Angiostrongylus vasorum Causing Severe Granulomatous Hepatitis with Concurrent Multiple Acquired PSS

Introduction

The adult Angiostrongylus vasorum (A. vasorum) metastrongyloid nematode resides in the right ventricle, pulmonary artery, or pulmonary arterioles of the definitive canine host. Eggs laid are carried to the pulmonary capillaries where they hatch to first-stage larvae. Those larvae migrate through the capillary wall and are expectorated, swallowed, and passed in the feces. Molts to third-stage larvae (L3) occur within the gastropod mollusk, and infection of the definitive host occurs by ingestion of infective L3 within the intermediate hosts (i.e., slugs, snails), paratenic hosts (i.e., frogs, mice, rats), or, potentially, the environment.^1–3

After ingestion, the L3 cross the intestinal wall and migrate to the mesenteric lymph nodes where they molt to fourth- and fifth-stage larvae. Fifth-stage larvae migrate via lymphatics, hepatic and portal veins, and the caudal vena cava to the right ventricle and pulmonary arteries where they reside and mature into adults.

Either aberrant or ectopic foci of adult A. vasorum worms have been reported in the pancreas and kidney, the femoral arteries, the pericardial sac, the bladder, and the eye.^4–7 Although larvae have also been reported in multiple other organs, including the liver, their significance is rarely elucidated.^4,6

The purpose of this case report is to describe the first reported case of severe hepatic injury associated with a significant A. vasorum burden in the liver. The hepatic damage caused may have contributed to the development of acquired portosystemic shunting (PSS), although underlying pre-existing liver pathology cannot be ruled out. Clearly, the full spectrum of disease inducible by infection with A. vasorum in dogs is far from understood. This report also serves as a stark reminder of the importance of anthelmintic use for a parasite now considered endemic to certain parts of the United Kingdom and appears to be spreading.^8–11

Case Report

A 14 mo old female mixed-breed Jack Russell terrier presented to Emergency Department of The Royal Veterinary College, University of London, Hatfield, United Kingdom with a 1 day history of inappetence, vomiting, and lethargy. No diarrhea or any other significant medical history was reported. Fipronil^a was applied regularly for flea control and vaccinations were up to date.

On physical examination, the dog was markedly lethargic, hypothermic (35.8°C) with a heart rate of 174 beats/min, and poor peripheral pulse quality. Thoracic auscultation was unremarkable, abdominal palpation revealed mild generalized discomfort only.

Blood gas and electrolyte analysis were unremarkable. Serum biochemical findings supportive of liver disease and PSS included low total protein (36 g/L; reference range, 49–71 g/L), elevated fasted serum bile acids (55.7 µmol/L; reference range, 0.1–5 µmol/L), mildly elevated alanine aminotransferase (ALT; 1.84ukat/L; reference range, 0.22–1.47 ukat/L), serum cholesterol of 2.5 mmol/L (reference range, 3.3–8.9 mmol/L), and an elevated serum aspartate aminotransferase (1.19ukat/L 71.3; reference range, 0.17–0.83 ukat/L).

Abdominal radiographs revealed mild gaseous distension of the ascending and descending colon. Abdominal ultrasonography revealed midjejunal corrugation with hyperechoic intraluminal material, a hypoechoic pancreas, mild mesenteric and pancreaticoduodenal lymphadenopathy, and intestinal hypomotility. A repeat abdominal ultrasound 4 hr later revealed passage of the previously observed intraluminal material to the distal jejunum, again with corrugation (Figure 1) and a small volume of hypoechoic peritoneal fluid. No liver lesions were observed either at that time or retrospectively. A quantitative canine pancreas-specific lipase test^b yielded 91 μg/L (reference range, 0–200 μg/L). Abdominocentesis revealed a sanguinous, neutrophilic, and macrophagic modified transudate. Due to worsening demeanor, further vomiting episodes, and progressive abdominal pain, an exploratory celiotomy was performed due to concerns relating to an intestinal obstruction.

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Under general anesthesia, the dog passed a large volume of hemorrhagic diarrhea with multiple long strands of grass. Surgical findings included a diffusely mottled liver that was normal in size, gross pancreatic inflammation, and multiple abnormal tortuous vessels in the region of the kidneys (Figure 2). No gross evidence of a single extrahepatic PSS vessel was identified, and a gastrointestinal foreign body was not found. A liver biopsy was taken and the dog remained on the previously instituted symptomatic therapies (i.e., IV fluids and gastroprotectants) and was administered postoperative pain relief).

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Histopathological examination revealed significant parasitic hepatitis attributable to larval nematode infection. At that point, fenbendazole^c and moxidectin^d were administered, and the dog made a full clinical recovery within 5 days of presentation and with complete resolution of clinical signs.

Polymerase chain reaction was performed on formalin-fixed hepatic tissue. Briefly, total genomic DNA was extracted using a kit^e (as recommended by the manufacturer) prior to PCR amplification of the internal transcribed spacer 2 sequence using primers NC1 and NC2.¹² Subsequent comparative sequence analysis identified A. vasorum as the best match (National Center for Biotechnology Information blastn: 100% identity, E 0.0). One day after presentation, a Baermann fecal floatation test was also positive for A. vasorum and thoracic radiographic imaging was unremarkable.

Histopathological findings included numerous portal/periportal granulomas or pyogranulomas surrounding viable or degenerate nematode larvae (Figure 3) and multifocal clusters of hemosiderophages attributable to bleeding induced by larval migration. Also within portal areas, numerous small caliber arterioles (arteriolar “reduplication”) were consistent with acquired portal shunting and moderate fibrosis indicated a chronic-active process.

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Four days after initiating treatment, the clinical signs of vomiting, lethargy, and abdominal pain had resolved. Re-examination at 2 and 9 wk revealed ongoing remission from clinical signs; however, serum biochemical analyses showed a progressive picture. Specifically, ALT measured 2 wk after presentation was 5.03 ukat/L and 10.47 ukat/L 9 wk after presentation. A bile acid stimulation test performed at the 9 wk examination revealed raised preprandial and postprandial serum bile acids. Specifically, prebile acid stimulation was 352.1 µmol/L (reference range, 0.1–5 µmol/L) and postbile acid stimulation was 190.4 µmol/L (reference range, 0.1–10 µmol/L). A Baermann fecal floatation test was negative at that time.

Discussion

A. vasorum infection in dogs often presents with clinical signs of respiratory disease, bleeding diatheses, and neurological dysfunction.^8,13,14 This case report highlights a hitherto unknown consequence of A. vasorum infection, namely hepatic dysfunction, which may have contributed to the development of acquired PSS.

Histopathological examination revealed multifocal granulomatous portal or periportal hepatitis centered on nematode larvae together with fibrosis, suggesting chronic liver disease induced by A. vasorum. The significant vascular proliferation was typical of that observed in PSS and supported the clinical, surgical, and biochemical findings of acquired PSS.¹⁵ The clinical history did not suggest chronic gastrointestinal complaints, hepatic encephalopathy, lower urinary tract signs, growth retardation, or coagulopathies, which are typically reported in congenital forms of shunting. In addition, the liver appeared normal in size at the time of surgery and on imaging. Usually, microhepatica is observed in cases of congenital PSS.

Unfortunately, because there had been no investigations prior to this dog's presentation to the Emergency Department, serum biochemical analysis, computed tomography, portovenography, or histopathological examination of the liver prior to presentation were unavailable to determine if there were any pre-existing congenital abnormalities, such as portal vein hypoplasia (microvascular dysplasia) or noncirrhotic portal hypertension. It is indeed feasible that a congenital disease affecting portal vein size and flow could facilitate trapping of migrating larvae. However, with such marked granulomatous inflammation, it could also be possible that A. vasorum infection caused multiple acquired shunts to develop during the development of portal hypertension, either with or without any underlying hepatic structural anomalies. Portal vein hypoplasia (microvascular dysplasia) is a histological diagnosis and is generally indistinguishable from changes found in animals with PSS, making further comment based on the histopathology difficult. It is difficult to establish a definitive causal relationship between hepatic A. vasorum burden and PSS.

The presence of a single pre-existing intrahepatic PSS is an unlikely possibility in light of the clinical history, liver size, and breed of dog. Furthermore, no clear single extrahepatic PSS was identified on exploratory laparotomy. Although multiple acquired PSS are usually thought to develop secondary to a primary liver disease, multiple congenital PSS are reported in the literature (although rarely) and could have been present in this dog prior to the acquisition of hepatic A. vasorum.¹⁶

The most common causes of acquired extrahepatic shunts are hepatic fibrosis (cirrhosis), portal vein hypoplasia with portal hypertension, and hepatic arteriovenous malformations.¹⁷ Severe liver disease due to drugs, toxins, metabolic defects, or inflammation can also result in acquired shunt development.^17,18 The study authors hypothesize that the acquired shunts were secondary to portal hypertension due to severity of the A. vasorum burden present and the secondary changes within the liver (granulomatous and fibrosing portal hepatitis), restricting hepatic vascular flow. However, as stated before, congenital multiple PSS and/or portal vein hypoplasia could have been the primary pathology. Further investigations, which were not appropriate or applicable at the time, would have included measurement of portal pressures and portovenography to further characterize the changes seen.

It also remains unclear whether this dog's presenting clinical signs were attributable to either the hepatic dysfunction and shunting or whether she presented for a pancreatitis and/or mild enteropathy and the liver lesions were simply incidental. The abdominal effusion present was a modified transudate, the exact origin of which remains unclear. It could feasibly have been a result of the liver lesions and portal hypertension and/or concurrent pancreatitis.

Retrospectively, explanations for worsening demeanor during hospitalization would include raised serum ammonia levels with gastrointestinal bleeding, which had not yet been appreciated, and impaired hepatic processing of methadone^f dosing (0.2 mg/kg) q 4 hr.

It is unknown whether such a severe but theoretically treatable lesion will resolve with time. Considering that acquired extrahepatic shunts are formed in response to severe and often terminal liver disease, this is rarely possible. Nine weeks after diagnosis and treatment with both moxidectin and fenbendazole, the dog was clinically normal but had significantly raised ALT values and raised pre- and postprandial serum bile acids, which may suggest worsening subclinical liver disease attributable to the extensive hepatic inflammation and repair that would occur upon nematode death and clearance.

Although in the short-term follow-up period, in the absence of A. vasorum infection, the presence of hepatic lesions and shunting is confirmed, the extent of those shunts and the potential for resolution is not certain. In dogs with congenital PSS, there is no association between the extent of histological changes and the long-term prognosis after attenuation.¹⁹ With the available information, the authors therefore cannot postulate a prognosis without long-term follow-up information. However, it remains important to be aware that A. vasorum is capable of inducing significant hepatic inflammation on its migratory route.

One further point for discussion relates back to the case presentation. Specifically, none of the presenting complaints, diagnostic investigations (including thoracic radiographs), or historical information suggested any respiratory compromise, bleeding diatheses, or syncopal episodes. In one study, 65% of dogs with A. vasorum infection presented with respiratory signs, and 18 out of 19 dogs had abnormal thoracic radiographs.⁸ In light of this, and considering that such a significant burden was apparent, it might seem reasonable to expect a respiratory component to this dog's clinical picture. This, together with anecdotal reports of increasingly varied A. vasorum presentations, serves to underline the clinical importance of an increasingly prevalent parasite.

Conclusion

This report describes the first case of hepatic A. vasorum with associated significant hepatic dysfunction. Whether the multiple PSS vessels seen were acquired secondary to the parasite-induced dysfunction or were an unusual congenital presentation of multiple shunting vessels predisposing the dog to infection remains unclear.