THE word antibiotic comes from two Greek words: anti – “against”, and bios – “life”. The practice of using natural substances was seen as far back as 2000 BC. Antibiotics are chemical substances produced by various species of microorganisms that suppress the growth of other microorganisms and may eventually destroy them.

Ancient people used diverse compounds such as vine plants and mud to combat infection. These are all documented in ancient manuscripts and these substances helped the healing process in wounds. Plants, through the passage of time were used to great effect. Plants contain antibacterial agents and were easily collected, treated and used as ointments.

Scientists in the mid 1940s showed that over 3,000 species of plants contained antibacterial agents and were used medicinally for centuries. Cheese mould was also shown in early times to have a beneficial effect in combating infection.

In the 1920s, a scientist named Alexander Fleming accidentally discovered the antibiotic penicillin and it is a derivative of the mould or fungus Penicillium. He showed that this had great antibacterial effect and thus began the start of antibiotics.

RESISTENCE CONCERNS

The term antibiotic is now used to include synthetic and semi-synthetic compounds. Penicillin enjoyed great success for a period until the bacteria developed methods of combating it. It was then imperative to introduce new antibiotics and this proved the case.

Other families of antibiotics, such as aminoglycosides, tetracyclines, cephalosporins, sulphonamides, quinolones and many others have been created to combat the increasing diseases in our world today. The use of antibiotics is limited due to the ability of bacteria to inactivate them.

When this happens, the much used term ‘resistance’ comes into prominence. The problem of resistance has become a major issue as over-use of these substances or inappropriate use has occurred over the past 30 years

The agriculture industry has been blamed for the spread of resistance from the animal population to the human population, but the agriculture industry is not the only reason for this.

Lately, it has been shown that the most resistant of bacteria, notably the Staphylococci, have been associated with hospital acquired infection. The organism MRSA is one which springs to mind, although this organism has no links with the agricultural industry.

Bacterial resistance to antimicrobials has become a global issue which is affecting veterinary treatment of animals, welfare of animals and can have economic implications. Antibiotic resistance is widely recognised as a serious threat to global health in the 21st century. The use of antibiotics in animals can lead to selection of resistance genes that can cause serious risk to human health due to the lack of ability to treat infections.

Resistant bacteria found in agricultural animals used for human consumption is a major concern for veterinarians and health professionals alike. If antimicrobial resistant genes were integrated into the genome of human pathogens and passed on through consumption of the animal or its by-products to humans, the ability to control bacterial infections becomes more complicated and zoonotic potential is very high.

Clinical practice in veterinary and human medicine has been radically improved due to the introduction of third generation cephalosporins in the early 1980s. However, as a result, resistance has emerged to extended spectrum cephalosporins.

Antimicrobial resistance has been identified in pathogenic and commensal bacteria in horses, including E. coli and Staphylococcus aureus.

This will have significant implications in the treatment of the animals but also poses significant implications to public health due to antimicrobial resistance in the bacterial flora of both horses entering the food chain along with those who are companion animals.

The existence of antibiotics is being threatened on a daily basis by resistant bacteria.

The judicial use and choice of antibiotic is vitally important both for humans and animals. Identification of the pathogen in a laboratory and a reliable sensitivity test will help in the choice of a suitable antibiotic.

Using antibiotics can also promote the emergence of other pathogens such as fungi. This is a result of killing the normal flora and allowing the opportunity for colonisation by fungi.

At the Irish Equine Centre, we have seen an increase in the number of fungal infections in horses over the past few years.

Antimicrobial resistance in pathogenic bacteria is a global problem and it is a concern for both animal and human health. The use of antibiotics in food producing animals has become a concern as they can contribute to the antibiotic resistant bacteria in animals.

Resistant bacteria have the potential to spread to humans through the food chain. Additionally, antibiotic resistance genes have the potential to transfer to human pathogenic bacteria and cause antibiotic treatment failure in human medicine, which can range from minor to life threatening.

The minimum inhibitory concentration (MIC) is defined as the lowest concentration of an antimicrobial agent required to inhibit the growth of an organism. MICs are considered the gold standard for determining the susceptibility of organisms to antimicrobials and detecting the resistance mechanisms.

A recent study carried out by the Irish Equine Centre investigated the resistance patterns of equine pathogens to a range of antibiotics over a 15-year period. This involved studying the most commonly associated pathogens of horses: Klebsiella pneumonia, Pseudomonas aeruginosa, E. coli, Staphylococcus aureus and Streptococcus equi or ‘Strangles’. A total of 2,100 individual MIC tests were analysed.

RESISTANCE PATTERNS (over a 14-year period)

Escherichia coli (green graphs)

The graphs above show the minimum inhibitory concentration (MIC) increase in resistance of E. coli to the antibiotics Gentamicin, Enrofloxacin, Amikacin and Tetracycline. Over a 10-year period, Gentamicin had an increase in resistance of 111%, Enrofloxacin increased by 42%, Tetracycline increased by 48% and Amikacin increased by 74%.

Klebsiella pneumoniae (red graph)

The graph above shows the minimum inhibitory concentration (MIC) increase in resistance of Klebsiella pneumoniae to the antibiotic Gentamicin. Over a seven year-period, Gentamicin had an increase in resistance of 966%.

Staphylococcus aureus (blue graph)

The graph above shows the minimum inhibitory concentration (MIC) increase in resistance of Staphylococcus aureus to the antibiotic Ampicillin. Over a seven-year period, Gentamicin had decreased in resistance by 43%.

As per the previous graphs, there is serious proof that antibiotic resistance is a major problem for the equine industry worldwide. In the USA, out of 506 drugs in development, only five are antibiotics. The reasons for this are a lack of return for pharmaceutical companies on their investment, a lack of research funding, and also problems associated with efficacy of the products.

Recent joint policies by the Departments of Agriculture and Health will help develop strategies for the appropriate use of antibiotics. There is also a need for new vaccines and also an awareness of biosecurity in equine premises on a daily basis.