Heartworm Biology
Heartworm (Dirofilaria immitis) is a serious threat to dog wellness. Over the last two decades, its recognition worldwide has increased dramatically. Heartworm infection is quite debilitating, even deadly. Furthermore, treatment is costly and difficult. Thus, prevention of heartworm is a key goal of pet wellness. In 1997 it was reported to the University of Guelph that there were 525 heartworm positive dogs in Canada. This number is not alarming in itself, but when it is revealed that 90% (474) of these positive dogs were not on a preventive program, the effectiveness and necessity of a preventive regime is evident. Incidence of heartworm is up from last year. In 1996 there were 443 positives reported in Canada. Increased awareness of the disease prompting an increase in the amount of testing done by veterinarians is partly responsible for the increase this year, but these numbers would certainly be much higher if prevention was not so widely used.

Heartworm is able to infect wild animals such as coyotes, wolves, foxes etc. As a result, once heartworm is introduced in an area it becomes permanent, as wild animals are not able to be treated with a preventive and transmission goes unchecked.

Life Cycle
Infective heartworm larvae are transmitted between animals via mosquitoes, and their continued transmission is directly related to environmental conditions within an area. The correct species of mosquito is necessary (about 70 are known to carry the disease) and a temperature of roughly 21° Celsius for at least 14 days is required for the heartworm larvae to complete the required molts within the mosquito. Typically, during a bloodmeal, a mosquito will pick up 1 or two microfilarie, and rarely 3. A moderate rate of infection, both in the intermediate host (mosquito) and the final host (the dog) is most conducive to heartworm transmission. If there are to many circulating microfilarie the mosquito will be killed by the numbers it ingests.

Adult worms range in size from six to twelve inches and generally live in the right ventricle of the heart and the pulmonary arteries, where their presence damages the arterial walls. Over time, the walls of the blood vessels of infected dogs begin to thicken and expand. Eventually, the dog develops increased blood pressure (hypertension) as a result of damage to its arteries. In very severe cases, the worms can actually enter the right side of the heart. If the number of worms or "worm burden" becomes significant, they can cause congestive heart failure as well as kidney and liver failure (called caval syndrome).

Click here to view the heartworm life cycle.

Legend
1. Adult heartworms in heart and lungs of infected animals (domestic or wild).
2. Fertile female heartworm releases microfilarie (tiny heartworm larvae) into bloodstream.
3. Mosquito swallows microfilarie when it bites dog.
4. Microfilarie grow to infective stage in mosquito.
5. The mosquito serves as an intermediate host for two molts of the larvae.
6. In the infective third stage, the larvae enter the mosquito's mouth parts, ready to be transmitted.
7. The mosquito bites the dog and the larvae are deposited via the saliva.
8. The third molt to L4 occurs 3 to 4 days after infection when the larvae begin their journey under the dog's skin.
9. The fourth molt (to L5) takes place 50 to 70 days after infection. During that time the larvae have been working their way toward the pulmonary arteries.
10. Larvae become adults in heart and lungs.
11. Within 70 to 100 days after infection, the worms arrive in the pulmonary arteries. After an additional 3 months the worms reach sexual maturity (L5 to adult), mate and release microfilarie, completing the life cycle.

The time from infection by a mosquito to an active adult is completed in approximately 6 months.
Incidence
The technical term for canine heartworm disease is dirofilariasis. It is only fairly recently that it was recognized as a serious problem. Until the early 1970s, it was confined primarily to the coastal areas of the southeastern and southern sections of the United States and a few localized areas in the Midwest and Northeast. Since that time the disease has spread so rapidly that it is now found in all but the mountainous and desert areas of the country. This spread is due to human migration from heartworm-endemic parts of the country (with then heartworm infected dogs) to other areas where heartworm was previously rare or unrecognized. The heaviest infestations of heartworm continue to be found along the major waterways and tributaries, due to the high prevalence of mosquitos, but the incidence is increasing across Canada.

The rate of transmission of heartworm is directly proportional to the number of mosquitoes in the area. It is also dependent on the number of infected dogs. Dogs are increasingly susceptible if they live outdoors, and susceptibility increases with years of exposure. Numerous mosquito species have demonstrated the ability to carry the heartworm larvae.

Dogs with an extremely high concentration of microfilaria (20,000 per milliliter or greater) are actually less efficient disease carriers than dogs with lower concentrations, because the mosquito can take in so many microfilaria that it dies.

For the past 65 years, heartworm disease has also been known to occur in cats, although only in the past 20 years has it been diagnosed with any regularity. The incidence is believed to be 10% or less of that of unprotected dogs living in the same area. Cats are less susceptible to heartworm than are dogs; however, arterial damage and clotting can be severe in some cats and may be a cause of sudden death.

Generally, infections in cats consist of a very light worm burden (1 to 3 adult worms), which may be of the same sex; therefore, no microfilarie are generated. Diagnosis of heartworm infection in cats is difficult due to ambiguous test results. However, chronic vomiting and coughing can be signs of an infection. Significant pulmonary disease can be identified from radiographs.

Heartworm infection also occurs in wild canines, ferrets, muskrats, raccoons, bears, horses, foxes, coyotes and the California sea lion. In many species, however, the life cycle of the worm ends at the adult stage. It is due to infestation in the wild animals of an area that heartworm becomes a permanent problem once introduced, as wild animals are not treated with preventives and the transmission by mosquitos is allowed to go unchecked.
Heartworm infection in humans usually consists of a single worm, often located just under the skin, although some do reach the pulmonary artery. The incidence in humans is probably far greater than what has been reported, primarily because only countries with advanced medical services are likely to recognize the problem.

Clinical Signs
The worms generally make a home for themselves in the pulmonary arteries. In the living dog, heartworms are not found in the right side of the heart except when the dog is very heavily infected. They may also be present in the veins connecting to the liver, where large numbers can cause severe damage.

Normal Progression of Clinical Signs
Adult heartworms living in the pulmonary arteries cause inflammation inside the vessels. Blood clots also may form within these arteries, which may completely obstruct segments of the vessels. As the damage progresses, smaller branches of arteries become affected. The inflammation and occlusion of blood vessels leads to increased blood pressure (pulmonary hypertension).

As the pressure increases, the pulmonary arteries try to expand to circulate needed blood. In addition, the right ventricle of the heart must strain to pump blood through the affected arteries. Eventually, the right side of the heart usually enlarges and may fail. Normally, 3 to 5 years is required for clinical pulmonary hypertension to develop as a result of heartworm disease. However, clinical signs may develop more quickly in active dogs.

Unfortunately, much of the internal damage to the dog occurs before any clinical signs are evident. When clinical signs do develop, they resemble signs of general heart failure consisting of sporadic coughing and lowered exercise tolerance. As time passes, the cough becomes chronic and is accompanied by noticeably difficult respiration.

In advanced cases of the disease, a dog will often collapse after even light physical exertion. Most dogs develop congestive heart failure and ascites (an accumulation of fluid in the abdomen). When infection becomes this severe, normal health may not be fully restored even after an effective treatment program. Thus, early detection of infection is of critical importance.

Liver Failure or Caval Syndrome
A fatal condition called caval syndrome is encountered in dogs exposed to a massive infection over a short period of time. Caval syndrome occurs when a large mass of worms cluster in the vena cava (the large blood vessel leading to the heart from the liver, kidneys and abdominal organs). These worms restrict blood flow to the heart and cause anemia and extensive liver and kidney damage. Two US studies have shown that 16 to 20% of dogs with heartworm disease develop caval syndrome. Most commonly found in young dogs who have acquired little or no protective immunity, the clinical signs include sudden weakness, rapid breathing, and brown urine. If the worms are not eliminated, death occurs in 24 to 72 hours. In these severe cases, immediate surgical removal is the only option for survival.

Diagnostic Options
A combination of methods, including physical examination, diagnostic tests and the history of the dog are combined when making a determination. Two extremely important factors are: