BIOSECURITY AND BIOTERRORISM: BIODEFENSE STRATEGY, PRACTICE, AND SCIENCEVolume 1, Number 1, 2003© Mary Ann Liebert, Inc.

Potential for Aerosol Dissemination of Biological Weapons:Lessons from Biological Control of Insects

DAVID B. LEVIN and GIOVANA VALADARES DE AMORIM

INTRODUCTION

THE ANTHRAX ATTACKS of 2001 have increased con-cerns that “weapons grade” biological agents can be

obtained or manufactured and disseminated by terrorists.In order to assist in planning for future attacks, bioterror-ist attack scenarios have been envisioned that involve theuse of aerosol-delivery technologies to target large civil-ian populations by air, inside buildings, or in mass transitsystems. However, there is relatively little unclassifieddata on which biodefense planners can base their under-standing of the potential consequences of a large-scalebioterrorist attack. A 1970 World Health Organization(WHO) study1 estimated that 50 kg of Bacillus anthracisreleased over an urban population of 5 million wouldsicken 250,000 and kill 100,000 people, and a 1993 Of-fice of Technology Assessment (OTA) study2 estimatedthat between 130,000 and 3 million deaths would followthe release of 100 kg of B. anthracis. However, neither ofthese analyses employed empirical data.

While aerosol technologies for large-scale dissemina-tion have been developed and tested by the United Statesduring the existence of its biological weapons program,and by the former Soviet Union3 and Iraq,4 few details ofthose tests are available to civilian planners. A declassi-fied report of a 1960s U.S. experiment performed nearJohnston Atoll in the South Pacific reported that a plane“sprayed a 32-mile long line of agent that traveled formore then 60 miles before it lost its infectiousness” tononhuman primates.5 However, there are no known un-classified data on human exposure to large-scale aerosolreleases of biological weapons. The only known aerosolrelease of B. anthracis spores resulting in large numbersof human deaths occurred in 1979, when anthrax sporeswere accidentally released from a military facility inSverdlovsk, in the former Soviet Union.6 Ninety-six peo-ple were reportedly sickened and 68 people died, al-though the death toll may have been as high as 105.3

Some analysts have questioned whether the B. an-thracis spores such as that used in the 2001 attacks couldbe produced and deployed effectively by terrorist groupswithout the support of a nation-state. Some have assertedthat to be used effectively as a biological weapon, B. an-thracis would have to be in a dry powdered form, highlyconcentrated, of uniform particle size, low electrostaticcharge, and treated to reduce clumping in order for thebacteria to penetrate the spaces of the deep lung. Someanalysts have also argued that foggers and crop dusterswould not be effective ways to disseminate B. anthracis,because the use of liquid formulations would require ahigh level of purity to prevent plugging of nozzles andwould create “globs” which would harmlessly fall to theground rather than staying suspended in the air. Finally, ithas been argued by others that the use of an aircraft to at-tack a large city with B. anthracis would be ineffectivebecause most of an urban population is inside buildingsat any given time, offering some level of protectionagainst breathing air contaminated by spores.

In the absence of empirical data, it is difficult to con-firm or refute these assertions. Experiments designed toexamine human exposure to biological weapons in civilianpopulations obviously cannot be conducted. Yet valuablelessons can be taken from the world of biological controlof defoliating insect pests. Over the past two decades, bac-teria, fungi, and viruses have been proven to be useful andeffective tools for such purposes.7–10 Biological agents areused routinely in forestry and agriculture, and they are of-ten used near densely populated urban areas. In this article,we summarize the empirical data from the human expo-sure that followed the outdoor spraying of the biologicalcontrol agent Bacillus thuringiensis, a bacteria that isclosely related to B. anthracis. Detailed results of this sci-entific assessment have been published in an earlier paper;however, the implications of this data for biodefense arethe focus of this article.11,12 The data from this report andfrom the larger body of information related to the biolog-

David B. Levin, Ph.D., and Giovana Valadares de Amorim, M.D., are with the Department of Biology, University of Victoria,Victoria, British Columbia, Canada.

37

ical control of insects suggest that a liquid biologicalagent can indeed be disseminated on a large scale usingcrop-dusting airplanes. Biodefense planners should takethis into account as they seek to prevent the use of bio-logical weapons and to plan to reduce the consequencesof such potential attacks.

THE SCIENCE OF BIOLOGICAL CONTROL

Bacillus thuringiensis is a ubiquitous, soil dwelling,spore-forming, gram-positive bacterium that produces in-tracellular “crystal proteins” during sporulation (formationof the spore). These crystal proteins are precursors to tox-ins that specifically attack certain groups of insects.13 Dif-ferent subspecies or varieties of B. thuringiensis producedifferent crystal proteins. For example, one subspecies ofB. thuringiensis produces a crystal protein that is a precur-sor to a toxin that kills Colorado potato beetles; anothersubspecies of B. thuringiensis produces a protein that killsflies and mosquitoes. Spores and crystal proteins lie dor-mant in the soil and on surfaces of plant roots and foliage.When insect larvae feed on the contaminated plant mater-ial, the crystal proteins are processed (proteolyticallycleaved) within the insect mid-gut to become potent toxins.

Aerial dissemination in Canada

In February 1999, the government of the province ofBritish Columbia (BC), Canada, authorized an aerial sprayprogram to control insurgent populations of the Europeangypsy moth, Lymantria dispar, found in and around thecity of Victoria, BC (Fig. 1). The gypsy moth populationsposed a serious economic threat to BC lumber product ex-ports to other provinces of Canada and to the neighboringstates of the U.S. If the insurgent gypsy moth populationwere to become established, it would risk an embargo ofBC lumber products and cause significant environmentaldamage. To eradicate the gypsy moth problem, the biolog-ical insecticide Foray 48B, which contained a 2.1% solu-tion of B. thuringiensis spores,14,15 was applied to morethan 12,204 hectares (approximately 30,157 acres) in thegreater Victoria region (Fig. 1). The spray zone included amix of residential and rural areas containing approxi-mately 75,420 people.16 For the purposes of this article, thevariety of B. thuringiensis used in this spray (B.thuringiensis var. kurstaki, strain HD1) will henceforth bereferred to as Btk HD1.

Unlike the closely related, highly pathogenic bacteriaB. anthracis, B. thuringiensis is rarely infectious to hu-mans or other mammals.17 However, there have been re-ports of human infection under extreme conditions: Acorneal ulcer developed in a previously healthy 18-year-old farmer who accidentally splashed a commercial B.thuringiensisproduct into his eye,18 and multiple thigh and

knee abscesses containing B. thuringiensiswere found in apreviously healthy soldier who was severely wounded by alandmine explosion.19 Animal experimentation has alsoshown that interperitoneal (abdominal) injection of B.thuringiensis can cause death in guinea pigs,20 and im-munocompromised mice are susceptible.21

To allay public concern about exposure to Btk HD1spores in the Foray 48B spray, a coordinated study of theshort-term health effects of the spray on the human popu-lation was undertaken by the Public Health Office of theCapital Health Region (CHR) during the spray period.16

The study included surveys of the health of asthmatic chil-dren and the general public, as well as the monitoring ofthe incidence and distribution of Btk HD1 in environmen-tal and in human samples collected before (pre-spray) andafter (post-spray) the three aerial applications of Foray48B. Pre-spray environmental samples and nasal swabswere collected in the study because it is known that com-mercially available insecticides containing Btk HD1 areused by gardeners in residential settings, and it was antici-pated that an environmental assessment would show BtkHD1 was present at low levels in the environment even be-fore any aerial dissemination of Foray 48B.

Assessing the results of spraying

Aerial applications of Foray 48B were carried out inVictoria, BC, on May 9–10, May 19–21, and June 8–9,1999.11 The spray was applied by aircraft (Cessna 188and DC 6) between 5:00 A.M. and 7:00 A.M. (Pacifictime), when the wind was at 10 kilometers/hour (approx-imately 6 miles/hour) or less. The spray was applied at 4liters/hectare (approximately 0.25 U.S. gallons/acre) at arate of 70 liters/minute (approximately 20 U.S. gal-lons/minute), and a droplet size of between 110 and 130microns. The objective was to apply the bacteria so itwould maximally stick to tree foliage.16 The applicationof biological control spray Foray 48B resulted in .99%mortality of the gypsy moth population.

Two hundred and fifty-six (256) bulk air samples weretaken during aerial application at outdoor locations (Fig.1), both within and down-wind of the spray zone,12 tomonitor the presence and concentration of airborne BtkHD1 spores during and after the spray application. Thesesamples were taken by pulling air through 0.5-micronpore-size Teflon filters mounted in 37-mm close-facecassettes using constant flow battery-powered pumps.The pumps were calibrated to a flow of 2 L/min (65%)before and after sampling using a rotameter (Matheson).The pumps were operated for a period of 30 minutes dur-ing the aerial application of Foray 48B; therefore, thevolume of air filtered was approximately 60 L.

To characterize the size distributions of the Btk HD1-containing aerosols, air samples were taken using asize-selective, six-stage cascade impactor mounted withtrypticase-soy-agar plates (Andersen plates). These air

LEVIN AND VALADARES DE AMORIM38

Vancouver Island

Vancouver

Victoria

N

B-1

A-1A-2

Victoria Aerial Spraying 12,203 Hectares

Scale

Legend

Kilometers2 0 2

B-2

British Columbia

Washington State

Canada

USA

Direction of prevailing winds during the time of arieal applications of Foray 48B

Inside Spray Zone

Outside Spray Zone

Downtown

W ill iam Head

Albert Head

Parry Bay

Royal Roads

Portege Inlet

Victoria Harbour

Direction of prevailing winds during the time of aerial applications of Foray 48B

Esquimalt Harbour

Esquimalt Harbour

FIG. 1. (A) Location of Victoria, British Columbia, in North America, and (B) details of the spray zone. The area of gypsy mothinfection (the spray zone) is indicated by the boxed light-gray area. The lanes flown by aircraft to apply the spray are indicated bythe letters A-1, A-2, B-1, and B-2, separated by vertical lines. Air and human sampling locations inside the spray zone are indicatedby small circles. Air and human sampling locations outside the spray zone are indicated by small squares. Small squares marked byXs indicate sample sites in the city center. N 5 North.

A

B

samples were drawn through 0.5-micron filters so thatparticles were separated and settled on the agar plates onthe basis of their airborne size. The Andersen samplerseparates the following size ranges of aerosol: Stage 1).7 microns; Stage 2) 4.7 to 7 microns; Stage 3) 3.3 to 4.7microns; Stage 4) 2.1 to 3.3 microns; Stage 5) 1.1 to 2.1microns; Stage 6) 0.65 to 1.1 microns. Air was drawn at arate of 28.3 L/min with a high volume pump maintainedby an orifice placed after the sixth stage of the Andersenplates.12 Samples were taken both outside and inside thehomes of participating residents during and after thespray application.

Nasal swabs were obtained from asthmatic childrenwhose families volunteered to perform these tests. Thesefamilies were located both within the spray zone and out-side the spray zone (down-wind).22 Post-spray swabswere taken the morning after the spray, after the familymembers had risen but before any doors or windowswere opened.

Environmental (air and water) and human (nasal swab)samples, collected before and after aerial applications ofForay 48B, were analyzed for the presence of Btk HD1.11

Random Amplified Polymorphic DNA (RAPD) analysis,cry gene specific polymerase chain reaction (PCR), andDot-blot DNA hybridization techniques were used toidentify over 11,000 isolates of bacteria. Identification ofBtk HD1 in air samples collected on the filters of the bulkair samplers and on the agar plates of the Andersen sam-plers permitted calculation of the concentrations of air-borne Btk HD1 spores both outside and inside homes.Identification of Btk HD1 in nasal swab samples permit-ted an estimation of the human exposure to the airbornespores. Bacillus species with genetic patterns consistentwith those of Btk HD1 used in the spray were identifiedin 85.4% of the isolates obtained from the air samplesand 76.6% isolates obtained from the human nasal swabsamples.

The average concentration of airborne Btk HD1 mea-sured outside of residences during the 30-minute periodof the spraying within the spray zone was 739 colonyforming units per cubic meter (CFU/m3) of air.12 (Thenumber of CFUs here is essentially equivalent to numberof spores.) Additional outdoor air samples were taken upto 9 days post-spray. The outdoor air concentrations ofBtk HD1 diminished in two phases. In the first few hours,the airborne Btk HD1 concentration declined by half theoriginal concentration. This was followed by a slowerrate of decreasing concentration that persisted over sev-eral days.

During and immediately after the spray application (asthe spray planes passed over the houses), the averageconcentration of airborne Btk HD1 measured inside resi-dences within the spray zone was 2.3 to 4.6 times lowerthan outside. Data from the Anderson samplers revealedthat Btk HD1 aerosols with a medium diameter of 4.3 to

7.3 microns were present in air outside residences within15 minutes after the spray application began. The aero-sols were not visible to the human eye and were smallenough to penetrate airways of the respiratory system.

How these small aerosols were generated is not clear atthis time. It is possible that the large droplets producedby the nozzles are broken up immediately upon their re-lease by air pressures created by the speed of the aircraft(370 km/hour 5 approx. 230 miles/hour). It is also possi-ble that the droplet size decreased due to evaporation ofwater during their descent. This hypothesis is supportedby evidence that airborne Btk HD1 concentrations ofsmall size (,7 microns) were higher with lower relativehumidity and higher temperatures.12 Particle size insidehouses were in the same sample range as those detectedoutside (in the 4.3 to 7.3 micron range). The observed de-crease in the outside Btk HD1 spore concentration mayhave resulted from a combination of dilution by wind,settling of larger particles, elevation of smaller particleson rising air heated by the sun as the day progressed, andby inactivation of spores by ultraviolet light from the sun.

Approximately 5 to 6 hours after the spray applicationbegan, indoor concentrations of airborne Btk HD1 in-creased and then exceeded the outdoor concentrations,with an average of 245 CFU/m3 of indoor air. During thistime period, outdoor air concentrations had begun to fallto an average of 77 CFU/m3.12 Given that the rate of in-halation of an adult at rest is approximately 20 m3/day or0.833 m3/hour, as of 5–6 hrs. post-spraying, people in thespray zones could be estimated to be inhaling approxi-mately 203 colony forming units of Btk HD1 (spores)/hour. There is insufficient data from this study to estimatethe average quantity of spores inhaled by people residingin homes that were part of this study. It should be empha-sized that no one fell ill as a result of this spraying. But it isclear that spore counts of Btk HD1 did rise in homes insidethe spray zone following spraying. And there were also asubstantial number of nasal swabs positive for Btk HD1 inpeople residing inside the spray zone.

CONCLUSIONS

The study of the B. thuringiensis spray in Canada in1999 provides data that refutes arguments asserting thatthere are technological barriers that would prevent all butmajor military programs from using B. anthracis as anaerosol disseminated bioweapon. These findings shouldbe understood by those with responsibility for preventingor responding to the consequences of bioterrorist attacks.These data provide evidence that it is technologicallyfeasible to disseminate biological agents from aircraft (orbackpack sprayers, or truck-mounted foggers).23–27 For-est protection personnel, mosquito control personnel, andfarmers have been doing so for over 2 decades. Spray

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formulations consist of readily available ingredients thatcan be obtained from agricultural supply stores, and theseformulations do not clog spray nozzles. And while mostof the droplets in the spray are large, in the range of 100to 150 microns, a significant amount of small dropletaerosolization occurs. Droplets of 2 to 7 microns areformed in sufficient quantities to penetrate houses andcontaminate the nasal passages of residents inside theirhomes. The concentration of airborne spores indoors in-creased within a few hours after the spray and ultimatelyexceeded the outdoor concentration.

This study was designed to provide information con-cerning human exposure to the Foray 48B spray in orderto assess the potential public health impact of sprayingwith B. thuringiensis. It was not designed, specifically, toanswer questions about a potential bioterrorist attack.Therefore, questions relevant to biosecurity remain unan-swered: How are droplets of ,7 microns generated dur-ing the spray application of much larger particles? Whatis the rate of decrease of airborne spores inside and out-side homes and buildings? What is the role of nasalswabs after such exposure, and what role could such testsprovide in triage and treatment after exposure? Is there adifference in exposure if the spray is applied by a truck-mounted fogger or from a backpack sprayer, comparedwith an aircraft? Additional studies would need to be de-signed and conducted to answer these and other ques-tions that bear directly on biosecurity.

In conclusion, monitoring ongoing routine human ex-posures to nonhuman pathogens that are applied in urbanareas may be our best hope of acquiring more detailedempirical human and environmental data about the po-tential consequences of aerosol-disseminated bioweapons.B. thuringiensis appears to be an excellent model foraerosol dissemination of B. anthracis. Opportunities toevaluate human exposure to nonhuman pathogens arisein North America every spring and summer, when bio-logical agents are applied to control insect pest popula-tions.

REFERENCES

1. WHO. Health Aspects of Chemical and BiologicalWeapons. Geneva, Switzerland: World Health Organiza-tion. 1970.

2. Office of Technology Assessment, US Congress. Prolifera-tion of Weapons of Mass Destruction. Washington, DC:US Government Printing Office; Publication OTA-ISC-559, 1993.

3. Alibek K, Handelman S: Biohazard: The Chilling TrueStory of the Largest Covert Biological Weapons Programin the World. New York, NY: Random House. 1999.

4. Zilinskas RA: Iraq’s biological weapons. JAMA 1997;278:418–424.

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6. Meselson M, Guillemin J, Hugh-Jones M, et al: TheSverdlovsk anthrax outbreak of 1979. Science 1994;266:1202–1208.

7. Streett DA, Woods SA, Erlandson MA: Entomopoxvirusesof grasshoppers and locusts: Biology and biological controlpotential. Mem Entomol Soc Can 1997;10:115–130.

8. Sosa-Gomez DR: Current status of the microbial control ofagricultural pests with entomopathogenic fungi. Rev So-cied Entomol Argentina 1999;58:295–300.

9. Miller LK: The Baculoviruses. New York, NY: PlenumPress, 1997.

10. Van Frankenhuyzen K. The challenge of Bacillus thuringien-sis. In, Bacillus thuringinensis, An Environmental Biopes-ticide: Theory and Practice. (PF Entwhistle, JS Cory, MJBailey, and SR Higgs, eds), Chichester, UK: John Wiley &Sons Ltd. 1993, pp. 1–35.

11. de Amorim GV, Whittome B, Shore B, Levin DB: Identifi-cation of Bacillus thuringiensis subspecies Kurstaki strainHD1-like bacteria from environmental and human samplesafter aerial spraying of Victoria, British Columbia, Canadawith Foray 48B. Appl Environ Microbiol 2001;67:1035–1043.

12. Teschke K, Chow Y, Bartlett K, Ross A, van Netten C:Spatial and temporal distribution of airborne Bacillusthuringiensis var. kurstaki during an aerial spray programfor gypsy moth eradication. Environ Health Perspect 2001;109:47–54.

13. Schnepf E, Crickmore N, Van Rie J, Lereclus D, Baum J,Feitelson J, Zeigler DR, Dean DH: Bacillus thuringiensisand its pesticidal crystal proteins. Microbiol Mol Biol Rev1998;62:775–806.

14. Otvos IS, Vanderveen S: Environmental report and currentstatus of Bacillus thuringiensis var. kurstaki use for controlof forest and agricultural pests. Victoria: Forests Canadaand Province of British Columbia Ministry of Forests.1993.

15. Abbott Laboratories. Foray 48B Material Safety DataSheet. North Chicago, IL: Abbott Laboratories. 1999.

16. Capital Region District (CRD). Human health surveillanceduring the aerial spraying for control of North Americangypsy moth on southern Vancouver Island, British Colum-bia. Report to the Administrator, Pesticide Control Act,Ministry of Environment, Lands, and Parks, Province ofBritish Columbia. Prepared by the Office of the MedicalHealth Officer and Director of Research, Capital HealthRegion, Victoria, British Columbia. 1999.

17. McClintock JT, Schaffer CR, Sjoblad RD: A comparativereview of the mammalian toxicity of Bacillus thuringien-sis-based pesticides. Pest Sci 1995;45:95–105.

18. Samples JR, Buettner H. Ocular infection caused by a bio-logical insecticide. J Infect Dis 1983;148:614.

19. Hernandez E, Ramisse F, Cruel T, Ducoureau JP, AlonsoJM, Cavallo JD: Bacillus thuringiensis serovar H34-konkurkian superinfection: report of one case and experi-mental evidence of pathogenicity in immunosuppressedmice. J Clin Microbiol 1998;36:2138–2139.

20. Fisher R, Rosner L: Toxicology of microbial insecticideThuricide. Agri Food Chem 1959;17:686–688.

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21. Hernandez E, Ramisse F, Cruel T, le Vagueresse R, Cav-allo JD: Bacillus thuringiensis H34 isolated from humanand insecticidal serotypes 3a3b and H14 can lead to deathof immunocompetent mice after pulmonary infection.FEMS Immunol Med Microbiol 1999;24:43–47.

22. Pearce MR, Habbick B, Williams J, Eastman M, NewmanM: The effect of aerial spraying with Bacillus thuringiensisvar. kurstaki on asthmatic children. Can J Pub Health 2002;93:21–25.

23. Behle RW, McGuire MR, Shasha BS: Extending the resid-ual toxicity of Bacillus thuringiensis with casein-based for-mulations. J Econ Entomol 1996;89:1399–1405.

24. Behle RW, McGuire MR, Gillespie RL, Shasha BS: Ef-fects of alkaline gluten on the insecticidal activity of Bacil-lus thuringiensis. J Econ Entomol 1997;90:354–360.

25. Van Frankenhuyzen K, Wiesner CJ, Riley CM, NystromC, Howard CA, Howse GM: Distribution and activity ofspray deposits in an oak canopy following aerial applica-tion of diluted and undiluted formulations of Bacillusthuringiensis Berliner against the gypsy moth, Lymantria

dispar L. (Lepidoptera: Lymantriidae). Pest Sci 1991;33:159–168.

26. Sundaram A: Drop-size spectra and deposits of four Bacil-lus thuringiensis formulations on simulated and natural firfoliage. Trans ASAE 1994;37:9–17.

27. Tamez-Guerra P, McGuire MR, Behle RW, Hamm JJ,Sumner HR, Shasha BS: Sunlight persistence and rainfast-ness of spray-dried formulations of baculovirus isolatedfrom Anagrapha falcifera (Lepidoptera: Noctuidae). JEcon Entomol 2000;93:210–218.

Address reprint requests to:Dr. David B. Levin

Department of BiologyUniversity of Victoria

P.O. Box 3020 STN CSCVictoria, British Columbia, Canada

E-mail: dlevin@uvic.ca

LEVIN AND VALADARES DE AMORIM42