Anthrax (bacillus anthracis infection)

Occurrence: Worldwide.
Species affected: Pigs, ruminants, humans, other animals.
Age affected: All, but most common grower to adult.
Causes: Gram positive spore-forming encapsulated bacterium, Bacillus anthracis.
Effects: Affected pigs may develop fever to 42˚C (107˚F), and swelling of the neck (pharyngeal form) followed by depression, vomiting and reluctance to eat, increasing difficulty in breathing 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 form. When the organisms cause septicaemia (septicaemic form), pigs may be found dead. Others in the group may have transient fever and recover completely, develop the intestinal form or have swollen necks.

Causes

Pigs of all ages are susceptible to this acute to chronic disease but it is most common in feeding pigs from grower to adult. The causal agent is Bacillus anthracis which is a spore-forming Gram-positive bacillus which possesses capsules and produces a protective factor and lethal toxins. 

Mode of transmission

Infection is normally by mouth but can occur following injection, scratches or insect bites. The spores are heat resistant and live for many years and feed, water or soil contamination with them is the normal source of infection. Spores form after an animal dies or after discharges are exposed to the air and infected pig flesh or urine and faeces from infected animals can spread the disease in a pen. The disease is most common in ruminants, but can occur in all mammalian species and can cause fatal infections in humans.


This scanning electron micrograph (SEM) depicted spores from the Sterne strain of Bacillus anthracis bacteria.
Photo credit: CDC - Janice Haney Carr

Clinical signs

Affected pigs may develop fever to 42˚C (107˚F), and swelling of the neck (pharyngeal form) followed by depression, vomiting and reluctance to eat, increasing difficulty in breathing 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 cause septicaemia (septicaemic form), pigs may be found dead. Others in the group may have transient fever and recover completely, develop the intestinal form or have swollen necks.

Postmortem lesions

Anthrax is a rare cause of sudden death, 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. If seen first at post-mortem examination, there is massive enlargement and reddening of the cervical lymph nodes which are surrounded by gelatinous oedema fluid in the pharyngeal form. 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, by culture of blood, oedema fluid or an affected organ or by polymerase chain reaction in an approved laboratory.


This photograph depicts the colonial growth displayed by Sterne strain members of the Gram-positive bacterium, Bacillus anthracis.
Photo credit: CDC - Dr Todd Parker, PhD 

Prevention

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 using an approved disinfectant which can kill the spores. Slurry from the affected pigs should be disinfected and disposed of safely. 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 a country in contaminated meat and bone meal, but the organism is so rare in traded product, that specific measures to prevent the disease are not worthwhile in most pig-rearing countries.

Special note

Anthrax is a Notifiable Disease in most countries and its presence is recorded by the home country, in Europe, by the European Union and worldwide by the World Organisation for Animal Health (OIE). As it is a zoonosis, diagnosis, culture and control must be carried out safely and usually under the supervision of state veterinarians.

Smear of blood showing the diagnostic capsule around the bacilli. 
Photo credit: David Taylor