Heartworm disease is caused by the internal parasite Dirofilaria immitis, better known as the heartworm. These parasites are long thin worms that live in the right side of the heart and the pulmonary arteries, the blood vessels that carry blood from the right side of the heart to the lungs. Dogs become infected when they are bitten by an infected mosquito that is carrying immature worms called larvae. Once they enter the dog, the larvae move through the tissues of the body, eventually entering the bloodstream and migrating to the right side of the heart. Within the heart, the worms complete their maturation, mate, and produce immature worms called microfilariae, which circulate in the bloodstream. When the infected dog is bitten by another mosquito, the microfilariae will pass from the dog to the mosquito. It takes about 5 – 6½ months for the heartworm life cycle to be completed.
The 2 most common tick fever strains in Singapore is Ehrlichia and Babesia. Both are spread from host to host by tick bites and their intracellular location makes them difficult to remove as most antibiotics do not penetrate to the inside of cells. Both bacteria cause the immune system to attack and destroy the body’s own platelets, cells important to normal blood clot formation.
Diagnosing ehrlichiosis is not always straightforward. Many dogs are bitten by Ehrlichia infected ticks without becoming noticeably ill, and the most commonly used diagnostic blood tests only determine whether or not a dog has been exposed to one or two Ehrlichia species. Therefore, both false positive and false negative results are not uncommon. Also, some dogs can develop clinical signs attributable to ehrlichiosis long after being bitten by an infected tick, so an apparent lack of recent tick exposure doesn’t rule out the disease as a cause of a dog’s symptoms.
Babesia organisms can be seen on a blood smear. Babesia canis organisms are tear-shaped and occur in pairs. Other Babesia species have several forms in which they appear. If Babesia organisms are found, the patient is definitely infected but in some cases an alternative method of diagnosis is needed. Antibody testing has been problematic as infected animals may have circulating antibodies long after the organism is gone or may have no antibodies circulating while the few organisms remain hidden inside red blood cells.