Whole-cell inactivated leptospirosis vaccine: future prospects.

Leptospirosis is an infectious disease of worldwide distribution that is caused by pathogenic spirochete bacteria of the genus Leptospira. It is transmitted by the urine of an infected animal and contagious in a moist environment. Epidemiological studies indicate that infection is commonly associated with certain occupational workers such as farmers, sewage workers, veterinarians, and animal handlers. The annual incidence is estimated at 0.1-1 per 100,000 in temperate climates to 10-100 per 100,000 in the humid tropics. A disease incidence of more than 100 per 100,000 is encountered during outbreaks and in high-exposure risk groups. The 11 countries in South-East Asia (SEA) together have a population of more than 1.7 billion and a work force of about 770 million with more than 450 million people engaged in agriculture. Because of the large number of serovars and infection sources and the wide differences in conditions of transmission, the control of leptospirosis is complicated and will depend on local conditions. The available leptospirosis vaccines are mono- or polyvalent cellular suspensions. These cells are inactivated by chemical agents like formaldehyde and phenol, or by physical agents like heat. The vaccine confers protection for not longer than about one year, while there are cases that need revaccination six months later during epidemic periods.