Inside the infected mosquito, these small worms mature into the infective larval stage. When the infected mosquito then bites a cat (or other animal) the larvae are inserted in the skin and start migrating inside their victim.
It takes about two months for the worms in cats to migrate through the connective tissue under the skin to get into the arteries of the lung. In total, it takes seven to eight or so months for the larvae to mature into adult worms that then begin producing their very own offspring (small worms / microfilariae).
The good news is that cats are not really good hosts for Heartworms since the adult worms can produce offspring in only about 20% of the cats and of these few make it to adulthood.
Also, unlike dogs, dogs that may suffer from Heartworm-induced heart and lung damage cats show only minimal adverse effect in the heart. Instead cats often show symptoms in the lungs.
FELINE HEARTWORM SYMPTOMS
It is nearly impossible to determine whether your cat has Heartworm by clinical signs alone. However, the generic signs of illness (applicable to many illnesses) are vomiting intermittently (food or foam, usually unrelated to eating), lethargy, loss of appetite / weight loss, coughing, asthma, rapid breathing (tachypnea) or gagging.
A Heartworm infection can also result in acute pulmonary inflammation response and lung injury. Signs associated with Stage 1 Heartworm disease (when the heartworms enter a blood vessel and are carried to the pulmonary arteries) are often wrongly diagnosed as asthma or allergic bronchitis. In fact these symptoms are caused by what is now called heartworm Associated Respiratory Disease (HARD).
DETECTING HEARTWORM IN CATS
According to the Heartworm Society diagnosis can be done by a physical examination, x-rays, echocardiography (ultrasounds), angio-cardiography (X-ray of the heart with injected contrast fluid), CBC (complete blood count), serologic testing (antigen and antibody study), microfilaria / worm testing.
PREVENTING FELINE HEARTWORM
Bio-Spot Defense has a widely used preventative. You can also consider an all-in-one preventative like Bayer Advantage II for Cats which helps prevent heartworm while also treating /controlling of fleas infestations, roundworms, hookworms and ear mites.
TREATING HEARTWORM IN CATS
According to the Heartworm Society There are no products in the United States approved for the treatment of feline heartworm. In fact the most commonly recommended treatment for cats that display no overt clinical signs (despite being diagnosed with Heartworm) is to allow time for a spontaneous cure to occur. This is followed by regular checkups / testing every 6 to 12 months.
Where there are signs of lung disease your vet may recommend a therapy using use "small, gradually decreasing doses of prednisone (a cortisone-like drug).
Best bet in all circumstances is to discuss your cat's Heartworm with your vet as many times no treatment is needed. I hope this is of help to you. Simba