Leptospira interrogans: Characteristics, Pathogenesis, Lab Diagnosis

Last updated on June 11th, 2021

Leptospira interrogans is a member of the class Spirochaetes order Spirochaetales and the family Leptospiraceae. There are two species of Leptospira; L. interrogans are the pathogenic species and are the causative agents of leptospirosis or Weil’s disease whereas L. biflexa is sprophytic in nature.

Morphological Characteristics

  • Tightly coiled fine spirochete:
    • L. interrogans have a size of 6-12 μm length x 0.1 μm width which allows them to pass through filters used to sterilize the culture medium.
    • They are tightly and regularly coiled, with characteristic hooked ends (hence the species name interrogans resembling interrogation or question mark).
    • Spirals have a wavelength (interval between spirals) of 0.5μm and an amplitude of 0.1μm.
    • They possess a single endoflagellum attached at the pole and are highly motile exhibiting spinning and translational movements.
  • Not Stained with dyes
    • They cannot be seen under the light microscope due to their thinness (leptos, meaning fine or thin).
    • They do not take up ordinary stains but may be observed by dark ground or phase-contrast microscope or stained by silver impregnation method and by immunofluorescence.
  • Antigenically complex: Divided into serogroups which were further divided into serovars. On the basis of agglutination testing with specific antisera against the surface LPS antigens, Leptospira is divided into:
    • Leptospira interrrogans: Comprises of 250 serogroups which further consists of over 250 serovars.
    • L. biflexa: 65 serovars arranged in 38 serogroups.


  • Source: Although more than 100 animals can be infected; but important sources of infection are rats dogs, cattle, and pigs. Even asymptomatic animals can transmit the infection via urine (persistent colonization of renal tubules of carrier animals.)
  • Transmission:
    • Zoonotic: Humans can become infected through direct or indirect contact with the urine (or other body fluids, except saliva) of infected animals.
    • Person to person transmission rare

3R’s: The three important epidemiological determinants for leptospirosis include exposure to rodents, rainfall, and rice field.

  • Risk factors:
    • Lower socioeconomic status
    • Urban and rural slum areas
    • Rainfall and floods
    • Occupational exposure to animal urine, e.g. rice field workers, farmers.
  • Global distribution: Worldwide, the highest burden of the disease has a reported from urban slums of Brazil, India, and Thailand.
  • Phases of Infections:
    • The acute or bacteremic/septicemic phase
    • Immune phase: Aseptic meningitis

First Phase: Acute or bacteremic/septicemic phase

  • Entry via mucous membranes of the eyes, nose, or mouth or through skin abrasion
  • Vascular damage: Spirochetes can be found in the walls of capillaries, medium, and large-sized vessels. The exact mechanism of vascular damage is not clear.
  • Penetration and invasion of tissues are due to active motility and release of hyaluronidase.
  • Invade bloodstream and hematogenous spread multiple sites of the body:
    • producing fever,
    • dysfunction of the liver (jaundice),
    • kidneys (uremia),
    • lungs (hemorrhage)
    • central nervous system (aseptic meningitis)

Second Phase (Immune Phase)

After seroconversion Spirochetes start to disappear from the blood but they may colonize in kidney: Leptospira become adherent to the proximal tubular brush border and is excreted in the urine.

Clinical Manifestations

Incubation period: 5-14 days

Two distinct clinical syndromes:

  • Mild anicteric febrile illness: 90% of patients. Biphasic in nature; septicemic phase followed by immune phase.
  • Weil’s disease (hepato-renal-hemorrhagic syndrome)
Weil’s Syndrome:
  • a severe form of the leptospirosis infection (icteric) which occurs in approximately 10% of patients
  • jaundice and significant liver damage
  • kidney and/or vascular dysfunction
  • lethal pulmonary hemorrhages
  • Typical biphasic course may not be present.
  • death is up to 10% of cases

Laboratory Diagnosis

Laboratory diagnosis of leptospirosis infection can be done by detecting L. interrogans in a clinical specimen (urine, blood, or CSF) by immunofluorescence, impregnation stains such as Fontana stain, and modified Steiner technique or by using dark-ground microscopy or phase-contrast microscopy.

(a) Growth from blood in EMJH semisolid media Leptospira is forming subsurface colonies in the tube on the left and no growth in the tube on the right; (b) dark field microscopy image of the culture showing spirochetes with morphology compatible with Leptospira; (c) Conventional PCR targeting lipL32 gene  (Image source: Christopher Ryan Larson)

It can be isolated in the lab by using Fletcher’s or EMJH (Ellinghausen, McCullough, Johnson, Harris) medium.

Antibodies may be detected in the blood within 5–7 days of symptom onset which can be demonstrated by the microscopic agglutination test (MAT). Molecular diagnosis includes amplification and detection of various genes such as 16S or 23S rRNA or IS1533 insertion sequence.

Find details about the laboratory diagnosis of leptospirosis in this post of microbeonline.

References and Further Readings

About Acharya Tankeshwar 474 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.

Be the first to comment

Do you have any queries? Please leave me in the comments section below. I will be happy to read your comments and reply.

This site uses Akismet to reduce spam. Learn how your comment data is processed.