Lecture #7a: THE TREMATODES OR FLUKES

General Characteristics

1.      They are leaflike, ovoid and non–segmented.

2.      They are provided with suckers:

a.      Anteriorly located – oral sucker

b.      Posteriorly located – ventral sucker / acetabulum (for attachment)

3.      They are hermaphrodite, i.e., both sexes in one organism

4.      All require body of water to complete the life cycle.

Physiologic characteristics

1.      Digestion is predominantly an extracellular process.

2.      Respiration is essentially anaerobic.

3.      Excretory system is bilaterally symmetrical and open at the posterior end of the body with flame cell or solenocytes.

4.      Nervous system is composed of a group of paired ganglion cells disposed like saddle on the dorsum of the pharynx or esophagus and main pair each posteriorly and anteriorly directed nerve trunk in the dorsal, ventral and lateral fields.

5.      Both male and female reproductive organ system have one common opening known as the genital pore.

a.      The male reproductive organ system is composed of the prostate enclosed by serous pouch or sac followed by dilation known as the seminal vesicle, leading to one vas deferens, then bifurcates into two vas deferens which ends into a pair of testes.

b.      Female reproductive system consist of sac–like structure called the uterus which opens into the seminal receptacle, then the ootype, then the oviduct ending in a single ovary.

c.       The sexually mature digenetic trematode is found in the definitive host.

General rule in classifying flukes

1.      All flukes are leaflike

2.      All flukes are hermaphrodite

3.      All require two intermediate host

4.      The infective stage is the metacercaria

5.      The mode of transmission is by ingestion of the infective form.

6.      All eggs are operculated (except for the Schistosomes)

Host requirements

1.      (1) Definitive host – harbors the adult stage of the parasite (in man).

2.      (2) Intermediate host:

a.      First I.H. – harbors the early larval stage of the parasite (in snails)

b.      Second I.H. – harbors the stage infective to man (in fishes)

Classification of flukes as to habitat

1.      Lung fluke

a.      Paragonimus westermanii

2.      Intestinal fluke

a.      Fasciolopsis buski

b.      Echinostoma ilocanum (immature egg)

c.       Heterophyd group (mature egg)

(1)   Heterophyes heterophyes

(2)   Metagonimus yokogawai

(3)   Haplorchis yokogawai

3.      Liver fluke

a.      Fasciola hepatica

b.      Chlonorchis sinensis

c.       Opistorchis felineus

4.      Blood fluke

a.      Schistosoma japonicum

b.      Schistosoma mansoni

c.       Schistosoma haematobium