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).
Naegleria fowleri. CDC. https://www.cdc.gov/parasites/naegleria/index.html.
Siddiqua, T., Habeeb, A.,
Neurocysticercosis. Saudi Journal of Kidney Diseases and
Transplantation
31, 254-258 (2020).
Taeniasis. CDC. https://www.cdc.gov/parasites/taeniasis/biology.html.
Toxoplasmosis. Mayo
Clinic. https://www.mayoclinic.org/diseases-conditions/toxoplasmosis/symptoms-causes/syc-20356249.
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).
No comments:
Post a Comment