Anthrax (Miltsiekte)Friday, 31st May 2019
By Dr Mackie Hobson
Anthrax is a highly contagious disease of both animals and humans caused by the bacterium Bacillus anthracis.
The disease was much more prevalent in the early 1900’s with major outbreaks up until the 1960’s after which the number of outbreaks reduced with the development of a vaccine and the use of penicillin antibiotics.
Outbreaks do however still occur and sporadic point outbreaks in animals are common in the Northern Cape Province and the northern Kruger National Park (KNP). Outbreaks have occurred during 2006 to 2009 and again during 2010 to 2014 in Northern Cape. At least nine major outbreaks have occurred in KNP since the 1960s. The most recent outbreak close to Angora goats have been in Lesotho.
Cattle, horses and donkeys are more resistant to anthrax than sheep and goats with goats being the most susceptible. Browsers are more susceptible than grazers.
How does Anthrax spread?
The success of the disease is due to the fatal septicaemia and subsequent dissemination of the spores when the carcass is opened due to the exposure to oxygen. Putrefaction and CO2 inhibits sporulation. Under good conditions spores can almost survive indefinitely but usually considered to be 3 to 4 years (possible to remain viable for hundreds of years). Even tanning and curing of hide does not free leather of spores.
The spores do not survive in a carcass for longer than 3 days at temperatures of >25-30C. The spores survive longer in skin (2 weeks) and bone marrow for a week.
When a carcass is opened the spores are liberated into the environment. These spores may be spread by water and vectors such as birds, animals, insects and humans. Even blowfly can distribute the spores 63km from a carcass at 2,35km a day. The droplets from the blowfly are usually deposited n twigs at 1-3m above the ground and hence browsers being more susceptible to the disease.
Infections usually peak during dry summer seasons although in the Kruger the peak appears to be late winter (before the rains) when animals congregate around watering holes.
Humans can be infected:
Butchers, workers in tanneries, handlers of wool and mohair may be most at risk. Infection is usually per cutaneous. In Zimbabwe in 1979-80 there were 10 000 human cases and 100 deaths. Biting flies may also spread the disease.
Pulmonary anthrax or woolsorter’s disease occurs when dust carrying spores from hair/wool is inhaled. Usually all untreated pulmonary cases are fatal.
The cutaneous form (usually 95% of infections) enters through a skin abrasion or insect bite. The incubation period is 1-12 days. Cutaneous lesions include a carbuncle or pustule. Central necrosis of the lesion occurs and ulceration follows. 90% of lesions recover and in 10% it spreads to the regional lymph nodes.
Different animals are affected in different:
Animals resistant to infection: dogs, pigs
Animals easily infected but resistant to the toxin: chimps, rabbits, mice, monkeys , goats, sheep, horses and cattle.
Incubation period varies from 1-14 days.
In goats, cattle, sheep, kudus, roan antelope and impala the disease is usually quick (under 2 hours) and the animal is found dead without signs.
Where signs are seen (usually wild herbivores) terminally the front legs extend rigidly and head is pulled back (torticollis). The course of the disease is usually less than 72 hrs.
- muscle tremors,
- dyspnoea, congestion of mucous membranes,
- Ruminal stasis and collapse.
- Blood stained fluid from nostrils, mouth and anus.
- An oedematous swelling occurs of the throat and neck follows primary infection of the pharyngeal tissues
In humans Ciprofloxacin or doxycycline are used as first line-treatment. Penicillin regimes may be used when antimicrobial testing confirms penicillin susceptibility. Antibiotics should be taken for 60 days to ensure protection from anthrax spores that may activate in the body
Post mortem lesions depend on the route of infection.
In ruminants the most consistent finds are:
- Rapid decomposition of carcass
- Incomplete rigor mortis
- Oozing of blood stained fluid from nose, mouth, and anus.
- Petechiae and echymoses (bleeding) throughout the carcass.
- Pulmonary oedema
- Blood tinged serous fluid in peritoneum, pericardial and pleural cavities
- Oedema and haemorrhage of lymph nodes
- Enlarged pulpy spleen
- Haemorrhage of gastro-intestinal tract
- Blood smear
Usually not able to see bacteria in blood smears of live animals. Cultures of the blood may identify the bacteria.
If a blood smear is not possible cut off the tip of the tongue or regional superficial lymph nodes are sample of choice in a post mortem. If Anthrax is suspected the carcass should not be opened due to the spread of the spores.
If PM opened then sample of spleen and lymph nodes.
- Vaccination (last 9 months, done annually in risk areas. Immunity develops after a week). In an outbreak infection can be controlled within 10 days by vaccinating the herd.
- Carcass must be burned (If not then buried 2m deep covered by lime)
Infectious Diseases of Livestock: Coetzer, Thomson and Tustin
National Institute of Notifiable diseases