Spirocercosis in dogs

Spirocercosis is a parasitic disease mainly of domestic dogs, caused by the nematode (worm) Spirocerca lupi. Spirocercosis occurs mainly in tropical and subtropical regions. It is a debilitating disease and can lead to acute death. There has been a marked increase of clinical spirocercosis among dogs in South Africa, in both urban and rural areas.

The adult spirurid nematode is a relatively large worm, pink-red in colour with males and females reaching 3-4cm and 6-7cm respectively.

Life cycle
The adult worm is embedded in a nodule in the thoracic (chest) esophagus. It passes eggs which are excreted in the dog’s faeces. Eggs hatch after ingestion by an intermediate host, a small black dung beetle. These small dung beetles are found in the grass around the faeces. The large dung beetles often seen rolling balls of dung on farms or game reserves are not the intermediate hosts. Birds, lizards, frogs, snakes, mice, rabbits and rats are sometimes incidental or paratenic hosts. The definitive host (dog) will become infected if it ingests either the beetle or an incidental host.

The life cycle within the dog takes approximately six months. After ingestion, the larvae penetrate the stomach wall, enter the arterial system and migrate to the aorta in the chest. They then migrate through wall of the aorta to the esophagus. A nodule (granuloma) forms in the esophagus.

Aberrant migrations have been described in the thoracic cavity organs as well as migration to the spinal cord.

Large breed dogs appear to be predisposed and, due to the lengthy life cycle, it is uncommon in dogs under a year of age. As can be seen from the life cycle, spirocercosis is not contagious between dogs.

Clinical signs
The nodules are generally situated in the esophagus between the heart and the diaphragm. These nodules result in the clinical signs such as vomiting, regurgitation (food comes up immediately after swallowing), the inability to swallow properly and weight loss. The larval migration may cause low-grade inflammation and infection, resulting in general malaise and fever. Other symptoms include coughing, enlarged salivary glands, excessive salivation, joint inflammation, spondylitis of the chest vertebrae. Some dogs can develop an aortic aneurism which results in acute death with bleeding into the chest. These cases may show no pre-existing symptoms. In long standing cases the nodule can transform into malignant cancerous masses.

Aberrant migrations cause atypical clinical signs of spirocercosis. Aberrant S lupi can be present without the characteristic signs, rendering the ante mortem diagnosis of aberrant migration difficult.

Diagnosis
Any dog with a history of chronic weight loss, regurgitation, difficulty in swallowing, abnormal salivation or coughing should be evaluated by a veterinarian for Spirocercosis.

Normal overview or contrast radiographs (using barium) are helpful in diagnosing large nodules and extensive disease. This is relatively easy to perform, safe and does not require sedation or anesthesia. Small nodules in the early stages of the disease could be missed however.

Endoscopy is a sensitive method, earlier lesions (small nodules) can be detected and the extent of the disease can be evaluated. Biopsies of the nodule(s) can be done using an endoscope. This method however requires general anesthesia and is more expensive.

Faecal flotation tests can be helpful, but are not very reliable. The eggs are difficult to detect and infection will be missed if the worm is not shedding eggs at the time of the faecal examination.

Treatment
The only drug at present to which the worm is sensitive is Doramectin and ivermectin. At this stage relatively little research has been done and there is no standardized treatment protocol. Owners should take note that Doramectin is a drug used for sheep, and is not registered for the use in dogs. Therefore owners have their dogs treated with this drug at their own risk.

The drug is safe in all dog breeds, except in collie dogs and herding dogs. This includes border collies, rough collie, smooth collie, bearded collie, old English sheepdogs as well as Australian shepherd, cattle dogs and possibly less common herding breeds. These dogs may carry a mutation of the multiple drug resistance – 1 (MDR-1) gene. Should they carry the MDR-1 gene, it results in Doramectin causing neurological symptoms and possible coma.

There is however a laboratory test available to detect the MDR-1 gene mutation and it is advised that all collie, collie crosses and herding breeds are tested before the drug is used. Should the dog test positive a special low dose protocol is used.

All confirmed cases, as well as other dogs in the same household are treated with Doramectin. One of the protocols is to inject the dogs with Doramectin subcutaneously every two weeks for six treatments. The drug may also be given orally.

Affected dogs should be re-evaluated for the success of the treatment. A nodule that shows no regression in size after four to six treatments of therapy should be biopsied to test for cancer formation.

Side effects of treatment
As discussed above, the drug appears to be safe in all dogs with the exception of collie breeds and herding breeds which should be tested first. Rarely the following symptoms may be seen while the dog receives the Doramectin treatment: dilated pupils, apparent blindness, muscle tremors, disorientation or coma. Should any of these symptoms be noted the treatment must be stopped and the owner should seek veterinary advice immediately.

Prevention
As can be seen from the life cycle the most important method for prevention of infection is to remove dog faeces as soon as possible. The intermediate host is the very small beetle that lives in the ground, and if the faeces is removed promptly, the life cycle of Spirocerca lupi is interrupted. A dog can only become infected by eating a beetle in which the Spirocerca egg has developed into an infective larva.

Due to the increase in prevalence in South Africa, it has become an acceptable approach to treat all dogs as a preventative measure with a Doramectin course twice a year. Bear in mind that routine deworming does not affect Spirocerca lupi. The dog should be weighed to establish the correct drug dosage.

The treatment and prevention of spirocercosis is an area which requires further research.

References