In 1851, Bilharz (physician) reported seeing microscopic eggs with a
pointed spine in the female worm. He also found that the eggs would
hatch to release a small, ciliated larva that swam around for about and
hour and then disintegrated.
The suspicion remained that human acquired the infection either by
eating infected snails or by drinking water containing the ciliated larvae,
In 1870 egg were obtained from a young girl living in the Cape of Good
Hope and found that, although the eggs would not hatch in urine, they did
so in fresh or brackish water. Then, in about 1904, Japanese physicians
found that a related blood fluke, could also infect humans, but this species
had eggs without spine.
In 1905 another type of schistosome egg, one with a spine on its side.
The life cycle and mode of transmission of the schistosome to a human was
first demonstrated between 1980 and 1910 in Japan. At the outbreak of
World War I, the British became concerned about the potential deleterious
effects of schistosomiasis on their troops in Egypt. In 1915 the British War
Office sent Robert Leiper to Cairo “to investigate bilharzia”. He and his
team identified the snails Bulinus and Biomphalaria as the vectors. His
study suggested that the infection was acquired by bathing in infested
water. Leiper was also able to show that when cercariae were placed in
dilute hydrochloric acid (similar in concentration to that found in the
human stomach) they were killed, this route of infection seemed most
*The life cycle of snail fever was finally known: on reaching fresh water, the
discharged eggs release a swimming larva, the miacidium. Miracidia are
short-lived, but if they encounter a suitable snail they penetrate the soft
tissues (usually the foot), migrate to the liver, and change in form (sporocyst);
for 6 to 7 weeks, by asexual reproduction, the numbers of parasite increase.
During this time, the snail sheds thousands of fork-tailed cercariae, which
can swim and directly penetrate human skin, and in 5 to 8 weeks they
develop into adult worms.
Snail fever, the disease
Schistosomes have separate sexes and they inhabit blood vessels.
Both males and females have two suckers at the head end of the worm,
and the more anterior one surrounds the mouth.