Introduction

Overview of parasitic diseases

The microorganisms that live upon any other organisms are commonly known as parasites and host organisms, respectively. Parasites cause different types of infections known as parasitic infections or parasitic diseases. Such infections can occur in the intestines, blood, brain, and skin of children and adults.

Their clinical manifestations vary with both age groups. Three main types of parasites cause human infections.¹ They include:

• Protozoa which are single-cell organisms like the parasite Plasmodium, Entamoeba hystolitica, Leishmania parasite, and Trypanosoma parasite
• Helminths are parasitic worms. Types of Helminths include trematodes, for example, Schistosoma mansoni; nematodes, for instance, Ascaris lumbricoid; and cestodes, for example, Tenia solium
• Ectoparasites are those that live on the external surface of their host. Ticks, fleas, lice, and mice are some examples^1,2

Parasites are transmitted through the following methods:

• Blood transfusions
• Insect bites: Mosquito, flies, ticks
• Contaminated food and water
• Contaminated soil
• Unprotected sex¹

Parasitic diseases account for 48.4 cases, and 59,724 deaths annually as shown by Torgerson et al.³ Intestinal parasitic infections alone affect approximately 3.5 billion people worldwide causing over 450 million hospitalized cases annually.⁴

Parasitic Disease Prevalence in Children vs. Adults

Factors Influencing Prevalence

Children

Over 260 million preschool-age children and 654 million school-age children live in areas with high transmission of parasitic diseases. These areas are associated with:^5,6

• Poverty
• Poor hygiene and sanitation
• Low development: about 12% of infections in children are caused by intestinal parasites in less developed countries
• Deworming measures not strictly applied

Preschool and school-age children are also susceptible to contracting parasitic infections with higher prevalence in low and middle-income countries.

Adults

Factors influencing the prevalence of parasitic diseases in adults include:

• Environmental and Personal sanitation practices
• Socioeconomic status 
• Availability of clean portable water
• Food sanitation
• Climatic conditions that favour vector growth and multiplication. Extreme low temperatures are not favourable for insect growth 
• Public health initiatives to create awareness^5,7,8

Common Parasitic Infections in Children

Common parasitic infections in children include

• Giardiasis is caused by Giardia duodenalis, an intestinal parasite obtained from contaminated water, food, surfaces, or objects. It usually manifests as diarrhea
• Pinworm infection, also known as Enterobius vermicularis, is a roundworm that infects the intestines. Symptoms are usually mild and prevention can be done by washing hands¹
• Scabies is a skin infection caused by the ectoparasite Sarcoptes scabiei. It usually manifests as intense puritis mostly at night, with a skin rash looking like a pimple. Prevention is by avoiding skin contact with contaminated persons
• Head lice is caused by Pediculus humanus capitis which hides in the hair of humans and feeds on their blood. It usually manifests by puritis. Transmission is by direct contact with infected persons, or indirectly through sharing of combs, clothes, beds, etc. with an infected person¹
• Cryptosporidiosis, caused by the parasite Cryptosporidium, is transmitted through contaminated surfaces, as well as contaminated food and water. It manifests as watery diarrhea. The primary preventive measure is washing hands
• Toxoplasmosis is an infection caused by Toxoplasma gondii. It is usually asymptomatic in children. Transmission is through food, from animals like cats, or from mother to child during pregnancy¹ 
• Soil-transmitted worms like Ascaris lubricoides (roundworm), and whipworm cause intestinal infections manifesting as diarrhea
• Schistosomiasis is caused by the Schistosomia species, usually found in water snails. Humans contract this infection when their skin comes in contact with contaminated water

In sub-Saharan Africa, Malaria caused by Plasmodium parasite is a common parasitic infection and a major cause of death in children.⁹

Common Parasitic Infections in Adults

Common parasitic infections in adults include:

• Pinworm infection: In adults, it mostly affects people in institutional settings and caretakers of persons infected with pinworm. Transmission is by contact with contaminated feces and ingestion by mouth without proper washing of hands¹
• Trichomoniasis: This is a sexually transmitted parasitic infection caused by Trichomonas vaginalis. It leads to vulvovaginitis in females and urethritis in males. Mostly asymptomatic, but sometimes present as painful urination, vaginal discharge, or penile for males¹⁰
• Toxoplasmosis: Usually asymptomatic in healthy adults but causes severe disease in immunocompromised patients. It causes damage to the brain, eyes, and other organs. The parasite is contracted from contaminated food, animals such as cats, and transmission from a mother to her child during pregnancy
• Giardiasis
• Cryptosporidiosis
• Threadworm infection in the UK
• Malaria in sub-Saharan Africa
• Strongyloidiasis: the causative agent is Strongyloides stercoralis obtained from contaminated soil. It is usually asymptomatic
• Chagas disease: the causative parasite is Trypanosoma cruzi. It is common in Mexico, and also in Central and South America. Globally, close to 8 million people have this disease. It is sometimes asymptomatic but may present with mild symptoms like swelling of the eyelid, fever, body rash, and headache. If left untreated, 20 to 30% develop serious heart and digestive problems¹¹
• Leishmaniasis: the causative agent is the Leishmania parasite. It affects humans in  3 different forms: skin, mucosal, and visceral forms presenting as sores on the skin, in the mouth, nose, and throat, or affecting internal organs. It is a vectorborne parasite transmitted by sand flies¹²

Treatment Approaches for Parasitic Diseases in Children vs. Adults

The two principal treatment approaches for parasitic infections are symptomatic treatment and causative agent eradication, in both children and adults. 

Symptomatic treatment varies with the clinical presentation of the infection.

Causative agent eradication is done by antiparasitic agents. The physician adjusts the doses for the various age groups. Depending on the causative agent, treatment may vary.

Protozoa are treated with antiprotozoal agents. These include: 

• Antimalarial medications like quinine, amodiaquine, and artemisinin combination therapy for the treatment of plasmodium infection
• Antiamoebic medications such as metronidazole, and tinidazole for the treatment of Amebiasis
• Antigiardial medications such as tinidazole, albendazole for the treatment of giardiasis
• Trypanocidal agents like benznidazole and suramin to treat Chagas disease
• An antileishmanial agent such as Sodium stibogluconate is the first-line treatment for visceral and cutaneous leishmaniasis and alternatives are pentamidine or amphotericin B
• Anti-toxoplasma agents such as Sulfadiazine combined with pyrimethamine for the management of Toxoplasmosis
• Metronidazole is also used to treat Trichomoniasis¹³

Helminths are treated with anti-helmintic agents. These are medications that either kill the intestinal worms (vermicides) or expel the worms from the body alive (vermifuges). Examples include:

• Praziquantel
• Niclosamide
• Albendazole

These are used for the treatment of tapeworms, roundworms, trichuriasis, trichinosis, hookworms, pinworms, and flukes.¹³

Ectoparasites are treated with Ectoparasiticides. These include:

• Antiscabietic agents such as permethrin, benzyl benzoate, ivermectin, and lindane are used to manage scabies
• Pediculicides like permethrin and pyrethrins are used for the management of lice¹³

Challenges in Treatment Accessibility and Effectiveness

Drug Accessibility and Health System Limitations

Certain anti-parasitic agents are not available in all regions of the world, especially in low and middle-income countries. An example of drugs not available in the USA include meglumine antimoniate which is used in the treatment of leishmaniasis.¹³

Due to health system limitations and the low socioeconomic status of low and middle-income countries, certain parasitic medications are not affordable and as such rely on third parties for subventions to be able to access these medications.¹⁴  

Drug Resistance

There is growing drug resistance to parasitic agents such as permethrin for the treatment of lice and scabies. Resistance to lindane which is also used for scabies management has also increased over the years.¹³

Strategies to prevent and control Parasitic disease in children and adults

Preventive measures in children

• Educate children on hygiene with emphasis on the importance of washing hands
• Programs and projects to deworm children in schools and communities
• Vaccination of all children
• Parental and community involvement in the hygienic conditions of the environment ensuring its neatness and insects free for the safety of their children¹⁵

Prevention strategies for adults

• Health education through campaigns on personal and environmental hygiene
• Health control of travellers to prevent migration of parasitic agents
• Use of insecticides, insecticide-treated nets, and other available preventive medications
• The government should ensure the availability of clean and portable water in all localities¹⁵

Summary

Parasitic disease prevalence varies in children and adults according to the hygienic conditions, and deworming initiatives for school-age children. It also depends on climate, socioeconomic status, as well as the available public health initiatives. Treatment approaches vary depending on the symptomatology, causative agent and well and availability of the medication to treat these parasitic infections. Personal and environmental hygiene is very important in the prevention of parasitic diseases both in children and adults.