Ricktasia

Rickettsia
General Features
• The rickettsia are bacteria which are obligate intracellular parasites. They are considered a separate group of bacteria because they have the common feature of being spread by arthropod vectors (lice, fleas, mites and ticks).

• The cells are rod-shaped, coccoid and often pleomorphic microorganisms which have typical bacterial cell walls, no flagella, are multiply via binary fission only inside host cells. They occur singly, in pairs, or in strands. Most species are found only in the cytoplasm of host cells, but those which cause spotted fevers multiply in nuclei as well as in cytoplasm.
The family Rickettsiaceae is taxonomically divided into three genera:
1. Rickettsia - obligate intracellular parasites which do not multiply within vacuoles and do not parasitize white blood cells.
2. Ehrlichia - obligate intracellular parasites which do not multiply within vacuoles but do parasitize white blood cells.
3. Coxiella --obligate intracellular parasite which grows preferentially in vacuoles of host cells.
Metabolism
the medium is enriched with potassium, serum albumin and sucrose, used for isolation of organisms that survive for many hours. If ATP is added to the solution, the organisms metabolize and consume oxygen. The basis for the obligate parasitism of these cells is that they require the rich cytoplasm to stabilize an unusually permeable cell membrane.
The rickettsia have many of the metabolic capabilities of bacteria, but require an exogenous supply of cofactors to express these capabilities.

Pathogenicity
In their arthropod vectors, the rickettsia multiply in the epithelium of the intestinal tract; they are excreted in the feces, but gain access to the arthropods salivary glands. They are transmitted to man, via the arthropod saliva, through a bite. In their mammalian host, they are found in the endothelium of the small blood vessels, in those of the brain, skin and heart. Hyperplasia of endothelial cells and localized thrombus formation lead to obstruction of blood flow, with escape of RBC s into the surrounding tissue. Inflammatory cells also accumulate about affected segments of blood vessels. This angiitis appears to account for some of the more prominent clinical manifestations, such as petechial rash, stupor and terminal shock.
Death is ascribed to damage of endothelial cells, resulting in leakage of plasma, decrease in blood volume, and shock. It is assumed that the observed clinical manifestations of a rickettsial infection are due to production of an endotoxin.
Laboratory Diagnosis
Presumptive diagnosis is based on the finding of rickettsial-like organisms in tissue or blood. Although the organisms are gram-negative, they only weakly take the counter stain, safranin. Therefore, special staining procedures are used. Infected tissue may be stained with:
1. Macchiavello stain--organisms are bright red against the blue background of the tissue.
2. Castaneda stain--blue organisms against a red background.
3. Giemsa stain-- purple organisms.
Confirmative diagnosis is based on a serological
1- Weil-Felix reaction in which the titer of the agglutinins in the patient s serum against the Proteus strains OX-19, OX-2 and OX-K are determined. These Proteus strains have no etiological role in rickettsial infections, but appear to share antigens in common with certain rickettsia. This test is usually positive seven days after the initial infection.
2-A more specific complement fixation test is available but does not show positive results until 14 days into the infection.
3-The indirect fluorescent antibody test is also useful for the detection of IgM and IgG antibodies against rickettsia.
Diseases
The rickettsial diseases of man are classified according to the arthropod vector as seen in table below.


Treatment
The drugs of choice for the treatment of rickettsial diseases are chloramphenicol and tetracycline.



Chlamydia
General Characteristics
The chlamydia, which are incorrectly called the PLT viruses or basophilic viruses, are bacteria which are obligate intracellular parasites of higher animals (mammals and birds). The members of this group share a unique development cycle, a common morphology and a common family antigen. They are not transmitted by arthropods.
These organisms are termed basophilic because they take up the Giemsa stain (i.e., they stain blue). They are gram-negative, non-motile and multiply in the cytoplasm of the host cell. These organisms generally parasitize epithelial cells.




Metabolism
It appears of obligate intracellular parasitism is due to a lack of ATP-generating ability and the need to obtain ATP from the host cell. The cells can synthesize DNA, RNA and protein.
Characteristics of the elementary and reticulate bodies of Chlamydia can be found in the tablebelow.
ELEMENTARY BODY (EB) RETICULATE BODY (RB)
Infectious Not infectious
Adapted for extracellular survival Adapted for intracellular growth
Hemagglutinin present Hemagglutinin absent
Induces endocytosis Does not induce endocytosis
Metabolically inactive Metabolically active

Life cycle of chlamydia

Pathogenicity
Subgroup A organisms primarily infect the mucous membranes of the eye or the genitourinary tract of humans. Subgroup B organisms, although primarily parasites of birds, can be transmitted to man where they cause a lung infection.
The mechanism by which chlamydia cause disease or injure cells is unknown. Chlamydial infections of mucous membranes cause damage to tissues deep in the epithelial layer; for example, in trachoma, scarring of the tarsal plate occurs frequently.
There are three species in the genus Chlamydia:
1. Chlamydia trachomatis Causes a spectrum of diseases:
• ocular infections – neonatal conjunctivitis
keratoconjunctivitis, blindness( trachoma). Trachoma is a major cause of blindness in the developing world.
• genital infections – non-specific urethritis, the most common sexually transmitted disease in the UK. In the tropics, it causes lymphogranuloma venereum
• pneumonia – in neonates.
2. Chlamydia pneumoniae causes acute respiratory tract infection.
3. Chlamydia psittaci primarily causes disease (psittacosis)in birds such as pet parrots from which humans contract the infection. The human infection, also known as psittacosis, takes the form of a primary atypical pneumonia.
Laboratory Diagnosis
1. Isolation of the organism from infected tissue. Identified by tissue culture (e.g. HeLa cells),
2. serology (complement fixation test) and fluorescent antibody staining of smears from the lesion.
3. Characteristic cytoplasmic inclusion bodies infected cells.
Antibiotic sensitivity
Tetracycline is effective for all chlamydial infections.