Whenever prescribing antibiotics, ideally their selection should be based on results of culture and MIC susceptibility. The minimum inhibitory concentration (MIC) is the concentration of a particular antibiotic that prevents visible growth of the bacteria in question after incubation for up to 18 hours. Bacteria are considered to be susceptible if growth is inhibited with 1/2 the average clinically obtainable blood concentration of the drug (or 1/4 peak plasma concentration).
In cases where antibiotics must be administered before culture results are obtained and/or are given empirically, it is imperative to identify the following:
1. If the patient has a treatable microbial infection
2. Location of infection
3. Identification of most likely pathogens
4. Identification of antibiotics most likely to be effective
5. Determining the appropriate route of administration
6. Determining treatment length
If at all possible, a sample of the infected material should be obtained and gram stained. This will give a minimum database of information to guide antibiotic selection. Then the antibiotic should be selected based on the knowledge of the most likely pathogen present and usual antibiotic susceptibility patterns.
Antibiotic resistance can develop in most bacterial lines by one of two mechanisms, being either chromosomal or plasmid mediated. If antibiotics are improperly dosed or given for inappropriate lengths of time then conditions favorable for these chromosomal mutations are provided. Underdosing antibiotics in either fashion kills off the susceptible bacteria and leaves the resistant ones intact, offering them a survival advantage. These resistant bacterial strains, usually present in low numbers, can now propagate freely, creating a stronger, more resistant strain.
Hospital Acquired Infections
Hospital-acquired infections are defined as infections that were not present or incubating at the time of hospital admission. Typically, these infections develop more than 48 hours after admission due to the incubation period of most hospital-acquired pathogens. As a general rule, infections are considered hospital-acquired if they develop within 30 days of hospitalization (1 year if an implant placed).
There is a vast and rapidly growing body of knowledge pertaining to hospital-acquired infections in human medicine. The veterinary literature is much more limited and consists mostly of reports describing epidemic outbreaks from isolated sources within the hospital. Little is known about the role that patient colonization and patient to patient transmission plays in veterinary hospital-acquired infections.
Epidemic vs. Endemic Infections
Epidemic infections represent those infections that develop due to contamination of a point source. It is estimated that these epidemic outbreaks represent less than 2% of all hospital-acquired infections in people. The remaining 98% of infections represent what are referred to as endemic infections. These infections occur continuously over time and come from multiple sources. Some endemic infections are not preventable and represent the minimal failure rate for certain procedures. For example, no matter how meticulous the surgical technique a small number of surgical patients will always develop surgical site infections. Other endemic infections occur as a result of preventable causes such as poor technique during intravenous or urinary catheter placement or inappropriate peri-operative antimicrobial prophylaxis.
Pathogenesis
Most epidemic outbreaks result from contamination or colonization of a point source. The contaminated source then spreads the organism to patients where it causes infections. These outbreaks can be investigated using well described epidemiologic methods in an effort to identify the point source.
Endemic infections typically begin with bacterial colonization of critically ill patients. Critical illness, particularly when combined with antimicrobial therapy, acts to alter the normal host flora. The flora of the oropharynx, gastrointestinal tract, and occasionally the skin is replaced by potential pathogens. This patient colonization represents the major reservoir for nosocomial pathogens within the ICU environment. Microorganisms are transferred from patient to patient either through direct contact or transient contamination of the environment or hospital personnel. The environment does not typically represent a long-term reservoir for microbial growth, but is instead repeatedly contaminated by colonized patients.
The high level of antimicrobial use in ICU patients predisposes to the evolution of antimicrobial resistant microorganisms. Infection with these organisms is associated with increased morbidity and mortality in numerous human studies. Numerous strategies have been proposed to limit the development of antimicrobial resistance including limiting antibiotic use, rotation of antibiotic choices, and reserving specific drugs for the treatment of hospital-acquired infections.
Prevention of Hospital Acquired Infections
Most hospital-acquired infection fall into one of the following categories:
1. Pneumonia
2. Surgical site infections
3. Catheter related infections
4. Urinary tract infections
Since these four categories represent the vast majority of infections most prevention measures are focused towards them. The most important step in preventing hospital-acquired infections is the prevention of colonization. It is believed that the veterinary staff represents the major source of this transmission. Hand washing between patient contact is the single most important preventative measure. Glove wearing augments this effect but does not replace the need to wash hands.
Pneumonia
Hospital-acquired pneumonias represent the second most common hospital-acquired infection in people but account for almost 50% of mortality. As a result, much effort has been focused on their prevention and treatment. Numerous potential risk factors have been identified including intubation, positive pressure ventilation, neurologic injury/surgery, use of H2 blockers, NG tube placement, propofol use, and sedation.
Many of these risk factors represent breaches in the normal defense mechanisms that are designed to prevent aspiration of potentially infectious material. Such breaches, especially when combined with colonization of the oropharynx by pathogenic bacteria, greatly increase the risk of gross or micro-aspiration either of which can lead to the development of pneumonia.
Surgical Site Infections
Surgical site infections (SSI) are probably the best studied of the veterinary hospital-acquired infections. A combination of duration of anaesthesia, degree of wound contamination, proper surgical technique, and appropriate antimicrobial prophylaxis represent the most significant risk factors. Of these, duration of anaesthesia and degree of wound contamination are probably the most important. Lengths of anaesthesia greater the 90–120 minutes are associated with significantly higher SSI rates. Appropriate antimicrobial prophylaxis involves administration of antibiotics so that they have peak blood levels at the time of the first incision. This requires that the antibiotics be given 45 minutes prior to the start of surgery (not anaesthesia). Antibiotics should be re-dosed every 90 minutes. Continuation of prophylaxis for more than 24 hours in wounds that are not infected is not associated with a lower rate of SSI.
One of the most effective controls of SSI in humans has been found to be prospective surveillance to determine the rate of SSI for the surgery department as a whole and reporting of individual surgeon SSI rates to each surgeon. This necessitates a high degree of confidentiality since each surgeon should be supplied with his/her SSI rate as well as the department’s SSI rate without being informed of other surgeons’ SSI rates. This approach is thought to encourage self-policing by the surgical staff, but should not be viewed as a witch hunt. It should always be remembered that surgeon who routinely fixes open fractures will have a higher SSI rate than one who does mainly spays and neuters.
Intravenous Catheter-Related Infection
Catheter related infections (CRI) are thought to occur most frequently due to catheter contamination at the time of placement. They may also occur due to migration of bacteria along the catheter or from contamination of the catheter tip by systemic bacteraemia. The vast majority of these cases resolve spontaneously upon catheter removal. True catheter related blood stream infections are thought to be rare in veterinary medicine. Prevention of CRI should focus on appropriate catheter placement and maintenance. Unfortunately, there is little research into the best way to place and manage intravenous catheters in veterinary patients.
Urinary Tract Infections
Urinary tract infections (UTI) are the most common hospital-acquired infection in people, but they are associated with the least morbidity and mortality. It is important to distinguish between colonization and contamination when making the diagnosis of UTI. Intermittent catheterization has been shown to be superior to indwelling catheters in people but no studies have examined this in veterinary patients.
Control of UTI should focus on minimisation of urinary catheter use, and adherence to sterile technique during placement and maintenance. Antimicrobial prophylaxis does not prevent the development of UTI but rather guarantees that the infection will be resistant the antimicrobial employed and should be avoided.
Treatment of Hospital Acquired Infections
Treatment of hospital acquired infections requires a knowledge of the antimicrobial resistance pattern of the likely pathogens. Such resistance patterns are highly hospital specific and result from the individual hospital’s antibiotic choices and the intensity of antibiotic use. A review of several months culture may aid in the appreciation of resistance patterns. Suspected cases of hospital- acquired infections should be treated with an appropriate antimicrobial. This should be continued until culture results are available and antimicrobial therapy can be further refined.