What dangers lie within your dental units? - Veterinary Practice
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What dangers lie within your dental units?

Harmful biofilms found within veterinary dental units could be a health hazard to veterinary professionals

As a veterinary dentist, I am preoccupied with biofilms. The commonest biofilm I come across is, of course, plaque: the thin layer that forms on teeth within minutes of any scaling procedure and which is the primary agent responsible for periodontal disease.

Plaque, like all biofilms, consists of various bacteria enmeshed in a hydrated matrix. This matrix has many important roles – but provides protection for the bacteria (from antibiotics and many disinfectants) and allows complex communication between the contained individual bacteria, the different colonies of bacteria and different species of bacteria. This helps provide further protection by “sharing” resistance and allowing phenotypical changes to occur.

However, biofilms are not limited to the mouth. They are seen on indwelling catheters and other medical devices. And that is the tip of the iceberg; biofilms are found just about everywhere from oil pipelines in frozen Alaska to the lid of your kitchen waste bin. One important biofilm, which is often completely ignored in veterinary practice, lines the pipes within your dental units (dental unit water lines; DUWLs). These are generally quite narrow and create a laminar flow in use. They are also virtually impossible to fully dry out. These conditions are almost designed to encourage biofilm development.

The biofilm “sludge”, if dislodged, can pass down into your handpieces, resulting in blockages and damage. This is not an uncommon reason for handpieces to be sent for service as “faulty”. From a health and safety viewpoint, far more important are the bacteria (and bacterial products) that are contained within the biofilm. It is not surprising that water sampled from DUWLs typically yields fungi and protozoa. In addition, soil and aquatic environmental bacteria, typically gram-negative, are in abundance. Opportunist pathogens such as Legionella spp., Pseudomonas aeruginosa and non-tuberculous Mycobacterium spp. are also often found.

Given the abundance of gram-negative bacteria, it is not surprising that high levels of endotoxins are often found. On the human dental side, research into the potential harmful effects of contaminated DUWLs has yielded a lot of information. However, until now, I am unaware of anyone even considering the possible harm that could result from contamination of veterinary DUWLs. The reason for this appears to be summed up by the veterinary technician who stated that “infected water lines don’t matter, as you are treating animals not people”. Unfortunately, this widespread attitude portrays a frightening ignorance of the issues and potential problems.

Unlike our human dental colleagues, our patients are intubated. Therefore, their airways are generally pretty well protected from the harmful bacteria within our DUWLs. However, it is not uncommon for veterinary surgeons, nurses and assistants to perform dental procedures (or be within a two- to three-metre radius of a dental procedure) without wearing effective face masks.

All these veterinary staff are potentially victims of the contaminated aerosols produced by the dental machines. It does not matter whether the staff are operating on human or veterinary patients – many of the risks come from the dental machines, not the patients.

Let us consider the findings of the human dental research. Legionella spp. were isolated from 68 percent of DUWLs, with L. pneumophilia present in 8 percent. Pseudomonas aeruginosa was isolated from 24 percent of DUWLs. Worryingly, despite “thorough” cleansing of the units, it was repeatedly isolated. This correlates with the fact that Pseudomonas can grow in distilled water or even dilute chlorhexidine and iodoform solutions. Non-tuberculous Mycobacterium spp. were found at a mean level of 365 colony forming units/ml.

Endotoxins often originate in the cell walls of gramnegative bacteria (the most common bacteria in DUWLs). Endotoxin levels in DUWLs reached 2,560 EU/ml; this can be compared to 66 EU/ml in tap water or <0.25 EU/ml in water for injection.

Given that water from DUWLs is commonly used in veterinary practice for surgical tooth extraction, there are obvious worries about infection and the effects of the endotoxins on wound healing. However, the staff health issues are perhaps more pressing. Detailed epidemiological studies are rare. There are some proven links directly between disease, even death, and infections derived from DUWLs in the human sphere.

Clearing protective biofilms is difficult; it requires a “shocking” procedure followed by ongoing maintenance. With the high levels of contamination recorded in veterinary practices occasionally, even when using the VetDentist Starter Kit, a second deep clean is required. Follow-up consists of regular quarterly testing of the water. Conventional bacteriology samples give inaccurate results – special low nutrient culture media, at room temperature, has to be used.

Dentists and their staff have higher rates of respiratory infections than the general public; they also have higher rates of seropositivity for Legionella antibodies than the general public, with titres appearing related to the time spent in clinics. Occupational asthma rates also show some links to clinic exposure times. Most alarming are the deaths of a patient in Italy and of a dentist in California directly linked to the Legionella infection in the DUWLS.

My concern is that it might take a veterinary nurse’s death from Legionella for our profession to wake up to the dangers in our dental units. Apart from the personal, moral and ethical tragedy that would represent – what about the potential litigation issues?

Given that effective control measures are mandatory in the human dental field and that an effective preventive system would only cost around 40p per day, can you imagine what the likely punitive damages would be on a business that sacrificed the life of a staff member to save 40p per day?


The most useful standards are the ASTM F2100–11 (2018). These cover the possible penetration of aerosol-borne bacteria. Most single layer masks used in veterinary practice would not reach even Level 1 and are therefore suitable only for dry procedures.

ASTM Level 1 implies a bacterial filtration efficiency (BFE) of ≥95 percent and a particulate filtration efficiency (PFE) of ≥95 percent at 0.1 micron. However, there is little protection from splashes of liquids. This equates to the European Standard of EN14683 Rating – Type II. Remember that this standard means that potentially 5 percent of airborne pathogens will still pass through the mask.

ASTM Level 2 has an increased BFE and PFE rating of 98 percent and includes a moderate degree of water resistance (120mmHg). This is roughly equivalent to the upgraded European Standard of EN14683 Rating – Type IIR.ASTM Level 3 maintains the BFE and PFE at 98 percent – but increases the fluid resistance rating to 160mmHg of pressure.

The highest level of protection, which is used when there is likely exposure to pathogens such as TB, is NIOSH Approved N95, which provides 99.7 percent BFE and PFE; at this level, only 0.3 percent of pathogens might pass through the mask, together with fluid resistance at 160mmHg of pressure.

For routine veterinary dental procedures, I would recommend at least adopting ASTM Level 2 (or EN14683 Rating – Type IIR (the R is important!)) standard masks.

For “dirty” mouths, or where Pseudomonas or similar infections are suspected, I would recommend ASTM Level 3 or above.

For information on testing DUWLs and how to protect staff and patients, visit: VetDentist.info

Bob Partridge


Bob Partridge, BVM&S, DipEVDC, MRCVS, is a European and RCVS Recognised Specialist in Veterinary Dentistry. Bob graduated from the Royal Dick in 1984 and became a European Diplomate in 2011. He teaches around the world and sees cases at his referral centre in Leeds, UK.

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