An introduction to medical parasitology

How should you study this module?
We suggest that start with the learning objectives and try to keep these in mind as you go through the module slide by slide, in order.
You should research any issues that you are unsure about. Look in your textbooks, access the on-line resources indicated at the end of the module and discuss with your peers and teachers.
Finally, enjoy your learning! We hope that this module will be enjoyable to study and complement your learning about TB from other sources.

Learning Outcomes
After completing this SDL, you should be able to:
Discuss how important parasites can be classified according to kingdom and phylum
State the meaning of commonly-used terms
Describe how parasitic infections affect communities in poor countries and that knowledge of their life cycle is necessary for effective prevention and control
Discuss the epidemiology, basic life cycle, clinical presentation, management and control of some important parasitic infections
Note: This SDL will contain many unfamiliar terms. You are NOT expected to remember the classifications and names of all of the different parasite species. The emphasis is on understanding basic concepts and being able to illustrate these with some important examples.
After completing this SDL, try the associated quiz to assess your learning.

Key definitions: What is ….?
Medical parasitology: “the study and medical implications of parasites that infect humans”
A parasite: “a living organism that acquires some of its basic nutritional requirements through its intimate contact with another living organism”. Parasites may be simple unicellular protozoa or complex multicellular metazoa
Eukaryote: a cell with a well-defined chromosome in a membrane-bound nucleus. All parasitic organisms are eukaryotes
Protozoa: unicellular organisms, e.g. Plasmodium (malaria)
Metazoa: multicellular organisms, e.g. helminths (worms) and arthropods (ticks, lice)
An endoparasite: “a parasite that lives within another living organism” – e.g. malaria, Giardia
An ectoparasite: “a parasite that lives on the external surface of another living organism” – e.g. lice, ticks

Key definitions: What is ….?
Host: “the organism in, or on, which the parasite lives and causes harm”
Definitive host: “the organism in which the adult or sexually mature stage of the parasite lives”
Intermediate host: “the organism in which the parasite lives during a period of its development only”
Zoonosis: “a parasitic disease in which an animal is normally the host - but which also infects man”
Vector: “a living carrier (e.g.an arthropod) that transports a pathogenic organism from an infected to a non-infected host”. A typical example is the female Anopheles mosquito that transmits malaria

Epidemiology
Although parasitic infections occur globally, the majority occur in tropical regions, where there is poverty, poor sanitation and personal hygiene
Often entire communities may be infected with multiple, different organisms which remain untreated because treatment is neither accessible nor affordable
Effective prevention and control requires "mass intervention strategies” and intense community education. Examples include:
General improved sanitation: pit latrines, fresh water wells, piped water
Vector control: insecticide impregnated bed nets, spraying of houses with residual insecticides, drainage, landfill
Mass screening and drug administration programmes which may need to be repeated at regular intervals

The burden of some major parasitic infections

Taxonomic classification of parasitic organisms
The classification of parasites is controversial - there is no universally accepted system
Parasites form part of the animal kingdom which comprises some 800,000 identified species categorised into 33 phyla (but it is estimated that there may be ~10m species in total)
The parasitic organisms that are of importance for human health are eukaryotes - they have a well defined chromosome in a nuclear membrane (as opposed to prokaryotes which have no nuclear membrane, e.g. bacteria)
Parasites are classified into 2 sub-kingdoms: protozoa (unicellular) and metazoa (multicellular)
Protozoan (unicellular) parasites are classified according to morphology and means of locomotion. There are 45,000 protozoa species. Most species that cause human disease belong to the phylums sarcomastigophora and apicomplexa
Metazoa (multicellular) include the worms (helminths) and arthropoda (posses an external skeleton) e.g. ticks, lice
Note that the genus starts with a capital letter and the species is always written in italics, e.g. Plasmodium falciparum, Giardia lamblia

Taxonomic classification of protozoa

Examples of important intestinal protozoa
Transmitted by the faecal-oral route and cause diarrhoea
Giardia lamblia: world-wide distribution, lives in the small intestine and results in malabsorption
Entamoeba histolytica: may invade the colon and cause bloody diarrhoea – amoebic dysentery. Also causes ameobic liver abscess.
Cryptosporidium parvum: more prevalent in the immunocompromised
Cyclospora cyatenensis - parasitises the small intestinal mucosa and may cause diarrhoea for several weeks
Balantidium coli: a large motile ciliated parasite that lives in the colon of pigs, humans and rodents and can lead to colonic ulceration
Enterocytozoon bienusi: a microsporidian that parasitises the small intestine. Also more common in the immunocompromised.
Typical flask-shaped ulcer due to E. histolytica in the colonic mucosa
Electron micrograph of G. lamblia trophozoites (feeding stage)

Examples of important systemic protozoa
Detected in the blood
Plasmodium: the cause of malaria. There are 4 species that infect man: P. falciparum, P. vivax, P. ovale and P. malariae
Toxoplasma gondi: transmitted by the ingestion of oocysts from cat faeces. Infection can lead to ocular problems and is also a cause of neonatal toxoplasmosis
Leishmania: transmitted by sand flies, can lead to visceral, cutaneous and mucocutaneous leishmaniasis
Trypanosoma: haemoflagellates which cause
In Africa - sleeping sickness (transmitted by the Tsetse fly)
In South America - Chagas disease (transmitted by the Reduviid bug)
Typical lesion of cutaneous leishmaniasis
Tsetse fly – the vector of African trypanosomiasis It has a painful bite!

Taxonomic classification of helminths

Examples of important metazoa – intestinal nematodes
Trichuris (whipworm)
A soil transmitted helminth
prevalent in warm, humid conditions
Can cause diarrhoea, rectal prolapse and anaemia in heavily-infected people
Ancylostoma and Necator (hookworms)
A major cause of anaemia in the tropics
Strongyloides
inhabits the small bowel
infection more severe in immunospressed people (e.g. HIV/AIDS, malnutrition, intercurrent disease)
Enterobius (pinworm or threadworm)
prevalent in cold and temperate climates but rare in the tropics
found mainly in children
Ascaris (roundworm)
Found world-wide in conditions of poor hygiene, transmitted by the faecal- oral route
Adult worms lives in the small intestine
Causes eosinophilia
Heavy intestinal infections may occur with Ascaris. Adult worms can be several cms long.

Examples of important metazoa –systemic nematodes
Filaria including:
Onchocerca volvulus – Transmitted by the simulium black fly, this microfilarial parasite can cause visual impairment, blindness and severe itching of the skin in those infected
Wuchereria bancrofti – The major causative agent of lymphatic filariasis
Brugia malayi – Another microfilarial parasite that causes lymphatic filariasis
Toxocara
A world-wide infection of dogs and cats
Human infection occurs when embryonated eggs are ingested from dog or cat faeces
It is common in children and can cause visceral larva migrans (VLM)

Examples of important flatworms - cestodes
Intestinal - (“tapeworms”)
Taenia saginata
worldwide
acquired by ingestion of contaminated, uncooked beef
a common infection but causes minimal symptoms
Taenia solium
worldwide
acquired by ingestion of contaminated, uncooked pork that contains cystercerci
Less common, but causes cystercicosis – a systemic disease where cysticerci encyst in muscles and in the brain – may lead to epilepsy
2. Systemic
Echinococcus granulosus (dog tapeworm) and Echinicoccus multilocularis (rodent tapeworm)
Hydatid disease occurs when the larval stages of these organisms are ingested
The larvae may develop in the human host and cause space-occupying lesions in several organs, e.g. liver, brain

Examples of important metazoa –trematodes (flukes)
Intestinal
Fasciolopsis buski - A common parasite of humans and pigs in South- east Asia. This parasite is one of the largest trematodes to infect man (8cm in length) and lives in the upper intestine. Chronic infection leads to inflammation, ulceration and haemorrhage of the small intestine
Fasciola hepatica (liver fluke)- Primarily, a parasite of sheep, humans become infected when they ingest metacercariae that have encysted on watercress. The adult trematode lives in the intra-hepatic bile ducts of the liver. “Fascioliasis” can lead to severe anaemia in humans
Clonorchis sinensis (liver fluke)- Widespread in China, Japan, Korea and Taiwan, this parasite is acquired by ingestion of infective metacercariae in raw or pickled fish
Paragonimus westermani ( lung fluke)- Widespread in the Far East and South east Asia, the parasite is acquired by ingestion of infective metacercariae in raw or pickled crustaceans
Schistosoma haematobium, S. mansoni and S. japonicum – see below
Adult Fasciolopsis buski trematode
© Dr. Peter Darben, Queensland University of Technology

Schistosomiasis (bilharzia)

Schistosomiasis (1)
Epidemiology
200m people in 74 countries infected, 85% of whom live in sub-Saharan Africa
S. haematobium - Africa and middle east (most prevalent)
S. Mansoni - Africa and Latin America
S. japonicum – Pacific region
Life cycle
Transmission occurs in fresh water
Infective cercariae released from snails of the genera Bulinus, Biomphalaria Oncomelania
Cercariae penetrate the skin of people who drink, swim or bathe in infected water
Adult worm live in the veins that drain the urinary system (S. haematobium) or mesentric blood vessels intestines (S. mansoni, S. japonicum) and release eggs into water in urine or faeces
Eggs develop into miracidia which then infect snails
Bulinus globosus, the intermediate host snail for S. haematobium (source: WHO/TDR/Stammers)
S. haematobium cercaria (differential interference contrast microscopy; source: WHO/TDR/Stammers)

Schistosomiasis (2)
Pathology
Shistosoma eggs become trapped in the tissues of the urinary tract (S. haematobium) and intestines (S. mansoni, S. japonicum)
This results in inflammatory response and tissue damage
Adult worms of S. haematobium are 1-2 cm long (source: WHO/TDR/Stammers)
S. haematobium eggs measure 140 x 50 μm (differential interference contrast microscopy; source: WHO/TDR/Stammers)

Schistosomiasis (3)
Symptoms and signs
Urinary Schistosomiasis:
S. hamatobium
Gross haematuria
Dysuria
Bladder, ureters and kidneys damaged Cancer of the bladder is common
Intestinal schistosomiasis
S. mansoni, S. japonicum
Gradual enlargement of liver and spleen intestines
hypertension of the abdominal blood vessels which begin to bleed
Blood in the stools
Diagnosis
Microscopy for eggs:
urine sediment for S. haematobium
concentrated stool samples for S. mansoni, S. japonicum
Serology – all species
Rectal biopsy - S. mansoni, S. japonicum
Treatment
Praziquantel: effective against all species
S. haematobium: single dose (40 mg/kg)
S. mansoni, S. japonicum: 2-3 doses of 30mg/kg
Haematuria due to S. haematobium (source: WHO/TDR)

Schistosomiasis (4)
Prevention and control
Educate people to not urinate or defecate in fresh water supplies
Eliminate snail vectors by making the water habitat unsuitable (increase water flow, remove vegetation)
Provide piped water to avoid direct contact with cercariae
Mass drug treatment of communities to reduce reservoir of infection
Concrete irrigation ditches do not allow breeding of Oncomelania snails (source: WHO/TDR/Crump)

Hookworm (1)
Epidemiology
>1200m infections each year of which 100m are symptomatic
It is due to 2 parasites both of which occur worldwide:
Necator americanus - predominant species in sub-Saharan Africa, south Asia and the Pacific
Ancylostoma duodenale –predominant in S. Europe, N. Africa, western Asia, northern China, Japan and the west coast of America
Hookworm is a major cause of anaemia

Hookworm (2)
Life cycle
Adult worms live in the intestine and excrete eggs in the faeces
In the absence of latrines, eggs contaminate soil and develop in warm, damp conditions
eggs hatch and infective filariform larvae develop in about one week and remain infective in soil for many weeks
filariform larvae penetrate the skin when a person walks barefoot in the soil
larva migrate from the skin to the lungs via the lymphatic and blood systems
larvae penetrate the capillary wall to enter the alveolus
Larvae are propelled up the respiratory tree to the epiglottis where they are swallowed
Develops to adult stage in upper intestine; adult worms are fully mature after about 5 weeks
Eggs are excreted in the faeces
Note: eating soil (pica) is a common practice. Ingested filariform larvae of A. duodenale can pass directly to the gut mucosa
Egg of A. duodenale in faecal smear (size 57-76 µm by 35-47 µm)
© Dr. Peter Darben, Queensland University of Technology
Filariform larvae
© Dr. Peter Darben, Queensland University of Technology

Hookworm (3)
Pathology
Hookworms move several times a day to different attachment sites in the upper intestinal mucosa to ingest blood
They secrete an anticoagulant which causes the old attachment sites to continue to bleed
Heavy hookworm infection results in chronic haemorrhage from the duodenal and jejunal mucosa
The combination of constant blood loss due to hookworm infection and poor iron intake in the diet results in iron deficiency anaemia
A. duodenale ingests 4-5 times more blood each day than N. americanus
In a child, the continued daily loss of 10ml of blood can lead to severe anaemia
Adult male and female worms of A. duodenale
© Dr. Peter Darben, Queensland University of Technology

Hookworm (4)
Symptoms and signs
Minor
Often itchy papules are found at the site where the larva penetrated the skin
There may be cough and wheezing as the larva migrates through the lungs
Major
Hookworm anaemia
Tiredness, aches and pains
Pallor
Breathlessness
Oedema
Diagnosis
Microscopic examination of faecal smears to demonstrate significant numbers of hook worm eggs
Measure Hb, serum ferritin, iron
Exclude other causes of anaemia
Prevention and control
Health education and improve sanitation facilities – install pit latrines
Encourage use of protective footwear
Discourage soil eating (pica)
Mass drug treatment of communities
Iron supplementation in areas of low iron intake
Treatment
Mebendazole (cheap) – 100mg, twice daily for 3 days
Mebendazole is contraindicated in pregnancy – use Bephenium hydroxynaphthoate “alcopar”
For anaemia: ferrous sulphate 200-400 mg three times a day for 3 months (adult regimen)

Lymphatic filariasis (1)
Epidemiology
120m people infected in >80 countries in Africa, Asia, the Pacific islands and South and Central America
40m of those infected are disfigured or severely incapacitated
95% cases due to Wuchereria bancrofti, other species include Brugia malayi and Brugia timori
A female Anopheles mosquito taking a blood meal

Lymphatic filariasis (2)
Life cycle
Wuchereria bancrofti is mainly transmitted by
Culex mosquitoes in India
Anopheline mosquitoes in Africa
B. malayi and B. timori are transmitted mainly by Mansonia mosquitoes
Larval forms of the parasite (microfilariae) are taken up by a female mosquito when it takes a blood meal from a human infected with adult worms
The microfilariae develop inside the mosquito
When the mosquito takes another blood meal the infective filariform larvae enter the bite wound
Filariform larvae migrate to the lymphatics and lymph glands
Larvae develop into sexually mature adult worms over 3-12 months depending on the species of filarial worm
Microfilaria of B. malayi in thick blood film (H&E stain; source: CDC)
Adult worms of B. malayi in section in a lymph node (source: Univ South Carolina)

Lymphatic filariasis (3)
Pathology
Adult worms live in the afferent lymphatic vessels and cause severe disruption to the lymphatic system
Scrotal damage and massive swelling may occur when adult Wuchereria bancrofti lodge in the lymphatics of the spermatic cord
Late stage disease is typified by elephantiasis – painful and disfiguring swelling of the limbs
Trauma and secondary bacterial infection of affected tissues is common
Elephantiasis of the leg (source: WHO/TDR/Crump)

Lymphatic filariasis (4)
Symptoms and signs – 3 stages
1. Asymptomatic stage
There is internal damage to the lymphatics and kidneys
2. Acute stage – Filarial lymphangitis
Characterised by bouts of fever
heat, redness, pain, swelling and tenderness of the lymph nodes and ducts
3. Chronic stage
Usually results in elephantiasis as a result of chronic lymphoedema
There is a massive overgrowth of tissue resulting in severe deformities
The legs are often affected and result in inability to walk
The scrotum is often affected in men and the breasts and vulva in women
Elderly male with massive hydrocoele, and elephantiasis of the leg. Also has nodules in the groin due to onchocerciasis (source: WHO/TDR/Crump)

Lymphatic filariasis (5)
Prevention and control
Rapid diagnosis and treatment of infected individuals
Mass drug administration to at risk communities
Vector control: eliminate mosquito breeding sites through improved sanitation and enviromental management
Personal protection against mosquito bites by insecticides, bednets and repellants
Diagnosis
Microscopic examination of Giemsa stained thick blood films for the presence of microfilariae
W. bancrofti shows marked nocturnal periodicity, so it’s best to collect blood samples between 10pm and 1 am
Serology
Treatment
Diethylcarbamazine (DEC) rapidly kills microfilariae and will kill adult worms if given in full dosage over 3 weeks
Release of antigens from dying microfilaria causes allergic-type reactions – add an antihistamine and aspirin to treatment regimen
Other treatment options are
ivermectin
combination of DEC and albendazole

Sources of information
The Special Programme for Research and Training in Tropical Diseases (TDR UNICEF, UNDP, World Bank, WHO) website: ww.who.int/tdr/media/image.html
University of South Carolina School of Medicine: http://pathmicro.med.sc.edu/book/parasit-sta.htm
Lecture notes on Tropical Medicine, Dion R Bell,
Fourth edition, 1996, Blackwell Science.
Parasites and human disease, W. Crewe and D.R.W. Haddock, 1985, First edition, Edward Arnold.