Monday, April 20, 2020


Parasitic Infections of the Brain 
By Eleyna Poteet
Recent studies have suggested that parasites make up over 50% of animal life on Earth, and are therefore very important to understand. Some parasites cause infections in the brain, and can even modify the behaviors of the host. So, what are some neurological diseases that can be caused by parasites? 

Neurocysticercosis 
         Neurocysticercosis is a common parasitic disease that occurs when infected with the encysted larval stage of the helminth parasite Taenia solium (Loker and Hofkin 2015). These parasites can cause infection through the consumption of undercooked pork or water that is contaminated with the parasitic eggs (Siddiqua and Habeeb 2020). Taenia solium has two infectious forms: the adult (intestinal tapeworm) form, and the larval (cysticercus) form. Infection with the adult tapeworms causes minor symptoms and is referred to as Taeniasis. However, infection with T. solium larvae is more serious because the cysts are often formed in the central nervous system, causing neurocysticercosis. The symptoms of neurocysticercosis include headache, dizziness, vision problems, dementia, and seizures (Siddiqua and Habeeb 2020, Loker and Hofkin 2015). Neurocysticercosis is also the leading cause of acquired epilepsy in the world, and seizures occur in 50%-80% of patients that develop cysts of the brain (Loker and Hofkin 2015, Siddiqua and Habeeb 2020). It is a leading health problem in many areas of the world, including India, Latin America, and Asia. It is becoming more prevalent in other areas as well due to travel and migration (Siddiqua and Habeeb 2020).
How is Neurocysticercosis transmitted? 
Taenia solium has an indirect lifecycle, meaning that it requires more than one host. The intermediate hosts, in which the parasite develops somewhat but does not sexually reproduce, are pigs. The definitive hosts, in which the parasite fully develops and usually sexually reproduces, are humans (Siddiqua and Habeeb 2020). Eggs are consumed by pigs fecal-orally who come into contact with infected feces; this is common because the eggs can survive for long periods of time in the environment (“Taeniasis”). The eggs, or oncospheres, hatch in the animal’s intestine and invade the intestinal wall. They then migrate to the muscle tissue and develop into cysticerci (sac-like stage of the larva). Humans become infected by consuming undercooked meat containing these cysticerci. At this stage, the parasite may either develop into an adult tapeworm (only in humans), or lodge itself along the central nervous system, causing neurocysticercosis (Loker and Hofkin 2015, Siddiqua and Habeeb 2020, “Taeniasis”). 

Treatments for Neurocysticercosis 
Neurocysticercosis, like many other parasitic diseases, is treated with multiple pharmacological methods. Antiparasitic drugs like Niclosamide and Praziquantel are used to try to kill any adult T. solium tapeworms. Niclosamide is often preferred because it does not provoke any brain cysts that are not yet showing symptoms, and can therefore help prevent further symptoms worsened by treatment. Symptoms of neurocysticercosis can be treated using steroids, along with antiepileptic drugs if seizures are occurring (Loker and Hofkin 2015).


Toxoplasmosis 
Toxoplasmosis is a common disease that is the result of infection by the protozoan parasite Toxoplasma gondii. Infection occurs by consumption of undercooked meat containing cysts, exposure to infected cat feces, or sometimes from mother to child during pregnancy (”Toxoplasmosis”). Healthy individuals with strong immune systems usually experience mild flu-like symptoms, including body aches, fever, headache, and fatigue (“Parasites”, ”Toxoplasmosis”). However, immuno-compromised individuals may have more severe symptoms, including headache, confusion, poor coordination, seizures, blurred vision, and lung problems (”Toxoplasmosis”). Some studies also suggest that Toxoplasmosis could even potentially have effects on human behavior, personality, and psychomotor performance (Flegyr 2007). It is thought to infect 40 million Americans at any given time, due to the fact that many people experience little to no symptoms unless they have weakened immune systems (“Parasites”).  
How is Toxoplasmosis transmitted? 
Toxoplasma gondii is an obligate intracellular parasite that is transmitted vertically through contaminated food or water (Mendez and Koshy 2017). The definitive hosts, in which the parasite can reproduce and complete its life cycle, are cats; T. gondii can also infect most warm-blooded animals, like rodents, birds, and humans, and use them as intermediate hosts (Mendez and Koshy 2017, “Parasites”). Cysts are shed through the feces of cats and are passed on to other species upon ingestion of water, soil or plants contaminated with the infectious oocysts. The oocysts (life stage 1) will transform into their tachyzoite stage (life stage 2) shortly after being ingested and migrate to different parts of the hosts body, including muscle tissue and the central nervous system. After lodging in the hosts tissues, the tachyzoites will develop into cyst bradyzoites (life stage 3) that can once again infect other organisms if the host animal is eaten. If a human becomes infected, the parasite forms tissue cysts in the skeletal muscle, cardiac muscle, eyes, and brain. (Mendez and Koshy 2017, “Parasites”, ”Toxoplasmosis”).  
 

Does Toxoplasmosis cause mind control in rodents? 
Rodents can become infected with T. gondii upon exposure to cat feces, and the parasite can migrate throughout the rodents’ body and into the brain. T. gondii needs to end up back inside of a cat to complete its lifecycle, and therefore alters the brain chemistry of the rodent to achieve its goal (Vyas et al. 2007). Typically, rodents are repelled by the smell of cat urine and will steer clear at any sign of it; however, T. gondii blocks the rat’s aversion to cat urine, increasing the likelihood that the rat will become prey and that the parasite can reproduce within the bowel of the cat (Vyas et al. 2007).  

Primary Amebic Meningoencephalitis: The Brain Eating Amoeba
Perhaps one of the most well-known parasites is Naegleria fowleri, the brain eating amoeba. This single-celled parasite is free-living and is also microscopic, and can be found in warm freshwater (rivers, lakes, and hot springs) (“Naegleria”, Loker and Hofkin 2015). This parasite gained its infamous name of the brain eating amoeba because it consumes the nervous tissue of infected individuals, causing primary amebic meningoencephalitis. (Loker and Hofkin 2015). To become infected, an individual has to come into contact with the parasite through the nasal passage, usually by sniffing contaminated water. The parasite then migrates through the nasal passage and gains access to the central nervous system (Loker and Hofkin 2015). While infection of N. fowleri is rare, there have been a handful of high profile cases in news headlines over the past decade (Grace et al. 2015, “Naegleria”). The CDC reports 145 known cases in the United States between 1962 and 2018, with at least one known case in 2019. All but four of these cases were fatal (“Naegleria”).




Treatment and Survival
Primary amebic meningoencephalitis is difficult to diagnose for multiple reasons. The symptoms of infection, including headache, fever, nausea, vomiting, seizure, and hallucinations, are similar to symptoms of bacterial meningitis (“Naegleria”). The disease also progresses rapidly, with death occurring between 1-18 days after symptoms begin. For these reasons, diagnosis of infection is usually made after death occurs (Grace et al. 2015, “Naegleria”). Infected individuals who survived were diagnosed promptly and treated with miltefosine, a broad-spectrum antimicrobial, along with cooling the body down below its normal temperature (“Naegleria”).

What Can You Do to Prevent a Parasitic Infection of the Brain?
            After learning the ways in which some brain-affecting parasites can infect humans, individuals can exercise precaution by thoroughly cooking meat, frequently cleaning their pet’s litter boxes, frequently washing their hands, and avoiding dunking their head underwater in high-risk areas!



References 
Flegyr, J., Effects of Toxoplasma on Human Behavior . Schizophrenia Bulletin 3, 757-760  
(2007).  
Grace, E., Asbill, S., Virga, K. Naegleria fowleri: Pathogenesis, Diagnosis and Treatment
Options. Antimicrobial Agents and Chemotherapy 11, 6671-6681 (2015).
Loker, E.S., Hofkin, B.V. (2015). Parasitology: A Conceptual Approach. Garland Science.
Mendez, O.A., Koshy, A.A., Toxoplasma gandii: Entry, Association, and Physiological
Influence on the Central Nervous System. PLoS Pathogens 7 (2017). 
Parasites: Toxoplasmosis. CDChttps://www.cdc.gov/parasites/toxoplasmosis/ 
Siddiqua, T., Habeeb, A., Neurocysticercosis. Saudi Journal of Kidney Diseases and
Transplantation 31, 254-258 (2020).
Vyas, A., Kim, S-K., Sapolsky, R.M., The Effects of Toxoplasma Infection on Rodent Behavior
are Dependent on Dose of the Stimulus. Neuroscience 2, 342-348 (2007).  

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