Anthrax is an often fatal infectious disease that can infect all warm-blooded animals, including horses and humans. Underdiagnosis and unreliable reporting make it difficult to estimate the true frequency of anthrax worldwide; however, anthrax has been reported from nearly every continent. Under normal circumstances, anthrax outbreaks in the United States are extremely rare. Anthrax received much attention in 2001 in relation to the terrorist attacks on the United States because of its potential use as a biological weapon.

Anthrax is caused by infection with a bacterium known as Bacillus anthracis. This bacterium forms spores, which make it extremely resistant to environmental conditions such as heating, freezing, chemical disinfection, or dehydration that typically destroy other types of bacteria. Thus, it can persist for a long time within or on a contaminated environment or object. Horses may consume the spores while grazing in areas where anthrax has been a problem. Optimal growth conditions for the bacteria often occur in neutral or alkaline, calcium- or lime-rich soils. Flies and other insects may also spread the disease from infected animals to other animals.

After exposure, the typical incubation period is from 3 to 7 days. Once the bacteria infect an animal or human, the organisms multiply and spread throughout the body. They produce a potent and lethal poison (toxin) that causes cell death and breakdown of the infected tissues. This results in inflammation and organ damage eventually leading to organ failure. The bacteria spread throughout the body through the blood and lymphatic (immune) system.

Signs occur rapidly in a previously healthy animal. High fever and agitation are quickly followed by chills, severe colic, loss of appetite, depression, disorientation, difficulty breathing or exercise intolerance, muscle weakness, and seizures. Bloody diarrhea may be observed. Swelling often occurs around the neck and may be so severe that suffocation is possible. Swelling of the chest, lower abdomen, and external genitals may also occur. If spores infect open cuts or abrasions, a localized skin infection occurs. Without rapid treatment, death usually occurs within 2 to 3 days of onset.

A diagnosis based on signs is difficult because many infections and other conditions (such as colic, sunstroke, or acute infectious anemia in horses) may have signs similar to anthrax. Diagnosis thus requires laboratory analysis of blood samples from the potentially infected animal or human to confirm the presence of the bacteria.

Human cases of anthrax may follow contact with contaminated animals or animal products. You should use strict precautions (wearing gloves, protective clothing, goggles, and masks) when handling potentially infected animals or their remains.

Anthrax is controlled through vaccination programs, rapid detection and reporting, quarantine, antibiotic treatment of any animals exposed to the bacteria, and the burning or burial of dead animals that had suspected or confirmed anthrax infection. Early treatment and vigorous implementation of a preventive program are essential. Vaccination of horses is done only when horses are pastured in an area that is known to be contaminated. In most countries, all cases of anthrax must be reported to the appropriate regulatory officials. Uninfected horses should be moved to another pasture away from where infected animals had pastured and from any possible site of soil contamination. Stables and equipment must be cleaned and disinfected.

Last full review/revision July 2011 by Otto M. Radostits, CM, DVM, MSc, DACVIM (Deceased); Delores E. Hill, PhD; Barton W. Rohrbach, VMD, MPH, DACVPM; Charles J. Issel, DVM, PhD; Max J. Appel, DMV, PhD; David A. Ashford, DVM, MPH, DS; Daniela Bedenice, DrVetMed, DACVIM, DACVECC; Farouk M. Hamdy, DVM, MSc, PhD, MPA (Deceased); Kenneth R. Harkin, DVM, DACVIM; Johnny D. Hoskins, DVM, PhD; Eugene D. Janzen, DVM, MVS; Jodie Low Choy, BVMS; John E. Madigan, DVM, MS; Dale A. Moore, MS, DVM, MPVM, PhD; J. Glenn Songer, PhD; Joseph Taboada, DVM, DACVIM; Charles O. Thoen, DVM, PhD; John F. Timoney, MVB, PhD, Dsc, MRCVS; Ian Tizard, BVMS, PhD, DACVM; Brian J. McCluskey, DVM, MS, PhD, DACVPM; Bert E. Stromberg, PhD; Peter J. Timoney, MVB, MS, PhD, FRCVS