The following points highlight the five major diseases caused by flukes. The human diseases are: 1. Schistosomiasis 2. Opisthorchiasis 3. Paragonimiasis 4. Fasciolopsiasis 5. Fascioliasis.
Human Disease # 1. Schistosomiasis:
Schistosomiasis (Bilharziasis) is caused by the parasitic infection of flukes of three principal species namely Schistosoma mansoni, S. japonicum and S.haematobium. Schistosomiasis is increasing in prevalence, affecting about 10 per cent of the world’s population and ranking second only to malaria as a cause of morbidity and mortality.
S. mansoni is found in tropical Africa, parts of Southwest Asia, South America and Caribbean island; S. japonicum in parts of Japan, China, Philippines, India and several countries of Southeast Asia, and S. haematobium in large regions of tropical Africa and parts of Southwest Asia.
The blood flukes pair migrates to the small venule, where the female fluke deposits immature eggs. S. mansoni and S. japonicum lay eggs in the intestine and S. haematobium in the urinary bladder.
Embryos develop during the passage of eggs through the tissues and the larvae are mature when the eggs pass through the wall of intestine or the urinary bladder. The eggs hatch in freshwater and liberate miracidia and get into the snail.
Here these develop into cercariae which leave the snail, swim freely in water and penetrate the skin of man. Through the blood circulation the cercaria reach the lungs to become adult flukes. The adult flukes then get into the blood stream.
Intestinal schistosomiasis caused by S. mansoni and S. japonicum is associated with granulomas in the lamina propria and sub-mucosa. The flukes cause dysentery and liver diseases followed by fever, sweating, diarrhoea, weight loss and lack of appetite. Urinogenital schistosomiasis is caused by S. haematobium.
In this disease, the eggs are enormous in the bladder, ureters and seminal vesicles, they may also reach the lungs, colon and appendix, Eggs in ureter can cause obstructive uropathy. There is high incidence of carcinoma of the bladder in patients with urogenital schistosomiasis. The diagnosis is made by finding characteristic schistosome eggs in the faeces and urine.
Chemotherapy is now highly effective. The drugs having specific actions on the schistosomes are nitrothiazole compound nitridazole, nilodin, hycanthone and antimony compounds are recommended.
Human Disease # 2. Opisthorchiasis (= Clanorchiasis):
Opisthorchiasis or clanorchiasis is caused by the infection of Opisthorchis (= Clanorchis) sinensis (Chinese liver fluke). This disease is widespread in China, Japan, Korea, Vietnam and India. Human infection is acquired by eating raw or undercooked fish which harbour metacercariae. Adult worms are flat and transparent, live in the bile ducts, and pass eggs to the intestine and finally come out in the faeces.
After ingestion by an appropriate snail, the egg hatches in a miracidium. Cercariae escape from the snail and seek out certain fish which they penetrate and in which they encyst. When human host eat the fish the cercariae emerge in the duodenum, enter the common bile duct and mature in the distal bile ducts to an adult fluke.
The symptoms vary from mild to severe depending upon the number of flukes. An onset with chills and fever indicate bacterial infection from biliary obstruction.
Patients with opisthorchiasis may die from a variety of complications including biliary obstruction, bacterial cholangitis, pancreatitis and cholangiocarcinoma. With massive infection the liver may be up to three times of the normal size. The diagnosis is made by identifying the eggs of O. sinensis in the stools. Antimony compounds, chloroquine and bithionol have been found to be effective remedy for opisthorchiasis.
Human Disease # 3. Paragonimiasis:
Paragonimiasis is caused by an infection of the oriental lung fluke Paragonimus westermani. The infection is widespread in Asia, Africa and South America. In India it is reported from Bengal, Assam and South India. Adult flukes live in the respiratory tract (lungs) of man.
Human hosts acquire the infection by eating raw or undercooked infected crustaceans (crabs and crayfishes). Paragonimus eggs are coughed up from the lungs, swallowed and passed in the stool. Miracidia emerge in water and infect a snail intermediate host after which a sporocyst and two generations of rediae being finally transformed into cercariae.
Infective cercariae emerge and penetrate the gills of a crustacean (freshwater crab and crayfish), the second intermediate host. The cercariae migrate to soft tissue and encyst. After a human host ingests the cyst, a metacercaria emerges and penetrates the wall of the stomach, migrates to the diaphragm, bores through the pleura and settles in the lungs, where it matures into an adult worm, which survives for 20 years.
The lung flukes cause chronic cough with recurring attacks of haemoptysis and eggs in sputa and stool. Night sweats, severe chest pain and pleural effusions are common. Although the adult flukes cause pulmonary disease, the larvae occasionally produce lesions in the brain, liver, gut, skeletal muscles, testes and lymph nodes.
In generalized paragonimiasis there is a fever, generalized lymphadenitis and cutaneous ulceration. Diagnosis may be made by the presence of eggs in sputa or stools. Emetine and chloroquine have both been used for paragonimiasis. Bithionol has been found to give encouraging results. Praziquantel, the drug of choice, is effective against pulmonary paragonimiasis.
Human Disease # 4. Fasciolopsiasis:
Fasciolopsiasis is caused by an infection of Fasciolopsis buski, the giant intestinal fluke.
It has been reported from China, Thailand and Malaysia, Bengal, Assam and other oriental regions. The adult worm lives in the small intestine of man and pig. The normal host is pig which serves as the reservoir of infection for man. It is the largest trematode parasitising man and measures 2.2 to 7.5 cm in length and 8.0 to 20.0 mm in breadth.
Intermediate host is a snail in which it passes through an elaborate developmental cycle, producing metacercariae that leave the snail to be located on fresh water plants particularly the water nuts. Man, the definitive host, acquires the infection of fasciolopsiasis by eating uncooked aquatic vegetables contaminated with the encysted cercaria of F. buski.
The flukes attach to the duodenal and jejunal wall. The point of attachment may ulcerate and become infected, causing pain that resembles to that of a peptic ulcer. Acute symptoms may be caused by intestinal obstruction or by toxins released by large numbers of worms. This is followed by chronic diarrhoea, nausea, and vomiting.
Diagnosis is made by identifying the eggs of F. buski in the stool. Specific antihelminthic drug, tetrachloroethylene, is useful in the eradication of intestinal flukes. Praziquantel is used for the treatment of fasciolopsiasis.
Human Disease # 5. Fascioliasis:
Fascioliasis is caused by the infection of Fasciola hepatica, the sheep liver fluke.
This disease is cosmopolitan. Human may acquire the infection wherever sheep are raised. The eggs, passed by sheep in their faeces, require two weeks in freshwater before a miracidium emerges. Miracidium infects a molluscan intermediate host (lymnaeid snail), after which infective cercariae emerge from the snail and encyst on submerged vegetation.
Humans become infected by eating vegetation that is contaminated by the cysts. Metacercariae encysts in the duodenum, pass through the wall in the peritoneal cavity, penetrate the liver and migrate through hepatic parenchyma into the bile ducts.
The larvae mature to adults and live in hepatic bile ducts. Later the adult flukes penetrate through the wall of the bile ducts and wander back into the liver parenchyma where they feed on liver cells and deposit eggs. The eggs lead to abscesses formation, followed by a granuloma.
The flukes induce hyperplasia of the lining epithelium of the bile ducts, portal and periductal fibrosis, proliferation of the bile ductules and varying degrees of biliary obstruction. Eosinophilia, vomiting and acute epigastric pain are the main symptoms. Severe untreated infections may be fatal.
The diagnosis is made by recovering eggs from stool or from biliary tract. Early diagnosis and aggressive treatment with praziquantel prevents irreparable damage to liver. Bithionol is now the drug of choice for fascioliasis.