Dog Owner's Home Veterinary Handbook.pdf - Mr. Walnuts

344 •DOG OWNER’S HOME VETERINARY HANDBOOK
dead worms can be carried by the bloodstream into the pulmonary circulation,
resulting in a similar severe and sometimes fatal reaction. Chronic pulmonary
thromboembolism causes loss of lung tissue and right-sided
congestive heart failure. Dogs with thromboembolism may cough up bloody
sputum.
Worms entwined about the heart valves can interfere with the mechanics
of the heart and produce effects similar to those of chronic valvular disease.
Worm clumps in the vena cava or hepatic veins are responsible for a condition
called vena cava syndrome, which causes liver failure with jaundice,
ascites, spontaneous bleeding, and anemia. Collapse and death can occur in
two to three days.
DIAGNOSIS
The signs of heartworm disease depend on the number of worms and the size
of the dog. Dogs with a light infestation involving only a few worms may
remain asymptomatic.
The typical early signs of heartworm infestation are tiring easily, exercise
intolerance, and a soft, deep cough. As the disease progresses these symptoms
become more severe and the dog loses weight, breathes more rapidly, and may
cough after exercise to the point of fainting. The ribs become prominent and
the chest starts to bulge. Acute vena cava syndrome or episode of thromboembolism
can lead to collapse and death.
A number of blood tests are available to diagnose heartworms. The most
accurate is the heartworm antigen test, which identifies an antigen produced
by the adult female heartworm. False negatives occur in dogs with early infections
(before the appearance of mature worms), in light infections with fewer
than five adult worms, and in infections in which only males are present.
False positives are rare.
Another important heartworm test is the microfilarial concentration test,
in which parasites in a sample of blood are identified under the microscope.
Although a positive test definitely indicates heartworms, a negative test does
not rule out the diagnosis because typically 10 to 25 percent of infected dogs
do not have microfilariae circulating in the peripheral blood.
Dogs with a negative microfilarial concentration test who do have heartworms
are said to be suffering from an occult infection. There are a number of
explanations for occult infection. One is that the dog is receiving a heartworm
preventive. Preventives kill microfilaria but not adult worms. Thus
these dogs will have a positive heartworm antigen test and a negative microfilarial
concentration test. Also, a dog could be infected with adult heartworms
of just one sex, which means no reproduction is taking place.
There is yet another type of microfilaria that can be present in dogs tested
for heartworms. It is called Dipetalonema. It is a harmless worm living under