Today, we are going to look at parasites and their role throughout human history.
Our ancient ancestors were subject to every manner of disease and accident that one can
imagine. Although many of them did not realize it at the time, they were also infested with
parasites. Treatments for many of these are now available, but the same parasites that
plagued our ancestors are still abundant in the natural world today.
Parasites tend to evoke a primal response in humans. One of universal disgust, along
with a desire to avoid them at all costs.
To fully understand how they have impacted humans, we need to first understand more about
parasites in general.
A parasite is an organism that lives within or on a host. It uses resources from the host
to sustain itself, and also uses those same resources to support its reproduction efforts.
Although parasites have undoubtedly caused much disfigurement and death, it is also possible that they are (surprisingly) not without their benefits.
To fully understand how they have impacted humans, we need to first understand more about parasites in general.
They vary widely in size. Seventy percent of known parasites are not visible to the naked eye.
Some worm parasites, however, can reach over 30 meters (or 98 feet) in length.
All parasites fall into one of two primary categories.
Endoparasites live within or on the host. They usually rely on a carrier organism to
move from one host to another.
Epiparasites feed on other parasites. For instance, although a flea might live on a
dog, another parasite might live in the flea. Thankfully this category is not often relevant
where humans are concerned.
Once an organism is classified as a type of parasite, it can be further divided into additional
categories. We shall discuss the most common three.
Protozoa are single-celled organisms that can only multiply and divide within a host.
The organism that causes Malaria is one example.
Helminths are worm parasites. Most of the human related parasites we discuss will fall
into this category.
Ectoparasites live on, rather than inside their hosts. Lice and fleas qualify as ectoparasites.
We certainly don’t have time to discuss every parasite, but we will discuss a few
specific types. First, let’s look at some common endoparasites that infect humans.
Giardia, also known as “beaver fever,'' affects the lumen of the small intestine.
It can cause diarrhea, dehydration, and other unpleasant abdominal pains. Ingesting food
or water infected with cysts is the primary means of transmission. This parasite is very
common, even in modern western societies. Usually it can be eliminated with proper treatment
and is not fatal.
Leishmaniasis is a parasite transmitted by some types of sandflies. It can be fatal under
certain conditions. It can affect the skin, as well as mucous membranes of the mouth,
nose, and throat.
Among the worst is possibly primary amoebic meningoencephalitis. This is passed on through
an amoeba known as naegleria fowleri. It affects the brain and nervous system and is usually
fatal in between one and eighteen days. It is transmitted by inhaling contaminated soil,
and contaminated water.
If there was a contender for the highest body count, Malaria would certainly be the victor.
It is a parasite transmitted by mosquitoes, most commonly in tropical regions. Malaria
causes fever and hallucination, and a variety of other side effects. It is not always fatal,
although it can be in many cases.
Toxoplasmosis is a parasitic pneumonia caused by the parasite toxoplasma gondii. It occurs
all over the world and affects the liver, heart, and brain. It can be acquired via undercooked
meat or milk, or through contact with food or soil that has been in contact with cat
feces.
As if being a woman did not already come with a long list of complications, you can add
trichomoniasis to the list. It is a sexually transmitted infection caused by the parasite
trichomonas vaginalis. Men can carry this parasite as well, but usually exhibit no symptoms.
Trypanosomiasis, or sleeping sickness, is passed to humans by the tsetse fly. This mostly
affects the central nervous system and changes sleeping patterns. It is fatal if left untreated.
All of the parasites mentioned so far comprise just a small number of the endoparasites that
plague humanity. Most ectoparasites, by contrast, live on their hosts and survive by feeding
off the blood or tissues of humans they encounter.
Bedbugs, lice, and scabies are some of the more common ectoparasites that humans will
encounter. They spread via skin-to-skin contact, and via sharing clothing or bedding that has
been in contact with the parasites.
The vast majority of humans who have hosted a parasite did so by providing a home for
worms. Given this fact, it might be useful to examine the most common worms that prefer
to live in people. It might also be comforting to know that modern medicine can treat most
of them.
It is possible to be infected with a parasitic worm and have no symptoms at all. Some general
symptoms that may be encountered are: nausea, lack of appetite, diaherra, abdominal pain,
weight loss and general weakness. There are additional symptoms specific to each type
of worm.
Tapeworms are typically ingested by drinking contaminated water or eating contaminated
meat. A tapeworm will embed its head into the intestinal wall and stay there. One specimen
was measured at nearly 80 feet in length. It is also possible for a tapeworm to live
in a person for up to 30 years. When they cause symptoms, tapeworms can result in: lumps
or bumps, allergic reaction, fever, and neurological symptoms such as seizures.
Flukes are more rare in humans. Usually contaminated water is where they can be found. They like
to live in the blood, intestines, or other tissues. They reach a few inches in size at
the most. It can take weeks or months to notice a fluke infection. They may cause fever and
fatigue.
Hookworms are transmitted via feces and contaminated soil. The most common method for catching
them is to walk barefoot on soil that contains hookworm larvae. They live in the small intestine
and are usually less than an inch long. Hookworms can cause a rash, anemia, and fatigue.
Pinworms are most commonly found in children. They are mostly harmless and live in the colon
and rectum. The females tend to lay eggs near the anus, usually at night when the host is
sleeping. When pinworms are not harmless, they can cause appendicitis or symptoms of
appendicitis.
Eating undercooked meat could allow one to catch trichinosis. The larvae mature in the
intestines, but then move on to other tissues. They are not what one would consider a pleasant
guest. The symptoms related to trichinosis are: fever, swelling of the face, muscle pain
and tenderness, headache, light sensitivity, and conjunctivitis.
Throughout most of human history, it was difficult if not impossible to diagnose parasitic worms.
Modern medicine has some techniques to do so which have their own forms of unpleasantness.
A fecal test can be used to find worms. This checks feces of the patient for live parasites,
larvae, or eggs.
If the fecal test doesn’t show evidence of parasites as a cause for diarrhea, then
it might be necessary to perform a colonoscopy. Blood tests, as well as MRI and CT scans are
also used to find parasites.
One special diagnostic method used to find pinworms is called the tape test. Clear tape
is placed around the anus. The tape can then be examined under a microscope to see if there
is evidence of pinworm eggs. Sometimes, however, the eggs can be seen with the naked eye.
Treatment typically involves taking a medication designed to eradicate the worm. In some severe
cases, surgery may be required, however.
Understanding medical conditions in the ancient world is always difficult. The field of paleoparasitology
has been able to determine with surprising precision the types of parasites that infected
our ancestors. Paleoparasitology primarily examines human feces, latrines, cesspools,
and soil from burials.
The experts in this field were able to determine that in ancient China tapeworm and roundworm
were very common. Unfortunately, so were flukes, hookworms, and head lice. Some unlucky victims
in ancient China led an uncomfortable existence.
Ancient Rome, it seems, was also infested with parasites. What is truly surprising is
that Imperial Romans had higher numbers of whipworm and roundworm than their Bronze Age
and Iron Age ancestors. Rome had some semi-modern sanitation practices, which would suggest
it should have fewer worm problems. Although nobody knows for certain, a popular theory
is that baths were the source of worm infestations. The water was kept warm and changed very rarely.
Also, uncooked fermented fish sauce was a popular item in the Empire. This is considered
an explanation for why fish tapeworm was also common.
In some cases, studying parasites illustrates changes in a population’s diet. Researchers
studying a medieval latrine in Germany first found a high number of tapeworms that come
from freshwater fish. Further analysis showed that in the 1300s, the tapeworms switched
to a type normally associated with beef. The people in this area switched from fishing
to farming for reasons not entirely understood.
Ancient droppings were found on the Greek island of Kea. These droppings were found
to contain eggs for whipworm and roundworm. Hippocrates first described these over 2500
years ago. But researchers found the presence of the eggs extended to the Bronze Age, which
showed the problem existed long before Hippocrates.
Sometimes studying parasite DNA can reveal where a population has traveled. Researchers
found that Northern European whipworms from 1000 AD to 1700 AD were very closely related
to ones found in present day Uganda. Cleary the two populations were in close proximity
at some point.
Although most people prefer to avoid parasites, for some having them might be beneficial.
There is a claim that modern hygiene has caused some people to have greater instances of allergies
and autoimmune disorders. This is known as the hygiene hypothesis.
There are some studies which support this hypothesis. One study showed that people with
parasites, or people that recently had parasites, have fewer autoimmune conditions. Clinical
trials with ulcerative colitis and Crohn’s disease patients reduced disease activity
after exposure to intestinal parasites. The mechanism isn’t known for certain. But advocates
claim that the worms reset the host’s immune system, which in turn reduces autoimmune disease
symptoms.
Although humans cannot see most parasites, it has been theorized that there is a mechanism
called the behavioral immune system which helps with parasite avoidance. This mechanism
is simply reacting in a hypersensitive manner to the perception of parasites in the immediate
environment.
To put it in a modern context, imagine you see somebody with head lice next to you. You
will probably feel a desire to escape the area, and may even feel as if your head is
itching due to imagining the lice infestation. This would be a manifestation of the behavioral
immune system.
Overall, humans are potential hosts to nearly 300 species of worms, and over 70 species
of protozoa. Some of these come from our primate ancestors. However, as the study of parasite
DNA has shown, the majority of our parasites arrived in the past 11,000 years. Living in
larger groups and the discovery of agriculture and animal husbandry was almost certainly
the catalyst.
Clearly being infected with parasites is a problem. There is a risk of very unpleasant
symptoms under the best of circumstances. At worst, a slow and painful death is the
result. However, due to removing parasites as a concern for most humans within the past
few hundred years, it seems another set of problems might now exist. Our ancestors suffered
because of parasites. We suffer because our immune systems might not have been trained
properly without exposure to those same parasites.
Science concerning the hygiene hypothesis is not settled. So, it would definitely not
be a good idea to purposely infect yourself with worms. What the existing evidence does
demonstrate is that humans and parasites evolved with each other. The people of the past were
almost never without parasites of some sort. Those of us in modern times who do not have
any parasites are among the privileged few when placed into historical context.
The evidence also clearly shows that parasites evolve as humans change their environment.
Thus, much like with antibiotic resistance, it is quite possible that current treatments
for worms and other parasites may start to lose effectiveness. The time during which
people can live parasite-free may be quite short if treatment options are not able to
match the pace of natural selection.
Do you think that modern humans can win the fight against parasites? Or is it a battle
we are destined to lose? Please let us know in the comments.
Thank you for watching Bad Things in History!