There are three genera of medical importance:
- Treponema,
- Leptospira
- Borrelia.
GENUS Treponema
Regularly coiled spirochetes with a longer wavelength than Leptospira. Several species and subspecies important human pathogens; others are members of the normal flora, especially in the mouth. T. pallidum and its subspecies pertenue and T. carateum are most important species.
Characteristics:
Individual cells too small to visualize by direct light microscopy; can be seen with dark ground illumination, or after silver impregnation or immunofluorescent staining. Cells are actively motile by means of flagella contained within periplasmic sheath.
Transmission:
Very susceptible to heat and drying, so successful transmission depends upon very close contact. T. pallidum is spread by close sexual contact and may also be vertically transmitted in utero. Yaws and pinta spread by direct contact from infected skin lesions. No animal reservoir.
Diseases:
T. pallidum: syphilis.
T. pallidum-pertenue.
T. carateum: the non-sexually transmitted treponematoses, yaws and pinta.
Laboratory Identification:
T. pallidum and closely related species cannot be grown in artificial media; diagnosis of infection depends upon microscopic examination of fluid from primary lesions and on serology.
GENUS Leptospira
Two species:
- L. interrogans
- L. biflexa
L. interrogans is parasitic, L. biflexa contains free-living species. Within the species interrogans there are several different serogroups and serovars responsible for disease in humans and animals.
Leptospira interrogans:
Characteristics:
Finely coiled spirochetes with hooked ends. Cells 0.1-0.2 µm in diameter, up to 20 µm in length. Not visible by direct light microscopy unless stained by silver impregnation or immunofluorescent methods. Dark ground microscopy reveals rotational and directional motility by means of periplasmic flagella.
Transmission:
Leptospirosis in humans is a zoonosis, usual host being rodents, bats, cattle, sheep, goats and other domestic animals. Leptospires excreted in urine contaminate food and water. Infection occurs by contact with either through occupation or recreation. Organisms may penetrate unabraded skin and conjunctiva.
Diseases:
Leptospirosis or Weil’s disease in humans and animals.
Laboratory Identification:
Direct microscopy of blood and urine possible, but difficult to interpret. Leptospira can be grown, with difficulty, in special serum-containing media. Serologic diagnosis is usual.
GENUS Borrelia:
Two species of Borrelia of importance in humans, B. recurrentis causes relapsing fever and B. burgdorferi causes Lyme disease.
Characteristics:
Less finely coiled than the leptospires. Cells 0.2-0.5 µm in diameter; stain readily, so are visible by light microscopy.
Transmission:
B. recurrentis spread from person to person by lice. Lyme disease is a zoonosis transmitted to humans by hard ticks (Ixodes spp.).
Diseases:
In relapsing fever the relapsing element may be due to antigen switching. Lyme disease slowly progressive rather than relapsing. Characteristic skin lesion ‘erythema chronicum migrans’ occur in approximately 50% of cases. Joint pains and fatigue common and later, in untreated cases, neurologic and cardiac manifestations.
Laboratory Identification:
B. recurrentis demonstrated in blood smears by staining with Giemsa or acridine orange. B. burgdorferi much more difficult to visualize. Culture from biopsy material possible, but difficult; diagnosis usually by serology.
