This article throws light upon the three main drugs acting against heart worms. They are: 1. Thiacetarsamide Sodium 2. Dithiazanine 3. Diethylcarbamazine Citrate.
Drug # 1. Thiacetarsamide Sodium:
It consists of the sodium salt of S, S-diester of p-carbamolydithiobenzene aronous acid with mercaptoacetic acid. The chemical structure is depicted in Fig. 35.5.
Therapeutic Effectiveness:
After 4 doses of thiacetarsamide adult worms die within 5-7 days. In some cases they take a longer period of 14 days. The dead worms are eliminated from heart by the flowing blood and lodge in the pulmonary artery especially in diaphragmatic lobes.
The lodged worms disappear after a period of 2-3 months by phagocytosis. Complete rest during the first 14 days is essential and limited exercise is allowed. If body temperature is raised accompanied by coughing, it indicates a pulmonary embolism.
Dose:
2.2 mg/kg, i.v. twice daily for 2 days.
Toxicity:
Thiacetarsamide is nephrotoxic and hepatotoxic. While injecting this drug, it should be remembered that perivascular leakage can cause huge swelling and tissue sloughing. Dogs are allowed their feed 1 hr before the injection of thiacetarsamide.
If persistant vomiting or discoloration of urine is noticed after the first injection the treatment should be discontinued. Because a single dose (2.2 mg/kg) of thiacetarsamide consists of 0.44 mg elemental arsenic per kg, the symptoms consist of persistent vomiting, discoloration of urine (orange color), icterus.
The toxicity produced by thiacetarsamide is treated with BAL (Dimercaprol) with a dose of 8.8 mg/kg/day i.v. in four divided doses.
Drug # 2. Dithiazanine:
It is effective against the heartworm in dogs. It is a cyanine dye. The iodide salt is used for the above purpose and is poorly soluble in water. The chemical structure of dithiazanine iodide is depicted in fig. 35.6.
Dose:
6.6- 11 mg/kg/day orally with feed once daily for 7 days. To avoid the toxicity, 6.6 mg/kg (lowest dose) is given. Spliting the daily dose into two (3.3 mg/kg twice) can minimise the occurrence of toxicity.
Mechanism of Action:
It is not clear how dithiazanine eliminates the circulating microfilariae from the blood. It is suspected that the worms loose their motility, trapped in the capillary beds and are phagocytized by host cells. The biochemical studies indicate that dithiazanine inhibits glucose absorption irreversibly in worms and causes marked reduction in free glucose. Thus, it causes depletion of energy-rich phosphate bonds.
Toxicity:
High doses of dithiazanine produce vomiting, diarrhoea, anorexia and asthenia. Since, it is nephrotoxic and hepatotoxic death may occur in dogs.
Drug # 3. Diethylcarbamazine Citrate:
It is filaricide and piperazine derivative. It is available as a syrup, powder or tablet. The combination of diethylcarbamazine with oxbendazole protects the dogs against hookworm infections.
The use of diethylcarbamazine in microfilariae-positive dogs is discouraged and is contraindicated.
Dose:
6.6 mg/kg daily for prophylactic heartworm schedule.