The significance of Medical

Parasitology

The significance of Medical

Parasitology

• Prevalent infections worldwide

• Significant morbidity & mortality

• Significant impact on economic & social development

• Increased mobility of individuals & populations

• Drug resistance, climate change & opportunistic infections

Global estimates of parasitic diseases and disease burden (Topley & Wilson, 2005)

Definitions of parasitism

• Concept of metabolic dependence (Smyth) (nutrition, developmental stimuli, control of maturation)

• Genetic complementation

• Population dynamic approach (Crofton)- lethal level, overdispersion, reproductive rate

• Morphological adaptations

• Biochemical changes

• Specialized mechanisms for entry

• Complex life cycles and transmission opportunities

• Mechanisms for immune evasion

• Impact on host versus impact of host

Parasitic adaptations

Parasitic adaptations

Important parasitic groups• Microparasites

(Protozoa)– Kinetoplastida

– Ciliophora

– Apicomplexa

• Macroparasites (Helminths, Arthropods)– Platyhelminthes

– Nematoda

– Arthropoda

Important issues associated with parasites of human importance

• Life cycle and transmission

• Pathology and symptomatology

• Epidemiology, human behaviour and at risk groups

• Medical history

• Diagnostic difficulties

• Prevention and control

Approach to the patient

• Full history required particularly travel history

• Symptoms may be subtle and may change over time (repeated history taking)

• General history also important - occupation, hobby and leisure activities, diet, drugs taken.

• Clinical exam

Life cycle and routes of transmission

• Diverse routes of transmission which relate to human behaviour and risk factors• e.g. Toxoplasma gondii

• Diverse routes of transmission with implications for parasite survival and pathogenicity• e.g. Strongloides stercoralis

Toxoplasma Life cycle

Strongyloides life cycle

Symptomatology and pathology 1

• Parasites often provoke nonspecific and diverse symptoms

– e.g. Toxocariasis in children

• Microparasites often provoke more acute disease compared to the chronicity of macroparasites

– e.g. Plasmodium falciparum versus Strongyloides stercoralis in returned travellers

Symptomatology and pathology 2

• Differential pathology

• Immunocompromised versus Immunocompetent

hosts

• e.g. Toxoplasma gondii & Cryptosporidium parvum

Epidemiology

• Changes in human behaviour

– Migration and International travel

– Exotic foods

– Pet ownership (including exotic pets)

– Intensification of agricultural practices

• At risk groups

– Age, Occupation, Immune status, Pregnancy, Organ transplantation

Diagnostic dilemmas

• Different diagnostic tests for different patient groups

– Toxoplasma gondii

• Need for repeat diagnostic testing

– Giardia intestinalis

• Need for improved sensitivity of diagnostic methods

– Strongyloides stercoralis & Taenia solium

Prevention and control

Dracunculus medinensis : guinea worm

Prevalence of important Microparasites

• Malaria : 500 million (annual deaths 2.2 to 2.5 million)

• Giardiasis : 200 million• American Trypanosomiasis : 16-18

million (annual deaths 60,000) • Leishmaniasis : 12-13 million • Amoebiasis : 500 million

Prevalence of important Macroparasites

• Cestodiasis : 70 million

• Schistosomiasis : 200 million (annual deaths 500,000 to 1 million)

• Lymphatic filariasis : 78.6-90 million

• Onchocerciasis : 17.5 million

• Ascariasis : 1.3 billion

• Hookworm : 1 billion

• Strongyloidiasis : 80-100 million

The Big Three and Neglected tropical diseases (NTDC)

• Three vector-borne protozoa: Leishmaniasis, African Trypanosomiasis & Chagas disease

• Three bacterial infections: trachoma, leprosy and Buruli ulcer

• Seven helminth infections: hookworm, ascariasis, trichuriasis, lymphatic filariasis, onchocerciasis, guinea worm & schistosomiasis

• Cysticercosis, food-borne trematodiasis & some other parasitic infections could be includedFrom Hotez et al, 2006, PLoS Medicine

Implications of co-infections

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0 - 6 7 - 12 13 - 18 19 - 25

Age (months)

Prevalence (%)

Ascaris Malaria

• Rare diseases Echinococcus multilocularis : alveolar echinococcosis (AE)

• Emerging diseases

Challenges

• Toxoplasmosis• Malaria• Schistosomiasis• Neurocysticercosis• Lymphatic filariasis

Texts

• Topley and Wilson’s Microbiology and Microbial infections Parasitology (2005). Eds Cox, F.E.G., Wakelin, D., Gillespie, S.H and D.D. Despommier. 10th Edition. Hodder Arnold.

• Peters, W. and Pasvol, G. (2007). Tropical Medicine and Parasitology. Mosby 6th edition.