Examining the myth that rodents do not get post-operative infection

Why may the theory have arisen.

There are two possible reasons that would explain why numerous research personnel working with rodents hold this opinion: either post-operative infection does not occur in these species or there is a failure to diagnose it when it does occur. It is important that we consider these two possibilities and weigh up the evidence.

How does infection become established?

The following criteria contribute to bacterial infection becoming established.

• Sufficient size of a bacterial inoculum: for infection to occur an inoculum of the size 105 / g or per ml of body fluid is required. The provision of a large catchment area for microorganisms to reach their critical number facilitates this. Long duration of surgery will also contribute; the number of bacteria entering the surgical site is proportional to the time taken for surgery.
• Nutrients for growth: large quantities of nutrient material, such as blood or serum, must be present to provide a suitable medium in which bacteria can multiply.
• Surgeon’s skill level: the experience and expertise of the surgeon regulates the time taken for surgery and the magnitude of tissue trauma resulting from it.
• Decreased activity of the host’s defences: this stems from factors such as the host’s age, state of health, nutritional status, vitamin deficiency, corticosteroid levels (released by the body in stressful circumstances or administered as medication), and concurrent treatment with chemotherapy or radiation therapy.

Is there support for the theory that rodents are resistant to infection?

Referring to the list above, the risk of post-operative infection can be decreased by using animals that are young and healthy, creating small surgical incisions, ensuring surgery is of a short duration and that the quantity of blood and necrotic tissue present when it is finished is small. Rodent surgery often fulfils these criteria, largely because the majority of rodents used in experimental procedures are of high health status and are small. In addition, research workers become experienced and skilled in the surgery they are performing, particularly when it is a single-specialised technique and this limits tissue trauma and ensures surgery is rapid. It is certainly true that the risk of post-operative infection is often reduced by the nature of rodent surgery itself.

If we examine the published literature it becomes apparent that rodents are frequently used successfully as models for wound infection, and in addition, a few studies have been performed to examine the responses of rodents to post –operative wound infection. These studies show that postoperative infection can become established in these animals and suggest that if we only look for the typical gross signs of infection (those that occur in humans) we may be missing the indicators that occur specifically in rodent species.

Could there be a failure to diagnose postoperative infection?

There are a multitude of factors that may have contributed to a failure to diagnose post-operative infection in rodents.

• In cases where post-operative complications arise and the easily recognised signs of infection, e.g. pus or death with peritonitis, are not observed, it is assumed that there is no infection present. Many personnel carrying out routine experimental surgery do not have extensive medical or veterinary training and would fail to recognise the more subtle signs of wound infections and transient infection post-surgery, such as malaise and behavioural changes.
• Post-operative infection does not necessarily cause the death of an animal. It may merely result in depressed success rates of a technique. The link between surgery and the sequelae may not be made, particularly if signs of infection are inapparent.
• In some cases, accurate records of post-operative complications or mortality are not kept, and so the scale of a problem goes unrecognised, and it is not considered necessary to diagnose the cause. If post-mortems are not routinely undertaken and performed by trained personnel, this will contribute to under-diagnosis of disease.
• Batch surgery has been carried out in some facilities using the same set of surgical instruments for all the animals and using an “aseptic” technique that becomes progressively more compromised as the surgeon goes from one animal to the next. The justification put forward for this practice was that batch surgery made true aseptic technique impractical. In other cases the belief that ‘post-operative infection does not occur in rodents’ has been used to justify practices such as using non-sterile instruments or instruments which have been “flamed” in a Bunsen burner to perform rodent surgery. In both these circumstances there would be no advantage in diagnosing post-operative infection, as this would acknowledge that the procedures used were not satisfactory and needed to be revised.

Conclusion

To some extent there is evidence to support the belief that post-operative wound infection is observed less often in rodents, but it is not the case that rodents are inherently resistant to wound infection. It is therefore advisable to take reasonable precautions to limit wound contamination, as described earlier. When certain types of procedure (e.g. implanting catheters into blood vessels) are undertaken, full aseptic technique must be adopted.