Morphology and staining properties
The causative agent of leptospirosis (L. interrogans) belongs to the family Leptospiraceae. These are thin spirochetes, 6–24 µm long and no more than 0.15 µm thick.
- Structure: the cell has 20 to 40 primary coils. The ends of the bacterium are curved and outwardly resemble hooks. Spores and cysts are not formed.
- Motility: provided by single fibrils extending from each pole of the cell.
- Staining: leptospires take up aniline dyes extremely poorly and are difficult to distinguish by Gram staining or the Romanovsky–Giemsa method. The optimal visualization method is dark-field microscopy or phase-contrast microscopy.
Cultural properties and resistance
Leptospires are aerobes and grow slowly. The optimal temperature for their reproduction is 28–30 °C, which is lower than the normal human body temperature.
Lipids—long-chain fatty acids and alcohols—serve as their source of energy and carbon. Cultivation on artificial media strictly requires the addition of serum albumin or blood serum. In liquid media, they multiply by transverse fission without forming turbidity (growth remains transparent).
Environmental resistance:
| Factor | Effect on leptospires |
|---|---|
| Desiccation and low pH | Die rapidly |
| Heating | Perish at 56 °C in 25–30 minutes, instantly upon boiling |
| Moist environment (water bodies) | Remain viable for up to 30 days |
| Moist soil | Can survive for a long time—up to 280 days |
Epidemiology and pathogenesis
Leptospirosis is a natural focal zoonosis with a characteristic summer-autumn seasonality. The main reservoir consists of wild and synanthropic rodents, while domestic animals (dogs, pigs, cattle) serve as an additional reservoir. In animals, the infection proceeds chronically and asymptomatically: bacteria are excreted in the urine, contaminating soil and water.
Humans are highly susceptible but represent a "dead end" for the infection—transmission of the pathogen from a sick person to a healthy one is practically non-existent.
Phases of infection development:
- Portal of entry: intact mucous membranes of the gastrointestinal tract or microtraumas of the skin (waterborne transmission predominates).
- Primary dissemination: penetration into the bloodstream and seeding of organs of the reticuloendothelial system (kidneys, liver).
- Incubation period: takes 7–10 days; bacteria actively multiply in parenchymatous organs.
- Secondary bacteremia: massive release of leptospires and their toxins into the blood, which coincides with the acute onset of clinical manifestations.
- Organ pathology: bacteria damage capillaries, causing hemorrhagic syndrome, jaundice, aseptic meningitis, and renal failure. Case fatality ranges from 3 to 40%.
Diagnostics, treatment, and prevention
Material for laboratory testing (blood, urine, CSF, serum) is selected depending on the stage of the disease.
- Microbiological diagnostics: includes dark-field bacterioscopy, inoculation onto nutrient media, and PCR.
- Serology: the gold standard is the microscopic agglutination and lysis test (MALtest) with a panel of reference live cultures. ELISA is also used.
- Biological assay: performed by infecting suckling rabbits.
Treatment: The basis of etiotropic therapy is antibiotics of the penicillin and tetracycline groups. Additionally, specific heterologous antileptospiral immunoglobulin derived from bovine serum is used.
Prevention: Nonspecific measures include rodent control (deratization) and vaccination of farm animals. According to epidemiological indications, specific prevention for humans is carried out using a heat-killed whole-cell vaccine containing antigens of the four main serogroups.
Briefly: answers to topical questions
What pathogenicity (virulence) factors do leptospires have?
Leptospiral pathogenicity factors include endotoxins, exotoxin-like substances, and various aggressive enzymes.
- Endotoxin is a lipopolysaccharide complex (LPS) with low endotoxin activity, possessing pyrogenic, dermonecrotic, and lethal properties.
- Exotoxin-like substances possess cytotoxic and hemolytic properties (cytotoxins, hemolysins).
- Aggressive enzymes include fibrinolysin, plasmacoagulase, lipase, paraoxonase, collagenase, and phospholipase C.
What biological material is taken from a patient to diagnose leptospirosis in the first and second weeks of the disease?
The choice of biological material for diagnosing leptospirosis depends on the stage of the infectious process.
- First week of the disease — citrated blood is taken for examination.
- From the second week of the disease — urine is examined.
- In case of meningitis — cerebrospinal fluid (CSF) is examined.
What species of leptospires are distinguished in modern classification besides the pathogenic L. interrogans?
In modern taxonomy, saprophytic leptospires are distinguished in addition to the pathogenic species.
- Pathogenic species (L. interrogans) — a parasitic microorganism that causes disease in humans and animals.
- Saprophytic species (L. biflexa) — free-living microorganisms inhabiting the external environment (in soil and water). More than 60 serovars belong to this species.
Class and exam questions
How does a person contract leptospirosis?
The main route of transmission is waterborne, through swimming in contaminated bodies of water or consuming polluted water. Alimentary and contact routes are also possible.
Can you get infected with leptospirosis from a sick person?
No. Despite the fact that humans are extremely susceptible to the pathogen and excrete it into the environment, they are an epidemiological "dead end" and do not serve as a source of infection for other people.
Does immunity develop after recovering from the infection?
Yes, durable humoral immunity is formed. However, it is strictly serovar-specific—it protects only against the specific bacterial strain that caused the disease.
What else to study on this topic
- Antigenic structure of leptospires: genus-specific and variant-specific antigens
- Aggressins: hemolysins, lipase, fibrinolysin, and cytotoxins
- Features of performing and reading the microscopic agglutination and lysis test (MALtest)
- Principles of creation and application of heterologous immunoglobulins
- Differences between pathogenic leptospires and saprophytic species in water and soil
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