Canine Heartworm

Dirofilaria immitis or heartworm (HW) has been found around the world in 
Asia, Europe, North and South America as well as Australia. In North 
America it occurs primarily in the southeastern areas. HW is found 
throughout the U.S. and Canada including Hawaii. However, with the 
exception of the southeast the incidence is low. The incidence in the 
southeast and and along the gulf of Mexico is greater than 50% and 
persists 12 months of the year. Whereas, in many of the more northern 
areas the disease is only of concern in the warmer 6 months. 

Heartworm is a parasitic helminth that lives in the dog, fox, wolf and 
cat. It has also been diagnosed in humans in the more southeastern part 
of the US. Canidae are the natural host for the worm and the incidence is 
low in other species. The worm is quite long (25-30cm). The mosquito is 
essential for the transmission of the disease.The disease is spread by 
mosquitoes when a blood meal is taken from a host¹s tissues. Within an 
aliquot of blood immature microscopic HW (microfilaria)  are found. These 
microfilaria mature in the digestive tract of the mosquito and molt into 
an infective stage. This process of molting is dependent on daily 
temperatures greater than 64 degrees F over a months time and, is the 
reason why the disease is primarily seasonal in North America.  After 
several weeks in the mosquito and 2 molts, the third larval stage can 
infect the dog with the next blood meal. Upon infecting the new canine 
host, the microfilaria migrate to major vessels and by 5 months are 
entering the large vessels of the lungs. By six months adult heartworms 
are discharging their own microfilaria to begin the cycle again at the 
next mosquito bite. A host can act as its own reservoir where a mosquito 
bites the infected dog and 3 weeks later returns to bite and reinfest the 
same dog for a superimposed infection increasing the adult HW burden. 
Signs of the disease are related to the number of adult worms found in 
the dog and, burdens of 200+ worms have been reported. As the number of 
worms increase in the host, worms migrate into the right heart and major 
vessels obstructing blood flow. Obstruction to blood flow is associated 
with many deleterious effects: pulmonary hypertension, hemorrhage, kidney 
disease and pulmonary thromboembolism. 

Testing for HW is done approximately 6 months after the previous mosquito 
season; in the spring. Dogs in climates where the mosquito season is 
extended late into the year have the option of continuous prophylaxis or 
repeat testing in the middle summer.  Two basic tests for HW exist: 
direct microfilaria tests and antigen tests. Antigen tests are dependent 
on the microfilaria being greater than 6 months old. Antigen tests are 
extremely sensitive and require no more than 1 adult female worm. Almost 
all adult female worms produce an antigen response and thus, antigen 
tests are extremely reliable. However, it is possible to only have male 
adult worms. Addittionaly, approximately 1% of female worms produce no 
antigen response resulting in false negative antigen tests. Microfilaria 
without adults are possible in early stages of infection and in certain 
stages of ongoing HW therapy. Radiographs of the lungs can detect HW 
thromboembolisms but should only be used as a follow up to a confirmed 
case or an occult suspect. Microfilaria tests are very quick and 
inexpensive. They only test for circulating microfilaria and can miss HW 
infections that are not shedding microfilaria. Also, dogs using the once 
a month prophylaxis are free of microfilaria but may still have adult 
worms. Antigen testing is quickly becoming the screening test of choice. 
Certain instances exist where both tests should be employed.

Signs of heartworm disease are not apparent until several years post 
infection. They include coughing, difficulty breathing, occasionally 
coughing blood, fainting, and, exercise intolerance. Advanced disease can 
show abdominal fluid and severe bleeding. 

Treatment for the disease involves first destroying the adult worms and 
then eliminating the microfilaria. Risks of therapy are directly related 
to the quantity of the HW burden and lung involvement which increase the 
incidence of pulmonary thromboembolism. Adulticide therapy involves 
arsenic compounds and hence, potential complications. Complications 
include caval syndrome and pulmonary artery infections where the HW 
burden has been extended into the heart and main pulmonary arteries. 
Usually prior to adulticide treatment the adult heartworms are surgically 
retrieved from the heart. Historically intravenous administration of 
thiacetarsamide sodium has been the mainstay of  therapy. Four 
intravenous injections are given over 2 days. The toxicity of the drug is 
high to the patient and therefore, hospitalization and monitoring are 
important. Complications of severe vomiting and jaundice require 
suspension of treatment for several months before resuming the full 
treatment.

Recently, melarsomine has been released as an adulticide. Although this 
drug is also an arsenical it provides several advantages over 
thiacetarsamide. It is less toxic to the patient and seems to be more 
toxic to adult HW. Additionally, the administration is via an 
intramuscular injection in the lumbar muscles. Previous concerns with 
phlebitis from thiacetarsamide have been eliminated with melarsomine. The 
drug is administer twice over 2 days. Each adulticide can be associated 
with complications up to 4 weeks  post therapy that result from the 
pulmonary thromboemboli from the dead worms.  A patient should be 
restricted to mild activity for one month after adulticide therapy.

A microfilaricide is administered about 4 weeks after completing 
adulticide therapy. Some dogs with heavy burdens may experience side 
effects to the therapy which include: pale membranes, anorexia, vomiting 
and salivation. It is unlikely that these signs will progress to serious 
effect but, not impossible. Some dogs may need to be hospitalized for 
reaction to sudden death of large numbers of microfilaria. Two 
microfilaricides exist: ivermectim and milbemycin oxime. A third drug 
will soon be on the market. In addition to being used to clear the 
microfilaria of an existing HW infection they are also the mainstay in HW 
prophylaxis. 

HW prophylaxis is given after a negative HW test has been performed. 
Prophylactic drugs include the monthly use of ivermectim or milbemycin 
oxime or the daily administration of diethylcarbamazine citrate (DEC). 
DEC is administered daily starting just prior to the beginning of the 
mosquito season and extending one month beyond the mosquito season. The 
drug is decreasing in use since the introduction of monthly 
microfilaricides. DEC is very inexpensive and is safe providing the dog 
is tested negative prior to its use each year. It can cause very serious 
reactions, including shock and death, if given to dogs with current 
microfilaria. Annual retesting is important using this drug and should 
include in addition to the common antigen test ,a direct test for 
microfilaria in the blood.

Monthly microfilaricides are extremely safe and easy to use but they are 
significantly more expensive than DEC. The price difference is most 
pronounced in large dogs. Both ivermectim and milbemycin oxime also are 
effective dewormers for roundworms and hookworms. Milbemycin oxime also 
is effective against whipworms. Additionally, these drugs are highly 
effective in their use as microfilaricides. Aside from the initial HW 
test prior to their use, retesting beyond the first year is not essential 
if the owner has been diligent. Veterinarians may want to retest after 
the first year of use to insure successful prevention. Retesting every 
other year or third year is probably adequate.