Schistosomes

Family Schistosomatidae – Blood Flukes Chapter 16

Blood flukes belonging to the genus Schistosoma are important parasites of humans:

• disease is ____________________________________

• disease is named after German Theodor Bilharz who identified adult parasite in the 1851

Schistosomiasis in one of the major parasitic diseases in the world - 270 million persons are infected (1 out of every 12 persons).

Disease is on the increase in the world because:

1.

2.

3.

4.

Schistosomiasis is distributed in parts of the tropics.

It is not endemic in the continental U.S. but does occur in Puerto Rico and other Caribbean islands.

Cases in U.S.?

3 major species of human Schistosoma

Schistosoma mansoni - adults occur in the posterior mesenteric veins of the __________________________

Schistosoma japonicum - adults occur in the anterior mesenteric veins of the _____________________

Schistosoma haematobium - adults occur in the veins of the _____________________________

(There are 2 other minor species in humans we will not consider.)

Morphology of Adult Schistosoma

Schistosoma is ____________________.

Male worms are shorter and stouter than females.

• males have _____________________________ - ventral longitudinal groove in which the female resides

• several testes are located behind the acetabulum

Morphology of Adult Schistosoma

Female worms are thin and long – resides in the gynecophoral canal of male

• reproductive organs are arranged linearly

Both sexes have oral sucker and acetabulum.

Intestine (cecum) divides, then fuses near middle of the body.

Morphology of Adult Schistosoma

Differences of Adult Schistosoma

Adults of the 3 species differ in:

1.

2.

3.

4.

However, there is overlap of many characteristics.

We will not be responsible for identification to species.

Differences of Adult Schistosoma

Differences of Schistosoma Eggs

Specific diagnosis is determined by ID of eggs

S. mansoni - elliptical egg with ________________________________________

S. haematobium - elliptical egg ________________________________________

S. japonicum - round to oval egg with __________________________________

Life Cycle – similar in 3 spp.

1. Adult female enters small venules in wall of intestine or urinary bladder and deposits eggs.

2. Eggs work their way through the tissue by mechanical action and reach the lumen.

- Eggs of S. mansoni and S. japonicum are voided in _____________________.

- Eggs of S. haematobium are voided in __________________.

What happens to most eggs?

Life Cycle – similar in 3 spp.

3. Eggs embryonate during passage through the tissues and hatch soon after eggs are released into water.

4. _______________________ hatches from egg and swims for 1-6 hours to find a snail intermediate host.

The three species of schistosomes utilize 3 different snail intermediate hosts.

5. Within the snail, _________________________develops which produces _________________________.

(no redia stage)

Life Cycle – similar in 3 spp.

6. ______________________ ________________________ emerge from daughter sporocysts and leave the snail.

 - Over 200,000 cercariae may be produced in a snail infected with a single miracidium!

Life Cycle in Human

.7. Cercariae swim for 1-3 days until they find human host.

Cercariae are attracted to human by skin secretions.

8. On human skin, cercariae release histolytic enzymes from penetration glands and ________________________________

A cercaria loses its tail and becomes a ______________________________

9. Schistosomula enter circulation and reach the liver where they mature to adults.

Life Cycle in Human

.10. Adults migrate down the hepatic portal vein into the mesenteric veins of intestines or urinary bladder where they mate and females begin to produce eggs.

 - Egg production begins 5-8 weeks after cercarial penetration.

Differences in Life Cycles of 3 species

Schistosoma japonicum

Distribution - Orient (China, Southeast Asia, Philippines, and South Pacific Islands)

Adults inhabit the ______________ _____________________________ _____________________________

Adult female is proliferic egg layer - up to 3,000 eggs are deposited in the submucosa of the small intestine daily.

Pathology of Schistosoma japonicum

1. Eggs in the _______________ ________________________ cause host inflammatory reaction and fibrous scar tissue nodules are formed.

- these nodules may ulcerate and result in hemorrhaging

- some eggs migrate through the wall to be released in the feces

Pathology of Schistosoma japonicum

2. Eggs are swept up the hepatic portal vein into the ____________________

- ________________________ occurs around the eggs

- _______________________ - areas of scar tissue form around eggs

  - cirrhosis and hepatosplenomegaly result

Pathology of Schistosoma japonicum3. In 9% of cases, eggs reach the _______________causing

severe tissue reaction resulting in paralysis, coma, and death.

No pathology is caused by the adults.

Pathology of Schistosoma japonicum

Schistosoma japonicum is considered the most pathogenic of the three species:

1. Produces _______________________

2. Eggs cause greater pathology in ________________

3. Only species in which eggs can enter the ________________

Biology of Schistosoma japonicum

Many ______________________________________ serve to maintain the infection for possible human infection. Other hosts are cattle, pigs, goats, dogs, cats, deer, and rodents.

Thus, this disease is a ____________________.

CONTROL of S. japonicum is difficult due to:

(1) numerous reservoir hosts

(2) snail intermediate host is _______________________________ and only visits water to lay its eggs. Molluscicides are ineffective.

Schistosoma mansoni

Distribution - widely distributed in various parts of Africa, Middle East, South America, and Caribbean Islands  

Schistosoma mansoni

Adults live in the ___________________________________ _________________________________________________

Female releases eggs into the wall of the large intestine and eggs work their way into the lumen and are released in the feces.

Less than 100 eggs are produced each day, but egg deposition occurs for the life of the worm (4 years up to 26 years).

Pathology of Schistosoma mansoni

1. Eggs in the ______________________________________ stimulate host

inflammatory reaction followed by fibrous scar tissue deposition.

• Fibrous scar tissue forms nodules along the large intestine.

• Some eggs make their way through the intestine to enter the feces

Inflammatory reaction to eggs Fibrous scar tissue nodules

Pathology of Schistosoma mansoni

2. Eggs are swept up the hepatic portal

vein into the liver

• __________________________ is ollowed by scar tissue formation of ______________________

• cirrhosis and hepatosplenomegaly occur

Cirrhosis – areas of scar tissue in liver

Pathology of Schistosoma mansoni

• ___________________________________ (accumulation of fluid in

abdominal cavity) is common

• This produces the typical "swollen belly" associated with chronic schistosomiasis

On the left, a boy of 11 shows massive liver and spleen enlargement.  On the right, a boy of 9 in a well advanced stage of early chronic infection.

Pathology of Schistosoma mansoni

No brain involvement as eggs are too big to pass through the liver.

Again, no pathology is caused by the adults.

Disease is _____________________________ and years may elapse before the parasite will kill its human host.

Natural infections also occur in monkeys and baboons but disease is not a zoonosis as humans and primates do not live together. Thus, it is ________________________________.

Schistosoma haematobium

Distribution - through Africa and in parts of the Middle East

Co-exists with ____________________________________ in much of Africa especially along the Nile River Valley.

Schistosoma haematobium

Adults live in the ________________ ___________________________

Most eggs pass through the wall of the urinary bladder and are voided in the urine.

Invasion of other organs by eggs? ___________________________

Thus, this species is considered to be the least pathogenic of the 3 species.

Pathology of Schistosoma haematobium

1. _____________________________ - eggs in the tissues of the urinary bladder cause inflammation. Intense pain occurs at end of urination.

2. __________________________ - blood in the urine is common

Pathology of Schistosoma haematobium

3. _________________________ deposition around eggs in the urinary bladder wall is common and tissues lose their elasticity.

4. Infection also increases the likelihood of ___________________________

Again, all pathology is caused by the eggs.

Adults cause no pathology.

Biology of Schistosoma haematobium

It is thought that this famous individual was infected with Schistosoma haematobium.

Who is he?

How could he have gotten it?

Biology of Schistosoma haematobium

NO reservoir hosts. This parasite is _______________________________.

 Human infection is a result of infected persons urinating in ponds, lakes, streams, and irrigation ditches.

Irrigation practices in the Nile River Valley due to the building of the Aswan High Dam have increased the prevalence of this parasite from 5% to over 35%.

Movie on S. haematobium in Nigeria:http://www.cartercenter.org/news/multimedia/HealthPrograms/EndingtheAnguishofaSilentDisease2008.html

Diagnosis of Schistosomiasis 

Diagnosis is by ______________________________________________

 - this is augmented by use of concentration techniques or tissue biopsy.

Immunological tests to identify schistosome antigens have been developed recently and are also effective in diagnosis.

 - ELISA test is 100% effective

- most useful when?

Treatment of Schistosomiasis 

Drugs are available: 1. Antimony compounds

2. Anti-malarial drug Artemisinin

3. Oxamniquine

4. Praziquantel

5. Calcium-channel blockers ?

Drug treatment is difficult when large populations must be treated.

- some drugs are toxic

- all require long-term treatment and constant monitoring

- adults may migrate back into the liver where drugs are less effective

- drugs are expensive

- Praziquantel resistance is now appearing in ____________________

Drug treatment is not effective once liver involvement occurs.

Prevention of Schistosomiasis 

1. ___________________________________

• Human insistence on pollution of water with their wastes is difficult to change.

• People need to know to stay out of infected water

2. ____________________________________ - problems just mentioned

Prevention of Schistosomiasis 3. ______________________________________- avoid contact between

human wastes and water containing snails.

•  Livelihood around water (crop irrigation, fishing, washing clothes, bathing, etc.) perpetuates schistosomiasis.

•  Costs money to bring clean water into a village

Prevention of Schistosomiasis 

4. Snail control by use of _______________________________.

•  Application is tricky - too high a dose will kill fish

- too little may not kill all the snails and their numbers will be back to the same in a few months

•  Not effective against the amphibious snail of S. japonicum as snail can move onto land away from the molluscicide

Prevention of Schistosomiasis 

5. Introduction of ____________________________________ – shown some success

Prevention of Schistosomiasis 

No vaccine or drug preventative is yet available to prevent schistosomiasis

Nobel prize is medicine is waiting for someone who can find an effective vaccine.

Immunology of Schistosomiasis 

In many parasitic infections, the parasite or its eggs are detected by our immune system as being "foreign" (this is an ________________________).

If the antigen (parasite) is too large to be phagocytized, the host immune system attacks this foreign invader by producing inflammation and fibrous scar tissue around the parasite or its eggs to "wall it off" from other tissues.

This occurs in schistosome eggs, liver fluke infection, and lung fluke infection.

Immunology of Schistosomiasis 

Adult schistosomes live within the bloodstream and are in constant contact with host lymphocytes which detect antigens and start the immune response.

Why aren't adult schistosomes detected?

Immunology of Schistosomiasis 

Adults have the ability to acquire ______________________________________ (proteins in the bloodstream) and coat their _______________ with these molecules.

This coating takes 3 days but once it is accomplished, the parasite is recognized by lymphocytes as “a human cell" and is protected from a host immune response.

As a result, they are not detected by the immune system and no pathology is caused by the adults.

Swimmer’s Itch

Swimmer's itch is caused by _____________________________ ____________________________________________________

Swimmer's itch is common in Midwest and along coastal areas of the U.S.

- itch was first described at U. Michigan Biological Station in 1936

- extensively studied in Wisconsin in the 1930's & 40’s

- problem is common in Wisconsin lakes and occasional outbreaks occur (recent outbreak in Half Moon Lake in Eau Claire)

Swimmer’s Itch

3 genera of schistosomes cause swimmer's itch in the Midwest:

Gigantobilharzia - adult blood flukes occur in mesenteric veins of ____________________________________

Trichobilharzia - adults in mesenteric veins of _________________________________

Schistosomatium - adults in mesenteric veins of _________________________________

Life Cycle

Life Cycle of Swimmer’s Itch

Humans become infected when snail intermediate hosts of these parasites release_________________________which mistake humans for the definitive host.

Infection results when __________________ _____________________________________

 Snail hosts are common pond snails in the genera Physa and Lymnaea.

Pathology of Swimmer’s Itch

Cercariae penetrate epidermis but are unable to enter the dermis.

Cercariae die in the epidermis and an ______________________________ is produced against the dead cercariae

Formation of a ____________________________ (dermatitis) occurs at the sites of cercarial penetration.

Intense itching also occurs.

Pathology of Swimmer’s Itch

Dermatitis is mild in initial exposures but may become severe in persons exposed to previous outbreaks – called _______________________

Itching and breaking of skin may lead to secondary infections.

Rash has been misdiagnosed as the ______________________

Treatment and Prevention of Swimmer’s Itch

TREATMENT - topical creams such as ___________________________ reduce the itching and ______________________________ reduce the inflammatory response

PREVENTION – best way to reduce swimmer’s itch is to ____________________________________________________________________________________

• Snail control efforts (using copper sulfate as molluscicide) have been attempted but generally have been unsuccessful.

•  Removal of bird or mammal definitive hosts is not possible.

Swimmer’s Itch

Swimmer's itch is a problem to _________________________ __________________________________________________

- Swimming beaches are common sources of infection.

- First outbreaks occur in late June and early July when vacationers are numerous.

But problem is often kept hushed, as resort owners don’t want you to know that swimmer’s itch is a problem in their lake, as they don’t want to lose business.