Anthrax (Etiology, transmission, pathogenesis, clinical signs, diagnosis, treatment, and control)

 Synonyms

• Splenic fever, Milzbrand, Charbon, Wool sorters disease, and Malignant carbuncle.

What is anthrax disease?

1. It is a peracute, acute, and septicaemic disease of cattle and sheep leading to sudden death.
2. The extensive enlargement of the spleen given this disease the name splenic fever.
3. It is a zoonotic disease spread from infected animals to humans during the processing of wool and hides.

Etiology/ What causes anthrax?

1. Bacillus anthracis is a pathogenic strain causing anthrax in both humans and animals.
2. It is a non-motile, capsulated, aerobic, spore-forming, gram-positive, organism.
3. Appears as straight, very large rod-shaped, arranged in a small short chain.
4. The organism has two virulent factors such as, a capsule containing poly D-glutamic acid, which resists the phagocytosis induced by the host immune cells and is coded by the virulent plasmid pxo2, and the triple toxins such as edema factor-1, lethal factor-11, and the protective antigen factor- 111 and these are coded by the plasmid pXo1.
5. The organism produces spores when exposed to the external environment and can live for a prolonged period outside the host in the environment.

Predisposing factors

1. The pH of the infected soil, infected wool, materials, hides, and other materials.
2. In wet soil, and stagnant water spores are concentrated and suspended.
3. Alkaline soil (pH 9.0) under a warm climate preserves the organism to develop anthrax but acid soil destroys the organism.
4. Blood transfusion may also spread the disease.
5. Contaminated bone meal, pasture contaminated with tannery effluents, fodders grown on infected soil, turning up of soil, blood and other discharges from infected animals are the major source of infection.
6. Calcium-rich alluvial soil, organic, and poorly drained soils have the risk of causing endemic infection.

What is the source of infection?

• Contaminated soil, environment, infected excretions, and secretions from animals.

Transmission/ How is anthrax transmitted?

1. Ingestion, inhalation, and contamination of wounds by spores are the prime modes of spread.
2. Injury to oral mucosa permits the invasion of the organism.
3. Inhalation plays a less important role in animals in contrast to more spread in human beings and is responsible for causing wool sorters disease.
4. Mechanical transmission through Tabanid fly bite occurs.

Host affected

1. Cattle and sheep are highly susceptible as compared to goats and horses.
2. Human is less susceptible than cattle.
3. Pigs, dogs and cats are relatively resistant.
4. But once affected the case fatality rate is high in farm animals and pigs.
5. Hippopotamus, cape buffalo and elephants die in large numbers due to anthrax.
6. Persons working in wool and hide industries are mostly affected.
7. Predators act as a carrier for this infection.

Pathogenesis

1. Discharges and materials containing anthrax bacilli when exposed to air, spores are produced.
2. The spores are very resistant in the external environment but the un-germinated spores are considered not harmful they become vegetative form inside the host.
3. The organism is resistant to phagocytosis due to the presence of poly D glutamic acid-containing capsule, and triple toxin factors such as edema factor-I, lethal factor-II and protective antigen-III.
4. Primarily, spores proliferate in the regional draining lymph nodes, subsequently, travels via lymphatic vessels and reach the bloodstream and cause septicemia followed by massive invasion into all body tissues.
5. The lethal toxin of B.anthracis causes edema, tissue damage, and death as a result of shock, acute renal failure and terminal anoxia. The blockage of capillaries initiating death is commonly known as the “Log Jam” theory.
6. As per the evidence at least 3×106 bacteria/ml blood is necessary for causing death.
7. In pigs, localization of the organism occurs in the lymph nodes of the throat following invasion through the upper part of the digestive tract eventually lead to fatal septicemia.
8. Shock acute renal failure and terminal anoxia lead to death.

Clinical Findings/ What are the symptoms of anthrax?

Cattle and sheep

Peracute form

1. Sudden death without any premonitory signs. Upon close observation following signs will be seen.
2. Fever, muscle tremor, dyspnoea, congestion of mucous membrane are noticed.
3. Animal collapses and dies due to terminal convulsions.
4. After death, oozing of blood from nostrils, mouth, anus, and vulva is noticed.

Acute form

1. The incubation period of the disease is 1-2 weeks.
2. The course of this form is about 48 hours.
3. Fever (42°C), severe depression, listlessness, congestion and hemorrhage of the deep mucous membrane, increased heart rate, and rapid respiration were observed.
4. Affected cattle with mouth breathing due to oxygen hunger.
5. Abortion, decrease in milk yield, blood-stained or deep yellow color milk, diarrhea, local edema of the tongue, throat, sternum, perineum, and flank occur.

Pigs

1. Acutely and sub-acutely infected animals show high temperature, appear dull, anorectic with distinct inflammatory edema in the throat and face.
2. Painless swelling causes obstruction which leads to swallowing and respiration difficulty.
3. Bloodstained froth is present at the mouth.
4. Petechial hemorrhages of skin, dysentery without any edema of the throat occur.

Pulmonary form

1. In baby pigs, it develops lobar pneumonia and exudative pleurisy through inhalation of dust which leads to death in a day or two.
2. Occasionally, affected individuals may be alive for several days after infection.

Horse

1. Always horses are affected with acute infection by ingestion and lead to septicemia, enteritis and colic.
2. Painful, edematous and subcutaneous swellings appear in the throat over the lower neck, the floor of the thorax, abdomen, prepuce and mammary gland.
3. There is a high fever, severe depression, dyspnoea due to swelling of the throat, or colic due to intestinal irritation.
4. The course of the disease is about 48-96hours.

Zoonosis

1. It is a fatal disease of human beings.
2. B.anthracis is an agent of bioterrorism and is listed as a category A disease by OIE.
3. Occupational hazard by inhalation of spores occurs mostly while sorting out wool and processing of hides.

Necropsy findings

1. Absence of rigor mortis with gaseous decomposition, quick assumption of the characteristic sawhorse posture.
2. Un-clotted tarry-colored blood oozes out through natural orifices.
3. There is a soft and enlarged spleen with a blackberry jam consistency and is an important feature of anthrax.
4. The carcass suspicious of anthrax should not be opened before confirmation made by peripheral blood smear examination.
5. In horses and pigs enlargement of local lymph nodes, lesions in the soft tissues of the neck and pharynx are important and it is called a pharyngeal form of anthrax.

Diagnosis

1. Based on clinical signs and necropsy findings.
2. Isolation and identification of the organism.
3. Hematological and blood chemistry examinations should not be conducted because of the risk of human exposure.
4. The square-ended blue bacilli (chain) with metachromatic pink capsule detected by microscopic examination after staining with ripened polychrome methylene blue stain is called ‘McFadyean’s reaction’.
5. In the early stages, bacilli may not be present in the blood, so, culture can be done or the materials like infected fluid can be injected into guinea pigs would give a proper diagnostic result.
6. Fluorescent antibody technique using monoclonal antibody can be done to detect the disease.
7. In decomposed carcass vegetative bacilli may not be present, in such a condition diagnosis is difficult so that the protective antigen is detected by the immuno-chromatography test. It has high specificity but does not give positive results in recently vaccinated cattle.
8. Isolation of bacilli from infected soil is difficult but real-time quantitative PCR and Nested PCR have been found highly sensitive.
9. In humans, cutaneous anthrax is diagnosed by Immunohistochemical detection of the bacilli in the skin biopsies.
10. Gelatin stab culture: The growth resembles an ‘inverted fir tree appearance’ with slow liquefaction commencing from the top.
11. Nutrient agar media: The 5% blood agar show ‘medusa head colonies’.
12. Ascoli’s test: To demonstrate the antigen in the severely decayed carcass and the skin in which tube containing positive serum is overlayed with suspected sample develops a white precipitation ring at the junction of the two liquids.

Differential diagnosis

1. Lightning strike.
2. Peracute blackleg.
3. Malignant edema.
4. Bacillary hemoglobinuria.
5. Hypomagnesemia.

Treatment/ How to cure anthrax?

1. Recovery occurs when the infection is identified in the early stages and treated with penicillin 20,000 IU /kg BW twice daily for 3 days.
2. Streptomycin 8-10g/day in two divided doses IM for cattle is effective.
3. Oxytetracycline 5-10mg /kg/day IV or IM can be given.
4. Antiserum is given daily for 5 days however, it is expensive.

Prevention

1. Annual spore vaccine is recommended in an endemic area. An uncapsulated, avirulent, glycerol added live spore containing strains of B.anthracis vaccine can be administered to all animals. However, glycerol suspended vaccines are not safe for small ruminants especially, in goats. Because, it may cause irritation, gangrenous edema, septicemia, and sloughing of the tail at the site of injection (caudal fold). Montanide is an adjuvant added in the vaccine is highly suitable for all ruminants including sheep and goats. It produces higher immunity.
2. Anthrax spore vaccine: It contains 34F2 strains of B.anthracis, non-capsulated, avirulent (i.e. pX01+/pX02-), glycerol suspended spore vaccine. It can be used to protect all species of animals viz/- cattle, sheep, goats, horse, ass, elephant, pigs, and camel. The vaccine is used both in disease-free and in outbreak areas. Cattle, sheep, and goats 1ml, in the caudal fold, elephant and horses 1 ml, s/c followed by a booster 3ml, s/c one month following primary vaccination should be given.
3. IVPM Vaccine: Anthrax spore vaccine is a glycerinated suspension of live spores of an un-capsulated, avirulent strain of Bacillus anthracis. The vaccine is used to protect all species of animals such as cattle, sheep, goats, horses, ass, elephants, pigs, and camels respectively. The vaccine is used both in disease-free and outbreak areas. Cattle, sheep and goats, elephant and horses 1ml each subcutaneously given. In elephants and horses, it is advisable to give a second dose of 3ml after 1 month is advisable. The immunity following vaccination is established in about 10 days.
4. Following the outbreak, the animals can be consecutively vaccinated for three years for the prevention of further outbreaks in the region.

How to control anthrax disease?

1. Anthrax suspected carcass should not be opened.
2. Plugging of natural orifices with cotton to prevent exposure and spore formation.
3. The carcass must be disposed of by burning or deep burial.
4. Burial should be done 2 meters deep with the adequate application of quick lime, liberal application of 5% formaldehyde to control the spread of the spores.
5. Strong disinfection with 5% Lysol for 2 days, 5-10% solution of formalin, or 2% sodium hydroxide is highly effective.
6. Peracetic acid 3% solution of 8L/m2 is considered a very good sporicide and effective sterilant.
7. Maintain suspected animals for 2 weeks under quarantine.
8. Vaccination of survivors, administration of hyperimmune serum to contact animals should be carried out.
9. Livestock and animal byproducts like meat, milk should not be taken out of the farm during the quarantine period.