Anthrax – Characteristics and History
Anthrax is an acute zoonotic disease, primarily of herbivorous animals, which is transmissible to humans. The causative organism is Bacillus anthracis, often referred to in earlier, and especially in French, texts as bact´eridie, the name first bestowed on it by Casimir Davaine in 1863. Humans are infected only secondarily through contact with animals or animal products, and thus the disease must be considered in relation to anthrax in animals.
The species of domestic animals most commonly affected are cattle, sheep, and goats; pigs, dogs, and cats are less susceptible. An enlarged spleen is a classic observation in animals with anthrax, thus the disease has also been known as “splenic fever” or “splenic apoplexy.” In humans, the cutaneous form is known as “malignant pustule,” and the pulmonary or intestinal (industrial) type as “woolsorters’ dis-ease” or “industrial anthrax.” In French, the equivalent of splenic fever is sang de rate, in German Milzbrand; other French synonyms include charbon and pustule maligne.
Because B. anthracis produces resistant spores in suitable soils, the disease has long been endemic in many areas throughout the world, with most outbreaks occurring in Europe and Asia. Once contaminated with anthrax spores, an area can be extremely difficult to clear, as has been demonstrated on the island of Gruinard off the west coast of Scotland, which was experimentally contaminated during World War II. This is of prime importance for the epidemiology of the disease because it is rarely spread directly from animal to animal, but almost always through ingestion of contaminated food, either by grazing or, in cooler climates, through imported winter foodstuffs. The infectivity of the anthrax bacillus for people is low, and therefore, even where large numbers of spores and bacilli are found in an industrial environment, only relatively few cases occur.
Cutaneous anthrax, the nonindustrial type in humans, affects those in professions such as veterinary surgery, pathology, farming, butchery, and the like and takes the form of malignant pustule – a lesion caused by contamination of skin with material from infected animals. Pulmonary anthrax, the industrial type, may present as either malignant pustule or pulmonary disease and is acquired in the woolen industries, especially through contaminated air. The disease approached an epidemic situation during the late-eighteenth and nineteenth centuries in France and England in factories processing imported horsehair and sheep’s wool.
Outbreaks in animals in Europe (mainly cattle) and in Asia (sheep and goats) heavily out-weigh those in the United States and Africa, whereas Australia and Canada are rarely affected. Extensive enzootic areas with a constant presence of infection include China, Ethiopia, and Iran, and, in the Americas, Mexico and some South American countries. Available data suggest an annual average total of some 10,000 out-breaks throughout the world. Since World War II, fatal cases in humans have been substantially reduced by antibiotic therapy.
The immunogenic behavior of the anthrax bacillus is complex, and it is not certain whether or to what extent immunity develops in cases of recovery from infection. The existence of an extracellular toxin produced by B. anthracis (which in part determines its virulence) was demonstrated only during the 1980s. Certain strains of certain animal species possess high natural resistance, which introduced confusion and fed much of the controversy surrounding early work on anthrax. The live attenuated vaccines used by Louis Pasteur have undergone continued development and improvement over the years, but early claims of reductions in incidence and fatality rates following their use were not readily sustained. Thus, the early vaccines have gradually been replaced by spore-based vaccines and prepared antiserum. Un-til recently, vaccines have not been considered safe for use in humans, but serum treatment has been used extensively for prophylactic and therapeutic purposes. For occupational reasons, women are less liable to exposure than men, but the disease, when established, is more commonly fatal in females.
In its principal animal hosts, anthrax may take one of three forms: a peracute type (splenic apoplexy), where sudden death occurs almost simultaneously with the first symptoms; an acute type characterized by acute fever, usually followed by death after 2–12 days; and a sub-acute type often followed by recovery. Classical signs include fever, stupor, spasms, convulsions, intestinal disturbances, and respiratory or cardiac distress. Death follows septicemia and accompanying severe toxic manifestations.
Anthrax in people may take the form of a malignant pustule (cutaneous anthrax) where the bacilli enter through the skin, producing a primary lesion developing into a characteristic area of inflammation surrounding a dark necrotic center; or it may take the form of the pulmonary or – less commonly – the intestinal type, which follows inhalation of dust containing anthrax spores, as has occurred in the woolen industries. Monkeys exposed to artificially generated aerosols of anthrax spores develop symptoms mimicking woolsorters’ disease. Postmodern findings include hemorrhages in the lung, hemorrhagic mediastinitis and lymphadenitis, and sometimes hemorrhagic meningitis.
In the past, outbreaks of anthrax (along with other epizootic diseases) among animals have undoubtedly helped to prepare the way for major outbreaks of epidemic disease in humans. When anthrax has decimated herds of cattle or sheep, for example, human populations have faced starvation, which in turn has lowered their ability to resist those epidemics. Anthrax has been known from antiquity, although until relatively recently it was not clearly separated from other diseases with similar manifestations. Possibly, sudden death of animals at pasture, blamed by Aristotle (and subsequently by his followers over the centuries) on the shrew-mouse and its “poisonous bite,” may in many cases have resulted from the peracute form of anthrax commonly known as splenic apoplexy.
Nineteenth century authors speculated that the fifth and sixth plagues of the Egyptians (as described in Exodus), which struck their herds and the Egyptians themselves, might have been anthrax. Evidence centers on the Israelites, who were installed on sandy ground above the level of the Nile. They escaped the plagues, whereas those who did not lived in areas subject to flooding, which could have provided perfect conditions for growth of the bacillus. Three decades before the birth of Christ, Virgil vividly de-scribed an animal plague that had much in com-mon with anthrax and warned against its trans-mission to people through contact with infected hides.
Through the centuries, there are many records of animal plagues that almost certainly were anthrax but were often confused with other complaints. By 1769, when identification of epidemic diseases of animals and humans had become more precise, Jean Fournier in Dijon, France, classified a number of different lesions as a single disease entity (anthrax), which he called charbon malin. More importantly, he recognized the transmission of the disease to people and drew attention to cases occurring in workers who handled raw hair and wool, a theme developed in several French accounts during the following decade. From the mid-nineteenth century, the disease became a problem in English factories as well, and subsequently in Scotland. About the same time, the woolen industries began experiencing the problem as wool and hair from the East were introduced into British trade. Woolsorting, until then considered a particularly healthful occupation, suddenly produced an alarming increase in the number of deaths and extent of disease among workers. The workers themselves suspected an association between the disease and the grow-ing proportion of wool and hair imported from the East. By the late 1870s, concern in Yorkshire factories was acute, but by then the new bacteriology had identified the cause of anthrax: J. Bell demonstrated that both woolsorters’ dis-ease and malignant pustule in humans derived from anthrax in animals.
Bell’s work was made possible by the work of Davaine and that of Robert Koch in the 1860s and 1870s. During the nineteenth century, the study of anthrax and its use of animal models had become an important part of the framework for the emergence of bacteriology as an academic discipline. In France, Eloy Barth´elemy established the transmissibility of anthrax in 1823. From 1850 onward, study of the putative agent was pursued, beginning with the results obtained by Aloys Pollender, then by Pierre Rayer, and finally by Davaine who, during extensive work with guinea pigs in the 1860s, bestowed on it the name of bact´eridie, which survived in the literature for a long time. From 1876 onward, the anthrax bacillus be-came a cornerstone of both Koch’s theories and his development of pure culture methods; in the late 1870s, W. Greenfield and H. Toussaint reported studies of acquired immunity against anthrax in animals. Pasteur took over the field and in 1881 demonstrated that immunity could be produced through vaccination of sheep.