[NEW DELHI] A long-standing mystery about cholera vaccines — why they show promise in trials in the United States and Europe but perform disappointingly in endemic areas — may have been solved.
Researchers have shown that the vaccines are much less effective in people infected with intestinal worms.
People with worms produce fewer antibodies against the cholera bacterium Vibrio cholerae than those free of worms. De-worming programmes in areas where cholera is endemic could help counter this, they say.
The study tested the immune responses of cholera patients with and without worm infections. Tests were conducted on 361 patients in Dhaka, Bangladesh, of whom 53 had intestinal worm infections — mostly roundworms, hookworms and tapeworms.
The researchers — scientists at the International Centre for Diarrhoeal Disease Research, Bangladesh (ICDDR,B), Massachusetts General Hospital and Harvard Medical School in Boston, United States — tested immune responses to two components of a cholera vaccine.
One is based on part of a lethal toxin produced by V. cholerae. The other is a sugar-based molecule called lipopolysaccharide that makes the bacterium infectious.
They found that patients with intestinal worms had the lowest immunity to the cholera toxin and developed few antibodies to it in the intestines. Worm infection did not affect the immune response to lipopolysaccharide.
The researchers say the finding provides an additional reason for de-worming programmes in cholera endemic areas.
More than five million cases of cholera occur worldwide, leading to around 100,000 deaths — mostly in poor areas in developing countries. These countries also have a high incidence of parasitic worm infections in the digestive tract, such as hookworms and tapeworms.
Bangladesh, for example, records 200 cholera cases for every 100,000 people each year, while 80 per cent of children have intestinal worms. Hospital data from Kolkata, India, and Kathmandu, Nepal, show almost a third of children with diarrhoeal diseases also have worm infections.
Firdausi Qadri, an immunologist at ICDRR,B, told SciDev.Net that scientists at his institute are trying to identify new or novel substances that produce an immune response as well as studying factors that can improve the immune response to cholera vaccines in children in South Asia. They are also investigating the use of anti-parasitic drugs to improve the immune response to cholera vaccines.
The study was published last week (31 March) in PLoS Neglected Tropical Diseases.
PLoS Neglected Tropical Diseases doi:10.1371/journal.pntd.0000403 (2009)