Page 342 - Feline Cardiology

Chapter 23: Heartworm Disease 355

the cat (Dillon et al. 2008). Moreover, although cats • Exposed cats in which the parasite completed its life
may clear the infection so that no adult worms develop, cycle to the mature L 5 stage with evidence of heart-
pulmonary lesions can still be severe (Browne et al. worm disease potentially involving multiple organs,
2005). particularly the lungs (antibody positive, ± antigen
Thus, the disease process can be divided into 3 main positive, dirofilariasis or heartworm disease)
stages of disease in cats, with the first being associated
with the arrival of juvenile worms in the pulmonary Wolbachia are a vertically transmitted, gram-negative,
arteries and an acute parenchymal inflammatory reac- intracellular bacterium that are harbored by D. immitis
tion. This stage is followed by a relatively symptom-free larvae and adults. Wolbachia are endosymbionts, which
period after the worms mature to adults and the host are also found in several other filarial species including
immune response is suppressed. However, when the Onchocerca volvulus and Brugia malayi (Simon et al.
worm (or worms) die(s), the immune system is no 2007). The metabolic products and surface proteins
longer suppressed and an intense inflammatory response from Wolbachia can elicit an inflammatory response in
follows (stage 2), with sudden death reported in 10–20% the host dog or cat (Nelson et al. 2005), and exposure to
of affected cats (Nelson 2008a). Finally, if the cat sur- Wolbachia occurs whenever larvae or adult worms die
vives, it enters the final stage of disease with permanent or are killed. A recent study showed a strong IgG response
lung injury due to type II alveolar cell hyperplasia and against the surface protein of Wolbachia in heartworm-
chronic respiratory disease (Nelson 2008a). infected cats (experimentally and naturally infected)
A link between heartworm infection in cats and lower leading to the hypothesis that the bacteria could play a
airway disease, such as feline asthma, has been hypoth- large role in the feline inflammatory response to heart-
esized for a long time (Borgarelli et al. 1997). In fact, worm infection (Morchon et al. 2004). Moreover, the
thoracic radiography performed in the first stage of immune response to Wolbachia antigens was detected
heartworm disease often shows changes which may before antibodies against D. immitis further supporting
be misinterpreted as indicating allergic bronchitis or this theory. A study evaluating pretreatment with doxy-
feline asthma (pulmonary bronchointerstitial pattern, cycline prior to adulticide administration in dogs showed
hyperinflated pulmonary fields, etc). Furthermore, there was a reduction in pathology associated with
radiographic changes improve with glucocorticoid worm death (McCall et al. 2008; Nelson 2008b). No
administration, further supporting the misdiagnosis similar studies have been reported to date in cats;
(Nelson 2008a). Even if heartworm disease was suspected however, according to the American Heartworm Society,
and tested for, cats in this stage of disease will not have a studies to determine the effects of suppressing Wolbachia
positive antigen test (because adult worms are not populations with doxycycline in cats are in progress. Heartworm Disease
present) and a significant number of false negative results
are possible with the various antibody tests (see the HISTORY, CHIEF COMPLAINT, PHYSICAL
section “Differential Diagnosis” for further discussion). EXAMINATION
This evidence has led to a proposed classification for
heartworm disease, which reflects the difficulty of defin- The owner-reported history is highly variable. The owner
ing a heartworm-infected cat (Atkins 2007). In this rec- of an affected cat may report a wide range of clinical
ommended scheme, heartworm-exposed or infected signs such as chronic coughing, sudden or episodic
cats would be divided into the following groups: dyspnea, and/or vomiting (Atkins et al. 1998, 2000).
Vomiting and coughing are common clinical signs in
• Unexposed cats (antibody negative, antigen negative) heartworm-infected cats, and the combination of these
• Exposed cats treated with heartworm preventative signs in a feline patient warrants a high level of suspicion
(antibody positive, antigen negative, and free of clini- for heartworm infection. Lethargy, decreased appetite
cal signs of heartworm disease) and/or weight loss may also be reported. Occasionally
• Exposed cats with subclinical disease in which the central nervous system signs such as seizures or blind-
parasites developed to the immature L 5 stage in small ness may be present, particularly when aberrant migra-
numbers (antibody positive, antigen negative, and tion has occurred. Cardiac abnormalities such as
without radiographic lesions) pulmonary hypertension, cor pulmonale, or congestive
• Exposed, possibly clinically affected cats (parasites heart failure are uncommon. Asthmatic signs are common
developed into immature L 5 with secondary pulmo- manifestations and often occur about 3 to 4 months
nary lesions, but parasites did not mature to adults postinfection. Less commonly, ascites, hydrothorax, chy-
(antibody positive, antigen negative, “pulmonary lothorax, pneumothorax, ataxia, and syncope have all
larval dirofilariasis”) been reported to be associated with feline heartworm