Anthrax

In cattle, sheep and goats the usual first sign is sudden death with little warning. Where illness is seen it is peracute: fever, staggering, trembling, excitement or dullness, difficult breathing, collapse and death within two to three days. Horses show fever, colic, bloody diarrhoea and swellings; pigs develop throat swelling or intestinal disease. Carcasses may leak dark, unclotted blood from body openings and fail to stiffen. At necropsy the blood is dark and thick, with serosal haemorrhages and a large, soft, dark spleen.

Anthrax is caused by Bacillus anthracis, a spore-forming bacterium. Ingested or inhaled spores germinate into vegetative bacilli inside macrophages, travel through lymphatics to regional lymph nodes and multiply until the animal is septicaemic. Two plasmid-encoded factors drive disease: a glutamate capsule that blocks phagocytosis, and a three-part toxin. Protective antigen binds host cells and carries in oedema factor, which raises cellular cAMP to cause oedema and cripple neutrophils, and lethal factor, which inactivates protein kinases, kills macrophages and triggers inflammatory cytokine release. Lethal toxin also inhibits blood clotting and platelet function, which explains the dark, unclotted blood seen after death.

Domestic and wild herbivores are most at risk: cattle and sheep are the most susceptible, then goats and horses, while pigs are relatively resistant; carnivores and people are infected by eating contaminated meat. Spores persist in soil for years and the disease is found on every continent except Antarctica, endemic in parts of Africa and Asia. Animals are infected by ingesting spores from soil, forage or water, and outbreaks follow weather shifts such as heavy rain after drought or warm spells. It does not normally pass animal to animal. People acquire it through skin, inhalation or ingestion from infected animals or products. It is a serious zoonosis and a WOAH-listed, notifiable disease.

In ruminants: clostridial infections, lightning strike, bloat, anaplasmosis, bacillary haemoglobinuria, acute leptospirosis and lead poisoning. In horses: African horse sickness, colic, purpura and ionophore poisoning. In pigs: classical and African swine fever and malignant oedema of the throat. The separating clues are unclotted blood from orifices, absent rigor mortis and a soft enlarged spleen, confirmed by finding B. anthracis in blood by microscopy, culture or PCR.

Blood and vascular system, spleen, lymph nodes, gastrointestinal tract, respiratory tract, throat and subcutaneous tissues, skin

Annual vaccination with the live Sterne 34F2 spore vaccine in at-risk areas; specific carcass disposal controls, with burning preferred over deep burial; prompt reporting as a WOAH-listed disease.

Poor: disease is almost always fatal in farm animals, pigs excepted, because it progresses so fast. It is treatable with antibiotics; penicillin and oxytetracycline are the most effective under field conditions.