Literature review > Issue_3 > Review Patterson et al. 

Due to a lack of readily available diagnostic tests, the global epidemiology of chancroid is poorly understood [1]. Although UNAIDS and WHO estimate that the annual global incidence of chancroid is 6 million cases, these estimates are primarily based on the incidence of syphilis, and the relative estimates of the percentages of syphilis and chancroid in GUD cases from a limited number of studies. WHO does not include chancroid in their global estimates of sexually transmitted diseases because the data are inadequate. The most sensitive diagnostic test for chancroid is multiplex PCR (M-PCR). M-PCR has a resolved sensitivity and specificity for H. ducreyi of approximately 98.4% and 99.6%, respectively [1]. Unfortunately, M-PCR is not commercially available and requires special training and equipment. Development of a new diagnostic test for chancroid could be useful from several points of view.

The study by Patterson, et al. shows that monoclonal antibodies that bind to the hemoglobin receptor of the organism, called HgbA, can be used in an immunochromatography format to detect H. ducreyi. The performance of the test was determined using bacterial colonies lysed in a buffer containing Triton X-100 and SDS. Test strips containing an immobilized anti-HgbA MAb were exposed to the lysate for 15 minutes. The test strips were observed for the presence of a pink or purple line at the site of the captured antigen. The strips identified 26 of 26 H. ducreyi strains that were obtained over the past 4 decades from the United States, the Caribbean, Africa, and the Far East, and did not react to 8 other bacterial species that colonize or infect the genital tract or that can be recovered from swabs of genital ulcers plated on nonselective media. As currently configured, the lower limits of detection were 2 x 106 colony forming units of lysed H. ducreyi.

As pointed out by the authors, whether the limit of detection of the assay for H. ducreyi is sufficient for this test to be useful clinically is unclear. The authors express optimism that the test may perform well on clinical specimens, because it may not depend on the recovery of viable organisms on swabs as much as capture of HgbA on swabs. They suggest that HgbA is likely to be expressed by the organism in ulcers, as it is required for pustule formation in human volunteers. The authors fail to discuss the fact that in natural chancroid, samples of ulcers obtained with swabs or wire loops generally yield between 1 to 25 CFU [2,3]. In a study in which ulcers had to be sufficiently purulent so that a cotton swab could be saturated with ulcer pus, swabs of 9 ulcers generally yielded from 100 colonies to confluent growth [4]. If small numbers of colonies (25-100) are typically recovered by swabbing chancroidal ulcers, or HgbA does not withstand proteases released by PMNs in ulcers, then the immunochromatographic test strip as configured is unlikely to have a limit of detection that is low enough to detect H. ducreyi. However, if HgbA is upregulated in vivo, survives protease digestion, and is captured by a swab, the number of recoverable CFU could be irrelevant.

The authors point out that the greatest advantage of the test would be the rapidity of making a diagnosis of chancroid. However, even an inexpensive diagnostic test for screening GUD patients for chancroid may not be cost effective given the changing epidemiology of this disease. Historically, female sex workers are a reservoir for chancroid and play an important role in its epidemiology. In areas where syndromic treatment has been provided to sex workers, chancroid prevalence has fallen dramatically [5]. It may be more cost effective to use the highly sensitive M-PCR assay to screen a limited number of GUD patients when chancroid is suspected and provide syndromic management if chancroid is found rather than using a rapid assay to screen all GUD patients in low prevalence settings. In addition, approximately 15% of chancroidal ulcers diagnosed by M-PCR are coinfected with other agents, and the immunochromatography test will not aid in the diagnosis of other agents of GUD.

In summary, Patterson et al. provide a solid proof of principle that a particular set of monoclonal antibodies might be useful for the diagnosis of chancroid. The clinical utility of the test is unknown. The immunochromatography test will aid in our understanding of the epidemiology of chancroid only if it compares well to culture or M-PCR in the field

References:

1. Bong, C.T.H., Bauer, M.E. and Spinola, S.M. Haemophilus ducreyi: clinical features, epidemiology, and prospects for disease control. Microbes and Infection 4:1141-1148, 2002.

2. Hammond, G.W., Lian, C.J., Wilt, J.C. and Ronald, A.R. Comparison of specimen collection and Laboratory Techniques for Isolation of Haemophilus ducreyi. J.Clin.Microbiol. 7:39-43, 1978.

3. Schmid, G., Faur, Y., Valu, J., Sikandar, S. and McLaughlin, M. Enhanced recovery of Haemophilus ducreyi from clinical specimens by incubation at 33 versus 35oC. J.Clin.Microbiol. 33:3257-3259, 1995.

4. Nsanze, H., Pummer, F.A., Maggwa, A.B.N., Dylewski, J., Piot, P. and Ronald, A.R. Comparison of media for primary isolation of Haemophilus ducreyi. Sex.Transm.Dis. 11:6-9, 1984.

5. Steen, R. On eradicating chancroid. Bull.WHO. 79:818-826, 2001

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