Recent outbreaks of anthrax in the US have focused new attention on one of civilization's oldest and deadliest diseases. Anthrax remains a far greater threat to livestock than to people - while infected animals left untreated die within a few days, the most common form of anthrax in humans is a generally non-fatal skin infection that strikes workers handling infected animals or animal products. (That anthrax is largely an "occupational hazard" for humans is reflected in the common name given to its deadly pulmonary form: "woolsorter's disease", contracted through the inhalation of spores in fleece.) AG21 asked specialists in AG's Animal Health Service for a basic guide to anthrax in animals.
What is anthrax?
Anthrax is an acute infectious disease that can strike almost all warm-blooded animals, including humans, and affects a variety of tissues including the skin, intestine, kidney, meninges, conjunctiva and lymphatic system. It is caused by the bacterium Bacillus anthracis, which belongs to a group of bacteria that have the capability of forming spores. Spores are microscopic, resistant particles that enable the micro-organism to survive adverse environmental conditions. In animals, the disease usually causes sudden death, and in most countries it is required by law that outbreaks of anthrax be reported to the appropriate authorities.
Where does anthrax occur in the world?
The disease occurs worldwide. In some countries, specific areas are known to favour the survival of anthrax bacterium spores in the soil and are thus subject to occasional outbreaks. Alkaline soils are favourable to the survival of the spores. In these areas, the spores of the organism multiply when soil conditions - such as temperature, moisture and nutrition - are favourable. Re-occurrence of the disease is experienced in such locations periodically for many years.
Which animal species can be affected by anthrax?
The disease affects domestic animals - such as cattle, sheep, goats, horses, donkeys, pigs and dogs - as well as wild ruminants such as antelopes, gazelles and impalas. Even elephants and hippopotami are reported to have died from the disease in outbreaks in some parts of Africa. Wild carnivores such as lions, hyenas and jackals, are also susceptible. Birds, however, seem to be resistant to anthrax.
How does anthrax spread in animals?
Outbreaks of anthrax tend to occur in association with particular climatic and weather events, such as heavy rainfall, flooding and drought. In anthrax-prone areas, the close grazing of animals on fresh shoots of grass after rainfall often leads to outbreaks of the disease due to the ingestion of organisms picked from contaminated soils. During severe outbreaks, biting flies may transmit the disease from one animal to another but this is a very minor mode of transmission. The principal mode of transmission is ingestion of infective micro-organisms. Non-biting blowflies may contaminate vegetation by depositing vomit droplets after feeding on a carcass infected with B. anthracis. Animals feeding on such vegetation then become infected. Wild carnivores become infected through the consumption of infected animals that have died from anthrax. Outbreaks of anthrax have been reported in some animals (mainly pigs) after ingestion of feeds containing meat and bone meal based concentrates originating from carcasses contaminated with anthrax bacterial spores.
How does anthrax manifest itself?
The incubation period of anthrax is three to seven days, with a range of one to 14 days. A common feature of anthrax is that animals in apparently good condition die suddenly without showing overt signs of ill health. Acute cases in cattle, sheep and wild herbivores are characterized by fever, depression, difficulty in breathing and convulsions. Animals may die within two or three days if not treated. It is common to see bloody discharges from natural openings. In few instances, anthrax can manifest itself as a mild disease characterized by general malaise. In pigs, the disease is characterised by swelling of the throat, which may cause difficulties in breathing. In dogs, cats and wild carnivores, the disease resembles that seen in pigs.
What measures can be taken to control anthrax in animals?
Because anthrax is often fatal in domestic animals, a preventive strategy should be adopted involving annual vaccination of susceptible animals (usually cattle, sheep and goats) in areas prone to the disease. This is usually done two to four weeks before the onset of known period of outbreaks. In situations where animals show clinical signs of the disease, antibiotic treatment is recommended. Other measures to be adopted in addition to immunization and treatment are enforcement of quarantine regulations, prompt disposal of dead animals, bedding and contaminated materials, control of scavengers, and observation of general hygiene by people who have come in contact with diseased or dead animals.
What vaccine is used in controlling anthrax in animals?
The non-encapsulated Sterne strain vaccine is used. This live attenuated vaccine is incapable of causing clinical disease, but produces immunity against anthrax. A single vaccination produces immunity lasting for an average of nine months. Annual vaccination of susceptible animals is sufficient to control outbreaks of anthrax in defined localities. The Sterne vaccine strain is not hazardous to humans. Consequently, no specific protective measures are prescribed for veterinarians, veterinary technicians and farmers who handle the vaccine.
How dangerous is anthrax for humans?
Anthrax is an old disease of both animals and humans. It is not contagious - that is, it is not easily transmitted from person to person. However, one reason it causes concern is that the spore form of the bacterium can persist in the environment for a long period of time if conditions are favourable. Humans can develop localized skin lesions or cutaneous anthrax through the contact of broken skin with infected blood or tissues, or may acquire the highly fatal form from inhalation of spores. Humans can also acquire the intestinal form of anthrax by consuming poorly cooked contaminated meat.
Published December 2001