Introduction

Dirofilaria immitis is a highly pathogenic vascular nematode found globally, including North America.¹ Mosquitoes transmit L3 larvae to a canine host where the larvae mature to L4 in tissue and migrate to the bloodstream as developing adults and eventually mature in the heart and lungs.² Clinical signs vary depending on the worm burden and can be nonspecific or nonexistent.³ With heavy worm burden, right-sided heart failure with or without caval syndrome may occur. Microfilaria found in urine, termed microfilaruria, is an uncommon presentation of D immitis, and limited reports have previously described this presentation.^3,4 This case report details incidental microfilaruria secondary to hemorrhage.

Case Report

A 12 yr old female spayed American Staffordshire terrier initially presented to the primary veterinarian for stranguria and pollakiuria of 2 to 3 wk duration. An abdominal ultrasound was performed and identified a mass of unknown origin extending toward the trigone of the bladder with concern for impingement of the urethra and possibly the left ureter. On physical examination upon referral, the bladder was severely distended. The patient exhibited stranguria while dribbling brown to red, foul-smelling urine consistent with overflow incontinence. On rectal examination the urethra was normal on palpation, and no sublumbar lymphadenopathy was appreciated; however, a mass-like structure was appreciable in the caudal abdomen. A II/VI left systolic heart murmur was auscultated. An iStat Chem8^a, which includes electrolytes, urea nitrogen, creatinine, glucose, ionized calcium, anion gap, hematocrit, and hemoglobin, was unremarkable. Urinalysis from a cystocentesis sample was red and turbid with 4+ protein on urine dipstick. Sediment analysis revealed significant hematuria, pyuria, and bacteriuria (rods and cocci) with rare casts (granular and hyaline) and nematode larvae (Figure 1). A portion of the urine was reserved for culture and sensitivity.

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Direct smears and concentrated urine preparations stained with modified Wright-Giemsa contained numerous intracellular and extracellular bacteria that consisted primarily of cocci and lesser numbers of rods, occasional nematode larvae, and many degenerate neutrophils. Larvae were approximately 250 µm in length and 6–7 µm in width with a thin, tapering tail. A second, ultrasound-guided cystocentesis was performed several hours later and confirmed repeatable findings.

Two days following initial presentation, a complete blood count^b revealed a mild normocytic, normochromic nonregenerative anemia (hematocrit: 31.4%, reference interval [RI]: 37.5–60.3; reticulocytes 0.010 × 10⁶/µL, RI: 0.006–0.082) and a mild neutrophilia (12.24 × 10³/µL, RI: 2.10–11.20) with a moderate left shift (bands 6.80 × 10³/µL, RI: 0.0–0.13 and 0.23 × 10³/µL myelocytes) and mild monocytosis (1.59 × 10³/µL, RI: 0.0–1.20). A blood film contained rare metamyelocytes and 26 microfilariae that were ∼250 µm in length and ∼6–7 µm in width. A SNAP 4Dx^c was positive for D immitis antigen, and a modified Knott’s test was subsequently submitted to Cornell University’s Animal Health Diagnostic Center that confirmed D immitis larvae. A fecal float, direct fecal smear, and Baermann’s test were performed and were negative for parasites.

An abdominal ultrasound identified a caudal abdominal mass of indeterminant origin compressing the urethral lumen. This mass was aspirated for cytology and subsequently stained with Wright-Giemsa for evaluation by a clinical pathologist. The mass was composed of atypical round-shaped cells that were 1.5–3× the diameter of a neutrophil with light basophilic, frequently vacuolated cytoplasm, a moderate to high nuclear to cytoplasmic ratio with a round to reniform to indented central to eccentric nucleus, finely stippled chromatin, and one to three prominent, often round nucleoli. The mass was diagnosed as malignant neoplasia. Differentials included histiocytic sarcoma, lymphoma, an ectopic ovarian germ cell tumor, and an anaplastic carcinoma.

Owing to poor prognosis, the patient was euthanized and necropsy was performed. Two masses were identified within the urinary bladder mucosa, and a third mass was located left of the urinary bladder and extending into the pelvic canal, effacing the adipose tissue. Similar masses were found within the pulmonary parenchyma.

Histologically, the abdominal, bladder, and lung masses were composed of round to polygonal cells arranged in clusters and sheets with eosinophilic cytoplasm, an eccentric round to reniform nucleus, finely stippled chromatin, and variably prominent nucleoli. The cells exhibited moderate anisocytosis and anisokaryosis with a range of 27 mitotic figures per 10 high-power fields in the lung to 33 in the caudal abdominal mass (400×, high-power field; equivalent to 2.37 mm²). Tissue adjacent to blood vessels affected by neoplastic cells contained moderate hemorrhage. Immunohistochemistry was negative for Iba-1, CD18, factor VIII, and cytokeratin and diffusely positive for vimentin. The final histologic diagnosis was metastatic poorly differentiated sarcoma given the immunoreactivity to vimentin and lack of reactivity to other markers.

The hepatic sinusoids, necrotic areas of the left adrenal gland, alveolar septa, and rare glomerular capillaries contained larvae consistent with D immitis. Approximately 30 adult worms were identified in the vessels of the lungs; however, no adult worms were identified within the heart or the pulmonary artery.

Informed consent was obtained from the pet’s owners, and the patient was managed according to contemporary standards.

Discussion

In North America, D immitis and Acanthocheilonema reconditum are the most common microfilariae-producing nematodes, and differentiation is essential because of differing treatment and prevention strategies. Acanthocheilonema reconditum is a nematode transmitted primarily by fleas and is of little clinical significance as the adult worms live in the subcutaneous tissues. In contrast, D immitis⁵ is a highly pathogenic vascular nematode with adult worms located within the right heart and pulmonary artery. Clinical signs range from subclinical to respiratory disease, right-sided heart failure, and hemolytic crises.³

A modified Knott’s test is considered the gold standard for identification of microfilariae. Dirofilaria immitis microfilariae ranges in length from 295 to 325 µm and from 5 to 7.5 µm in width with a tapered head and straight tail.⁶ Although the microfilariae measured on this patient’s blood film were smaller than the range reported, standard measurements are determined by formalin-fixed samples, as in the Knott’s test,⁶ and lack of a fixative may be responsible for the discrepancy.

In this case, the patient presented with lower urinary tract signs rather than those typical of heartworm disease. Microfilariae were initially discovered on urinalysis, and fecal contamination via inadvertent colonic aspiration was initially considered a potential source of the larvae, particularly given the marked mixed bacteria. Strongyloides stercoralis, a gastrointestinal nematode released as L1 larvae in the feces,⁶ was a differential. However, a repeat ultrasound-guided cystocentesis yielded similar findings. To further exclude larvae secondary to fecal contamination, fecal testing was performed and was negative. Microfilaruria, a term meaning microfilariae in the urine, is rarely reported and can be secondary to inflammation or hemorrhage within the urinary tract during larval maturation.^4,7 Microfilaria can also accumulate in the glomerular capillaries,⁸ as was noted in this case, and it has been theorized they can penetrate the capillary walls and enter the urine; however, this has not been definitively proven.⁹ Hemorrhage was considered the primary cause of microfilaruria in this case owing to the high peripheral blood microfilarial burden and disruption of the urinary bladder vasculature by neoplastic cells, although penetration through glomerular capillaries with entry directly into urine filtrate cannot be completely eliminated.

Ultimately, when larvae are identified in the urine, fecal contamination needs to be excluded and microfilaruria should be considered.

Conclusion

This case report details microfilaria in the urine of dog thought to be primarily due to hemorrhage. Microfilaruria is a rare finding and fecal contamination needs to be excluded as a differential depending on sample collection technique, specifically and particularly blind cystocentesis. When microfilaruria is identified, this should prompt investigation of peripheral microfilaria.