Leptospirosis, Biology

Leptospirosis


Synonyms:
Weil’ disease, canicola fever, 7-day fever Leptospirosis is an important and widespread zoonosis. It was first described by Adolf Weil in 1886 when he reported an “acute infectious disease with enlargement of spleen, jaundice and nephritis. Rodents and some others species of wild animals act as carriers. Cattle and pigs are frequent carriers of leptospires and play a major role in the spread of infection among man and animals. Serological evidence indicates the prevalence of leptospirosis in India among domesticated animals, dog and man in different parts of the country.


The leptospiral group can be divided into saprophytic and parasitic subgroups represented by Leptospira biflexa and Leptospira interrogans, respectively. The organisms are long, coiled or spiral-shaped with hooked ends. The organisms are actively motile and show rotary or gliding movements due to flexibility. The organisms grow very poorly on solid media and better growth occurs in fluid or semi-solid media. Pathogenic strains need the presence of serum in the medium or substituted by various growth factors such as serum albumin fraction V, vitamins and Tween-80. The optimum temperature for growth is 290-320C. The multiplication of leptospires outside animal body is rare. Their survival outside animal body is markedly affected by slight changes in environment. The survival is highest during hot and humid weather. Some serovars can survive for several weeks in water and mud with neutral or slight basic pH. The members of the group have been divided into more than 280 different serovars belonging to 18 serogroups. The majority of spontaneous disease cases are associated with only a few serogroups.


Transmission: In carrier animals, the organism usually localises in the kidneys and is intermittently excreted in urine. The infected urine is the source of infection for other hosts. Pigs and rodents are frequent carriers of leptospires. The killing and eating of infected rats by dogs and pigs give rise to infection. Aborted foetuses, placenta and sometimes milk from pigs and cattle may also be responsible for the spread of infection.Leptospiral infection takes place through the intestinal tract after ingestion, and also through the skin which has become damaged or softened by contact with water or mud. Distribution via the urine is one of the main methods by which the disease becomes widespread, as organisms are frequently found in the urine after infection. Interspecies infection occurs between almost all domestic animals.


Symptoms: The symptoms shown by different species are discussed here.


Cattle: The disease may be acute or chronic in cattle. The acute form is characterized by rise in temperature, depression, diarrhoea, severe jaundice, dark urine, pneumonia and even meningitis. The chronic form may produce symptoms such as mastitis, abortions and repeat breeding in pregnant animals. The abortions are common and occur as epidemics. The mortality rate in acute cases is high.


Pigs:
The pigs are common carriers of leptospires. In some herds, leptospirosis is the cause of infertility. Occasionally, severe cases may show signs of jaundice, convulsions, conjunctivitis and abortions.


Horses:
Acute form of disease is characterized by rise in temperature, jaundice,haemaglobinurea and periodic ophthalmia. Abortions are seen in mares.


Dogs: The peracute form is characterized by sudden rise in temperature, depression, shivering and vomiting. The temperature falls and haemorrhages develop in and around month, lips, conjunctiva and skin. The icteric form is characterized by similar initial symptoms followed by severe jaundice. Haemorrhages are not so extensive as in peracute form. In subacute form, there is slight jaundice and symptoms of nephritis.


Lesions: In kidneys, there are sub mucous haemorrhages and interstitial nephritis in the form of small white areas in the renal cortex. Necrotic foci are seen in the liver of aborted foetus. Jaundice and haemoglobinuria are other lesions.


Diagnosis: Direct microscopic examination of plasma or the deposits of urine is carried out. Dark field microscopy and immunofluorescence microscopy is very useful for the examination of urine samples. Cultural methods, inoculation of animals like mice, hamsters and guinea-pigs, and serological tests like microscopic agglutination test (MAT) and ELISA are very useful methods in the diagnosis. Molecular techniques such as PCR are being used in recognized laboratories of the country.


Treatment: Penicillin, dihydrostreptomycin and other broad spectrum antibiotics are effective against leptospires. The treatment should be commenced during early stage of infection if the chronic renal infection is to be avoided. Dihydrostreptomycin is a more reliable antibiotic for preventing the development of chronic form of infection. Prolonged treatment also prevents development of the carrier state.


Control: The following methods may help in efficient control of leptospirosis
(i)  Efficient control of wild rodents.
(ii)  Maintenance of hygienic standards to prevent contamination of food and water with urine and other excreted material.
(iii)  Detection of infected animals by serological and bacteriological tests, and their separation from healthy animals.
(iv)  Immunization and use of antibiotics.

 

Killed vaccines prepared against different serovars prevent the development of  clinical infection and reduce the incidence of abortions in animals, but have little effect on symptom less carriers. In India no vaccination is carried out.

Posted Date: 9/17/2012 6:52:14 AM | Location : United States







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