ARTICLE - CHOLERA COMPLICATION SCARE

HAITI POLIO SCARE MAY BE RARE COMPLICATION OF CHOLERA
(New Scientist) - By Debora MacKenzie

Finally, some good news for Haiti: after three months, the cholera epidemic is starting to subside. And fears that polio had broken out may have been premature. A handful of people with polio-like symptoms may instead have had a previously unreported complication of cholera treatment.

Several people in the north of the country were reported to have developed paralysis late last year after being successfully treated for cholera. "Polio was one of the first possibilities looked into because of the public health implications," the World Health Organization's office for the Americas stated last week.

That is an understatement. Like cholera, polio – which causes paralysis – is carried in water and human faeces, and an outbreak would wreak havoc in Haiti, where sanitation is poor and diarrhoea and extreme poverty would help spread the virus. It would also threaten the worldwide eradication effort.

Reverted and dangerous
Polio was declared gone from the Americas in 1994, but in 2000 there were cases in Haiti and the neighbouring Dominican Republic. They were caused by the weakened, live virus in the polio vaccine, which had reverted to a dangerous form and begun circulating, as some health experts had warned it might.

But wild polio virus, still circulating in Africa and Asia, is a bigger threat – so 200,000 Haitian children have been vaccinated against the disease since the earthquake in the country in January 2010. Reports of paralysis woke fears that the vaccine virus had escaped again.

Last week, however, the WHO announced that on investigation, only four cases really looked like polio, and of those people three died – too high a death rate for polio. Pending results of tests for polio virus, it says it is "likely to rule out polio".

Salt in the blood
So what is it? The people developed paralysis one to three days after finishing cholera treatment. Niklas Danielsson of the European Centre for Disease Control and Prevention in Stockholm, Sweden, says this suggests osmotic demyelination syndrome (ODS).

ODS afflicts people whose blood salts are too low for two days or more, then return to normal too quickly. It results in destruction of the myelin sheaths around nerve cells, causing symptoms similar to the reports from Haiti.

"This has never been reported in cholera before, but I think it is a real possibility," says Mitchell Rosner, an ODS expert at the University of Virginia in Charlottesville.

Rare imbalance
If the concentration of sodium in blood falls, more water flows into cells by osmosis, causing them to swell. Nerves cells in the brain can be damaged if the glial cells that form the myelin sheaths around them swell.

If osmotic pressure persists, glial cells protect against this damage by reducing their own internal osmotic pressure so that they don't swell. However, if blood salt returns to normal quickly, the glial cells cannot adapt back fast enough. The normal osmotic pressure shrinks the cells, destroying them, says Rosner.

Both cholera and rehydration treatment cause low blood sodium which can last for several days, says Danielsson. ODS could be triggered when the blood returns to normal after treatment stops. An MRI scan of neural tissue could settle it, says Rosner, as ODS causes typical patterns of damage.

It's a devastating diagnosis for the people involved – but the good news for Haiti is that, unlike polio, ODS isn't infectious.