Anthrax

Description

Anthrax is a serious, often fatal zoonotic disease of wild and domestic mammals caused by the spore-forming bacterium Bacillus anthracis. The disease has been reported worldwide (particularly in the tropical countries) and often occurs in outbreaks. The disease primarly affectes herbivores but humans may be infected via contact with infected animal tissues, exposure to high concentrations of spores or contacted with infected animals.

Pathogenesis

The infected host sheds B. anthracis bacilli into the environment which sporulate on exposure to air. These spores are highly resistant and can survive in the environment for many years. Grazing animals may become infected if ingestion of a large number of spores occurs. Following inhalation, germination and localised multiplication of the spores takes place. Spread via the lymphatics and blood stream then occurs, leading to massive septicaemia. In addition to the above routes of infection, biting flies appear to have a role in transmission of spores in areas of endemic disease. Inhalation of dust-borne spores may also be of importance in the pathogenesis of the disease.

In general, carnivores are more resistant to disease than herbivores. In herbivores the disease commonly presents as a peracute onset septicaemia with a high mortality rate. In dogs, humans and pigs however, the onset is less acute.

Clinical signs

Ruminants and horses

The incubation period of the disease is approximately three to seven days. Often animals may be discovered dead in the field before any clinical signs have been observed. In general, in non-immunised ruminants and horses the disease is characterised by sudden death, peracute septicaemia, bleeding from orifices and subcutaneous haemorrhages. Other clinical signs including the following have been described:

• Acute onset severe pyrexia
• Depression
• Neurological signs such as staggering or trembling
• Cyanosis
• Dyspnoea
• Cessation of rumination
• Subcutaneous oedematous swellings
• Congested mucous membranes and petechiae
• Shivering and cramp-like clinical signs

Pigs

Pigs are relatively resistant to anthrax. The disease in pigs has two manifestations; a pharyngeal and an intestinal form. The pharyngeal disease is linked with scavenging or purposeful feeding of infected carcasses and often begins as an oedematous cellulitis of the the neck, head and regional lymph nodes. This may cause death by asphyxia. The intestinal form is thought to be associated with contaminated mineral supplements, and may produce less obvious clinical signs including diarrhoea, lethargy and anorexia. Animals affected by the intestinal form frequently recover.

Dogs

Dogs are rarely affected, but develop a similar disease to that found in pigs. Disease most often occurs due to scavenging of infected sheep or cattle carcasses. Clinical signs include severe inflammation and oedema of the pharyngeal region and the disease is usually not fatal.

Diagnosis

Diagnosis is confirmed in most affected animals (other than pigs) by demonstration of large numbers of rod-shaped B. anthracis in the blood and most other tissues. A tissue sample from a fresh carcass may be collected from an incision made in a well vascularised area such as the ear. In pigs, a small piece of lymphatic tissue should be collected aseptically and submitted. Diagnostic tests include bacterial culture, PCR and fluorescent antibody stains for demonstration of the bacillus in blood or tissue.

Pathology

Bloody discharges from body orifices may be observed in carcasses and Rigor mortis is often absent or incomplete in dead animals. Failure of the blood to clot and splenomegaly are other important post mortem findings. Lymph nodes are often swollen and oedematous. Haemorrhages may be found on the serosal surfaces of the thorax and abdomen as well as the endocardium.

Treatment

Penicillin and Streptomycin is the treatment of choice in animals suspected to be suffering from Anthrax. All livestock at risk from infection should be treated with a long-acting antibiotic and moved to uncontaminated pasture. Any suspected contaminated feed should be removed.

Control

Animals in endemic areas should be vaccinated on an annual basis with the non-encapsulated Sterne strain vaccine. As the vaccine is live, antibiotics should not be administered within one week of vaccination.

References

• Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial
• Turnbull, P (2007) Anthrax in Humans and Animals (Fourth Edition) World Health Organisation Press
• Jones, T. C., Hunt, R. D., King, N. W. (1997) Veterinary Pathology Wiley-Blackwell