Is there a link between COVID-19 and oral hygiene?

Professor Martin Addy and Dr Victoria Sampson look at the connection between good oral hygiene practices and COVID-19Professor Martin Addy and Dr Victoria Sampson look at the connection between good oral hygiene practices and COVID-19. 

A recent review (Sampson, Kamona and Sampson (2020) Could there be a link between oral hygiene and the severity of SARS-CoV-2 infections? BDJ 228: 12: 971) reminds the healthcare professions that oral micro-organisms can complicate viral respiratory tract infections, including COVID-19, through superinfection. The evidence has been derived, over many years, from influenza outbreaks, pneumonia in the elderly, ventilator associated pneumonia and now COVID-19. These superinfections by oral micro-organisms are usually – but not always – of bacterial origin. It could be a result of the aspiration or inhalation of salivary bacteria. This is possibly potentiated by an increase in the microbial load in the mouth.

Research has shown that 80% of patients in ICU exhibiting severe complications of COVID-19 also suffered bacterial superinfections. This shows that these superinfections account for a significant proportion of the morbidity and mortality in primary viral respiratory infections. This article will consider why the microbial load throughout the mouth and in saliva may increase during a COVID-19 infection. It will debate how oral care advice and oral hygiene products may help to prevent superinfection.

The microbiology of the oral cavity has and continues to be studied extensively. This is due to the two most prevalent oral conditions, namely caries and periodontal diseases. Much of the research has been in vivo, allowing findings to be directly applied to the clinical setting rather than extrapolating data from studies in vitro. Studies of plaque growth and salivary bacterial numbers are particularly useful when considering increased oral microbial load as a potential route for superinfection.

High levels of plaque

Virtually any interference with normal oral function and hygiene results in increases in bacterial plaque and salivary bacterial counts. At the extreme, suspension or reduction of oral hygiene practices has both effects. An example of this was recorded in jaw fracture patients with inter-maxillary fixation when salivary bacterial counts increased threefold and plaque scores nearly reached the maximum of the index used.

Studies in the elderly in residential homes showed very high levels of plaque, gingivitis, root caries and denture stomatitis. So much so that it has been concluded that one in 10 pneumonia-related deaths in care homes could have been prevented by improving oral hygiene.

At the other end of the spectrum, even the placement of a partial denture increases the rate of plaque accumulation and all studies of subgingival restorations have reported a reduction in gingival health at the site. These examples relate to adverse effects on oral bacteria. However, similar findings can be found with oral fungi. The carrier rate for candida is about 40% of the UK population: for denture wearers it is nearer 50%.

Moreover, in teenagers undergoing orthodontic treatment, the density of candida on mucosal surfaces increased during the appliance phase. The same applies to plaque scores. It returned to normal levels after completion of treatment and appliance removal. Importantly, some patients who were negative for candida became carriers during the appliance phase and returned to non-candida status when appliances were removed. There is a quote often made to dental students: ‘Candida is a clever clinician. It can detect local changes in the oral environment or systemic upset when it will cause true oral candida infections’.

Oral hygiene in hospitalised patients

These few examples of how the oral microbiota can be adversely affected should now be considered in relation to COVID-19. Data is not available on whether the oral flora is quantitatively or qualitatively altered in those infected with coronavirus. However, based on what has been discussed above, some reasonably sound points can be made.

The metagenome of patients severely infected with COVID-19 showed periodontopathic bacteria, namely Prevotella intermedia, Fusobacterium nucleatum and Staphylococcus aureus at abnormally high levels compared to patients without COVID-19 complications.

For those infected with the virus but symptomless, changes to the oral flora, besides the now presence of the virus in saliva, would seem unlikely. Those with symptoms are likely to show changes in the flora consequent to an adverse influence on oral hygiene practices. Indeed, those with severe respiratory symptoms may suspend oral hygiene practices entirely.

Hospitalised patients are likely to have an oxygen mask over the nose and mouth. They can even be intubated. Either of these would be expected to have significant adverse effects on the oral flora and predispose to superinfection. Given that a substantial number of deaths from COVID-19 can be attributed to superinfection by oral micro-organisms and not the virus, can anything be done to help prevent this phenomenon?

‘Wash your hands and brush your teeth’

Much has been discussed, written and recommended on ways to help prevent the spread of or infection by coronavirus. These include reducing contact spread by hand washing, and salivary droplet spread by oral hygiene practices. This involves toothpaste and/or mouth rinses. And recently in the UK, also the use of mouth and nose coverings. The slogan ‘wash your hands and brush your teeth’ should possibly be extended to say ‘wash your hands, brush your teeth and wear a mask’.

All of these recommendations have been based largely on extrapolations of data derived from studies in vitro or of surface disinfection. Mask wearing stems from decades of argument over their value. Here the debate over oral care products in helping prevent superinfection in COVID-19 cases can be based on a plethora of studies on their action on oral microbes – mainly bacteria, in vivo.

A lengthy review of the numerous antimicrobial agents present in toothpaste and mouth rinses is not necessary here. Instead, a summary of what they could achieve to reduce the likelihood of superinfection in COVID-19 patients will be presented.

Firstly, studies based on salivary bacterial counts show that antimicrobial compounds in toothpaste – notably detergents – and a range of antiseptics in mouth rinses reduce salivary bacterial load considerably and for variable time periods. Examples include: chlorhexidine in rinses exerts action for 12 hours and toothpaste detergents 3-5 hours. Another is povidone iodine in less than one hour.

Maintenance of oral care

In the inter-maxillary fixation study cited above, a chlorhexidine rinse used twice a day reduced baseline counts markedly. In comparison marked increases from baseline in salivary bacterial were reported in the control, saline rinse group. Secondly, studies of plaque and gingivitis show some toothpastes and mouth rinses reduce both parameters significantly compared to controls.

Examples would be triclosan and stannous fluoride in toothpaste and chlorhexidine and delmopinol in rinses. Although as debated in other articles, many agents in toothpaste and mouth rinses should kill coronavirus. These data lead onto what must be the major purpose of this article – to emphasise maintaining oral care across the whole UK population. But more specifically, in COVID-19 cases through the use of toothpaste and mouth rinses.

Thus, the dental team should continue to play their frontline role in advising twice daily, two minute tooth brushing with toothpaste. This is alongside the recommendation for the adjunctive use of mouth rinses when deemed necessary. Denture hygiene advice similarly should be reinforced by the team. Particularly at this time, the dental team is so important. It is best placed to provide oral care advice directly or indirectly to large numbers of people. This includes to those they consider at higher risk of COVID-19 infection.

For those individuals known to be infected by coronavirus, oral care needs to be provided through all healthcare professions. For COVID-19 cases self isolating at home, the actual practice of oral care is down to the individual. In severe disease, usually found in hospitalised patients, the oral care needs to be delivered by nurses. This is particularly important for those on ventilators.

Minimal media coverage

Indeed, chlorhexidine mouth wash swabbed around the mouth 2-3 times a day has been adopted prevent ventilator-associated pneumonia. Unfortunately, the potential role of oral care through the use of oral hygiene products both to help prevent both primary infection by the virus and superinfection by oral micro-organisms is hampered by a seemingly lack of knowledge in many healthcare groups.

This, with respect, appears to have arisen from a lack of knowledge or interest from the powers that be. There is also minimal coverage in the media. Thus, the slogan proposed at the beginning of this article to help in COVID-19 – ‘wash your hands, brush your teeth and wear a mask – is, to a majority of the population and, outside the dental team, a large proportion of healthcare workers, only known as ‘wash your hands and wear a mask’.

In conclusion, oral health should not be abandoned amidst this pandemic. Instead, it should be improved through toothpaste and mouth rinses to reduce the bacterial load in the mouth. Poor oral hygiene has shown a direct link with systemic disease and respiratory bacterial infections long before COVID-19. This must not be forgotten. If oral bacteria are allowed to increase, they could possibly introduce bacterial superinfections through aspiration or inhalation of salivary bacteria, creating further complications.


Martin Addy, emeritus professor, University of Bristol and Dr Victoria Sampson, dental practitioner, London.

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