Leptospirosis – a most dreaded zoonotic infection worldwide

Leptospirosis continues to be one of the most common and most dreaded zoonotic infections worldwide. Globalisation, in the context of international travel, particularly for recreational activities and military expeditions, has led to increased exposure of individuals from developed world to the disease, as recent outbreaks show. The Caribbean and Latin America, the Indian subcontinent, Southeast Asia, Oceania and to a lesser extent Eastern Europe, are the most significant foci of the disease, including areas that are popular travel destinations, says a report in the International Journal of Infectious Diseases.

Leptospirosis; also called Weil’s disease, infectious jaundice, pea picker’s disease, or swineherd’s disease, is an infectious disease of animals, occasionally communicable to humans, caused by a pathogenic spirochete of the genus leptospira. Rodents, domestic and wild animals form the reservoir of the infection where domestic animals such as cattle, dogs and pigs may act as carriers for several months (temporary carriers) while rodents usually remain carriers throughout their life (permanent carriers). The source of infection in farm animals is usually through pastures, drinking water, or feed, when contaminated by infected urine of animals inflicted with leptospirosis. It is a febrile disease most common in areas where the climate is warm and humid, soils are alkaline and there is abundant stagnant water. 

The leptospires are excreted in the urine of animals and they affect man when he comes into contact with urine of infected animals, directly or indirectly, when he is exposed to an environment contaminated by the urine of the infected animals such as soil and stagnant surface following rainfall. Therefore this illness commonly occurs during the monsoon months. The infection is transmitted to humans when they wade through stagnant rainwater contaminated by infected urine of animals. These organisms can survive for six hours in dry soil and for six months in flooded condition. They enter the host through the abrasions on the skin of the body or infect mucous membranes of eye.

Clinical evidence of the disease in human varies depending upon the infecting type of leptospire. Usually after an incubation period of about a week, fever, weakness and pains in the limbs, back and abdominal muscles are noted. Nausea, vomiting and diarrhoea are not uncommon. One characteristic symptom is congestion of the conjunctival blood vessels around the corneas of the eyes. Jaundice may occur after the first week of illness. The death rate is approximately 30% of the severely ill and jaundiced patients. 

Leptospirosis can occur in both urban and rural areas. In urban areas of developing countries, a contaminated environment due to various factors such as overcrowded slums, inadequate drainage and sanitation facilities for man and animals, presence of stray dogs, cattle, pigs, domestic rate and people walking barefoot contribute to the spread of the illness. In rural areas, high-risk groups are workers in rice fields, cane fields and other agricultural crops and animal husbandry staff. In addition, workers involved in mining, sewers’ cleaning and garbage collection are also at risk. History of animal contact is not essential for diagnosis, as it would be impossible to trace the source of infection and any person could be infected, irrespective of direct contact with animals, and due to contaminated environment. Therefore, the most important epidemiological factors are rainfall and contact with contaminated environment.

Persons of all ages and races are susceptible to the disease. Adult men, however, are more frequently infected because they tend to work in high-risk jobs. The number of cases in a region often fluctuates from year to year due to various factors such as rainfall, flooding and animal infections. Leptospiral infections tend to occur as individual, small cluster of cases or large outbreaks/epidemics.

Leptospirosis has long been considered important zoonotic disease in India. Since 1980s, the disease has been reported from various States during monsoon months in considerable epidemic proportions. The disease is endemic in Kerala, Tamil Nadu, Gujarat, Karnataka, Maharashtra, Andhra Pradesh, Orissa, West Bengal, Goa, et al. In Goa alone, on an average, around 1500 to 2000 suspected cases are reported, of which 50 to 60 are found to be positive, each year.

Health experts, however, point out that leptospirosis has been under reported and under-diagnosed in India due to lack of awareness of the disease and lack of appropriate laboratory diagnostic facilities in most parts of the country. Combining clinical expertise and awareness with confirmatory laboratory back up dramatically increases the recognition of patients with the disease. Clinical features of leptospirosis vary from mild illness to severe life threatening disease. The illness can be diagnosed only by laboratory tests as the clinical features are non-specific. But the laboratory tests are complex and hence definite guidelines for the diagnosis, treatment/management of human leptospirosis is greatly necessary.

According to the International Society for Infectious Diseases (ISID), leptospirosis is a re-emerging zoonosis of global importance and unique environmental and social correlations. Traditionally related to certain climatic or socio-economic conditions that favour endemicity in animal vectors and human exposure, it is generally confined to the developing parts of the world, being randomly reported from industrialised countries often as an important disease following international travel to exotic destinations. Therefore, attempts at global co-ordination and recognition of the true burden of the infectious disease, with significant mortality, should be reflected in efforts to define and utilise successful preventive policies.

(The writer is a freelance journalist)