Schistosomiasis Essay

7/22/2019 Schistosomiasis Essay

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Case Study Two Essay: Human Schistosomiasis

Laura Jenkins – 17428794

MED3MSA

Human Schistosomiasis is a common tropical di sease of parasiti c origin . Br iefl y descri be thi s disease

and discuss why schistosomiasis is considered to have such a high “burden of disease”. You should

include a discussion of what eff orts are being made to contr ol the spread of schistosomiasis.

Schistosomiasis is an infectious disease posing a major problem in 74 developing countries, with an

estimated 207+ million people affected (Stone, 2005). The infection is caused by schistosoma worms that

use snails present in fresh water as an intermediate host before infecting humans (Conlon, 2005).

There are five known schistosomes that have the ability of

causing human schistosomiasis and these are classified as

Schistosoma mansoni, haematobium, japonicum,

intercalatum and mekongi. The five species are known to

affect various regions of the world and infect different types

of intermediate snail hosts, however transmission is generally

the same (Ross et al., 2002). Transmission of human

schistosomiasis occurs in a cycle seen in figure 1, where

female schistosoma release their eggs into water, the eggs

release miracidium which find a snail intermediate host, the

miracidia in the host will multiply developing into cercarial

larvae. The larvae remains in the snail for a few weeks before

re-entering the water where within a week they will penetrate

the skin of a human host migrating via the bloodstream to

various parts of the body forming into adult schistosomes.

The schistosome is excreted from the human body and the

cycle will repeat (Gryseels et al., 2006). Figure 1: The c ycle of human schistosomiasis (Pearce and MacDonald, 2002)

Human schistosomiasis can be classified into two categories depending on onset and symptoms; this

includes acute and chronic schistosomiasis (James and Colley, 1995). Acute schistosomiasis is a

hyperactivity reaction to cercariae upon penetration of the skin, causing symptoms such as a rapid fever

and patchy infiltrates in the pulmonary system to arise weeks to months following primary infection.




Acute schistosomiasis is primarily present in travelers and is uncommon amongst individuals from

developing countries (Burke et al., 2009). Individuals from the endemic developing countries are more

susceptible to chronic schistosomiasis. This form of schistosomiasis is induced by granuloma formation

from the human immune system’s reaction to antigens released by schistosome eggs. The inflammation

induced by the immune system can promote the movement of the eggs to other regions of the body

inducing symptoms such as; tissue damage, diarrhea, liver disease and gastrointestinal disease (Ross et

al., 2002).

Schistosomiasis is a major issue for sub-

tropical developing countries, where

those most at risk are found in sub-

Saharan Africa, Asia, South America and

the Middle East, this can be visualized in

figure 2. Each year there are 200 million

people that are infected by the disease

however there are 600 million people

that have the threat of contracting the

infection (Conlon, 2005).

Figure 2: The worldwide distribution of human schistosomiasis (Conlon, 2005)

As so many individuals are affected by schistosomiasis, the infection is associated with a high burden of

disease which is contributed by both its mortality and morbidity. Burden of disease is a measure of a

population’s health status; the unit of measurement for burden of disease is the disability adjusted life

year (DALY’s) which takes into account years of life lost to the disease (YLL) and years of years lived

with disability (YLD) (Essink-Bot et al., 2002). The chronic form of schistosomiasis contributes highly to

the YLD, where the schistosome species may remain in the host for up to 40 years thus greatly affecting

the individuals’ quality of life (Ross et al., 2002). Each year there is an estimated 200,000 mortalities due

to human schistosomiasis this figure greatly contributes to the YLL aspect of burden of disease (Conlon,

2005). It is approximated that worldwide 1.532 million DALYs are lost to schistosomiasis with a majority

of these DALYs being associated with sub-Saharan Africa, these figures account for 0.1% of the global

burden of disease (Gryseels et al., 2006).

It is estimated with current data that the DALYs for schistosomiasis are to increase by up to thirty fold in

the future, therefore prevention, control and treatment are important in decreasing burden of disease

(Gryseels et al., 2006). Treatment and control for schistosomiasis can vary depending on the type of

infection the individual is infected with; for chronic schistosomiasis and mass-population treatment the




drug of choice is Praziquantel a broad spectrum antiheminthic drug with minimal side effects and a cure

rate of 85% (Conlon, 2005). Prazinquantel however has proven to be less effective in individuals with

acute schistosomiasis and may cause symptoms to worsen. Instead acute schistosomiasis is treated by

corticosteroids or a combined therapy of the drugs arthemeter and prazinquantel (Jaureguiberry et al.,

2010). Prevention of the infection also differs with the type of infection; as acute schistosomiasis is

prevalent in travelers it is recommend whilst visiting endemic developing countries, individuals avoid

exposure to fresh or contaminated water as it only takes five minutes of exposure to the water to contract

the infection (Jaureguiberry et al., 2010). Prevention of chronic schistosomiasis is more difficult to obtain

as the problem is more large scale. Eliminating the intermediate snail host using molluscicides, mass

population-chemotherapy and establishing a sanitary sewerage and water system would all work to

greatly reduce the spread of the infection, however all the methods are costly making them difficult to

achieve (Stone, 2005).

Human Schistosomiasis is a major continuing problem faced in sub-tropical developing countries,

contributing highly to the burden of disease of each of the affected individual countries and the global

burden of disease. Prevention and treatment methods have somewhat improved mortality and morbidity

rates, however further strides into providing a sanitary water system to control spread of the infection are

yet to be taken making the infection a constant threat to millions of people worldwide.




References

BURKE, M. L., JONES, M. K., GOBERT, G. N., LI, Y. S., ELLIS, M. K. & MCMANUS, D. P. 2009.

Immunopathogenesis of human schistosomiasis. Parasite Immunology, 31, 14.

CONLON, C. P. 2005. Schistosomiasis. Medicine, 33, 4.

ESSINK-BOT, M.-L., PERIERA, J., PACKER, C., SCHWARZINGER, M. & BURSTROM, K. 2002. Croos-national

comparability of burden of disease estimates: the European Disability Weights Project. World

Health Organization Bulletin of the World Health Organization, 80, 9.

GRYSEELS, B., POLMAN, K., CLERINX, J. & KERSTENS, L. 2006. Human Schistosomiasis. Seminar, 368, 13.

JAMES, S. & COLLEY, D. 1995. Schistosomiasis. Current Opinion in Infectious Diseases, 8, 5.

JAUREGUIBERRY, S., PARIS, L. & CAUMES, E. 2010. Acute Schistosomiasis, a diagnostic and therpeutic

challenge. Clinical Microbial Infection, 16, 7.

PEARCE, E. J. & MACDONALD, A. S. 2002. The Immunobiology of Schistosoimasis. Nature Review, 2, 14.

ROSS, A. G. P., BARTLEY, P. B., SLEIGH, A. C., OLDS, G. R., LI, Y., WILLIAMS, G. M. & MCMANUS, D. P.

2002. Schistosomiasis. The New England Journal of Medicine, 346, 10.

STONE, C. 2005. Schistosomiasis. Journal of Diagnostic Medical Sonography, 21, 5.

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Schistosomiasis Essay