Anthrax

What is anthrax?

Anthrax is a disease caused by the spore-forming bacteria Bacillus anthracis. The name of the bacteria derives from the Greek word for coal, because of the ulcers with dark centres that develop on the skin of affected people.

Anthrax occurs on all the continents and commonly causes high mortality, primarily in domestic and wild herbivores as well as most mammals and several bird species.

It is a serious zoonosis, meaning that it can be transmitted from animals to humans.

The bacteria produce extremely potent toxins, which are responsible for the ill effects. Anthrax spores are extremely resistant and can survive in the environment for decades, making the control or eradication of the disease very difficult.

Anthrax is an WOAH-listed disease and must be reported to the WOAH as indicated in its Terrestrial Animal Health Code.

Transmission and spread

Anthrax typically does not spread from animal to animal nor from person to person.

The bacteria produce spores on contact with oxygen. These spores are extremely resistant and survive for years in soil, or on wool or hair of infected animals. If ingested or inhaled by an animal, or after entering through cuts in the skin, they can germinate and cause disease. Because the blood of infected animals sometimes fails to clot and may leak from body orifices, insects can spread the bacteria to other animals.

Carnivores and humans can become infected by eating meat from an infected animal. Typically, however, animals become infected by ingesting spores that are in the soil or in feed.

Public health risk

More than 95% of human anthrax cases are of the cutaneous form and result from handling infected carcasses or the hides, hair, meat, or bones from such carcasses.

In humans, anthrax manifests itself in three distinct patterns (cutaneous, gastrointestinal and inhalational). The most common is a skin infection, where people become infected handling animals or animal products that contain spores. This can happen to veterinarians, agricultural workers, livestock producers or butchers dealing with sick animals, or when infection has been spread by wool or hides.

Bacillus anthracis is not invasive and requires a lesion to infect. The spores enter the body through cuts or scratches in the skin and cause a local infection that if not controlled may spread throughout the body. The digestive form occurs when the spores are eaten. Tragically, people who lose their animals may also lose their lives trying to salvage something and consuming the meat from an animal that died. Potentially the most deadly form is by inhalation. This has been called ‘wool sorters’ disease’ because spores on hides or hair can be inhaled. While inhalation anthrax is rare in nature, anthrax spores have been developed and used as a biological weapon. Clearly, preventing the disease in animals will protect human public health.

Clinical signs

Peracute, acute, subacute and, rarely, chronic forms of the disease are reported. Antemortem clinical signs may be virtually absent in peracute and acute forms of the disease. Meanwhile, the only sign in chronic form may be enlarged lymph glands.

Ruminant animals are often found dead with no indication that they had been ill. In this acute form, there may be high fever, muscle tremors and difficult breathing seen shortly before the animal collapses and dies. Unclotted blood may exude from body openings and the body may not stiffen after death. Subacute form may be accompanied by progressive fever, depression, inappetence, weakness, prostration and death.

In horses and (on occasions) in ruminants there may be digestive upsets and colic, fever, depression and sometimes swelling. These symptoms may last for up to four days before death results.

In carnivores when the animal feeds on an infected source there may be an intestinal form of the disease with fever and cramps from which animals sometimes recover.

Diagnosis

Anthrax is diagnosed by examining blood (or other tissues) for the presence of the bacteria. Samples must be collected carefully to avoid contamination of the environment and to prevent human exposure to the bacteria. Blood samples from relatively fresh carcasses will contain large numbers of B. anthracis, which can be seen under a microscope, cultured and isolated in a laboratory, or detected by rapid tests, e.g. polymerase chain reaction (PCR).

The WOAH Manual of Diagnostic Tests and Vaccines for Terrestrial Animals describes both the laboratory procedures to detect anthrax and the accepted methodology for production of vaccines.

Prevention and control

In addition to antibiotic therapy and immunisation, specific control procedures are necessary to contain the disease and prevent its spread. In particular:

• the proper disposal of dead animals is critical;
• the carcass should not be opened, since exposure to oxygen will allow the bacteria to form spores
• premises are to be quarantined until all susceptible animals are vaccinated and all carcasses disposed of preferably by incineration or alternatively by deep burial with quick lime.
• cleaning and disinfection are important as is control of insects and rodents.

Vaccination in endemic areas is very important. WOAH spells out the requirements for the manufacture and quality control of animal vaccines, in the WOAH Manual of Diagnostic Tests and Vaccines. Although vaccination will prevent outbreaks, Veterinary Services sometimes fail to vaccinate when the disease has not appeared for several years. But because the spores survive for such lengthy periods, the risk is always present.

Though anthrax is quite susceptible to antibiotic therapy, the clinical course is often so rapid that there may not be the opportunity to treat affected animals. Early detection of outbreaks, quarantine of affected premises, destruction of diseased animals and fomites, and implementation of appropriate sanitary procedures at abattoirs and dairy factories will ensure the safety of products of animal origin intended for human consumption.

Geographical distribution

Anthrax is found all over the world on all continents except Antarctica.

There are endemic areas with more frequent outbreaks, other areas are subject to sporadic outbreaks in response to unusual weather patterns which can cause spores that were dormant in the soil to come to the surface where they are ingested by ruminants, germinate and cause illnes

References

• WOAH Biological Threat Reduction Strategy
• The Center for Food Security and Public Health, Iowa State University
• Merck Veterinary Manual