transfuga has produced OLM in animals or human beings. It is known that the invasion of eyes by larvae of Toxocara canis and Baylisascaris procyonis may occur following oral infection with embryonated eggs. T. canis larvae caused uveitis, choroiditis and lesions of the retina in mice experimentally infected (Olson et al., 1970; Olson, 1976), while B. procyonis larvae produced choroidal granulomas, inflammation, degeneration, necrosis and disruption of the retina in subhuman primates (Kazacos et al., 1984). Both B. transfuga and B. procyonis larvae have evident lateral alae and grow considerably in mice, but the former are smaller: maximum measures of 900 /~m in length and 40 /xm in width are reported for B. transfuga versus 1.56 mm in length and 71 /zm in width for B. procyonis (Sprent, 1955; Bowman, 1987). Keys based on some morphological and histological features have been devised to aid in making an identification of these larvae (Sprent, 1964; Bowman, 1987). It has been established that a relatively high percentage of B. transfuga larvae enter the intestinal wall, body musculature and subcutaneous tissues after infection in mice (Sprent, 1951, 1952, 1953) and that a few larvae enter the brain (Sprent, 1955). In those studies oral dosages of 5000 or 2000-3000 eggs were used but the ocular localization was not investigated. Our research shows that localization in eyes is possible. The large egg dosage used in our study could be close to the level of natural exposures to B. transfuga eggs, taking into account the widespread infection in wild and captive bears (Addison et al., 1978; Dies, 1979; Abdel-Rasoul and Fowler, 1979, 1980; Duffy et al., 1994), the high number of eggs shed by infected bears (Vercruysse et al., 1976) and the high environmental contamination of eggs where faeces of infected bears are allowed to accumulate on the ground (Abdel-Rasoul and Fowler, 1979). We are inclined to believe that high doses of infective eggs may increase the chance of migration of B. transfuga larvae to the eye. Olson et al. (1970) and Kazakos et al. (1984) reported that experimental animals receiving higher dosages of T. canis and B. procyonis infective eggs, respectively, showed more severe localization of the larvae in eyes. Therefore,
320
R. Papini et al. / Veterinary Parasitology 61 (1996) 315-320
s u c h a n e v e n t s e e m s to b e d o s e d e p e n d e n t . It is c o n c l u d e d that t h e p o s s i b l e e x p o s u r e to B. t r a n s f u g a i n f e c t i v e e g g s o f p e o p l e ( t r a p p e r s , h u n t e r s , z o o a n d c i r c u s s t a f f ) h a v i n g contact for prolonged periods with infected bears should be investigated.
References Abdel-Rasoul, K. and Fowler, M., 1979. Epidemiology of ascarid infection in captive carnivores. Proc. Am. Assoc. Zoo Vet., October 1979, Denver, CO. American Association of Zoo Veterinarians, Philadelphia, pp. 165-169. Abdel-Rasoul, K. and Fowler, M., 1980. An epidemiologic approach to the control of Ascariasis in zoo carnivores. In: R. Ippen and H.D. Schroder (Editors), Verhandlungsbericht XXII. Internationales Symposium fiber die Erkrankungen der Zootiere, Arnhem, Netherlands. Akademie Verlag, Berlin, pp. 273-277. Addison, E.M., Pybus, M.J. and Rietveld, H.J., 1978. Helminth and arthropod parasites of black bear, Ursus americanus, in Central Ontario. Can. J. Zool., 56: 2122-2126. Bowman, D.D., 1987. Diagnostic morphology of four larval ascaridoid nematodes that may cause visceral larva migrans: Toxascaris leonina, Baylisascaris procyonis, Lagochilascaris sprenti, and Hexametra leidyi. J. Parasitol., 73: 1198-1215. Burren, C.H., 1971. The distribution of Toxocara canis in the central nervous system of the mouse. Trans. R. Soc. Trop. Med. Hyg., 65: 450-453. Dies, K.D., 1979. Helminths recovered from black bears in the Peace River region in North-Western Alberta. J. Wildl. Dis., 15: 49-50. Duffy, M.S., Greaves, T.A. and Burt, D.B., 1994. Helminths of Black Bear, Ursu americanus, in New Brunswick. J. Parasitol., 80: 478-480. Kazacos, K.R., Vestre, W.A. and Kazacos, E.A., 1984. Raccoon ascarid larvae (Baylisascaris procyonis) as a cause of Ocular Larva Migrans. Invest. Ophthalmol. Vis. Sci., 25:1177-1183. Olson, L.J., 1976. Ocular toxocariasis in mice: distribution of larvae and lesions. Int. J. Parasitol., 6: 247-251. Olson, L.J., Izzat, N.N., Petteway, M.B. and Reinhart, J.A., 1970. Ocular toxocariasis in mice. Am. J. Trop. Med. Hyg., 9: 238-362. Sprent, J.F.A., 1951. Observation on the migratory activity of the larvae of Toxascaris transfuga (Rud. 1819) Baylis&Daubney 1922. J. Parasitol., 37: 326-327. Sprent, J.F.A., 1952. On the migratory behaviour of the larvae of various Ascaris species in white mice. I. Distribution of larvae in tissues. J. Infect. Dis., 90: 165-172. Sprent, J.F.A., 1953. On the migratory behaviour of the larvae of various Ascaris species in white mice. II. Longevity of encapsulated larvae and their resistance to freezing and putrefaction. J. Infect. Dis., 92: 114-117. Sprent, J.F.A., 1955. On the invasion of the central nervous system by nematodes. I1. Invasion of the nervous system in ascariasis. Parasitology, 45: 41-55. Sprent, J.F.A., 1964. Observations relating to the diagnosis of visceral larva migrans. In: A. Corradetti (Editor), Proc. First Int. Congr. Parasitol., 21-26 September 1964, Rome, Vol. 2. Pergamon, Oxford/Tamburini, Milan, pp. 802-803. Sprent, J.F.A., 1968. Notes on Ascaris and To:~ascaris with a definition of Baylisascaris gen. nov. Parasitology, 58: 185-198. Vercruysse, J., Kumar, V., Ceulemans, F. and Mortelmans, J., 1976. Chemotherapy of helminthiasis among wild mammals. II. Baylisascaris transfuga (Rudolphi 1819) Sprent, 1968 infection of polar bear. Acta Zool. Pathol. Antwerp., 64:115-119.
Veterinary Parasitology 61 (1996) 315-320
Ocular larva migrans and histopathological lesions in mice experimentally infected with Baylisascaris transfuga embryonated eggs R. Papini a,*, G. Renzoni a, S. LO Piccolo b, L. Casarosa a a Dipartimento di Patologia Animale, Profilassi e lgiene degli Alimenti. Viale delle Piagge 2, 56100 Pisa, Italy b Clinica Pediatrica, Via del Vespro 127, 90127 Palermo, Italy
Received 21 December 1994; accepted 20 March 1995
Abstract The ability of Baylisascaris transfuga larvae to cause ocular larva migrans (OLM) in mice was examined. Mice were given approximately 3500 infective eggs of B. transfuga per os. Their eyes were removed and examined either microscopically or histologically at various intervals post-infection. Larvae were recovered beginning 7 days after infection, Histologically, free larvae were observed in the posterior chamber and within the ocular membranes. Larval granulomas were present in the choroid with involvement of retinal layers. It was concluded that B. transfuga larvae have the ability to produce OLM in mice following oral infection and should be considered as possible agents of OLM in other animals and in human beings. Keywords: Baylisascaris transfuga; Mouse; Larva migrans; Pathology-Nematoda
1. Introduction Baylisascaris transfuga is a common intestinal roundworm of bears (Sprent, 1968). The prolonged migration of B. transfuga larvae in deep tissues o f mice, causing visceral larva migrans, has been reported (Sprent, 1951, 1952, 1953, 1955). When infective eggs of B. transfuga are ingested by mice, hatching occurs in the gastrointestinal tract. Most of the larvae remain in the intestinal wall, but many of them invade the liver, lungs and
* Corresponding author. 0304-4017/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD1 0304-4017(95)00825-X
316
R. Papini et al./ Veterinary Parasitology 61 (1996) 315-320
musculature, and a few reach the brain and other organs. Sprent (1964) suggested the likelihood of infection of human beings with this parasite. Ocular larva migrans (OLM) is an ocular disease resulting from invasion of the eyes by nematode larvae (Olson et al., 1970; Olson, 1976; Kazakos et al., 1984). The invasion of eyes by B. transfuga larvae has not been investigated previously. The purpose of the present study was to examine the ability of B. transfuga larvae to cause OLM in mice as an indication of its possible role in causing ocular disease in other animals and in human beings.
2. Materials and methods
B. transfuga adults were obtained from naturally infected captive brown bears (Ursus arctos arctos), living in the town park of Leghorn (Italy), after deworming with piperazine adipate (200 mg kg-1 body weight; Izovermina®; Izo, Brescia, Italy). Eggs were obtained by dissection of the uteri from freshly recovered adult females. The eggs were incubated for 30-35 days at room temperature (22-25°C) in Petri dishes containing 0.1 N sulphuric acid at a depth of 2 - 3 mm. After embryonation, infectivity of the eggs was tested by infecting pairs of mice per os. Eggs were stored at + 5°C until use. Fresh solution was added to avoid loss of medium due to evaporation. Different lots of eggs were pooled before use. Mice were infected, under light ether anaesthesia, by administration of eggs suspended in tap water through a stomach tube. According to the method of Olson et al. (1970), three samples of each egg suspension were examined microscopically. Only eggs containing larvae were counted. The mean of these samples was used to estimate the total number of eggs per millilitre of suspension. The suspensions were adjusted by removal or addition of tap water to deliver a dose of approximately 3500 B. transfuga eggs to each mouse in a volume of 0.2-0.5 ml. Seventy-eight laboratory albino mice, of both sexes, weighing 20-25 g each, were selected. Mice were given commercial pellet food and water ad libitum. Groups of six infected mice were killed at various intervals after infection as shown in Table 1. At necropsy, eyeballs were removed with small forceps and placed in saline solution. Left eyes were squashed between two glass slides and examined for larvae at 40 X magnification (Burren, 1971); right eyes were fixed in 10% formalin and later processed for histology using paraffin embedding. Sections of each eyeball were cut at 6 /xm thickness and stained with haematoxylin-eosin and Van Gieson.
3. Results
Results are shown in Table 1. Eleven (14%) of 78 infected mice harbored B.
transfuga larvae in their eyes, as determined by tissue-press examination and serial histological sections. The number of positive mice ranged from 1 to 2 per group. In these mice, the total number of larvae in the eyes ranged from 2 to 6. Tissue-press examination showed that many of the recovered larvae were alive and motile. Histologi-
R. Papini et al./Veterinary Parasitology 61 (1996) 315-320
317
Table 1 Number of mice harboring B. transfuga larvae in one or both eyes, and total number of larvae in their eyes Days p.i.
No. mice with larvae in eyes
Total no. larvae in eyes
1 2 3 5 7 8 10 13 14 15 19 20 27
0 0 0 0 1 2 2 2 1 2 1 0 0
0 0 0 0 2 3 6 5 3 4 3 0 0
Mice were orally infected with approximately 3500 B. transfuga infective eggs and killed (n = 6 per day) at the times shown.
cally, examination of serial sections showed that B. transfuga larvae were located in the choroid (five mice), in the posterior chamber (three mice) or in the extraocular musculature (three mice). Seldom larvae found in the posterior chamber of the eye (Fig. 1) or within the ocular membranes (Fig. 2) were free of inflammatory cells. This was believed to represent a recent localization or a rapid migration, staying ahead of white blood cells migration. Lesions in the choroid consisted of focal chorioiditis with larvae undergoing granulomatous encapsulation. Larval granulomas had a centrally located
Fig. 1. Histological section of ocular globe from mouse killed on Day 7 p.i. Sections of B. transfuga larva free in the posterior chamber. Haematoxylin-eosin, X 80.
318
R. Papini et a L / Veterinary Parasitology 61 (1996) 315-320
Fig. 2. Histological section of ocular globe from mouse killed on Day 7 p.i.B, transfuga larva with evident lateral alae within ocular membranes free of any inflammatory reaction. Haematoxylin-eosin, X 500.
larva surrounded mainly by eosinophils and a few plasma cells. F r e q u e n t l y g r a n u l o m a tous chorioiditis resulted in retinal i n v o l v e m e n t with elevation, disruption and necrosis o f the retinal layers (Figs. 3(a) and 3(b)). In m o r e a d v a n c e d reactions, larvae w e r e
(al
Ibl
I
Fig. 3. (a and b) Histological sections of ocular globe from mouse killed on Day 10 p.i. Incipient chorioretinal granuloma containing a B. transfuga larva surrounded by moderate amount of eosinophils with elevation, focal disruption and necrosis of retinal layers. Haematoxylin-eosin, (a) x 125, (b) x 500.
R. Papiniet al. / VeterinaryParasitology61 (1996)315-320
319
surrounded by fibrous connective tissue in which inflammatory cells (mainly eosinophils) were scattered. Based on their internal structure and staining characteristics, we can suppose that many of the larvae were viable at the time of fixation.
4. Discussion The results of our experiments show that B. transfuga infective larvae have the ability to produce OLM in mice, so that B. transfuga larvae can presumably be indicated as possible agents of OLM in animals and human beings. We found that the eyes of infected mice harbored B. transfuga larvae as early as 7 days post-infection (p.i.). There was no indication of an increase or decrease in the number of mice harboring larvae in their eyes as the time after infection increased. Viable larvae were recovered from the eyes of mice until 19 days after infection. The life span of B. transfuga larvae in the mouse eye is not known. Sprent (1953) showed that B. transfuga larvae may remain in tissues of mice in viable conditions for several months. Therefore, we can suppose that after B. transfuga larvae invade the eye many of them may remain viable therein for a considerable length of time. We found no report that B. transfuga has produced OLM in animals or human beings. It is known that the invasion of eyes by larvae of Toxocara canis and Baylisascaris procyonis may occur following oral infection with embryonated eggs. T. canis larvae caused uveitis, choroiditis and lesions of the retina in mice experimentally infected (Olson et al., 1970; Olson, 1976), while B. procyonis larvae produced choroidal granulomas, inflammation, degeneration, necrosis and disruption of the retina in subhuman primates (Kazacos et al., 1984). Both B. transfuga and B. procyonis larvae have evident lateral alae and grow considerably in mice, but the former are smaller: maximum measures of 900 /~m in length and 40 /xm in width are reported for B. transfuga versus 1.56 mm in length and 71 /zm in width for B. procyonis (Sprent, 1955; Bowman, 1987). Keys based on some morphological and histological features have been devised to aid in making an identification of these larvae (Sprent, 1964; Bowman, 1987). It has been established that a relatively high percentage of B. transfuga larvae enter the intestinal wall, body musculature and subcutaneous tissues after infection in mice (Sprent, 1951, 1952, 1953) and that a few larvae enter the brain (Sprent, 1955). In those studies oral dosages of 5000 or 2000-3000 eggs were used but the ocular localization was not investigated. Our research shows that localization in eyes is possible. The large egg dosage used in our study could be close to the level of natural exposures to B. transfuga eggs, taking into account the widespread infection in wild and captive bears (Addison et al., 1978; Dies, 1979; Abdel-Rasoul and Fowler, 1979, 1980; Duffy et al., 1994), the high number of eggs shed by infected bears (Vercruysse et al., 1976) and the high environmental contamination of eggs where faeces of infected bears are allowed to accumulate on the ground (Abdel-Rasoul and Fowler, 1979). We are inclined to believe that high doses of infective eggs may increase the chance of migration of B. transfuga larvae to the eye. Olson et al. (1970) and Kazakos et al. (1984) reported that experimental animals receiving higher dosages of T. canis and B. procyonis infective eggs, respectively, showed more severe localization of the larvae in eyes. Therefore,
320
R. Papini et al. / Veterinary Parasitology 61 (1996) 315-320
s u c h a n e v e n t s e e m s to b e d o s e d e p e n d e n t . It is c o n c l u d e d that t h e p o s s i b l e e x p o s u r e to B. t r a n s f u g a i n f e c t i v e e g g s o f p e o p l e ( t r a p p e r s , h u n t e r s , z o o a n d c i r c u s s t a f f ) h a v i n g contact for prolonged periods with infected bears should be investigated.
References Abdel-Rasoul, K. and Fowler, M., 1979. Epidemiology of ascarid infection in captive carnivores. Proc. Am. Assoc. Zoo Vet., October 1979, Denver, CO. American Association of Zoo Veterinarians, Philadelphia, pp. 165-169. Abdel-Rasoul, K. and Fowler, M., 1980. An epidemiologic approach to the control of Ascariasis in zoo carnivores. In: R. Ippen and H.D. Schroder (Editors), Verhandlungsbericht XXII. Internationales Symposium fiber die Erkrankungen der Zootiere, Arnhem, Netherlands. Akademie Verlag, Berlin, pp. 273-277. Addison, E.M., Pybus, M.J. and Rietveld, H.J., 1978. Helminth and arthropod parasites of black bear, Ursus americanus, in Central Ontario. Can. J. Zool., 56: 2122-2126. Bowman, D.D., 1987. Diagnostic morphology of four larval ascaridoid nematodes that may cause visceral larva migrans: Toxascaris leonina, Baylisascaris procyonis, Lagochilascaris sprenti, and Hexametra leidyi. J. Parasitol., 73: 1198-1215. Burren, C.H., 1971. The distribution of Toxocara canis in the central nervous system of the mouse. Trans. R. Soc. Trop. Med. Hyg., 65: 450-453. Dies, K.D., 1979. Helminths recovered from black bears in the Peace River region in North-Western Alberta. J. Wildl. Dis., 15: 49-50. Duffy, M.S., Greaves, T.A. and Burt, D.B., 1994. Helminths of Black Bear, Ursu americanus, in New Brunswick. J. Parasitol., 80: 478-480. Kazacos, K.R., Vestre, W.A. and Kazacos, E.A., 1984. Raccoon ascarid larvae (Baylisascaris procyonis) as a cause of Ocular Larva Migrans. Invest. Ophthalmol. Vis. Sci., 25:1177-1183. Olson, L.J., 1976. Ocular toxocariasis in mice: distribution of larvae and lesions. Int. J. Parasitol., 6: 247-251. Olson, L.J., Izzat, N.N., Petteway, M.B. and Reinhart, J.A., 1970. Ocular toxocariasis in mice. Am. J. Trop. Med. Hyg., 9: 238-362. Sprent, J.F.A., 1951. Observation on the migratory activity of the larvae of Toxascaris transfuga (Rud. 1819) Baylis&Daubney 1922. J. Parasitol., 37: 326-327. Sprent, J.F.A., 1952. On the migratory behaviour of the larvae of various Ascaris species in white mice. I. Distribution of larvae in tissues. J. Infect. Dis., 90: 165-172. Sprent, J.F.A., 1953. On the migratory behaviour of the larvae of various Ascaris species in white mice. II. Longevity of encapsulated larvae and their resistance to freezing and putrefaction. J. Infect. Dis., 92: 114-117. Sprent, J.F.A., 1955. On the invasion of the central nervous system by nematodes. I1. Invasion of the nervous system in ascariasis. Parasitology, 45: 41-55. Sprent, J.F.A., 1964. Observations relating to the diagnosis of visceral larva migrans. In: A. Corradetti (Editor), Proc. First Int. Congr. Parasitol., 21-26 September 1964, Rome, Vol. 2. Pergamon, Oxford/Tamburini, Milan, pp. 802-803. Sprent, J.F.A., 1968. Notes on Ascaris and To:~ascaris with a definition of Baylisascaris gen. nov. Parasitology, 58: 185-198. Vercruysse, J., Kumar, V., Ceulemans, F. and Mortelmans, J., 1976. Chemotherapy of helminthiasis among wild mammals. II. Baylisascaris transfuga (Rudolphi 1819) Sprent, 1968 infection of polar bear. Acta Zool. Pathol. Antwerp., 64:115-119.