Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean
Infection of dogs with the heartworm, Dirofilaria immitis, has been reported on
the increase in Florida in recent years. Although this increase may be due to better
methods of diagnosis and reporting, surveys suggest that nearly 1 out of 3 dogs in
Florida may be infected with this parasite one or more times in their life. Canine
heartworm disease is often subclinical and not diagnosed until it has progressed to
the point of producing noticeable signs in the infected dog. Once these symptoms have
been detected, many irreversible changes in the dog's internal organs may have occurred
and the animal may be chronically sick. Tests have been developed to help detect the
presence of heartworm infection at early stages and some drug treatments are now avail-
able which destroy the worms. A method of prevention of infection by daily administra-
tion of small amounts of a drug is also effective. Early detection of infection, prompt
and adequate treatment, and the use of a program of prevention of infection based on
present knowledge is possible.
LIFE CYCLE OF THE DOG HEARTWORM, DIROFILARIA IMMITIS
Male and female heartworms live in the dog's heart and adjacent large blood vessels.
Females release living larvae (microfilariae) directly into the dog's blood stream.
These larvae are removed from the infected dog's blood stream by certain common mosqui-
tos when they ingest the blood. After a short period of development (10-14 days) in the
mosquito, larvae are transmitted to another dog when the mosquito takes another blood
meal. Once larvae enter the dog's system, they develop further and eventually reach
the heart as mature worms. This developmental period takes 6-7 months. When mature
male and female heartworms have developed, they produce microfilariae which can be de-
tected in the dog's blood by examining a sample under a microscope.
EARLY DIAGNOSIS OF HEARTWORM INFECTION
The earliest time that infections can be detected is when the mature heartworms
start producing microfilariae. One or more samples of blood are taken from the dog,
treated with chemicals, and centrifuged to concentrate any microfilariae present. This
preparation is examined under a microscope. This method is called the "Modified Knott
Method." There are also other methods of blood examination (such as the filter method)
that may be equally effective in detection of microfilariae.
A major difficulty in early diagnosis is the fact that dogs may also be infected
with another parasite that produces microfilariae, Dipetalonema reconditum. This
organism is transmitted by fleas and is apparently not a disease-producer. However,
- 2 -
HEALTHY DOGS WHI
MOSQUITOS INGEST M
FILARIAE WHILE SUC
ADULT WORM IN HEART
AND ADJACENT LARGE
1. Female heartworms in animal's system begin repro-
ductive cycle by producing larvae (microfilariae).
2. Larvae spread throughout animal's circulatory sys-
3. After they have been ingested by a mosquito, larvae
undergo a 10 to 14 day development period in the
mosquito before being passed to a new host.
4. Once infective larvae have entered the blood stream
of a healthy animal, they migrate to the heart and
mature in 6 or 7 months.
Life History of the Dog Heartworm (Dirofilaria immitis)
the microfilariae are similar to those of Dirofilaria immitis, thus careful study is
necessary to determine which is present in a given blood sample. There are some dis-
tinct size, shape, and movement characteristics of each of these types of microfilariae
which the veterinarian can use to differentiate between them.
SYMPTOMS OF INFECTION WITH HEARTWORM AND CANINE HEARTWORM DISEASE
The first symptoms of heartworm infection are usually shortness of breath, shallow
cough, and a tendency toward easy tiring. These are seldom seen when microfilariae
are first detectable in a dog, but appear only after adult worms have been present for
a period of time. These first symptoms are due to interference with normal blood cir-
culation by adults in the heart and the blood vessels that lead from it.
If the first symptoms are not detected and the infection properly treated, other
symptoms occur and the condition of the dog becomes worse. The dog with canine heart-
worm disease generally has a chronic cough, tires easily and may even gasp for breath,
especially under conditions of exertion. Permanent damage to the lungs, liver, and
kidneys follows, which is mostly due, according to present knowledge, to interference
with normal blood pressure and flow by presence of adult worms. There is also some
evidence that the presence of circulating heartworm microfilariae in a dog causes
damage to vital organs.
Unfortunately, the numbers of circulating heartworm microfilariae in a dog's blood
does not have any relationship to the numbers of adult worms in the heart. It is thus
not any measure of the severity of the disease. In fact, as many as 5-10 percent of
dogs infected with Dirofilaria imnitis may never have circulating microfilariae. This
presents a difficult problem for the veterinarian in diagnosis.
TREATMENT OF INFECTION WITH DIROFILARIA IMMITIS IN DOGS
Treatment is best divided into 2 parts and is dependent upon utilizing the best
clinical and laboratory examinations possible.
Pre-treatment: All of the medical history available should be utilized in the
pre-treatment examination of a dog infected with heartworm. Due to the time required
to complete the heartworm life cycle, infected dogs are seldom less than 12 months
old and are usually more "middle aged" (4-6 years old). The complete physical ex-
amination of the infected dog includes laboratory tests to determine the condition of
certain vital organs. Such laboratory tests usually include a complete blood count
for overall assessment of the dog's condition, blood chemistry for blood urea nitrogen
(BUN) level to check on efficiency of kidneys in eliminating wastes, and complete
urinalysis as an added check on kidney function. Blood chemistry for liver function
is sometimes indicated in chronic cases. X-rays of the heart and lungs may also be
desirable in dogs showing severe symptoms. Individual veterinarians will often perform
additional examinations to help make medical assessments of their patients.
Treatment: The only F.D.A.-approved drug for the destruction of adult Dirofilaria
ionitis in infected dogs is an organic arsenic compound, thiacetarsamide, which is
administered intravenously and is marketed under various trade names. Thiacetarsamide
(the generic name) is a potentially dangerous drug. Its mode of action against heart-
worms is essentially that of poisoning them. The margin of safety between poisoning
the heartworms and poisoning the dog is narrow, hence the greatest of care must be em-
ployed in its use. Since thiacetarsamide is directly toxic to liver and kidney cells,
the pre-treatment assessment of the condition of these organs is very important. In
fact, if liver or kidney function is judged to be poor, it might be best not to treat
Once the medical decision has been made to treat with thiacetarsamide, dosage
will be calculated based on the dog's weight. The time schedule for administering
the drug will vary among veterinarians depending on their experience. The drug will
probably be given in 4 doses, approximately 12 hours apart. Due to possibility of
toxicity, dogs under treatment must be under constant medical surveillance and re-
stricted in activity. If toxicity appears, prompt supportive treatment must be given.
If toxic symptoms continue, treatment will probably be stopped and the dog allowed
to recover. Treatment can often be successful 4-6 weeks later.
When a dog has received the full dose of thiacetarsamide, toxic reactions may
occur from the dying heartworms. Clinically, the dead heartworms are swept by blood
flow from the heart into the pulmonary blood vessels. This usually occurs within 8
days after treatment and may cause serious problems if the dead worms block vital
blood vessels in the lungs. This is the reason for close restriction of the dog's
activities during the first 8 days or so after treatment.
After successful destruction of the adult heartworms, microfilariae that had been
in the circulating blood will remain there since thiacetarsamide has no effect on them.
This requires treatment with another drug, dithiazanine iodide. Removal of these re-
maining microfilariae is important not only because they may be capable of disease
production, but also because they will remain in the blood stream of the dog to be
picked up by mosquitos and transmitted to other dogs, thus perpetuating the infection.
Dithiazanine iodide (the generic name) is the only F.D.A.-approved drug to use
against microfilariae at the present time. It is also a potential poison and should
be used under careful medical supervision. This drug is usually administered as a
calculated dose in tablet form for 7 days. A blood examination for microfilariae
should be done at the end of 7 days. If the test is positive, the 7-day treatment
with dithiazanine iodide should be repeated until the test results are negative. Pro-
per supervision by a veterinarian is essential. A new drug, levamisole (the generic
name) is being currently studied for use against microfilariae and also the adult
heartworms. It is not yet F.D.A.-approved for general use in dogs.
PREVENTION OF INFECTION WITH DIROFILARIA IMMITIS
Diethylcarbamazine is an effective preventive medication against heartworm in-
fection when administered on a daily basis at a calculated dose. Its activity is
against developing heartworm larvae between the time they enter the dog from a bite
of an infected mosquito and when the heartworms start to mature in the heart area.
A major drawback in using this drug is that the dog must be heartworm-free or a
severe reaction may result. The exact cause of the reaction is not understood, but
the drug often produces symptoms of "allergic shock" which can be fatal. Therefore,
it is imperative that dogs treated for adult heartworms with thiacetarsamide and for
microfilariae with dithiazanine iodide be free of both forms of the parasite before
starting the preventive dose of diethylcarbamazine. Considerable caution and close
observation should be made of treated dogs for the first few days of the diethyl-
The safest way to use diethylcarbamazine as a preventive drug against heartworm
is to start the dosage when dogs are less than 6 months of age, prior to the time that
heartworms could mature. In all cases where diethylcarbamazine is used as a heartworm
preventive, it is sound medical practice to have routine blood examinations made for
the presence of microfilariae every 6 months. Quite often this timing coincides with
regular health checkups, booster vaccinations, etc. by the veterinarian.
Mosquito control can be a part of the prevention program for heartworm infections.
In Florida during years when rainfall is abundant and temperature suitable, mosquitos
are present in all months of the year and, despite the use of insecticides and usual
mosquito control practices, dogs may be exposed to mosquito bites year round. Under
such conditions, the use of diethylcarbamazine as a preventive drug may be the only
way to control heartworm infections. On the other hand, in other regions where mos-
quito populations may be controlled by dry weather or colder temperatures, it is
possible that diethylcarbamazine be used only during the time when mosquitos are a-
bundant and for 2 months after.
By Richard E. Bradley, D.V.M., Ph.D., Veterinary Parasitologist, College of Veterinary
Medicine, IFAS, University of Florida. 77-4.
This public document was promulgated at an annual cost of $43.15, or 2.2 cents per copy, to inform pet owners and
the general public of research results and knowledge in veterinary medicine. 6 2M 80
COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORIDA, INSTITUTE OF FOOD AND AGRICULTURAL
SCIENCES, K. R. Tefertiller, director, In cooperation with the United States Department of Agriculture, publishes this Infor-
matlon to further the purpose of the May 8 and June 30, 1914 Acts of Congress; and is authorized to provide research, educa-
tional Information and other services only to individuals and institutions that function without regard to race, color, sex or
national origin. Single copies of Extension publications (excluding 4-H and Youth publications) are available free to Florida
residents from County Extension Offices. Information on bulk rates or copies for out-of-state purchasers is available from
C. M. Hinton, Publications Distribution Center, IFAS Building 664, University of Florida, Gainesvllle, Florida 32611. Before publicizing this
publication, editors should contact this address to determine availability.