Abstract
BackgroundInfection of cats with Dirofilaria immitis causes seroconversion on antibody tests and pulmonary pathology, often without subsequent development of adult heartworms. Consistent administration of topical 10% imidacloprid-1% moxidectin has been shown to result in sustained plasma levels of moxidectin in cats after three to five treatments, a pharmacokinetic behavior known as “steady state”.MethodsTo evaluate the ability of moxidectin at “steady state” to protect cats from subsequent infection with D. immitis, cats (n = 10) were treated with the labeled dose of topical 10% imidacloprid-1% moxidectin for four monthly treatments. Each cat was inoculated with 25 third-stage larvae of D. immitis 7, 14, 21, and 28 days after the last treatment; non-treated cats (n = 9) were inoculated on the same days, serving as infection controls. Blood samples were collected from each cat from 1 month prior to treatment until 7 months after the final inoculation and tested for antibody to, and antigen and microfilaria of, D. immitis.ResultsMeasurement of serum levels of moxidectin confirmed steady state in treated cats. Cats treated with topical 10% imidacloprid-1% moxidectin prior to trickle inoculation of D. immitis L3 larvae throughout the 28 day post-treatment period remained negative on antibody and antigen tests throughout the study and did not develop gross or histologic lesions characteristic of heartworm infection. A majority of non-treated cats tested antibody positive by 3–4 months post infection (6/9) and, after heat treatment, tested antigen positive by 6–7 months post-infection (5/9). Histologic lesions characteristic of D. immitis infection, including intimal and medial thickening of the pulmonary artery, were present in every cat with D. immitis antibodies (6/6), although adult D. immitis were confirmed in only 5/6 antibody-positive cats at necropsy. Microfilariae were not detected at any time.ConclusionsTaken together, these data indicate that prior treatment with 10% imidacloprid-1% moxidectin protected cats from subsequent infection with D. immitis for 28 days, preventing both formation of a detectable antibody response and development of pulmonary lesions by either immature stages of D. immitis or young adult heartworms.
Highlights
Infection of cats with Dirofilaria immitis causes seroconversion on antibody tests and pulmonary pathology, often without subsequent development of adult heartworms
Mean trough levels of moxidectin in samples collected from treated cats on study days −85, −57, −29, and −1, prior to first inoculation with D. immitis, were as follows: none detected, 16.5, 33.4, and 40.0 μg/L, respectively (Figure 1)
Antigen and antibody test results on 6 of the non-treated cats were reported in an earlier publication focusing on immune complexes in cats infected with D. immitis [14]
Summary
Infection of cats with Dirofilaria immitis causes seroconversion on antibody tests and pulmonary pathology, often without subsequent development of adult heartworms. Antibody testing, which reveals evidence of both past and current infection, documents a higher prevalence than antigen tests, in cats with respiratory disease. The moxidectin-based feline heartworm preventive is applied topically absorbed systemically, and is formulated in combination with imidacloprid, an insecticide primarily intended to control flea infestations [7]. Each of these heartworm preventives was approved based on the WAAVP-endorsed experimental approach whereby infection is first established by inoculation with third-stage larvae and 30 days later the preventive is administered [8]. Because infections are allowed to progress for one month using this approach, seroconversion on antibody tests may occur even if adult heartworms do not develop in treated cats
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