Antimicrobial Resistance – Using antibiotics wisely

Calls to diminish the use of antibiotics are becoming more frequent. The aim is not to prevent the use of antibiotics, but to use antibiotics as carefully as we can, so that we can prolong the time that current antibiotics remain useful, and also comply with residue regulations.

The global problem of antibiotic resistance is of concern to all. When antibiotics are misused, used for minor ailments or to replace a lack of hygiene, antimicrobial resistance in man and animals increases.

The first well-known antibiotic, penicillin, discovered in 1928, together with sulphonamides, saved the lives of soldiers during the Second World War. The almost miraculous revival of someone suffering from a life-threatening disease, has become commonplace. Nobody expects to die from a simple infected wound anymore.

Further antibiotics were developed over the years, but the source of new antibiotics has almost dried up. Many bacteria are resistant to the older as well as newly-developed antibiotics and disease-causing bacteria infect our animals as well as us. Some analysts predict that antibiotics will only be useful for less than a century in total.

What are the underlying causes of bacterial resistance?

Bacterial factors

Bacteria are similar to the cells of our and animal bodies, in that they multiply in the same way. They also double in size, and then split into two equal halves. They also carry their genetic material in DNA. Bacteria however, also often carry a plasmid, which is a tiny extra circle of DNA. Both plasmids and the normal gene carry antibiotic resistance genes. When a bacterial cell multiplies, the plasmid doubles to form two circles, and one ends up in each new cell. One plasmid often carries resistance genes for many different antibiotics. Plasmids can multiply more rapidly than bacteria, and the extra copy of the plasmid is then donated to another bacterium.

Pharmaceutical factors

There are very few new antibiotics presently being developed. Antibiotics have to target bacteria, and not animal cells, otherwise they would be too toxic for use. There are a limited number of differences between bacterial and mammalian or bird body cells, and many of the possible disruptions to bacterial multiplication have already resulted in useful antibiotics. Most of the current research is focused on modifying existing antibiotics, and not on producing new ones. Developing new drugs is an immensely expensive exercise.

What the farmer can do

  • Dosing – Antibiotics should always be used at the correct dose. Underdosing means that the animal is not properly treated, and resistance develops more easily, than when the right dose is used. Using a lower dose to save money is false economy. Using antibiotics for “just in case” should also be avoided.  
  • Biosecurity – When an animal is bought in, one usually quarantines it for a while, to make sure that it does not carry any diseases. Disease free animals can however carry a large number of normal bacteria that carry resistance genes. These could be transferred to the bacteria causing problems on the farm, and the same antibiotic that worked for years, is no longer effective.
  • Bacteria can also be brought to a farm by visitors, feed trucks and free-living animals such as rodents. Many bacteria can spread from a person or a rodent to your livestock. Limiting visitors, cleaning feed and other trucks before they enter your premises and rodent control are all part of biosecurity measures that a modern farmer implements on his or her well-run farm.
  • Controlling or limiting antibiotic use is something that farmers already do. Withdrawal time, which is the time which must elapse between the last treatment and marketing, is well-understood. Each antibiotic remains for a longer or shorter period in the body, and this differs according to the type of animal the antibiotic has been given to. It also differs for the different products, such as meat, milk or eggs.
  • Managing the marketing antibiotic free gap already ensures skills to enable further progress. Examine your production system critically. Instead of using a blanket antibiotic cover over a long time, antibiotics could be used for short bursts, at crucial times just before many animals are likely to become ill. If a few animals become ill during the antibiotic free gap, they could be treated individually.
  • Hygiene A single bacterium rarely causes disease. There usually have to be at least 100 bacteria, and more likely a thousand or more, before an animal becomes ill. Hygiene is therefore crucial to the control of disease. The fewer bacteria that vulnerable animals encounter, the smaller their chances of becoming ill are. Dung and pus carry many millions of bacteria. Cleaning both away, as quickly as possible, diminishes the chances of disease. Rodents carry many bacteria, and they readily infect feed with urine and faeces if uncontrolled. All in and all out systems decreases the build-up of resistant bacteria on a farm.
  • Hygiene in the milking parlour is another crucial area. Ensuring that disinfectant washes are at the correct strength, the milking machine is treated according to manufacturer standards, and milking cows with mastitis last, are simple measures that anyone could implement. Feeding raw milk from cows with mastitis to calves or other livestock is not a good idea. Bacteria can multiply rapidly in milk standing at ambient temperature for an hour or two, and spreading bacteria from treated quarters around the farm, will also spread antibiotic resistant bacteria. The low levels of antibiotics in mastitic milk promote new resistance gene development.
  • Stock density should be optimized. The bacteria that cause pneumonia are coughed or breathed out, and animals that are in close contact with each other can inhale a lethal amount. Animals that are more spread out, and not overcrowded, will inhale only a few bacteria.
  • Contaminated water may also carry virulent bacteria. This may be seasonal, with a higher concentration of bacteria during times of drought. Chlorinating the water would lower the numbers. Chlorination could be strategic, during dry seasons and crucial times for general farming. Dairy farmers would be wise to ensure that their water is always chlorinated.
  • Prevention of disease by vaccination is of course one that South African farmers have enthusiastically embraced, ever since Sir Arnold Theiler founded the Daspoort Bacteriological Laboratory in 1898, which lead to the founding of the Onderstepoort Veterinary Institute. Vaccines against stock diseases are readily available in South Africa.

Implementing these managemental strategies and others that are specific to your farming enterprise, should result in lower antibiotic use, and greater profits. Biosecurity does not only protect your farm from new diseases; it will also have a positive effect on lowering antibiotic use. When prospective new animals are being considered, include the antibiotic use on the farm of origin as well as their disease status and genetic potential.

The rational use of antibiotics does not mean that antibiotic use is forbidden. Rational use means precisely what it says. A rational person understands that antibiotics have become a scarce resource, and instead of paying more for increasingly limited quantities, there is a great deal that a farmer can do to lessen the impact of antibiotic resistance.

Written by: Dr Marijke M Henton (, Specialist veterinarian (Bacteriologist) at Vetdiagnostix (