The Two Millimetre Problem: Pharaoh Ants, Nosocomial Risk, and the Treatment That Makes It Worse

1. Introduction: a species that specialises in the wrong building

Most structural pests are a nuisance in most buildings and a serious problem in a few. The pharaoh ant inverts this. In an ordinary house it is an irritation. In a hospital it is a plausible route by which an organism reaches a patient who is already compromised.

The species has been described in the literature as the most persistent and difficult of all house-infesting ants to control or eradicate.⁶ That assessment dates from 1965 and has not been overturned.

1.1 What this paper covers

The vector evidence and its limits, the colony biology that defeats standard practice, the protocol that follows, and an honest account of what remains uncertain. It is written for facility managers and infection prevention staff as much as for operators.

2. How a tropical ant reached Winnipeg

This section connects directly to the urban ecology argument published elsewhere in this journal, because the pharaoh ant is the cleanest illustration of it.

The species is tropical. It extended its range to more temperate regions by establishing colonies in heated buildings.⁷ The conditions that favour it are warm at around 30 degrees Celsius and humid at 63 to 80 per cent relative humidity.⁷

Read that as a description of a hospital. Continuous climate control, stable humidity, warm mechanical spaces and service chases, maintained year-round regardless of what February is doing outside. A modern healthcare facility is closer to the pharaoh ant's native thermal environment than anywhere in Manitoba has ever been.

2.1 The implication for seasonality

Because the population is thermally decoupled from the outdoors, there is no seasonal low point in activity and no winter suppression to wait for. A pharaoh ant programme runs on the same footing in January as in July, which distinguishes it from almost everything else in a Canadian operator's caseload.

3. Where they have been found

The recovery locations reported in the literature are the reason this species is treated differently from other ants.

In hospitals, foraging ants have been found in surgical wounds, intravenous glucose solutions, sealed packs of sterile dressing, soft drinks, water in flower displays, and water pitchers.²

Two of those deserve emphasis. Surgical wounds means the ant reached an open wound on a patient. Sealed packs of sterile dressing means sterility was defeated inside intact packaging.

3.1 The size argument

The explanation is mechanical. At roughly two millimetres, the species can be found inside clean rooms and electrical equipment, where it can contaminate or destroy instruments.⁴

Its small size allows establishment of colonies in any suitable location, including unusual places such as between books and in stored clothing.⁷ The species has been described as a serious pest simply because of its ability to get into things, with reports of penetration even into recombinant DNA facilities.⁵

Barriers designed to exclude dust, airborne organisms or human contact are not designed to exclude a two millimetre insect that walks.

4. The vector evidence

The foundational study is Beatson, published in The Lancet in 1972.¹

4.1 Design and findings

Long-standing infestations of pharaoh ants in nine hospitals were sampled to determine whether the ants carried organisms of medical interest, specifically Salmonella species, Pseudomonas aeruginosa, Staphylococcus species, Streptococcus species and Clostridium species. Examples of all these bacteria were isolated.¹

That is not a partial result. Every organism the investigators looked for, they found.

4.2 Corroboration from Brazil

A substantial subsequent literature, much of it Brazilian, has extended the finding. Ants were collected across several wards of 13 hospitals in five states, and bacteriological work indicated the potential for mechanical vectoring of Staphylococcus, Serratia, Klebsiella, Acinetobacter, Enterobacter, Candida and Enterococcus.³

In one study, seven ant species were identified, of which the most frequent were M. pharaonis and Solenopsis saevissima, with 19 species of bacteria isolated from a single hospital.³ Ants in hospitals have been found to carry both Gram-positive and Gram-negative bacteria.³

4.3 The antibiotic resistance dimension

One finding sharpens the concern considerably. In work sampling Tapinoma melanocephalum, another tramp ant species found in hospitals, Pseudomonas, Staphylococcus and Group D Streptococcus were the microorganisms showing the highest resistance to the tested antibiotics.³

An ant foraging across a hospital is not sampling organisms at random. It is sampling from an environment where antibiotic selection has already concentrated resistant strains, and then walking them somewhere else.

4.4 How the literature frames the risk

The considered position in this literature is measured rather than alarmist. Ants are potential vehicles for pathogenic and opportunistic bacteria and can represent a risk factor in nosocomial infections.³ One assessment concludes that the risk presented by these ants is similar to that of any other mechanical vector of bacterial dissemination in a hospital.³

That comparison is worth sitting with rather than dismissing. Hospitals expend considerable effort controlling other mechanical vectors, including hands, surfaces and equipment. An ant is a mechanical vector that moves autonomously, at night, into sealed packaging, and is not addressed by any of those controls.

Authors of this work have recommended that methods of controlling urban ants be adopted and strictly adhered to in order to minimise infection risk to patients,³ and that special attention be given by nosocomial infection control commissions.³

5. Beyond carriage: a documented cross-infection

Carriage studies establish that ants can pick organisms up. They do not, on their own, establish that transmission to patients occurs.

The Beatson paper addressed this directly by reporting a cross-infection of Bordetella bronchiseptica in the isolation unit of a school of veterinary medicine, given as an example of the capacity of pharaoh ants to transmit disease.¹

5.1 What this does and does not prove

It is a single documented case from 1972, in a veterinary school rather than a human hospital. We are not going to represent it as more than that.

What it establishes is that the transmission pathway is real rather than theoretical. Combined with the carriage evidence¹³ and the recovery of ants from surgical wounds and sterile dressings,² the mechanism is complete: ants acquire organisms, ants reach patients and sterile materials, and in at least one documented instance an infection crossed between patients.

6. The colony structure

Everything difficult about controlling this species follows from how its colonies are organised.

Pharaoh ants are social insects with a unicolonial lifestyle. Colonies can contain vast numbers of individuals and hundreds of queens. Because of this structure they are extremely hard to eradicate.⁴

Colonies are polygyne, meaning multiple queens, and new colonies form through budding rather than through swarming flights.⁸

6.1 Why multiple queens defeats most tactics

A single-queen colony has a point of failure. Remove the queen and reproduction stops. This is the implicit model behind most ant control advice.

A colony with hundreds of queens has no such point.⁴ Killing a proportion of the queens reduces reproductive output temporarily and eliminates nothing. Every surviving queen is a complete reproductive unit.

6.2 Nesting inaccessibility

The difficulty in controlling pharaoh ants has been attributed to inaccessible nesting sites, rapid population growth and dispersion of colonies.⁷

In a healthcare building, inaccessible has a specific meaning: wall voids behind sealed finishes, service chases above suspended ceilings, equipment interiors, and spaces within controlled areas that cannot be opened without an infection control procedure of their own.

7. Budding and why spraying fails

This is the mechanism that makes pharaoh ants uniquely dangerous to treat badly.

Disturbing a pharaoh ant colony leads to budding, the phenomenon where part of the colony moves away from the nest and establishes a new sub-colony in an alternative location. The newly formed sub-colony often remains in contact with the mother colony, sharing food and other resources, but it can also survive separately.⁴

7.1 The arithmetic of a bad treatment

Apply a repellent insecticide to a visible trail. The treated workers die. The colony registers chemical stress. It buds. Where there was one nest there are now two or more, in locations nobody has identified, connected by resource sharing or independent.

The infestation has not been reduced. It has been distributed, and distribution inside a hospital means movement toward areas that were previously clean.

This is why the guidance around this species is unusually categorical. Do-it-yourself treatments typically make the problem worse by causing colony budding, and professional baiting programmes are described as the only effective solution.⁸ When dealing with pharaoh ant infestations, it is best to target the entire colony rather than the visible portion.⁴

7.2 The counterintuitive conclusion

For most pests, doing something is better than doing nothing while a proper programme is arranged. For pharaoh ants in a healthcare setting, the opposite holds. An untreated colony is a known, localised problem. A sprayed colony is an unknown, distributed one.

Facility staff who spray a trail they have noticed are acting reasonably and making the situation materially worse, and they have no way of knowing that from the packaging.

8. Why this species defeats ordinary practice

Collecting the properties above produces a profile that defeats nearly every standard tactic.

Residual sprays: trigger budding.⁴

Queen removal: meaningless against hundreds of queens.⁴

Nest location and treatment: nests are inaccessible and multiple.⁷

Exclusion: a two millimetre insect already inside the envelope, nesting within the structure.⁴

Sanitation alone: helpful but insufficient against an omnivorous forager taking proteins, sweets, grease and other insects,⁸ in a building that cannot eliminate moisture or organic material.

Waiting for winter: the population is thermally decoupled from outdoors.⁷

What remains is slow-acting bait carried back through the colony by the workers themselves, which is the only delivery mechanism that reaches inaccessible nests and multiple queens without triggering the stress response that causes budding.

9. The populations most exposed

The risk is not evenly distributed across a hospital.

In ant-infested hospitals, burn victims and newborns are subject to increased risk, because the pharaoh ant can transmit over a dozen pathogens including Salmonella, Staphylococcus and Streptococcus species.⁵

The logic is grim and straightforward. Burn patients have extensive compromised skin barrier, moist wound surfaces, and heightened susceptibility to exactly the organisms the ants carry. Neonates have immature immune function. Both groups are cared for in warm, humid, continuously heated units, which is to say in the conditions this species prefers.

9.1 Other high-consequence areas

By the same reasoning: intensive care, surgical suites and sterile processing, sterile supply storage where packaging is the only barrier, and pharmacy compounding. Each combines patient vulnerability or product sterility with the warmth and moisture the species requires.

9.2 The scale a facility can reach

Infestations are not necessarily local. In Texas, an extensive infestation was reported throughout a seven-floor medical centre.⁵ A building-wide infestation of a multi-storey hospital is the documented upper bound, not a hypothetical.

10. What a defensible protocol requires

Combining the evidence, a healthcare pharaoh ant programme should satisfy the following.

Baiting only, without exception. No repellent products anywhere in the building for the duration, including products applied by facility staff or other contractors.⁴⁸

A written no-spray instruction extending beyond the pest contractor. Budding caused by a well-meaning housekeeper is indistinguishable from budding caused by anyone else.

Building-wide scope from the outset. The documented seven-floor case⁵ and the unicolonial, resource-sharing colony structure⁴ both argue against unit-scoped response.

An extended timeline communicated in advance. Reaching hundreds of queens through worker-mediated bait transfer takes weeks. A programme judged at two weeks will be judged to have failed.

Bait stays available and undisturbed. Cleaning away placements mid-programme resets it.

Coordination with infection prevention and control. Given the documented vector role,¹³ this is an IPAC matter and not only a facilities matter, and the literature's own recommendation is that infection control bodies give it attention.³

Monitoring in high-consequence areas specifically, weighted toward burn units, neonatal care, sterile processing and supply storage.⁵

11. Emerging approaches

One line of research is worth noting because it sidesteps the budding problem entirely.

Work on insecticidal zeolite powders found that when applied to ants, zeolites adsorb part of the epicuticular wax layer, leaving the ants vulnerable to desiccation because that layer regulates water exchange.⁴

The mechanism is physical rather than neurotoxic, which places it alongside the desiccant dusts discussed elsewhere in this journal. A desiccant does not produce the chemical stress signal that triggers budding, and it is not subject to metabolic resistance.

We flag this as a research direction rather than a recommendation. Applying any dust in a healthcare setting raises its own questions about airborne particulate in clinical areas, and those would need answering before the approach could be considered.

12. The Manitoba position

Several things follow for healthcare facilities in this province.

The climate offers no protection. The species reached temperate latitudes precisely by occupying heated buildings.⁷ Manitoba's winter is irrelevant to a colony in a hospital mechanical room.

Our building stock suits it. Continuously heated facilities with extensive service chases, suspended ceilings and stable humidity are the habitat this species prefers.⁷

No local prevalence data exist. We have found no published survey of pharaoh ant occurrence in Canadian healthcare facilities, let alone Manitoba ones. Given the vector evidence, that gap is more surprising than it should be.

The protocol is a procurement question. Because the decisive requirement is that nobody sprays, a facility's exposure depends on what its cleaning contractors and maintenance staff do, not only on its pest contract. That is an institutional control rather than a technical one.

13. Limitations and open questions

The foundational study is from 1972. Beatson's work predates modern infection control practice, current building standards and contemporary microbiological methods.¹ Its findings have been corroborated by later work,³ but a reader should know the age of the primary source.

Carriage is better established than transmission. The carriage literature is extensive.¹³ The documented cross-infection is a single case in a veterinary school.¹ We have described the risk as the literature does, as comparable to other mechanical vectors,³ rather than quantifying an attributable infection rate, because no such figure exists that we could find.

Much corroborating work is Brazilian. Climate, building stock and ant species composition differ.³ M. pharaonis is common to both settings, which supports transfer of the mechanism if not the prevalence.

Some sources are trade publications. Where we have used those,⁸ we have limited the claims drawn from them to colony biology that is also supported in the academic literature, and we have not reproduced claims we could not corroborate.

The chart compares unlike things. The scale figure places values from separate studies with different designs side by side for legibility, and it should be read as an indication of scope rather than as a comparison.

14. Conclusion

Monomorium pharaonis is a tropical species that reached this latitude by moving into heated buildings,⁷ which makes a Manitoba hospital a better approximation of its native environment than anywhere outdoors in the province.

Sampling across nine hospitals isolated Salmonella, Pseudomonas aeruginosa, Staphylococcus, Streptococcus and Clostridium, and a cross-infection of Bordetella bronchiseptica in an isolation unit demonstrated the transmission pathway.¹ Later work across 13 hospitals extended the organism list and found the highest antibiotic resistance among precisely the genera of greatest concern.³ Ants have been recovered from surgical wounds, intravenous solutions and sealed packs of sterile dressing.² Burn patients and newborns carry the greatest exposure.⁵

And the colony will not be sprayed away. Hundreds of queens leave no reproductive point of failure,⁴ nests are inaccessible and multiple,⁷ and chemical disturbance causes budding that distributes the colony further through the building.⁴

Which yields an unusual practical conclusion. In a healthcare facility with pharaoh ants, the most important instruction is not what to apply. It is that nobody applies anything except bait, and that the instruction reaches every person in the building with access to a cabinet and an aerosol can. The species' defining vulnerability is the one that ordinary competence, applied without knowing better, reliably triggers.

References

1. Beatson, S.H. (1972). Pharaoh's ants as pathogen vectors in hospitals. The Lancet, 1, 425–427. Source for the sampling of long-standing infestations across nine hospitals, the isolation of Salmonella spp., Pseudomonas aeruginosa, Staphylococcus spp., Streptococcus spp. and Clostridium spp., and the documented cross-infection of Bordetella bronchiseptica in the isolation unit of a school of veterinary medicine. https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(72)90869-0/fulltext
2. Ohio State University Extension, Ohioline. Pharaoh Ant fact sheet HYG-2136. Source for the recovery of foraging ants from surgical wounds, intravenous glucose solutions, sealed packs of sterile dressing, soft drinks, water in flower displays and water pitchers. https://ohioline.osu.edu/factsheet/HYG-2136-10
3. Fowler, H.G., Bueno, O.C. and related Brazilian hospital ant studies. Source for ants collected across several wards of 13 hospitals in five states, the mechanical vectoring potential for Staphylococcus, Serratia, Klebsiella, Acinetobacter, Enterobacter, Candida and Enterococcus, the identification of seven ant species with 19 bacterial species from one hospital, the Tapinoma melanocephalum antibiotic resistance finding, the Gram-positive and Gram-negative carriage, the framing of risk as comparable to other mechanical vectors, and the recommendations directed at nosocomial infection control commissions. https://www.semanticscholar.org/paper/Ants-As-Potential-Vectors-of-Pathogens-in-Hospitals-Fowler-Bueno/de84482ed754ce5f0ef5e39647853e3f9e0ea09e
4. Van Den Noortgate, H., Lagrain, B., Wenseleers, T. & Martens, J.A. (2018). Analysis of Cuticular Lipids of the Pharaoh Ant (Monomorium pharaonis) and Their Selective Adsorption on Insecticidal Zeolite Powders. International Journal of Molecular Sciences, 19(9), 2797. doi:10.3390/ijms19092797. Source for the two millimetre size and access to clean rooms and electrical equipment, the unicolonial lifestyle with hundreds of queens, the budding response to colony disturbance including resource sharing with the mother colony, the recommendation to target the entire colony, and the zeolite desiccation mechanism. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6165539/
5. University of Florida IFAS Extension. Pharaoh Ant, Monomorium pharaonis (Linnaeus). Publication IN533. Source for the Wilson and Booth (1981) report of extensive infestation throughout a seven-floor Texas medical centre, the increased risk to burn victims and newborns, transmission of over a dozen pathogens, and the species' documented ability to penetrate secured facilities. https://ask.ifas.ufl.edu/publication/IN533/pdf
6. Smith, M.R. (1965). USDA Agricultural Research Service Technical Bulletin No. 1326, describing the pharaoh ant as the most persistent and difficult of all house-infesting ants to control or eradicate, as cited in the subsequent literature. https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/6077937
7. Ebeling, W. (1978). Urban Entomology, University of California Press, and associated literature. Source for the species' tropical origin and range extension into temperate regions by establishing colonies in heated buildings, the favourable conditions of approximately 30 degrees Celsius and 63 to 80 per cent relative humidity, the attribution of control difficulty to inaccessible nesting sites, rapid population growth and colony dispersion, and nesting in unusual locations including between books and in stored clothing. https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/6077937
8. Pest identification reference for Monomorium pharaonis. Trade source used for colony biology corroborated elsewhere: polygyne colony structure, colony formation through budding rather than swarming, omnivorous foraging across proteins, sweets, grease and other insects, and the guidance that do-it-yourself treatment worsens infestations by causing budding while professional baiting is the effective approach. https://romexpest.com/resources/pest-library/pharaoh-ant