Moxidectin steady state prior to inoculation protects cats from subsequent,repeated infection with Dirofilaria immitis

Background: infection 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. methods: to 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. results: measurement 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. conclusions: taken 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. © 2015 little et al.; licensee biomed central.