|
|
Letter
Screening Blood Donors at
Risk for Malaria: Reply to Hänscheid et al.
Read original
letter, http://www.cdc.gov/ncidod/eid/vol7no6/benito_letter.htm
Read Hänscheid's
letter, http://www.cdc.gov/ncidod/EID/vol8no8/02-0025.htm
Suggested citation for this article: Benito A, Rubio
JM. Screening blood donors at risk for malaria: reply to Hänscheid et
al. Emerg Infect Dis [serial online] 2002 Aug [date cited];8.
Available from: URL: http://www.cdc.gov/ncidod/EID/vol8no8/02-0200.htm
To the Editor: The letter to editor by Hänscheid et al. addresses
our suggestion that polymerase chain reaction (PCR) could serve as a reference
test for screening blood donations. At present, PCR is the most sensitive
and specific method for parasite detection in malaria-endemic areas. However,
additional measures should be taken into account, such as serologic testing,
refining donor history, defining at-risk locations, and delimiting malaria-endemic
areas. Therefore, we do not suggest that only PCR should be used as a
reference method to exclude blood donors at risk, but it could help shorten
the deferral period for blood donor (currently 3 years after an asymptomatic
person leaves the malaria-endemic area). PCR should be accompanied by
serologic tests and the elimination of actual or possible plasmodial infection
in the blood donor.
In our laboratory, we use the indirect fluorescent antibody test (IFAT)
for antigens of the four plasmodia species, in addition to PCR screening.
At present, we have analyzed a total of 531 blood samples (406 more than
the 125 described in our previous letter to the editor) from possible
donors at risk for malaria, and only the five described in the letter
were malaria positive. Moreover, 40% (50 of 125) of these sera were negative
by IFAT (unpub. data), a fact that indicates the importance of having
a complete donor history and being certain of the patient’s origin in
the context of malaria endemicity (i.e., several geographic areas without
malaria transmission in some Central and South American countries could
be excluded as malaria-risk areas; these areas coincide with sera negative
by IFAT).
On the other hand, with a standard 450-mL blood donation, parasitemias
<90 could test negative by PCR; but, as previously described, this
technique should be accompanied by careful questioning, serologic testing,
and eliminating parasites from the recipient during blood processing and
storage.
We take for granted that, theoretically, any method would have to detect
a single parasite per unit of blood to be safe and that little is known
about the frequency of low parasitemias. In nature, and in accordance
with the parasitologic definition of equilibrium between parasite and
host (defined over thousands or millions of years according to different
phylogenetic theories), one of the main strategies for parasite survival
is sustained malaria transmission, which allows low parasitemias to be
ingested by the anopheline vector (the amount of blood ingested by the
female anopheline varies from 1.3 to 3.0 µL) (1). In
this way, the amount of blood should have sufficient parasites to continue
the cycle inside the vector. This fact explains the stability of malaria
transmission during dry seasons. Plamodium falciparum infections can persist
for at least 1 year in a substantial proportion (10%) of the host (2).
In two-thirds of the cases cited by Mungai et al. (3),
the donor-screening process failed, illustrating the difficulties in obtaining
accurate travel and immigration histories from donors. In this paper,
serologic tests were positive retrospectively in 98% of tested donors,
indicating that serologic tests should be a useful screening technique
for malaria blood donors; 35% showed parasitemia in blood smears, a level
that would have increased if the blood had been analyzed on the day of
transfusion and not in retrospective study (after the degradation or deformation
of the parasites or loss of staining of parasite chromatin).
Moreover, two of the three cases described by Slinger et al. (4)
were in blood donors positive by microscopy or PCR (the other potential
blood donor was not available for follow-up). These results show that
parasites could have been detected in these cases, reflecting that all
blood samples had detectable parasitemias.
Serologic tests in Spain indicate that approximately 50% of the referral
donations could be used in transfusion, and travel histories should distinguish
the specific destinations or the level of malaria transmission in the
area. Most histories are based on the wide areas of transmission listed
in travel guidelines. These data should decrease the cost of testing per
blood donation when we add the value of the blood donation to the real
cost of death prevented.
In conclusion, our initial results, now accompanied by serologic results,
justify the exclusion criteria we first reported (5).
Screening tests for blood donors (e.g., PCR) should be used as a reference
technique that could shorten the deferral period for blood donors. In
all well-reported cases (complete studies with follow-up) of transfusion-associated
malaria described in Canada and the United States, PCR could have detected
the parasites in blood.
Finally, the study of donors at risk could serve as indirect surveillance
for asymptomatic infections and could play an important role in detecting
autochthonous malaria transmission in the United States (5)
or Spain where local anopheline vectors exist. An additional benefit for
parasite detection is that it would permit the donor to be treated and
locally acquired malaria to be eliminated. The Anopheline mosquito vectors
of malaria still exist in the United States at levels sufficient to sustain
malaria transmission, and dozens of cases of autochthonous malaria transmission
have been reported in the United States over the past 15 years (6).
A. Benito and J.M. Rubio
Institute of Health Carlos III, Madrid, Spain
References
- Gilles HM, Warrell DA. Bruce-Chwatt`s essential malariology.
London: Edward Arnold, division of Hodder and Stoughton; 1993. p. 340.
- Arez AP, Snounou G, Pinto J, Sousa CA, Modiano D, Ribeiro H, et al.
A
clonal Plasmodium falciparum population in an isolated outbreak of malaria
in the Republic of Cabo Vorde. Parasitology 1999;118:347–55.
- Mungai M, Tegtmeier G, Chamberland M, Parise M. Transfusion-transmitted
malaria in the United States from 1963 through 1999. N Engl J Med
2001;344:1973–8.
- Slinger R, Giulivi A, Bodie-Collins M, Hindieh F, St. John R, Sher
G, et al. Transfusion-transmitted
malaria in Canada. CMAJ 2001;164:377–9.
- Benito A, Rubio JM. The
usefulness of the seminested malaria-PCR to screen blood donors at risk
in Spain. Emerg Infect Dis 2001;7:1068.
- Zucker JR. Changing
patterns of autochthonous malaria transmission in the United States:
a review of recent outbreaks. Emerg Infect Dis 1996;2:37–43.
|
