Rickettsiae

RICKETTSIAL DISEASE:

Scrub typhus

• Sera from endemic and epidemic typhus agglutinate OX 19 and sometimes OX2
• In tick borne spotted fever both OX 19 and OX2 are agglutinated
• In scrub typhus only OX K agglutinins are only found. This test is negative in rickettsial pox, trench fever and Q fever

Amongst the options provided ‘Louse’ is the insect vector only for Rickettsia prowazekii.

Ref: Jawetz, Melnick and Adelberg’s Medical Microbiology, 19th Edition, Page 295; Textbook of Microbiology By Ananthanarayan, 6th Edition, Page 414

Rickettsial pox, caused by Rickettsia akari, is transmitted to humans by the house mouse mite Liponyssoides sanguineus.

It is also known as vesicular rickettsiosis, a disease similar to chicken pox.

The disorder begins with the bite of an infected mite, which results in a black eschar. The classic triad of rickettsialpox consists of fever, eschar, and rash. It is endemic in US. It is self-limited over 7-10 days.

Rickettsia rickettsia is the cause of the tick-borne Rocky Mountain spotted fever.

R. typhi is the flea-borne cause of endemic typhus.

Rickettsia conori causes Mediterranean spotted fever, also known as African tick typhus.

Rash is seen in 

Ans. is ‘b’ i.e., Gram +ve bacillus 

Rickettsiae are

• Gram negative
• Pleomorphic coccobacilli
• Obligate intracellular organisms
• Non motile
• Non capsulated
• Stain bluish purple with Giemsa and Castaneda stains
• Stain deep red with Machiavella and Gimenez stains

About other options

.  Rickettsiae are transmitted to humans by arthropod vectors through their bite or feces 

.  Serological diagnosis of Rickettsial infection may be done by

.  Heterophile Weil felix reaction or

.  Specific tests using Rickettsial antigens.

Ans. is ‘a’ i.e., R. Quintana 

.  Rickettisiae are obligate intracelluar organisms. So they cannot grow in cell free medium. But, there is one exception to this rule, R. Quintana which is not an intracellular parasite, so can grow in cell free media.

Ans. is ‘b’ i.e., Endothelial cells of small vessels 

Pathogenicity

• On entry into the human body, the rickettsiae multiply locally and enter the blood. They become localized chiefly in vascular endothelial cells, which enlarge, degenerate and cause thrombus formation, with partial or complete occlusion of the vascular lumen.

.   The overall pathological features of the rickettsia! diseases are due to the damage of vascular endothelium.

Ans. is ‘c’ i.e., High OX – K 

.  Fever, headache, erythmatous lesion along with generalized lymphadenopathy are suggestive of scrub typhus.

.  In scrub typhus, Weil – Felix reaction is strongly positive with the proteus strain OX – K.

Note —> In this question the key word is lymphadenopathy. Amongst the typhus group, lymphadenopathy is seen only in scrub typhus.

Ans. is ‘a’ i.e., Multiply only within living cell; ‘c’ i.e., Transmitted by athropod vectors

Option ‘a’

.                       Rickettsiae are unable to grow in cell free media. Growth generally occurs in the cytoplasm of infected cells. (In case of spotted fever, growth may take place in the nucleus as well).

.  Ricketsiae are cultivated in –

i)         Yolk sac of developing chick embryo.

ii)       Mouse fibroblast

di) Continous cell lines -4 HeLa, HEP-2, Detroid-6

iv)     Tissue cultures are not satisfactory.

v)       Guinea pigs and mice are useful for isolation.

Option ‘b’

.  Rickettsiae produce typhus fever :

1)       Epidemic typhus

2)       Endemic typhus

3)       Scrub typhus

Option ‘c’

.  Rickettsiae are transmiteed to human by arthropod vectors (Except Q fever)

Ans. is ‘a’ i.e., Weil-Felix reaction 

Ans. is ‘d’ i.e., Cephalosporins are drugs of choice 

.  Tetracyclines (e.g., doxycycline are DOCs).

Ans. is ‘c’ i.e., Rickettsia prowazeki 

Ans. is ‘b’ i.e., Proteus

Weil felix reaction

• This reaction is an agglutination test in which sera are tested for agglutinins to 0 antigens of certain nonmotile proteus strains OX -19, OX – 2 and OX – K.
• The basis of the test is the sharing of an alkali – stable carbohydrate antigen by some rickettsiae and by certain strains of proteus, P. vulgaris OX – 19 and OX – 2 and P. mirabilis OX – K.
• The test is usually done as a tube agglutination, though rapid slide agglutination methods have been employed for screening

Ans. is ‘a’ i.e., Endothelial injury

Rickettsial infections are associated with a high morbidity and mortality.

The mortality rate can be as high as 20% of Rocky Mountain Spotted fever and 30% for epidemic typhus.

On entry into the human body, the rickettsiae multiply locally and enter the blood. They become localized chiefly in vascular endothelial cells, which enlarge, degenerate and cause thrombus formation, with partial or complete occlusion of the vascular lumen.

The overall pathological features of the rickettsial diseases are due to the damage of vascular endothelium.