Blastomycosis is a potentially fatal fungal disease of humans and dogs caused by the dimorphic fungus Blastomyces dermatitidis. The organism exists in a mycelial phase in nature and converts to a yeast phase at body temperature.1 Incubation periods in humans and dogs are long, ranging from 3 to 15 weeks. The severity of the disease can range from subclinical or self-limiting to severe clinical disease with pulmonary, cutaneous, or ocular involvement. Although cases of primary cutaneous blastomycosis among laboratory workers and veterinarians have been reported, the respiratory tract is considered to be the predominant portal of entry.2,3 Blastomyces dermatitidis is known to be endemic in the Ohio and Mississippi river valleys and in the states and provinces that border the Great Lakes and the Saint Lawrence River. Although the endemic areas have been identified, the organism's specific ecological niche remains poorly understood.4–6
Isolation of the organism in the field has proven to be difficult because the environmental conditions that promote the growth of the mycelial phase are often short-lived and difficult to predict. Bacteriologic culture of soil samples from endemic areas and samples collected after outbreaks is often nonproductive. However, results of studies7–11 focusing on the organism's ecological characteristics indicate that B dermatitidis can be more readily isolated from soil contaminated with rotting wood, decayed vegetation, and abundant organic matter. Conditions that appear to encourage its growth include proximity to sandy and acidic soils, lower elevation waterways, and riverbanks; recent rainfall; and high humidity12–15 In 2 epidemiological studies16,17 of outbreaks in Wisconsin, a significant association between human cases of blastomycosis and involvement in digging projects was identified. Data from at least 2 studies17,18 have suggested that meteorologic conditions within localized areas of an endemic area may periodically promote growth of the mycelial phase of the Blastomyces organism and increase potential exposure of humans and animals.
From north to south, Illinois is 395 miles long and is subject to substantial variation in climate, topography, and soil type; soil types vary from the highly fertile, deep, nutrient-enriched surface mollisols with high organic matter in the northern two-thirds of the state to the less fertile, clay-enriched alfisols in the southern third of the state. The state can be divided into 14 natural regions on the basis of bedrock geology, glaciation, soils, climate, drainage, and plant distribution.19
Humans and dogs are susceptible to disease after exposure to Blastomyces spores in the environment. It is known that the prevalence of blastomycosis in dogs may provide valuable information about the prevalence of blastomycosis in humans. MacDonald et al20 reported a temporal association between canine and human cases of blastomycosis in 2002. Baumgardner and Paretsky7 and Furcolow et al5 reported geographic clustering of canine cases of blastomycosis similar to that reported for humans. Dogs appear to be at a much higher risk for developing blastomycosis, compared with humans; the estimated prevalence among dogs is 10 times as great as the prevalence among humans.5 In 1998, Sarosi et al21 suggested that blastomycosis in dogs may be an important epidemiological marker for concomitant blastomycosis in humans. This apparent increased susceptibility along with a similar distribution of human and canine cases suggests that dogs could serve as an indicator species for human cases. Because dogs usually do not travel great distances as often as their owners, the distribution of dogs with blastomycosis might give a good approximation of the distribution of Blastomyces organisms in the local environment.21–24
To our knowledge, large-scale prevalence studies on blastomycosis in humans and dogs were last done in the 1970s. In those studies,4,5 only prevalences of human and canine cases were determined, with no mention of temporal and spatial relationships among cases, controls, and potential risk factors. Several studies11–13,20,21,24,25 have investigated these associations but only on a limited basis and often after outbreaks. One recent study18 compared suspected exposure sites of 2 geographically and ecologically distinct areas in Wisconsin in which blastomycosis was endemic. However, a statewide spatial and period prevalence survey of canine and human cases was not attempted.
Among Illinois health practitioners, northern Wisconsin has been recognized as a highly endemic area for blastomycosis, and recent travel to that part of Wisconsin may be among the history questions asked of humans with respiratory complaints and owners of pet dogs that are evaluated for respiratory tract signs. However, previous research has also established Illinois as an endemic area for blastomycosis in humans.26 Nonetheless, because there has been no statewide blastomycosis survey in dogs in Illinois to date, the level of exposure of dogs to blastomycosis within Illinois is unknown.
The objective of the study reported here was to compare the temporal and spatial distribution of blastomycosis cases among humans and dogs in Illinois. To this end, mean annual incidence of B dermatitidis infection in human and canine patients from 2001 through 2007 in Illinois was estimated. Survey information was used to determine demographic, spatial, and temporal associations between blastomycosis in humans and dogs and to evaluate the hypothesis that dogs with blastomycosis could act as an indicator for human infections with B dermatitidis.
Materials and Methods
Blastomycosis in humans was a reportable disease in Illinois from July 1994 until April 2008, when it was removed from the state list of reportable diseases. The case definition for blastomycosis in humans during the period in which it was reportable was B dermatitidis-positive results of bacterial culture of a non-skin-associated sample or detection of organisms resembling a Blastomyces sp in symptomatic individuals. Records of reported human cases of blastomycosis from 2001 through 2007 were accessed from the Illinois Department of Public Health. Human cases were reported by the Illinois Department of Public Health only as number of cases per county along with associated demographic information (gender, racial or ethnic group, and age). All human case data were analyzed in aggregate with no personal identifiers. The present study received a Title 45 Part 46 exemption from the Institutional Review Board of the University of Illinois at Urbana-Champaign.
To estimate annual incidence of blastomycosis in humans in Illinois, the number of human cases reported by county in each year from 2001 through 2007 was divided by the population of that county in each year. The population for Illinois counties each year was found by use of state-based information compiled by the US Census Bureau.27
For ease of data analysis, the 14 natural regions of the state were consolidated into 6 major regions: Grand Prairie, Mississippi and Illinois Rivers, Northeast Morainal, Shawnee Hills-Coastal Plain, Southern Till Plain, and Wisconsin Driftless-Rock River. Counties that overlapped regions were assigned to the natural region that covered most of the county area. Human cases of blastomycosis were segregated by ZIP code to these 6 consolidated natural regions of the state.
Blastomycosis in dogs is not a reportable animal disease in Illinois, and there is no existing database from which case information is available. For this study, canine cases were defined as those dogs from which samples yielded positive results of culture for Blastomyces spp or those dogs for which the yeast form of the organism was seen during microscopic examination of a biopsy specimen or culture plate growth or impression smear from samples (including those taken from cutaneous lesions). Dogs for which diagnoses were made on the basis of radiographic, ultrasonographic, or serologic evidence alone or in combination were excluded from the study.
A 2-part survey was used to estimate the annual incidence of blastomycosis in dogs for each year during the study period. The initial statewide survey was designed to establish in which Illinois counties canine cases of blastomycosis were recorded and to estimate the overall annual incidence of blastomycosis in dogs in Illinois. The follow-up practice-based survey was designed to investigate annual disease patterns in those practices, describe demographic characteristics, and analyze environmental factors associated with blastomycosis cases during the study period.
The initial survey was based on a random sample of the 747 private and corporate veterinary medical practices in Illinois. A listing of practices was obtained from the Illinois State Veterinary Medical Association.a A standard calculation for survey sample sizeb was used to determine the minimum sample size (197 practices) necessary to assure with 95% confidence that the sample was representative of veterinary practices in Illinois. Each practice in Illinois was assigned a random number,c and practices were contacted in ascending numeric order. Practices failing to respond after 2 requests for information were dropped from the survey. Referral hospitals were included in the survey; however, cases that were referred from another practice included in the survey were cross-matched and attributed only to the referring practice.
The initial survey was conducted via telephone interview, fax, or e-mail to determine whether the diagnoses of blastomycosis in dogs were in accordance with the study's case definition during the period of interest. The initial survey took approximately 5 minutes to complete and was pilot tested on veterinarian colleagues in academic and private practice prior to use. To estimate annual incidence, each practice counted blastomycosis cases for each year of the period of interest. Case counts were grouped by county and region and served as the numerator for incidences. The total number of dogs in each county for a given year was estimated by use of a human population-based formula developed by the AVMA.28 Responses from the initial survey identified counties in which blastomycosis in dogs had been identified and provided an estimate of statewide annual incidence of blastomycosis in dogs during the study period.
A follow-up survey was conducted with a subset of veterinary medical practices from the initial survey that were able to provide case records containing information regarding demographic and potential environmental risk factors for cases. All practices included in the follow-up survey were able to determine, through computer-generated patient censuses or rabies vaccination tag records, the number of canine patients in the practice each year during the study period. Estimated annual incidence for each practice in each year during the period of interest was calculated as the number of canine cases of blastomycosis per each year divided by the number of canine patients per each year. Again, county data were aggregated into 1 of 6 natural regions in Illinois. Responses from the follow-up survey were used to estimate annual incidences and patterns of blastomycosis case count in dogs within each practice; for dogs with signs of blastomycosis consistent with the case definition used in the study, age, sex, breed, place of residence, exposure to geographic features, and participation in activities such as hunting and herding livestock were noted.
Data were analyzed by use of commercial software.d To test the hypothesis that there were no mean or proportional differences in temporal and spatial distribution of human and canine case rates, t tests (assuming unequal variances) were used. Also, t tests (assuming unequal variances) were used to test the strength of association between breed (sporting vs nonsporting) and canine cases. χ2 Analysise was conducted to determine whether gender or race was significantly associated with diagnosis of blastomycosis in humans. Odds ratios and 95% CIs were calculated to determine the strength of these associations.e Single-factor ANOVAd was used to test the strength of association between canine cases and sex of dogs, quarter of the year in which diagnosis was made, or residence and environmental exposures. Simple linear regression modelingd was used to analyze trends of human and canine cases over the study period with year as the independent variable and estimated incidence of blastomycosis as the dependent variable. A value of P < 0.05 was considered significant.
Results
Geographic distribution of cases of blastomycosis—The distribution of human and canine cases of blastomycosis in 2001 through 2007 was determined on the basis of Illinois county data that were consolidated into 6 major natural regions. The distribution patterns were mapped by county as follows: human and canine cases reported, only human cases reported, only canine cases reported, or neither human nor canine cases reported (Figure 1).
Cases of blastomycosis in humans—The mean estimated annual incidence of blastomycosis in humans in Illinois from 2001 through 2007 was 7.5 cases/1 million persons/y. The number of reported cases per year during the period of interest increased from 48 (3.8 cases/1 million persons) in 2001 to 138 (10.7 cases/1 million persons) in 2007. From 1993 through 2000, the number of cases per year increased from 24 to 64, and the estimated annual incidence increased from 2.0 to 6.4 cases/1 million persons/y26 (Figure 2). The mean estimated annual incidence of human cases in some regions during the period of interest varied significantly from the statewide mean estimated annual incidence. In the Wisconsin Driftless-Rock River region, in the northwest corner of the state, the mean estimated annual incidence (11.7 cases/1 million persons/y) was significantly (P = 0.024) greater than that of the entire state, with 36 of the 40 (90%) cases in this region reported in Winnebago County. The mean estimated annual incidences for the Mississippi and Illinois Rivers (1.2 cases/1 million persons/y) and Southern Till Plain (2.6 cases/1 million persons/y) regions in the western and extreme southern parts of the state, respectively, were significantly (P ≤ 0.001) lower than the statewide mean estimated annual incidence. The mean estimated annual incidences for the Northeast Morainal region (8.9 cases/1 million persons/y), Grand Prairie (5.7 cases/1 million persons/y), and Shawnee Hills-Coastal Plain (4.3 cases/1 million persons/y) regions did not differ significantly from the statewide mean estimated annual incidence (Table 1). In 2007, there were 2.8 times as many cases reported (n = 138) as there were in 2001 (48) and 6.0 times as many cases reported as there were in 1993 (24).
Distribution of reported blastomycosis cases in humans by region in Illinois from 2001 through 2007.
Region | 2001 | 2002 | 2003 | 2004 | 2005 | 2006 | 2007 | Total | Mean regional estimated annual incidence (No. of cases/1 million persons/y) |
---|---|---|---|---|---|---|---|---|---|
Grand Prairie | 7 | 11 | 6 | 11 | 8 | 9 | 22 | 74 | 5.72 |
Mississippi and Illinois Rivers | 1 | 1 | 0 | 1 | 1 | 0 | 2 | 6 | 1.16* |
Northeast Morainal | 37 | 68 | 72 | 70 | 83 | 94 | 97 | 521 | 8.9 |
Shawnee Hills-Coastal Plain | 0 | 1 | 2 | 0 | 0 | 1 | 0 | 4 | 4.27 |
Southern Till Plain | 0 | 1 | 1 | 4 | 4 | 3 | 7 | 20 | 2.61* |
Wisconsin Driftless-Rock River | 3 | 5 | 8 | 5 | 6 | 5 | 8 | 40 | 11.69* |
No residence information available | 0 | 0 | 0 | 0 | 0 | 0 | 2 | 2 | 2 |
Total | 48 | 87 | 89 | 91 | 102 | 112 | 138 | 667 | 7.5 |
Estimated statewide annual incidence (No. of cases/1 million persons/y) | 3.8 | 6.9 | 7.0 | 7.2 | 8.1 | 8.7 | 10.7 | 10.7 | 10.7 |
Records of reported human cases of blastomycosis during the period of interest were accessed from the Illinois Department of Public Health. Data comparisons were performed by use of a t test, assuming unequal variances.
Significantly (P < 0.05) different from mean statewide estimated annual incidence.
Of the 667 humans with blastomycosis reported in Illinois from 2001 through 2007, 64.3% were male; ages ranged from 4 to 89 years (mean age, 44 years; median age, 44 years). The odds of reported infection occurring in a male during the study period were 1.88 (95% CI, 1.24 to 2.85; P = 0.003). Most cases (521 [78%]) were reported in the northeast region of the state (ie, the most populous region of the state), and 272 (41%) cases were reported in Cook County, which includes Chicago and accounts for approximately 41% of the state's population.
The largest number (335 [50.2%]) of human cases of blastomycosis involved persons who were reported as White. However, because White residents made up the largest proportion of the state's population (78%) during the period of interest, the odds of reported infection in a White resident, compared with infection in a non-White resident, were 0.313 (95% CI, 0.210 to 0.468; P < 0.001). Although persons who were reported as Black made up only 15% of the state's population during the period of interest, that group accounted for 27.7% of blastomycosis cases. Estimated annual incidence for Black residents was evenly distributed throughout the state. There was no significant difference in estimated annual incidence between the region with the highest proportional Black population (Northeast Morainal) and the other regions of the state. The mean estimated annual incidence for Black residents was 13.8 cases/1 million persons/y, which was 2.7 times that for White residents. The odds of reported infection in a Black resident during the period of interest, compared with infection in a non-Black resident, were 2.14 (95% CI, 1.36 to 3.34; P = 0.001). Asian residents comprised approximately 4.3% of the state's population in 2001 through 2007 and accounted for almost 5% of the cases. The mean estimated annual incidence for Asian residents was 10.9 cases/1 million persons/y, which was 2.1 times that for White residents. The odds of a case of blastomycosis occurring in an Asian resident during the period of interest, compared with a non-Asian resident, were 1.37 (95% CI, 0.57 to 3.28; P = 0.493). There was no significant difference in mean estimated annual incidence between the region with the highest proportional Asian population (Northeast Morainal) and the rest of the state. There was no significant difference between the mean estimated annual incidence for Black residents and that for Asian residents for any year during the period of interest (Table 2).
Gender and ethnicity distribution and mean gender- and race-specific estimated annual incidences of blastomycosis in humans in Illinois from 2001 through 2007.
Characteristic | No. of persons (%) | Mean estimated annual incidence (No. of cases/1 million persons/y) | Odds ratio† (95% CI) | P value |
---|---|---|---|---|
Sex | ||||
Male | 429 (64.3) | 9.9 | 1.88 (1.24–2.85) | 0.003 |
Female | 236 (35.5) | 5.3 | Reference | — |
Unknown | 2 | — | — | — |
Race | ||||
White | 335 (50.2) | 5.1 | 0.31 (0.21–0.47) | 0.0001 |
Black | 185(27.7) | 13.8 | 2.14 (1.37–3.34) | 0.002 |
Asian | 33 (4.9) | 10.9 | 1.37 (0.57–3.28) | 0.49 |
Other or unknown | 114(17.1) | — | — | — |
Total No. of persons (statewide) | 667(100.0) | 7.5 | — | — |
Population characteristics were derived from 2004 state census data.
Odds ratios and 95% CIs were calculated for the following comparisons of mean estimated annual incidence of blastomycosis: male versus female, White versus non-White, Black versus non-Black, and Asian versus non-Asian.
— = Not calculated.
Cases of blastomycosis in dogs—Of the 747 private veterinary medical practices in Illinois, 303 randomly selected practices were contacted for the initial survey. Responses were received from 221 (72.9%) practices; no response was received from 82 practices after 2 contacts. The distribution of responding and nonresponding practices was not related to region. The sample size of 221 responding practices was greater than the 197 required to assure with 95% confidence that the sample was representative of the practices in Illinois.
Of the practices that participated in the study, 107 (48%) had made a diagnosis of blastomycosis in ≥ 1 dog according to the established case definition during the study period. The percentage of practices with at least 1 case of blastomycosis in a dog during 2001 through 2007 and the estimated number of cases per year varied by region but did not differ significantly. The Shawnee Hills-Coastal Plain region, in the southern portion of the state, had the highest mean estimated annual incidence, and consistent with the human case data, the Mississippi and Illinois Rivers region, in the western part of the state, had the lowest mean estimated annual incidence. Regional estimated annual incidences were not significantly different from statewide estimated annual incidences.
During the 2001–2007 period, the mean estimated annual incidence of blastomycosis in dogs (62.5 cases/100,000 dogs/y) from the initial survey was 8.3 times the mean annual incidence of blastomycosis in humans, with similar incidence trends and regional distributions (Table 3).
Distribution of reported blastomycosis cases in dogs by region in Illinois from 2001 through 2007.
Region | No. of practices in region | No. of practices contacted | No. of practices that responded to survey (%*) | No. of practices with blastomycosis cases (%†) | Estimated mean No. of cases per year | Estimated No. of dogs in region | Mean estimated annual incidence (No. of cases/100,000 dogs/y) |
---|---|---|---|---|---|---|---|
Grand Prairie | 167 | 82 | 65 (38.9) | 44(67.7) | 73 | 462,507 | 15.8 |
Mississippi and Illinois Rivers | 56 | 35 | 25 (44.6) | 5(20.0) | 3 | 187,683 | 1.6 |
Northeast Morainal | 389 | 101 | 62(15.9) | 31 (50.0) | 76 | 2,113,896 | 3.6 |
Shawnee Hills-Coastal Plain | 9 | 9 | 9(100.0) | 1(11.1) | 8 | 32,834 | 24.4 |
Southern Till Plain | 87 | 46 | 32 (36.8) | 9(28.1) | 21 | 274,103 | 7.7 |
Wisconsin Driftless-Rock River | 39 | 30 | 28(71.8) | 17(60.7) | 18.5 | 123,067 | 15.0 |
Total (statewide) | 747 | 303 | 221 (29.6) | 107(48.4) | 199.5 | 3,194,090 | 6.25 |
Canine case counts were obtained from results of a survey of Illinois veterinary practices.
Percentage of practices that responded to the survey among all practices in the region.
Percentage of practices in which blastomycosis was diagnosed in ≥ 1 dog among the total number of practices that responded to the survey in the region.
Of the practices that responded to the initial survey, 58 (26%) had 203 case records that were sufficiently complete to allow assessment of demographic and environmental risk factors through completion of a follow-up survey. This follow-up survey was completed via either fax or e-mail. Because practices in which blastomycosis had not been diagnosed in any dogs were eliminated from the second survey, estimated annual incidences were considerably higher than those reported for all practices in the initial survey.
Similar to the blastomycosis data for humans, data for dogs obtained from the follow-up survey indicated a significant upward trend in the number of cases reported per year. The mean estimated annual incidence reported by Illinois practices in the follow-up survey increased from 2.4 cases/100,000 dogs/y in 2001 to 121.5 cases/100,000 dogs/y in 2007 (Figure 3). Follow-up survey data from veterinary practices were used for trend analysis because the numerator (cases of blastomycosis in dogs) and denominator (dog census by practice) were not estimates.
Most infections with B dermatitidis were diagnosed in male dogs (n = 119 [58.8%]), with neutered male dogs accounting for the majority of cases (73 [36.6%]). The proportion of cases of blastomycosis that occurred in male and female dogs was not significantly (P = 0.51) different. More than half of the infected dogs (111 [54.9%]) resided in rural areas, more than a third (70 [34.7%]) resided in suburban areas, and the remainder resided in urban areas (21 [10.4%]). Differences in case counts segregated by residence were not significant (P = 0.098). The mean age of infected dogs was 4.8 years, and the largest proportion of infections (32.8%) were diagnosed during the third quarter of the year; however, this seasonal pattern was not significant (P = 0.91). A significant (P = 0.04) proportion of diagnoses were made in sporting breeds (n = 99 [48.8%]), which is consistent with previous reports.29,30 Among these sporting breeds, blastomycosis was most commonly diagnosed in retrievers and retriever mixes (n = 73 [76%]).
One hundred thirteen of the 203 case records included in the follow-up survey contained data on social and environmental factors for the 6 months prior to the diagnosis of blastomycosis. Of these, 80 (70.8%) dogs had exposure to waterways, and 90 (79.6%) had exposure to disturbed ground through access to gardens, construction sites, or farm fields. Five (4.4%) case animals had exposure to dog parks, and 8 (7.1%) case animals had exposure to livestock arenas, pastures, or pens. Differences between groups of environmental exposures were not significant (P = 0.29; Table 4). Additionally, 65 (32.3%) case records contained information on travel outside of the county of residence. Among those dogs, 4 (6.2%) had traveled outside of their county of residence in the 12 months prior to diagnosis of blastomycosis.
Demographic and risk factor data regarding cases of blastomycosis in dogs in 2001 through 2007 collected via a follow-up survey of 58 veterinary practices in Illinois.
Characteristic | No. of dogs (%) | P value |
---|---|---|
Sex | 0.51* | |
Sexually intact male | 46(22.2) | |
Neutered male | 72 (36.6) | |
Sexually intactfemale | 27(13.4) | |
Neutered female | 57 (28.9) | |
Quarter of year† | 0.91 | |
First | 44(22) | |
Second | 47 (23) | |
Third | 66 (33) | |
Fourth | 44(22) | |
Breed group | 0.04‡ | |
Sporting | 99 (48.8) | |
Hound | 6(2.9) | |
Working | 41 (20.2) | |
Terrier | 6(2.9) | |
Toy | 7 (3.5) | |
Nonsporting | 15(7.4) | |
Herding | 14(6.9) | |
Mixed | 15(7.4) | |
Residence | 0.098* | |
Urban | 21 (10.4) | |
Suburban | 70 (34.7) | |
Rural | 111 (54.9) | |
Environmental exposures | 0.29* | |
Waterways | 80 (70.8) | |
Disturbed ground | 90 (79.6) | |
Outdoor pursuits | 36(31.9) | |
Dog parks | 5 (4.4) | |
Bats | 32 (28.3) | |
Livestock arenas | 8(7.1) |
Calculated on the basis of single-factor ANOVA of sex designation, residence, and exposure to environmental risk factors of case animals.
Year of diagnosis.
Calculated on the basis of a t test that assumed unequal variances, comparing sporting with nonsporting breeds.
During the 7-year period covered by the survey, 32 of the 102 counties in Illinois reported human and canine cases of blastomycosis, and 33 counties reported neither. In 19 counties, blastomycosis was diagnosed in humans but not in dogs, according to the reported data. In 18 counties, the converse was true. Of the 37 counties in which either human or canine cases but not both were reported, 17 were among the 35 counties of the 2 most southern regions in the state (Southern Till Plain and Shawnee Hills-Coastal Plain). The remaining 20 counties in which either human or canine cases but not both were reported were spread out over the other 4 regions and 65 counties. In 6 counties in the western portion of the Southern Till Plain region, blastomycosis was diagnosed in humans but not in dogs. In 4 counties in the eastern portion of the same region, blastomycosis was diagnosed in dogs but not in humans (Figure 1).
Discussion
The mean estimated annual incidence of blastomycosis in humans in Illinois during the period of interest (7.5 cases/1 million persons/y) was in the middle of the range of annual incidences previously reported for other states in the Mississippi watershed in which the disease is considered to be endemic. In previous studies,31–34 the annual incidence of blastomycosis in humans ranged from 14 and 13 cases/1 million persons/y in Wisconsin and Mississippi, respectively, to 2.3 and 2.0 cases/1 million persons/y in Louisiana and Missouri, respectively. The findings of the present study have provided additional evidence that the number of human cases of blastomycosis in Illinois continues to increase. Using the same case definition, Dworkin et al26 found a mean annual incidence of 3.8 cases/1 million persons/y from 1993 through 2002, approximately half the rate determined in the present study. The mean age (44 years; median, 44 years) and gender distribution of cases (64.3% were male) in the present study were consistent with results of previous studies.11,26,32,33
Similar to findings of previous studies25,31,34 in other states, there were regional differences in estimated annual incidence of blastomycosis in humans within Illinois. The northern and northeastern regions of the state reported the highest mean estimated annual incidences, whereas the western region of the state, along the Mississippi River, reported the lowest mean estimated annual incidence. Within regions of the state, there were areas in which a high number of cases were reported. In the Grand Prairie region, in east central Illinois, Champaign County accounted for 23% of all reported cases of blastomycosis in humans (mean estimated annual incidence, 12.9 cases/1 million persons/y) and in the Wisconsin Driftless-Rock River region, in the far northern part of the state, Winnebago County accounted for 90% of all cases of blastomycosis in humans (mean estimated annual incidence, 17.9 cases/1 million persons/y). The mean estimated annual incidences in both of those counties were considerably higher than the statewide mean estimated annual incidence. Regional variations in the calculated blastomycosis incidences for humans and dogs in Illinois should be interpreted with caution because the 6 regions used in this analysis included combinations of natural regions, and the data, although the best available to us, were derived from passive human surveillance and retrospective reporting of canine cases.
The present study provided relevant demographic information on differential case rates among the Black and Asian populations in the state. Because the Illinois Department of Public Health did not specify Hispanic ethnicity in the blastomycosis data during the period of interest, we were not able to analyze Hispanic ethnicity as a risk factor for blastomycosis in humans.
The association of Black race with blastomycotic infection has been reported previously.31,32,34 However, the higher case rate among Asian residents of Illinois has not been previously reported, to our knowledge. An assumption that genetic factors may be involved in an apparent increased susceptibility of people of Black race to blastomycosis was made in earlier studies.31 This assumption cannot be ruled out, but there is also a possibility that sociodemographic factors, such as occupation in manual labor trades and participation in leisure activities such as hunting and fishing, may be involved as well. A previous case-control study31 did not reveal an association between blastomycosis and recreational and occupational exposures. However, that study31 assessed relatively small sample sizes, and individuals were interviewed years after the exposure had occurred, introducing a potential recall bias in interviews about potential exposures. The authors of that report31 identified another limitation in determining environmental and socioeconomic risk factors for a disease that is endemic to an area: many potential risk factors of interest are common aspects of everyday life and would require a large, long-term, prospective cohort study to generate enough statistical power to determine strength of association. Ultimately, the apparent greater susceptibility of some racial and ethnic groups to development of blastomycosis may be attributable to the interplay between individual genetics and multiple social, biological, chemical, and physical exposures.
To our knowledge, this is the first report on the burden of blastomycosis in dogs in Illinois. Our attempt to estimate annual incidences over the period of interest was subject to a number of limitations. Because blastomycosis is not a reportable animal disease in Illinois, there is no statewide or local registry for case reporting. Second, the total number of cases reported by many of the practice respondents in the initial survey could have been a considerable underestimation because practitioners and staff could only verify cases that they remembered and for which they had paper or computer-based records. Third, there is no formalized canine census in Illinois. We attempted to use canine rabies immunization records as the basis of calculating canine populations in each county but found that, although rabies immunization of dogs is required by state law, not all of the 102 counties in Illinois require registration with a county agency. Finally, the formula used to calculate canine populations in communities, although well researched and documented by the AVMA, is only an estimate. In effect, the numerator (cases) and the denominator (canine population) used to determine the incidences of blastomycosis among dogs from initial survey data were based on estimates.
The mean estimated annual incidence of blastomycosis in dogs in Illinois from the initial survey was significantly higher than that for humans and was in agreement with results of previous studies.5 Dog data from the initial survey indicated that there were regional differences in the calculated incidences similar to those identified from the human data; however, these regional differences in estimated annual incidence of blastomycosis in dogs were not significant, with P values (t test) ranging from 0.5 to 0.8.
Consistent with the statewide human case data, there was an increase in the estimated annual incidence of blastomycosis in dogs in the period of interest as reported by the subset of veterinary practices that responded to the follow-up survey. The data obtained via the follow-up included demographic and environmental risk factor data for canine cases. However, practices in which blastomycosis had not been diagnosed in any dogs were excluded, so follow-up survey data could not be used to calculate statewide estimated annual incidences.
More detailed information regarding the breeds of dogs infected with blastomycosis was available from the follow-up survey. Although the highest number of cases was among sporting breeds, it was impossible to assess breed-related risks for blastomycosis infection without knowing the absolute numbers of each breed in Illinois. Such data are not currently available. Although most dogs reported to have blastomycosis had previous exposure to waterways and disturbed ground, it was impossible to determine the strength of association between reported environmental factors and case rates in dogs on the basis of the follow-up survey data because no survey data were obtained for a control group.
Survey data may underestimate the true numbers of human and canine cases in each population. In the present study, both case definitions excluded patients with blastomycosis that were diagnosed on the basis of serologic evidence, detection of urine antigen, or radio-graphic evidence alone or in combination. Additionally, because of the nonspecific and often mild clinical signs of blastomycosis, physicians and veterinarians may not recognize cases and may not pursue diagnostic tests required to support the case definitions used in our study.
If dogs with blastomycosis are indicators that humans are at risk for the disease, there should be few counties in endemic areas of Illinois in which there are reports of human cases but no canine cases. Almost half (17) of the 37 (46%) counties in which either human or canine cases but not both were reported were in the southern third of the state. A diagnosis of blastomycosis in either a human or dog indicates that, at some time during the period of interest covered by the survey, environmental conditions were adequate to support the growth of Blastomyces organisms and that exposure of humans or dogs was sufficient to cause clinical disease. Evidently, local human and veterinary health-care providers should be considering this diagnosis in any patient with appropriate clinical signs. Throughout Illinois, veterinarians, physicians, and public health agencies should be encouraged to communicate with each other regarding diagnoses of blastomycosis in either species. Perhaps this could best be accomplished through communication with the local health department as an intermediary. Delay of diagnosis after onset of illness in humans has been associated with an increase in mortality rate.26 Although the disease may not always be reportable in states where it is endemic, there is a case to be made for heightening awareness of the disease because delay of diagnosis may be responsible for prolonged morbidity or even death. If health-care providers were made aware of reports of recent canine cases of blastomycosis, physician awareness might be heightened, and diagnostic testing in persons with compatible illness might occur sooner.
ABBREVIATION
CI | Confidence interval |
Illinois State Veterinary Medical Association member directory [database online]. Springfield, Ill: Illinois State Veterinary Medical Association, 2007. Available at: isvmaimpak.networkats.com/members_online/members/directoryi.asp. Accessed Mar 13, 2008.
Survey Sample Calculator, Raosoft Inc, Seattle, Wash.
Microsoft Excel, Microsoft Inc, Redmond, Wash.
SPSS, version 17.0, SPSS Inc, Chicago, Ill.
StatPages.org 2-Way Contingency Table Analysis, John C. Pezzullo, Department of Medicine, Georgetown University, Washington, DC. Available at: statpages.org/ctab2×2.html. Accessed Aug 4, 2009.
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