We provide genotypic evidence that the strain dynamics of C. gattii isolates involved in the Pacific Northwest C. gattii emergence may be different from those seen on Vancouver Island, with VGIIb and VGIIc subtypes playing roles of equal significance to VGIIa types. We also show that human and animal infections caused by genotypes VGIIa, VGIIb, and VGIIc are occurring along the northern Pacific coast, now appear in California, and may have expanded eastward to Idaho. Because the described surveillance system is passive and not active, this study does not encompass the full scope of C. gattii disease in the Pacific Northwest United States, but it does provide a snapshot of human and animal cases that are occurring at this time.
There is no subtype bias in any one state in this study for either VGIIa or VGIIb, with each being found in Washington, Oregon, and California. This is the first report of the contemporary occurrence of genotypes VGIIa and VGIIb in California. As noted earlier by Byrnes and coworkers (4
), we also found that the VGIIc subtype is unique to Oregon with the exception of the single isolate from Idaho. The lone case-patient from Idaho had a travel history to Oregon and Washington and might have acquired the infection in either state. However, it must be noted that the CAP59
allele from the Idaho isolate differs from all of the other VGIIc CAP59
There were no significant differences in the distribution of subtypes between human and veterinary cases. Many of the veterinary cases had no history of travel, so it can be assumed that those cases arose from direct local environmental exposure. Because neither the strain types nor the MICs differed significantly between human and animal isolates, the animal cases may serve as a good sentinel for human cases in any particular area. Although we have no environmental isolates for comparison, earlier studies from both British Columbia and Washington state indicated that the genotypes of environmental isolates roughly correlated with the genotypes of patient isolates by the percentage of each genotype found (16
). It is interesting to note that no VGIII isolates were found during the environmental testing in British Columbia and Washington State (16
), and yet this type was not infrequent in either human or veterinary isolates in our study. Clearly, there is a need for more environmental testing in the Pacific Northwest United States.
This is the first analysis of antifungal susceptibility of C. gattii
isolates from the ongoing emergence in the Pacific Northwest and the first time that MICs were compared among subtypes. While much research has been conducted on the antifungal susceptibilities of C. neoformans
, very few studies have addressed the susceptibilities of C. gattii
and none have compared the susceptibilities of specific subtypes. Only a few studies have used CLSI broth microdilution to report the susceptibilities of C. gattii
), and none of these studies involved isolates from the Pacific Northwest emergence. As we found in our study, none of the earlier publications described elevated MICs to flucytosine or amphotericin B. Chen and coworkers (6
) reported a higher geometric mean MIC of their 21 Taiwan isolates to amphotericin B than was found for the isolates in this study, but the mean was still within the range considered susceptible. Although we did not find any isolates with fluconazole MICs of >32 μg/ml, Trilles and coworkers reported fluconazole MICs of ≥64 μg/ml in their study of 57 C. gattii
isolates from Brazil (28
), although the proportion of their isolates with very high MICs was not given. Chen and coworkers also reported that 20% of the 18 C. gattii
isolates from Australia that they tested had fluconazole MICs of ≥64 μg/ml (5
). Our finding that 23.3% of the Pacific Northwest isolates have fluconazole MICs of 16 to 32 μg/ml is roughly double the 12.7% of isolates in this category reported from Spain (24
) and reflects the findings of De Bedout and coworkers, who tested 15 C. gattii
isolates from Colombia (9
) and reported that 33% of their isolates had fluconazole MICs of 16 to 32 μg/ml. Voriconazole MICs in this study were relatively low, with the overall geometric mean MIC of 0.1 μg/ml and only a single isolate with an MIC of ≥1 μg/ml. Even so, our values are still higher than the geometric mean of 0.03 μg/ml reported for the isolates from Spain (24
). Twenty-three percent of our isolates had itraconazole MICs that would be considered resistant for Candida
species, but the clinical significance of these values for C. gattii
The most interesting aspect of our study is that the MICs to fluconazole could be correlated with subtype. All of the VGI and VGIII isolates had comparatively low fluconazole MICs, while the majority of isolates with MICs of 16 to 32 μg/ml were of the subtype VGIIc. Clearly, there are differences between the subtypes, and these differences need to be further explored.
Unfortunately, what is not known is the relationship between elevated MICs and clinical outcome. While there have been no studies to date which correlate C. gattii
MICs to clinical outcome, two recent studies of C. neoformans
compared fluconazole MICs to clinical outcome (1
). The authors of these manuscripts reached opposite conclusions, with one study stating that among AIDS patients high fluconazole MICs may be predictive for patients who will not respond to fluconazole therapy (1
), and the other study stating that among HIV-positive and -negative patients treated with fluconazole alone, there was no correlation between MICs and clinical success or failure (8
). This is an area where more research is needed.
While fluconazole MICs of 16 and 32 μg/ml seem high compared to typical MICs for C. albicans
and fluconazole, in the largest global study of C. neoformans
susceptibility to date (1,811 human isolates) 44% of the isolates had an MIC to fluconazole of ≥8 μg/ml (26
). Given this study and the studies cited above, it is clear that Cryptococcus
species may have elevated MICs to fluconazole compared to values and breakpoints for Candida
species. While we are not seeing MICs in the 64- to 256-μg/ml range, predictive of clinical failure for Candida
species, VGIIb and VGIIc isolates from the Pacific Northwest have elevated MICs to fluconazole and comprise almost half of the collected isolates from the Pacific Northwest United States emergence. Unfortunately, very little is known about the clinical course and outcome for patients infected with these strains.
Treatment of C. gattii
patients in the Pacific Northwest can be challenging, and these patients may have a difficult clinical course (31
). In light of the difficulty in treating these patients and the 8.7% case fatality rate for these types of patients seen in British Columbia (13
), it is prudent that physicians in the new areas of C. gattii
emergence are aware that their patients may have an infection with C. gattii
rather than with C. neoformans
. It is also critical that we develop a better understanding of the correlation, if any, between MICs and clinical outcome. Public health surveillance for C. gattii
infection, including tracking further spread of subtypes, may help us solve some of these problems.