eChapter Name: Anthrax
9789389571721
eBook Name: ZOONOTIC DISEASES
by Sukdeb Nandi
Synonyms: Splenic fever, wool sorter's disease, Tarka
It is an infectious disease of livestock and worldwide in distribution. This is a zoonotic disease. It is also known as splenic fever because of enlargement of the spleen (splenomegaly).
Etiology: It is caused by a rod shaped bacteria of 4-8 µm X 1-1.2 µm known as Bacillus anthracis. It is non-motile, capsulated, aerobic, spore bearing and Gm+ in nature. The organisms remain in chains. It was the first pathogenic bacterium to be observed under the microscope, the first communicable disease to be transmitted by inoculation of infected blood, the first bacillus to be isolated in pure culture and shown to possess spores and the first bacterium used for preparation of an attenuated vaccine.
The capsule is polypeptide in nature being composed of a polymer of d(-) glutamic acid. Capsules are not formed under ordinary conditions of culture but only if the media contain added bicarbonate or are incubated under 10-25% CO2. Capsules are formed in the body of an infected animal.
Spores are formed in culture or in the soil, but never in the animal body during life. Spores are formed only in the presence of O2. Spores are central, elliptical or oval in shape. The spores are very resistant to physical and chemical agents. They have been isolated from naturally infected soil for as long as 60 years. They resist dry heat at 140°C for 1-3 h and boiling for 10 min.
Anthrax bacilli is Gm + and non-acid fast. When blood films containing anthrax bacilli are stained with polychrome methylene blue for a few seconds and examined under the microscope, an amorphous purplish material (capsule) is noticed around the bacilli called M'Fadyean's reaction. On agar places, irregularly round colonies are formed 2-3 mm in diameter raised, dull opaque, grayish white with a frosted glass appearance. Under the low power microscope, the edge of the colony is composed of long, interlacing chain of bacilli, resembling locks of matted hair. This is called "Medusa head appearance." Virulent capsulated strains form rough colonies while avirulent or attenuated strains form smooth colonies. On gelatin stab culture, a characteristic 'inverted fir tree' appearance is seen with slow liquefaction commencing from the top.