Earlier in the series, we learned about the clade Neodermata, and within this clade we can find the
parasitic flatworms of Trematoda, Monogenea, and Cestoda. These are highly specialized parasites
that descended from a lineage of free-living flatworms within Rhabditophora, though they
have secondarily lost their rhabdites. All members of Trematoda and Monogenea
are ectoparasites collectively known as flukes. More recent phylogenetic analyses have revealed
that Monogeneans are more closely related to the tapeworms, though this is still debated.
Monogeneans, unlike the Trematodes, cause considerably less damage to their hosts,
since their life-cycle within the host is generally quite a simple one,
unlike some trematodes which undergo complex migrations, causing extensive damage along
the way. However, dense infestations can cause severe irritation, and extremely
dense infestations can cause extreme damage and even death. They are commonly found on the skin,
gills, or fins of fish, and occasionally within the tissues of amphibians and hippos.
They have a direct life cycle and do not require an intermediate host.
Nearly all adult Trematodes parasitize vertebrates, including humans. Though we’ve
discussed parasites in this series before, these are the first human parasites we have encountered.
As a warning, some of the images we will be looking at are a bit graphic, and the
information might be disturbing to some viewers. Morphologically, adult trematodes are tear-drop
or leaf-shaped with adhesive suckers. They are well adapted to parasitism, wielding various types
of penetrating organs, cyst-producing glands, and adhesive organs such as suckers and hooks.
In addition, up to 80% of an adult fluke’s body is devoted to their complex reproductive system.
Trematodes are such well-adapted parasites that some of them have existed for longer
than their hosts. For example, the members of genus Schistosoma have been dedicated parasites
for at least 150 million years, which is several orders of magnitude older than the evolutionary
history of many of their current definitive and intermediate host species, including humans.
Perhaps the most infamous flukes are those of clade Digenea, which includes about 6,000 species,
nearly all of which have a complex life cycle that begins with a free-living aquatic,
or semi-aquatic intermediate host, usually a mollusk, and a definitive host, or final host,
that is almost always a vertebrate where the flukes reproduce. Reproduction only takes place
in the definitive host. Some species also use secondary and even tertiary intermediate hosts
to get to their definitive host. In addition, some may also have paratenic hosts, meaning one that
is not essential to the life cycle. For example, Paragonimus westermani, or the oriental lungworm,
has aquatic snail and crustacean intermediate hosts, but may also have an additional paratenic
host, pigs in particular, from which humans can become infected through consumption.
Individual species of digenean flukes are usually very host-specific. That is, they can only inhabit
specific species. Even within an individual species a fluke will only inhabit a specific
site within their host’s body. Some target the digestive system, others the respiratory system,
still others the circulatory system, and others the reproductive system. Many, like Clonorchis
sinensis, or the Chinese Liver Fluke, infect different systems in different hosts.
However, like everything in biology, there are exceptions. Fasciola hepatica, for example,
can infect a huge range of mammals, and is even reported in some bird species.
Let’s examine a typical life cycle to get a better understanding. We’ll start with an individual egg
that has passed through an infected human’s fecal matter. If this egg does not reach the water,
or if the water into which it is deposited is sufficiently treated,
it will not develop further. If it does make it to the water, it may live for several weeks before
it must be ingested by an intermediate host so that the embryonic egg membrane can be dissolved,
and the ciliated miracidium can emerge. This mobile stage can move about within a
snail host’s body and eventually penetrates the intestine, then the digestive gland,
where it metamorphoses into a sporocyst. Sporocysts reproduce asexually and can produce
additional sporocysts or rediae. Rediae can also reproduce asexually to create more rediae,
or the infectious cercariae that attack fish from the family cyprinidae, such as carps and minnows.
A single miracidium can give rise to 250,000 individual cercaria. This is an
incredibly important point when considering the epidemiological risk posed by flukes. The clonal
expansion within the intermediate host means that a location can go from being low or no risk to
high risk with minimal environmental contamination from eggs derived from the definitive host.
If an individual cercaria encounters a suitable fish host, it will bore under the scales into the
muscles where they lose their tails and encrust themselves in protective cysts, then remaining
dormant as metacercariae. If a human, or other mammal, consumes the flesh of raw, undercooked,
salted, pickled, or smoked freshwater fish, the protective cyst is often ruptured by the juvenile
fluke as they excyst themselves through enzymatic releases initiated by some external change, such
as a change in pH once in the digestive system. The juvenile fluke then migrates to the bile
tract where they become adults and may live for an additional 15 to 30 years. All of this describes
the life cycle of just one species of trematode. There are many others that connect a wide range
of hosts. Some, like the green-banded broodsac, force themselves into the eyestalks of a snail
to attract birds. Others, like the lung flukes, infect a wide range of wild carnivores, humans,
pigs, and rodents, and cause respiratory problems which commonly result in mortality.
Some, like the liver flukes Fasciola hepatica and F. gigantica are a major global issue for
livestock in terms of mortality, morbidity, and production losses. They cause an estimated
annual cost of three billion dollars to the global economy, which is likely a significant
underestimate, and it is a significant cause of human disease in some developing countries.
Other species of horn snail trematodes castrate their intermediate hosts and use their bodies
to reproduce, eventually releasing cercariae that burrow into the brains of killifish, forcing them
to flash their shiny sides upward to attract their final host, a predatory bird, where they
reproduce and begin the cycle anew. This form of parasite-induced behavior alteration is generally
done to an intermediate host, in order to get it killed or maimed, so the parasite can enter its
final host in order to sexually reproduce. Another example is the lancet liver fluke, which begins
as an egg in cow dung, that, if eaten by a snail, reproduces asexually until cercaria exit the snail
through slime balls that are consumed by ants. Within the ants, the fluke burrows into the brain
and induces behavior alteration, forcing the ants to fasten their mandibles to a blade of grass
until they are consumed by a passing herbivorous mammal, such as a cow. The ants will behave
as part of their colony during the day, and only move up to the tip of a blade of grass at night.
This is likely an evolutionary mechanism that maximizes transmission potential,
as if the ant was on top of the grass in the day it would likely desiccate and die more quickly.
Interestingly enough, some flukes also encyst themselves on aquatic plants such as the water
chestnut, water caltrop, lotus, bamboo, and others until they are consumed by a mammal,
where they develop in the duodenum in about three months and attach to the intestinal wall.
Another clade of trematodes, the blood flukes, cause a disease known as schistosomiasis, which
infects more than 252 million people globally and leads to thousands or even hundreds of thousands
of annual deaths. They are considered by the World Health Organization to be the second-most
socioeconomically devastating parasitic disease after malaria. Individuals who are
infected develop a rash or itchy skin. Fever, chills, cough, and muscle aches
can begin within 1-2 months of infection. Eggs that are produced usually travel to the intestine,
liver, or bladder, causing inflammation or scarring. Children who are repeatedly
infected can develop anemia, malnutrition, and learning difficulties. After years of infection,
the parasite can also damage the liver, intestine, lungs, and bladder. In some cases, eggs are
found in the brain or spinal cord and can cause seizures, paralysis, or spinal cord inflammation.
Blood flukes differ from most other flukes in that they are dioecious, that is, they are male
or female, but not both. Three species of blood flukes, S. haematobium, which resides within
the veins of the urinary liver, S. japonicum, which lives in the veins of the small intestine,
and S. mansoni, which lives in the veins of the small intestine, account for most cases
of schistosomiasis in humans, though there are many other species and even reports of
hybridization between existing species. All species of blood flukes infect their
hosts in a similar fashion. Thousands of eggs are released by an infected human
in urine or feces to fresh water. Larvae must then pass through an intermediate snail host,
where they metamorphosize into a sporocyst that asexually releases cercariae into the water.
Free-swimming cercariae infect a new mammalian host by directly penetrating the skin, causing
a transient skin irritation commonly referred to as swimmer’s itch. Within the host they reach
the liver and migrate to their characteristic site. Once there, the male surrounds the female
and encloses her within his gynacophoric canal. The two will remain in this bond for their entire
life. As the male feeds on the host’s blood, he passes some of it to the female and the
two continually sexually reproduce. The female lays tiny, barbed eggs that extrude through the
walls of the veins and enter the gut or bladder lining where they are expelled from the body.
Eggs that are not released may cycle back into the blood where they cause inflammation,
tissue damage, impede blood flow, and cause fibrotic reactions and granulomas.
And with that rather jarring image, we will conclude our introduction to the parasitic
flukes. Keep in mind, this is just a sample of a few of the thousands of known trematode species,
many of which have not yet had their full life-cycle analyzed.
It’s possible they are responsible for even more intricate relationships between
hosts. In any case, let’s move forward and wrap up our discussion of phylum Platyhelminthes.