Of all the current policies for emergency disease control (EDC), the Australian and New Zealand governments employ the most authoritative approach with their AUSVETPLAN, which forms the basis for plans in other countries, including Canada, the USA and Mexico, as well as the Office International des Epizooties (OIE) – the world organisation for animal health.

The AUSVETPLAN classifies eligible disinfectants into four categories and specifically cites eight products by name as being particularly suitable for EDC. As shown in Table 1, nearly all the disinfectants selected by the AUSVETPLAN are so called ‘basic’ chemicals. Only one – Virkon S – is a modern formulated brand.

This article reviews the stringent requirements of disinfectants for EDC use and considers to what extent the products named in the AUSVETPLAN measure up.

The key emergency diseases

The OIE lists 15 animal diseases as ‘List A’. These are defined as ‘transmissible diseases with the potential for very serious and rapid spread, irrespective of national borders, that are of serious socio-economic or public health consequences and of major importance in the international trade of animals and animal products’ (see Table 2).

Disinfectant choice

Biological activity/approved uses.
In the critical early hours of an emergency disease outbreak, the diagnosis may not yet have been confirmed and it is vital that any disinfectant chosen can be used ‘blind’ – that is that its biocidal spectrum is broad enough to cover all possible causative pathogens.

Having a single tried and tested disinfectant on hand also greatly assists the training of first responders. Since nearly all the OIE List A diseases are viral, a broad virucidal activity spectrum is crucial.

Good independent comparative data for disinfectant efficacy against emergency animal diseases are surprisingly rare, but a useful indication is the dilution level approved by governmental bodies such as the UK’s DEFRA (Department of the Environment, Food and Rural Affairs) (see Table 3).

Figures supplied are based on independent verified testing (the higher the number, the more potent the product and the less required).

Table 3. UK DEFRA approved dilutions for AUSVETPLAN named disinfectants against three representative OIE List A diseases (Note: the OIE itself currently has no official disinfectant recommendations for its List A diseases, but simply states which non-formulated basic chemicals will inactivate the causative organisms).

It can be seen that, of the AUSVETPLAN named disinfectants, only Virkon S has UK DEFRA approval for all three representative OIE List A diseases, and is effective in all cases at high dilution. For disinfectant classes not specifically endorsed by the AUSVETPLAN – such as synthetic phenols, quaternary ammonium compounds (QACs), organic chlorine release compounds, biguanides and cresols – DEFRA approvals for OIE List A diseases are few and far between.

In many cases, these products completely fail to achieve the required biocidal standard or do so only at impractically high concentrations.

The AUSVETPLAN specifically recommends Virkon S for all viral OIE List A diseases where disinfection is recognised to be important, stating ‘Virkon S has outstanding virucidal properties. It is an excellent disinfectant, active against all 17 virus families’.

In the US, Virkon S was the only product approved for foot and mouth preventative biosecurity during the UK foot and mouth epidemic of 2001, and the US Department of Agriculture/Animal and Plant Health Inspection Service (USDA/ APHIS) secured rapid approval from its sister organisation, the US Environmental Protection Agency (US EPA), for additional uses of Virkon S including vehicle and footwear disinfection.

Virkon S now has US EPA official labelling for more OIE List A diseases than any other disinfectant and, during the 2001 UK foot and mouth outbreak, it was used at border controls by over 30 countries.

As well as considering activity against current OIE List A pathogens, it is worth remembering that the global disease pattern is constantly evolving. While the nature of future emergency diseases cannot be predicted, an assessment of disinfectant performance against the most resilient viruses currently known – such as the non-enveloped parvovirus – provides a valuable indicator of likely activity against emergency diseases of the future.

In independent tests comparing a number of commercially available products, Virkon S was the only disinfectant capable of inactivating the parvovirus within 60 seconds.

In other independent tests, Virkon S proved the only agent effective against porcine circovirus 2 (PCV2) – the cause of Post-weaning Multisystemic Wasting Syndrome (PMWS) in pigs – and the only disinfectant to achieve 100% kill (within 30 minutes) of the astrovirus implicated in Poult Enteritis and Mortality Syndrome (PEMS) in turkeys.

PWMS and PEMS – both emerging animal diseases unknown in the past – are rapidly increasing in importance and have been the cause of major financial losses in their respective industries in recent years.

Environmental and climatic variables: effect on efficacy.
For a disinfectant to work on farms it must be able to penetrate organic materials such as soil, straw, milk, blood and manure, which can inactivate some disinfectants or protect micro-organisms from their effects.

Chlorine based disinfectants (such as sodium and calcium hypochlorite) and citric acid are particularly susceptible to this pitfall (as are QACs, synthetic phenols, iodophors and biguanides).

Even hard water can reduce or destroy the activity of some disinfectants. Farmers also need to ensure that the disinfectants they use will remain active in different operating temperatures. Formaldehyde is particularly badly affected by cold and is unable to stop bacterial growth – even at double the usual dilution – when temperatures fall to 10ºC. The same applies to glutaraldehyde. Conversely, decomposition of hypochlorite disinfectants (for example bleach) is accelerated by heat.

These problems are not shared by Virkon S which has been independently proven to be effective in the presence of organic challenge, in hard water, at low temperatures and across the pH spectrum.

Health, safety and the environment.
When using disinfectants, the safety of operatives, animals and consumers is paramount.

A number of basic chemicals are highly corrosive, irritant and possibly carcinogenic and the higher concentrations often required for adequate activity exacerbate these problems.

Sodium hydroxide, hydrochloric acid, the aldehydes and hypochlorites all pose a serious threat to human and animal health, requiring stringent precautionary measures and use only in depopulated livestock housing.

In addition, glutaraldehyde, sodium carbonate, citric acid and the hypochlorites are known ecological hazards, with detrimental effects on plants and aquatic life.

In contrast Virkon S has been recognised as environmentally friendly and is over 90% biodegradable.

It has no significant health and safety concerns, with the AUSVETPLAN stating only that ‘reasonable care is necessary’ during use (see Table 4).

Table 4. Special considerations needed when using disinfectants in the AUSVETPLAN.

Practicalities of usage.
Speed of action, stability and versatility in different disinfection scenarios are all crucial considerations in the EDC context. Glutaraldehyde and the hypochlorites typically require prolonged contact time to ensure efficacy.

The hypochlorites also have poor stability/ short shelf life. Sodium hydroxide powder attracts moisture readily under field conditions, causing problems with dissolution in water.

Citric acid, the hypochlorites and particularly sodium hydroxide and hydrochloric acid are corrosive to certain metals and may damage vehicles and equipment.

In addition, independent field trials have established the impracticality of many formulated disinfectants for use in footdips.

Virkon S was formulated specifically with these practical challenges in mind. Supplied in powdered form allowing dilution at the site of a disease outbreak, it is the fastest acting emergency disinfectant available.

Extensive testing of Virkon S on the structural components of vehicles was recently undertaken by the UK Motor Industry Research Association (MIRA), with the resulting report confirming the long-term corrosive effects to be insignificant.

In addition, in a US footdip trial comparing the performance of disinfectants from six leading classes, only Virkon S achieved effective disinfection after boot cleaning within the practical timeframe of 30 seconds.

Virkon S can be used for all prevention and decontamination procedures – footdips, vehicles, surfaces and aerial disinfection (even in the presence of livestock).

It can also be shipped easily and stored for long periods, making it eminently suitable for stockpiling.

Within hours of the 2001 UK foot and mouth outbreak, demand for farm disinfectants had increased by at least 30 times and manufacturers were unable to respond promptly.

Stockpiling of EDC disinfectants is therefore a crucial part of contingency planning.

In conclusion, disinfectants for EDC use must be selected according to a number of stringent criteria.

Ultimately, the most costly disinfectant is one that does not work and, in EDC, farmers and livestock producers can not afford to fail.

Further Information