Guarding the Heart
, scientifically known as Dirofilaria immitis, is a species of parasitic worm that lives and reproduces in the heart chambers of certain animals.
Indoor pets like dogs may be predisposed to the disease, and male dogs may be more likely to be infected than female dogs, due in part to their habit of roaming and spending more time outdoors. The most important predisposing factor is failure to receive heartworm preventative medication. All dogs living in an area where heartworm disease exists are at risk, eve if they live entirely indoors.
A Vicious Cycle
Heartworm is more common in the tropical regions of the world, where the conditions of warmth and moisture are ideal for mosquitoes, which serves as intermediate hosts for the larval stage of the worm.
Female heartworms bear live young – thousands of them in a day. These young – the microfilariae – circulate in the bloodstream for as long as three years, waiting to hitch a ride in a bloodsucking mosquito. These microfilariae then burrow into the dog and undergo several changes to reach adult form, after which they will travel to the right side of the heart through a vein and await the opportunity to reproduce.
Adult heartworms can reach 30 cm in length and can remain in the dog’s heart for several years. The time lag between the initial infestation of microfilariae and reproduction by adult worms living in the heart is six to seven months in dogs.
Signs and Symptoms
The first signs of heartworm infestation may not manifest for a year after infection, and even then the soft cough that increases with exercise may be dismissed as unimportant by the owner. But this cough will worsen and with exertion, the infected dog may actually faint. The owner will find that the dog tires easily, is weak and listless, loses weight and condition, and may cough up blood.
Breathing becomes more difficult as the disease progresses. The progression is traumatic: the dog’s quality of life diminishes drastically and it can no longer run after a ball or take a long walk in the park without respiratory distress. Congestive heart failure ensues, and the once active, outgoing pet will be in grave danger.
Detection and Treatment
Heartworms can be detected by a simple blood test. The filtration test finds microfilariae in the blood; the occult test locates adult worms in the heart. Many veterinarians prefer to do both tests as the absence of microfilariae in the blood does not necessarily mean that there are no adult worms in the heart. Both tests are done with a single blood draw. X-rays can also detect the presence of adult heartworms in the heart and lungs.
If a blood test or the onset of symptoms alerts the owner and veterinarian to the presence of this devastating parasite, treatment is possible and successful as long as the disease is detected early.
The first step is to evaluate the dog and treat any secondary problems of heart failure or liver or kidney insufficiency so that it can withstand the treatment. Only then can the adult worms be killed with an arsenic compound. Surgical removal of the adult heartworms is also possible and may be indicated in advance cases with heart involvement.
For a Healthy Heart
Despite its seriousness, heartworm is a disease that can be easily prevented. Many veterinarians recommend a year-round heartworm prevention program to guard against the ever-present threat in Singapore’s tropical climate. This is a small expense that could mean a world of difference between life and death for your beloved pet dog!
