Print ISSN: 2476-535X, Online ISSN: 2717-2910

Document Type : Original Article


1 Department of Animal Health Management School of Veterinary Medicine, Shiraz University Shiraz 71441-11731 IRAN

2 Shiraz University

3 Department of Clinical Sciences, School of Veterinary Medicine, Shiraz Universi


Toxoplasma is an intracellular parasite that can cause toxoplasmosis in humans and animals. In this disease, parasitic cysts of Toxoplasma gondii can remain present in various tissues such as the brain throughout the host's life. The potential role of toxoplasmosis should be considered in many neurological diseases with unknown mechanisms. In horses, behavioral disorders like crib-biting have multifactorial causes and may be due to neurophysiological dysfunction. This research aimed to determine the role of toxoplasmosis in the manifestation of crib-biting behavior in horses. A case-control study was conducted in horse riding clubs in Fars province, Iran, near the city of Shiraz. Ten horses with crib-biting behavior and 10 clinically healthy horses matched for sex, age, and breed were enrolled in the study. Blood samples were collected from cribbers and healthy horses, along with a thorough history-taking. An enzyme-linked immunosorbent assay (ELISA) was performed on sera to detect T. gondii-specific Immunoglobulin G (IgG) and Immunoglobulin M (IgM). The results of this study revealed that all horses were free of toxoplasmosis. It is concluded that this relation was not detected in the horse farms of Shiraz suburbs. Further research is required to explore more aspects of crib-biting behavior and its relationship with causative factors.


Main Subjects


Toxoplasmosis is a common parasitic zoonosis found in humans and other warm-blooded vertebrates worldwide. Toxoplasma gondii is a coccidian protozoan capable of infecting a wide range of hosts and host cells through various routes of transmission (1). Infection by this protozoan results in the formation of parasite cysts that persist in several tissues, primarily in neural and muscular tissues including the brain, eyes, and skeletal and cardiac muscles throughout the host's life (2, 3). T. gondii shows a strong affinity for brain tissue (4), and the fundamental aspects of the interactions between the protozoan and the host are still largely unknown (5). A chronic infection state is established by T. gondii in the brain and skeletal muscle of its mammalian host. The blood-brain barrier is crossed by Toxoplasma as either extracellular tachyzoites that infect and replicate with brain endothelial cells or within an infected monocyte, but there is limited understanding of how Toxoplasma infections of the brain and skeletal muscle and the resulting inflammation impact the tissues' function. Recent studies have revealed that both excitatory and inhibitory neurotransmission in the central nervous system are altered by Toxoplasma, and these changes lead to unbalanced synaptic activity and seizures (6). Therefore, the potential role of toxoplasmosis should be considered in many related diseases with unknown mechanisms. The behavior of hosts can be altered and the risk of predation and transmission to other hosts is potentially increased by T. gondii. In rodents, infection has been associated with a reduction in innate fear and an increase in risk-taking behavior toward cat odor. In foxes, a similar phenomenon known as Dopey Fox Syndrome has been observed, with abnormal behavior such as a lack of fear towards humans and an increase in daytime activity exhibited by infected animals. Toxoplasma infection may be a contributing factor to this behavior, potentially manipulating the host's immune system and altering neurotransmitter levels in the brain, although the precise mechanisms are not fully understood (7).

The seroprevalence of T. gondii in horses has been studied in various regions, including Iran (8), Turkey (9), Saudi Arabia (10), China (11), and Brazil (12). These studies highlight the global distribution of toxoplasmosis in equine populations. Additionally, the detection of anti-T. gondii antibodies in horses underscores the importance of understanding the prevalence and potential implications of this parasitic infection in horses. Crib biting, a stereotypic oral-based behavior characterized by grasping a solid object with incisor teeth and inhaling air with an audible grunt (13), is associated with altered neurophysiological function (14, 15) and is similarly associated with brain dysfunction in schizophrenia, autism, and caged parrots (16). While poor welfare and suboptimal management may contribute to the appearance of crib-biting, the mechanisms of formation and causes of equine stereotypy remain largely speculative (17). In a mouse model of chronic T. gondii infection, neurologic and behavioral abnormalities secondary to inflammation and loss of brain parenchyma were observed by Hermes et al. (18).

Yolken et al. (19) reported significantly increased levels of anti-Toxoplasma antibodies in individuals with a first episode of schizophrenia compared with control subjects, suggesting a possible relationship between Toxoplasma infection and the occurrence of schizophrenia. The diagnosis of toxoplasmosis is typically made by detecting immunoglobulins G and M (IgG and IgM) antibodies in serum samples using various methods, including enzyme-linked immunosorbent assay (ELISA), enzyme-linked fluorescence assay (ELFA), immunosorbent agglutination assay (ISAGA), and IgM-indirect fluorescent-antibody (IgM-IFA) (20). IgG antibodies are typically detected within one to two weeks after infection acquisition and persist throughout the host's life, while IgM antibodies may appear earlier but decline more rapidly (21). While the studies provide valuable information on crib-biting behavior and toxoplasmosis in horses, further research is needed to establish a definitive link between these two conditions. Given the worldwide importance of T. gondii and the limited data on the correlation between these parasites and certain diseases, this study aimed to examine the potential relationship between toxoplasmosis and crib-biting behavior in the horse farms of Shiraz suburbs.


Materials and Methods

This study was conducted Shiraz suburb, Fars province, south of Iran, around the city of Shiraz with the approval of the State Committee on Animal Ethics at Shiraz University, which followed the guidelines for the protection of animals used for experimental purposes outlined in the European Council Directive 86/609/EC (1986). The study included a total of ten established crib-biting horses, consisting of 7 stallions, 2 mares, and 1 gelding, of different breeds (including crossbreds, Arabian, Turkmen, and Dareshouri), ranging in age from 2 to 14 years old. The horse underwent an examination based on a history of over a year of involvement in crib-biting disease. The symptoms and signs of crib-biting disease were identified and implemented as per Omidi et al. (22). The horses were housed individually in conventional horse boxes located in various riding stables in the vicinity of Shiraz. The owners reported that the crib-biting behavior had been ongoing for at least one year, although the full history of the behavior was unclear for all of the horses. A control group of ten age- and sex-matched horses with no history of stereotypic behavior, and kept under the same housing conditions, were also included in the study. All horses, including crib biters and controls, received similar feeding, with concentrate included in their daily ration. All blood sampling was performed by a qualified veterinarian. In this study, 10 mL blood samples were collected from the jugular vein of horses, and sera were harvested by centrifuging the blood samples at 750g for 15 minutes at room temperature. The sera were stored at -80°C until tested. The anti-T. gondii-specific IgM and IgG serum antibodies were analyzed by ELISA using a commercially available kit (Euroimmun, Germany), which included standard positive and negative controls provided in the kits. Antibody levels >10 IU/mL were considered positive.


Toxoplasmosis is a parasitic infection caused by the protozoan parasite T. gondii. The infection can affect both humans and animals and is widespread globally (23). In horses, toxoplasmosis is typically a subclinical or latent infection, but it can cause severe illness in pregnant mares and young foals. While the symptoms of toxoplasmosis in horses are generally mild, the infection can have more severe effects on other species, such as cats, where it can cause severe neurological symptoms (24). Detection of T. gondii antibodies is the main way to diagnose the infection. The results of the study indicate that the serum IgG and IgM antibodies against T. gondii were at low levels in horses with crib-biting behavior, suggesting that this relation was not detected in the horse farms of Shiraz suburbs. The vital signs and characteristics of healthy horses were similar to those affected by crib-biting, and statistically, there was no difference between them. The formation of T. gondii cysts in the brain can result in various changes in brain function, including alterations in anatomy, pathology, immunity, neurotransmitters, and gene expression (25). A wide range of behavioral changes in humans and animal models have been attributed to toxoplasmosis. While some studies suggest a possible association between toxoplasmosis and certain diseases such as schizophrenia, depression, anxiety, Alzheimer's disease, Parkinson's disease, and epilepsy, the direction of causality remains controversial and requires further investigation (26). Despite some studies supporting a relationship between toxoplasmosis and behavioral changes in mice models and other animals, this study did not find any evidence of T. gondii infection in horses with crib-biting behavior. The relationship between toxoplasmosis and behavioral changes in humans and animals is a controversial area of research. Some studies have suggested a possible link between toxoplasmosis and changes in behavior. However, the direction of causality is not clear, and it is not yet clear whether toxoplasmosis is the cause of these behavioral changes or whether other factors are responsible (27). While it may seem that a sample size of ten affected horses is small for comparison, it is important to note that the disease prevalence is low. The ten horses were selected from various stables that had confirmed crib-biting behavior for at least one year, and a similar horse in terms of age, gender, and breed was chosen for comparison. Therefore, the results appear to be reliable and accurate at least for the Shiraz region. Nevertheless, crib-biting disease in horses still has an unclear etiology, and such research with various hypotheses will help us get closer to the primary causative factors of the disease. Understanding the underlying causes of stereotypic behaviors in horses is important for developing effective prevention and treatment strategies.


Ethical approval

All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.


This study was financially supported by Shiraz UniversityShiraz, Iran. The authors express their utmost gratitude to the horse owners in the vicinity of Shiraz who allowed them to examine and sample their horses.

Conflicts of interest

The authors declare that they have no conflict of interest in sampling, analyses, and interpretation of data.


  1. References

    1. Tenter AM, Heckeroth AR, Weiss LM. Toxoplasma gondii: from animals to humans. Int J Parasitol.  2000 Feb 30; (12-13): 1217-1258. (00)00124-7
    2. Dubey JP. Long-term persistence of Toxoplasma gondii in tissues of pigs inoculated with T gondii oocysts and effect of freezing on viability of tissue cysts in pork. Am J Vet Res.  1988 Jun; 49(6): 910-913.
    3. Pavesio CE, Lightman S. Toxoplasma gondii and ocular toxoplasmosis: pathogenesis. Br J Ophthalmol.  1996 Dec; 80 (12): 1099.
    4. Del Grande C, Galli L, Schiavi E, Dell’Osso L, Bruschi F. Is Toxoplasma gondii a trigger of bipolar disorder?. Pathogens, 2017 Jan; 6(1): 3.
    5. Crotta M, Limon G, Blake DP, Guitian J. Knowledge gaps in host-parasite interaction preclude accurate assessment of meat-borne exposure to Toxoplasma gondii. Int J Food Microbiol.  2017 Nov 16; 261: 95-101.
    6. Wohlfert EA, Blader IJ, Wilson EH. Brains and brawn: toxoplasma infections of the central nervous system and skeletal muscle. Trends Parasitol.  2017 Jul; 33(7): 519-531.
    7. Milne G, Fujimoto C, Bean T, Peters HJ, Hemmington M, Taylor C, et al. Infectious causation of abnormal host behavior: Toxoplasma gondii and its potential association with Dopey Fox syndrome. Front Psychiatry.  2020 Sep 16; 11: 513536.
    8. Asgari Q, Sarkari B, Amerinia M, Panahi S, Mohammadpour I, Sarvestani AS. Toxoplasma infection in farm animals: a seroepidemiological survey in fars province, south of iran. Jundishapur J Microbiol.. 2013, 6 (3): 269-272.
    9. Karatepe B, Babür C, Karatepe M, Kılıç S. Seroprevalence of toxoplasmosis in horses in niğde province of turkey. Trop Anim Health Prod. 2010 Mar; 42(3): 385-389.
    10. Alanazi AD, Alyousif MS. Prevalence of antibodies to Toxoplasma gondii in horses in riyadh province, saudi arabia. J Parasitol. 2011 Oct; 97(5): 943-945.
    11. Miao Q, Wang X, She L, Fan Y, Yuan F, Yang J, et al. Seroprevalence of Toxoplasma gondii in horses and donkeys in yunnan province, southwestern china. Parasit Vectors. 2013 Jun 6; (1) :168.
    12. Finger MA, Villalobos EM, Lara Mdo C, Cunha EM, Barros Filho IR, Deconto I, et al. Detection of anti-Toxoplasma gondii antibodies in carthorses in the metropolitan region of Curitiba, Paraná, Brazil. Rev Bras Parasitol Vet. 2013 Jan-Mar; 22(1):179-81.
    13. Parker M, McBride SD, Redhead ES, Goodwin D. Differential place and response learning in horses displaying an oral stereotypy. Behav Brain Res. 2009 Jun 8; 200(1): 100-105.
    14. Hemmings A, McBride SD, Hale CE. Perseverative responding and the aetiology of equine oral stereotypy. Appl Anim Behav Sci. 2007 Apr; 104(1-2): 143-150.
    15. Omidi A, Vakili S, Nazifi S, Parker MO. Acute-phase proteins, oxidative stress, and antioxidant defense in crib-biting horses. J Vet Behav, 2017 Sep 6; 20: 31-36.
    16. Garner JP, Meehan CL, Mench JA. Stereotypies in caged parrots, schizophrenia and autism: evidence for a common mechanism. Behav Brain Res. 2003 Oct 17; 145 (1-2): 125-134. (03)00115-3
    17. Sarrafchi A, Blokhuis HJ. Equine stereotypic behaviors: Causation, occurrence, and prevention. J Vet Behav. 2013 Sep; 8(5): 386-394.
    18. Hermes G, Ajioka JW, Kelly KA, Mui E, Roberts F, Kasza K., et al. Neurological and behavioral abnormalities, ventricular dilatation, altered cellular functions, inflammation, and neuronal injury in brains of mice due to common, persistent, parasitic infection. J Neuroinflammation. 2008 Oct 23; 5: 1-37.
    19. Yolken RH, Bachmann S, Rouslanova I, Lillehoj E, Ford G, Torrey EF, et al. Antibodies to Toxoplasma gondii in individuals with first-episode schizophrenia. Clin Infect Dis. 2001 Mar 1; 32(5): 842-844.
    20. Gharavi MJ, Jalali S, Khademvatan S, Heydari S. Detection of IgM and IgG anti-Toxoplasma antibodies in renal transplant recipients using ELFA, ELISA and ISAGA methods: comparison of pre-and post-transplantation status. Ann Trop Med Parasitol. 2011 Jul; 105(5): 367-371.
    21. Montoya JG. Laboratory diagnosis of Toxoplasma gondii infection and toxoplasmosis. J Infect Dis. 2002 Feb 15; 185(1): S73-S82.
    22. Omidi A, Jafari R, Nazifi S, Parker MO. Potential role for selenium in the pathophysiology of crib-biting behavior in horses. J vet behav, 2018; 23: 10-14.
    23. Al-Malki ES. Toxoplasmosis: stages of the protozoan life cycle and risk assessment in humans and animals for an enhanced awareness and an improved socio-economic status. Saudi J Biol Sci. 2021 Jan; 28(1): 962-969.
    24. Tassi P. Toxoplasma gondii infection in horses. A review. Parassitologia. 2007 Jun; 49 (1-2): 7-15.
    25. Nayeri T, Sarvi S, Daryani A. Toxoplasmosis: targeting neurotransmitter systems in psychiatric disorders. Metab Brain Dis. 2022 Jan; 37 (1): 123-146.
    26. Daher D, Shaghlil A, Sobh E, Hamie  M, Hassan  ME, Moumneh  MB, et al.  Comprehensive overview of Toxoplasma gondii -induced and associated diseases. Pathogens. 2021 Oct 20; 10(11): 1351.

    27. Bay-Richter C., Petersen E., Liebenberg N., Elfving B., Wegener G. Latent toxoplasmosis aggravates anxiety and depressive-like behaviour and suggest a role of gene-environment interactions in the behavioural response to the parasite. Behav Brain Res. 2019 May 17; 364: 133-139.