Snail fever (Schistosomiasis) - parasites that can penetrate intact skin

Riding in a mokoro in the Okavango Delta
Before my African trip I had been trying to research whether schistosomiasis is present in the places we were to visit. We had a three day camping trip within the Okavango Delta in Botswana with activities including swimming and riding in mokoro (dug out canoes). We also planned to visit Victoria Falls and had a free day for perhaps a white water rafting trip on the Zambezi River or a visit to the Devil's pool on the edge of Victoria Falls.

The Devil's pool sounded like a very cool place - the ultimate infinity pool, but also the most dangerous pool in the world. It is only accessible for a few months in the year when the water level is low. You dive into the pool only to be stopped by a submerged rocky lip that separates you from the 100m drop.
Looking across the falls, seeing people in Devil's pool


So what is Schistosomiasis or Bilharzia?

Schistosomiasis, or Bilharzia, is a parasitic fluke transmitted in infected fresh water. The larva form of the worm is released by infected fresh water snails, and can penetrate intact human skin! Once in the body, the larvae migrate in the blood vessels and eventually mature into adult worms that lay eggs into the bladder and bowels to continue the life cycle. Symptoms can take 2-10 weeks to develop, can be subtle and may include fever, cough, fatigue, weight loss, abdominal pain, blood in urine or stools. Chronic infection can lead to liver or kidney failure or even neurological symptoms.

Schistosomiasis occurs throughout Sub Saharan Africa and also in tropical/subtropical areas in the Middle East, South East Asia and South America. For my particular destinations, IAMAT states that it is present in the Okavango River and marshlands. Infection rates peaked in the 1980s (up to 80%) and seems to be related to water flow in the river system. Appleton (2008) predicted there may be another epidemic around 2020. Schistosomiasis is endemic in the Zambezi River.


Credit: WHO Map Library


Prevention

The snails that carry the infected larva are only present in fresh water so swimming in the ocean or in chlorinated pools is safe. Avoid swimming, wading, paddling or water sports in fresh water. There is no way to distinguish infected from non-infected water.

Whilst some may say white water rafting is safe, this is not true. All rafting begins and ends with wading through relatively still water while getting into and out of the river. Schwartz (2005) reported outbreaks of schistosomiasis in three rafting trips on the Omo River in Ethiopia and found that up to 76% became infected often after just one exposure. Another study by Morgan (2010) screened swimmers and kayakers/rafters with recreational exposure to water on the upper Nile River in Jinja District in Uganda, reviewing seroconversion rates of 13% and 15% respectively. So basically no big difference in infection rates between the two groups.

Avoid drinking untreated water - stick with bottled, boiled or treated water. Heat bathing water to 50 degrees for 5 minutes or treat the water with iodine or chlorine tablets. Vigorous towel drying immediately after exposure may remove the schistosoma larva before they can penetrate the skin but there is no study to look at the effectiveness of this. Application of 50% DEET may also block penetration but again there is no guarantee.

Combing through the travel forums I noticed a lot of people downplay the seriousness of schistosomiasis, most basing their opinions on being lucky not to catch it! As well, some local businesses will try to present an area as safe when there is actually no way of knowing. Some local resorts claim to sell schistosomiasis treatment but the problem is that the treatment will only work on adult worms and not eggs or larvae. So taking the treatment immediately before or after exposure will not do anything! Secondly the drugs sold may be counterfeit, a major problem in Africa.

I saw a recent episode of The Amazing Race (season 19) where the contestants were tasked to unload cargo from a boat on Lake Malawi and carry it onto the shore. The episode also shows locals swimming and playing in the lake. Schistosomiasis is endemic in Lake Malawi and infection rate in tourists can approach 30% (Cetron 1996). In my opinion it was totally irresponsible for the show not to mention this potentially deadly disease in this episode nor were the contestants given information and advice regarding prevention, testing or treatment. 


Investigations and Treatment

Blood test (serology) can be done on return from endemic regions, or urine or stool testing done if symptomatic. It can take 6-8 weeks for the blood test to become positive so testing should be deferred for 8 weeks after your last exposure. Praziquantel is the treatment of choice. It is only active against mature worms so a repeat treatment a few weeks later may be required.

Sunset on the Okavango Delta


The Aftermath

We did not end up doing any water activities at Victoria Falls and opted for a helicopter ride over the falls instead. It was a great decision as the bird's eye view gave us a better perspective of the falls and made us truly appreciate its grandeur and power.

Unfortunately I could not resist going into the water with everybody in the Okavango Delta; afterall 40 degrees heat and 3 days without showers in a bush camp was a daunting combination. I went into the water knowing the risks, but I admit I was a bit worried what I've been exposed to. I had a blood test 2 months after coming home and am happy to report it was negative.




References

  1. International Association for Medical Assistance for Travellers (IAMAT)
  2. Appleton et al (2008) Epidemic transmission of intestinal schistosomiasis in the seasonal part of Okavango Delta, Botswana. Ann Trop Med Parasitol 102 (7):611-23
  3. Cetron et al (1996) Schistosomiasis in Lake Malawi. Lancet 348 (9037):1274-1278
  4. Morgan et al (2010) Schistosomiasis among recreational users of upper Nile River, Uganda, 2007. Emerg Infect Dis 16 (5)
  5. Schwartz et al (2005) Schistosome infection among river rafters on Omo River, Ethiopia. J Travel Med 12 (1):3-8



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