Identification of Aero-allergens using the Burkard Spore Trap
Written by Ms D Berman
Many different methods of monitoring the pollen and fungal content of the atmosphere are in current use. Where a daily count is required the exposure of a sticky slide each day is still the method of choice. In situations where retrospective reading are acceptable, the Burkard seven day recording spore trap is highly reliable.
- Principle of operation:
The Burkard spore trap is fitted with a drum, around which is placed a sticky cellulose strip. Clockwork mechanism moves the drum at a rate of 2 mm per hour past a narrow slit in the casing which encloses the drum. By means of a vacuum pump, air is ducked through this slit at a constant rate of ten litres per minute. This volume of air flows over the trapping surface, depositing pollen grains and fungal spores on the sticky strip.
- Preparation of the trapping surface
It is essential that the trapping surface be correctly coated to ensure the adherence of pollen grains and fungal spores. The manufacturers of the Burkard spore trap recommend an undercoat of Gelvatol, a water-soluble plastic. This, in turn is coated with vaseline, ensuring a surface that is considered optimal in terms of exposure to weather and adhesiveness. Where high humidity is a problem, 0.1% Tecnazine (4-chloro nitrobenzene) should be added to prevent the germination of spores. If high temperatures are experienced, a higher melting point paraffin wax may be used instead of vaseline.
- Locating the spore trap
The height at which the trap is positioned is critical. This determines the pollen harvest. Too high an elevation will yield a high proportion of tree pollens. When the trap is too close to the ground, airspora from local sources predominate. (Ref.1) We have used a height of 3 metres above the ground for our aero-allergen counting program at the Red Cross Children’s Hospital.
- Preparation of slides for microscopy
The deposit on the trapping surfaces is macroscopically visible and is known as the “trace”. The cellulose strip is removed from the drum after seven days and divided into several equal sections, each representing a 24-hour period. There are several methods of preparing the strips for microscopy.
a. The acetolysis method eliminates extraneous debris, but is time-consuming. Permanently mounted slides may be prepared by adding a glycerine jelly mountant beneath the cover-slip and heat-sealing. A basic fuchsin stain may be used to facilitate the identification of pollen grains. It is essential that the stain, or mountant, be compatible with the adhesive used on the strip.
b. The count is determined by reading 3 longitudinal bands (each 1/3 mm in diameter) using an objective lens (x40) and ocular lenses (x10). This is equivalent to 1m3 of air (Ref.4).
c. When counts re used to estimate potentially allergenic airspora the aerobiologist should be familiar with known allergens. This should be done by correlating counts with skin test results. There are several pollen and fungi that appear on the slide in large numbers, but are not considered allergenic, e.g. Basidiospores.
- Identification
- Fungi
The identification of airspora can be tedious and time-consuming. There are a number of excellent handbooks and atlases available as guides but long experience is the most useful aid. Access to a mycology laboratory is invaluable in identifying fungi.
Workers who identify fungi from agar cultures will find this very helpful in identifying single spores, found on slides. Again it is useful to establish which fungi are allergenic.
Three important fungi are:
- Alternaria
- Cladosporium, and,
- Epicoccum
The clinical significance of aero-allergen identification in the Western Cape has been addressed in a recent study by Potter et al (Ref.2).
- Pollens
Pollen grains are identified morphologically. They are far more difficult to identify than fungal spores because of the similarity of different grains. A pine pollen is shown in Fig. 1.
It is not possible to distinguish different grass species using light microscopy. A magnification of 400x is adequate. The eyepiece of the microscope should be fitted with a graticule so that pollen grains may be measured according to the calibrations. Features of pollen grains used in identification are size, shape, thickness of the exine (outer wall) and intine (inner wall) of the pollen grain.
The number and distribution of pores and furrows and whether the pollen occurs as a simple or compound grain are also important. Grains are measured according to their polar and transverse axes. A list of important aerospores is given in Table 1. Good communication with a botany department is invaluable.
- Meteorological effects
Meteorological data are only relevant if the data is obtainable from a site close to the sampling site. Meteorological parameters such as daily average temperature, humidity and atmospheric pressure, wind speed and direction and rainfall figures, may have a marked effect in the data obtained by the spore trap.
Airspora that consistently appear in significant numbers i.e. greater than 50 grains per cu.m.
Fungi
Pollens
Alternaria
Cladosporium
Epicoccum
Helminthosporium
Pithomyces
Stemphyllium
Compositae
Fagaceae
Grass
Pinaceae
Platanaceae
Plantaginaceae
Urticaceae
References:
- J Charpin, R Surinyach & A W Frankland. Atlas Europeen des pollens allergisants, p 11 Laboratories, Sandoz S.A.R.L. 1974
- PC Potter, D Berman, A Toerien, D Malherbe, EG Weinberg. Clinical significance of aeroallergen identification in the Western Cape. South African Medical Journal. In press (1990).
- Davies, RR and Smith LP. Forecasting the start and severity of the hayfever season. 1973. Clinical Allergy, Vol 3, p 263-267.
- F Spieksma. GRANA 22: 119-128, 1983.