B051 - ANTHRAX

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B051 - ANTHRAX

Nature of the disease
Anthrax is an acute, usually septicaemic infectious disease caused by a large, spore-forming bacterium Bacillus anthracis.
Classification
OIE List B disease
Susceptible species
Anthrax can affect a wide variety of domestic and wild animals, and humans. Of the domestic animals, goats, cattle, sheep and horses in that order are the most susceptible.
Distribution
Anthrax is endemic in many countries of the world, particularly in tropical and sub-tropical areas, in the Region it is present in some area of Australia and Papua New Guinea. 

The disease commonly occurs in well defined endemic areas where environmental conditions appear particularly favourable for the survival of the spores.

Clinical signs 
Anthrax cases usually sudden deaths, often with few, if any premonitory signs. When present, the following signs can be seen:

Ruminants: 

  • Sudden death, 
  • Bleeding from orifices and subcutaneous haemorrhage
  • Brief period of fever 
  • Disorientation

Equines and some wild herbivores: 

  • Fever, 
  • Restlessness,
  • Dyspnoea, 
  • Agitation.

Pigs, carnivores, primates: 

  • Local oedemas and swelling of face and neck or of lymph nodes, particularly mandibular and pharyngeal and/or mesenteric.
Post-mortem findings 
NB Because of the zoonotic risk, autopsy of suspected anthrax cases is not recommended. If undertaken, the maximum level of protection should be followed.

Characteristic findings include:

  • Rapid putrefaction of the carcase
  • Bloody discharges from muzzle and anus
  • Dark tarry blood that does not clot properly
  • Haemorrhages in subcutaneous tissues, serous membranes and viscera
  • Enlarged and pulpy spleen
Differential diagnosis 
Anthrax may be difficult to differentiate from other cause of sudden death such as:
  • Lightning strikes
  • Peracute blackleg
  • Acute leptospirosis
  • Bacilliary haemaglobinuria
  • Peracute lead poisoning
  • Hypomagnesaemic tetany
  • Acute bloat
Specimens required for diagnosis 
A diagnosis of anthrax is confirmed by microscopic examination of blood smears for the presence of typical encapsulated Anthrax bacilli. Smears should be made from peripheral blood of freshly dead carcases and stained with polychrome methylene blue, an ELISA test is also available
Transmission   
The source of anthrax outbreaks in livestock is usually contaminated soil, water, or foodstuffs. Ingestion of spores is the common method of infection. Infection can also result from cutaneous infection, with spores entering through skin wounds. Pulmonary anthrax due to inhalation of spore-laden dust has been reported.

Biting insects have been incriminated as vectors of spread in some epidemics in horses.

Anthrax spores are very resistant to inactivation and can persist in soil for many years, particularly in warm climates and in soils with a neutral to alkaline pH and which contain organic matter. They may also persist for long periods in contaminated animal products such as hides, air, wool, bones, feedstuffs or handicrafts.

Risk of introduction   
Anthrax is most likely to be introduced into a previously-free area via spore-contaminated materials or equipment.

For example, feeding unsterilised bone meal as a mineral supplement, and using bone dust as a fertiliser have spread anthrax to new areas.

Therefore hides, air, wool, feedstuffs, handicrafts and soil sample should be considered at risk when coming from and endemic area.

Recently anthrax has been used as biological weapon.

Control / vaccines  
In situations where anthrax occurs carcases of clinical cases should be buried or burned. The rest of the flock or herd can be vaccinated using a live, avirulent, non-encapsulated vaccine.
The attenuated spore vaccination is highly effective in controlling outbreaks and in preventing the disease in endemic areas.
Suspected clinical cases may be successfully treated with antibiotics if detected early enough.
References
  • Anthrax, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 432-435
  • Anthrax, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 671-676
  • World Health Organization, Guidelines for the Surveillance and Control of Anthrax in Humans and Animals, WHO/EMC/ZDI/98.6