Properties

Pathogenesis

Clinical findings

Laboratory diagnosis

Treatment & Prevention

Small gram negative bacilli.

Obligate intracellular pathogens.

All primarily reproduce in endothelial cells.

Divide by binary fission within the host cell.

Transmission:

- maintained in nature by ticks, lice, fleas & mice.

- exception: C.burnetii is transmitted by aerosol & inhaled into lungs.

- almost all are zoonoses except epidemic typhus, which occurs only in humans.

- typhus group: louse/tick.

- spotted fever group: tick or mouse mite.

Typical lesion is a vasculitis in endothelial lining of vessel wall.

Damage to skin vessels results in:

- characteristic rash.

- edema & hemorrhage due to increased capillary permeability.

No exotoxins found.

Diseases:

- typhus: epidemic, endemic & scrub.

- Q fever, Rocky Mountain spotted fever.

Typhus

Forms:

- louse-borne epidemic: R.prowazekii

- flea-borne endemic: R.typhi

- chigger-borne scrub: R.tsutsugamushi

Symptoms:

- begins with sudden onset of chills, fever & headache 1-3 weeks after louse bite.

- 5-9 days: maculopapular rash begins on trunk & spreads peripherally.

- characteristic eschar at site of primary infection: an ulcer with a dark or black crust.

- second week: severe meningoencephalitis, delirium & coma.

- death results from peripheral vascular collapse or bacterial pneumonia.

Epidemic typhus is transmitted from person to person by the human body louse, Pediculus.

Brill-Zinsser disease: a recurrent form of epidemic typhus with similar but less severe signs.

In conditions of overcrowding & poor hygiene, lice proliferate & the introduction of an infected patient or a case of Brill-Zinsser disease can start an epidemic.

Rocky Mountain spotted fever:

- acute onset of fever, severe headache, myalgias & prostration.

- rash begins with macules that progress to petechiae.

- CNS changes: delirium & coma.

- disseminated intravascular coagulation, edema, & circulatory collapse.

Laboratory diagnosis is usually by serology (complement fixation).

Antibiotics:

- tetracycline

- chloramphenicol

Prevention:

- personal hygiene

- delousing with DDT.

- reduce exposure to arthropod vector by wearing protective clothing & using insect repellent.

Typhus vaccine:

- contain formalin-killed R.prowazekii organisms.

- used in military & persons at high risks.

Coxiella burnetii

Obligate intracellular pathogen.

Replicates inside an acidified vesicle within macrophages.

Transmission:

- host: sheep, cattle & goats.

- forms resistant spore-like structures & survives drying.

- spread by airborne route or by drinking infected milk.

Disease: Q fever

Symptoms:

- acute febrile illness

- atypical pneumonia

- hepatitis

- ‘culture-negative’ endocarditis

- neurological disease

- osteomyelitis

By serology

Acute infection: doxycycline

Endocarditis:

- combination of drugs used for long periods.

- valve replacement

Prevention: pasteurization of cow’s milk.

Ehrlichia species

Obligate intracellular pathogens of white blood cells in man & mammals.

Site of infection:

- human macrophages: E.chaffeensis & E.canis

- granulocytes: E.phagocytophila & E.ewingii.

E.chaffeensis/E.canis/E.phagocytophila/E.ewingii:

- acquired by tick bite; animal reservoirs.

- patients develop, fever, rash & some with severe mutli-system disease.

E.sennetsu:

- observed only in Japan & Malaysia.

- acquired by eating raw fish.

- infects macrophages.

- a glandular fever-like illness develops with chills, fever & enlarged cervical lymph nodes.

- usually a mild infection.

Diagnosis is usually by serology.

Sometimes a ‘morula’ can be seen in the cytoplasm of leukocytes: an intracellular inclusion which contains replicating bacteria.

Doxycycline