Spirocerca lupi infestation in dogs.

Spirocerca lupi is a roundworm, or nematode parasite, of mostly the dog family (Canidae), that has a world wide distribution. There is no sex or age preference, although the long developmental cycle makes it unlikely to occur in dogs less than six months old.

The worm has a heteroxynous life cycle, in that it uses both an intermediate and a final host. Intermediate hosts include dung beetles but are not limited to these, and there are a number of transport hosts. In order to understand the concept of final, intermediate and transport hosts it is necessary to give some definitions. A final host is the host that ingests the infective stage of the nematode that occurs in the intermediate host, and in which the nematode develops to the adult, reproductive stage, in this case a dog. The intermediate host is the host that ingests the egg, that occurs in the faeces of the dog, and the larva that subsequently hatches undergoes maturation changes. For example, the egg is ingested by the intermediate host, it hatches into the first stage, the larva grows and moults to the second stage, which in turn grows and moults to the third stage. This third stage is known as the infective stage, and in the majority of the nematodes, it is only the third stage larva that is able to infect a new host. All these changes have taken place in the intermediate host, and in order for the final host, the dog, to become infected, the intermediate host must be eaten.

The main intermediate host of Spirocerca lupi is a variety of dung-beetle. It is also found in other small animals, such as mice, lizards and birds, including chickens, but these would rather be the paratenic, or transport, hosts. The worm larvae are released during digestion and move along the stomach wall, penetrate it and end up in the small arteries that branch off from the aorta, the main artery in the body. Here they mature before eventually moving down to the posterior part of the oesophagus. It is here that thy will stay, mature and lay eggs, which move through a small hole into the oesophagus to be passed with the faeces and re-infect he environment.

The worm causes disease in many ways:

• The earliest sign if infestation may just be a fever and lethargy. This can be caused if a large number of worms are migrating simultaneously through the stomach, as this can cause a great deal of inflammation.
• As the worms reach the aorta, they cause damage to the wall of the blood vessels due to their migration. The wall of the aorta becomes scarred and fibrosed and an aneurysm develops. It is not uncommon for the aorta to rupture at these weak spots. Aortic rupture will result in death within minutes. On post mortem the chest will be filled with blood and the worms are often seen.
• Once the worms have reached their goal, the wall of the oesophagus, they form a nodule, or granuloma, just below the lining of the oesopahus. A granuloma is a collection of inflammatory (defence/ white blood cells) cells and fibroblasts (scar tissue), indicating that the body is trying to encapsulate the parasite. These granulomas range in size from 0.5 cm to large growths, 8-10 cm in diameter.
• In this stage of disease the patient often regurgitates/vomits frequently, shortly after being fed, and appears to have difficulty in swallowing. This difficulty appears unrelated to the size of the growth, as small growths can also cause severe irritation. Additional complications which may be associated with the growth in the oesophagus are enlarged and painful salivary glands (at the corner of the jaw bone) and inflammation and swelling of the legs (a condition associated with cancer). Some dogs also have difficulty in breathing, with flaring of the ribcage and sitting with an extended head.
• The larger the lump becomes the more likely it is to become neoplastic (cancerous). This type of cancer is either a malignant osteosarcoma, that spreads to the lungs, or a fibrosarcoma that occurs at the site of infection in the oesophagus. It seems, however, that the occurrence of oesophageal neoplasms is uncommon.
• Additional disease is caused by aberrant migration. Conditions include rupture of major blood vessels in the chest, abscesses in the kidneys or in the chest and even migration into the spinal cord, causing neurological signs.
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Diagnosis and Treatment
There is, to date, no method of detecting infection with this parasite prior to it settling in the oesophagus and starting to pass eggs. These eggs can be detected in faecal examinations using special flotation fluids, but the test is not very sensitive. Factors influencing the sensitivity include the short period that the female worm lays eggs and the presence of an opening in the granuloma through which the eggs can be passed into the digestive tract. The eggs are quite small and contain a first stage larva. If the first flotation is negative it is best to wait a day or two and then repeat the test.

Once clinical signs of vomiting have been detected chest x-rays will often show a suspicious area behind the heart. Confirmation of the diagnosis is usually by passing an endoscope (camera) into the oesophagus to observe the size and extent of the granulomas growing into the oesophagus.

Treatment of the granulomas is by killing the worm. Two drugs belonging to the macrocyclic lactone group of anthelmintics seem to be quite effective. There are several regimens by which the drugs are given, and which one used will depend very much on the veterinarian treating the dog. The smaller and smooth nodules respond very well to treatment.
 
Once the nodule in the oesophagus has transformed into a tumour surgical removal with post-operative chemotherapy is the only option. The prognosis is not good as there are severe complications and the survival time after resection of the oesophagus is less than 1 year. Chemotherapy given to dogs where  osteosarcomas have transferred to a limb is moderately successful after amputation but it appears that fibrosarcomas react poorly to chemotherapy.

Prevention
The practical way to prevent spirocercosis is by removing dog faeces every dayand disposing of them in such a manner that dung beetles cannot access it. By doing this one prevents the intermediate host from becoming infected, and this will result in the paratenic, or transport hosts, also not being infected. In addition, all offal, especially that from chickens should be thoroughly cooked before feeding to the dog.  This will result in the infective larvae being killed and they can therefore not infect the dog. The use of anthelmintic drugs is being investigated and it may well be that in future a small dose given once a month may prevent the dog form becoming infected.

References
Dvir, E., Kirberger, R.M. & Malleczek, D. 2001. Radiographic and computed tomographic changes and clinical presentation of spirocercosis in the dog, Veterinary Radiology and Ultrasound 42, pp. 119–129.

Mazaki-Tovi, M., Baneth, G., Aroch, I., Harrus, S., Kass, P.H., Ben Ari, T., Zur, G., Aizenberg, I., Bark, H. & Lavy, E. 2002. Canine spirocercosis: clinical, diagnostic, pathologic and epidemiologic characteristics, Veterinary Parasitology 107, pp. 235–250.

Van Der Merwe,  L.L., Kirberger, R.M., Clift, S., Williams, M.C., Keller, N. & Vinny Naidoo, V. 2008. Spirocerca lupi infection in the dog: A review.  www.up.ac.za/dspace/handle/2263/9337