120 Overview of Parasitic Infections
120.1 Introduction
Parasitic infections remain a major public health concern in the tropics, particularly in Ghana and the wider sub-Saharan African region, where they significantly contribute to morbidity and mortality among children. These infections are closely linked to environmental, socioeconomic, and behavioural factors such as poverty, inadequate sanitation, unsafe water supply, and limited access to healthcare.
In paediatric practice, parasitic diseases commonly present as anaemia, malnutrition, growth failure, diarrhoea, and neurological complications, often exacerbating other ongoing health conditions. Understanding the basic biology, classification, and epidemiology of parasites is crucial for diagnosis, treatment, and preventive interventions in endemic areas.
This chapter provides an overview of parasitic infections, defines key concepts, and discusses the classification, modes of transmission, and epidemiological patterns with specific reference to Ghana and sub-Saharan Africa.
120.2 Basic Definitions
A parasite is an organism that lives on or within another living organism, known as the host, deriving benefit at the host’s expense. The relationship is typically one-sided, with the parasite gaining nourishment, shelter, or other support while causing harm or disease to the host.
120.2.1 Key Terminologies
Host: The organism that harbours the parasite.
- Definitive host – harbours the adult or sexually mature stage.
- Intermediate host – harbours the larval or asexual stage.
- Paratenic (transport) host – carries the parasite without further development.
- Definitive host – harbours the adult or sexually mature stage.
Vector: An organism, often an arthropod (e.g., mosquito, tsetse fly), that transmits the parasite from one host to another.
Reservoir host: Animals that harbour parasites infectious to humans, maintaining the life cycle in nature.
Infection vs. Infestation:
- Infection refers to internal invasion by protozoa or helminths.
- Infestation refers to external colonization by ectoparasites (e.g., lice, ticks, mites).
- Infection refers to internal invasion by protozoa or helminths.
Endoparasites live inside the host’s body (e.g., Plasmodium, Ascaris lumbricoides), while ectoparasites live on the host’s body surface (e.g., Pediculus humanus capitis).
Parasites have evolved to coexist with their hosts, but in humans—especially in young or malnourished children—these relationships often result in overt disease.
120.3 Classification of Parasites
Parasites of medical importance are broadly classified into three groups: protozoa, helminths, and arthropods.
120.3.1 Protozoa
Protozoa are unicellular microscopic organisms that reproduce by binary fission or multiple fission. Many are motile and can survive in host tissues or extracellular fluids. They are responsible for some of the most severe parasitic diseases in children.
120.3.1.1 Major Groups of Protozoa
| Group | Locomotory Organ | Example | Disease |
|---|---|---|---|
| Amoebae | Pseudopodia | Entamoeba histolytica | Amoebic dysentery, liver abscess |
| Flagellates | Flagella | Giardia lamblia, Trypanosoma brucei, Leishmania donovani | Giardiasis, African sleeping sickness, leishmaniasis |
| Ciliates | Cilia | Balantidium coli | Balantidiasis |
| Sporozoa | None (complex life cycle) | Plasmodium spp., Toxoplasma gondii, Cryptosporidium spp. | Malaria, toxoplasmosis, cryptosporidiosis |
120.3.1.2 Clinical Relevance in Ghana
In Ghana, malaria, caused by Plasmodium falciparum, is by far the most important protozoal infection. It accounts for a significant proportion of outpatient visits and hospital admissions in children, with high mortality if untreated. Other protozoal infections such as giardiasis and cryptosporidiosis are increasingly recognized as causes of persistent diarrhoea in both immunocompetent and immunocompromised children, including those with HIV infection.
120.3.2 Helminths
Helminths are multicellular worms that infect various tissues and organs. They are typically large enough to be seen with the naked eye and are classified into three main groups: nematodes, cestodes, and trematodes.
120.3.2.1 Nematodes – Roundworms
These are cylindrical worms with separate sexes.
Examples: - Ascaris lumbricoides (roundworm) - Ancylostoma duodenale and Necator americanus (hookworms) - Trichuris trichiura (whipworm) - Strongyloides stercoralis - Enterobius vermicularis (pinworm)
Clinical relevance: In Ghana, soil-transmitted helminth infections remain endemic in many rural areas, contributing to chronic anaemia, malnutrition, and impaired cognitive development among school-aged children.
120.3.2.2 Cestodes – Tapeworms
These are flat, ribbon-like worms with segmented bodies called proglottids.
Examples: - Taenia saginata (beef tapeworm) - Taenia solium (pork tapeworm) - Diphyllobothrium latum (fish tapeworm) - Hymenolepis nana (dwarf tapeworm)
Clinical relevance: Tapeworm infections are relatively uncommon in Ghana compared to nematodes, but Taenia solium poses a risk of neurocysticercosis, an emerging cause of seizures in children and adults.
120.3.2.3 Trematodes – Flukes
These are leaf-shaped flatworms that require at least one intermediate host (usually a snail).
Examples: - Schistosoma haematobium – urinary schistosomiasis - Schistosoma mansoni – intestinal schistosomiasis
Clinical relevance: Schistosomiasis remains endemic in the Volta Basin, northern Ghana, and other water-rich communities. In children, it leads to haematuria, hepatosplenomegaly, anaemia, and growth retardation.
120.3.3 Arthropods
Arthropods are invertebrates with jointed appendages and chitinous exoskeletons. They are important as vectors, intermediate hosts, or direct causes of disease through bites, stings, or infestation.
120.3.3.1 Common Arthropods in Medical Practice
| Group | Example | Role in Disease |
|---|---|---|
| Mosquitoes | Anopheles, Aedes, Culex | Vectors of malaria, filariasis, dengue |
| Tsetse flies | Glossina spp. | Vector of African trypanosomiasis |
| Sandflies | Phlebotomus spp. | Vector of leishmaniasis |
| Blackflies | Simulium spp. | Vector of onchocerciasis |
| Fleas, lice, ticks, mites | Various species | Ectoparasitic infestations; vectors of typhus, rickettsial infections |
In Ghana, malaria-transmitting Anopheles mosquitoes are the most significant arthropod vector, while blackflies are responsible for onchocerciasis (river blindness) in some riverine communities.
120.4 Modes of Transmission of Parasitic Infections
Parasites employ diverse routes to infect humans. Understanding these is crucial for preventive strategies.
- Faeco–oral transmission – ingestion of cysts, oocysts, or eggs from contaminated water or food (E. histolytica, Giardia lamblia, Ascaris lumbricoides).
- Vector-borne transmission – via arthropods such as mosquitoes, tsetse flies, and sandflies (Plasmodium, Trypanosoma, Leishmania).
- Skin penetration – infective larvae penetrate intact skin (Schistosoma, hookworms, Strongyloides).
- Transplacental or perinatal transmission – Toxoplasma gondii, Trypanosoma cruzi.
- Ingestion of undercooked meat or fish – Taenia, Trichinella, Diphyllobothrium.
- Person-to-person contact or fomites – Enterobius vermicularis, lice, scabies.
In Ghana, the faeco-oral route and vector-borne transmission dominate, reflecting poor sanitation and high vector density due to favourable climatic conditions.
120.5 Epidemiology of Parasitic Infections in Ghana and Sub-Saharan Africa
120.5.1 Burden and Impact
Parasitic infections constitute one of the most widespread causes of childhood morbidity in sub-Saharan Africa. They coexist with malnutrition, anaemia, and chronic infections like tuberculosis and HIV, creating a vicious cycle of ill health and poor development.
In Ghana, the Ghana Health Service (GHS) and Neglected Tropical Disease Control Programme have prioritized the control of malaria, schistosomiasis, lymphatic filariasis, and soil-transmitted helminths through mass drug administration, insecticide-treated nets, and health education.
120.5.2 Key Epidemiological Highlights
| Disease | Agent | Mode of Transmission | Epidemiology in Ghana |
|---|---|---|---|
| Malaria | Plasmodium falciparum | Bite of female Anopheles mosquito | Endemic nationwide, seasonal peaks during rains |
| Schistosomiasis | S. haematobium, S. mansoni | Skin penetration in infested water | Endemic in Volta, Northern, and Ashanti regions |
| Soil-transmitted helminths | Ascaris, Trichuris, Hookworm | Faeco-oral or skin penetration | Common in school-aged children in rural areas |
| Onchocerciasis | Onchocerca volvulus | Blackfly bite | Focal along river basins |
| Filariasis | Wuchereria bancrofti | Mosquito bite | Endemic in parts of Northern Ghana |
120.6 Control and Prevention Strategies
Effective control requires multisectoral interventions integrating health, water, sanitation, and education systems.
- Environmental sanitation: Proper disposal of faeces, improved water supply, and waste management.
- Vector control: Use of insecticide-treated bed nets, indoor residual spraying, and elimination of vector breeding sites.
- Mass drug administration (MDA): Periodic deworming with albendazole, praziquantel, and ivermectin under the Ghana NTD programme.
- Health education: Promoting hand hygiene, safe food practices, and community awareness.
- Nutrition support: Addressing micronutrient deficiencies to reduce susceptibility to infection.
- Surveillance and research: Strengthening laboratory capacity and epidemiological monitoring, especially in children.
120.7 Summary
Parasitic infections remain a significant public health challenge in Ghana and across sub-Saharan Africa, with children being the most vulnerable group. Understanding their biology, modes of transmission, and epidemiological context is essential for effective prevention, diagnosis, and treatment.
Sustained control will require continued investment in sanitation, health education, and access to quality paediatric care.
120.8 Further Reading
- Kliegman RM et al. Nelson Textbook of Pediatrics, 22nd Edition. Elsevier; 2023.
- Ghana Health Service. Neglected Tropical Diseases Control Programme Annual Report, 2022.
- WHO. World Malaria Report 2023. Geneva: World Health Organization.
- Hotez PJ, Kamath A. Neglected Tropical Diseases in Sub-Saharan Africa: Review of Their Burden and Distribution. PLoS Neglected Tropical Diseases. 2009;3(8):e412.
- Addo KK et al. Intestinal Parasites among School Children in Accra. Ghana Medical Journal. 2018;52(2):64–70.
- Brooker SJ et al. Soil-Transmitted Helminth Infections in Africa: Distribution, Burden and Control. PLoS Negl Trop Dis. 2015;9(3):e0003565.