Oesophagostomum is a genus of parasitic nematodes (roundworms) of the family Strongylidae. These worms occur in Africa, Brazil, China, Indonesia and the Philippines. The majority of human infection with Oesophagostomum is localized to northern Togo and Ghana. Because the eggs may be indistinguishable from those of the hookworms (which are widely distributed and can also rarely cause helminthomas), the species causing human helminthomas are rarely identified with accuracy. Oesophagostomum, especially O. bifurcum, are common parasites of livestock and animals like goats, pigs and non-human primates, although it seems that humans are increasingly becoming favorable hosts as well. The disease they cause, oesophagostomiasis, is known for the nodule formation it causes in the intestines of its infected hosts, which can lead to more serious problems such as dysentery. Although the routes of human infection have yet to be elucidated sufficiently, it is believed that transmission occurs through oral-fecal means, with infected humans unknowingly ingesting soil containing the infectious filariform larvae.
Oesophagostomum infection is largely localized to northern Togo and Ghana in western Africa where it is a serious public health problem. Because it is so localized, research on intervention measures and the implementation of effective public health interventions have been lacking. In recent years, however, there have been advances in the diagnosis of Oesophagostomum infection with PCR assays and ultrasound and recent interventions involving mass treatment with albendazole shows promise for controlling and possibly eliminating Oesophagostomum infection in northern Togo and Ghana.
Taxonomy and classification
Oesophagostomum is part of the phylum Nematoda. This phylum is composed of five orders: Ascaridida, Enoplida, Oxyurida, Spirurida, Strongylida and Rhabditida.John, David T., Krotoski, Wojciech A. and Markell, Edward K. Markell and Voge's Medical Parasitology. St. Louis: Saunders Elsevier, 2006. 9th ed. Strongylida has three superfamilies - Ancyclostomatoidea, Trichostrongyloidea and Strongylidea. Oesophagostomum lie within the Strongyloidae.Wooten, Darcy. Oesophagostomum. 5 Feb 2009.
Synonyms
The prominent, single-nodule form of oesophagostomiasis is often referred to as Dapaong tumor named after a city in northern Togo. Within the villages of Togo, villagers often refer to it as ‘Kounkoul’, which means ‘turtle’ in the native Moba-language; the name aptly describes the hard, round mass in the patient's abdomen.Polderman, A. M., and J. Blotkamp. “Oesophagostomum infections in humans.” Parasitology Today 11.12 (1995): 451-456.
History of discovery
The first case of infection by Oesophagostomum spp. was reported in 1905 by Railliet and Henry, describing parasites found in the tumors of the caecum and colon of a male hailing from the Omo River in Southern Ethiopia. In 1910, H. Wolferstan Thomas reported the second known case, describing the macroscopical and microscopical pathology of Oesophagostomum stephanostomum. His descriptions were based on his observations regarding the post mortem of an infected Brazilian man who died from extreme dysentery.Thomas, H.W. (1910). The pathological report of a case of oesophagostomiasis in man. Annals of Tropical Medicine and Parasitology. 4: 57-88. In subsequent decades, several more cases of Oesophagostomum spp. infection in humans were found in Brazil, Indonesia, Canada, and several areas of Africa, particularly Northern Ghana, Nigeria and Brazil. Of all reported Oesophagostomum human infections, only O. aculeatum, O. bifurcum and O. stephanostomum have been identified with certainty.
Clinical presentation in humans
There is no overarching clinical picture for symptoms of oesophagostomiasis; however, most patients experience pain in the lower right quadrant, accompanied by the presence of one or several protruding abdominal masses.Ash, Lawrence R. and Orihel, Thomas C. Parasites in Human Tissues. Chicago: ASCP Press, 1995. In oesophagostomiasis, larvae can invade the colon wall, potentially causing two pervading types of nodular pathology. Multinodular disease is characterized by the formation of many tiny nodular lesions containing worms and pus along the colon wall. About 15% of patients have this form of oesophagostomiasis.GIDEON Infectious Diseases – Diseases (oesophagostomiasis). GIDEON Infectious Disease Database. 5 Feb 2009. Nodules themselves are usually not a problem, but they can give rise to further complications, such as bowel obstruction, peritonitis and intestinal volvulus. In rare cases, serious disease can occur including emaciation, fluid in the pericardium, cardiomegaly, hepatosplenomegaly, perisplenitis and enlargement of the appendix.
Single-nodular disease, more commonly known as Dapaong disease, is characterized by the development of a single mass that develops throughout the colon wall. This is the most common form of oesophagostomiasis, affecting 85% of patients. This nodule can instigate intense tissue reactions that result in the formation of painful projecting masses.
Common misdiagnoses include carcinoma, appendicitis, amebiasis and tuberculosis.Sun, Tsieh. Parasitic Disorders: Pathology, Diagnosis, and Management. Baltimore: William and Wilkins, 1999. 2nd ed.
The following is a summary of the second recorded case of oesophagostomiasis, as reported by H. Wolferstan Thomas in 1910:
Patient: male, 36 years old, native of the Rio Purus region in the Amazon State Chief complaints: suffering from acute dysentery, later experiencing deliriousness Outcome: Died within the three days following his admission. Major Findings: Lungs were emphysematous. Heart had evidence of hypertrophy in muscle, with some atheromatous patches along the aortic ring. Exterior of the small intestine was lined with several prominent tumors dark in color, 37 in total. Most of the tumors were found between the outside muscular layer and the bowel's peritoneal covering of the bowel. The tumors were generally small and varied in shape, from smooth ovular masses to flat button shapes to elongated masses akin to a leech; they were elevated by as much as 6 to 8 mm above the bowl surface. These nodules were found to contain one worm each, no more, no less. In the interior of the small intestine, twenty nodules were found along the walls, causing a discernible bulging of the mucous membrane. The caecum walls were irregularly thickened and dark in color, with three ovular tumors containing immature adult Oesophagostomum. Interior of the caecum was filled with rope-like opaque masses of rows of cystic tumors, which caused great thickening of the walls. Examination of the exterior of the ascending colon revealed the formation of thick adhesions spanning the whole length of the colon; these adhesions were filled with fat, enlarged glands and omental tissue. Underneath were a multitude of small cystic tumors that ruptured upon disturbance of the adhesion, disclosing small worms. The interior of the colon was most affected, with tumors of widely varying shapes and sizes occupying the walls and floor of the gut, causing as much as 5 mm of thickening of colon walls.
Transmission
Transmission of Oesophagostomum is believed to be oral-fecal for both humans and animals, largely because percutaneous infection with Oesophagostomum has never been reported.Ziem, J.B. Controlling human oesophagostomiasis in northern Ghana. (Doctoral thesis) Leiden University. 2006. It is unclear whether or not parasite transmission is specifically waterborne, foodborne, or both. Regardless, introduction of the stage three infective larvae is necessary for human infection. Much about the biological mechanism of transmission is still unknown, and current knowledge of oral-fecal transmission mechanisms does not explain why Oesophagostomum are mostly localized to Northern Togo and Ghana. It is possible that there are behavioral factors or unique soil conditions that facilitate larval development and are not found outside the current endemic areas. Oesophagostomiasis is generally classified as a zoonotic disease, which is an infectious disease that can be transmitted between animals and humans. This has been called into question recently, as recent research has found that human-to-human transmission is possible.
Reservoir
Oesophagostomum are carried predominantly by non-humans, infecting cattle, sheep, goats, wild pigs, and primates. Humans are largely presumed to be an accidental host, as they are not suitable for completion of the Oesophagostomum development; however, the extreme localization of oesophagostomiasis to northern Togo and Ghana in Africa suggests the possibility that the Oesophagostomum is increasingly exhibiting preference for human hosts.
Until recently it was believed that primates were the main reservoirs of human-infecting Oesophagostomum in northern Togo and Ghana, as these particular species have a considerable concentration in non-human primate reservoirs. A 2005 study done by van Lieshout and de Gruijter found that O. bifurcum in humans from northern Ghana is distinct from the O. bifurcum found in olive baboons and mona monkeys outside the endemic area. They used species-specific PCR and microscopy to establish the identification of two separate species of O. bifurcum. These results are significant, as they necessitate further research to determine the definitive reservoirs of human-infecting O. bifurcum.
Vector
Oesophagostomiasis has no vector.
Incubation period
The life-cycle of Oesophagostomum can usually be completed in less than 60 days. When the eggs are passed into the feces to the outside environment, they hatch into stage one larve. The stage two larve then molt twice, developing into infective stage three larva in 6–7 days. These stage three larvae can survive extended periods of desiccation by shrinking within their sheaths.
Morphology
Adult worms of all Oesophagostomum spp. exhibit a cephalic groove by its proximal gut as well as a visible secretory pore, or stomum, at the same level of the oesophagus19. Like other nematodes, Oesophagostomum spp. contain a developed, multi-nucleate digestive tract as well as a reproductive system. Their developed buccal capsule and club-shaped oesophagus are useful for distinguishing Oesophagostomum spp. from hookworms.
Both sexes of adults have a cephalic inflation and an oral opening lined with both internal and external leaf crowns. Female adults, which have a length range of 6.5–24 mm, are generally larger than their male counterparts, with a length range of 6–16.6 mm. Males can be distinguished by their bell-like copulatory bursa, located in the tail, and their paired rodlike spicules.
Eggs are ovular in shape and range from 50 to 100 micrometres in size; they closely resemble those of hookworms, which renders diagnosis via stool analysis useless in areas co-infected with both Oesophagostomum and hookworm.
Life cycle
For non-human hosts, the life cycle of Oesophagostomum begins with the passing of eggs in the animal feces. From there the eggs develop into stage one larvae. These larvae then spend 6–7 days in the environment developing into stage two and then infectious stage three larvae. Infection begins with the ingestion of soil contaminated with stage three larvae. After ingestion the larvae end up in the large intestine, unsheathing and penetrating the intestinal wall to form nodules. The resulting adult worms that remain in the intestinal lumen copulate; the eggs from the female are then deposited in the feces. Females usually lay around 5,000 eggs per day, which is on par with reproductive rates of other nematodes within Strongyloidea.