Heartworm Disease and
Heartworm
disease (HWD) is caused by infection with the nematode (roundworm) known
scientifically as Dirofilaria immitis. Dogs become infected after
being bitten by an infected mosquito. About half of the mosquito species
in the
Dogs
with heartworm infection are frequently asymptomatic (show no signs),
particularly when worm burdens are low. Common symptoms of HWD in dogs
include cough, lethargy, and exercise intolerance (tire rapidly). Prolonged
or severe infections may also result in weight loss, fainting or collapse
(syncope), and accumulation of fluid within the abdomen (ascites) resulting in a
bloated appearance. The worst cases may result in respiratory difficulty,
formation of blood clots in the lungs, liver enlargement, heart failure, and
death.
Any
dog that is not taking heartworm preventative in an area where heartworm is
found is susceptible to infection. While infection can occur at any age,
most dogs are between 3 and 8 years old at the time of diagnosis. Gundogs
and working hounds are likely to be at increased risk due to the fact that many
spend a good portion of their time outdoors when mosquitoes are most active.
Wild canines, domestic (and presumably wild) cats, and ferrets may be infected
with heartworm disease in the same manner as dogs. Humans are a “dead
end” host for heartworms; while they can be infected “accidentally,”
people do not become visibly ill or serve as reservoirs for disease.
There
are areas where heartworms are endemic (present at all times) throughout the
country. While Arizona is generally considered to have low prevalence of
HWD, there are several “pockets” of endemism in the Valley of the Sun, most
of which are in or near agricultural (or formerly agricultural) areas using
canals and flood irrigation on the urban fringe. The risk in Arizona
increases every year, however, as more and more people—nearly 1000 per month
in both Gilbert and Chandler—move from California and the East Coast, where
nearly 100% of unprotected dogs can be expected to contract HWD eventually.
Add this reservoir of infected dogs to the mosquito hatcheries (that developers
call water features) built into most subdivisions these days, and it’s easy to
see why the risk to dogs in
We
are fortunate to have safe and effective preventatives for HWD, which may be
given on daily, monthly, or six-month intervals (depending upon the medication).
By and large, these preventatives are inexpensive, safe, and very effective.
Many of these medications also control at least a portion of the intestinal
parasites common to dogs (either on their own or as a result of additional
parasiticides). Most dogs should be tested for heartworm infection prior
to beginning any preventative. The American Heartworm Association also
recommends annual testing for dogs on preventative to detect infections that can
occur as a result of treatment lapses or failures.
Unprevented
heartworm infections can be life-threatening, resulting in inflammation in the
lungs, liver congestion, and heart or kidney failure. Established
infections are typically treated with compounds related to arsenic. In
addition to being expensive, treatment often carries considerable risk and the
need for several weeks’ rest. Mild to moderate infections carry a good
prognosis when treated appropriately, but complications are relatively common in
more-severely infected patients. Re-infection can occur if a treated dog
is not taking appropriate preventatives.
For
these reasons, I typically recommend that my patients be on year-round heartworm
preventative. While using preventatives from May through December may
effectively prevent infection in
BG
Billy Griswold
Chandler, AZ 85225-1643
(480) 782-9522
(phone/fax)
(480) 734-5959 (mobile)