Spirocerca lupi in Small Animals
Adult Spirocerca lupi are bright red worms, 40 mm (male) to 70 mm (female) long, generally located within nodules in the esophageal, gastric, or aortic walls. Infections are seen in southern areas of the USA as well as in many tropical and subtropical regions worldwide (eg, Greece, India, Israel, Japan, South Africa). Dogs are infected by eating an intermediate host (usually dung beetle) or a transport host (eg, chickens, reptiles, or rodents). The larvae migrate via the wall of the celiac artery to the thoracic aorta, where they usually remain for ~3 mo. Eggs are passed in feces ~5–6 mo after infection.
Most dogs with S lupi infection show no clinical signs, but when signs are present, they most commonly include weight loss, coughing, and dyspnea. When the esophageal lesion is very large (usually when it has become neoplastic), the dog has difficulty swallowing and may vomit repeatedly after trying to eat. Such dogs salivate profusely and eventually become emaciated. In addition, dogs may develop thickening of the long bones characteristic of hypertrophic osteopathy. These clinical signs are suggestive of spirocercosis with associated neoplasia in regions where the parasite is prevalent. Occasionally, a dog dies suddenly as the result of massive hemorrhage into the thorax after rupture of the aorta damaged by the developing worms.
The characteristic lesions are aneurysm of the thoracic aorta, reactive granulomas of variable size around worms in the esophagus, and exostoses that bridge between ventral aspects of thoracic vertebrae. Esophageal sarcoma, often with metastases, is sometimes associated (apparently causally) with S lupi infection, particularly in hound breeds. Dogs with Spirocerca-related sarcoma often develop hypertrophic osteopathy (see Hypertrophic Osteopathy in Small Animals).
Diagnosis can be made by demonstrating the characteristic small (11–15 × 30–38 μm), elongated eggs (by NaNO3 [specific gravity 1.36] or sugar flotation) that contain larvae in the feces. However, eggs are sporadically voided in feces and can be difficult to find. Gastroscopy occasionally reveals a nodule or an adult worm. A presumptive diagnosis can be made by radiographic examination when it reveals dense masses in the esophagus; a positive-contrast barium study may help define the lesion. CT is an additional useful diagnostic tool, with a higher level of sensitivity than thoracic radiography for S lupi. However, while CT generally provides more information on the location and severity of the infection than radiographs, the specificity of the findings for S lupi is currently unclear.
Many infections are not diagnosed until necropsy. The granulomas vary greatly in size and location in the esophagus but usually are sufficiently characteristic to be diagnostic, even if the worms are no longer present. Worms and granulomas may be present in the lungs, trachea, mediastinum, stomach wall, or other abnormal locations. Healed aneurysms of the aorta persist for the life of the dog and are diagnostic of previous infection. When sarcomas are associated with the infection, the esophageal lesion usually is larger and often contains cartilage or bone; metastases frequently are present in the lungs, lymph nodes, heart, liver, or kidneys.
In endemic areas, dogs should be prevented from eating dung beetles, frogs, mice, lizards, etc, and not fed raw chicken scraps. In Europe, monthly treatment with topical moxidectin/imidacloprid is approved for use in dogs as a preventive for S lupi infection. Treatment of clinical cases is often not practical. However, efficacy has been demonstrated with doramectin (0.2 mg/kg, SC, three doses at 2-wk intervals; 0.4 mg/kg, SC, six doses at 2-wk intervals; 0.5 mg/kg, SC, two doses 2 wk apart; 0.5 mg/kg/day, PO, for 42 days; 0.8 mg/kg, SC, two doses 1 wk apart; additional treatments may be required), and with ivermectin (0.6 mg/kg, SC, two doses 2 wk apart) combined with prednisolone (0.5 mg/kg, PO, bid for 2 wk and then tapered), although none of these treatments is approved. The specific breed toxicity associated with ivermectin in Collies and other herding dog breeds also occurs with doramectin. Surgical removal usually is unsuccessful because of the large areas of the esophagus involved.