Geographic distribution of Pythium insidiosum infections in the United States

Don Nguyen Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI

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Raquel Vilela Biomedical Laboratory Diagnostics Program, Michigan State University, East Lansing, MI
Faculty of Pharmacy, Federal University of Minas Gerais, Minas Gerais, Brazil

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Bruno M. Miraglia Faculty of Pharmacy, Federal University of Minas Gerais, Minas Gerais, Brazil

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Gabriella Vilela Biomedical Laboratory Diagnostics Program, Michigan State University, East Lansing, MI

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Noora Jasem-Alali Biomedical Laboratory Diagnostics Program, Michigan State University, East Lansing, MI

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Riann Rohn Biomedical Laboratory Diagnostics Program, Michigan State University, East Lansing, MI

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Robert Glass Pan American Veterinary Laboratories, Lexington, TX

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Richard D. Hansen SolidTech Animal Health, Inc, Newcastle, OK

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Leonel Mendoza Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI
Biomedical Laboratory Diagnostics Program, Michigan State University, East Lansing, MI

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Abstract

OBJECTIVE

To describe the geographic distribution of infections caused by Pythium insidiosum in dogs, horses, and other animal species in the US.

ANIMALS

For the last 20 years, we have collected data from cases of pythiosis in 1,150 horses, 467 dogs, and other species (59) from various geographic locations in the US.

PROCEDURES

Due to lost data (from 2006 to 2016), the selected cases include years 2000 to 2005 and 2016 to 2020. The selection of cases was based on infected host clinical features, serum samples demonstrating strong positive anti–P insidiosum IgG titers in serologic assays, and positive results on ≥ 1 of the following diagnostic modalities: microbial culture on 2% Sabouraud dextrose agar, histologic evaluation, PCR assay, and wet mount cytologic evaluation (with potassium hydroxide).

RESULTS

Most confirmed P insidiosum infections were found in horses and dogs in the southeastern US. Interestingly, in Texas, no cases were found west of longitude 100°W. Few pythiosis cases were diagnosed in west-coast states. Equine cases were more often diagnosed during summer and fall months, but canine cases were more often diagnosed between September and February. Cases in other species were discovered in the same geographic areas as those in dogs and horses.

CLINICAL RELEVANCE

To our knowledge, this is the first report providing the ecological distribution of P insidiosum infection in affected species in the US. Results of this study illustrated the importance of including P insidiosum in the differential diagnostic scheme of nonhealing skin lesions or intestinal granulomatous masses, particularly in dogs and horses inhabiting or having visited endemic areas.

Abstract

OBJECTIVE

To describe the geographic distribution of infections caused by Pythium insidiosum in dogs, horses, and other animal species in the US.

ANIMALS

For the last 20 years, we have collected data from cases of pythiosis in 1,150 horses, 467 dogs, and other species (59) from various geographic locations in the US.

PROCEDURES

Due to lost data (from 2006 to 2016), the selected cases include years 2000 to 2005 and 2016 to 2020. The selection of cases was based on infected host clinical features, serum samples demonstrating strong positive anti–P insidiosum IgG titers in serologic assays, and positive results on ≥ 1 of the following diagnostic modalities: microbial culture on 2% Sabouraud dextrose agar, histologic evaluation, PCR assay, and wet mount cytologic evaluation (with potassium hydroxide).

RESULTS

Most confirmed P insidiosum infections were found in horses and dogs in the southeastern US. Interestingly, in Texas, no cases were found west of longitude 100°W. Few pythiosis cases were diagnosed in west-coast states. Equine cases were more often diagnosed during summer and fall months, but canine cases were more often diagnosed between September and February. Cases in other species were discovered in the same geographic areas as those in dogs and horses.

CLINICAL RELEVANCE

To our knowledge, this is the first report providing the ecological distribution of P insidiosum infection in affected species in the US. Results of this study illustrated the importance of including P insidiosum in the differential diagnostic scheme of nonhealing skin lesions or intestinal granulomatous masses, particularly in dogs and horses inhabiting or having visited endemic areas.

Introduction

Pythiosis is a life-threatening disease of humans and animals caused by the oomycete Pythium insidiosum.14 This oomycete pathogen is acquired through open skin after contacting contaminated wet environments containing P insidiosum zoospores (the infecting propagules).5 Dogs may also be infected by drinking zoospores-contaminated water or eating plants, especially grasses, where encysted zoospores could attach. Infected hosts exhibit superficial, cutaneous, subcutaneous, intestinal, and/or systemic infections.1,46 Phenotypically, P insidiosum resembles a filamentous fungus, but is a mold-like pathogen (phylum, Oomycota) currently classified within the SAR (Stramenopiles, Alveolates, and Rhizaria) supergroup.2,4,5,7 Several tools, including microbial culture, histologic evaluation, molecular assays, and serologic testing, are available for the accurate diagnosis and identification of this pathogen in the laboratory.4,811

Pythiosis in animals is common in tropical and subtropical areas of the world.1,3,4,11,12 Recently, pythiosis in the US has been diagnosed with increasing frequency in the same regions mentioned by earlier investigators.1,4,6,1219 However, details on the geographic distribution and the epidemiology of infections caused by P insidiosum in animals in the US has yet to be addressed. Thus, we conducted an epidemiological study utilizing data collected from confirmed cases of the disease with the intent to identify the geographic distribution and frequency of P insidiosum infections in endemic areas.

Materials and Methods

Data collection and recording

Between 2000 and 2005, Biomedical Laboratory Diagnostics at Michigan State University (BLD-MSU) offered microbial culture on 2% Sabouraud dextrose agar, wet mount cytologic evaluation using 10% potassium hydroxide, histologic evaluation, serologic testing (by immunodiffusion), and molecular diagnostics using a PCR assay to diagnose pythiosis for the veterinary and human medical communities. Pan American Veterinary Laboratories (Lexington, Texas) took over the serologic diagnosis using an ELISA between 2006 and 2020. During the same period, BLD-MSU retained the other described diagnostic capabilities. It is important to note that due to computer failure, we lost the accumulated data of 2006 to 2016; thus, the study included 9 full years of collected data (2000 to 2005 and 2016 to 2020). Selection of cases in this study was based on the clinical features including skin ulcers and/or intestinal granulomas in the infected species, positive ELISA serologic reactivity detecting high anti–P insidiosum IgG titers, the presence of 2 to 3 identity bands on an immunodiffusion assay, and positive results in ≥ 1 of the following diagnostic assays: microbial culture, histologic evaluation, PCR assay, and wet mount cytologic evaluation. Cases in this study exhibited skin ulcers or intestinal granulomas and had a positive ELISA result and positive results for ≥ 1 of the mentioned assays. The data for each sample were recorded using a template that included type of lesions, species, tests performed, ZIP code listed on the submission form, physician or veterinarian, owner, date of diagnosis, and treatment outcome. The described diagnostic assays were performed either by Pan American Veterinary Laboratories (ELISA) or by BLD-MSU (microbial culture, histologic evaluation, immunodiffusion, PCR assay, and wet mount cytologic evaluation). These entities are well-recognized reference laboratories in the US, focusing on the diagnosis of P insidiosum infection.1,4,10,11

Criteria for the selection of cases

Cases of pythiosis in cats (n = 25), cattle (18), dogs (467), and horses (1,150) included in this study were unpublished cases originally diagnosed in our institutions from 2000 to 2020 as described. All selected individuals in this study exhibited cutaneous or intestinal granulomas, a strong positive serologic reaction, and positive results on additional assays. The number of cases by state and species following the above criteria are listed (Table 1). The location was determined based on the ZIP code where the case was first clinically diagnosed, the date when the diagnosis was confirmed (clinical features plus serologic testing plus other assays, as described). In cases where the original location of the patient was not available, the office address of the physician or veterinarian in charge was selected as the ZIP code for geographic location.

Table 1

Number of pythiosis cases by state and species over a 9-year period.

State No. of animals with pythiosis
Horses Dogs Other species
Alabama 29 3 0
Arizona 6 5 0
Arkansas 47 17 0
California 1 6 2
Colorado 0 1 0
Florida 401 104 15
Georgia 35 22 1
Idaho 2 2 0
Illinois 5 5 2
Indiana 8 3 0
Iowa 1 0 0
Kansas 0 1 0
Kentucky 5 1 0
Louisiana 46 21 5
Maryland 1 2 0
Michigan 6 1 0
Minnesota 0 3 0
Mississippi 35 25 2
Missouri 17 11 0
Nebraska 2 0 0
New Jersey 1 0 0
New Mexico 1 0 0
New York 2 5 0
North Carolina 20 31 0
North Dakota 0 2 0
Ohio 0 6 0
Oklahoma 13 7 0
Oregon 1 1 2
Pennsylvania 3 9 4
South Carolina 25 15 1
Tennessee 6 6 5
Texas 392 143 20
Virginia 37 7 0
Wisconsin 2 2 0
Total 1,150 467 59

Due to epidemiological relevance, information on published pythiosis cases involving captive animals and a California bird are provided along with 3 cases of pythiosis in camels that were diagnosed at our laboratory (Appendix).

Assembly of cases by year

The number of cases diagnosed each month during the 9 years of the study (data collection and recording) were compiled and tabulated using a commercially available software program (Excel 2016; Microsoft Corp).

Results

Pythiosis in dogs and horses

The species more frequently diagnosed with P insidiosum infections in our study were dogs (n = 467) and horses (1,150; Table 1). Pythium insidiosum lesions in horses were more commonly found on their limbs, but other anatomic areas in contact with wet environments containing propagules of P insidiosum were also reported (ie, abdomen, chest, face, prepuce, and other). In dogs, a variety of skin areas were affected, including limbs and virtually all other areas (80%; 374/467), whereas the intestinal form was less common (20%; 93/467). Pythiosis in horses and dogs displayed similar geographic distributions, with southeastern states (Arkansas, Alabama, Florida, Georgia, Louisiana, Mississippi, and Texas) followed by Oklahoma and nearby states having the highest number of cases. Most cases in these 2 species occurred in Florida and Texas, with clusters in Louisiana, Mississippi, and southern Oklahoma. Interestingly, several cases of pythiosis in dogs were also recorded in North Carolina, South Carolina, and Virginia. In this study, we found few cases from western states, specifically California, Idaho, and Oregon. In Texas, no cases were found west of longitude 100°W, which also was the case for the northern states at the same longitude. According to the consulted literature, states with no previous cases of pythiosis in these species included Idaho, Minnesota, Montana, Nebraska, North Dakota, and Oregon.

A significant number of canine cases were diagnosed during January and February, with lower numbers in summer months in this evaluation (Figure 1), whereas most equine cases were diagnosed between summer and fall during the 9-year period. Overall, horses showed the highest number of cases during summer and fall, while dogs displayed more cases in fall and winter. The geographic distribution of all infected species (Table 1) showed the disease is more frequent in southern states east of longitude 100°W, a dramatic feature best observed in Texas.

Figure 1
Figure 1

Numbers of cases of pythiosis by month in dogs (panel A) and horses (panel B) during a 9-year period. Blue bars comprise winter and spring months, whereas red bars include summer and fall months.

Citation: Journal of the American Veterinary Medical Association 260, 5; 10.2460/javma.20.10.0595

Pythiosis in other species

Compared to horses and dogs, few pythiosis cases were found in cattle, cats, and other species. Pythiosis in cattle and cats showed the same state distributions as canine and equine cases (Florida [cats = 8, cattle = 3], Louisiana [cats = 2, cattle = 3], Mississippi [cats = 1], and Texas [cats =4, cattle = 10). Interestingly, some cases of pythiosis in cats were also diagnosed on the west coast (California and Oregon), and 3 cases were diagnosed in Pennsylvania. States without previously known pythiosis cases in cattle and cats were Oregon and Idaho. These cases occurred mostly during the summer and fall months (data not shown). Diagnosis of pythiosis in captive species in our laboratories included 3 zoo camels in Tennessee (n = 2) and Florida (1; Appendix).

Discussion

The study found pythiosis in dogs is more frequently diagnosed between October and February. This could be in part because the clinical features of cutaneous and intestinal canine pythiosis mimic other cutaneous and intestinal infections caused by common eukaryotic and prokaryotic pathogens other than P insidiosum. Additionally, the clinical diagnosis of this disease can be challenging, particularly for those veterinarians unfamiliar with the syndromes caused by P insidiosum; thus, diagnosis of pythiosis in dogs is usually achieved later during the infection. Some dogs may be infected during the warm summer months (June to August), but an accurate diagnosis may not be achieved until 6 to 7 months later. This could explain in part why cases of pythiosis in dogs showed higher incidence between October and February. This finding contrasts with pythiosis in horses, where accurate diagnosis appears to be accomplished earlier in the course of the disease, perhaps due to a greater awareness of the disease among equine practitioners in southern states. It is important to mention 2 additional emerging mammalian oomycetes, Lagenidium giganteum and Paralagenidium spp.2528 These pathogens also infect the skin and intestinal tract of affected dogs where eosinophilic granulomas developed, thus mimicking the clinical features of pythiosis, potentially delaying accurate diagnosis. These pathogenic oomycetes cannot reliably be distinguished from P insidiosum in histopathological preparations.2528

Based on reports of pythiosis cases in the US in several species and this study, the southern states, particularly Alabama, Florida, Louisiana, Mississippi, and Texas, are heavy endemic areas of P insidiosum, with sporadic cases in adjoining states.1,4,6,12,1423,25,28,29 However, based on the cases diagnosed in our laboratories, the distribution of the disease within each state did not always involve the entire state. Texas is a prime example since no cases were diagnosed west of longitude 100°W, where the state is considerably more arid. Pythium insidiosum requires water to complete its life cycle in nature,5 so likely this is the main reason that Texas lacked cases west of 100°W longitude and that cases diagnosed in the eastern section of the state occurred frequently during wet summer months. Diagnosed cases from Arizona and New Mexico (both arid regions and west of longitude 100°W) were due to access to well-irrigated areas. A contrasting example is Florida, where pythiosis in all species is widely distributed throughout the state, although cases are more prevalent in central Florida. This was an expected outcome since warm, wet environments have the ideal ecological features for this oomycete to thrive in nature. Recent isolation of P insidiosum from several freshwater lakes in Florida in part explains why this pathogen is widely distributed in the central part of the state, thus validating early reports and the findings of this study.6

Pythiosis in northern states (Illinois, Indiana, Kentucky, Minnesota, New York, Ohio, Pennsylvania, Virginia, and Wisconsin) corroborated recent reports of the disease in several species.1,4,12,20,30 The findings of this study and those reported in the literature suggest P insidiosum colonizes waterway environments during summer months, and the organism likely survives freezing temperatures during the winter. Under this assumption, P insidiosum settles down in northern areas of the country during warm summer months and thus potentially is adapting to warmer winters. Diagnosed cases of pythiosis in the western US are a rather recent event mostly recorded after 2000.23,3033 According to our data, diagnosed cases occurred in Idaho, Oregon, and western Montana in nontravelled individuals.31 To the best of our knowledge, these are states with no previous reported cases.

Pythiosis in captive species, such as bears, camels, and big cats, are very rare.2022,24 In our laboratories, we identified only 3 cases in camels from zoos in Tennessee and Florida; the other cases were published in various journals, with one as a personal communication (Robert Glass, BS, Pan American Veterinary Laboratories, email, July 15, 2010). Except for few cases, including wildlife species (nestling white-faced ibis bird) diagnosed in California,23,3133 most pythiosis cases occurred in mammals inhabiting southeastern states.

On the basis of the reviewed literature, this is the first study in the US dealing with the geographic distribution of P insidiosum infections in multiple animal species. The objective of this epidemiological study was not only to demonstrate the areas where the disease is more prevalent, but also to call the attention of the veterinary community to the geographic areas where and the time of the year when pythiosis is commonly diagnosed. We hope this study encourages veterinarians to consider pythiosis in their differential diagnostic scheme of nonhealing skin lesions or intestinal granulomatous masses, particularly in dogs and horses inhabiting the endemic areas.

Acknowledgments

No funding was received in connection with this study or the writing of the manuscript.

Mr. Robert Glass is involved in the diagnostic ELISA for pythiosis at Pan American Veterinary Laboratories. Dr. Leonel Mendoza holds several patents on the immunotherapy of pythiosis. Dr. Richard D. Hansen is president, shareholder, and director of SolidTech Animal Health Inc, which is involved in the development of a Pythium insidiosum immunotherapeutic product to treat pythiosis and a vaccine to prevent the disease in at-risk horses and dogs; these products are current candidates for USDA commercial licensure. The remaining authors declare no conflict of interest.

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Cases of pythiosis in the US reported in wildlife and captive species.

Year State Host Disease form Reference No. Available isolate
2001 North Carolina Bear Subcutaneous 1 ATCC 90478 (CDC B-542)
2002 Florida Camel Subcutaneous and intestinal 20 None
2004 Mississippi Jaguar Pulmonary 21 None
2006 Florida Bengal tiger Intestinal 22 None
2008 California White-faced ibis Subcutaneous 23 None
2010 Florida Camel Vulvar 24 Lost
2010 Florida Camel Vulvar 24 MTPI-37*
2010 North Carolina Bear Subcutaneous None

MTPI: Biomedical Laboratory Diagnostics, Michigan State University Pythium insidiosum collection.

Personal communication: R Glass, BS, Pan American Veterinary Laboratories, email, July 15, 2010.

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