Anthrax – do we still need to worry?

Anthrax is an often-fatal disease caused by the bacterium Bacillus anthracis. It can affect both animals and humans and is found worldwide on all continents. Anthrax is considered by many in Europe as a disease of the past as outbreaks are rare and sporadic in this region. However, incidences do still occur and, as it is a World Organisation for Animal Health (WOAH) listed disease, all incidents must be reported. In Great Britain (GB), anthrax is a notifiable disease, and all suspicion of disease must be reported to the competent authority.

Agent

B. anthracis is a Gram-positive rod-shaped bacterium that on contact with oxygen can form dormant, highly resistant spores that can persist in soil, wool and hair for decades. It is this resistant nature and the fatal consequences of inhalation of these spores that have made it a plausible agent of biological warfare and bioterrorism. In 1942, weaponised anthrax was tested on a remote Scottish island, and such was the resistance of the bacterial spores that the island was not declared safe until 1990 after a significant decontamination effort in 1986.

Once in the body, [anthrax] spores are “activated” into the vegetative form of the bacteria which can then multiply and begin to produce toxins

Anthrax spores are not invasive and thus require an entry route into the body, for example through small cuts and wounds or through inhalation or ingestion. Once in the body, the spores are “activated” into the vegetative form of the bacteria which can then multiply and begin to produce toxins.

Clinical signs

The toxins produced by B. anthracis bacteria give rise to the clinical signs seen once an animal (or a person) has become infected. Infection in grazing animals often occurs where the soil has been disturbed by flooding, landslides or excavation, most notably in areas where there have been previous incidents. This disturbance brings any spores to the surface of the ground where the spores may be either inhaled or ingested by grazing animals. Infection, both in animals and humans, can also occur when spores are present in food, hides or wool and are aerosolised or ingested, or come in contact with wounds when these products are handled.  

Anthrax generally causes high mortality, especially in herbivores. Most mammals, and some birds (eg ostriches), can be affected by the disease but to different severities. Cattle are often among the animals worst affected, while dogs and pigs are among those mammals that are affected the least.

Disease type

The disease can present in several forms in animals. Often in peracute and acute disease, the animal is found dead with no obvious premonitory clinical signs. In acute disease, if detected before the animal’s death, tremors, fever and respiratory distress may be evident. Further, blood clotting may be reduced resulting in blood seeping from orifices which, if the temperature is appropriate, increases the risk of spores forming when bacteria in the contaminated blood come into contact with oxygen. While black tarry blood leaking from orifices is considered a classic sign of anthrax infection, it is seen in a minority of cases and its absence cannot rule out anthrax.

Subacute disease is characterised by similar signs, but with a slower progression. A progressive fever may develop, and the animal may show inappetence and malaise followed by weakness, collapse and death. Milking animals can show milk drop or cease milk production altogether. Horses can have colic for several days before they die while also showing fever and malaise. Both horses and pigs may show a hot and painful swelling around the throat. Some pigs will appear unwell but then show a complete recovery, often only to relapse and die some weeks later if they become stressed.

Chronic anthrax is not commonly seen. Carnivores that have ingested contaminated meat can get an intestinal form of anthrax characterised by fevers and cramps, and some may recover completely from this.

Diagnosis

Differential diagnosis

In GB, suspicion of anthrax is most commonly reported in the sudden death of grazing livestock. It is important in these instances to rule out any more common causes of death in these animals as very young or chronically unwell animals are more likely to have died from other causes. Periparturient animals should be checked for signs of dystocia and recently dead, bloated carcasses may indicate animals that have succumbed to bloat. The carcass and local area should be checked for sources of poisoning and for signs of scorching, in case of sudden death by lightning strike. The condition of the ground should also be assessed: signs of fitting/scrabbling when the grass is lush may indicate hypomagnesaemia, and recent spread of chicken litter, particularly where several animals are affected, may increase suspicion of botulism.

Suspicion of anthrax is most commonly reported in the sudden death of grazing livestock. It is important in these instances to rule out any more common causes of death

Conversely, if the ground has recently been flooded or dug over, this may increase suspicion of anthrax, as spores may have recently been brought to the surface. Equally, whether indoors or outdoors, several animals with the same food source dying at once could indicate anthrax contamination of fodder once other causes of death have been ruled out (such as electrocution in the milking parlour).

Laboratory diagnostics

Animals that die of anthrax usually have large numbers of anthrax bacteria in their blood; however, B. anthracis in its vegetative form is not very resilient. Conditions caused by putrefying bacteria in a carcass mean that B. anthracis is unlikely to survive in the blood after 72 hours at 10°C or 24 to 48 hours between 25 and 40°C. Therefore, when investigating suspicion of anthrax, it is important to sample the carcass as soon as possible after death. Similarly, B. anthracis doesn’t survive for long in the surrounding environment unless in its sporulated form, in which case, as described above, it can survive for years.

Anthrax bacteria can be quickly identified by microscopic examination of a stained blood smear from a recently deceased animal by trained personnel. When anthrax is suspected in a case of sudden death it is extremely important that the carcass is not opened, or a post-mortem examination performed, before the presence of B. anthracis has been ruled out. This is both for the safety of the investigating vet and to prevent spread into the surrounding environment. A few drops of blood are to be taken after nicking a superficial vein by a trained investigator wearing appropriate personal protective equipment (PPE).

When anthrax is suspected in a case of sudden death it is extremely important that the carcass is not opened, or a post-mortem examination performed, before the presence of B. anthracis has been ruled out

Laboratory diagnostic techniques include culture and isolation of B. anthracis in specialist laboratories and detection by molecular techniques such as PCR. Serum-based antibody tests are also available but are more often used as research tools rather than for diagnostics.

A pen-side rapid immunochromatographic test (ICT), which detects part of the circulating anthrax toxin in the blood of infected animals, has been developed by the US Naval Medical Research Center. The ICT allows rapid results in the field with limited equipment and is now being used by veterinary authorities in several countries (Burans et al., 1995; Hornitzky and Muller, 2010). Vaccines against anthrax are available and successful, often used for the control of anthrax in areas where the disease is endemic or areas that are deemed as high risk.

       

Current status in Great Britain

Although anthrax is now very rare in GB, occasional cases arise where spores are still present in the soil. The latest case was in 2015 in two cows on a farm in Wiltshire that had a history of previous cases (most recently in 1996). Before the 2015 case, incidents occurred in South Wales in 2006, where two cows died, and then in Wrexham in 2002.

An epidemiological investigation of the 2015 Wiltshire case found that the pasture the cows had been grazing on was 1.5km downstream of a former tannery site where excavation work had been carried out close by. This area and the farm were placed under quarantine, which was lifted on the farm after 20 days, but for the next three years the farm had to vaccinate animals grazing the affected areas and only feed forage from these areas to vaccinated animals, and were not able to use the areas for arable purposes (Public Health England, 2017).

These types of restrictions, along with safe carcass disposal, are essential to reduce contamination of the environment and stop any spread of the disease to other animals or people when these isolated incidents occur. Ensuring that GB is entirely free of the risk of anthrax from dormant spores is almost impossible but stringent surveillance and control measures such as those outlined above form the backbone of GB disease control policy.

Conclusion

Anthrax is a zoonotic disease preventable by vaccines and treatable by antibiotics but is nevertheless more often fatal, especially in cattle. It is still a genuine threat to both human and animal health in GB and worldwide. Therefore, anthrax should remain firmly on livestock keepers’ and vets’ radar when faced with sudden death in livestock.