Discussion by Gerald Berg
Chief of Virology, National Environmental Research Center,
Environmental Protection Agency, Cincinnati, Ohio
Because of the ubiquity of viruses in the polluted water environment and the high infective
capability of these agents, it has been a matter of some importance for several years now to improve
virus detection technology. It has become a generally accepted goal to develop methods capable of
detecting one infective unit of virus in 100 gallons of water;
Since 1965, when the problems of viruses in water were first critically assessed in a unified
coordinated concept
1 , the Baylor laboratory has been one of the more productive innovators of
detection methodology, contributing technics that have served important needs in survey,
enforcement, and in experimental areas.
The complexity of the problem of recovering quantitatively very small amounts of viruses from
large volumes of waters of many qualities has made achievement of a totally effective universal technic
one of slow, step-by-step progression. The new methodology described in this paper is one more step
in this progression intended, in large part, to meet the need for sampling large volumes of water
quantitatively.
There are essentially three parameters that must be considered in the resolution of this problem.
The system that is devised for this purpose must be capable of testing volumes of at least one-hundred
gallons, it must be capable of adsorbing or filtering out all of the viruses in the samples, and a way
must be determined for recovering the adsorbed or filtered viruses for assay.
The apparatus that Metcalf, Wallis, and Melnick described was designed with depth filters of much
greater adsorptive area than the surface filters heretofore used. The process rate is superior to any yet
reported for any other filtration system designed for quantitative virus recovery. Among the methods
under development for recovering small amounts of viruses from large volumes of water, this apparatus
represents a new generation. Of course, this first series of experiments, demonstrating high efficiencies
of recovery of large quantities of poliovirus 1 from seeded natural waters, will need to be followed by
tests with small amounts of a variety of appropriate viruses. Past experience has shown that recovery
efficiencies with large quantities of viruses may not reflect efficiencies with small amounts of viruses.
2
Moreover, recovery efficiencies for a system may vary from one virus to another.
It will need to be determined also whether orlon, polyester, and polypropylene filters, which
passed 8 0 - to 90% of large amounts of seeded poliovirus 1, will pass equally high percentages when
only small amounts of viruses are present in the water, and whether viruses other than poliovirus 1 will
pass as easily.
The most important problem that has to be resolved, however, is that of recovering viruses from
solids. Viruses, free in the water environment, may absorb to certain solids or they may be imbedded
deeply within small particles of fecal material. In either event, in waters with large amounts of sewage
solids, we detect more viruses on the solids than in the water samples from which the solids are
obtained.
3
In rivers, even far below outfalls, when viruses are detected, we often detect as many viruses on the
solids as in the waters from which the solids are obtained.
The methods we have used to recover viruses from waters have varied widely, but all have been
considerably more efficient than the methods we have available for recovering viruses from
so lids-methods with a recovery efficiency of less than one percent. The amounts of viruses on the
solids are probably orders of magnitude greater than we detect and quite likely constitute the larger
share of viruses present in waters that contain certain solids. To what degree the solids are
proportional to the virus content of a water, however, probably depends on the kinds of solids that are
present. Sand, for example, does not itself adsorb viruses. Fecal material may contain viruses naturally.
Many other solids vary in their adsorptive capacity.
In any event, it is clear that solids in water will often contain a large percentage of the viruses in
the water, and recovery of the viruses from the solids will often be more important than recovery of
the viruses from the water itself. All new technic development for recovering viruses from water with
any appreciable quantities of solids will have to be designed accordingly.
The apparatus described by the authors was not specifically designed for recovering solids, but it
can undoubtedly be made to do so.
As awareness of the relatively large amounts of viruses adsorbed onto or trapped within solids
becomes widespread, we can expect that as much attention will be directed towards recovering solids
and the viruses adsorbed or entrapped upon and within the solids as to recovery of viruses from the
water itself.
REFERENCES
1. BERG, G. Ed. "Transmission of Viruses by the Water Route." John Wiley & Sons, New York,
1967.
117
Chief of Virology, National Environmental Research Center,
Environmental Protection Agency, Cincinnati, Ohio
Because of the ubiquity of viruses in the polluted water environment and the high infective
capability of these agents, it has been a matter of some importance for several years now to improve
virus detection technology. It has become a generally accepted goal to develop methods capable of
detecting one infective unit of virus in 100 gallons of water;
Since 1965, when the problems of viruses in water were first critically assessed in a unified
coordinated concept
1 , the Baylor laboratory has been one of the more productive innovators of
detection methodology, contributing technics that have served important needs in survey,
enforcement, and in experimental areas.
The complexity of the problem of recovering quantitatively very small amounts of viruses from
large volumes of waters of many qualities has made achievement of a totally effective universal technic
one of slow, step-by-step progression. The new methodology described in this paper is one more step
in this progression intended, in large part, to meet the need for sampling large volumes of water
quantitatively.
There are essentially three parameters that must be considered in the resolution of this problem.
The system that is devised for this purpose must be capable of testing volumes of at least one-hundred
gallons, it must be capable of adsorbing or filtering out all of the viruses in the samples, and a way
must be determined for recovering the adsorbed or filtered viruses for assay.
The apparatus that Metcalf, Wallis, and Melnick described was designed with depth filters of much
greater adsorptive area than the surface filters heretofore used. The process rate is superior to any yet
reported for any other filtration system designed for quantitative virus recovery. Among the methods
under development for recovering small amounts of viruses from large volumes of water, this apparatus
represents a new generation. Of course, this first series of experiments, demonstrating high efficiencies
of recovery of large quantities of poliovirus 1 from seeded natural waters, will need to be followed by
tests with small amounts of a variety of appropriate viruses. Past experience has shown that recovery
efficiencies with large quantities of viruses may not reflect efficiencies with small amounts of viruses.
2
Moreover, recovery efficiencies for a system may vary from one virus to another.
It will need to be determined also whether orlon, polyester, and polypropylene filters, which
passed 8 0 - to 90% of large amounts of seeded poliovirus 1, will pass equally high percentages when
only small amounts of viruses are present in the water, and whether viruses other than poliovirus 1 will
pass as easily.
The most important problem that has to be resolved, however, is that of recovering viruses from
solids. Viruses, free in the water environment, may absorb to certain solids or they may be imbedded
deeply within small particles of fecal material. In either event, in waters with large amounts of sewage
solids, we detect more viruses on the solids than in the water samples from which the solids are
obtained.
3
In rivers, even far below outfalls, when viruses are detected, we often detect as many viruses on the
solids as in the waters from which the solids are obtained.
The methods we have used to recover viruses from waters have varied widely, but all have been
considerably more efficient than the methods we have available for recovering viruses from
so lids-methods with a recovery efficiency of less than one percent. The amounts of viruses on the
solids are probably orders of magnitude greater than we detect and quite likely constitute the larger
share of viruses present in waters that contain certain solids. To what degree the solids are
proportional to the virus content of a water, however, probably depends on the kinds of solids that are
present. Sand, for example, does not itself adsorb viruses. Fecal material may contain viruses naturally.
Many other solids vary in their adsorptive capacity.
In any event, it is clear that solids in water will often contain a large percentage of the viruses in
the water, and recovery of the viruses from the solids will often be more important than recovery of
the viruses from the water itself. All new technic development for recovering viruses from water with
any appreciable quantities of solids will have to be designed accordingly.
The apparatus described by the authors was not specifically designed for recovering solids, but it
can undoubtedly be made to do so.
As awareness of the relatively large amounts of viruses adsorbed onto or trapped within solids
becomes widespread, we can expect that as much attention will be directed towards recovering solids
and the viruses adsorbed or entrapped upon and within the solids as to recovery of viruses from the
water itself.
REFERENCES
1. BERG, G. Ed. "Transmission of Viruses by the Water Route." John Wiley & Sons, New York,
1967.
117