Indian Schistosomes: A Need for Further Investigations
Blood flukes cause devastating pathologies in hosts, and are difficult to . were employed to test the relationship between the number of snail. the biological basis of the snail/parasite relationship with the hope of finding novel ways to Species of the human blood fluke Schistosoma are estimated to infect approximately . (ANOVA) followed by least significant difference [LSD] test. In water, the eggs release miracidia which infect snails and undergo asexual Parasite morphology: Blood flukes form five different developmental stages: eggs , . in host samples; some tests showing good correlations with parasite burdens.
Human Schistosomiasis Schistosomiasis is considered only next to malaria among parasitic diseases of man.
Obviously, those schistosome species, namely, S. In the beginning of twentieth century, India also attracted attention of the scientists on the subject as this was the period of new discoveries on the schistosomes and also because of reporting of several cases of human schistosomiasis from different parts of the country.
Soon, it was realized that schistosomiasis cannot establish on the Indian soil as the known intermediaries of human schistosomes are not prevalent; further, trials of infecting local snails with miracidia of S.
These findings resulted in fading of the interest of Indian scientists on schistosomiasis. Notwithstanding the above facts, a confirmed endemic focus of urinary schistosomiasis was demonstrated in Gimvi village of Ratnagiri district, Maharashtra state of the country [ 4 ]. As the eggs were oval shaped and were present in the urine of the patients, the parasite was termed as Schistosoma haematobium.
But no species of Bulinus snail, the intermediate host of S. These works resulted in identifying Ferrissia tenuis as its intermediate host. Much later, a WHO team, in collaboration with Government of India was able to eliminate the infection from Gimvi village and now it is a dead infection. However, there are many questions which have remained unanswered in this case [ 57 ]. The basic question is about the species of schistosome capable of infecting human beings in Gimvi and also why we have considered it confined only to Gimvi or adjacent villages.
The main objection of considering it as S. Therefore, it will be confusing to call the parasite as S. For this reason, we have suggested to call it a new schistosome species, that is, Schistosoma gimvicum [ 9 ]. There is no reason to consider this blood-fluke to be confined only to Gimvi village, as Ferrissia tenuis snails, so widely prevalent in India [ 10 ], were not screened at other places of the country for furcocercous cercariae.
If Ferrissia species is reconfirmed as snail host for any schistosome species from any part of the geography, it will lead to basic changes in our understanding in life cycle of the blood flukes. We have no information if ever comprehensive surveillance of remote areas for urinary schistosomiasis has ever been made in India. This no-reporting does not confirm absence of the infection at other places but may reflect lack of scientific interest in the subject.
It is not that cases of only urinary schistosomiasis were reported from India. Even schistosome infections responsible for hepatic form have been reported from human beings from the country [ 5 ].
A confirmed infection of S. Another support of hepatic infection came forward by reporting schistosome eggs in human stools in Punjab state [ 12 ] and also from a village of Andhra Pradesh [ 13 ].
But these reports failed to identify new endemic areas of human schistosomiasis, thereby failing to generate interest in the subject by any scientific agency.
The later work on the subject has also not ruled out the probability of occurring human schistosomiasis in India. However, this work was not restricted to examining human stools or urine but was extended to search for cercarial dermatitis. As Indian schistosomes have not developed in line with river system but are using snails inhabiting stagnant waters, the search was made in human beings located near to the ponds and tanks and using them for their routine domestic work including recreational activities.
The cercarial dermatitis may have different sequences as per compatibility of schistosome species involved. In considerable cases and particularly with avian schistosomes, the schistosomulae are destroyed in the skin of man itself.
In other cases, at least some schistosomulae are able to traverse to lung or liver where they may be trapped. Still, there is a parasite like S. What is the sequence of cercarial dermatitis in India? It is difficult to predict the same with conformity but positivity of the cases by CHR diagnostic test suggested presence of circulating antibodies hence presence of alive schistosomes in the body of the concerned person [ 18 ].
Additionally, five cases turned positive for presence of schistosome antigen in urine when it was tested by a single step diagnostic kit developed by the Netherland laboratory [ 19 ]. Portal hypertension often leads to hepatomegaly, splenomegaly, ascites, and sometimes gross enlargement of oesophageal and gastric veins varices which may burst.
Cerebral granulomas have been associated with focal epileptic convulsions, while spinal cord granulomas may cause transverse myelitis. Schistosomes have indirect digenetic life-cycles, involving sexual reproduction in vertebrate definitive hosts and asexual reproduction in snail intermediate hosts.
Parasites are transmitted between hosts by motile aquatic stages which actively seek hosts. Female worms produce numerous eggsper day which seek to exit the host by penetrating the gut or bladder wall and being passed with host faeces or urine.
When deposited in water, the embryonated eggs hatch releasing free-swimming miracidia which only live for several hours. In that time, they actively seek suitable intermediate hosts amphibious snails using chemotaxis and phototaxis despite absence of eyespots. The miracidia invade the soft tissues of the snail and form a mother sporocyst near the site of penetration.
Daughter sporocysts are produced weeks after infection and they migrate to other organs in the snail. Schistosomes do not produce redia stages; instead the sporocysts produce cercariae which are released into the water in their thousands beginning 4 weeks after infection.
The fork-tailed cercariae are rapid swimmers and they periodically swim to surface of the water and then sink to bottom for up to three days. They are attracted to skin secretions and when they come into contact with a prospective definitive host, they attach and actively penetrate the skin within minutes, losing their tails in the process. Young worms then pair and migrate to their predilection sites in the veins of the gut or bladder.
The digestive system consists of a simple sac with a mouth either at the anterior end or in the middle of the ventral surface. An anus is usually absent, but some species have one or two anal pores.
The nervous system consists of a pair of anterior ganglia, or nerve centres, and usually three pairs of lengthwise nerve cords. Most species are hermaphroditic; i. In some, however, the sexes are separate.
Most species pass through egg, larval, and mature stages.
Schistosomes and Other Trematodes - Medical Microbiology - NCBI Bookshelf
Blood flukes occur in most types of vertebrates; three species attack humans: The human diseases caused by them are known as schistosomiasis bilharziasis ; they affect millions of persons, particularly in Africa and east Asia.
The urinary blood fluke S. Eggs, laid in the veins, break through the vein wall into the bladder and are voided during urination.
The larval fluke develops in the body of a snail chiefly of the genera Bulinus and Physopsisthe intermediate host.