In light of my last post about Brazil releasing a vaccine for Schistosomiasis, I felt it would be a good idea to do a post on Schistosoma itself. Schitosomiasis has a long history with humans. In fact, some sources assert that it was a common cause of death amongst the ancient Egyptians during the Greco-Roman period. It continues to be a problem today that about 1/6 of our world’s population has to deal with. It is to blame for lowered productivity and social stigmas that drag down the economies of developing countries. Its population even exploded as man feebly attempted to divert the great waters of the Nile. (*Side note*: As cases mounted due to Nile irrigation projects from the 1950s-1980s, people were being treated with shots of tartar emetic, which later increased the spread of hepatitis C via unclean needles….good job, Egypt!)
Like all helminths, this worm belongs to kingdom Animalia. It is a type of flatworm known as a “fluke” putting it in phylum Platyhelminthes and class Trematoda. Other platyhelminths include tapeworms and some nifty non-parasites called turbellarians (google it…they are really neat!) The term “fluke” is synonymous with the term “trematode” and I might use them interchangeably, so beware! This particular parasite is in order Strigeidida under the suborder Strigeata. It further belongs to the family Schistosomatidae. Members of this family spend their larval stages within molluscs and their adult stages within vertebrates. All members are dioecious (meaning they have separate male and female sexes) unlike many other flatworms that are hermaphroditic (having male and female organs within a single individual). The family has14 genera, 9 of which infect birds.
|Unamused Hippo is Unamused|
The only genus that infects humans is the genus Schistosoma. Seven species within Schistosoma infect humans, but there are other mammals that carry Schistosoma. Most of these other animal are ruminants, but there are two species that infect hippos. (I’ll list one of them here because the name is intuitive, so you will never forget it: Schistosoma hippopotami.)
These parasites are known as “blood flukes”…and I bet you can guess why! (Because you are a smart cookie!) The first person to ever describe the full life cycle was Dr. Pirajá da Silva in 1908.
|S. haematobium egg|
|S. mansoni egg|
Let’s start with the eggs. The eggs of these little guys are pretty scary-looking. They have tiny spines that may be found in various places on the egg depending on the species of Schistosoma from which they came. Eggs are passed via urine or feces from an infected host into a fresh water system. The eggs then hatch, releasing a miracidium that then proceeds to penetrate a snail host (which is often species specific). Inside the snail, the parasite undergoes a few different morphological changes before popping out as a tailed cercaria. These cercariae swim about in the water until they find a suitable host and then they burrow their way into that host through the host’s skin. They then wiggle their way into the circulatory system and eventually make their way to a specific destination in the body where they mate and spend the rest of their lives eating and pumping out babies. The destination depends on the species of Schistosoma that is infecting its host. Some like the liver, some like veins associated with the intestines, and some like the bladder.
(Female's head is in orange
poking out of the canal .)
One of the more interesting aspects of the life cycle for S. mansoni is that these parasites are monogamous. The males have a special canal that runs the length of their bodies which is referred to as the “gynecophoric canal”. The smaller, more slender female finds a mate and slips into this canal to live out her life, literally, inside of the male. This life style is called living “encopula”. They only take one mate, and if the female dies, the male retains her dead body within his canal until he too dies. He eats a ton of glucose that he passes on to her as she pumps out eggs that invade the liver or are passed out of the host to start the life cycle over again. Homosexuality has also been observed in these animals even though the anatomical fit isn’t quite as perfect. When curious scientists tried to separate the homosexual couples mechanically, they always went right back to one another. How cool is that?!?
As previously stated, there are seven species that infect humans. These infections are aptly named “schistosomiasis” despite which species causes the infection. Most of the species are found in or near South East Asia or Africa, but there is a species that spans from Africa and the Middle East all the way to the Caribbean and into South America. This disease infects a large portion of the human population. In fact, it runs right behind malaria in terms of the devastating socioeconomic impacts. The CDC and the WHO both classify schistosomiasis as a NTD (Neglected Tropical Disease). This is WHY it was such a BIG DEAL when Brazil announced that they had finally found a vaccine for this detrimental and disfiguring disease. That and the whole first-vaccine-against-a-worm thing…that was important too. :p Anyway, back to the disease.
Schistosomiasis is sometimes called “Bilharzia” (after the first physician to describe the cause of urinary schistosomiasis in 1851: Theodor Bilharz) or “Snail Fever”. S. mansoni and S. intercalatum cause intestinal schistosomiasis. S. haematobium causes urinary schistosomiasis. S. guineensis can cause liver disease. S. malayensis is rarely known to infect humans at its preferred host is von Muller’s rat. And finally, S. japonicum and S. mekongi cause Asian intestinal schistosomiasis.
The disease itself is often chronic and debilitating, but not necessarily fatal.
|Skin lesions left from Schistosoma penetration.|
Symptoms include abdominal pain and diarrhea, coughing, anemia and malnutrition, elevated white blood cell counts, fevers, fatigue, enlargement of the liver or spleen, genital sores, and dermatitis caused by host skin penetration. The longer the infection, the more detrimental the parasites are to their hosts, often leading to calcifications and cancers if left unchecked for too long. (*Side Note*: Because young boys often pick up S. haematobium when they reach puberty and start working in rice fields, they will often experience bloody urine. Infection in these cultures is so pervasive, that this is not seen as abnormal, but as a form of “male menstration”. There are even instances of boys who didn’t go into working in rice fields, thus they didn’t get infected, and parents taking them to doctors to find out why their sons weren’t menstruating! Wild huh??)
Diagnosis, Treatment, and Prevention
To diagnose schistosomiasis, most hospitals test for antigens using ELISA and a patient’s blood sample. This is the best method for diagnosis, but it can also be diagnosed via demonstration in a stool sample, or (rarely) a urine sample. Sometimes a tissue biopsy is performed, but is less commonly used because it is more invasive.
To treat patients diagnosed with schistosomiasis, most places recommend a single, yearly dose of a de-wormer known as praziquantel. Outside of the US, some places have developed species specific treatment such as oxaminiquine for S. mansoni and metrifonate for S. haematobium.
To prevent the transmission of this disease, some areas treat fresh water sources with chemicals to kill off the snail intermediate hosts. However, these chemicals often kill more than just the snails. Even if they didn’t, the snails are still a vital part of the ecosystem, so every time this prevention method is used there are ecological effects that forever change the flora and fauna of the freshwater sources.
Now, because Brazil is awesome, we have…for the first time ever…an effective vaccine against schistosomiasis. Check out my last blog post for more details.
Moral of the Story
If your son didn’t get his period, it’s totally normal! :p If he did, see a doctor and get rid of it! (Reference from The Jerk, anyone?) Anyway, the vaccine for this is super awesome, and super important. It will be interesting to see the socioeconomic impacts that this vaccine has over the next few years! :D
Another thing we can take away from this is that there are great examples of monogamy among animals...there's even homosexual monogamy! Ahh true parasitophilia in the strictest sense of the made-up word! It's enough to make you want to weep with joy!