Mosquito season in full swing: Heartworm risk, not just limited to dogs

This year, it appears as if the mosquitos are finally taking over. At least down here in Florida, I thought initially. But with record sales of pesticides at the on-line store it looks like the problem is all over the country. Related to this, the American Heartworm Society continues to remind pet owners of heartworm risk. Here is an example of their official campaign letter:

“As temperatures grow warmer, the American Heartworm Society (AHS) wants every pet owner to be prepared for mosquito season and the heartworm disease risk it carries for pets.
Although the risk of heartworm disease is heightened in warmer months when the mosquito population increases, the AHS recommends year round prevention for both dogs and cats. By giving heartworm prevention every month, forgetful pet owners will have their pets protected when they need it most.

"Surveys show only about 75 percent of pets are given the full dosage recommended by a veterinarian," Sheldon Rubin, DVM, AHS president and Chicago practitioner, said. "With year-round prevention, if doses are accidentally skipped, the drug is still effective."
Recently, researchers discovered that respiratory signs in cats, which are often diagnosed as feline asthma or allergic bronchitis, may actually be caused by the presence of heartworms in either larval or adult stages. The acronym "HARD" is the term for this clinical presentation and stands for Heartworm Associated Respiratory Disease. Heartworm preventive medications are the only option for cats, as there is no approved treatment for feline heartworm disease.
Heartworm disease treatments are available for dogs, but treating for heartworms is much more costly and dangerous to the animal than simply preventing it. Yet another reason to use year-round prevention is that many heartworm preventives also have activity against other intestinal and common parasites, such as roundworms and fleas.
Options for preventing heartworm infection in both dogs and cats include daily and monthly tablets and chewables as well as monthly topicals. These methods are effective, easy to administer and inexpensive. These medications interrupt heartworm development before larvae and adult worms reach the lungs and cause disease. When administered properly and on a timely schedule, heartworm infection can be prevented.
The American Heartworm Societyis the global resource for the prevention, diagnosis and treatment of heartworm disease. It was formed during the Heartworm Symposium of 1974. The American Heartworm Society stimulates and financially supports research, which furthers knowledge and understanding of the disease. Its headquarters are located in Batavia, Ill.”

Heartworm disease is a serious and potentially fatal condition caused by parasitic worms living in the arteries of the lungs and occasionally in the right side of the heart of dogs, cats and other species of mammals, including wolves, foxes, ferrets, sea lions and (in rare instances) humans. Heartworms are classified as nematodes (roundworms) and are filarids, one of many species of roundworms. Dogs and cats of any age or breed are susceptible to infection.
The American Heartworm Society together with the American Association of Feline Practitioners and Pfizer Animal Health has made it one of its priorities to educate and alert cat owners of this serious problem.

“Countless cat owners throughout the United States are misinformed about feline heartworm disease, posing serious risk to their feline friends. Cats may be misdiagnosed with feline asthma, or test negative for heartworm antigens and antibodies but still have heartworms in their systems. Now, due to new research, veterinarians recognize this infectious agent is doing a whole lot more damage than previously thought.

The KNOW Heartworms campaign is a public awareness campaign sponsored by the American Heartworm Society (AHS) and the American Association of Feline Practitioners (AAFP), and funded by an educational grant from Pfizer Animal Health. The program stresses the importance of understanding the five myths and misunderstandings about feline heartworm disease:

1. Dogs vs. Cats: Heartworm is not just a canine disease, and it affects cats differently than dogs.
2. Indoor vs. Outdoor Cats: Heartworm disease is mosquito-borne and evidence has shown indoor cats are just as susceptible to it as outdoor animals. In a North Carolina study, 28 percent of the cats diagnosed with heartworm were inside-only cats.
3. It's a Heart Disease: “Heartworm disease” is a misnomer; it mostly affects the lungs, not just the heart. The disease frequently is mistaken for asthma and other respiratory diseases.
4. Adult Heartworms vs. Larvae: New research shows that heartworm larvae at all stages, not just adult worms, can cause serious health problems.
5. Diagnosis: Accurate diagnosis can be difficult, since negative antigen and antibody tests don’t automatically rule out the presence of heartworms.

Feline Heartworm Disease
Heartworm infection takes place when a mosquito carrying infective, microscopic-size heartworm larvae, bites into a cat for a blood meal. The larvae then actively migrate into the new host and develop further as they travel through the subcutaneous tissue in the cat's body. At about 3-4 months, they usually settle into the arteries and blood vessels of the lungs, where they continue to develop to sexual mature male and female worms (Dirofilaria immitis). The average time from when the microscopic parasites enter the host until the females develop into mature worms and produce offspring is approximately eight months and is referred to as the prepatent period. This is about one month longer than in dogs.

As adults, the heartworms can mate and the females can release offspring called microfilariae (pronounced: micro-fil-ar-ee-a) into the blood stream. The cycle begins again when a mosquito takes a blood meal from the newly infected cat and draws the microfilariae into its system.

Cats are resistant hosts of heartworms, and microfilaremia, (the presence of heartworm offspring in the blood of the host animal), is uncommon (usually less than 20% of cases). When present, microfilaremia is inconsistent and short-lived. Some cats appear to be able to rid themselves of the infection spontaneously. It is assumed that such cats may have developed a strong immune response to the heartworms, which causes the death of the parasites. These heartworms may die as a result of an inability to thrive within a given cat's body.

Cats typically have fewer and smaller worms than dogs and the life span of worms is shorter, approximately two to three years, compared to five to seven years in dogs. In experimental infections of heartworm larvae in cats, the percentage of worms developing into the adult stage is low (0% to 25%) compared to dogs (40% to 90%).

However, heartworms do not need to develop into adults to cause significant pulmonary damage in cats, and consequences can still be very serious when cats are infected by mosquitoes carrying heartworm larvae. Newly arriving worms and the subsequent death of most of these same worms can result in acute pulmonary inflammation response and lung injury. This initial phase is often misdiagnosed as asthma or allergic bronchitis but in actuality is part of a syndrome now known as Heartworm Associated Respiratory Disease (HARD).

Which Cats Are Susceptible?
Although outdoor cats are at greater risk of being infected, a relatively high percentage of cats considered by their owners to be totally indoor pets also become infected. Overall, the distribution of feline heartworm infection in the United States seems to parallel that of dogs but with lower total numbers. There is no predictable age in cats for becoming infected with heartworms. Cases have been reported in cats from nine months to 17 years of age, the average being four years at diagnosis or death.

Clinical Signs
The clinical signs of heartworm infection in cats can be very non-specific, and may mimic many other feline diseases. Diagnosis by clinical signs alone is nearly impossible, but a cat may exhibit generic signs of illness, such as vomiting intermittently (food or foam, usually unrelated to eating), lethargy, anorexia (lack of appetite), weight loss, coughing, asthma-like signs (intermittent difficulty in breathing, panting, open-mouthed breathing), gagging, difficulty breathing (dyspnea) or rapid breathing (tachypnea).
Signs associated the first stage of heartworm disease, when the heartworms enter a blood vessel and are carried to the pulmonary arteries, are often misdiagnosed as asthma or allergic bronchitis, when in fact they are actually due to a syndrome newly defined as Heartworm Associated Respiratory Disease (HARD).
Some cats exhibit acute clinical signs, with disease often related to the organs where the adult heartworms are thriving. Occasionally such infected cats die quickly without allowing sufficient time to make a diagnosis or offer appropriate treatment.

Diagnosis
Heartworm infection in cats is harder to diagnose than it is in dogs and it is easy to overlook. Diagnostic tests have limitations, so negative test results do not necessarily rule out an infection. Antigen tests, for example, only detect adult female or dying male worms. Immature or male-only worm infections are rarely detected.
The diagnostic plan for heartworm disease in cats can include, but is not limited to, a physical examination, radiography (X-ray), echocardiography (ultrasound readings of the heart), angiocardiography (X-ray of the heart with injected contrast fluid), CBC (complete blood count), serologic testing (antigen and antibody study), microfilaria testing, and necropsy (after death).

Physical Examination
The results of a physical examination may appear to be perfectly normal in cats infected with heartworms. Harsh lung sounds are a frequent abnormal finding and may be present in cats without any respiratory signs. The presence of a heart murmur or abnormal rhythm is uncommon. Only rarely, have there been reports of ascites (fluid in the abdomen), exercise intolerance and signs of right-sided heart failure. In cats, the primary response to the presence of heartworms occurs in the lungs.

Treatments
Currently, there are no products in the United States approved for the treatment of feline heartworm infection. Most cats with heartworm infection that are not demonstrating clinical signs are allowed the time for a spontaneous cure to occur. If there is evidence of disease in the lungs and their blood vessels consistent with feline heartworm infection, such cases (possibly in the early stage) can be monitored with chest X-rays every six to twelve months as needed. Supportive therapy with small, gradually decreasing doses of prednisone (a cortisone-like drug) is recommended for cats with radiographic or clinical evidence of lung disease.
Cats with severe manifestations of feline heartworm disease may require additional supportive therapy, and may benefit from intravenous fluids, oxygen therapy, cage confinement, bronchodilators (which expand the air passages of the lungs), cardiovascular drugs, antibiotics and nursing care.

Heartworm extraction with various surgical devices has been performed in cats in which the worms can be visualized with ultrasound at the tricuspid valve or in the right atrium (of the heart), and especially in those rare instances of caval syndrome (obstruction of blood flow affecting the heart and the liver.

Preventives
It is generally recommended that all cats be tested for both antigens and antibodies (serology) prior to administration of a heartworm preventive. There are four heartworm disease preventive products approved by the FDA for use in cats, (see your vet for details). All of these products are considered effective in preventing the development of adult heartworms when administered properly on a monthly basis relative to the period of transmission.”

Covering the same topic, however with emphasis on latest scientific research, Veterinary Practice News recently published an article on feline heartworms. In it, Gary D. Norsworthy, DVM, Dipl. ABVP (feline) who owns and practices at Alamo Feline Health Center in San Antonio, Texas and also frequently speaks about cat health at veterinary conferences and seminar, reports of a study performed at Auburn University which has shown a new side of feline heartworms. This study, performed with financial backing from Pfizer Animal Health in NY, came to the following conclusions:
“Life CycleWhen a mosquito bites a cat, larvae (L3 stage) are deposited on the cat’s skin. Within minutes they enter the subcutaneous tissue through the bite wound. The L3 molt within a couple of days to fourth stage larvae.

L4 migrate subcutaneously in fat and muscle for two months, then molt to become a juvenile or immature adult worm. Immature adult worms enter circulation via a peripheral vein. This occurs about 60 days after infection. An antibody response begins about this time; some cats test antibody positive.

Within the next 15 to 30 days, 75 to 90 days post infection, the immature adult worms arrive in the pulmonary arteries. The vast majority of the juvenile worms die, are carried by blood flow into the lungs and cause an intense inflammatory response affecting the pulmonary arterioles, bronchi and alveoli.

It is estimated that about 3 to 4 percent of the immature adults become 6 inch long adult heartworms and live for two to four years before dying spontaneously.
HARD
Heartworm associated respiratory disease, or HARD, is unique to the cat. It is defined as vascular, airway and interstitial lung lesions caused by the death of immature adult worms, and the inflammation may last up to eight months.
The study revealed: If 100 infective larvae are administered to a dog, 75 will mature to adults. If 100 infective larvae are administered to a cat, many will become immature adult heartworms; however, only three to four will mature to adults. A very large number of immature adult worms develop but never make it to adulthood due to the effects of the cat’s immune system. Severe lung lesions are present but:
No adult worms will be present on necropsy. The immature adult worms disintegrate within the lung tissue and are very difficult to find on necropsy. Antibodies disappear very quickly. Antigen tests will be negative because there have been no adults. Radiographically, these cats may look similar to cats with allergic bronchitis. Interstitial or bronchial patterns may be present, and the caudal pulmonary arteries may be enlarged and blunted. In some cats, apparent enlargement may be due to periarterial inflammation. Repeated exposure to immature adult heartworms results in severe interstitial and bronchial disease.”

It is no wonder that this disease has eluded detection until we learned of the results of this study.

Let’s recap, in summary the 3 important points are:
1. By about three months post infection, 2 inch long immature adult heartworms are in the pulmonary arteries. .
2. Most of these are killed by the immune system, never becoming adult heartworms. They are carried by blood flow into the lungs..
3. About 3 percent to 4 percent of the immature adults become 6 inch long adult heartworms.

Dr. Norsworthy continues:
“For every 10 heartworm-infected dogs in a given locale, one cat has adult heartworms. However, it is likely that only about 10 percent of heartworm-infected cats have an adult worm. That makes the exposure and infection rates of dogs and cats about the same.

Heartworm Testing
Antibody Tests: Antibodies are produced by the presence of immature adults, and they begin to wane as the immature adult worms die. If the immature adult worms mature to adults, the adult worms suppress the immune system, causing antibodies to dissipate. Most antibody tests turn negative about four months later as long as new infections do not occur. A positive antibody test means one or more of these possibilities: A current infection with late L4.; a current infection with immature adult heartworms, a current infection with adult heartworms; a previous heartworm infection. Antibody persists about four months.

Many cats with HARD are antibody positive and antigen negative. However, many test negative on both antigen and antibody tests, making differentiation from cats with allergic bronchitis virtually impossible.

Antigen Tests: A positive antigen test means (one or both): One or more adult female heartworms and/or one or more dying adult female heartworms.

Heartworm tests are inclusionary, not exclusionary. If they are positive, they are meaningful. If they are negative, they are not meaningful.

Antigen or antibody testing is not necessary to begin heartworm prevention because there is not a reaction between current heartworm prevention products and any stage of the heartworm.

Microfilaria Testing: In contrast to dogs, very few microfilaria circulate in cats. This test has very poor sensitivity in cats. This also explains why cats are very poor reservoirs for heartworm infections to other cats or to dogs.

Summary
Positive antibody test: This cat is or has been infected with heartworms that progressed at least to the immature adult stage. It is clearly at risk of future infections
Positive antigen test: This cat is infected with adult heartworms. It is clearly at risk of future infections.
Microfilaria testing: This test has a very poor diagnostic yield.
Most cats with HARD have mild coughing, but a severe respiratory crisis can occur when a large number of immature adult heartworms die at once. These cats should be placed on a heartworm preventive product to prevent new infections.”

I know, it all sounds very scientific and can be very confusing. However, this is a topic I really want every cat owner to become familiar with. You need to know about this, heartworm is a serious problem you don’t want your pet to suffer from and have to go through the lengthy, painful treatment. What kind of surprises me is that how seemingly little we know or knew about this problem related to felines. The best site to find more details and kind of easy to understand info including graphical illustrations is the one of the American Heartworm Society. Besides background info, the site features a lot of downloadable materials (for free) and even features an educational section “Just for Kids”.