Zoonoses disease-anthrax, Biology

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Anthrax is primarily a disease of the herbivores and occurs in almost all parts of the world. The disease has declined recently throughout the world as a result of the development of an effective vaccine and the use of antibiotics. As per the World Health Organization (WHO) 20,000-100,000 cases of human anthrax are reported annually throughout the world.The causative agent of anthrax is Bacillus anthracis. The organism forms spores in contact with air, which are resistant to heat, low temperature, many disinfectants and prolonged drying; thus survive for long periods in soil, feed, animal products, etc.

In comparison to the herbivores, man is moderately resistant to anthrax. Human anthrax has been classified as non-industrial anthrax and occurs in butchers, farmers, pathologists and veterinarians as a result of close contact with infected animals. Industrial anthrax occurs in those employed in the processing of wool, hair, hides, bones or other animals products.
Non-industrial anthrax results from handling of infected carcasses and usually manifests as cutaneous form. Water-borne and insect –transmitted anthrax also take the cutaneous form. Intestinal anthrax resulting from the consumption of infected meat also belongs to the non-industrial category. Industrial anthrax may be cutaneous but generally occurs as pulmonary form due to inhalation of spore-laden dust. Man almost invariably acquires anthrax directly or indirectly from infected animals. Laboratory-acquired infections have been reported. The occurrence of anthrax appears to be unrelated to age or sex.

Clinical features:
Anthrax occurs in 3 forms in man – the cutaneous form, the intestinal and the pulmonary form. Cutaneous form accounts for most human cases throughout the world. All the three forms are potentially fatal but the cutaneous types are often self – limiting.

Cutaneous anthrax: The incubation period is usually 2-3 days. The infection occurs via a cut, abrasion or insect bite. The lesion develops as a papule which turns black and may contain pus if secondarily infected with pyogenic organisms. The lesion may remain small but occasionally becomes very extensive. It is always surrounded by oedema and the black eschar firmly adherent to the underlying tissues. In cases that become septicaemic, temperature rises to 40oC and then may fall to below normal within a few hours. The patient becomes toxaemic and shocked. Dyspnoea, cyanosis and collapse precede death.

Intestinal anthrax: The incubation period is usually 2-5 days. The lesions develop after the ingestion of B. anthracis spores in infected meat, milk, or other food stuffs. The characteristic eschar, or malignant carbuncle occurs in most part of the duodenum. Nausea, vomiting, anorexia, fever, abdominal pain and bloody diarrhoea are often seen. This form of anthrax is more fatal than cutaneous anthrax.

Pulmonary anthrax:
This form of anthrax is almost always caused by industrial exposure to spores (Woolsorter’s disease). Illness begins 2-5 days after exposure with mild fever, fatigue and malaise. The patient may vomit or cough up a little blood. There is rapid development of dysponoea, cyanosis and severe pyrexia followed by coma and death. The disease is usually fatal.

Laboratory diagnosis: The organism may be seen in smears of exudates stained with polychrome methylene blue (McFadyean reaction) and may tentatively be identified by their characteristic morphology. Anthrax bacilli may be readily cultured from the skin lesions.

Susceptibility of B. anthracis to a specific gamma bacteriophage is helpful for confirmation of the organism. Immunofluorescence and enzyme-linked immunosorbent assay may be conducted for diagnosis. Nucleic acid based diagnosis like PCR may be used for rapid and accurate diagnosis.

Control and prevention: These methods are discussed here.
1.  Avoiding contact with infected animals.
2.  Proper disposal of dead animals.
3.  Prevention of development of anthrax in farm animals through the use of anthrax vaccine.
4.  Vaccination of workers at risk.

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