Outbreaks of Anthrax: Part 2

This article highlights very briefly some reports of recent outbreaks of anthrax in Europe and Asia. Outbreaks in Europe have been reported mainly from France and Italy. The interested reader may note that several outbreaks which have not been noted as an interesting study in a prominent scientific journal would tend not to be reported. This article may serve only to give a few faint clues as to the true nature of the magnitude of the disease. In the concluding part of this article, brief observations about the nature of the toxin produced, the mechanisms involved in toxin production and some results and effects of prophylaxis will also be very briefly highlighted.

Anthrax outbreaks in France In France, two outbreaks of anthrax zoonoses occurred in 1997 with as many as 94 animals dying while three nonfatal cases were detected in humans. According to Patra et al at the Institut Pasteur, all the strains of Bacillus anthracis in animal and soil samples identified by a multiplex PCR assay belonged to the variable-number tandem repeat (VNTR) group (VNTR)3.

Anthrax outbreaks in Italy As many as three cases of anthrax were reported by de Lalla et al from an agricultural area in northern Italy in 1992. In other reports outbreaks of malignant pustule have been reported from different districts, in Lombardia and in Province of Milan. In a significant number of cases, the cause of the outbreak has been traced to contaminated leather imported from African countries

Anthrax outbreaks in India Animal anthrax is very common in many parts of India, but human anthrax has been recognized in only certain limited locations. In the Chittoor and North Arcot districts, its prevalence has increased in recent years.

Sekhar et al have reported 30 human anthrax cases from Ramabhadrapuram village of Chittoor district in Andhra Pradesh during November-December, 1989. The human cases followed in the wake of an anthrax epidemic among cattle and sheep of the village. The villagers who ate the contaminated meat were infected with anthrax. All affected individuals survived after treatment with penicillin.

George et al at the department of medicine, Christian Medical College, Vellore have reported an outbreak of anthrax meningo-encephalitis via ingestion of infected sheep meat in Chittoor district in Andhra Pradesh, in October 1990. All patients who were exposed to the carcasses or raw sheep meat suffered with anthrax meningo-encephalitis and died. However, only one patient with a malignant pustule recovered. A large number of people who cooked or ate the cooked meat of the dead sheep did not show any signs of the disease.

Anthrax outbreaks in Thailand Kunanusont et al at the Epidemiology Division of the Ministry of Public Health, Bangkok have reported an outbreak of anthrax among 14 persons exposed to the meat of two water buffalo which had died from anthrax, in two neighbouring villages in the northeastern region of Thailand. Three of the affected individuals had eaten raw meat; one of them died from gastric anthrax with severe haematemesis. All the others were successfully treated with penicillin. The incubation period varied between two and 11 days.

Sirisnathana et al have reported an outbreak of 52 cases of cutaneous anthrax and 24 cases of oral-oropharyngeal anthrax in rural Northern Thailand in 1982, caused by contaminated water buffalo meat. In their study ELISA was used to measure antibody titers to components of anthrax oedema toxin (edema factor [EF] and protective antigen [PA]), lethal toxin (lethal factor [LF] and PA), and poly-D-glutamic acid capsule. Nine patients with cutaneous anthrax, 10 patients with oral-oropharyngeal anthrax, and 43 healthy unexposed Thai control villagers were studied.

Detection Methods
At the department of veterinary microbiology, Obihiro University of Agriculture and Veterinary Medicine, Hokkaido, Japan, Cheun et al have developed a rapid and simple detection system for the isolation of Bacillus anthracis from meat and tissue. These results could be useful for detecting animals with latent anthrax, and meat contaminated with B. anthracis, rapidly and simply.

In another report, Kiel et al have developed culture techniques for rapid isolation and identification of "live" anthrax from the environment. By using a special medium (3AT medium), the researchers have been able to differentiate between closely related bacilli and non-pathogenic strains. In the study carried out by Kiel et al, nitrate was found to be a primary factor influencing spore formation in Bacillus anthracis.

Toxicity of Bacillus anthracis
The toxicity of B.anthracis has been identified to three components, protective antigen (PA), lethal factor (LF), and edema factor (EF). PA is the target-cell binding protein and is common to the two effector molecules, LF and EF, which exert their toxic effects once in the cytosol by PA. PA is the major component of vaccines against anthrax since it confers protective immunity.

Three proteins produced by Bacillus anthracis, the protective antigen (PA), the lethal factor (LF), and the edema factor (EF), combine in pairs to produce the lethal (PA+LF) and edema (PA+EF) toxins. In tissue culture systems, Chauhan et al at the Centre for Biotechnology, Jawaharlal Nehru University, New Delhi have been able to obtain as much as 125 mg of recombinant protective antigen (rPA) protein per liter of batch culture.

Anthrax infections occur due to dormant endospores entering the mammalian host and becoming engulfed by regional macrophages (Mphi). During systemic anthrax, late stage events include vegetative growth in the blood and the synthesis of the anthrax exotoxin complex, which causes disease symptoms and death.

The anthrax lethal toxin, at high concentrations, induces lysis of macrophages in vitro but shows little or no effect on other cells. It appears that systemic shock and death from anthrax happens because of high levels of cytokines, especially IL-1, produced by macrophages stimulated by the anthrax lethal toxin.

Efficacy of respirators against infectious aerosols
According to Nicas et al, on the basis of their studies, a full-face-piece powered air-purifying respirator would be the best air-purifying device for responding to an anthrax spore release.

Adverse reactions to vaccination
There have been conflicting results about the ability of killed vaccines to induce adverse reactions. The killed vaccine has been associated with a higher incidence and severity of adverse effects. Killed anthrax vaccines appear to be effective in reducing the risk of contracting anthrax with a relatively low rate of adverse effects.

Vaccination
Vaccines which are efficacious against anthrax, such as the human vaccine, Anthrax Vaccine Absorbed (AVA), contain the protective antigen (PA) component of the anthrax toxins as the major protective immunogen. Although AVA protects against inhalational anthrax, the immune responses to and role in protection of PA and possibly other antigens have yet to be fully understood

Anthrax-protective antigen is the central moiety of the anthrax toxin complex that allows the entry of the other two toxin components, lethal factor and edema factor into the cells. It is also the main immunogen of the cell-free vaccine against anthrax. Killed anthrax vaccines appear to be effective in reducing the risk of contracting anthrax with a relatively low rate of adverse effects. Currently available human vaccines in the United States and Europe are made up of alum-precipitated supernatant material from cultures of toxigenic, non-encapsulated strains of Bacillus anthracis.

Protective antigen (PA) of anthrax toxin is the major component of human anthrax vaccine. The mutant PA protein holds out hope for the development of an effective recombinant vaccine against anthrax.

Price et al at the department of microbiology, Ohio State University have shown that DNA-based immunization alone protects against a lethal toxin challenge and that DNA immunization against the LF antigen alone can give complete protection.

Cohen et al have developed several highly attenuated spore-forming non-toxinogenic and non-encapsulated Bacillus anthracis vaccines differing in levels of expression of recombinant protective antigen (rPA). The investigators found the immune response to be long lasting (at least 12 months) providing protection against a lethal challenge of virulent (Vollum) anthrax spores. The recombinant B. anthracis spore vaccine appears to be more efficacious than the vegetative cell vaccine.

Conclusion
There is still much to be understood about the mechanisms of toxicity and immunogenecity in devising more efficient prophylactic measures for the prevention and control of anthrax, in animals.

References