Anthrax

Previous authors: V DE VOS AND P C B TURNBULL

GENERAL INTRODUCTION: ENDOSPORE-FORMING GRAM-POSITIVE RODS AND COCCI

Introduction

Anthrax is a peracute, acute or subacute, highly contagious disease of domestic and wild animals and humans caused by the bacterium Bacillus anthracis. In most species of animals it is characterized terminally by the development of a rapidly fatal septicaemia, resulting in sudden death. The principal lesions are those of widespread oedema, haemorrhage and necrosis. An anthrax-like disease has  been recorded in several mammal species in western and central Africa, caused by Bacillus cereus biovar anthracis.^{6, 179}

Epidemics occur almost every year worldwide resulting in numerous  deaths in animals and humans. ²²⁹ Martin (2010) estimated human anthrax cases globally to range between 2000 to 20000 annually. Humans are most often infected by exposure to infected animals or their products.

A global historical perspective

The first record indicative of anthrax is considered to be in the Bible (Exodus, Chapters 7 to 9), which refers to the fifth and sixth plagues of Egypt, this murrain, which is believed to be systemic and cutaneous anthrax respectively, occurred in about 1491 BC.^{100, 183, 192, 193, 312, 393} The next record was provided by Virgil (70 to 19 BC), who not only described major periodic outbreaks indicative of anthrax in animals and humans in ancient Rome, but also associated the occurrence of anthrax in humans with the consumption of meat or contact with hair or hides of animals that had died of the disease.⁹² Reference is also made to “anthrax” in the ‘Hippokratika’, a collection of veterinary documents dating from the tenth century.¹⁸³ Earlier writings therefore pointed to the probable origin of anthrax being in erstwhile Mesopotamia and northern Africa.^{183, 192, 193, 312} While the Nile valley is the locality where the disease was first recorded its further spread was thought to be associated with the process of domestication of wild ungulates and the dispersion of domestic cattle into the rest of Africa,^{102, 120, 275} and subsequently into areas such as Eurasia,¹⁹³ the USA,^{141, 319, 393} and Australia.²⁹⁵ It is, however, also possible that the native tribes and their domesticated animals, during their southern migrations through Africa, encountered anthrax along the way, where it already existed endemically among free-living wild animals.  The genotypic grouping of anthrax isolates supports this hypothesis and suggests that the geographic origin of B. anthracis may be in sub-Saharan Africa. ^{171, 308}

Phylogenetic methods have since been used to trace the prehistoric timing of B. anthracis diversification, and the introduction and spread of anthrax globally. ³⁶⁷ It  is endemic in many regions of the world especially sub-Saharan Africa, Asia and Central and South America.³⁹¹ Bacillus anthracis was estimated to have spread widely about 3000-6000 years ago although further evidence suggests this was an underestimation The earliest introduction into North America is estimated to have occurred around 13 000 years ago, entering from Asia via the Beringian Steppe ecosystem.¹⁷⁴ Isolates from across China suggest that B. anthracis movement across Eurasia was associated with ancient trade routes along the Silk Road.³⁰²

The more recent history of anthrax is intimately associated with the history of microbiology and infectious diseases.^{183, 200, 312, 349, 394} Anthrax appears to be the first disease of humans and animals that was shown to be caused by a microorganism. Barthelémy in 1823 demonstrated its transmissibility by inoculation, and Rayer and Davaine in 1850 observed filiform bodies in the blood of animals that had died of the disease. This organism, referred to as Bacterium by Davaine in 1864, was named Bacillus anthracis by Cohn in 1875.^{64, 394} Final proof of its causative role was however furnished by Koch in 1876.¹⁹⁰ In this classic masterpiece, subsequently referred to as ‘Koch’s postulates’, he described not only the organism, its ability to form resistant spores, and its cultivation in vitro, but also the reproduction of the disease by injection of pure cultures, and the recovery of the organism at necropsy from animals that had died as a result of experimental infection.¹⁹⁰ Anthrax was also the first disease against which avirulent strains of the causative organism were used as immunizing agents in vaccines.^{266, 342}

In spite of the fact that Pasteur’s 1881 vaccine against anthrax²⁶⁶ has long been credited with being history’s first bacterial vaccine, it has since been pointed out that Greenfield prepared an effective vaccine against anthrax and described his results some months before Pasteur’s experiments were conducted.^{100, 183, 192, 193, 312, 342} Pasteur’s vaccine however, met with disappointing results, and in 1920, the introduction of an improved attenuated spore vaccine, after the method of Cienskowsky...