Surviving lifelong copulation in a hostile milieu
Mammalian blood is among the most hostile habitats for an alien organism. An intricate arsenal of immunological weapons, cellular and humoral, can usually destroy trespassers. A creature that not only dwells in the human blood stream, but incessantly mates in this unfriendly love nest is the blood fluke called schistosoma. The schistosomatic couple affords a romantic sight in copula under the microscope. Genus Schistosoma (Latin: split body) belongs to phylum Platyhelminthes (flat worms), class Trematoda
This parasite causes schistosomiasis, a major source of morbidity and mortality for developing countries in Africa, South America, the Caribbean, the Middle East, and Asia. It is a chronic disease that damages the urinary tract, intestines and liver in infected individuals. Most human schistosomiasis is caused by the species Schistosoma haematobium, Schistosoma mansoni, or Schistosoma japonicum. Named after the German pathologist in Cairo, Theodor Bilharz, who first described the cause of urinary schistosomiasis, bilharzia, or bilharziosis, is the eponym for schistosomiasis in many countries. At least 200 million people in at least 74 countries have active schistosomal infection. Of these, approximately 120 million have symptoms, and 20 million are severely ill. Disease prevalence tends to be worse in areas with poor sanitation, increased freshwater irrigation usage, and heavy schistosomal infestation of human and/or snail populations.
Romantic worms. Anatomic adaptation
The schistosomes are dioecious
, i.e., the sexes are separate and the two sexes display a strong degree of sexual dimorphism
, being dissimilar in appearance. Their body structure particularly that of the female, is clearly an adaptation to an intravascular existence. Female flukes
are long and slender (1.2 to 2.6 cm in length). The ovary is situated behind the midpoint of the body and the uterus contains 20 to 30 eggs. The male is shorter (from 0.6 to 2.2 cm long) and possesses 4 to 8 testes
. While the body is incurved ventrally to form a groove called gynaecophoric canal
. The male surrounds the female and embraces her within his gynaecophoric canal
for the entire adult lives of the worms in monogamy
, reproducing sexually. The anatomy of the slender worms is clearly streamlined to locomotion in blood vessels.
In addition to their structural adaptation, nature has provided these parasites immunological strategies to survive in the blood vessels. They are masters at evading their host’s immune system, surviving for years in the blood stream. The worms manipulate their surfaces to deceive the host to be accepted by the receptors of the venous endothelium and develop armours ( teguments) that are highly resistant to host immune responses. They steal and incorporate host proteins, including blood group antigens, in their integuments. Coated in host antigens, and apparently unaffected by host immune responses, schistosomes can live comfortably for years in the mammalian blood stream, in some cases, for up to 40 years. As a result, those who live in endemic areas are at constant risk of re-infection, which can be very rapid.
When it facilitates reproduction, the parasites shed their immune mask and trigger inflammatory response of the host tissue. The parasite's continued transmission is actually dependent on their host's immune response. Schistosoma eggs secrete antigens to induce an intense granulomatous response that facilitates their passage from the blood stream to the gut lumen and, thus, to the outside world. Eggs require this intense immune response to aid their migration through the body.
Living in the blood stream, these flukes drink blood. The origin of the word parasite is derived from the respectable Greek word parasitos, meaning quite mundanely a 'fellow-diner' or 'guest' to the house. Biologically, parasitism is defined as a type of symbiosis where two (or more) organisms from different species live in close proximity to one another, in which one member depends on another for its nutrients, protection, and/or other life functions. The dependent member (the parasite) benefits from the relationship while the other one, the host, is harmed by it. ( BioTech Dictionary at http://biotech.icmb.utexas.edu/.)
Power struggle between man and worm
Biological theories attempt to explain sexual reproduction as a source of variability that drives evolutionary processes as a result of organisms attempting to boost their resistance to parasite infection. This struggle for survival between host and parasite, in which both parties evolve as fast as they can in order to live and prosper, has been described as the Red Queen Effect by the late evolutionary biologist William Donald Hamilton, who postulated that sex evolved because new combinations of genes could be presented to parasites - organisms with sex were able to continously run away from parasites. In his book The Red Queen, Matt Ridley expounds on this and interesting aspect of parasitism. Parasites evolve toward greater infectious capacity just as quickly as hosts evolve stronger resistance. Hamilton and Ridley argue that sex has been humanity’s most effective weapon for the war on parasites, both internal and external.
Eggs passed in the urine or feaces of an infected person and hatch in fresh water to release larvae, which infect the intermediary host, freshwater snails, which release large numbers of fork-tailed free-swimming larvae called cercariae into water. Here they may find the definitive host, a human wading in water, or die. Cercariae bore through the skin to veins and pass into the lungs before reaching the portal vein, where they metamorphose and mature, shedding their tails. Here, the permanent copulation starts. Together, the male embracing the female in his gynaecophoric canal, they happily swim against the blood current to the veins of the gut or bladder, where the female begins to lay egg, which migrate through the bowel or bladder wall to be shed via feaces or urine. Eggs that are not shed successfully may remain in the tissues or be swept back to the liver or to lungs.
The pathology of chronic schistosomiasis, which is far more common than the acute form of the infection, results from immune response to the eggs, including granuloma formation and fibrotic changes. Schistosomiasis is precancerous. Cancer of the urinary bladder often develops in chronic S haematobium infection.
The therapy of choice is with Praziquantel, which destroys the worms by increasing their cell membrane permeability, resulting in loss of intracellular calcium, massive contractions, and paralysis of their musculature, followed by attachment of phagocytes to the parasite and its death. Owing to reinfection, children in endemic areas, should be treated twice a year.
Prevention strategies and prospects
The prevalence of schistosomiasis is increasing world wide. Transmission is usually associated with poor socio-economic conditions. In highly endemic areas children start to accumulate worms as soon as they are old enough to have water contact and may, because of the chronic nature of the infection and continued susceptibility to re-infection, remain infected throughout their lives.
Mass treatment of the population (especially children) is effective in decreasing the egg load in the community. Attempts to control or eradicate the intermediate snail host by mollusciciding chemotherapy have generally been unsuccessful.
The progress made has been more than counterbalanced by increases in the incidence of schistosomiasis that have accompanied social dislocation and mass migrations caused by war, drought and famine, and by the effects of man-made ecological changes, unfortunately including many water resource development projects, with the engineers refusing to listen to epidemiologists, although dams can be made less ‘snail-friendly’.
The only lasting solution would be global access to a safe water supply, improved sanitation, including the provision of latrines, and health education. The reality is that billions of people lack this luxury. As long as the mankind accepts and neglects poverty, and humans parasitize humans, this vision will remain an illusion. As a species, humans appear to be losing the collective evolutionary race against many other parasites.