Signalment:  

2 year old, female, Jersey, bos taurus, bovineThree of 100 mature Jersey cattle died with no observable clinical signs. They were female dry cattle at pasture. The clinical differential diagnosis was clostridial disease, botulism or anthrax. The clinician opened the carcass and quickly discovered an enlarged spleen. A 2 by 3 inch sample of spleen was removed and the clinician made his own smears and submitted the spleen sample and smears to the laboratory as a possible case of anthrax.

Gross Description:  

The cow had an enlarged and swollen spleen that oozed thick dark blood from the cut surface.

Histopathologic Description:

The spleen was enlarged and congested with large numbers of extracellular erythrocytes filling the red pulp with a marked reduction in amount of the white pulp and small lymphoid follicles. Widely dispersed throughout the spleen were myriads of extracellular bacteria. The bacteria were large and uniform in size bacilli, approximately 3um long and 1um wide. There were large numbers of widely dispersed macrophages that were filled with pigment, confirmed as hemosiderin. There was disruption of the parenchyma, due to autolysis of the tissue.

Morphologic Diagnosis:  

Acute diffuse splenic haemorrhage with numerous extracellular bacilli.

Lab Results:  

A smear from the spleen was prepared and stained with polychrome methylene blue. There were numerous large square ended bacilli with a pink capsule. Blood from the spleen was cultured on sheep blood agar plates and incubated aerobically. Within 12 hours of incubation, there were large numbers of small dry irregularly shaped ground glass colonies. The bacterial colonies were sensitive to penicillin. These findings are consistent with Bacillus anthracis.

Condition:  

Bacillus anthracis

Contributor Comment:  

Anthrax is a peracute, acute or subacute highly contagious disease of domestic animals and humans caused by the bacterium Bacillus anthracis.¹ Necropsy of animals infected with Bacillus anthracis is not recommended because exposure to air allows the bacteria to sporulate, resulting in extremely resistant anthrax spores that contaminate the environment for years.² The recommended means of diagnosis is collection of a peripheral blood smear without opening the carcass. Peripheral blood smears in anthrax cases have large numbers of Gram positive rod shaped bacteria, with square ends and a pink capsule stained by methylene blue, Giemsa or Schaeffer and Fulton's malachite green technique. The bacteria need to be differentiated from Clostridial bacteria, that have rounded corners and no capsule. The presence of Bacillus anthracis in a blood smear can be confirmed by microbiological culture or PCR. USA Naval Medical Research Centre developed diagnostic tests for anthrax are being trialed for their suitability, in Australian conditions, for the 'pen-side' diagnosis of anthrax in livestock.³

Anthrax is a disease syndrome recognized for centuries and a pathogen that is widely distributed around the world. In 1823 anthrax was the first disease of humans and animals shown to be caused by a micro-organism.¹ Anthrax occurs sporadically in Australia affecting sheep, cattle, infrequently pigs and rarely goats and horses.⁴ It is largely confined to the "anthrax belt" which extends through the middle of the Australian states of New South Wales and into northern and central Victoria.^3,4 This laboratory in Victoria would typically diagnose 2 or 3 cases of anthrax per year. In January and February 2007, there was an unusual outbreak of anthrax in central Victoria with this laboratory diagnosing 37 positive anthrax cases, on eight farms from approximately 300 submissions from the surveillance area. The last significant outbreak of anthrax in Victoria was between January and March 1997, when anthrax was diagnosed on 83 properties with 202 cattle and 4 sheep confirmed to have died of anthrax.⁵ In Australia, effective control of anthrax infection is achieved by vaccination of in contact farms and livestock.

Ruminants are typically infected with anthrax by ingestion of spores that germinate in the intestinal tract to form encapsulating vegetative cells that replicate and spread to the regional lymph nodes and then disseminate systemically.² Infection may also occur by cutaneous abrasion and insect bites.¹ Extremely rarely it is possible, in cattle, to initiate an infection by inhaling spores while grazing dry dusty contaminated sites.¹ Bacillus anthracis produces exotoxins termed lethal toxin and edema toxin. The toxins and the capsule of the bacteria inhibit phagocytosis, increase capillary endothelial permeability and delay clotting.¹ Animal species vary in their susceptibility to anthrax infection. Species easily infected with anthrax include cattle, goats, sheep, monkey, mouse, guinea pigs, horses and chimpanzees. Species resistant to anthrax but once infection is establish are highly susceptible to effects of the exotoxins include dog, pig and NIH black and Fisher rats.¹ Humans can be infected with anthrax by inhalation, ingestion or cutaneous abrasions. Human cases of anthrax are rare in Australia and there have been only four cases in the last ten years, all have been the cutaneous form and most of the cases have been in farmers or rendering plant workers.⁶

JPC Diagnosis:  

Spleen: Congestion, acute, diffuse, severe, with lymphocytolysis, and myriad bacilli, Jersey (Bos taurus), bovine.

Conference Comment:  

The Centers for Disease Control and Prevention classifies anthrax as a Category A agent of bioterrorism. Category A agents have the potential to pose a threat against public health, spread across a large area or need public awareness, and need a great deal of planning to protect the publics health. Despite this potential, humans are relatively resistant to natural infection. 

Infection of both humans and animals can occur through ingestion, percutaneously, or more rarely through inhalation of anthrax spores. Under certain conditions, spores have been known to remain viable in the soil up to 200 _50 years. Germination of spores occurs between 20o - 40o C and in conditions of greater than 80% relative humidity. Upon ingestion of spores, the organisms quickly germinate to the encapsulated toxin-producing vegetative form. The capsule is a poly-D-glutamate capsule that inhibits phagocytosis.¹

Lethal toxin inhibits mitogen-activated protein kinase-kinase and results in terminal shock through the release of tumor necrosis factor (TNF) and interleukin-1 (IL-1). Edema factor results in altered intracellular water and ion concentrations through the abnormal production of c-AMP. Edema factor has also been implicated in preventing mobilization and activation of leukocytes. The presence of the capsule and two toxins effectively results in prevention of phagocytosis, increased capillary endothelial permeability and decreased blood clotting ability.¹

References:

1. De Vos V. Turnbill PCB. Anthrax. In Infectious Diseases of Livestock. 2nd edn. eds. Coetzer, JAW and Tustin, RC. pp. 1788-1818. Oxford University Press South Africa, Cape Town, South Africa, 2004.
2. Fry MM, McGavin MD. Bone marrow, blood cells and lymphatic system. In Pathologic Basis of Veterinary Disease. 4th ed McGavin MD, Zachary JF. p811. Mosby Elsevier, St. Louis, 2007.
3. Muller JD, Wilks CR, O'Riley KJ et al. Specificity of an immunochromatographic test for anthrax. Aust Vet J 82 (4) 220-222. 2004.
4. Snedden HR, Albiston HE. Bacterial diseases. Diseases of Domestic Animals in Australia Part 5 Volume 1. pp. 12-26. Commonwealth Government Printer, Canberra, Australia. 1965.
5. Turner AJ, Galvin JW, Rubira RJ et al. Anthrax explodes in an Australian summer. J Appl Microbiol 87(2) 196-199. 1999.
6. Kolbe A, Yuen MG, Doyle BK. A case of human cutaneous anthrax. Med J Aust. 185 (5) 281-282. 2006.

A virtual slide is not available for this case.

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anthrax plate

anthrax smear

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