Field efficacy and safety of a combination of moxidectin and imidacloprid for the prevention of feline heartworm (Dirofilaria immitis) infection

Throughout the end of March to beginning of May 2006, 212 owned cats and 608 owned dogs from a heavy endemic area for canine heartworm (HW) disease in northern Italy have been examined to assess HW infection prevalence. Both cats and dogs were clinically examined and blood samples were taken from each animal to be examined for HW antibody (Ab). Ab-positive cats were further examined for circulating microfilariae, HW antigens (Ag) and by echocardiography (ECHO) to assess the presence of adult worms. Dogs were clinically examined and blood samples taken from each animal were examined for circulating microfilariae and for HW Ag. Ten cats (4.7%) were found Ab positive. Of these, 6 cats were Ag positive (2.6%) and in 4 (1.8%) the worms were visualized by ECHO. HW prevalence in dogs was 36% (221/608). One hundred and seventy-six (29%) were both microfilaraemic and Ag positive, 40 (7%) had occult infections (no circulating microfilariae) and 7 (1%) were microfilaraemic but Ag negative. Upon owners’ consent, 132 cats (including cats Ab and/or Ag and ECHO positive) were prophylactically treated against HW disease with an imidacloprid/moxidectin spot-on combination (10% imidacloprid/1% moxidectin) monthly administered for 6 months. Cats were re-examined for HW infection in November, 1 month after the last drug administration, and in May–June 2007, 7–8 months after the last treatment. All 122 cats found HW negative before treatment, were found negative at the two examinations at the end of study. The 4 cats Ab positive, 2 cats Ab and Ag positive and 1 Ab, Ag and ECHO positive at the beginning of treatment were found negative. Throughout the treatment, transitory hypersalivation and generic signs of annoyance were reported by owners in 6 cats (4.5%). All signs regressed spontaneously.     

Six-month prophylactic efficacy of moxidectin sustained release (SR) injectable for dogs against experimental heartworm infection in growing puppies

The purpose of this study was to assess the efficacy of moxidectin sustained release injectable for dogs (moxidectin SR, Fort Dodge Animal Health) in protecting growing puppies from experimental infection with the heartworm, Dirofilaria immitis, six months after treatment. The study involved 27 puppies, approximately 12 weeks of age at the beginning of the study, with nine puppies in each of three size classes. The small breed class included eight Pekingese and one purpose-bred small breed mongrel; the medium breed class included nine purpose-bred mongrels, and the large breed class included nine puppies with an anticipated adult weight >or=30-35 kg. Both genders were included with no attempt made to have equal numbers of male and female puppies. Puppies were blocked by weight within each size class and randomly assigned to three treatment groups of nine dogs. On Day 0, pups in two groups were injected subcutaneously with moxidectin SR, dosed to deliver 0.17 mg moxidectin/kg b.w. The third group was injected with sterile saline. Personnel making observations were blinded to the treatment status of the animals. Following treatment, puppies were observed for signs of adverse local and systemic reactions. Puppy weights and serum moxidectin levels were also monitored. On Day 180, puppies in all treatment groups were inoculated subcutaneously with 50 third-stage larvae of D. immitis. On Days 348 and 349, puppies were euthanatized and necropsied. Hearts and lungs were examined for adult heartworms. All animals in the saline control group were infected with an arithmetic mean of 39.22 adult heartworms each. Seventeen of 18 dogs in the moxidectin SR-treated groups were uninfected. One treated puppy was infected with a single adult heartworm. This infected individual was from the large breed size class and had the second highest percent increase in body weight. Based on arithmetic means, the heartworm recovery from all treated puppies represents a 99.86% reduction relative to the saline control. There were no adverse local or systemic reactions to treatment in any animal.     

Efficacy of selamectin in the prevention of adult heartworm (Dirofilaria immitis) infection in dogs in northern Italy

The efficacy of a novel avermectin, selamectin, was evaluated for the prevention of heartworm disease (adult Dirofilaria immitis infection) in 120 dogs (aged 9 months to 13 years at enrolment) presented as veterinary patients. The study was conducted at five veterinary practices in a heartworm hyperendemic region of northern Italy. Dogs were allocated randomly in a 2:1 ratio to treatment with either selamectin or ivermectin. Treatments were administered at monthly intervals for 6 months during the heartworm transmission season (May-November). Selamectin was applied topically in a single spot to the skin on each animal's back at the base of the neck in front of the scapulae as a unit dose that provided at least the minimum recommended dosage of 6mgkg(-1) (range, 6-12mgkg(-1)). Ivermectin (6microgkg(-1) of body weight) was administered orally at monthly intervals, in accordance with the manufacturer's product label recommendations. Study day 0 was defined individually for each dog as the day of first treatment administration. Efficacy was assessed on the basis of the absence of D. immitis microfilariae and adult heartworm (D. immitis) antigen in tests conducted on days 180 and 300. There were no adverse clinical signs arising due to treatment with selamectin and no drug-related mortalities. The prevention rate for D. immitis microfilariae and adult heartworm antigen was 100% for both selamectin and ivermectin. Thus, selamectin administered as a unit dose, providing at least the recommended minimum dosage of 6mgkg(-1), at monthly intervals during the heartworm transmission season was safe and 100% effective in the prevention of heartworm disease in dogs presented as veterinary patients.     

Ivermectin and milbemycin oxime in experimental adult heartworm (Dirofilaria immitis) infection of dogs

Background: The US Food and Drug Administration reports an increase in the frequency of reports of lack of effectiveness claims for heartworm (HW) prevention products. Hypothesis: At their labeled doses, single doses of commercially available HW prevention products are not completely effective against all field isolates of HW. Animals: Forty‐two HW‐free dogs experimentally inoculated with a recent HW field isolate. Methods: Placebo‐controlled, blinded laboratory clinical trial. Dogs randomly allocated to 1 of 3 treatment groups with 14 dogs per group. Groups were untreated control or PO dosed with milbemycin oxime (MBO) or ivermectin (IVM). Dogs were inoculated with 50 HW third stage larvae 30 days before dosing and necropsy was performed on Day 123 after treatment to enumerate adult HW. Results: Thirteen of 14 control dogs had adult HW detected at necropsy with a geometric mean worm count of 22.3. One HW was found in 1 dog in each of the MBO and IVM treatment groups. Conclusions and Clinical Importance: Two currently approved macrocyclic lactone HW preventives used at their labeled dose rates were <100% effective against a recent HW field isolate, supporting the hypothesis that the effectiveness of a single dose of these preventives can vary. This is important in guiding clients on expectations of product effectiveness.     

Surgical transplantation of adult Dirofilaria immitis to study heartworm infection and disease in dogs

Heartworm infections were established in dogs by surgical transplantation of adult heartworms into the external jugular vein. The heartworms were obtained from donor dogs that had been infected with large numbers of infective larvae. This heartworm model standardized the infection with a known number of heartworms of a known age and sex. This paper presents the methods for obtaining heartworms from donor dogs, for selection of recipient dogs, and for transplantation of heartworms.     

Percutaneous heartworm removal from dogs with severe heart worm (Dirofilaria immitis) infestation

Canine heart worm disease is often life-threatening due to its various complications, including right side heart failure, caval syndrome and pulmonary eosinophilic granulomatosis. Several preventive medications and melarsomine have been developed and they are very effective to control heartworm infestation. However, in a case of severe infestation, melarsomine therapy often results in an unfavorable outcome because of the severe immune reaction caused by rapid killing of the adult worm. Surgical removal and an interventional method using flexible alligator forceps have been well described in the literature. Despite the usefulness of mechanical removal using flexible alligator forceps, the methodology still needs to be upgraded for increasing the applicability for treating dogs with severe infestation. We describe herein a newly developed percutaneous removal method for heartworms and this was successfully applied to 4 dogs with severe heartworm infestation. The follow-up studies also showed favorable outcomes with no complications.     

Survey of 11 western states for heartworm (Dirofilaria immitis) infection, heartworm diagnostic and prevention protocols, and fecal examination protocols for gastrointestinal parasites

Heartworm infection in dogs and cats in the western United States is a fairly new phenomenon, and for this reason it is often considered to be of minimal significance.The purpose of this survey was to collect data from 11 western states (Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming) in an effort to assess awareness of heartworm disease and identify areas in which improvement in understanding is needed. To accomplish this goal, veterinary clinics and hospitals in these states were sent a one-page survey in early 2006. The results of the survey demonstrate that cases of heartworm disease have been reported in all 11 states, illustrating the importance of annual testing and the routine use of preventives.     

Activity of an injectable, sustained-release formulation of moxidectin administered prophylactically to mixed-breed dogs to prevent infection with Dirofilaria immitis.

OBJECTIVE: To test the ability of a single injection of a sustained-release formulation of moxidectin (moxidectin SR) to protect dogs against heartworm infection for 180 days after inoculation with infective third-stage larvae (L3) of Dirofilaria immitis. ANIMALS: 32 adult mixed-breed dogs. PROCEDURE: Dogs were allocated to 4 groups on the basis of weight and sex. Dogs were injected SC with saline (0.9% NaCl) solution or moxidectin SR at the rate of 0.06, 0.17, or 0.5 mg/kg of body weight (day 0). Each dog was inoculated SC with 50 D immitis L3 180 days later. On days 330 and 331, dogs were euthanatized. The heart, lungs, and thoracic cavity were examined, and number and sex of heartworms were determined. RESULTS: A mean of 35.9 heartworms was recovered from untreated control dogs. Fourteen worms were recovered from 1 of 8 dogs given moxidectin SR at the lowest dosage, and none of the dogs in the 2 highest moxidectin treatment groups were infected. Small barely palpable granulomas were detected at injection sites of moxidectin-treated dogs. Frequency and size of granulomas were positively correlated with dose of moxidectin administered. CONCLUSIONS AND CLINICAL RELEVANCE: A single dose of moxidectin SR at a dosage as low as 0.17 mg/kg can safely and reliably confer complete protection against infection after challenge-exposure with D. immitis L3, and protection lasts for at least 180 days. This mode of prophylactic treatment against infection with heartworms effectively eliminates failure of prophylaxis that results from erratic administration of medications designed for monthly administration.     

Immune response to and tissue localization of the Wolbachia surface protein (WSP) in dogs with natural heartworm (Dirofilaria immitis) infection

Human and animal parasitic filarial nematodes, including the agent of canine and feline heartworm disease Dirofilaria immitis, harbour intracellular bacteria of the genus Wolbachia (Rickettsiaies). It is thought that these bacteria play an important role in the pathogenesis and immune response to filarial infection. Immunoglobulin G (total IgG, IgG1, IgG2) production against and immunohistochemical staining of tissues for the Wolbachia surface protein (WSP) from dogs with natural heartworm infection were evaluated. All infected dogs had significant total anti-WSP IgG levels compared to healthy controls. Interestingly, WSP was recognized by the IgG2 subclass in both microfilariemic dogs and in dogs with no circulating microfilariae (occult infection). However, microfilariemic dogs also produced gG1 antibodies. Positive staining for WSP was observed in lungs, liver and kidneys, in particular in glomerular capillaries of naturally infected dogs who had died from heartworm disease. Our results show for the first time that Wolbachia is recognized specifically by D. immitis--infected dogs and that the bacteria is released into host tissue. Furthermore, microfilariemic status appears to effect immune responses to this endosymbiont.     

Evaluation of a single injection of a sustained-release formulation of moxidectin for prevention of experimental heartworm infection after 12 months in dogs

OBJECTIVE: To evaluate the efficacy of a single injection of a sustained-release formulation of moxidectin in preventing heartworm (Dirofilaria immitis) infection for 12 months in dogs. ANIMALS: 14 healthy dogs. PROCEDURE: Group A (nontreated control dogs; n = 6) received sterile vehicle administered SC, and group B (treated dogs; n = 6) received a sustained-release formulation of moxidectin administered SC. All dogs were housed in a heartworm-endemic area for 11.5 months, and heartworm antigen and modified Knott tests were performed monthly. All dogs (including 2 additional control dogs [group C]) were then inoculated with infective-stage larvae (L3) of D. immitis, and 4.5 months later, all dogs were euthanatized and post-mortem examinations were performed. Adult D. immitis were counted and measured, and their age was estimated. RESULTS: All dogs in groups A and C were infected with young (4- to 4.5-month old) adult male and female D. immitis. No dogs in group B were infected with heartworms. CONCLUSIONS AND. CLINICAL RELEVANCE: The age of heartworms recovered suggests that infection was the result of experimental inoculation and not natural exposure to mosquitoes during the 11.5-month period the dogs resided in a heartworm-endemic area. A single SC injection of a sustained-release formulation of moxidectin was effective in providing protection against heartworm infection after 12 months in dogs. This formulation is a convenient method of heartworm prophylaxis that could eliminate the problem of poor owner compliance.     

Influence of immunosuppressants against Dirofilaria immitis infection in dogs

Two immunosuppressive drugs, azathiopurine (AZP) and prednisolone (PDS), were examined on dogs experimentally infected with Dirofilaria immitis in order to estimate the involvement of immunological assail in rejecting the parasite by the host. AZP was orally administered to 3 dogs daily at a dosage of 1-10 mg/kg for a period from 3 days before infection until the end of the experiment. The dose was then varied and transiently ceased according to the severity of the side effects. PDS was subcutaneously administered daily to 2 dogs. They were administered 10 mg/kg of PDS from 3 days before infection to day 15 and 5.0-8.5 mg/kg from day 60 after infection to day 70. The serum D. immitis-specific antibody level assessed by an indirect hemagglutination test was steadily decreased in the AZP-medicated dogs. However, in the PDS-medicated dogs, the antibody titer was decreased until day 32 and, thereafter, was recovered. When all dogs were sacrificed between days 145-148, an average recovery rate of worms in both the AZP- and PDS-medicated dogs was 52.5% and 49.6%, respectively, while the controls showed 42.5%. The statistical analysis revealed no significant difference among the groups, indicating that the administration of AZP and PDS was not effective in protecting the larvae from the host's immune attack.     

No evidence of endemic infection with Dirofilaria immitis in dogs

Extract

Dirofilaria immitis is periodically recorded in dogs imported from Australia into New Zealand. In this country, environmental conditions are marginal for the transmission of this species, but potential mosquito intermediate hosts are known to be present⁽¹⁾. Development in the intermediate host is temperature dependent, with most reports indicating that a mean temperature of at least 18 °C is required^(2)(3)(4). Therefore, the most likely locations in New Zealand for D. immitis to establish are the Auckland region and areas north of this.     

Influence of immune treatments against Dirofilaria immitis infection in dogs

Mongrel dogs were inoculated with two kinds of antigenic substances. The first was a phosphate buffered saline extract of whole Dirofilaria immitis mixed with aluminum hydroxide gel (group 1), and the second was an orally administered live Metastrongylus apri infective larvae (L3) (group 2). Both groups were then infected with D. immitis L3. Indirect hemagglutination (IHA) tests showed that the antibody was produced by these inoculations before the infection was introduced, even in dogs inoculated with M. apri. This suggests a cross-reactivity between D. immitis and M. apri. The initial passive cutaneous anaphylaxis (PCA) antibody production was markedly delayed by about 70 days in group 2 compared with the production in the infected control dogs (group 3). The appearance of microfilaremia was also delayed by about one month in group 2 compared with that in the above control group. All dogs were sacrificed after the termination of the observation and worms recovered from the right ventricle and pulmonary arteries were counted and measured. The results indicated that immunization resulting from the homologous worm-somatic antigen might accelerate the growth of the infected larvae, whereas immunization resulting from the heterologous worm antigen, but cross-reactive to D. immitis, might disadvantageously affect the growth.     

Feline heartworm (Dirofilaria immitis) infection: a statistical elaboration of the duration of the infection and life expectancy in asymptomatic cats

A study was conducted to assess the duration and the outcome (self-cure or death) of feline heartworm infection and the life expectancy of infected cats. To be included in the study, cats had to be positive for heartworm antibody (Ab) and heartworm antigen (Ag) and had to demonstrate the presence of worms by echocardiography. Self-cure was defined as (1) negative for heartworm Ag and (2) no further visualization of worms by echocardiography. Of the 1962 eligible cats, 364 (18.5%) were positive for heartworm Ag and 131 were positive for heartworm Ag and for echocardiography diagnosis (prevalence 6.7%). None of the cats was microfilaremic. Of 43 asymptomatic cats included into the follow-up study with owners' consent, 34 (79%) self-cured and nine (21%) died. Eleven (26%) cats remained asymptomatic and self cured within 21-48 months, 23 (53%) showed symptoms but self-cured within 18-49 months, 6 (14%) died within 8-41 months of follow-up and 3 (7%) suddenly died after 38-40 months, which was related to heartworm infection. The probability for death or sudden death increased significantly with age at diagnosis, but no difference was detected by gender, survival time after diagnosis, or the presence or absence of symptoms. The presence/absence of symptoms showed significant interaction with the age at diagnosis (i.e., symptomatic cats showed increasing duration of heartworm infection along with age at diagnosis compared to that for asymptomatic cats. Heartworm-infected cats survived significantly longer than heartworm-negative cats affected by hypertrophic cardiomyopathy, chronic renal failure, or neoplastic diseases.