Schistosoma mekongi

Schistosoma mekongi is a species of trematodes, also known as flukes. It is one of the five major schistosomes that account for all human infections, the other four being S. haematobium, S. mansoni, S. japonicum, and S. intercalatum. This trematode causes schistosomiasis in humans.

Schistosoma mekongi
Scientific classification
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S. mekongi
Binomial name
Schistosoma mekongi
Voge, Bruckner & Bruce, 1978

Freshwater snail Neotricula aperta serves as an intermediate host for Schistosoma mekongi.[1]

History

Schistosomiasis was first reported in the Mekong river basin region in 1957.[2] It was believed that the cause of these cases was Schistosoma japonicum until 1978, when Neotricula aperta was discovered and it was determined that the Schistosome was a unique species, Schistosoma mekongi.[2] In 1967, a WHO mission was sent to Champasack confirming the infections and putting focus on how it was being transferred. In 1989, the Ministry of health initiated its first chemotherapy-based intervention in the communities in Khong and Mounlapamok. One third of children were found to be positive for S. mekongi. This launched an effort to implement health information, education, and communication of the disease in addition to chemotherapy. This intervention was performed annually until 1995 and continued to 1999. After several rounds of praziquantel the prevalence of S. mekongi has dropped to as low as 2% in some villages. The first national policy and strategy for control of helminth infections was developed and endorsed by the MoH in 2009. This served as the backbone of helminth control. A specific national schistosomiasis elimination action plan for the period 2016–2020 has been developed as a guidance for the National Control Program. This plan is supported by a Technical Taskforce at the central, provincial and district levels, and the task-force members are experts from the ministries, which are already involved by the National Committee for NTD Control. [3]

Characteristics

S. mekongi shares many general characteristics with other schistosomes, particularly S. japonicum, but it does have crucial differences. S. mekongi eggs are 30-55 μm and have a diminutive spine, and only 95 per mating pair are produced per day, whereas S. japonicum eggs are larger and produce on average 250 per day. N. aperta infected release on 42 cercaria per day, far lower than other Schistosomes.[4]

Epidemiolgoy

S. mekongi is found in the Mekong river basin region, from Kratie province, Cambodia, to Khong Island.[1] Schistosomiasis mekongi has also been found in the province of Champasack which is in the southern regions of Laos and has also been found in eastern Thailand. In fact, S. mekongi has shown that it has the smallest distribution of any schistosome species. Transmission of Schistosoma mekongi occurs in the low water season in this geographical area. Approximately 140,000 people are at risk for S. mekongi infection. This is quite a small number, but infection and re-infection sustain the disease in these populations. These affect children the most because of their high level of water contact. Reservoir hosts also play a big role in keeping the disease around, although the number of reservoir host for the Mekong species is less than the other species. With the use of stool examinations only about 1500 people are shown to be infected but it is projected that more than 10 times this number is more accurate.[5] In 1989 a universal mass treatment with praziqauntel of the people of Khong Island, Laos was performed. In 1995 a similar treatment was performed in Cambodia. In some areas this treatment was highly effective, eliminating S. mekongi. In other regions, particularly Khong Island, there was little effect.[2]

Attempting to control the intermediate host with a molluscicides also had no long-term effectiveness.[2]

Life Cycle/Etiology

Schistisoma mekongi infections are caused by the penetration of a humans skin by the cercariae stage of the parasite. Eggs are released into the water via urine or fecal matter. These eggs hatch and release a meriacidia which penetrates into the intermediate snail host Neotricula aperta. Sporocysts will then be released in the water by either successive generations of snails or, by predatory birds in the area eating then snails and pooping out the sporocysts in the water. Where they will then form cercariae that will penetrate into the human/animal skin and turn into a schistosomulae. The schistosomulae will circulate in the host blood and turn into adults once it finds its normal place. Schistisoma mekongi is much like Schistisoma japonicum in that they are found as adults more frequently found in the superior mesenteric veins, but can be found in the CNS. The cycle then repeats its self. [6]

Treatment/Prevention

Treatment for all types of schistosomiasis are the same, but first step to treatment is always to see a physician. They will likely treat you with praziquantel for 1-2 days to treat all schistosomiasis infections. To prevent yourself from getting the parasite you should boil all drinking water and try to restrain from swimming in areas of slow current or evidence of rotting wood in the dry season. [7]

Reservoir Hosts

The reservoir hosts for Schistosoma mekongi are dogs and pigs. This species is thought to not be able to use cattle, such as water buffalo, as an effective reservoir host like its close cousin Schistosoma japonicum.

References

  1. Attwood SW, Fatih FA, Upatham ES (2008). "DNA-sequence variation among Schistosoma mekongi populations and related taxa; phylogeography and the current distribution of Asian schistosomiasis". PLoS Negl Trop Dis. 2 (3): e200. doi:10.1371/journal.pntd.0000200. PMC 2265426. PMID 18350111.
  2. Ohmae, Hiroshi; Sinuon, Muth; Kirinoki, Masashi; Matsumoto, Jun; Chigusa, Yuichi; Socheat, Duong; Matsuda, Hajime (2004-06-01). "Schistosomiasis mekongi: from discovery to control". Parasitology International. Centenary Symposium to Celebrate the Discovery of Schistosoma japonicum Part II. 53 (2): 135–142. doi:10.1016/j.parint.2004.01.004. PMID 15081945.
  3. Virak Khieu;Somphou Sayasone; Siuon Muth; Masashi Kirinoki; Sakhone Laymanivong; Hiroshi Ohmae; Rekol Huy; Thipphavanh Chanthapaseuth; Aya Yajima; Rattanaxay Phetsouvanh; Robert Bergquist; Peter Odermatt(2019/02/7) "Elimination of Schistosomiasis Mekongi from Endemic Areas in Cambodia and the Lao People's Democratic Republic: Current Status and Plans". Nation Center for Parasitology
  4. Hebner, Rebecca. "Schistosoma mekongi". Parasites and Pestilence. Stanford University. Retrieved 2017-04-24.
  5. Virak Khieu;Somphou Sayasone; Siuon Muth; Masashi Kirinoki; Sakhone Laymanivong; Hiroshi Ohmae; Rekol Huy; Thipphavanh Chanthapaseuth; Aya Yajima; Rattanaxay Phetsouvanh; Robert Bergquist; Peter Odermatt(2019/02/7) "Elimination of Schistosomiasis Mekongi from Endemic Areas in Cambodia and the Lao People's Democratic Republic: Current Status and Plans". Nation Center for Parisitology
  6. CDC (2018/12/18)
  7. CDC (2018/12/18)


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