Diagnosis and Treatment of Heterobilharzia americana in Dogs

Canine schistosomiasis, caused by the trematode Heterobilharzia americana, is an infectious disease that is currently expanding its known range across the United States. Historically considered to be confined to the Gulf Coast and southern Atlantic states, H americana has recently established new endemic areas in the Southwest,^1,2 a finding that inspired prospective surveillance work currently in progress at the authors’ diagnostic laboratory. The preliminary findings support unexpectedly high prevalence in areas of southern California and western Arizona that border the Colorado River. While many infected dogs remain asymptomatic, some progress to severe clinical complications, leading to multiorgan failure and fatal outcomes.

H americana is not detected by routine fecal screenings, and no preventives yet exist to reliably protect dogs from contracting the parasite in freshwater environments. Raising awareness within the dog-owning and veterinary medical communities is essential for early detection and effective management of the disease. This article shares insights from the authors’ ongoing research to help equip veterinarians with the necessary tools for accurate diagnosis and successful treatment of canine schistosomiasis.

Life Cycle and Routes of Transmission of H americana

H americana can infect a wide range of mammals.^3-6 It is frequently found in raccoons, nutria, and dogs.^7,8 The ability of this blood fluke to establish itself in an area depends on the presence of an intermediate snail host of the Lymnaeidae family that is capable of supporting the parasite’s life cycle.^1,2,8 Upon excretion in the feces of an infected animal, fluke eggs hatch and release free-swimming miracidia when immersed in water. Miracidia penetrate the intermediate snail host to carry out asexual development into infectious cercariae, which are then released from the snail.

Dogs are at risk of contracting H americana when they wade or swim in cercariae-infested fresh water. A diagram of the life cycle of H americana is shown in FIGURE 1. Infection occurs when cercariae penetrate a dog’s intact skin. Inside the dermis, the cercariae transform into a new juvenile state that enters dermal vessels and migrates to the lungs in approximately 5 to 9 days before reaching the liver between days 7 and 45 postinfection. Here, the parasites mature into adult male and female flukes. Adults settle in the mesenteric vasculature, mate, and produce eggs.⁸ The eggs disseminate through the bloodstream to visceral organs or penetrate the intestinal wall to enter the lumen, from which they are subsequently excreted in feces approximately 84 days after infection.⁹

Figure 1. Life cycle of Heterobilharzia americana. Courtesy Anil Baniya, PhD, Connor Goldy, Department of Nematology at the University of California, and Jiranun Ardpairin, Department of Microbiology and Parasitology, Faculty of Medical Science, Naresuan University, Phitsanulok, Thailand

Even in long-recognized endemic areas, the prevalence of H americana remains unknown. This led to prospective prevalence research in Texas that is currently in progress. The observation from a recent study confirming the presence of H americana in a snail host in California² led to an additional targeted prevalence investigation in states located in the southwestern United States and with access to the Colorado River. Preliminary results of these studies strongly suggest that H americana is a parasitic infection of importance in dogs frequenting fresh water in these geographic regions. However, samples from dogs from all over the country have tested positive for H americana in the authors’ diagnostic laboratory.

Clinical Manifestations of H americana Infection

Canine schistosomiasis can remain asymptomatic or manifest with a range of clinical signs and abnormalities. Schistosomiasis should be considered in dogs presenting with signs of persistent intestinal and liver disease. Frequently observed clinical signs include weight loss, anorexia, lethargy, diarrhea, hematochezia, vomiting, polyuria, and polydipsia.^10,11

Disease progression is driven by the deposition of fluke eggs in visceral organs, leading to granulomatous inflammatory responses, mineralization, and fibrosis of affected tissues. Eggs can be located in the intestines, liver, pancreas, lymph nodes, and lungs, disrupting organ function and leading to corresponding hematologic and biochemical abnormalities.⁹

Among the key hematologic abnormalities are anemia, which may be regenerative or nonregenerative; lymphopenia; eosinophilia or eosinopenia; and thrombocytopenia. Biochemical changes to look for include decreased albumin and elevated globulins. Dogs frequently present with hypercalcemia, although hypocalcemia has also been observed in dogs with schistosomiasis.^10,11

To assess the risk of past exposure to H americana, the authors encourage clinicians to routinely inquire about their patients’ swimming history. Once the infection is confirmed, housemates should be tested as well given their likely similar lifestyle and swimming habits.¹²

Diagnosis of H americana Infection

Fecal saline sedimentation, a commercially available PCR assay, and histopathologic examinations of biopsy samples can be used to diagnose canine schistosomiasis.

Fecal Testing

In the authors’ experience, fecal saline sedimentation can be a very reliable noninvasive diagnostic test for asymptomatic dogs, which may be shedding fewer eggs or have a lower parasite burden. Intact fluke eggs do not float and therefore cannot be detected by flotation methods. While fecal saline sedimentation does not require elaborate technical equipment, H americana eggs come in varying colors, shapes, and sizes, making it challenging for clinicians and laboratory personnel to accurately identify them. H americana eggs have a thin shell and can appear clear, golden, or brown, with a round or ovoid shape (FIGURE 2). The most significant morphologic feature is the larval miracidium that can be observed within the egg.¹³

Figure 2. Two Heterobilharzia americana eggs, each harboring a fully developed miracidium (40× magnification). Courtesy Joe Luksovsky, Diagnostic Parasitology, College of Veterinary Medicine & Biomedical Sciences, Texas A&M University

A quantitative PCR test for detecting H americana in fecal samples is offered by the Gastrointestinal Laboratory at Texas A&M University¹⁴ and performs well in dogs that are symptomatic with H americana infection.

The authors are currently evaluating the utility of PCR as a screening test in asymptomatic dogs.

Histopathology

Histopathologic examination of tissue samples is the most reliable, albeit most invasive, method of confirming the presence of H americana. Eggs are commonly found infiltrating the small and large intestine, liver, lymph nodes, pancreas, stomach, spleen, and lungs. Eggs are often partially mineralized and surrounded by granulomatous inflammation (FIGURE 3).^9,10,15

Figure 3. Trematode eggs, characteristic of Heterobilharzia americana, surrounded by inflammatory cells in the jejunum (hematoxylin and eosin stain, 200× magnification).

Diagnostic Imaging

One study demonstrated that certain sonographic findings can serve as highly specific indicators of schistosomiasis and warrant diagnostic testing when observed.¹⁶ These features primarily consist of pinpoint hyperechoic foci in the small intestine, hepatic parenchyma, or mesenteric lymph nodes (FIGURE 4). In the small intestine, this aberration is commonly accompanied by an abnormal and heterogenous layering of the intestinal wall. In infected dogs, pinpoint hyperechoic foci may also be found in the large intestine, pancreas, and kidneys. They indicate areas of mineralization that consist of calcified eggs and surrounding granulomatous inflammatory reactions.¹⁵

Figure 4A. Ultrasonographic image showing diffuse hepatic parenchymal heterogeneity with multifocal to coalescing punctate hyperechoic foci (arrows).

Figure 4B. Ultrasonographic image showing severe thickening of the small intestinal submucosa with numerous submucosal and adjacent mucosal punctate hyperechoic foci (arrows).

Abdominal radiography can further aid in visualizing areas of mineralization. In the small intestine, these can be organized linearly within the intestinal wall (FIGURE 5).^17,18 Previous studies have confirmed an enlargement of the liver in schistosomiasis-positive dogs.^18,19 Slight abdominal distention and blurred serosal margins, indicative of peritoneal effusion, have also been observed.¹⁷

Figure 5. Abdominal radiograph showing severe multifocal stippled mineralization of the small intestinal walls (arrows).

Treatment of H americana Infection

Praziquantel combined with fenbendazole is routinely used to treat H americana infection. However, several key factors need to be considered when electing a treatment strategy.

Drugs and Dosage

At present, there are no approved veterinary medications for the treatment of canine schistosomiasis. The authors follow the dosing protocols presented in TABLE 1.

.

Use of Droncit (Elanco, elanco.com) 34-mg tablets as the source of praziquantel poses a challenge for owners of large-breed dogs due to the sheer number of tablets required to achieve the necessary dose. Use of Biltricide (Bayer, bayer.com) 600-mg tablets can mitigate this problem. In many cases, this treatment regimen will need to be repeated a second time based on results of follow-up testing.

Cost

One way to reduce the financial burden of treating schistosomiasis can be the use of compounded praziquantel from compounding pharmacies. Another cost-reducing alternative for treating asymptomatic dogs is a low-dose protocol (TABLE 1) that has proven partially successful in a recent study.¹²

Retesting and Timing

Juvenile schistosomes are more resistant to praziquantel and therefore can persist after a first round of treatment.²⁰ Infected dogs living in endemic areas are at risk of contracting new schistosomes with every new exposure to fresh water. This can lead to dogs harboring multiple life stages. Retesting 4 weeks after completion of the first round of treatment and, if necessary, a second round of treatment is therefore strongly recommended, regardless of the treatment protocol elected. In the authors’ experience, testing more than 1 fecal sample after treatment can increase the likelihood of detecting low numbers or intermittent shedding of eggs.

Summary

Schistosomiasis is a parasitic infection of emerging importance in dogs. Infection occurs during contact with contaminated freshwater sources. Patients may be asymptomatic for some time before demonstrating nonspecific gastrointestinal or other symptoms. Infection is confirmed by detecting the presence of H americana eggs via fecal sedimentation or PCR assay.  Treatment consists of administering praziquantel and fenbendazole for several days and may require repeating to eradicate infection. Symptomatic and supportive care is advised to help the dog better tolerate drug therapy and promote a smoother recovery from infection.