Background
Species of the genus
Coccidioides, primarily
Coccidioides immitis and
C. posadasii, cause coccidioidomycosis, which is a disease endemic to North and South America [
1]. This mycosis is most prevalent in the Southwestern United States (US), Northern Mexico (MX), Central America and the foothills region of South America [
2]. The habitat conditions that permit the development of the saprophytic phase of these fungi are sandy soils in arid areas where the annual rainfall is less than 500 mm [
3‐
5].
Coccidioidomycosis infection in humans and other mammals is caused by the inhalation of arthroconidia. The illness begins with acute respiratory symptoms that are typically benign and vanish spontaneously; however, the disease can evolve into progressive clinical forms that spread to the skin and subcutaneous, visceral and skeletal tissues. These severe progressive forms cause high morbimortality and are commonly associated with immunocompromised patients [
6].
Coccidioidomycosis is an emerging disease because increased infection rates have been recorded in recent years from MX [
7] and Argentina (AR) [
8]. In endemic areas in the US, more than 100,000 primary human infections by
Coccidioides spp. are estimated each year; a considerable increase in the incidence of this disease has been noted in recent years, particularly in California and Arizona. The increased incidence of the disease has been associated with a rise in the migration of individuals who have not previously been exposed to the fungus into the endemic areas [
9].
The current epidemiology for coccidioidomycosis in MX is unknown, because no prevalence studies have been conducted in most Mexican states since 1960. However, according to the information that is available, more than 1,500 cases of primary coccidioidomycosis and 15 cases of disseminated disease are estimated annually. It is important to note that this estimate was based on epidemiological studies prior to 1994; since 1995, there are no records of coccidioidomycosis incidence in MX because this infection was excluded from reports prepared for the national epidemiological registry [
10]. This suggests that the disease may have developed much as it has in the US, where prevalence and incidence rates have soared since the early 1990s [
11].
Epidemiology in AR is similar to that in MX: there are few existing studies, although in recent years, several epidemiological studies have been performed. These studies include an investigation conducted by Canteros
et al. [
8] that sought to identify areas of endemic mycoses in 10 rural communities from the Teuco-Bermejito interfluve, which is in the Chaco province. Although results from this study demonstrated that
Histoplasma capsulatum was the principal agent of endemic mycoses, the researchers also indicated that the climatic conditions of the area are optimal for
Coccidioides development and thus that the potential for
Coccidioides infections in the area should not be dismissed. In another recent study conducted by Canteros
et al. [
12], the authors performed a comprehensive retrospective review of all documented coccidioidomycosis cases in AR between 1892 and 2009. This review demonstrated that between 2006 and 2009, the disease incidence in the Catamarca province increased from a historical rate of less than 0.5 cases per 100,000 inhabitants to 2 cases per 100,000 inhabitants, indicating that coccidioidomycosis is an emerging disease in this region.
Because of the increased incidence of this disease in North and South America, several studies in recent decades have sought to apply molecular techniques to better understand the taxonomy and population biology of
Coccidioides. Numerous studies
, principally in the US, have focused on the genetic variability of
C. immitis isolates and concluded that this fungus has high genetic recombination and no genetic structure among fungal isolates; however, the recombination process has never been observed [
13‐
17]. A recent study that supports the presence of recombination was conducted by Jewell
et al. [
18], who used microsatellites to determine that outbreaks of coccidioidomycosis in Arizona, US, were caused by a single fungal clone that was likely hypervirulent, possessed a high level of genetic variation and showed no dominant subtypes among its isolates. The absence of genetic structure among
C. immitis isolates and the presence of cryptic sex in both species of this fungus led to an investigation of whether isolates of
C. posadasii, which is the dominant species in Latin American countries, have the same behavior as isolates of
C. immitis, which is the dominant species in the Southwestern US. To find evidence that would indicate whether there is an expansion of the fungus in MX and AR, this study aimed to determine the pheno- and genotypic variability and the population structure of two populations (MX and AR).
Discussion
The spread of coccidioidomycosis over recent years in endemic areas of MX and AR is sufficient to warrant attention given that there are so few studies of the disease. Therefore, it is important to be well informed about the different aspects of
Coccidioides spp
. to carefully manage the disease. In this study, we estimated the genetic variability among
C. posadasii isolates from MX and AR. Our results indicate that the partial
Ag2/PRA Coccidioides spp. sequences obtained by Bialek
et al. [
21] method are useful for identify
C. posadasii from different geographical origins. This fragment can also be used to identify
C. immitis because this species presents in this region a deletion of 12 bases [
36]. Thus, this fragment is not suitable to diversity or genetic structure studies, because it presents scarce variation. On the other hand, the results of phenotypic characterisation parameters (growth rates using different NaCl concentrations and arthroconidial size) showed no differences among the
C. posadasii isolates.
The genetic diversity of the
C. posadasii isolates from MX and AR showed high genetic variability using polymorphic AFLP. However, AMOVA indicated that this variation was not geographically structured. The results from this study suggested high rates of gene flow between isolates in MX and AR, which explains the scarce differentiation found among them. A probable explanation about these findings may be the constant flow of genes between these populations favoring the air-borne dissemination of the fungus since the hyphae that constitute the saprobe or infectious stage fragment even with the lightest air currents, freeing the arthroconidia and easily travelling large distances in the wind [
37,
38]. The viability of the conidia in the environment is favored by the tolerability of the fungi to high temperatures (50°C) and their resistance to UV light (due to their melanine content) benefiting survival and longevity. Another possible explanation maybe high gene flow is a consequence of the constant movement of people across the continent or due to the migration of mammals [
39], among these specific bat species which are long-distance migratory. This hypothesis is supported by the recent findings by Cordeiro
et al. [
40] who demonstrated the infection by
Coccidioides spp. in these mammals.
The no-association among the
C. posadasii isolates from MX and AR with their geographical origin, was supported by the dendrogram and haplotype network, confirm the scarce genetic differentiation observed between them. These findings partially concur with Fisher
et al. [
39], who found low variability among
Coccidioides spp. isolates and little genetic differentiation among isolates from South America and the US. Notwithstanding, the technique used in this study was the AFLP. This is a useful tool for establishing the changes in the genome of the fungi isolates allowing for simultaneously analysing many loci and detecting a greater number of polymorphic DNA markers than any other method based on PCR. However, a limiting factor in this study was the impossibility of comparing results with other studies on the structure of
Coccidioides spp. populations due to the use of different methodologies. It is recommended that future studies use markers that have been validated and employed by other authors in studies of genetic variability of
Coccidioides spp. in order to compare the results obtained with isolates from different geographic regions.
On the other hand, the high genetic variability found among the isolates studied maybe explained by a recombinant sexual reproductive system (
I
A
= 0.0287), as was suggested by other authors [
1,
13‐
17,
41,
42]. This mode of reproduction was also supported by the presence of the potentially functional MAT idiomorph loci, MAT1-2 (HMG) and MAT1–1 (alpha-box) [
41,
42]. Thus, even though this species’ sexual phase remains undescribed, studies indicate that, based on high variability, these fungi recombine, gaining advantages such as adaption to new environments; thus, virulent or resistant strains could emerge, which may also explain the numerous recent epidemic outbreaks.
The observed high variability may also be explained by the possibility that the different genotypes could adapt to other environments under inappropriate developmental conditions, which is true for the isolate “MA”, found in a patient originally from the state of Campeche, located in the Southeast region of MX, who claimed to have never left the state. This region has climatic conditions different from the preferred fungal growth conditions. Similarly, adaptation to new environments facilitates the appearance of hypervirulent strains, as suggested by Fisher
et al. [
1] and Jewell
et al. [
18].
It is important to understand variability among these fungi, which has also been investigated in recent publications, to determine genotype distributions among populations, monitor outbreaks, assess variations in virulence and predict disease progression [
18,
43,
44]. Several lines of research that pertain to this issue are in progress that will further our understanding of the underlying biology of these pathogens and their interactions with other living species.
Acknowledgements
This paper is a partial fulfilment for the graduate Program in Biological Sciences of the National Autonomous University of Mexico (UNAM). E. Duarte-Escalante acknowledges support from the Program in Biological Sciences.
This project was funded by PAPIIT-DGAPA (IN215509-3).
The authors would like to thank to Actuary Dolores Hernández who provided statistical analysis support.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
MRRM, EDE were involved in the study design, analised and interpreted the results and drafted the manuscript. EDE and MGFDL performed the experiments. GZ conducted data analysis and participated in drafting the manuscript and provided a critical review of the manuscript. CC participated in the study design and provided a critical review of the manuscript. RCO performed microscopic fungal identification. MRRM conceptualized and coordinated the project. All authors read and approved the final manuscript.