Parasitology

OVERVIEW OF PUBLIC HEALTH

Written by MicroDok

Public health is a branch of medical sciences that primarily deals with the surveillance, control, detection and prevention of the emergence and spread of infectious diseases in a defined human population. It involves the protection and improvement of the health of the general public. In clinical medicine, doctors and nurses practice their profession by looking at the health issues of individuals who turn out to be their patients, but in the practice of public health, the entire community and not individuals are regarded as patients. Public health professionals go beyond treating individuals when they fall sick or become injured (which is usually the main task of clinicians) to preventing infectious diseases and other health-related emergency from either occurring or re-occurring within a specific human population. It is the scientific discipline that promotes the health of the general public by preventing, detecting and controlling infectious diseases as they emerge. Public health is a field of the medical sciences that ensures improved quality of life for the general public by making sure that people imbibe good environmental sanitation and personal hygiene aside other relevant precautionary measures as sustainable means of restrain the emergence and spread of infectious diseases.

Infectious diseases are a leading cause of morbidity and mortality across the globe, and they have more ravaging effects in developing or low income countries where environmental sanitation, water supply, standard of living, medical care amongst other developmental factors are still poor. Infectious diseases including malaria, HIV/AIDS, cholera, Ebola virus, Lassa fever, multidrug resistant bacteria, hepatitis, gonorrhea, influenza, tuberculosis and food borne diseases impose colossal economic burden on the entire society. They can cripple the workforce of any nation if taken lightly, and thus timely detection and containment is paramount amongst all other factors. A well trained, prepared and proactive public health labor force is critical in any nation as a panacea to dealing with the threats of emerging and re-emerging infectious diseases.

Some key functions of public health personnel’s include the following:

  • To protect the health of the entire community (including a village, local community/district or an entire nation).
  • To improve the health status of an entire community through health education and promotion of personal and communal hygiene.
  • To focus on the general populations rather than individuals in the research and development of preventive measures against contracting infectious diseases.
  • To be proactive, and develop therapeutic and defensive measures against the emergence and spread of infectious diseases in the community.

The development and use of vaccines, antibiotics, and other antimicrobial agents in the fight against infectious agents (including bacteria, viruses and fungi) have significantly saved countless number of people from the fatality caused by infectious diseases. However efficacious and powerful these antimicrobial agents are, and however proactive the activities of public health specialists are in containing the menace of infectious diseases; infectious diseases cannot be completely eradicated from planet earth. This is due in part to the ingenuity of microorganisms (including viruses, protozoa, bacteria and fungi) in evolving new ways of overcoming the protective capacity of the body’s immune system. Secondly, microorganisms are constantly evolving and re-evolving, and many have even developed resistant to some available therapeutic measures.

Pathogenic microorganisms have become more sophisticated and can now survive even in the face of antimicrobial onslaught. Some have even developed mechanisms that allow them to remain undetected in vivo or use the body’s immune system to their own advantage. The entire population still remains at high risk of the menaces of infectious diseases, with the women, children, and the immunocompromised remaining the most vulnerable. New infectious diseases are emerging and even old ones which were taught to have been eradicated are now re-emerging and have remained obstinate in the human population. Irrespective of the sophistication of pathogenic microorganisms and their inventiveness in evading the body’s immune system and the action of antimicrobial onslaught, it still remains the duty of public health professionals to intervene in the cycle of infectious disease development and spread in the entire community. This can be made possible through improving available public health systems, encouraging environmental sanitation and safety, and also to develop sustainable measures of containing infectious diseases whenever and wherever they may occur. Since public health professional’s regards and look at the general population as its patient, it is vital that relevant health information’s are constantly made available to local communities, vulnerable groups, and the entire population on how to keep infectious organisms at bay, and on how to act when there is any disease outbreak in their locality.

The distribution of parasitic diseases which are of public concern are influenced by many factors including climatic changes, increased globalization and free movement/transport of people across the globe which allows pathogens to be moved from one geographical location to another. Public health microbiologists help to control the spread of communicable diseases including tropical and subtropical parasitic diseases through timely detection and reporting. They also monitor food establishments and water supplies in an attempt to keep them safe and free from infectious agents including microorganisms. In this book, parasitic diseases as it affects the many parts of the African continent and other parts of the tropic and subtropical regions including Asia and South America shall be expanded as a key public health issue in this book. The occurrence and widespread distribution of parasitic diseases across the tropical and subtropical countries of the world is a public health challenge of great importance confronting these regions, and which calls for concerted global efforts to assuage the malady.

Though some of these diseases (termed infectious diseases of poverty, IDP) are no longer a headache in most developed economies such as China, the USA, UK and the rest, the technological and medical advances in these nations coupled with their successes in other areas (the environment, water supply, and sanitation) which have long freed their regions from some of these diseases are also vital to the eradication of IDPs in the developing world, most especially in the African continent. However, it still rest on the shoulders of the African people themselves and other developing economies who are currently undergoing the menaces of IDPs (such as malaria, tuberculosis, schistosomiasis, leishmaniasis, filariasis and onchocerciasis) to come up with innovative ways of totally eradicating or bringing under control the IDPs in their regions since  a good chunk of these diseases (though still of global health importance) are no longer of great concern to the developed countries of the world who have in their own approach to things eradicated some of these diseases from their nations. The World Health Organization (WHO) in view of the fact that IDPs (though a problem of a specific region of the world) still portend a grave danger to the public health of everybody (developed or developing nations), has through the World Bank and its sister agencies started the Special Programme for Research and Training in Tropical Diseases (TDR) which has helped the developing countries (Africa in particular) with the technical know-how, resources, trainings and expertise directed at eradication or containing IDPs in the region.

Most importantly, apart from providing research grants and other resources to tackle the diseases of poverty in the sub region, the TDR programme of WHO has also helped to develop and equip indigenous and skilled manpower (including scientists, researchers, academics, health professionals and the rest) who will harness local resources and technology to the effective containment of IDPs in their countries.  Parasitology is the study of parasites and their interaction with other organisms (i.e. their hosts). It is the study of parasites, their ecology, transmission and life cycle. The study of parasites is as ancient as mankind as man became conscious of his immediate environment. The field of Parasitology was fully established and widely appreciated in the early 1860’s. During this period, scientists as at the time postulated that protozoa and helminthes (which are both parasites) were actually responsible for some diseases of man and animals. Diseases and infections caused by parasites are by far amongst the top-most ever-present malady that man and his domestic animals encounter on almost daily basis aside bacterial, viral or fungal infections. The disease burden of parasitic infections cannot be over-emphasized, and this menace is experienced most in tropical developing countries where both natural and man-made activities has made it possible for the parasites to continue their devastating effects almost unperturbed.

Though some bacteria, fungi and viruses are pathogenic, and cause disease and infections in humans; these microorganisms are not covered in the study of Parasitology even though they can coexist parasitically with man. Not all parasites are microorganisms (such flea and tick) but all microorganisms are parasitic in nature, and they cause harm or disease in their host. There are about 10,000 species of protozoal parasites with different life cycles, and that cause a variety of diseases in human beings and animals. Parasitic infections (example malaria) accounts for billions of resource waste in most parts of the world where they are a great source of concern and burden to health practitioners because they account to a colossal amount of mortality and morbidity in these regions (in the African and Asian continent where these organisms are most prevalent). Understanding the life cycles of these parasites and the mechanisms of their pathogenesis is critical to the containment of the diseases that they cause. There spread and transmission in any population can also be better controlled; and a reduction in economic wastages due to them will be achieved as well.

Parasites are living organisms that establish a physiological association with the tissues on the surface or inside the body of another organism called the host (example man, animal and host). The association between a parasite and a host is usually one that inflicts some level of injury and discomfort to the host, and this occasionally results to a disease or infection in the affected organism or host. This kind of association or relationship between a parasite and a host is called parasitism. In such relationships, only the parasites benefit while the host is often left diseased in the cause of the relationship. The host does not benefit but rather suffers the sole dependability of the parasite on it for food, water, shelter and protection. The host of a parasite can be definitive host, intermediate host or reservoir host.

Reservoir hosts store or harbour the parasite, and they serve as medium via which other susceptible human or animal hosts become infected by the protozoa. Definitive hosts are hosts that maintain the mature infective form of the parasite; and in some cases some clinically important parasites have humans as their only definitive hosts (example Trichinella spiralis). Sexual development of parasites takes place in the definitive host. Intermediate hosts are the second hosts of a parasite apart from the definitive host; and they are known to substitute with the definitive host and are usually known to harbour some reproductive stages of the parasite like the definitive host. Asexual development of parasites takes place in the intermediate host. In establishing a disease condition, parasites usually replicate intracellularly (i.e. inside the cells of its human host) as is seen in the Plasmodium species which solely multiply in the blood or extracellularly (outside the cells of its host) as is seen in Giardia species that develop in the lumen of the intestinal tract. There are a wide variety of parasites or protozoa that parasitize humans and cause diseases or parasitic infections (Table 1); and some of these infections are subclinical in nature while the others are symptomatic and are presented with some clinical episodes that make the individual seek for medical help. The parasites that infect man and other animals are usually found all over the world and close to their immediate environments; and such organisms constitute one of the world’s greatest danger to the socioeconomic and well being of humans.

Table 1: Some clinically important parasites and their clinical features

Parasite Group Morphological form Host Disease
Ascaris lumbricoides Roundworm/

Intestinal nematode

Eggs and adult worms

 

Humans Ascariasis
Ancylostoma duodenale Intestinal

hookworm

Eggs Humans Hookworm

disease

 

Acanthamoeba species* Amoeba Cyst and trophozoite Amoebiasis

 

Balantidium coli* Ciliate Cyst and trophozoite Humans and pig Balantidial

dysentery

Brugia species* Filarial

nematode

Microfilaria Cat, humans and monkey Lymphatic

filariasis

Clonorchis sinensis* Trematodes/

fluke

Eggs Cat, pig, dog, humans, mouse, fish and snail Clonorch

iasis

Cyclospora cayetanensis* Intestinal

coccidian

Oocysts Humans Diarrhea in immunoco

mpro

mised host

Cryptosporidium parvum* Intestinal

coccidian

Oocysts Humans, livestock, domestic and wild animals Cryptosp

oridios

is and diar

rhea

in immun

ocomp

romised

host

Dracunculus medinensis* Guinea

worm or

filarial

nematode

Larva Humans and Cyclops Dracuncul

iasis

 

Diphyllobothrium latum* Tapeworm/

cestode

Eggs Cat, humans, dog, crustaceans and fish Diphyllob

othria

sis

 

Entamoeba histolytica* Amoebae Cyst and trophozoite

 

Humans Amoebic

dysentery

 

Enterobius vermicularis Threadworm/

intestinal

nematode

Eggs and adult worms

 

Humans Enterobiasis
Echinococcus granulosus Tapeworm/

cestode

Larva Sheep, humans, dog, and wild animals Echinoco

ccosis

Fasciolopsis buski Trematodes/

fluke

Eggs Snails, humans and pig

 

Fasciolops

iasis

Fasciola hepatica Trematodes/

fluke

Eggs Snail, humans and cattle Fascioliasis
Giardia lamblia* Flagellate Trophozoite and cyst

 

Humans Giardiasis
Hymenolepis nana  Tapeworm Eggs Humans Hymeno

lepiasis

 

Isospora belli* Intestinal

coccidian

Oocysts Humans Isosporiasis

and diarrhea in immunocom

promised

host

 

Leishmania species* Flagellates Amastigote Sandfly, humans, dog and rodent

 

Leishmania

sis

Loa loa Filarial

nematode

Microfilaria Chrysops and humans Loiasis or

calabar

swelling

Necator americanus Hookworm/
intestinalnematode
Eggs Humans Hookworm disease
Onchocerca volvulus Filarial

nematode

Microfilaria Blackfly and humans River blind

ness or

onchocerc

iasis

 

Opisthorchis viverrini Trematodes/

fluke

Eggs Snail, humans, cat and fish Opisthor

chiasis

Paragonimus westermani Trematodes/

fluke

Eggs Snail, humans, carnivores, crab and other crustaceans Paragonim

iasis

Plasmodium species* Blood/tissue

coccidian

Trophozoite, schizont and gametocyte Mosquitoes and humans Malaria
Schistosoma species Trematodes/

fluke

Eggs Snail, humans, cat, rodent and cattle Intestinal & urinary

schistoso

miasis

 

Strongyloides stercoralis Intestinal

nematode

Larva Humans Strongyl

oidiasis

Taenia solium Tapeworm Eggs Pig and humans Taeniasis
Taenia saginata Tapeworm Eggs Cattle and humans Taeniasis
Trichinella spiralis Filarial

nematode

Larva Humans, pig, rodent and wild animals Trichinel

losis

Trichomonas vaginalis*   Flagellate Trophozoite Humans Vaginitis (in females) and urethritis (in males)
Trypanosoma species* Flagellate Trypomastigote Tsetse fly, humans, triatomine bugs, cat and dog African trypanoso

miasis

(sleeping sickness) and American trypanoso

miasis

(Chagas

disease)

Toxoplasma gondii* Blood/tissue

coccidian

Oocysts Humans, cat, pigs, birds and rodents

 

Toxoplas

mosis

Trichuris trichiura  Whipworm/

intestinal

nematode

 

Eggs Humans Trichuriasis
Wuchereria bancrofti Filarial/tissue nematode Microfilaria Mosquitoes and  humans Bancroftian filariasis (elephant

iasis)

Most of these parasites are usually located and distributed in the tropical regions of the world because of some peculiar features of these areas. Typically examples include the Plasmodium parasites which are inherently found in tropical regions of the world especially in the African continent (Nigeria for example). Plasmodium which causes malaria in man has over the years even now impacted negatively on the wellbeing and socioeconomic welfare of human beings, and this scenario has resulted to the high morbidity and mortality rates recorded in malaria disease cases across Africa. Most parasitic diseases (malaria) are major diseases of the tropical and subtropical regions; and these diseases which are often known as tropical infections are serious public health problems in these parts of the world where the diseases are endemic. Aside the climatic conditions of the tropical and subtropical regions which make them more prone to most parasitic infections than the temperate world; poor health policies, poor environmental sanitation and poverty amongst other factors are amongst the key reasons why some of these diseases are still widespread in these regions, and the situation is pathetic in most developing parts of Africa. In this section, some of the important tropical diseases (especially those that are caused by protozoa) as well as their pathogenesis, control and preventive measures shall be discussed.

PLEASE CHECK SUBSEQUENT SECTIONS FOR DETAILED EXPLANATION OF THE MANY PARASITIC DISEASES AS COVERED IN PARASITOLOGY

REFERENCES

World Health Organization Control of the leishmaniases: report of a meeting of the WHO expert committee on the control of the leishmaniases. World Health Organ Tech Rep Ser. 2010; 949:1–186.

World Health Organization, WHO (2013). Dracunculiasis eradication: global surveillance summary, 2012. Wkly Epidemiol Rec, 88:189–200.

World Health Organization. World Malaria Report 2009. Geneva, Switzerland: World Health Organization; 2009. Available at http://www.who.int/malaria/world_malaria_report_2009/en/index.html. Accessed January 04, 2014.

Taylor LH, Latham SM, Woolhouse ME (2001). Risk factors for disease emergence. Philos Trans R Soc Lond B Biol Sci, 356:983–989.

Stedman’s medical dictionary, 27th edition. Philadelphia: Lippincott, Williams and Wilkins.

Steketee R.W, Campbell C.C (2010). Impact of national malaria control scale-up programmes in Africa: magnitude and attribution of effects. Malar J, 9:299.

Summers W.C (2000). History of microbiology. In Encyclopedia of microbiology, vol. 2, J. Lederberg, editor, 677–97. San Diego: Academic Press.

Snow R.W, Guerra C.A, Noor A.M, Myint H.Y and Hay S.I (2005). The global distribution of clinical episodes of Plasmodium falciparum malaria. Nature, 434:214-217.

Snow R.W, Trape J.F and Marsh K (2001). The past, present and future of childhood malaria mortality in Africa. Trends Parasitol, 17:593-597.

Schneider M.J (2011). Introduction to Public Health. Third edition. Jones and Bartlett Publishers, Sudbury, Massachusetts, USA.

Ruiz-Tiben E, Hopkins D.R (2006). Dracunculiasis (Guinea worm disease) eradication. Adv Parasitol, 61:275–309.

Roberts L, Janovy J (Jr) and Nadler S (2012). Foundations of Parasitology. Ninth edition. McGraw-Hill Publishers, USA.

Rothman K.J and Greenland S (1998). Modern epidemiology, 2nd edition. Philadelphia: Lippincott-Raven.

Principles and practice of clinical Parasitology. Edited by Stephen H. Gillespie and Richard D. Pearson. John Wiley and Sons Ltd. Chichester, New York.

Palatnik-de-Sousa CB, Day MJ (2011). One health: the global challenge of epidemic and endemic leishmaniasis. Parasit Vectors, 4:197.

Nelson K.E and Williams C (2013). Infectious Disease Epidemiology: Theory and Practice. Third edition. Jones and Bartleh Learning

Mandell G.L., Bennett J.E and Dolin R (2000). Principles and practice of infectious diseases, 5th edition. New York: Churchill Livingstone.

Molyneux, D.H., D.R. Hopkins, and N. Zagaria (2004) Disease eradication, elimination and control: the need for accurate and consistent usage. Trends Parasitol, 20(8):347-51.

Lucas A.O and Gilles H.M (2003). Short Textbook of Public Health Medicine for the tropics. Fourth edition. Hodder Arnold Publication, UK.

MacMahon   B.,   Trichopoulos   D (1996). Epidemiology Principles and Methods.   2nd ed. Boston, MA: Little, Brown and Company. USA.

Leventhal R and Cheadle R.F (2013). Medical Parasitology. Fifth edition. F.A. Davis Publishers,

Lee JW (2005). Public health is a social issue. Lancet. 365:1005-6.

John D and Petri W.A Jr (2013). Markell and Voge’s Medical Parasitology. Ninth edition.

Gillespie S.H and Pearson R.D (2001). Principles and Practice of Clinical Parasitology. John Wiley and Sons Ltd. West Sussex, England.

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