My colleagues (from SSHSPH) and I had performed a review of leptospirosis a few months back, and coincidentally I was asked to provide some input for the mainstream media (behind a paywall). It is a very interesting infection from the academic/clinical perspective, and there are a number of excellent reviews, notably here and here (behind a paywall). The experience immediately up north is summarised here.
Leptospirosis periodically appears in local news. In 2012, a young Bangladeshi worker living in cramped, unhygienic conditions in an illegal makeshift structure in Little India died from the disease. In 2016, an outbreak among dogs was linked to Sunny Heights Day Care Centre. In March this year, the Gunung Belumut Recreational Forest Area in Kluang, Malaysia, was closed when two young children developed leptospirosis after having visited the area with their family.
The disease is not well known to the general public, but is widely regarded as the most common zoonosis (a disease in animals that can be transmitted to humans) worldwide. In 2015, the World Health Organization’s Leptospirosis Burden Epidemiology Group estimated that there are approximately 1.03 million cases with 58,900 deaths each year, the bulk of which occurs in tropical countries.
The family of corkscrew-shaped bacteria that causes leptospirosis (Leptospira species; leptospires) are widespread in the natural environment. More than 250 subgroups (also termed “serovars”) have been identified that cause infections in animals and humans. Small mammals such as rodents are commonly “maintenance hosts”, where infection is chronic, mild and endemic. The leptospires reside in the kidneys of these animals and are shed into the environment through urine. “Accidental hosts”, which include humans and domestic animals such as farm animals and dogs, are infected when they come into direct or indirect contact with pathogenic leptospires in the environment. Humans can also shed the bacteria in their urine when infection occurs, but only for a short period of time – the infection is quickly cleared in the majority of cases, or else the host dies from infection.
The major risk factors for infection in humans are occupational and recreational activities that greatly increase the risk of exposure to environmental leptospires. Rice harvesting was one of the earliest at-risk occupations identified, and the Japanese term “akiyami” (autumn fever) still remains in the medical lexicon. Local studies published in 1987 showed that wet market/food centre cleaners, sewer workers and abattoir workers (no available online copy) were at higher risk of exposure. Outbreaks have also been linked to recreational activities with significant freshwater contact – the most well known being the Eco-Challenge-Sabah 2000 multisport endurance race, where 79 of 189 athletes developed leptospirosis. In the urban setting, severe flooding is associated with outbreaks of leptospirosis, presumably by exposing more people to rat urine-contaminated water. Beyond occupation and recreation, dogs (but rarely cats) may be infected with leptospirosis, with attendant potential risk of transmission to their owners.
The incubation period of leptospirosis is between 2 and 30 days, with the majority occurring between 5 and 12 days after known exposure. It is often difficult to pinpoint the exact location and time of exposure given the highly variable incubation period and ubiquitous nature of the bacteria.
Most infections are believed to be mild and self-limiting, where infected persons either do not seek medical attention or are misdiagnosed (but recover anyway). Fever, headache, muscle aches, and nausea with vomiting are the most commonly reported symptoms. Rarely, a devastating form of disease occurs with kidney failure and liver damage resulting in yellowing of skin and eyes (jaundice), usually accompanied by lung injury. This classic syndrome is named after German physician Adolf Weil who described it in 1886. The risk of dying from Weil’s disease, especially with lung injury, exceeds 10% even in the best medical centres.
Leptospirosis is endemic in Singapore – a study by the National Environment Agency and DSO Laboratories more than a decade ago showed that 41% of 1,200 rats were infected – but the number of known human infections is small. It was officially made a notifiable disease here in September 2016. According to the Ministry of Health’s weekly infectious disease bulletin, there have been no cases of leptospirosis reported this year, whereas 54 cases were notified in 2017 – well within the range of reported cases (30-64 each year) between 2012-2015. Approximately 40% of cases are imported.
Prevention of leptospirosis is based on the understanding of how the bacteria are transmitted. In an urban setting, housing construction and infrastructure that minimizes exposure to rodents as well as flooding will significantly reduce the risk of outbreaks. Workers in high-risk occupations should be issued with personal protective equipment that prevents exposure of skin to contaminated surfaces and water. Rodent control is an important way of minimizing the risk of human leptospirosis in cities, but this has proven challenging globally and will require considerable resources and innovation. In rural and pastoral settings, avoiding activities in water bodies that might be contaminated with animal urine and barefoot walking in endemic rodent-infested areas will reduce the risk of leptospirosis. In New Zealand, vaccination of domestic flocks is strongly advocated in order to reduce the risk of human infection. Unlike rabies, strategic vaccination of wild animals has not been attempted and is unlikely to work because current vaccines only protect against a handful of leptospiral serovars for a short period of time.
Leptospirosis vaccines are available for dogs in Singapore, but annual vaccination is required. Human vaccines are only available in a handful of countries, including Cuba, France and China. These are directed towards specific major serovars in each country and offer relatively short-term protection (2 years in the case of the French vaccine), and thus are generally only recommended for workers in high-risk occupations. The antibiotic doxycycline has been employed for prophylaxis against leptospirosis in militaries training in endemic areas and in civilians after high-risk exposures, i.e. floods, but the evidence for efficacy is not strong, and breakthrough infections have been reported.
In conclusion, leptospirosis is a global zoonosis that is endemic in rodents in Singapore. There are relatively few local cases each year, but fatalities are reported every few years. The risk of infection can be further reduced by controlling the rodent population, maintaining and improving on current high standards of housing construction and infrastructure, and ensuring adequate protection of workers in high-risk occupations as well as those engaging in recreational activities that involve exposure to animal urine-contaminated water or soil. Dog owners should vaccinate their pets annually.