In Southeast Asia, a total of 59 species of foodborne intestinal flukes have been known to occur in humans. The largest group is the family Heterophyidae, which constitutes 22 species belonging to 9 genera (Centrocestus, Haplorchis, Heterophyes, Heterophyopsis, Metagonimus, Procerovum, Pygidiopsis, Stellantchasmus, and Stictodora). The next is the family Echinostomatidae, which includes 20 species in 8 genera (Artyfechinostomum, Acanthoparyphium, Echinochasmus, Echinoparyphium, Echinostoma, Episthmium, Euparyphium, and Hypoderaeum). The family Plagiorchiidae follows the next containing 5 species in 1 genus (Plagiorchis). The family Lecithodendriidae includes 3 species in 2 genera (Phaneropsolus and Prosthodendrium). In 9 other families, 1 species in 1 genus each is involved; Cathaemaciidae (Cathaemacia), Fasciolidae (Fasciolopsis), Gastrodiscidae (Gastrodiscoides), Gymnophallidae (Gymnophalloides), Microphallidae (Spelotrema), Neodiplostomidae (Neodiplostomum), Paramphistomatidae (Fischoederius), Psilostomidae (Psilorchis), and Strigeidae (Cotylurus). Various types of foods are sources of human infections. They include freshwater fish, brackish water fish, fresh water snails, brackish water snails (including the oyster), amphibians, terrestrial snakes, aquatic insects, and aquatic plants. The reservoir hosts include various species of mammals or birds.The host-parasite relationships have been studied in Metagonimus yokogawai, Echinostoma hortense, Fasciolopsis buski, Neodiplostomum seoulense, and Gymnophalloides seoi; however, the pathogenicity of each parasite species and host mucosal defense mechanisms are yet poorly understood. Clinical aspects of each parasite infection need more clarification. Differential diagnosis by fecal examination is difficult because of morphological similarity of eggs. Praziquantel is effective for most intestinal fluke infections. Continued efforts to understand epidemiological significance of intestinal fluke infections, with detection of further human cases, are required.

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Foodborne Intestinal Flukes in Southeast Asia

Fluke egg characteristics for the diagnosis of human and animal fascioliasis by Fasciola hepatica and F. gigantica

A total of 20 species belonging to nine genera of Echinostomatidae are known to cause human infections around the world. They constitute an important group of food-borne trematodes of public health importance chiefly in the Southeast Asia and Far East. The largest genus is Echinostoma (seven species), followed by Echinochasmus (five species) and others (1–2 species for each genus), i.e., Acanthoparyphium, Artyfechinostomum, Echinoparyphium, Episthmium, Himasthla, Hypoderaeum, and Isthmiophora. Various types of food animals, including freshwater fish, brackish water fish, freshwater snails, brackish water snails (gastropods and bivalves), and amphibians are the sources of human infection. The pathogenicity and host-parasite relationships have been studied extensively in several species, including Echinostoma hortense (human-infecting species) and Echinostoma trivolvis and Echinostoma caproni (animal-infecting species). Mechanical damages by the worms and toxic effects by their metabolites seem to be important factors in the pathogenesis of human echinostomiasis. Mucosal ulcerations and bleeding at the upper part of the duodenum or at the distal part of the stomach are occasionally encountered in gastroduodenal endoscopy of human E. hortense infections. However, clinical importance of each echinostome species, as well as mucosal defense mechanisms of different hosts, is yet poorly understood. Diagnosis of echinostomiasis can be done by recovery of eggs in the feces, but specific diagnosis is difficult because of morphological similarity of eggs. Praziquantel has been proved to be effective against echinostomiasis. Epidemiological surveys and detection of human infections are required for better understanding of the geographical distribution and endemicity of each echinostome species.

Echinostomes in humans

Echinostomiasis, caused by trematodes belonging to the family Echinostomatidae, is an important intestinal foodborne parasitic disease. Humans become infected after ingestion of raw or insufficiently cooked molluscs, fish, crustaceans and amphibians, thus, understanding eating habits is essential to determine the distribution of the disease. Despite the public health impact of echinostomiasis, it has been neglected for years. Traditionally, echinostomiasis has been considered as a minor disease confined to low-income areas, mainly in Asia. However, the geographical boundaries and the population at risk are currently expanding and changing in relation to factors such as growing international markets, improved transportation systems, new eating habits in developed countries and demographic changes. These factors make it necessary to improve our understanding of intestinal trematode infections. Herein, we review the main features of human echinostomiasis in relation to their biology, epidemiology, host-parasite relationships, pathogenicity, clinical aspects, diagnosis, treatment and control.

An update on human echinostomiasis.

This chapter examines the significant literature on the biology of echinostomes The members of the family Echinostomatidae are medically and veterinary-important parasitic flatworms that invade humans, domestic animals and wildlife and also parasitize in their larval stages numerous invertebrate and cold-blooded vertebrate hosts All echinostomes possess a complicated lifecycle expressed by: (i) alternation of seven generations known as the adult, egg, miracidium, sporocyst, redia, cercaria and metacercaria, and (ii) inclusion of three host categories known as the definitive host and first and second intermediate hosts Moreover, echinostomes have served as experimental models in parasitology at all levels of organization We discuss recent advances in several areas of the biological sciences that feature studies on echinostomes Initially, we consider aspects of the lifecycle, development and systematics of selected members of the Echinostomatidae We then highlight host-parasite interactions between echinostomes and their intermediate and definitive hosts with emphasis on the application of novel techniques to these topics

Chapter 3. Recent advances in the biology of echinostomes.

To the Editor: In 2002, the Department of Health-Provincial Health Team (DOH-PHT) of Surigao del Norte reported 102 cases of fasciolid-like infections in the municipality of Santa Monica, Siargao Island, the Philippines. The reports were based on characteristic large operculate eggs having been found in routine stool examinations conducted during schistosomiasis surveys. Fasciola hepatica infection was the initial diagnosis considered. In 2005, a collaborative team from DOH-PHT, the National Institutes of Health, the University of the Philippines Manila, and the Local Government Unit/Rural Health Unit conducted a field investigation to determine the cumulative prevalence of intestinal helminthic infections in adult patients whose conditions had been previously diagnosed as fasciolid-like infection and to determine the causative species of trematode.

The study group consisted of 70 adult patients from the barangay (local government unit of 50–100 families) of Libertad, for whom fasciolid infections had been noted on previous surveys. Researchers confirmed infections and collected adult trematodes for species identification. All patients underwent bowel preparation with bisacodyl (Dulcolax) 10-mg tablets (2 tablets taken immediately after a meal on night before deworming), followed by praziquantel (25 mg/kg in 2 doses 4 h apart), and 30-g magnesium sulfate granules, dissolved in 1 cup of milk, given 1 h after the second dose of praziquantel. Stools were processed by using the Kato-Katz method (1) and examined microscopically by medical technologists from the Diagnostic Parasitology Laboratory, College of Public Health, University of the Philippines Manila, for intestinal helminth ova. In addition, a clinical history was taken and a complete physical examination was conducted for each patient volunteer after stool submission. Eating preferences and habits were specifically noted.

The research was approved by the Department of Health Center for Health Development of the Caraga region. Informed consent was obtained before procedures were done and treatment was given to infected patients.

Cumulative prevalence for soil-transmitted helminth infections among the 70 patients was 51.4%. Prevalence according to species was Trichuris spp. 42.9%, Ascaris spp. 17.1%, and hookworm spp. 1.4%. Schistosoma japonicum infection rate was 10%. Stool samples from 8 (11.4%) patients had large (120–130 µm × 80–90 µm), brownish, operculated eggs; 3 had a total of 13 adult flukes. Microscopy showed small leaflike flukes 8–9 mm long and 2.5–3.5 mm wide. After the organisms were processed and stained with aceto-carmine and fast green stains, diagnostic features of Echinostoma malayanum (Leiper 1911) were noted. Adult trematodes were within known species size range (5–10 mm × 2.5–3.0 mm) and had elongated bodies and bluntly rounded ends. The ventral sucker (acetabulum) was prominent and larger than the anterior oral sucker. Paired testes were deeply branched and positioned high in the posterior half of the body, extending above the midplane with a single anterior globular ovary. The uterus was entirely anterior to the ovary, and vitellaria (glands) were abundant along both lateral portions of the worm, ending just posterior to the esophagus. The oral sucker had a horseshoe-shaped anterior collar with 43 circumoral spines, which differentiates this species from E. ilocanum (49–51 collar spines), another trematode species endemic to the Philippines.

In terms of eating habits, patients reported that fish were commonly eaten raw, after being dipped in a salt and vinegar mixture, locally known as kinilaw. Other methods of fish preparation were tinola (boiled), ginataan (stewed in coconut milk), and sinugba (charcoal-grilled). All echinostome-infected patients had a history of having eaten snails, kuhol and kiambu-ay, prepared raw with coconut milk and lime juice (kinilaw), especially when found in greater abundance during the rainy season.

Human echinostome infection results from ingestion of metacercariae that encyst in secondary intermediate hosts, usually freshwater snails, tadpoles, or fish. E. malayanum uses various species of gastropod mollusks for primary and secondary intermediate developmental stages (2–5). Certain species of fish may also serve as secondary intermediate hosts (2). Several mollusks that may serve as primary and secondary intermediate hosts have been identified in the Philippines, including Lymnaea (Bullastra) cumingiana, Radix quadrasi, and Physastra hungerfordiana for E. malayanum, and Pila luzonica for E. ilocanum (6,7).

To our knowledge, this is the first report of E. malayanum infections in the southern Philippines. Local eating habits are a strong factor in echinostome infections. The general lack of awareness by health staff and the community was a big factor in the poor identification of the disease. Clinical and laboratory staff and healthcare providers need training about echinostome infections and other intestinal foodborne trematode infections. Similar environmental, sanitary, and eating practices in the region suggest that the same parasitoses should be considered to be widespread in the area. Redirecting vital resources of the local health and government units of the Caraga region to the periphery and building local capacity will help empower authorities to provide public health services in rural areas, strengthen public health programs, and further develop public health infrastructure. Moreover, a successful control program against chronic intestinal parasitoses could serve as a paradigm for local health system development of effective control measures for other endemic diseases.

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Adriano U Suba-an

Papers

Echinostoma malayanum infection, the Philippines.