The relationship between the parasitic infection, Toxoplasma gondii and psychiatric disorders
There have been significant efforts at identifying causes of mental health disorders such as depression, anxiety, bipolar disorder, and schizophrenia. Most causes are often related to biological/genetic and/or psychological factors. However, there is increasing research to suggest that exposure to a common parasite may also be a potential factor to consider.
Toxoplasma gondii (T. gondii) is a single-celled parasite capable of infecting warm-blooded animals (including humans). In humans, T. gondii is one of the most common parasites in developed countries with serological studies demonstrating 30–50% of the global population has been exposed to and may be chronically infected with T. gondii [1].
Figure reference: Esch and Petersen [2]
The main host in Australia and developed countries is the domestic cat, which becomes infected mainly through eating small infected mammals, including mice and birds. The parasite can sometimes move from cats to humans, through contact with cat faeces (e.g., litter boxes and in the garden). Other sources of exposure include[3]:
Consuming raw or undercooked meat containing T. gondii tissue cysts (particularly undercooked pork)
Ingesting contaminated water, soil, vegetables, or anything contaminated with oocysts shed in the faeces of an infected animal.
Blood transfusion or organ transplant
Transplacental transmission from mother to foetus, particularly when T. gondii is contracted during pregnancy
Drinking unpasteurised goat milk
The T. gondii parasite can persist for long periods of time in the body of humans possibly even for a lifetime. Most people who are infected with T. gondii develop very few symptoms as their immune system can keep the parasite from causing illness. However, susceptible individuals include pregnant women and people with a compromised immune system. In such individuals, T. gondii infection can cause serious health problems. Some symptoms include:
Swollen lymph glands, especially around the neck
Muscle aches and pains
Headaches
Fever
A general feeling of being unwell
Inflammation of the lungs
Inflammation of the heart muscle
Inflammation of the eye, for example, the retina (at the back of the eye)
Behavioural or personality changes
Central nervous system disease, such as encephalitis (brain inflammation).
It has been confirmed via several epidemiological studies and meta-analyses that there are increased odds of T. gondii exposure in individuals with a psychiatric illness. For example, increased exposure rates have been identified in [4-6]:
Bipolar disorder – people with T. gondii exposure have a 2 to 4 times greater prevalence of bipolar disorder
Recent onset psychosis
Self-directed violence
Suicidal attempts
Generalised anxiety disorder
Obsessive-compulsive disorder
Autism
Mixed anxiety and depressive disorder
Depression during pregnancy
It is important to note that these studies confirm an association of the above disorders with T. gondii. However, this does not mean that T. gondii is a cause of these disorders. More research is required to determine the reasons for this high association.
How to test for T. gondii
Detection of T. gondii is often undertaken by measuring for anti-toxoplasma antibodies in the blood. However, these tests only provide a marker of past exposure. According to Xiao et al [4], as T. gondii persists largely in the form of tissue cysts in the brain, it is often not possible to directly detect organisms in infected individuals. They reported that standard tests and assay methods may substantially underestimate the prevalence of T. gondii infections.
How does T. gondii affect mental health?
According to researchers [4,6], there are several ways T. gondii exposure can potentially contribute to psychiatric illnesses. These include:
Increasing brain inflammation and damage to certain parts of the brain associated with mood, behaviour, and personality
Reducing serotonin levels. Serotonin is a neurotransmitter implicated in many mental health disorders including depression and anxiety
Increasing glutamate activity. Glutamate is an excitatory neurotransmitter and is believed to be associated with mental health problems such as bipolar disorder, depression, and schizophrenia
Affecting dopamine concentrations. This neurotransmitter is associated with most mental health problems. Excess dopamine is implicated with some mental health problems (e.g., bipolar disorder) while reduced levels are associated with other disorders (e.g., depression)
Increasing neurotoxic metabolites, eventually leading to neurodegeneration
Altering gut bacteria (intestinal microbiome). There is a strong bi-directional link between the gut and the brain
How to prevent T. gondii exposure
According to the American Center for Disease Prevention[3] T. gondii exposure can be prevented/minimised by:
Cooking food to safe temperatures
Peeling or washing fruits and vegetables thoroughly before eating
Not eating raw or undercooked oysters, mussels, or clams (these may be contaminated with T. gondii that has washed into seawater).
Not drinking unpasteurised goat’s milk
Washing cutting boards, dishes, counters, utensils, and hands with soapy water after contact with raw meat, poultry, seafood, or unwashed fruits or vegetables
Wearing gloves when gardening and during any contact with soil or sand because it might be contaminated with cat faeces that contain T. gondii
Ensuring that the cat litter box is changed daily
In addition, it is important to boost your immune system by eating a healthy, nutritious diet, optimising body nutrient levels, getting good sleep, reducing or managing stress, and engaging in regular exercise.
Treatment for toxoplasmosis
Sulfonamides and pyrimethamine (Daraprim) are two drugs widely used to treat toxoplasmosis in humans. In addition to this, it is important to take steps to boost your immune system (as detailed previously).
Interestingly, there is some research to suggest that medications used to treat conditions such as bipolar disorder and psychosis have anti-parasitic effects. These include some mood stabilisers and anti-psychotic medications such as fluphenazine, zuclopenthixol, haloperidol, valproic acid, and risperidone [7].
Natural medicinal herbs have also been used to target T. gondii such as Artemisia annua (sweet wormwood), Glycyrrhiza glabra (Liquorice), and curcumin extracts [8,9].
References
Hill D, and Dubey JP. Toxoplasma gondii: transmission, diagnosis and prevention. Clin Microbiol Infect. 2002 Oct;8(10):634-40. link to article
Esch KJ and Petersen CA. Transmission and epidemiology of zoonotic protozoal diseases of companion animals. Clin Microbiol Rev. 2013 Jan;26(1):58-85. link to article
Parasites – Toxoplasmosis (Toxoplasma infection) (Center for Disease Prevention) link to page
Xiao J, et al. Toxoplasma gondii: Biological Parameters of the Connection to Schizophrenia. Schizophr Bull. 2018 Aug 20;44(5):983-992. link to article
de Barros J, et al. Is there any association between Toxoplasma gondii infection and bipolar disorder? A systematic review and meta-analysis. J Affect Disord. 2017 Feb;209:59-65. link to article
Del Grande, et al. Is Toxoplasma gondii a trigger of bipolar disorder? Pathogens. 2017 Jan 10;6(1). pii: E3. link to article
Fond G, et a. Comparative analysis of anti-toxoplasmic activity of antipsychotic drugs and valproate. Eur Arch Psychiatry Clin Neurosci. 2014 Mar;264(2):179-83. link to article
Sharif M, et al. The efficacy of herbal medicines against Toxoplasma gondii during the last 3 decades: a systematic review. Can J Physiol Pharmacol. 2016 Dec;94(12):1237-1248. link to article
Al Nasr I, et al. Toxoplasmosis and anti-Toxoplasma effects of medicinal plant extracts-A mini-review. Asian Pac J Trop Med. 2016 Aug;9(8):730-4. Link to article
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