World Wide Epidemiology of Helminths Infection World Wide Epidemiology of Helminths Infection

Helminths (from the Greek Helmins, meaning worm) include three groups of parasitic worm, large multicellular organisms with complex tissues and organs. Helminths do not replicate within the human host except Strongyloides stercoralis . Prevalence is commonly combined with worm burden (intensity of infection), which is commonly measured by the number of eggs per gram (EPGs) of faeces for intestinal helminths and schisto‐ somes. Based on EPGs and their association with morbidity, individuals are classified into categories of light, moderate and heavy infection by the WHO. In the case of soil‐ transmitted helminths, the WHO recommends use of both prevalence and intensity of infection to classify communities into transmission categories—category I (high), cate‐ gory II (medium), and category III (low). The neglected status of the helminthiasis should be addressed on community levels and globally all over the world.

Studying epidemiology of helminths is highly important because they have a wide geographical distribution in different regions especially in sub-Saharan Africa, South America, Central America, Middle East, the Caribbean and Asia [2].
In 1998, the WHO Collaborating Centre for the Epidemiology of Intestinal Parasites at University of Oxford and WHO launched Global Atlas of Helminth Infections (GAHI) which is an initiative aimed at collating the available survey data as both a database and graphical tool (www.thiswormyworld.org) [2].
Based on GAHI, data collected between 1998 and 2010, on prevalence of helminths infections. Some data were collected older than these dates if no available data for a particular country.
Some countries were excluded based on their socioeconomic levels. And also three countries were excluded according to the comprehensive control such as Mauritius, Maldives and Mayotte. Seven countries in Oceania had no data, so they were assigned mean prevalence based on the regional data. Also eight countries had no data were assigned mean prevalence based on data of neighbouring countries with similar eco-epidemiological situation such as Georgia, Iraq and Turkmenistan data from Iran; Algeria data from Morocco; Syria and Tunisia data from Turkey; PDR Korea from Jilin Sheng (A province in China); and Timor Leste from Indonesia [2,3]. Table 1 shows the distribution-prevalence of different helminths all over the world.

Fasciola
They are highly distributed worldwide, and their reservoir hosts are not the human.
There are four species of Fasciola: • Fasciola hepatica: It is found all over the world even in countries with high socioeconomic levels such as the USA and Europe. The unwashed watercress which is contaminated by the excreta of sheep is the mode of infection by eating it. It mainly affects the liver.
• Fasciola gigantica: It is found in Africa, Eastern Asia and Hawaii. Eating raw vegetables contaminated by the excreta of cattle and some mammals is the mode of infection. It mainly affects the liver.
• Fasciolopsis buski: It is found with high distribution in Far East and Indian Subcontinent especially Taiwan, Thailand, India and Bangladesh. Eating unwashed vegetables contaminated by human excreta or excreta of some pig mammals is the mode of infection. It mainly affects the intestine.
• Echinostoma species: They are found in South East Asia like Philippines and also in Japan. The mode of infection is ingestion of fresh water snails containing metacercaria. They mainly affect the intestine [1,4].

Paragonimus westermani
It is a lung fluke which is found in South America and Eastern Asia. Eating raw seafood is the mode of infection. It mainly affects the lung and presents there encapsulated [1,4].

Schistosomes
They are blood-dwelling trematodes. They have separate sexes.
It is estimated that 250 million people infected with Schistosomiasis by its major types: mansoni, haematobium and japonicum, more than 90% of them are in sub-Saharan Africa.
Also 500 million or more people are exposed to infection. The mode of infection is penetration of skin by cercaria. They mainly affect liver, lung, intestine and bladder.
There are three major species of Schistosomes: • Schistosoma haematobium: It is found in Africa, Middle East and India. It affects the urinary tract.
• Schistosoma mansoni: It is found Central and West Africa, Egypt, Brazil, the Arabian Peninsula, Surinam, Venezuela, Madagascar and the West Indies. It affects the intestine and the liver. Also the infection starts with Katayama fever.
• Schistosoma japonicum: It is found in China, Japan, Indonesia and the Philippines. There are also another two species but they have the same symptoms like Schistosoma mansoni: • Schistosoma intercalatum: It is found in Central and West Africa.
• Figure 1 showing the current global distribution of schistosomiasis worldwide [5].

The oriental liver flukes
They are found in the Far East and Europe. The mode of infection is eating raw fish. They affect the liver.

Heterophyes heterophyes
It is found in the Far East. The mode of infection is eating raw or pickled fish. It affects the small intestine [1].

Metagonimus yokogawai
It is found in the Far East. Eating raw or pickled fish is the mode of infection. It affects the small intestine [1].   Table 3 shows the prevalence of human Cestodiases around the world.

Taenia species
They are the most common cestodes infect humans. They affect the intestine especially the duodenum.

Clonorchiasis Opisthorchiasis Fascioliasis
Parasite  There are two species of Taenia: • Taenia saginata: It has a cosmopolitan distribution, but is more commonly found in developing countries due to poor socioeconomic levels and bad hygiene. The mode of infection is eating improperly cooked beef.
• Taenia solium: It is almost not present in the USA and Europe. The mode of infection is eating improperly cooked pork [6].

Hymenolepididae
They have a cosmopolitan distribution. The mode of infection by them is eating raw vegetables. They affect the small intestine.
There are four species of Hymenolepididae: • Hymenolepis nana: It is thought to be the most common Cestode in the world. It affects children more than adults.
• Hymenolepis diminuta: It has a cosmopolitan distribution but it is found infrequently in humans.

Diphyllobothriidae
There are three species of Diphyllobothriidae: • Diphyllobothrium latum: It has a cosmopolitan distribution but it is found mostly in countries bordering the Baltic Sea in Europe. The mode of infection is eating raw fish. It affects the small intestine.

Dipylidium caninum
Rarely, human is infected with Dipylidium caninum. Almost all reported cases are children.
The mode of infection is eating raw vegetables. It affects the small intestine [7].

Larval cestodes
• Echinococcus granulosus: It is a larval stage of dog cestode. It causes Echinococcosis which is also known as hydatid disease due to development of hydatid cysts. These cysts can be located in the liver, the lung, the brain and the kidney. This disease is found commonly in Africa, the Middle East, South America and Australia. The mode of infection is eating raw vegetables.
• Echinococcus multilocularis: It is found in Europe, in the highlands like Switzerland, and also it is found in Alaska and Canada. The mode of infection is eating raw vegetables. Cysts are formed in the liver but they can later be located in the lung or the brain.
• Multiceps multiceps: It has a cosmopolitan distribution but mainly presents in the UK and the USA. The mode of infection is eating raw vegetables. Cysts are formed and located mainly in the spinal cord, the brain and the subcutaneous tissue [6]. Table 4 shows the prevalence of nematodes around the world.

Ascaris lumbricoides
It is the largest intestinal nematodes found in humans. It has a cosmopolitan distribution, but it is more common in tropical and subtropical zones. The mode of infection is eating of raw vegetables. It affects the intestine and the lung [8].

Hookworm species
They are considered most destructive parasitic helminths infect human. Penetration of skin by Filariform larva is the mode of infection.
• Necator americanus: It is a New World hookworm. It is found in Far East, Asia, Africa, South America, and Oceania. It affects the small intestine.
• Ancylostoma duodenale: It is an Old World hookworm. It is found in the Middle East, North China, Europe, the Mediterranean countries, Africa, Asia and South America. It affects the small intestine.
• Cutaneous Larva migrans (Ancylostoma braziliense or Ancylostoma caninum): The mode of infection is contacting with hookworms larva of the dog or cat. They affect the skin [8]. Figure 2 showing the life cycle of Ancylostoma.

Trichuris trichiura
It is known as the whipworm. Children are most commonly affected. It is found in tropical zones with poor sanitation. The mode of infection is eating raw vegetables. It affects the caecum and the upper part of the colon [1].

Strongyloides stercoralis
It is found in sub-tropical zones. Most of the 52 species of Strongyloides do not infect humans. S stercoralis is the most common human pathogen. Other species include S myopotami and S procyonis. These species have animal hosts and are thus responsible for zoonotic infections.
Infections are initiated when exposed skin contacts contaminated soil. Autoinfection commonly occurs allowing infection to persist decades. Hyperinfection is typically triggered by drug-induced or disease-associated defects in cellular immunity, which allows a massive increase in parasite burden and dissemination to nearly all organ systems [9].

Angiostrongylus (Parastrongylus) cantonensis
It is also known as the rat nematodes. It is found in the Pacific areas, Central America and Cuba. The mode of infection is eating raw vegetables. It causes eosinophilic meningitis [1].

Angiostrongylus (Parastrongylus) costaricensis
It is found mainly in Costa Rica, but also some cases have been reported in Mexico, Central and South America. The mode of infection is eating raw vegetables. It affects the bowel wall [1].

Anisakiasis
It is found in Netherlands, North America, Canada, Chile, and the UK with the increasing spread of popularity of 'sushi'. There are two species causing human anisakiasis: Anisakis simplex and Pseudoterranova osculatum. The mode of infection is eating raw fish. It affects the intestine [1].

Gnathostoma spinigerum
It is found in Japan, China, the Far East and the Philippines; the mode of infection is eating raw freshwater fish. It affects the intestine and the cutaneous tissues [1].

Trichinella spiralis
It has a cosmopolitan distribution, and it is most commonly found in China and Europe. The mode of infection is eating raw pork. It affects the small intestine, the blood and the central nervous system.
According to the zones, temperature and infectivity, there are four types of it: Trichinella spiralis spiralis, Trichinella spiralis nelson, Trichinella spiralis nativa and Trichinella spiralis pseudospiralis [1].

Wuchereria bancrofti
It causes lymphatic filariasis. It is found in tropics and subtropics zones. The mode of transmission is puncturing the skin by the vector, a mosquito.

Mosquitoes responsible for its transmission are Culex quinquefasciatus, Anopheles gambiae and Anopheles funestus.
It causes Lymphangitis commonly in the lower extremities and obstruction of the genital organs.
There are two strains of Wuchereria bancrofti: • The nocturnal periodic strain which is found in Africa, India and the Far East.
• The sub-periodic strain which is found in the Pacific region [1].

Brugia malayi
It is found in South East Asia. There are two strains of Brugia malayi: • The nocturnal periodic strain which is spread widely in Asia.
• The sub-periodic strain which is found in Malaysia, Indonesia and the Philippines.
The mode of transmission is puncturing the skin by the vector, Mansonia.
It affects the lymph vessels causing the same clinical picture of Wuchereria bancrofti [1].

Brugia timori
It is found in Indonesia. The mode of transmission is puncturing the skin by the vector, Mansonia. It affects the lymph vessels [1].

Loa loa
It is known as the African eye worm as it is found in West and Central Africa. The mode of transmission is puncturing the skin by the vector, Chrysops species.
It affects the skin causing Calabar swellings, which are inflammatory swellings resulting in a localized subcutaneous oedema [1].

Mansonella species
It is found in tropics and subtropics zones. The mode of transmission is puncturing the skin by the vector, Culicoides species. It affects the skin.

Onchocerca volvulus
It is found in central and West Africa. It is found also in South America.
The mode of transmission is puncturing the skin by the vector, Simulium (black flies), especially Simulium damnosum. It affects the skin and the eye [1,8].

Dracunculus medinensis
It is found in Iran, India and Ghana in zones of little clean drinking water. The mode of transmission is ingestion of unclean water containing Cyclops.
It affects intestine and the lung [8].

Capillaria philippinensis
It is found in mainly Philippines and Thailand, but there were also some few unusual reports from Japan, Taiwan, Egypt and Iran. The source of infection is eating raw or improperly cooked fish. It affects the intestine [9][10][11].

Enterobius vermicularis
It has a cosmopolitan distribution. The modes of transmission are finger suckling, nail biting and eat raw vegetables.
Here are some examples of Nematodes' prevalence in different parts of the world:

a. Mazandaran province, northern Iran
Data analysis performed after examining unwashed raw vegetables for soil transmitted helminths showed below: • The overall prevalence = 14.89% • Contamination in warm seasons = 20.5% • Contamination in cold seasons = 9.32% • The prevalence of contamination is higher in leafy vegetables than root vegetables [12].

b. Kampala, Uganda
Cross sectional survey in September and October 2013 applied on five groups of people: • Workers maintaining wastewater facilities.
• Urban farmers. • Slum dwellers at risk of flooding.
• Slum dwellers without work of flooding.
• They were tested using Kato-Katz stool sampling which showed: • Highest prevalence in urban farmers = 75.9% • Lowest prevalence in workers managing faecal sludge = 35.8% • Hookworm is the predominant by 27.8% • In urban farmers, the prevalence of Trichuris, Schistosoma mansoni and Ascaris was 15% and above [13].
Mansonella ozzardi is a rare nematode but mainly present in The Americas from southern Mexico to northwestern Argentina with prevalence rate (0-46%) [16].

Future perspectives
There is being some changes in several countries due to the efforts to treat actively these parasites. Nevertheless, some negative circumstances such as wars, climate changes and poverty may influence in this growing problem.
The neglected status of the helminthiasis should be addressed on community levels and globally all over the world.