Important Definitions for understanding Heartworm

*  Life Cycle – The unfolding of one’s life from the beginning to the end including development, sexual maturity, environment, the ability to reproduce, and all that is required for a full cycle of development and maturity.
*  Host – Is an animal or plant that harbors or nourishes another organism (parasite).
*  Parasite – A plant or animal which lives upon or within another living organism at whose expense it obtains some advantage.
*  Vector – A carrier, especially an animal or insect that transfers an infective agent from one host to another. 
*  Biological Vector is an arthropod vector (in the case of Heartworm the mosquito) in whose body the infecting organism develops or multiplies before becoming infective to the final host.
*  Microfilaria – This is the name given to the microscopic, snake-like prelarval stages of the Filaroidea worm in the blood of man and animals and in the tissues of the vector.  Microfilaria are the offspring of adult heartworm reproduction. 
*  Larva/larvae– An independent, motile, sometimes feeding, developmental stage (immature stage) in the life history of a worm or insect.  Usually undergoes metamorphosis or a molt to become an adult.  Heartworms have several stages of larvae.  L1 are the microfilaria in the blood of the infected animal and these are sucked up by the mosquito while it is feeding.  It molts from an L1 to L2 and again from an L2 to L3 in the mosquito before it can infect an animal.  L3 larvae are expelled from the mosquito’s mouthparts while the mosquito is feeding and burrow through the skin of the host. Inside the host it molts twice, from an L3 to an L4 and then from an L4 to an L5.  The L5 larva will become the adult once it matures. 
*  Adult worm – A worm that is sexually mature enough to produce young. The worm may still continue to grow in length and size after reaching sexual maturity.  Adult heartworms live in the dog for up to 7 years.  In the cat they are thought to live a much shorter life, around 2 years.  Adult Heartworms prefer to live floating freely in the blood in the right ventricle of the heart and in the associated pulmonary blood vessels. The right heart ventricle receives the blood returning from the body that is low in oxygen.  The right ventricle’s function is to pump this blood through the lungs to absorb oxygen and to deliver the oxygenated blood to the left ventricle.  From this chamber blood is pumped back throughout the body.  The returning, poorly oxygenated blood flows from the large veins (the Vena Cava Vein) to the right side of the heart, to the lung arteries and capillaries (called the pulmonary arteries). The Heartworm organisms often move back and forth anywhere along this pathway from the Vena Cava to the right ventricle to the pulmonary arteries.  The signs an animal shows depends on the number of worms present, the animal’s reaction to their presence, and general health factors in the patient such as age, size, and obesity. 
*  Aberrant migration or sometimes called abnormal migration – This term refers to the larval stage of the worm or other parasite where it wanders or deviates from the usual or normal course. The parasite ends up in locations that are different from where they would normally be present, such as under the skin, in the eye, or stomach.  This could occur in any animal but is more common in host animals that are not the preferred host for the parasite.  Think of aberrant migration as taking a wrong turn on the wrong road and ending up in an unintended location permanently.
*  Pre-patent Period – The time from initial infection with a parasite until the young adults are sexually mature and start to produce either larva or eggs.  In the Heartworm, this takes about 7 months in the dog and 9 months or more in the cat.
*  Patent Heartworm infection – Heartworm adults are sexually mature and producing microfilaria.
*  Microfilaremia – A condition where there are microfilaria circulating in the blood of an infected animal. 
*  Occult Heartworm infection – The infected animal has sexually mature adult Heartworms present but for some reason no microfilaria are present.  The most common reasons are:  1) The worms are sexually mature but the infected animal’s immune system kills and removes the microfilaria as soon as they leave the female worm’s body, leaving none to be detected in a blood sample.  2) There is a single sex Heartworm infection and therefore no microfilaria are produced.  3) Aberrant migration resulted in no microfilaria production even though the worms are sexually mature.
*  Antigen – Any substance which under appropriate conditions is capable of inducing the formation of antibodies and of reacting specifically in some detectable manner with the antibodies induced.   Basically an antigen is anything the body can recognize as foreign and therefore stimulates its immune system to eliminate what has been recognized. A commonly used Heartworm test detects an antigen produced by the female heartworm that is producing microfilaria.

Heartworm Tests
There are several kinds of Heartworm tests performed on a blood sample.  The most common ones are mentioned below.

 #1   The most common one in practice is a Heartworm antigen test that looks for the presence of a protein given off by the sexually mature female worm actively producing microfilaria.  There are several manufacturers that produce tests of variable sensitivity.    They may miss infections if there are only 1-2 adult female worms present. 

#2    Other types of tests look for the presence of microfilaria in the blood sample.  One is a filter test that traps the tiny microfilaria in the filter. The treated blood is forced through a filter to trap the microfilaria for microscopic analysis.  This test and the Knotts test treat the blood with a chemical to break down the blood cells. In the Knotts test, the solution is then spun in a centrifuge, the top part poured away, and the contents in the bottom portion of the test tube are examined under a microscope for the presence of Microfilaria.  A positive diagnosis is made if microfilaria are present, however, Heartworm can be missed if there are very few microfilaria or the dog or cat has an Occult Heartworm infection. 

#3    A third type of test looks for the presence of antibodies to the larval stage of the Heartworm.  The down side of this test is that it only shows that the animal has had larval stages of Heartworm in its body at some time in the past.  The antibodies persist for a while after all worms are dead.   Therefore, a positive test is not proof of a current live Heartworm infection.  This test just confirms exposure to Heartworms at some time in the pet’s life. 

Heartworm Infection -  versus -  Heartworm Disease

Heartworm infection – The host (dog, cat, or other animal) is parasitized by one or more stages of the Heartworm (Dirofilaria immitis).  This does not mean that the host is sick, or has adults in the heart.

Heartworm Disease – The host is sick.  Heartworm infection is resulting in any number of problems caused by damage due to the presence of Heartworms in the heart and nearby blood vessels.

Signs - A sign is any objective evidence of a disease that is perceptible to the examining doctor or person. Therefore it is what someone other than the patient observes about the patient and it may indicate a disease process. An example would be hearing another person cough or sneeze and thinking they may have a cold.

Symptoms - A symptom is what a patient perceives and communicates to someone what they perceive. Therefore a symptom is something experienced and felt by the patient and described by them.

Technically, signs and symptoms are used in human medicine and only signs in veterinary medicine. It is not unusual to see them used interchangeably even though technically they are completely different.

WHERE DO HEARTWORMS COME FROM?

We are not sure where they evolved originally but we do know they have been around for a long time.  The first written canine documentation was in the “Western Journal of Medicine” in 1847. The first documented cat infection was in the early 1900’s.   We have learned much more about them and now know that in areas where dogs contract heartworm, cats can become infected too.  Cats are simply less susceptible to becoming infected when exposed to the larvae deposited by a Heartworm-harboring mosquito.

The education of pet owners about Heartworms has increased significantly over the past twenty years.   The spread of the disease has accelerated due to the trend of pets, and especially dogs, to travel everywhere with their owners.  Pet owners often were unaware they were visiting Heartworm country, their pets became infected and the worm was brought back to their home environment unknowingly.  Thus territories that never experienced Heartworm infections in the past started to see them.   Heartworm disease is now wide spread and has been diagnosed in all 50 states in the USA as well as worldwide.  Actually, cats have been found NOT to be a big factor in the spread of the heartworm disease.

WHAT ANIMALS ACQUIRE HEARTWORMS?

Most parasites have a specific host they prefer to infest. The normal host for Heartworm is the canine.  Heartworms can live in animals other than canines but often have a shorter life, vary in size, or end up in odd locations due to aberrant migration.   This is true of Heartworm infections in the feline.   Some of the other animals that can acquire heartworm include the wolf, coyote, fox, bobcat, jaguar, tiger, muskrat, raccoon, ferret, otter, bear, horse, orangutan, gibbon, sea lion, and man.

View more information about Heartworm disease in dogs on the next page.  More on Heartworm in cats is on this page.

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