Page 348 - Feline Cardiology

Chapter 23: Heartworm Disease 361

worms are present in the distal pulmonary arteries, OUTCOME AND PROGNOSIS
retrieval is much more difficult due to the complex
three-dimension arborization of the distal arteries). In one field study, which followed 34 asymptomatic cats
Moreover, it makes theoretic sense that reducing worm diagnosed with feline heartworm infection by antibody,
biomass is preferential to adulticide therapy, particularly antigen, and echocardiographic examination every 3
in cats that are predisposed to acute inflammatory reac- months until self-cure or death, over 80% of the cats
tions when worms die and degrade. Multiple worm self-cured and 21 of the 34 remained asymptomatic
retrieval approaches have been described in the litera- throughout the study. Six of the 34 cats (18%) died.
ture and extraction of the worms often results in rapid Infections lasted over 3 years in most of the cats enrolled
and dramatic clinical improvement, including clinical in the study (Venco et al. 2008). This study shows that a
signs secondary to right-sided congestive heart failure large proportion of heartworm-infected cats can survive
(Borgarelli et al. 1997; Atwell and Litster 2002; Small the infection with supportive care. Moreover, many cats
et al. 2008). While it is preferable to remove heartworms remain asymptomatic through the course of disease.
rather than destroy them in situ, damage to a worm
during the retrieval process can result in shocklike signs Prevention
and death (Nelson et al. 2005) and the clinician must Cat owners should be advised regarding the potential
take care to avoid damaging worms during the proce- risk of heartworm infection in their community and the
dure. Additional challenges include the tenuous physi- comparative ease of preventing the disease vs the diffi-
ologic status of many of these patients (and associated culty of treatment. When monthly heartworm chemo-
anesthesia risk) and the small jugular size, which makes prophylaxis is elected, it should be initiated at least 30
passage of the Ishihara flexible heartworm retrieval days following the estimated seasonal onset of transmis-
forceps difficult in small patients. Other devices that sion and continued for 30 days after that period has
have been successfully implemented for retrieval include: ended. Year-round administration has been recom-
basket-type retrieval forcepts (Borgarelli et al. 1997), mended by some because 1) depending on the product
nitinol gooseneck snare catheter (Small et al. 2008), chosen, the cat may also be protected from intestinal or
horsehair brushes, endoscopic grasping forceps, and external parasites; 2) studies suggest owner compliance
rigid alligator forceps (Litster et al. 2008). is better; and 3) there is retroactive efficacy (reach-back)
if doses are missed inadvertently. The reach–back effect
covers the same age of heartworms as those killed under
chemical prophylaxis (larvae 8 months postinfection
MONITORING
and older) (McCall 2005). See Table 23.2 for a summa- Heartworm Disease
Periodic monitoring at 6- to 12-month intervals with rized comparison of the “reach-back” effect for the
repeat Ag and Ab testing and thoracic radiography is various preventatives. Currently four macrocytic lactone
warranted. After clearance of adult heartworms (iatro- products are registered for feline heartworm chemopro-
genic or spontaneous), infected cats will usually become phylaxis: ivermectin, moxidectin, milbemycin, and sela-
antigen negative within 4 or 5 months. However, it mectin. All of these products are safe and effective
is important to review the results carefully for each options for cats living in areas where canine heartworm
individual case recognizing the inherent weaknesses disease is considered endemic, and each is administered
of the available tests. The American Heartworm Society monthly. Ivermectin and milbemycin oxime are admin-
states that antibody testing becomes optional once istered orally while moxidectin and selamectin are
the cat is antigen negative because antibody persists for topical. All of these drugs can be safely administered in
an indefinite period after the worms are gone, and con- seropositive cats.
tinued exposure (even when the cat is receiving preven- Ivermectin is an effective preventive drug with the
tative therapy) will result in a positive antibody test. most potent reach-back effect of the various prophylaxis
Moreover, as already stated above, the performance of options. This effect means that prolonged administra-
different brands of antibody tests is variable. At least tion of the preventative can kill young larvae, older
regression of radiographic signs and seroconversion of larvae, immature adults and/or older adult worms. This
a positive Ag test suggest the period of risk of an acute effect is particularly advantageous when the heartworm
crisis following worm death and embolization is over. infection status is unknown at the time treatment is
Follow-up CBC and/or echocardiography may lend commenced or in cases of questionable owner compli-
further useful information in a small number of cases ance. A dose of 24 µg/kg administered monthly is an
but rarely will alter therapeutic recommendations and effective heartworm prophylaxis in cats, and it also con-
are therefore not cost effective. trols hookworms. Milbemycin, at a dosage of 2 mg/kg,