Schistosoma pdf

Cerebral granulomatous disease may be caused by ectopic S. Continuing infection may cause granulomatous reactions and fibrosis in the affected organs e. Pathology associated with S.

Pathology of S. Contact Us. Skip directly to site content Skip directly to page options Skip directly to A-Z link. Parasites - Schistosomiasis. Section Navigation. Facebook Twitter LinkedIn Syndicate. Minus Related Pages. Causal Agents Schistosomiasis Bilharziasis is caused by some species of blood trematodes flukes in the genus Schistosoma.

Credit: DPDx. Contact Us. Skip directly to site content Skip directly to page options Skip directly to A-Z link. Parasites - Schistosomiasis. Section Navigation. Facebook Twitter LinkedIn Syndicate. Globally, more than million people are infected with one of three Schistosome species, including an estimated 40 million women of reproductive age.

In Brazil, about 12 million children require preventive chemotherapy with anthelmintic. PZQ is the drug of choice because it presents as a high-spectrum anthelmintic, used in the treatment of all known species of schistosomiasis and some species of cestodes and trematodes. OXA, however, is not active against the three Schistosome species. All the 23 known Schistosoma species are parasites of mammals with 6 of these having high medical importance.

Schistosomes are classically spliced into 4 clades referred as S. Recently it has been proposed to split schistosomes into 6 clades according to phylogenetic relationships and geographical distribution Lawton et al. Physical description depends on the life cycle stage considered. The mammal infecting stage is a worm. Among the thousands species of trematodes, schistosomes have the particularity to have separate sexes.

The female lives in a ventral groove of the male called gynaecophoral canal. The black color of the female is due to the disposal product of hemoglobin degradation called hemozoin resulting from red blood cell ingestion Portela et al. This product is similar to the disposal product of malaria called malarial pigment. Miracidia can be confused with a ciliated protozoa but miracidium is a complex multicellular organism Figure 4.

The free larval stage infecting mammals cercaria is divided into two parts: tail and body. The main characteristic of schistosome eggs used in diagnosis are the absence of operculum, the presence of fully developed miracidia and the presence of spine: terminal S.

Figure 4. It is a free swimming larva thanks to its epidermal ciliated plates. The terebratorium apical papilla facilitates attachment to the snail host tegument. Figure 5. It is a swimming larva thanks to its bifurcated tail. The oral sucker and the ventral sucker known as acetabulum facilitate attachment to the tegument of the definitive host. Pre- and post-acetabular glands facilitate host penetration thanks to their proteases.

The tail is shed upon penetration. Human schistosomes have a complex lifecycle including two obligatory hosts Figure 6 : a vertebrate definitive host, the human or other mammalians as reservoir hosts , where the adult schistosomes pair and reproduce and a gastropod snail intermediate host where the larvae, called sporocysts, multiply asexually. Transmission from the vertebrate host to the freshwater snail vector is ensured by the miracidium larva. Transmission from the snail host to the vertebrate host is ensured by the cercaria larva during exposure to water with active penetration.

Figure 6. Life cycle of Schistosoma mansoni. When the stool of an infected human reaches freshwater, each egg releases a free swimming ciliated larva called miracidium there is no incubation period. This larva has just a few hours to find and actively penetrate a freshwater snail of the genus Biomphalaria. Inside the headfoot region, the miracidium develops and becomes mother sporocyst or sporocyst 1 in which many daughter-sporocysts or sporocyst 2 develop by asexual multiplication.

These larval stages leave the headfoot region to colonize different target organs as the hepatopancreas and the ovotestis of the snail host. The development of a single miracidium may result in thousands of cercariae, each one able to penetrate human skin actively. After penetration, the cercaria becomes schistosomulum and undertakes a complex migration through the body in the bloodstream that ends ultimately to the liver where males and females mate, become sexually mature then migrate to the egg-laying site, mesenteric veins for S.

Egg must pass through the vessel and the intestine walls to be discharged outside with the stool. They live in the mesenteric vein vessels irrigating the intestine all human schistosomes, except S. The eggs are laid in the blood hundreds or even thousands per day.

The mechanisms by which eggs move through the vessel walls and the tissues to reach the lumen are not well understood.

The egg upon reaching freshwater, the miracidium hatches and has few hours to find and actively penetrate the appropriate snail host.

In case of good host parasite compatibility, the miracidium becomes a mother sporocyst or sporocyst 1; Figure 7A producing daughter sporocysts or sporocysts 2; Figure 7B. Figure 7. The S. Germinal cells will develop into daughter sporocysts; b Young worm-like daughter sporocyst in the hepatopancreas of the intermediate host Biomphalaria.

The daughter sporocysts live preferentially in the hepatopancreas and ovotestis of the snail host; they undergo at least two multiplication generations and give rise to cercariae that are freed from the snail body into the water and chemotax to the human host skin. The cercariae then burrow through the skin and enter the bloodstream. The snail intermediate hosts belong to different genera depending on the species of schistosome: Biomphalaria for Schistosoma mansoni , Bulinus for S.

Humans are the vertebrate definitive hosts but other vertebrates may be found naturally infected by a species of human schistosomes and need to be considered as potential reservoirs for these diseases. Tables 2 to 4 show the species that have been found naturally infected with human schistosomes.

They belong to 9 orders: primates 16 species , lagomorpha 3 species , rodentia 58 species , carnivora 13 species , cetartiodactyla 11 species , perissodactyla 2 species , insectivora 6 species , xenarthra 1 species and didelphimorphia 3 species.

In summary, S. We should bear in mind that efforts to analyze these natural hosts in the field may be different for the different species of schistosomes. A plausible explanation for this is the ability of S.

The increase in the definitive host range can be interpreted as the result of hybrid vigor after hybridization of S.

Although no definitive natural host was found to be infected with S. The same lack of investigation can be applied for S. The last three species have a limited geographical distribution: gulf of lower Guinea for S. Table 2. Mammalian definitive hosts other than human found naturally infected by human schistosome S.

Cercopithecus aethiops a , C. Pan troglodytes a,c , Papio Anubis a,c , P. Akodon arviculoides h,i , Arvicanthis niloticus a,c,k,l,m,n,o , Bolomys Lasiurus a,i , Calomys expulsus i , Cavia aperea a,I,p , Damysmys b , Dasymys incomtus a,c , Gerbillus pyramidum a,c,q , Holochilus brasiliensis a,i,p , H.

Didelphis albiventris a , D. NOTE: S. Table 3. Mammalian definitive hosts other than human found naturally infected by the human schistosome S. Cercopithecus aethiops b , C. Table 4. Mammalian definitive hosts other than human found naturally infected by human schistosomes S.

Lepus cuniculus b , L. Apodemus argarius d , A. Canis familiaris d,e,f , Civet cat a , Felis bengalensis a,d , F. Bos Taurus a,d , B. Crocidura attenuate a,d , Erinaceus europaeus a,d , Suncus suncus d , S. In the definitive host, the prepatent period can be defined as the duration between cercarial penetration and output of eggs. This period takes around weeks after infection. In the intermediate snail host, the prepatent period is defined as the duration between miracidial penetration in the snail and cercarial shedding.

This period takes 28 days for S. In schistosomes, the shedding level of the larvae responsible for human infections is called cercarial production, i.

This number is highly variable, from a few to thousands of cercariae per day. Cercarial production during the patent period follows a sinuous rhythm alternating high and low production phases. This corresponds to the duration of the development of a new cercarial generation in the daughter sporocysts, 28 days in S.

The snail in which the parasite develops influences the production rhythm. For example, in S. The snail in which the parasite develops influences the quantity of cercariae produced: for example, in S. Shedding levels may also vary according to the environment of the snail hosts. For example, the cercarial production of S. Cercarial shedding patterns vary according to the water usage behaviors of the desired host as summarized in Figure 8.

Interestingly, all human schistosomes have adapted themselves to human behavior and shed their cercariae from the late morning to the afternoon when human targets use freshwater for multiple uses including bathing, fishing, washing, recreation and other purposes.

Nevertheless, some of these species may also use other vertebrate hosts and adapt their behaviors to those hosts. The species S. The adaptation of this species to the behavior of its hosts is exemplary: in Guadeloupe, the populations using human hosts primarily shed their cercariae with an average emergence peak around noon, the ones using a rodent host, Rattus rattus , shed their cercariae with an average emergence peak around 4pm.

Infection of snails by more than one species of schistosome is possible and each species keeps its proper rhythm. For example, S. In experimentally mixed infections, the average emergence peaks were 8 am and noon for S. More recently, another more extreme dichotomous behavior was found in the Sultanate of Oman where one population of S. This new behavior from S. Figure 8. Temporal extent of the daily cercarial emission in some Schistosoma species: S.

Cercarial shedding polymorphism has not been shown to be correlated with the species of snail infected by S. However, a correlation has been found between chronobiology and eco-geography forest versus savanna zones in S. In high endemic areas, presumably, individuals are frequently re-infected when they come into contact with cercariae emitted by infected snails. Penetration is active and occurs through the skin Figure 9.