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Anthrax - Medical Details - Part 2 |
Immunity
Considerable variation in genetic susceptibility to anthrax exists among animal species. Resistant animals fall into two groups: (1) resistant to establishment of anthrax but sensitive to the toxin and (2) resistant to the toxin but susceptible to establishment of disease.
Animals surviving naturally-acquired anthrax are immune to reinfection. Second attacks are extremely rare. Permanent immunity to anthrax seems to require antibodies to both the toxin and the capsular polypeptide, but the relative importance of the two kinds of antibodies appears to vary widely in different animals.
Vaccines composed of killed bacilli and/or capsular antigens produce no significant immunity. A nonencapsulated toxigenic strain has been used effectively in livestock. The Sterne Strain of Bacillus anthracis produces sublethal amounts of the toxin that induce formation of protective antibody. The best vaccine for humans is a preparation of the protective antigen of the lethal toxin recovered from culture filtrates of the bacteria. In either case, frequent boosters are necessary to maintain resistance to anthrax challenge.
Recently, the toxin of Bacillus anthracis, specifically its cell-binding domains, has been exploited to transport molecules into selected types of eukaryotic cells, in the search for new vaccines aimed against intracellular parasites. In this case, researchers fused parasite (bacterial or viral) antigens to the anthrax toxin's cell-binding components, thereby creating a model pathogen molecule which is able to recognize and be taken up by T-cells, but which is unable to produce disease.
Such types of vaccines are known as intracellular vaccines, and they theoretically have the potential to stimulate protective CMI (as opposed to AMI), which is rarely accomplished with most present vaccines. Though still in early stages of testing, the vaccines show promise, and this work may lead to an entirely new class of human vaccines against most viruses, certain bacteria, and parasites.
Anthrax and Biological Warfare
U.S. military forces have been vaccinated recently against anthrax, reflecting the concern about the prospect of anthrax spores being used in defense against them. Iraq, Russia and as many as ten nations have the capability to load spores of B. anthracis into weapons.
The spores of B. anthracis can be produced and stored in a dry form and remain viable for decades in storage or after release. When released, the spores are easily dispersed in air for inhalation by unprotected troops (or civilians downwind) and may remain in soil for many years. Anthrax spores are the top choice in biological weapons for "germ warfare".
Disinfection of contaminated articles may be accomplished using a 0.05% hypochlorite solution (1 tbps. bleach per gallon of water).
Spore destruction requires steam sterilization.
The military chemical protective mask is effective against inhalation of all Biological Warfare Agents.
Clinical Manifestions
Cutaneous anthrax usually occurs through contamination of a cut or abrasion, although in some countries biting flies may also transmit the disease. After a 2- to 3-day incubation period, a small pimple or papule appears at the inoculation site. A surrounding ring of vesicles develops. Over the next few days, the central papule ulcerates, dries, and blackens to form the characteristic eschar (Fig. 15-1). The lesion is painless and is surrounded by marked edema that may extend for some distance. Pus and pain appear only if the lesion becomes infected by a pyogenic organism. Similarly, marked lymphangitis and fever usually point to a secondary infection. In most cases the disease remains limited to the initial lesion and resolves spontaneously. The main dangers are that a lesion on the face or neck may swell to occlude the airway or may give rise to secondary meningitis. If host defenses fail to contain the infection, however, fulminating septicemia develops. Approximately 20 percent of untreated cases of cutaneous anthrax progress to fatal septicemia. However, B anthracis is susceptible to penicillin and other common antibiotics, so effective treatment is almost always available.
FIGURES (see below) - Evolution of an anthrax eschar in a 4-year-old boy. (A&B) the lesion when first seen (day 0). Note the arm swollen from the characteristic edema. (C) Day 6. (D) Day 10. (E) Day 15. Although penicillin treatment was begun immediately and the lesion was sterile by about 24 hours, it continued to evolve and resolve as seen. (Photographs kindly supplied by W.E. Kobuch, M.D., St. Luke's Hospital, Lupane, Bulawayo, Zimbabwe.)
Intestinal anthrax is analogous to cutaneous anthrax but occurs on the intestinal mucosa. As in cutaneous anthrax, the organisms probably invade the mucosa through a preexisting lesion. organisms spread from the mucosal lesion to the lymphatic system. In pulmonary anthrax, inhaled spores are transported by alveolar macrophages to the mediastinal lymph nodes, where they germinate and multiply to initiate systemic disease. Gastrointestinal and pulmonary anthrax are both more dangerous than the cutaneous form because they are usually identified too late for treatment to be effective.
Herbivorous animals, the primary hosts of B anthracis, contract the infection by ingesting spores on forage plants; the spores are derived from soil or dust or are deposited on leaves by flies after feeding on an anthrax-infected carcass. If the spores enter a lesion in the gastrointestinal mucosa, they germinate and are taken into the bloodstream and lymphatics, finally producing systemic anthrax, which is usually fatal.
Symptoms prior to fulminant systemic anthrax may be absent or mild, consisting, for example, of malaise, low fever, and mild gastrointestinal symptoms in the case of gastrointestinal disease. During this phase the organism is multiplying and producing toxin in the regional lymph nodes and spleen. Released toxin causes breakdown of these organs probably of the spleen in particular. This causes the sudden onset of hyperacute illness with dyspnea, cyanosis, high fever, and disorientation, which progress in a few hours to shock, coma, and death. Although symptoms vary somewhat with the host species, this final acute phase is marked by a high-grade bacteremia. In humans, blood cultures are not always positive.
Symptoms:
Symptoms of disease vary depending on how the disease was contracted, but symptoms usually occur within 7 days.
Cutaneous: Most (about 95%) anthrax infections occur when the bacterium enters a cut or abrasion on the skin, such as when handling contaminated wool, hides, leather or hair products (especially goat hair) of infected animals. Skin infection begins as a raised itchy bump that resembles an insect bite but within 1-2 days develops into a vesicle and then a painless ulcer, usually 1-3 cm in diameter, with a characteristic black necrotic (dying) area in the center. Lymph glands in the adjacent area may swell. About 20% of untreated cases of cutaneous anthrax will result in death. Deaths are rare with appropriate antimicrobial therapy.
Inhalation: Initial symptoms may resemble a common cold. After several days, the symptoms may progress to severe breathing problems and shock. Inhalation anthrax is usually fatal.
Intestinal: The intestinal disease form of anthrax may follow the consumption of contaminated meat and is characterized by an acute inflammation of the intestinal tract. Initial signs of nausea, loss of appetite, vomiting, fever are followed by abdominal pain, vomiting of blood, and severe diarrhea. Intestinal anthrax results in death in 25% to 60% of cases.
Medical countermeasures:
There is a licensed human Anthrax vaccine that consists of a series of six doses with yearly boosters. The first vaccine of the series must be given at least four weeks before exposure to the disease. This vaccine protects against Anthrax that is acquired through the skin in an occupational environment. It is believed that it would also be effective against inhaled spores in a BW situation.
For unvaccinated individuals, antibiotics are given if the individual is exposed to Anthrax. Pencillin is the drug of choice. Antibiotic treatment is known to lessen the severity of the illness in workers who acquire Anthrax through the skin.
Anthrax was formerly thought to be nearly 100% fatal despite antibiotic treatment, particularly if treatment is started after symptoms appear. A recent Army study resulted in successful treatment of monkeys with antibiotic therapy after being exposed to Anthrax spores. The antibiotic therapy was begun one day after exposure.
This study implies antibiotic therapy may be useful in a BW setting if begun soon after the attack.
There is no evidence of person-to person transmission of Anthrax. Quarantine of affected individuals is not recommended. Anthrax spores may survive in the soil, water and on surfaces for many years. Spores can only be destroyed by steam sterilization or burning, but not by disinfectants.
An infection of local animal populations such as sheep and cattle could follow a biological attack with spores. Infected animals could then transmit the disease to humans through the human's skin, mouth or nose. Veterinarians should be made aware of this possibility. Local health officials should take appropriate measures (published elsewhere) to prevent Anthrax outbreak among animals and an ensuing human epidemic."
Part
3
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