Vol. 22, No. 4
JOURNAL OF CLINICAL MICROBIOLOGY, OCt. 1985, p. 587-589
Copyright © 1985, American Society for Microbiology
Comparison of Sedimentation and Flotation Techniques for
Identification of Cryptosporidium sp. Oocysts in a Large Outbreak
of Human Diarrhea
SCOTT J. N. McNABB,l* DIANE M. HENSEL,2 DAVID F. WELCH,2,3 HARALD HEIJBEL,4 GARRY L. MCKEE,S6
AND GREGORY R. ISTRE7
Diagnostic Microbiology Division' and Public Health Laboratory Services,5 Oklahoma State Department ofHealth,
Oklahoma City, Oklahoma 73124; Microbiology Laboratory, Oklahoma Children's Memorial Hospital, Oklahoma City,
Oklahoma 731262; Division ofPediatric Infectious Diseases3 and Department ofMicrobiology and Immunology,6
University of Oklahoma College ofMedicine, Oklahoma City, Oklahoma 73190; Division ofField Services, Epidemiology
Program Office, Centers for Disease Control, Atlanta, Georgia 303334; and Epidemiology Service, Oklahoma State
Department ofHealth, Oklahoma City, Oklahoma 731527
Received 1 April 1985/Accepted 15 July 1985
Cryptosporidiosis, previously seen mostly among immunocompromised patients, is now recognized among
immunocompetent patients. During a large outbreak ofcryptosporidiosis in two day-care centers, we compared
two procedures for the demonstration of the organism in preserved stool specimens. Of 703 stool specimens
tested by both techniques, Sheather sucrose flotation (SSF) identified 127 (18.1%) as positive for
Cryptosporidium sp. oocysts. Ritchie Formalin-ethyl acetate sedimentation (F/EA) plus a modified cold Kinyoun
acid-fast stain (MCK) of the sediment identified 129 (18.4%) as positive for Cryptosporidium sp. oocysts. The
degree of agreement between the two tests was statistically highly significant (P < 0.0001). A total of 161
(22.9%) were positive by one technique or the other; 95 (13.5%) were positive by both techniques. A total of
32 specimens were positive by SSF but negative by F/EA plus MCK, and 34 specimens were positive by F/EA
plus MCK but negative by SSF. The discrepancies between the two techniques occurred in stool specimens that
contained rare to a few oocysts. Other parasitic forms were found by both techniques. F/EA plus trichrome
staining recovered 126 (17.9%) specimens with Giardia lamblia, whereas SSF recovered only 42 (6.0%)
specimens with G. lamblia. No association (X2 = 0.02, P = 0.89) was observed between the presence of G.
lamblia and Cryptosporidium sp. in these stool specimens. We concluded that F/EA plus MCK of the sediment
was as effective in the concentration and identification of Cryptosporidium sp. oocysts as SSF. F/EA plus MCK
may be advantageous as a single concentration method for general parasitology when Cryptosporidium sp. is
also being sought.
Until recently, human infections by the coccidian parasite
Cryptosporidium sp. have been reported infrequently. The
first two documented cases of cryptosporidiosis were re-
ported in 1976 (11, 13). Case studies of this disease link its
occurrence with exposure to infected animals (5, 14). Im-
munosuppression (17) or immunodeficiency (9), including
acquired immunodeficiency syndrome (3), are other predis-
posing conditions. Cryptosporidium sp. oocysts were impli-
cated recently in an outbreak of diarrhea among im-
munocompetent children in a Philadelphia day-care center
(1). The Centers for Disease Control has reported several
investigations of a similar nature in normal children (4).
The diagnosis of cryptosporidiosis was based originally on
finding the organism in histological sections of intestinal
tissue obtained by biopsy. Unstained and iodine-stained wet
mounts ofdirectly prepared or concentrated fecal material or
permanent staining techniques (e.g. trichrome or iron
hematoxylin) failed to demonstrate the organism. Cryp-
tosporidium sp. oocysts are small (4 to 6 ,um) and may easily
be confused with yeasts or artifacts.
In 1980 oocysts were found in human stool specimens (16).
In 1981 the acid-fast nature of the organism was demon-
strated (8). Since this time, new and varied concentration
and staining techniques (2, 6, 10) for demonstrating the
organism in stools have been reported, eliminating the
necessity for tissue biopsy techniques and allowing routine
diagnosis by examination of freshly passed or preserved
feces. However, these studies dealt with only a small num-
ber of positive cases, i.e., 10 confirmed cases in one study
(10) and 45 confirmed or suspected cases in another (6).
Little data are available as to the most effective routine
techniques for the recovery and identification of Cryp-
tosporidium sp. oocysts, especially in terms of sensitivity,
specificity, technical ease, and reproducibility.
A large-scale outbreak of cryptosporidiosis in two Okla-
homa day-care centers recently provided us with the oppor-
tunity to compare two concentration techniques currently
recommended for use in the diagnosis of Cryptosporidium
sp. infections: (i) The Ritchie Formalin-ethyl acetate (F/EA)
(7, 12) concentration procedure combined with a modified
cold Kinyoun acid-fast stain (MCK) of the sediment and (ii)
Sheather sucrose flotation (SSF) (15), a specialized proce-
dure adapted from veterinary science to clinical microbiol-
ogy laboratories (14) and currently recommended for the
diagnosis of cryptosporidiosis (10).
MATERIALS AND METHODS
Specimen collection and handling. Of over 1,100 stools
collected in this outbreak, the first 703 were chosen to
evaluate the two methods. The fresh stool specimens were
divided and placed into 10% buffered Formalin or polyvinyl
alcohol preservative. The specimens
in Formalin were
McNABB ET AL.
TABLE 1. Recovery and quantitation ofCrptosporidiumn sp.
oocysts by SSF and F/EA plus MCK in 703 specimens obtained in
Oklahoma in 1984
No. of oocysts/
No. of specimens with indicated no. of oocysts/high-
no. of oil
power field detected by SSF
detected by F
EA plus MCK
1/10 to 100
1/1 to 10
10 to 49
subdivided into two parts. One part was processed by F/EA
sedimentation, and the other part was processed by SSF. The
two concentration procedures and examination ofslides were
performed independently in two laboratories by experienced
parasitologists who were unaware of the results obtained in
the other laboratory until the end of the study. A permanent
was made from the polyvinyl alcohol-
preserved specimens and examined under oil immersion
magnification for the presence of other parasites.
F/EA and MCK. The F/EA concentration procedure was
performed as described previously (7, 12). Briefly, 4 to 5 ml
of the Formalin-treated stool specimen was washed in 10%
Formalin-saline, and the sediment, collected by centrifuga-
tion at 650 x
Formalin-saline-3 ml of ethyl acetate (certified American
Chemical Society grade; Fisher Scientific Co.). This mixture
was vortexed vigorously for 3 min and centrifuged at 500 x
g for 5 min, resulting in four layers: a layer of ethyl acetate,
a plug of debris, a layer of Formalin-saline, and sediment.
The plug was rimmed with an applicator stick, and the top
three layers were decanted. One portion of the sediment was
placed on a microscope slide and dried for the acid-fast stain.
The remainder of the sediment was examined at 100 and
450x for eggs and cysts in saline and iodine wet mounts.
The protocol for MCK has been described previously (10).
Smears were fixed in methanol for 1 min, stained in the
primary dye (Kinyoun carbolfuschin) for 15 min, and then
decolorized for 1 min in 10% H2S04. The counterstain was
light-green SF yellowish stain. Each MCK-stained slide was
examined under oil immersion power for 15 min. Results
were recorded as the average number of oocysts observed
per oil immersion field.
SSF. Approximately 2 ml of the Formalin-treated stool
suspension was strained through two layers of gauze into a
conical tube. Eight milliliters of Sheather sugar solution (500
g of sucrose, 6.5 g of phenol, 320 ml of distilled water) was
added, and the suspension was mixed thoroughly by inver-
sion. The tube was capped and centrifuged at 500 x g for 15
min. After centrifugation, material at the surface of the
suspension was removed with a loop bent at a 900 angle.
Three loopfuls were placed on a slide and covered with a
cover slip (22 by 22 mm). Each slide was examined for
approximately 5 min for pink, refractile oocysts by bright-
field microscopy and with the 45x objective lens of an
American Optical Corp. microscope. Results were recorded
as the average number of oocysts observed per high-power
Statistical analysis. The McNemar test for matched pairs
was used for statistical analysis of the data for the presence
of Cryptosporidium sp. oocysts and G. lamblia in the stool
g for 5 min, was suspended in 8 ml of
specimens. The Kappa statistic was used to evaluate the
A total of 703 stool specimens were examined by the two
procedures (Table 1), and Cryptosporidium sp. oocysts were
identified in 161 (22.9%) specimens. Using SSF, we identi-
fied oocysts in 127 (18.1%) specimens. F/EA sedimentation
plus MCK identified oocysts in 129 (18.4%) specimens. A
total of 542 specimens were negative for Cryptosporidium
sp. oocysts by both techniques. A total of 22 specimens that
were positive by SSF but negative by F/EA plus MCK
contained less than one oocyst per high-power field; 10 such
specimens contained one to nine oocysts per high-power
field. Of 34 specimens from which oocysts were recovered
by only F/EA plus MCK, 26 contained one oocyst per 10 to
100 oil immersion fields, 5 contained one oocyst per 1 to 10
oil immersion fields, and 3 contained one to five oocysts per
1 oil immersion field.
We found no correlation between the presence of
Ciyptosporidiiiin sp. and G. lamblia in stool specimens in
this study. Of 129 F/EA-concentrated specimens with
lainblia, whereas of 574 specimens without Cryptosporidiurm
sp. present, 105 (18.3%) contained G. lamnblia (X2 = 0.02, P
Parasitic forms other than Cryptosporidiuim sp. were also
found in the 703 stool specimens. A total of 126 G. lamnblia,
9 Entainoeba coli, 6 Enidolimaox nana, 8 Blastocystis
hominis, and 2 Dienitamoeba fragilis were found by exami-
nation of wet mounts of material concentrated in F/EA or of
polyvinyl alcohol-preserved material stained with trichrome.
Many of these parasites were not observed (e.g., only 42 G.
lamtnblia) in material examined by SSF.
sp. present, 21 (16.3%) contained G.
During an outbreak of diarrheal disease in a day-care
center we had the opportunity to investigate the comparative
sensitivities of two methods for the recovery of CGyp-
tiosporidiumn sp. from stool specimens. A general-purpose
technique for parasite concentration, the F/EA concentra-
tion procedure, was compared with the more classical
method (SSF) for the detection of Cryptosporidium sp.
There was no significant difference between these two
oocysts. All disparate results were associated with the
presence of very few oocysts. The use of both techniques
resulted in a 22.9% positivity rate, as compared with rates of
approximately 18% for either technique alone. In view of the
large number of specimens included in this study, this
difference in positivity rates may appear to warrant the
recommendation that both methods be used for the optimal
detection of Cryptoosporidiumn sp. However, an examination
of multiple specimens per patient by either technique may
provide a similar increase in sensitivity. As the F/EA con-
centration procedure is already performed routinely in most
clinical microbiology laboratories, preparation of the acid-
fast smear for detecting Crvptosporidiium sp. in the same
sediment may reduce cost and technical time. The acid-fast
smear provides a permanent record of the results, but the
preparation and reading of the smear require some technical
expertise. SSF concentration is easy to perform, and the
pink, refractile oocysts are easily recognized under high-
power magnification, but the wet mounts should be exam-
ined within 15 min after preparation or the oocysts may
in rates of recovery of Crs'ptosporidiuin
J. CLIN. MICROBIOL.
RECOVERY METHODS FOR CRYPTOSPORIDIUM SP. OOCYSTS
collapse. This difficulty may prevent batching specimens if
this method is used for a large number of stools.
In contrast to previously reported findings (18), we did not
find an association between Cryptosporidium sp. and G.
lamblia. Only 21 (3%) of the 703 stool specimens contained
both parasites, whereas 213 (30%) ofthe 703 stool specimens
contained one or the other parasite. The occurrence of
commensal parasites suggests hand-to-mouth transmission
and is probably an indication of ingestion of contaminated
feces in this day-care center.
We examined each MCK-stained slide for 15 min under oil
immersion magnification to cover approximately 100 fields.
This amount was roughly equal to the area covered when we
examined each SSF wet mount for 5 min under high-power
dry magnification. We chose these two parameters to stan-
dardize our procedures and make our data available for
comparison. However, because of time constraints and the
amount of stool material required, we were unable to per-
form reproducibility studies of these two techniques during
In conclusion, FlEA sedimentation plus MCK was as
sensitive as the specialized SSF technique for the detection
of Cryptosporidium sp. oocysts. The method selected for use
by individual laboratories may depend on the desirability of
incorporating an examination for Cryptosporidium
oocysts into general parasitology and the number of speci-
mens received for examination.
We thank George Healy, Centers for Disease Control, Atlanta,
Ga., and Pearl Ma, St. Vincent's Hospital, New York, N.Y., for
assistance with positive control material and techniques. We thank
Jerelyn Jacks and Everett D. Dodd for technical assistance. We also
thank Cathy Cuellar and her staffand Fred Reynolds, Edgar Cleaver,
Bonnie Seigel, and William Gibbons and their staff at the Tulsa
City-County Health Department and Pam Wall and Kathy Slaine,
Epidemiology Division, Oklahoma State Department of Health, for
assistance during the outbreak.
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J. Clin. Microbiol.
VOL. 22, 1985