Occurrence: Worldwide, notifiable in many countries

Age affected: All ages, human risk

 

Effects:  Illness, fever, bloody diarrhoea, swollen neck, breathing difficulty, death.

• Detailed causes
• Clinical signs
• Diagnosis
• Treatment and control

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Detailed causes

Anthrax is caused by Bacillus anthracis, a large rod-shaped bacterium capable of very fast growth inside and outside the body and surviving for many years by means of a spore which can resist boiling. It can be cultivated easily in the laboratory. Infection is oral, but can occur through scratches or by aerosol. The organism usually causes disease at the point it actually enters the body of the pig (tonsil or intestine) but can invade the blood to causes septicaemia. Organisms resist attack by the while cells of the body by means of a capsule of polyglutamic acid which is present around the cell and cause disease by means of 3 powerful toxins. The affected tissue become inflamed, filled with bloodstained fluid and swells. When septicaemia occurs, the infection often results in death. B. anthracis may be shed in urine and faeces and in discharges or in blood from dead animals and may remain for a period in lymph nodes of recovered pigs. Recovery and solid immunity may occur but damage to the skin over affected areas of the neck may remain in some cases. Anthrax bacilli can infect man through scratches and cause local blackened abscesses or in dust by aerosol to give pneumonia or by eating infected meat. It can be fatal.

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Clinical signs

When the organisms localises in the tonsils, affected pigs develop fever to 42˚C, (107˚F) and swelling of the neck (pharyngeal form) As the swollen neck increases in size, depression, vomiting and reluctance to eat occur, breathing becomes increasingly difficult and affected pigs may die within 24 hours. Some pigs recover, but blackened skin may remain over the site of the swelling. When the infection localises in the intestine (intestinal form), fever, digestive disturbance, loss of appetite and the passage of bloody faeces may be seen. Death is uncommon in this from. When the organisms causes septicaemia (septicaemic form), pigs may be found dead. Others in the group may have transient fever and recover completely or develop the intestinal form or have swollen necks.

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Diagnosis

Anthrax is rare and is therefore not usually suspected when sudden death occurs, but should be considered if fevered animals develop blood-stained diarrhoea or the swelling of the neck associated with the pharyngeal form of the disease. If anthrax is suspected, the carcass should not be opened and a blood sample should be taken from an ear or tissue fluid taken from the swollen neck for laboratory examination by a state veterinarian. Anthrax is often first suspected following post-mortem examination. In the phyryngeal form there is massive enlargement and reddening of the cervical lymph nodes which are surrounded by gelatinous oedema fluid. In the intestinal form, copious peritoneal fluid may be present and there may be blackened infarcted areas in the spleen, thickening of the intestinal wall, swollen mediastinal (intestinal) lymph nodes and adhesion between pieces of intestine. The intestinal lining may be covered with necrotic (dead) tissue. In the septicaemic form, the carcass and its lymph nodes may be reddened, the spleen enlarged, and the kidneys petechiated (spotted with blood). Laboratory confirmation is by demonstration of the organism with its characteristic capsule in smears made safe by fixation and by culture of blood, oedema fluid or an affected organ.

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Treatment and control

Anthrax responds to penicillin treatment and tetracyclines have been used by injection and in feed and water to treat affected animals and eliminate disease from affected groups. The disease is controlled by state veterinarians in many countries as anthrax affects man and is a risk to other livestock. The affected farm is usually isolated while the disease is controlled. Affected pigs are treated or killed. The bodies of dead affected pigs should be incinerated or buried unopened as exposure to the air allows the resistant spores to form. All areas in contact with the carcasses should be disinfected suing an approved disinfectant which can kill the spores. Slurry from the affected pigs should be disinfected and disposed of safety. In the longer term the Anthrax Spore Vaccine can protect pigs from the disease. Recent improvements in food safety mean that meat plants are unwilling to accept pigs from anthrax infected herds and the whole herd may have to be slaughtered, thus increasing the significance of the disease for the pig industry. The disease usually enters in contaminated meat and bone meal, but the organisms is so rare in traded product, that measures to prevent diseases are not worthwhile in most pig-rearing countries.

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