Bacterial diseases-anthrax, Biology


Synonym              :    Wool sorter’s disease
Species affected    :    Cattle, sheep, goats, pig, horse and majority of the mammals.

Fowls are resistant to anthrax. Man is susceptible but is not a primary host.The causative agent Bacillus anthracis is relatively large, rod shaped and non- motile gram positive organisms with roughly rectangular rods with square ends. The capsule of B. anthracis consists of D-glutamyl polypeptide. Inside the animal body, the organism becomes more rounded and produces capsule. Sporulation occurs outside the body in the presence of oxygen. The spores are highly resistant and are not easily killed by heat, light and disinfectants. The spores remain viable and are infective for years. The animals are infected by ingestion of food and water contaminated with spores. It rarely spreads directly from animal to animal. Man contracts infection from infected animals or animal products like meat, bones, wool, hides and bristles. B. anthracis produces an extra-cellular toxin. The most important toxins responsible for  the production of disease are the lethal toxin and the edema toxin. The edema toxin is composed of protective antigen (PA) and edema factor (EF).

The acute form of the disease is characterized by septicaemia and sudden death with the exudation of tarry blood from natural orifices of the dead animal Failure of the blood to clot, absence of rigor mortis and presence of spleenomegaly are important necropsy findings. The skin form is occasionally seen in horses and cattle when wound or abrasions become infected.

Sporadic cases of anthrax occur almost throughout India, although it is more prevalent in certain hot and humid areas. The disease is always fatal in all farm animals except swine and even in swine the death rate is high. Anthrax may impede the export trade of bones, hides and skins as a consequence of the wide prevalence of the disease in the country. In man, the disease produces localized cutaneous lesions (malignant pustule or carbuncle) but it may take a fatal pneumonic form (woolsorter’s disease) if acquired by inhaling spores. Occasionally, intestinal form or acute meningitis syndrome develops in human beings.

The mode of infection is through inhalation, wound, scratches and through unbroken skin. In cattle, death occurs within minutes of collapsing to the ground with convulsions. These are called the peracute cases. In less acute cases, there is high rise of temperature, shivering and signs of abdominal pain; before death the blood oozes out from rectum and nose. The disease is similar in sheep and goats. The disease in horse usually occurs in acute form. There is high temperature, severe colic, bloody diarrhoea and oedematous swelling in throat region. Acute form normally ends in death in a day or two while subacute form may lead to death in 3 to 5 days or longer.

The disease among pigs is usually chronic. There is oedematous swelling in the region of throat and neck. This swelling interferes in respiration. Frequently there is a blood-stained discharge from the mouth and sometimes enteritis also develops. Some of the affected animals die without showing any signs of illness. Infection in dogs gives rise to gastroenteritis due to ingestion of contaminated meat. Sometimes the throat is inflammed and swollen.The carcasses of herbivores died of anthrax rapidly decompose and become bloated. All natural orifices usually exude dark, tarry blood which does not clot. If the death is suspected to be due to anthrax, the carcass should not be opened for post-mortem examination. If accidentally opened, the spleen is found enlarged 10 to 15 times its normal size. There is severe enteritis. Haemorrhages throughout the body tissues and blood-stained fluid can be seen in the body cavities. There is also enlargement of lymph nodes. In such cases the carcass and all the materials should be burnt.

Sudden death of animals, acute bloat and exudation of blood from orifices of carcass should be suspected for anthrax. Necropsy of a carcass suspected to have died of anthrax is prohibited by Law, as the spore of the organisms can live in the soil for years and cause disease. Therefore, post-mortem examination should not be made on any animal suspected to have died of anthrax.

Blood smears: The blood smears made by puncturing a superficial vein of ear or  the region of foot, and stained with Loeffler’s methylene blue stain or Leishman’s stain may show large blue rods with pink capsules in positive cases. The organisms appear as truncated or with square-cut ends occurring single or in short chains of 2 or
3 organisms. Swabs of blood or exudates are prepared from superficial sites and despatched to laboratory for bacteriological examination/biological test.

A small piece of ear or muzzle should also be sent to laboratory for conducting serological test/biological test. The materials should be properly packed in suitable double containers to avoid any risk of breaking of the containers, spilling of their contents and spread of infection.

Serological test    :    Ascoli’s thermoprecipitin test

Biological test: Heat treated (80°C for 30 min in a water bath) clinical material is inoculated into guinea pigs and observed for a minimum period of 10 days. Animal dies in 3-7 days in positive cases. The characteristic organisms are seen under the microscopic examination of peripheral blood smear.

Treatment and control:
The treatment is usually not possible in acute cases. Subacute cases are treated with antibiotics and anti-anthrax serum. Penicillin, tetracycline and streptomycin in large doses are recommended. A live spore Sterne’s vaccine prepared from virulent uncapsulated strains of B. anthracis is safe for all species of animals. In endemic areas, especially in anthrax districts, annual prophylactic vaccination of susceptible animals should be carried out. Vaccination reaction usually consists of thermal and local reaction. Earlier, anti-anthrax serum was used for treatment of affected animals but this practice has since been discontinued.

Hygiene is an important factor in the prevention and spread of the disease. As the anthrax bacilli sporulate only if exposed to oxygen, the vegetative forms present in the tissues and body fluids die out if the carcass is not opened, but is burnt or buried deep in lime.

Posted Date: 9/17/2012 5:36:16 AM | Location : United States

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