Spirocerca lupi - The “worm in the throat”

 

A worm called Spirocerca lupi is becoming more common in South Africa’s summer rainfall areas, leading to an almost endemic situation. This worm lives in the dog’s oesophagus where it forms a nodule. Many worms can live inside a nodule and numerous nodules can form.

The female lays eggs and these are passed in the stool of the dog (host). These eggs have to be eaten by a dung beetle (intermediate host) for the lifecycle to continue. The small dung beetles (scarab beetles) living in the stool are part of the lifecycle, not the dung rollers we are more familiar with. he beetle can be eaten by the dog or another small mammal. The egg hatches and matures to a larval stage three in the beetle and stays in that phase even if eaten by other small mammals, lizards or birds. Further maturation will only occur if the host (beetle or transport host) is eaten by a dog.

The third stage larvae hatch and burrow into the stomach wall where they migrate along the arteries back towards the chest portion of the aorta, the main blood vessel in the body. This takes about two weeks. Once in the aortic wall, the worms mature and stay there for about three month. Extensive damage is caused to the aorta during this period and aneurysms develop. These may rupture and cause acute death. Once the worms are mature, they migrate directly across the body tissues to the oesophagus which lies adjacent in the chest. At this stage these worms are about 4-5 cm long and can cause considerable damage, resulting in rupture of large blood vessels, inflammation of the lining structures of the lungs and infection. The worms then establish themselves in the oesophagus where they live, mate and lay eggs.

Dogs react differently to these nodules: some dogs, especially fox terriers, show signs of sever irritation with even small nodules (gagging, swallowing, retching) whereas other dogs may show no symptoms until the nodules are large. Other symptoms associated with the migration can be fevers, joint pain, coughing, and difficulty breathing. With time, and due to the chronic irritation caused by the worm in the tissue, these nodules can become cancerous. This is a serious condition which may or may not respond to surgery depending on the extent of the cancer.

Diagnosis is best made by doing an oesophageal endoscopy (putting a camera into the oesophagus) and observing for a nodule. Chest X-rays can also help, but may miss some nodules. Not all nodules grow into the oesophagus and a CT scan might also be used to check for these. The worm sometimes gets lost and can make nodules in just about any organ. This is called aberrant migration. If one of your dogs is diagnosed with this worm it is advisable to have all your other dogs checked.

Preventative therapy is not yet proven beyond a doubt but evidence shows that you will decrease your chances of re-infection by 80% if you deworm your dogs monthly with a dewormer containing milbemycin oxime (Milbemax®), or use a cattle dewormer doramectin. These anti-parasiticide agents are the only proven drugs to cure and decrease the infection with Spirocera lupi. Spot-on preparations containing these drugs do exist, but there is no evidence that they are effective in treating spirocercosis. Worms that are exposed to these drugs also lay fewer eggs, even before they die, thus decreasing environmental contamination.

Deworming your dog monthly is the preferred method of controlling all other worms (round and tapeworm) as their lifecycles are about 21 days long and monthly deworming prevents them from maturing and laying eggs, contaminating your environment. This disease has not been documented to affect humans, and only rarely cats. So take the bull by the horns with this devastating disease and get your dogs checked by your veterinarian and start a monthly preventative program. My dogs are on it . .

Compiled by: Dr Liesel van der Merwe, Valley Farm Animal Hospital, Pretoria