Organism Leptospira interrogans with a large number of serovars, the most important of which is icterohaemorrhagiae. It is passed in rat's urine and can contaminate any area that they frequent. For the survival of the organism, there must be moisture, such as a canal or sewer, or else in damp soil, the washings of abattoirs or similar conditions. The pH of the soil or water is important and the Leptospira cannot survive in an acid environment. Leptospirosis is, therefore, commoner in places where the soil is alkaline. Salt water and chlorine solutions rapidly kill the organism.

Clinical features Commencing with fever, malaise, vomiting and myalgia, jaundice subsequently develops and there may be haemorrhages into the skin, mucous membranes and internal organs. The disease may progress to a more serious form with liver failure, renal failure or meningitis. However, many people only have mild infections and the vast majority do not exhibit any symptoms at all. In endemic areas, children are probably most commonly infected.

Diagnosis is by finding the motile organism by dark field illumination in a wet blood film during the first week of the disease.

After this time, serological tests or animal inoculation can be used, whereas Leptospira may be found in the urine from the third week onwards. Culture of the organism can take up to 1 month.

Transmission The Leptospira enters the skin of humans through minor abrasions or mucous membranes, although it does appear to be able to enter unbroken skin as well. Infection results from exposure to contaminated moist areas, such as swimming in canals, or walking barefoot over damp rat-infested soil. A direct rat bite can transmit the disease, as also an aerosol of contaminated fluid or ingestion of contaminated food.

Other animals can become infected with different serovars: cattle and water buffalo with hardjo, dogs with canicola and pigs with pomana. These domestic animals subsequently excrete Leptospira in their urine, contaminating the surroundings.

Incubation period is 4-19 days, usually 10 days.

Period of communicability Leptospira are excreted in the urine for several months, but person-to-person spread is rare.

Occurrence and distribution Where rats are common and conditions are favourable, the infection is widespread. In many areas surveyed, Leptospira antibodies have been found in a large percentage of the population, endemic in the community with the occasional severe case. It is common in the tropics particularly where the soil is alkaline or irrigation is used for agriculture. Infection is, therefore, common in rice paddy areas and sugarcane estates. This association of the disease with certain occupations is helpful in making the diagnosis. Such occupations as mine workers, farmers, canal cleaners, sewer workers, people employed in the cleaning and preparation of fish or in abattoirs, are at greater risk.

Flooding can widen the area of contamination leading to outbreaks in people not normally at risk. Disasters and any alteration in conditions that leads to an increase in the rat population will have a similar, but more long-term effect.

Leptospirosis is a very widespread zoo-notic infection of animals, endemic in many rodents, especially rats. The organism has been found in a variety of other animals, opossums, mongooses, skunks, hedgehogs, squirrels, rabbits and dogs, to name but a few, but the two domestic rats Rattus rattus and R. norvegicus are by far and away the most important reservoirs.

Control and prevention is the avoidance of areas contaminated with rat and animal urine, often a difficult thing to achieve. Various measures are:

• the reduction of rats by extermination and protection of buildings, especially those used for preparing meat, fish and housing domestic animals (rat control in Box 16.1);

• burning of sugarcane fields after harvest and the drying out of rice fields;

• wearing of protective clothing to reduce abrasions and contamination;

• avoiding canals, lakes and bodies of water known to be infected;

• controlling the number of dogs;

• providing proper pens with drainage for domestic animals so that urine does not collect and make the surroundings sodden;

• wash down food premises with a solution of chlorine or salt water.

Vaccination of persons at risk has been achieved in some countries using the specific serovar. Doxycycline prophylaxis can be used where short-term exposure is expected (e.g. in troops).

Treatment is with benzylpenicillin 2 million IU every 6h or doxycycline 100 mg for 7 days, preferably within the first week of the illness.

Surveillance Leptospirosis is a notifiable disease in many developed countries. Where outbreaks occur, the cause should be investigated and specific control measures instituted.

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