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Leptospirosis

 

Leptospirosis is a zoonosis of ubiquitous distribution, caused by pathogenic organisms belonging to the genus Leptospira, that are transmitted directly or indirectly from animals to humans. Leptospirosis is a major direct zoonosis. Human-to-human transmission occurs only very rarely. Leptospires are bacteria which can be either pathogenic (i.e. having the potential to cause disease in animals and humans) or saprophytic (i.e. free living and generally considered not to cause disease). Pathogenic leptospires are maintained in nature in the renal tubules and genital tracts of certain animals.

 

 

Leptospires

(www.ecdc.europa.eu)

 

 

So far, there are over 250 pathogenic serovars. Serovars having antigenic similarities are formed into serogroups, and all serovars have been divided into 25 serogroups. Different strains with small antigenic differences can sometimes be found within certain serovars.

Epidemiology

Leptospirosis is an emerging health problem in Thailand, with dramatic increases in reported incidence since 1996. Data from disease notification reports indicated a drastically increase in leptospirosis cases between 1995 and 2003, with a peak in 2000. There was an increase in the incidence rate from less than 0.3 per 100,000 in 1995 to 23.7 in 2000, with a drop in subsequent years. High morbidity of leptospirosis has been reported from provinces situated in the lower part of the Northeast region. Most infections occur in agricultural workers, primarily rice producers. The main risk factor of this disease is the bare-footed human activity during daily work in rice fields. Outbreaks of leptospirosis correspond with the rainy season, with an increase in cases beginning in August and decreasing in November. As of 1 January-5 September, 2014, a total 1370 cases have been reported with 10 deaths from 65 provinces out of 77 (The below graph). The overall morbidity rate is 2.16 per 100,000 population, mortality rate is 0.02 per 100,000 population and male to female ratio is 1 : 0.24. The highest morbidity rate was in the South (3.94 / 100,000 population), the Northeast (3.61 / 100,000 population), the North (1.44 / 100,000 population) , and the Central (0.35 / 100,000 population). The highest proportion of cases by age group were 35-44 years old (21.53%), 45-54 years old (21.17%), and 55-64 years old (17.01%).

 

 

Source: 506 Report, Bureau of Epidemiology, Department of Disease Control, Ministry of Public Health

Clinical Manifestations

The clinical manifestations are highly variable. In general, the disease presents in four broad clinical categories:

(i) a mild, influenza-like illness.

(ii) Weil’s syndrome characterized by jaundice, renal failure, haemorrhage and myocarditis with arrhythmias.

(iii) meningitis/meningoencephalitis.

(iv) pulmonary haemorrhage with respiratory failure.

The incubation period is usually 7–10 days, with a range of 2–30 days. The typical course of leptospirosis with an acute septicaemic phase followed by the immune phase as shown in Figure 1.

 

Figure 1: Typical course of leptospirosis

Incubation period

Septicaemic phase

Interphase

Immune phase

2-10 days

4-7 days

1-3 days

0-30+ days

Bacteria enter body

through cuts or mucosal surfaces; bacterial flagellae aid tissue penetration

Abrupt onset of fever, headache,

muscle pain, nausea; Ieptospires isolated

from blood, CSF, and most tissues; Mostly anicteric, 5-10% have

jaundice

Fever and other

symptoms resolve

temporarily prior to onset of immune phase

Recurring fever and CNS involvement (meningitis) primarily humoral response;

antileptospiral antibodies lead to clearance of the

organism from most tissues except kidney tubules; leptospires may continue to shed in the urine for long periods

Courtesy: Dr Richard A. Collins, Hong Kong

Mode of transmission

Human leptospiral infections result primarily from direct or indirect exposure to the urine of infected animals. Moisture is an important factor of the survival of the leptospires in the environment. Other modes of transmission of infection, such as handling infected animal tissues and ingestion of contaminated food and water, are also possible. Leptospires can gain entry into humans through cuts and abrasions in the skin, through intact mucous membranes (nose, mouth, eyes) and perhaps through waterlogged skin. They may occasionally enter the human body via the inhalation of droplets of urine or via drinking-water. Leptospires be transmitted from human to human but rarely. They can be transmitted from human to human by sexual intercourse, transplacentally from the mother to the fetus and via breast milk to a child. Urine from a patient suffering from leptospirosis should be considered infectious. As leptospires can be cultured from blood, this should be viewed as infectious for some time before the onset of symptoms and during the first 7–10 days of illness.


Cycle of leptospirosis (Faine et al. 1999)

Prevention and control

Because of the large number of serovars and infection sources and the wide differences in transmission conditions, the control of leptospirosis is complicated and will depend on the local conditions. Control can be achieved by controlling the reservoir or reducing infection in animal reservoir populations such as dogs or livestock. Control of wild animals may be difficult. Preventive measures must be based on a knowledge of the groups at particular risk of infection and the local epidemiological factors. Prevention and control should be targeted at : (a) the infection source; (b) the route of transmission between the infection source and the human host; or (c) infection or disease in the human host.

Control of infection source

It is important to establish what animal species are the infection sources in a particular area. Control measures can then be targeted to the local reservoir species of animals. Such measures include:

the reduction of certain animal reservoir populations, e.g. rats;

the separation of animal reservoirs from human habitations by means of fences

and screens;

the immunization of dogs and livestock;

the removal of rubbish and keeping areas around human habitations clean;

encouraging people not to leave food around, especially in recreational areas

where rats may be present. 

Interruption of transmission

It is important to be aware of the risk factors for human infection and, if possible, the

infection source. Risk of infection is minimized by avoiding contact with animal urine, infected animals or an infected environment. Where appropriate, protective clothing should be worn and wounds covered with waterproof dressings to reduce the chance of infection if exposure is likely, e.g. occupational or recreational exposure.

Human protection

Much depends on detailed knowledge of how, where and when humans may become infected in a particular area. One possibility is to increase awareness of the disease

among the population, risk groups and health care providers, so that the disease can be recognized and treated as soon as possible. Doxycycline has been reported to give some protection against infection and disease. In certain countries, vaccines for humans are available, but it should be remembered that they may only provoke immune responses to the serovars included in the vaccine.Human Immunization

Immunization by means of vaccines seems to provide a certain degree of protection. Vaccines are, in principle, suspensions of killed leptospires. Protection is largely serovar-specific. In areas where many serovars are causing leptospirosis, a vaccine must consist of different serovars matching those circulating locally. In some countries, e.g. China, where many serovars occur, vaccines consist of a mixture of a few of the most prevalent. Protective antibodies are produced only against the serovars present in the particular vaccine used.


 
 
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