Medical Parasitology for Ist year medical student

Snail transmitted parasitic

disease

Presented By:

Dr. Manar Mostafa Dr. Reham Ahmed Lecturer of Parasitology Lecturer of Parasitology

ILOs

By the end of the lecture the student will be able to:

• State general Structure of snails.

• List different stages of parasites inside snails

• List community impact of snail transmitted trematodes

General structure of the snails:

• Snails are metazoan animals with fleshy bodies. They may be marine, brackish, fresh water, terrestrial or amphibious.

• But the snails that are intermediate hosts of trematodes are mostly aquatic except that Onchomelania of Schistosoma japonicum is amphibious.

• The snail is composed of: Solid part (shell)

and soft part (Body)

1- Shell or Exo-skeleton.

Composed of whorls separated by grooves

(sutures). The last and the largest whorl is

the body whorl which ends by an opening.

2- Soft tissues constituting the body. The body is composed of head, foot and a visceral hump, the first two protrude from the shell while the visceral hump is enclosed within the spiral shell.

General structure of shell

•Snails are either right sided (Dextral) or left sided (Sinistral) according to the side of their aperture towards an imaginary line, when the apex is upwards and the aperture is downwards and anterior (facing the observer).

Different stages of the parasite formed inside the snail:

For every trematode there is a proper snail host. The

miracidium recognizes its snail host, probably through

chemotaxis. Inside the snail, asexual development

occurs by formation of sporocyt.

Sporocyst may develop to redia then cercaria eg

( Fasciola spp., Heterophyes heerophyes, Paragonimas spp.)or,

Sporocyst may develop to daughter sporocyst then

cercaria as in schistosoma spp. with no radia stage

• Sporocyst is a simple elongated sac, contain germ cells.

Asexually it may give daughter sporocysts or rupture

releasing rediae.

•Redia is a cylindrical motile feeding stage. Inside its body

either daughter rediae or cercariae develop and get out

through the birth pore. Redia stage is not formed in

Schistosoma species.

• Cercaria is formed of a body and a tail used for swimming in water. Various types of

glands may be found eg: escape glands, cystogenous glands, penetration glands.

• Types of cercariae:

• 1-leptocercus cercaria (Fasciola spp).

2- Lophocercus cercaria (Heterophyes heterophyes).

3- Microcercus cercaria (Paragonimas westermani).

4- Furcocercus cercaria (Schistosoma spp.)

Cercariae leave the snail and are capable of swimming in water

only for short time and they die within 24 – 48 hours if unable to

find the suitable host (eg: Schistosoma spp).

Or transform to encysted metacercaria either on green vegetable

(eg: Fasciola)

or

Inside fish as in Heterophyes heterophes

Example: Life cycle of Fasciola hepatica

Medically important snails that are found in Egypt:

• 1.Lymnaea cailiaudi:

• Dextral (opening is right).

• Thin and fragile.

• Prominent apex.

• Few whorls and large body whorl.

• Aperture 2/3 body length (large)

• Intermediate host (IH) of Fasciola gigantica.

• Fresh water snail.

• Surface feeder , susceptible to molluscicides

2. Lymnaea truncatula: • Dextral (opening is right).

• Thin.

• Prominent apex.

• Few whorls.

• Aperture ½ body length

• Intermediate host (IH) of Fasciola hepatica.

• Fresh water snail.

• Surface feeder , susceptible to molluscicides

• Dextral (right handed opening).

• With siphonal notch in the lower margin of the opening.

• Several whorls having prominent tubercles

• Thick shell.

• First intermediate host (IH) of Heterophyes heterophyes.

• Brackish water snail.

• Bottom feeder.

• Resistant to molluscicides.

3. Pirenella conica:

• Sinistral(opening is left handed).

• Blunt apex with shoulder.

• With umbilicus.

• Intermediate host (IH) of Schistosoma haematobium.

• Fresh water snail.

• Various level of water.

• Susceptible to molluscicides.

4. Bulinus truncates:

• Sinistral (opening is left handed).

• Discoid.

• Intermediate host (IH)

of Schistosoma mansoni.

• Fresh water snail.

• Surface feeder.

• Susceptible to molluscicides

5. Biomphalaria alexandrina:

Snail control :

a) Physical methods: • Clearance of canals from weeds.

• Pitching banks of canals by concrete to prevent plant growth.

• Drying canals or use alternate canals from time to time.

• Traps of palm leaves at canal inlet to collect snails and then destroy them.

• Winter closure of canals.

b) Biological method:

• Introduction of natural enemies, which eat snails as ducks and geese or snails as Marisa spp.

• Plantation of some plants as Balanites aegytiaca, Damseesa, which are toxic to snails.

c) Chemical methods: (Molluscicides)

An ideal molluscicide should be cheap, easy to apply, effective in low concentration and non-toxic to man, animals, fishes and plants. The most commonly used are:

• Copper sulfate: 10-20 part per million.

• Sodium pentachlorophenate: 5-10 p.p.m.

• Bayluscide: 2 p.p.m.