Spirocerca lupi (Park Worm)

Spirocerca lupi (Park Worm): Morphology, Life Cycle, Pathogenesis, Clinical Signs & Treatment

Spirocerca lupi, commonly known as the Park worm or esophageal worm, is a pathogenic spirurid nematode that primarily infects dogs. The parasite is characterized by its unique migratory life cycle, during which the larvae travel through the arterial system before establishing within the wall of the esophagus. Infection with S. lupi is clinically significant because it can cause esophageal nodules, aortic lesions, aneurysms, and, in chronic cases, malignant neoplastic transformation such as fibrosarcoma or osteosarcoma.

The life cycle of Spirocerca lupi involves coprophagous beetles as intermediate hosts, while a variety of vertebrates, including reptiles, birds, and small mammals, may serve as paratenic hosts. Dogs become infected by ingesting either infected beetles or paratenic hosts. Diagnosis is based on clinical findings, fecal examination, imaging techniques, and endoscopy, whereas treatment includes the use of appropriate anthelmintics and supportive management.

This article provides comprehensive veterinary parasitology notes on the morphology, taxonomy, life cycle, pathogenesis, clinical signs, diagnosis, treatment, control, and prevention of Spirocerca lupi, making it a valuable resource for vet students, veterinary practitioners, and competitive examination preparation.

Parasite Overview

• Host: Common in dogs; also reported in goats and donkeys.
• Intermediate Host: Coprophagous beetles (Scarabaeus spp.).
• Paratenic Host: Garden lizards (Calotes spp.) and chickens.
• Predilection Site: Walls of the esophagus, stomach, and aorta; rarely found free within the lumen of the stomach.

Taxonomical Classification

• Kingdom: Animalia
• Phylum: Nematoda
• Class: Chromadorea
• Order: Spirurida
• Superfamily: Spiruroidea
• Family: Spirocercidae
• Genus: Spirocerca
• Species: Spirocerca lupi
• Common Name: Esophageal worm, Park worm

Morphology

• Usually spirally coiled and pink in color when fresh.
• Male: 30–50 mm; Female: 54–80 mm.
• Worms are stout.
• Lips are trilobed, and the pharynx is short.
• The male tail has small lateral alae, four pairs of precloacal papillae, and one unpaired precloacal papilla.
• Two pairs of postcloacal papillae are present.
• A group of small papillae occurs at the tip of the tail.
• Spicules are unequal.

The female is ovoviviparous. The eggs are thick-shelled, capsule-shaped, and contain fully developed larvae.

Lifecycle

Eggs are passed in the feces of the host. They hatch only after being ingested by coprophagous beetles.

In the beetles, the larvae develop to the infective stage and encyst primarily on the tracheal tubes. If an infected beetle is ingested by an unsuitable host (paratenic host), such as amphibians, reptiles (garden lizards), domestic and wild birds, or small mammals such as hedgehogs, mice, and rabbits, the larvae again encyst in the esophagus, mesentery, and other visceral organs. The larvae can also be transmitted from one paratenic host to another.

Domestic chickens are considered important paratenic hosts in the transmission of infection to dogs.

Dogs acquire infection by ingesting either infected beetles or infected paratenic hosts. Following ingestion, the larvae are liberated in the stomach, penetrate the stomach wall, reach the arteries, and migrate through the walls of the gastric and gastroepiploic arteries to the celiac artery and thoracic aorta within approximately 3 weeks after infection.

Approximately 3 months after infection, the larvae migrate within the aorta, reaching the upper thoracic aorta. After 2–3 months, the majority of the larvae migrate to the esophagus within 102–124 days after infection, where they form cystic nodules connected to the esophageal lumen by a fistulous tract. They subsequently develop into adult worms.

Eggs are laid within the cysts, pass into the lumen of the esophagus, and are subsequently excreted in the feces. The prepatent period is 5–6 months. Over time, a granuloma forms around the cyst and may eventually undergo neoplastic transformation into a sarcoma.

Pathogenesis

The migrating larvae cause hemorrhage, inflammation, necrosis, abscess formation, and purulent streaks in various organs. These lesions may heal rapidly after the larvae migrate away, but vascular stenosis may persist.

Adult worms cause nodule formation in the walls of the esophagus, stomach, and aorta. These nodules contain cavities that harbor one or two worms.

The lesions and thoracic aortic scarring are considered pathognomonic for S. lupi infection. The aortic intima becomes rough and granular, with an eosinophilic granulomatous reaction resulting in nodule formation around the parasite.

Initially, the elastic tissue of the aorta degenerates and is replaced by collagen. Subsequently, calcification and ossification of the vessel wall result in stenosis, rupture of the vessel, or formation of aortic aneurysms.

In severe infections, the esophageal nodule may become large, pedunculated, and protrude into the esophageal lumen, leading to digestive disturbances. Persistent vomiting and progressive emaciation may also occur.

A secondary complication of S. lupi infection is the development of malignant esophageal tumors, particularly fibrosarcoma or osteosarcoma. Fibrosarcoma may metastasize to the lungs and other organs.

Long-standing cases may develop hypertrophic pulmonary osteoarthropathy affecting the long bones. Spondylitis may occur due to chronic irritation of the periosteal tissues by the parasite and obstruction of the intervertebral arteries.

The disease may also manifest with neurological disorders due to aberrant migration of the worms into the spine, marked weight loss, and even sudden death resulting from rupture of an aortic aneurysm and subsequent internal hemorrhage. Other complications include pyemic nephritis and aplastic anemia.

Clinical Signs

• Large esophageal tumors may interfere with swallowing, respiration, and circulation, leading to progressive wasting and emaciation.
• Persistent vomiting containing adult worms and rapid loss of body condition.
• Hemorrhage from the esophageal region may result in anemia and hemoptysis. Rupture of the weakened esophageal wall with leakage of ingesta into the pleural cavity may cause pleuritis. General weakness, emaciation, and anemia are common findings.
• In less severe cases, difficulty swallowing (dysphagia) may be observed.

Diagnosis

• Examination of feces and vomitus
• Contrast radiography
• Endoscopy

Treatment

• Disophenol: 10 mg/kg; two doses administered one week apart.
• Diethylcarbamazine (DEC): 20 mg/kg body weight, orally.
• Ivermectin: 200 μg/kg body weight, administered subcutaneously once weekly for 3–6 weeks.

Control

• Affected dogs should be isolated.
• Feces should be disposed of properly.
• Dogs should be prevented from consuming paratenic hosts.

Prevention

There are currently no 100% effective measures to prevent Spirocerca lupi infection.