Classification of Rickettsioses
In human pathology, three diseases hold the greatest epidemiological and clinical significance: epidemic typhus, endemic (murine) typhus, and Q fever.
Other notable rickettsioses include:
- Rocky Mountain spotted fever;
- Mediterranean spotted fever (caused by Rickettsia conorii);
- North Asian tick typhus and Flinders Island spotted fever;
- Rickettsialpox;
- Scrub typhus and trench fever;
- Tick-borne paroxysmal rickettsiosis.
General Pathogenesis of Vascular Lesions
At the cellular level, rickettsiae bind to cholesterol-containing receptors and enter endothelial cells via endocytosis. Intracellular replication of the pathogen inevitably leads to endothelial cell destruction.
This process triggers a cascade of severe vascular changes:
- Segmental or circumferential fibrinoid necrosis develops in the vessel wall, accompanied by perivascular granulomatous inflammation.
- Endotoxin release activates the kallikrein-kinin system, causing local blood coagulation.
- Disseminated vascular thrombosis develops. Ischemia and necrosis of perivascular tissues clinically manifest as a characteristic maculopapular rash.
- Vascular permeability increases, leading to edema and hemorrhages. Activation of the coagulation system frequently results in DIC.
Generalized microvascular damage causes meningoencephalitis, depression of the vasomotor center, sympathetic nervous system, and adrenal glands. This leads to a precipitous drop in blood pressure and respiratory failure.
Epidemic Typhus and Its Morphology
This is a highly dangerous infection caused by Rickettsia prowazekii. The source of infection is an infected human, and the vectors are body lice or (less commonly) head lice.
- Macroscopic findings: Non-specific; reddish-brown rash, dull and edematous meninges, hyperemic brain, enlarged spleen, and fatty change in parenchymal organs.
- Primary morphological substrate: Generalized typhus vasculitis, developing in four stages:
- Swelling, destruction, and desquamation of the endothelium (destructive endovasculitis).
- Thrombus formation in capillaries and arterioles within damaged areas (destructive endothrombovasculitis).
- Proliferation of endothelium and pericytes (destructive-proliferative endothrombovasculitis).
- Formation of lymphoid infiltrates in the vessel walls.
- Histological hallmark: Popov typhus granulomas. These are nodular foci of endo- and perivascular infiltration abundantly formed in the central nervous system (especially the medulla oblongata), myocardium, and endocrine glands.
Rocky Mountain Spotted Fever and Q Fever
- Rocky Mountain spotted fever: Transmitted by ixodid ticks. A primary affect in the form of an eschar forms at the site of the bite. After the pathogen enters the bloodstream, generalized vasculitis with fibrinoid necrosis develops. Thrombosis of large cerebral arterioles can lead to demyelination and brain infarctions. Pneumonitis may also occur.
- Q fever: Caused by Coxiella burnetii. Like other rickettsioses, it targets the microvasculature. However, it is characterized by the formation of perivascular macrophage granulomas in the liver, spleen, bone marrow, and brain, as well as interstitial pneumonia that morphologically resembles viral pneumonia.