SciELO - Scientific Electronic Library Online

Home Pagealphabetic serial listing  

Services on Demand




Related links


Revista do Instituto de Medicina Tropical de São Paulo

On-line version ISSN 1678-9946

Rev. Inst. Med. trop. S. Paulo vol.56 no.4 São Paulo July/Aug. 2014 


BEHAVIORAL AND MEMORY CHANGES IN Mus musculus COINFECTED BY Toxocara canis AND Toxoplasma gondii

Alterações comportamentais e na memória de Mus musculus coinfectado por Toxocara canis e Toxoplasma gondii

Flávia Motta Corrêa(1) 

Pedro Paulo Chieffi(1)  (2) 

Susana A. Zevallos Lescano(1)  (2) 

Sergio Vieira dos Santos(1)  (2) 

(1)Faculdade de Ciências Médicas da Santa Casa de São Paulo. São Paulo, SP, Brazil. E-mail:

(2)Instituto de Medicina Tropical de São Paulo (LIM-06). São Paulo, SP, Brazil


Several researchers have stated that parasites can alter the behavior of their hosts, in order to increase the transmission rate, principally when prey-predator relationships are a reliable way of infection transmission. The aim of this study was to verify the occurrence of changes in anxiety and short-term memory patterns in experimentally infected Mus musculus by Toxocara canis and/or Toxoplasma gondii. Forty male Mus musculus (Balb/c) eight-week-old were divided into four groups of 10 mice each. One group was infected with 300 eggs of Toxocara canis; a second group was submitted to infection with 10 cysts of Toxoplasma gondii; a third group was concomitantly infected with both parasites with the same inoculums and the last group was maintained without infection. The anxiety levels were evaluated using an elevated plus maze and an actometer; the short-term memory was determined by a two-way active avoidance equipment. The determination of anxiety levels were conducted 40 and 70 days after infection and the short-term memory was evaluated 140 days after infection. Mice chronically infected by Toxoplasma gondii showed impaired learning and short-term memory, but no significant differences were found in mice infected by Toxocara canis or concomitantly infected by Toxocara canis and Toxoplasma gondii when compared to non infected mice.

Key words: Mus musculus ; Toxocara canis ; Toxoplasma gondii ; Concomitant infections; Behavior alterations


Pesquisadores afirmam que parasitos podem alterar o comportamento de seus hospedeiros a fim de aumentar a sua taxa de transmissão. O objetivo deste estudo foi verificar a ocorrência de alterações na ansiedade e padrões de memória de curta duração em Mus musculus experimentalmente infectados por Toxocara canis e/ou Toxoplasma gondii. Utilizaram-se 40 camundongos da espécie Mus musculus machos (Balb/c) com oito semanas de idade, divididos em quatro grupos de 10 ratos cada. Um grupo foi infectado com 300 ovos de Toxocara canis, um segundo grupo foi submetido à infecção com 10 cistos de T. gondii, um terceiro grupo foi infectado concomitantemente com ambos os parasitas e o último grupo foi mantido sem infecção. Os níveis de ansiedade foram avaliados por meio de labirinto em cruz elevado e actômetro, a memória de curta duração foi determinada por esquiva aversiva. A determinação dos níveis de ansiedade foi realizada 40 e 70 dias após infecção e a memória de curto prazo foi avaliada 140 dias após a infecção. Camundongos cronicamente infectados por Toxoplasma gondii mostraram deficiência de aprendizagem e memória de curto prazo, mas não foram encontradas diferenças significantes em camundongos infectados por Toxocara canis ou concomitantemente infectados por Toxocara canis e Toxoplasma gondii quando comparados com camundongos não infectados.


According to the manipulation hypothesis, a parasite can alter the behavior of their hosts specifically to increase the transmission. This hypothesis requires that the change of behavior is a sophisticated product resulting in the parasite manipulation of the host, rather than a by-product of other physiological activities of the parasite3,32.

There are many examples in the literature of behavioral changes in insects, crustaceans, and fish acting as intermediate hosts in the life cycle of several species of parasites; however, little is known about behavioral changes in mammals13,5,21,36.

Some studies show evidence of behavioral changes in rodents in experimental protocols with single infections; however, in natural conditions, the occurrence of co-infection or multiple parasite infections of the same host should be common4,8,26,33,35,40,42.

Toxoplasma gondii is a protozoan parasite, whose definitive hosts are felines, but other warm-blooded vertebrates may act as intermediate hosts. Toxocara canis is a nematode parasite of dogs that eventually infects small mammals, which do not reach maturity. These mammals act as paratenic hosts, because they maintain the larvae in their organs for a long time and have an important role in the transmission of the parasite through the predator-prey relationship. Both parasites can cross the blood-brain barrier, settling in areas of the central nervous system of rodents related to the control of anxiety and locomotion9,16,22.

QUEIROZ et al.33 investigated the behavior of Rattus norvegicus infected by Toxocara canis and/or Toxoplasma gondii, and concluded that both parasites influenced rodent behavior. However, when the rats were concomitantly infected, a behavior modulation was observed, resulting in slight absence of behavioral alterations.

Due to the high frequency of Mus musculus infection by both parasites in natural conditions and the importance of these rodents as paratenic hosts, the aim of the present study was to verify the occurrence of changes in anxiety and short-term memory patterns in experimentally infected Mus musculus by Toxocara canis and/or Toxoplasma gondii.


Forty male Mus musculus (Balb/c) eight-week-old were obtained from the Central Animal Laboratory of the Faculty of Medicine, University of São Paulo. The eggs of T. canis were obtained by dissecting female worms, recovered from naturally infected dogs captured by the Center for Zoonosis Control of Guarulhos (CCZ/GRU). The cysts of T. gondii (ME49 strain cystogenic referring to genotype II) were provided by the Laboratory of Protozoology of the Institute of Tropical Medicine of São Paulo.

The mice were divided into four groups, namely, Toxocara: 10 mice infected with 300 eggs of Toxocara canis, Toxoplasma: 10 mice infected with 10 cysts of Toxoplasma gondii; Concomitant infection: 10 mice infected with 300 eggs of T. canis and 10 cysts of T. gondii, and Control: 10 mice without infection.

The behavior of the mice was assessed by testing in the elevated plus maze, to determine the levels of anxiety, using the technique described by PELLOW & FILE30. The motor activity, another behavioral parameter, was measured using the technique of determination of motor activity in the open field, using an Actometer, as described by SILVA et al.34, NASELLO et al.29 and GUARALDO et al.18.

The performance evaluation was conducted on two occasions: 40 and 70 days after infection.

To assess learning and memory consolidation, a two-way active avoidance equipment (Ugo Basile, Comerio, Italy) was used on two occasions, as described by KORTE & DE BOER28, 140 days after infection.

At the end of the experiment, all the rats were euthanized and the carcasses were submitted to digestion with HCl 0.5% (XI & JIN, 1998)44 and the central nervous system was macerated in saline solution 0.9% for the recovery of Toxocara canis larvae and Toxoplasma gondii cysts, respectively.

The data were expressed as the mean ± standard deviation. Statistical comparisons were performed using the Two-way ANOVA for the behavioral variables and the t-Student test for the evaluation of short-term memory. Only probability values which were (p) smaller than 0.05 were considered as statistically significant.

All care procedures were performed strictly according to the guidelines for animal experimentation, as stipulated in the Guide for the Care and Use of Laboratory Animals (National Institute of Health Publication Number 86–23, Bethesda, MD). The experimental protocol was approved by the Research Ethics Committee on Animal Experiments of the São Paulo Institute of Tropical Medicine (process no. 2011/098).


The analysis of the variables in the elevated plus maze showed a significant difference (p < 0.05) between the group infected by Toxoplasma gondii and the control group at 40 dpi; on the other hand, the group with concomitant infections did not show any significant difference in comparison to the non-infected control group (Fig. 1).

Fig. 1 - Frequency of entries into the open arms of the Elevated Plus Maze Mus musculus infected with Toxocara canis and/or Toxoplasma gondii at 40 dpi. *p < 0.05 related to the control. 

On evaluation of the frequency of entries into the closed arms of the elevated plus maze, there was a significant difference (p < 0.05) between the groups infected with T. canis and T. gondii, but not with the group with concomitant infections, in comparison to the control group (Fig. 2).

Fig. 2 - Frequency of entries into the closed arms of the Elevated Plus Maze Mus musculus infected with Toxocara canis and/or Toxoplasma gondii at 40 dpi. *p < 0.05 related to the control. 

The same variables observed in an Elevated Plus Maze at 70 days post-infection showed no significant difference among all the groups.

In aversive avoidance, there was no significant difference between the groups, however, only the group infected with T. gondii showed a difference between the first and second test (Fig. 3).

Fig. 3 - Time difference between training and test Mus musculus infected with Toxocara canis and/or Toxoplasma gondii in aversive avoidance test. * = difference between training and control test, T. canis, T. gondii and concomitant groups. p < 0.05. 

All mice of the infected groups showed, at least, a Toxocara canis larvae and/or Toxoplasma gondii cyst into the brain.


Since the 70's, interest from researchers has been increasing on the behavioral changes shown by infected rodents4,8,15,19,25,27.

Several studies show changes in the behavior of rodents infected with Toxocara canis and Toxoplasma gondii. These changes can probably be considered as a means of facilitating the transmission of both parasites to their paratenic hosts by behavioral manipulation4,7,10,1214,20,39.

Mus musculus plays an important role in the life cycle of Toxoplasma gondii and Toxocara canis, because it can harbor cysts and larvae, respectively, in the muscles and other organs, like the central nervous system, for a long time. Moreover, these rodents are part of the food chain of definitive hosts of both parasites23,41 and can be transmitted to their definitive host through prey-predator relationships.

COX & HOLLAND11 suggest that the decreased levels of aggression linked to decreased levels of anxiety and lack of inhibition to open environments increases the risk of predation of these rodents and, consequently, could facilitate the transmission of parasites through a prey-predator relationship, a possible way of Toxocara canis and Toxoplasma gondii transmission, respectively, to dogs and cats.

In the present study, mice infected with T. gondii evaluated in the elevated plus maze were less anxious due to higher input frequency in the open arms. However, the same was neither observed in the group infected with T. canis, nor in the group concomitantly infected with both parasites. One hypothesis regarding the lack of significance in the behavioral data of the group infected with T. canis is the quantity of eggs used for the mice infection. However, QUEIROZ et al.33 had already found similar results in Rattus norvegicus concomitantly infected by Toxocara canis and Toxoplasma gondii, suggesting occurrence of a modulation in behavioral changes when rats were concomitantly infected. On the other hand, COX & HOLLAND12,13 found that mice with large amount of T. canis larvae in the brain (inoculums of 3,000, embryonated) present increased behavioral alterations than mice infected by smaller quantities of larvae.

Infection with T. canis and T. gondii can also influence the memory in rodents24. Several surveys conducted on rodents report the presence of T. canis larvae in the telencephalon and cerebellum, and cysts of T. gondii distributed in various brain regions, but with a higher incidence in the region of the amygdale, areas related to learning, memory, coordination and control of voluntary movements17,35,38.

In this study, no significant differences were observed in the groups infected with T. canis and with concomitant infection when the short-term memory was evaluated by aversive avoidance. However, the animals infected with T. gondii showed a significant difference in this test. These results support the hypothesis that animals chronically infected with T. gondii have impaired learning and memory31,43 and reinforces the hypothesis of QUEIROZ et al.33 concerning the occurrence of modulation in the behavioral response when rodents were co-infected by both parasites.


The authors declare that there are no conflicts of interest.


Funding for this study was provided by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).


1. Adamo SA, Shocmaker KL. The effects of parasitism on the octopaminergic system of Manduca sexta: a possible mechanism underlying host behavioral change. Can J Zool. 2000;78:1-8. [ Links ]

2. Adamo, SA. Modulating the modulators: parasites, neuromodulators and host behavioral change. Brain Bahav Evol. 2002;60:370-7. [ Links ]

3. Barnard CJ. Parasitic relationships. In: Barnard CJ, Behnke JM, editors. Parasitism and host behaviour. London: Taylor and Francis; 1990. p. 1-33. [ Links ]

4. Berdoy M, Webster JP, Macdonald DW. Fatal attraction in rats infected with Toxoplasma gondii. Proc Biol Sci. 2000;267:1591-4. [ Links ]

5. Biron DG, Maché L, Ponton F, Loxdale HD, Galéotti N, Renault L, et al. Behavioural manipulation in a grasshopper harbouring hairworm: a proteomics approach. Proc Biol Sci. 2005;272:2117-26. [ Links ]

6. Burren CH. The distribution of Toxocara canis larvae in the central nervous system of the mouse. Trans R Soc Trop Med Hyg. 1971;65:450-3. [ Links ]

7. Burright RG, Donovick PJ, Dolinsky Z, Hurd Y. Behavioral changes in mice infected with Toxocara canis. J Toxicol Environ Health. 1982;10:621-6. [ Links ]

8. Chieffi PP, Aquino RTR, Paschoalotti MA, Ribeiro MC, Nasello AG. Muscular strength decrease in Rattus norvegicus experimentally infected by Toxocara canis. Rev Inst Med Trop São Paulo. 2009;51:73-5. [ Links ]

9. Chieffi PP, Aquino RTR, Paschoalotti MA, Ribeiro MCSA, Nasello AG. Behavioral change in Rattus norvegicus experimentally infected by Toxocara canis larvae. Rev Inst Med Trop São Paulo. 2010;52:243-6. [ Links ]

10. Chieffi PP, Santos SV, Queiroz ML, Lescano, SAZ. Human toxocariasis: contribution by Brazilian researchers. Rev Inst Med Trop São Paulo 2009;51:301-8. [ Links ]

11. Cox DM, Holland CV. Relationship between three intensity levels of Toxocara canis larvae in the brain effects on exploration, anxiety, learning and memory in the murine host. J. Helminthol. 2001;75:33-41. [ Links ]

12. Cox DM, Holland CV. The influence of mouse strain, infective dose and larval burden in the brain on activity in Toxocara-infected mice. J Helminthol. 2001;75:23-32. [ Links ]

13. Cox DM, Holland CV. The relationship between numbers of larvae recovered from the brain of Toxocara canis-infected mice and social behaviour and anxiety in the host. Parasitology. 1998;116:579-94. [ Links ]

14. Dolinsky ZS, Burright RG, Donovick PJ, Glickman LT, Babish J, Summer B, et al. Behavioral effects of lead and Toxocara canis in mice. Science. 1981;213(4512):1142-4. [ Links ]

15. Donovick PJ, Burright RG. The consequences of parasitic infection for the behavior of the mammalian host. Environ Health Perspect. 1987;73:247-50. [ Links ]

16. Dubey JP. Toxoplasmosis: a waterborne zoonosis. Vet Parasitol. 2004;126:57-72. [ Links ]

17. Good B, Holland CV, Stafford P. The influence of inoculum size and time post-infection on the number and position of Toxocara canis larvae recovered from the brains of outbred CD1 mice. J Helminthol. 2001;75:175-81. [ Links ]

18. Guaraldo L, Chagas DA, Konno AC, Kom GP, Pfiffer T, Nasello AG. Hydroalcoholic extract and fractions of Davilla rugosa Poiret: effects on spontaneous motor activity and elevated plus-maze behavior. J Ethnopharmacol. 2000;72:61-7. [ Links ]

19. Havlícek J, Gasová ZG, Smith AP, Zvára K, Flegr J. Decrease of psychomotor performance in subjects with latent “asymptomatic” toxoplasmosis. Parasitology. 2001;122:515-20. [ Links ]

20. Hay J, Arnott MA, Aitken PP, Kendall AT. Experimental toxocariasis and hyperactivity in mice. Z Parasitenkd. 1986;72:115-20. [ Links ]

21. Helluy S, Holmes JC. Serotonin, octopamine and the clinging behavior induced by the parasite Polymorphus paradoxus (Acanthocephala) in Gammarus lacustris (Crustacea). Can J Zool. 1990;68:1214-20. [ Links ]

22. Henriquez SA, Brett R, Alexander J, Pratt J, Roberts CW. Neuropsychiatric disease and Toxoplasma gondii infection. Neuroimmunomodulation. 2009;16:122-33. [ Links ]

23. Holland CV, Cox DM. Toxocara in the mouse: a model for parasite-altered host behaviour? J Helminthol. 2001;75:125-35. [ Links ]

24. Holland CV, Hamilton CM. The significance of cerebral toxocariasis: a model system for exploting the link between brain involvement, behaviour and the immune response. J Exp Biol. 2013; 216(PT 1):78-83. [ Links ]

25. Holmes JC, Bethel WM. Modification of intermediate host behaviour by parasites. In: Canning EU, Wright CA, editors. Behavioural aspects of parasite transmission. London: Academic Press; 1972. p. 123-49. [ Links ]

26. House PK, Vyas A, Sapolsky R. Predator cat odors activate sexual arousal pathways in brains of Toxoplasma gondii infected rats. PLoS ONE. 2011;6:e23277. doi:10.1371/journal.pone.0023277. [ Links ]

27. Keymer AE, Read AF. Behavioral ecology: the impact of parasitism. In: Toft CA, Aeschlimann A, Bolis L, editors. Parasite-host associations: coexistence or conflict? Oxford: Oxford University Press; 1991. p. 37-61. [ Links ]

28. Korte SM, De Boer SF. A robust animal model of state anxiety: fear-potentiated behaviour in the elevated plus-maze. Eur J Pharmacol. 2003;463:163-75. [ Links ]

29. Nasello AG, Machado C, Bastos JF, Felício LF. Sudden darkness induces a high activity-low anxiety state in male and female rats. Phisiol Behav. 1998;63:451-4. [ Links ]

30. Pellow S, File SE. Anxiolytic and anxiogenic drug effects on exploratory activity in elevated plus-maze: a novel test of anxiety in the rat. Pharmacol Biochem Behav. 1986;24:525-9. [ Links ]

31. Piekarski G, Zippelius HM, Witting PA. Auswirkungen einer letenten Toxoplasma infection auf das Lernvermogen von weiben Laboratoriumsratten und mausen. Z Parasitenkd. 1978;57:1-15. [ Links ]

32. Poulin R. The evolution of parasite manipulation of host behaviour: a theoretical analysis. Parasitology. 1994;109(Suppl):S109-18. [ Links ]

33. Queiroz ML, Viel, TA, Papa CHG, Lescano SAZ, Chieffi PP. Behavioral changes in Rattus norvegicus coinfected by Toxocara canis and Toxoplasma gondii. Rev Inst Med Trop São Paulo. 2013;55:51-3. [ Links ]

34. Silva MRP, Bernardi MM, Nasello AG, Felício LF. Influence of lactation on motor activity and elevated plus-maze behavior. Braz J Med Bio Res. 1997;30:241-4. [ Links ]

35. Skallová A, Kodym P, Frynta D, Flegr J. The role of dopamine in Toxoplasma-induced behavioural alterations in mice: an ethological and ethopharmacological study. Parasitology. 2006;133:525-35. [ Links ]

36. Thomas F, Schimidt-Rhaesa A, Martin G, Manu C, Durand P, Renaud F. Do hairworms (Nematomorpha) manipulate the water seeking behaviour of their terrestrial host? J Evol Biol. 2002;15:356-61. [ Links ]

37. Thomas F, Ulitsky P, Augier R, Dusticier N, Samuel D, Strambi C, et al. Biochemical and histological changes in the brain of the criket Nemobius sylvestris infected by the manipulative parasite Paragordius tricuspidatus (Nematomorpha). Int J Parasitol. 2003;33:435-43. [ Links ]

38. Vyas A, Pillai AG, Chattarji S. Recovery after chronic stress fails to reverse amygdaloid neuronal hypertrophy and enhanced anxiety-like behavior. Neuroscience. 2004;128:667-73. [ Links ]

39. Vyas A, Kim SK, Giacomini N, Boothroyd JC, Sapolsky RM. Behavioral changes induced by Toxoplasma infection of rodents are highly specific to aversion of cat odors. Proc Natl Acad Sci USA. 2007;104:6442-7. [ Links ]

40. Vyas A, Sapolsky R. Manipulation of host behaviour by Toxoplasma gondii: what is the minimum a proposed proximate mechanism should explain? Folia Parasitol (Praha). 2010;57:88-94. [ Links ]

41. Webster JP. Rats, cats, people and parasites: the impact of latent toxoplasmosis on behaviour. Microbes Infect. 2001;3:1037-45. [ Links ]

42. Webster JP. The effect of Toxoplasma gondii on animal behavior: playing cat and mouse. Schizophr Bull. 2007;33:752-6. [ Links ]

43. Witting PA. Learning capacity and memory of normal and Toxoplasma-infected laboratory rats and mice. Z Parasitenkd. 1979;61:29-51. [ Links ]

44. Xi WG, Jin LZ. A novel method for the recovery of Toxocara canis in mice. J Helminthol. 1998;72:183-4. [ Links ]

Received: November 27, 2013; Accepted: March 14, 2014

Correspondence to: Sergio Vieira dos Santos. E-mail:

Creative Commons License This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.