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.     

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.     

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.     

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.     

Full season efficacy of moxidectin microsphere sustained release formulation for the prevention of heartworm (Dirofilaria immitis) infection in dogs

The authors report the efficacy of an injectable, moxidectin sustained release (SR) formulation for the prevention of canine heartworm infection in endemic areas in northern and central Italy. Three field trials were carried out on a total of 324 dogs. Two hundred forty-three dogs were treated with moxidectin SR 6 months apart and 81 dogs (positive controls) with moxidectin tablets given monthly for 5 consecutive months during the risk season each year throughout the study. Results of testing for microfilariae and circulating adult female antigens were negative for all the experimentally treated dogs at the 6, 7, 11 and 19 months after the last injection. No adverse reactions to moxidectin SR were observed but a moderate pain at palpation and swelling (5-6 cm) at the injection site after the first treatment. In the study areas, prevalence of Dirofilaria immitis infection calculated by testing dogs which had no preventive treatment in the previous transmission season ranged from 33 to 63%. This study confirms the efficacy and safety of injectable, moxidectin SR formulation in the prevention of heartworm infection in dogs and demonstrates that the prophylactic efficacy lasts for the full season and strongly suggests that the product gives 1-year protection.     

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.     

Efficacy of a single milbemycin oxime administration in combination with praziquantel against experimentally induced heartworm (Dirofilaria immitis) infection in cats

The efficacy of a combination of milbemycin oxime and praziquantel in preventing the establishment of experimentally induced heartworm (Dirofilaria immitis) infection was investigated in a study involving 24 young domestic short-hair cats. The animals were inoculated with 50 infective larvae on day 0. Subsequently they were divided into two groups of 12 animals each. The animals in group 1 were treated once with medicated tablets containing 4 mg milbemycin (minimum dose 2 mg/kg body weight) and 10 mg praziquantel (MILBEMAX) on day 30 after infection. Cats in group 2 received placebo tablets on the same day. On day 183 post-infection a blood sample was taken from each animal before euthanasia and necropsy. The blood samples were tested for the presence of microfilariae and the necropsied animals were examined for the presence of adult worms. Microfilariae were not found in any of the investigated cats. No heartworms were found in the animals in group 1 (treated with medicated tablets). Out of the 12 placebo-treated cats 1 was heartworm-free, whereas all the others were found to be infected with 1-3 adult heartworms.     

Prevalence of Dirofilaria immitis infection among shelter cats

OBJECTIVE: To determine prevalence of Dirofilaria immitis infection among shelter cats. DESIGN: Cross-sectional study. ANIMALS: 239 cats euthanatized at an animal shelter in southeastern Michigan. PROCEDURE: A gross necropsy focusing on the thoracic cavity, heart, lungs, and pulmonary vessels was performed within 5 hours after cats were euthanatized. Blood was collected directly from the heart of cats found to be infected and tested, using a filter test for microfilariae. Serum was tested for D immitis antigens by use of 2 antigen detection kits and for D immitis-specific antibodies by use of 2 antibody detection kits. RESULTS: Cats ranged from approximately 4 months to 15 years old. Adult D immitis were found in 6 (2.5%) cats. Blood could not be recovered from 1 of the cats with heartworm infection. For the 5 other cats, results of the filter test were negative, and results of both antigen and both antibody tests were positive. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that cats living in an urban area in the northern part of the United States have a low prevalence of adult D immitis infection. However, this is likely to be an underestimate of the true prevalence of infection, because no attempts were made to identify cats infected with larval or juvenile stages of D immitis.     

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.     

Prevention of experimentally induced heartworm (Dirofilaria immitis) infections in dogs and cats with a single topical application of selamectin

In a series of six controlled studies (four in dogs, two in cats), heartworm-free dogs and cats were inoculated with Dirofilaria immitis larvae (L(3)) prior to topical treatment with the novel avermectin selamectin or a negative control containing inert formulation ingredients (vehicle). Selamectin and negative-control treatments were administered topically to the skin at the base of the neck in front of the scapulae. In dogs, selamectin was applied topically at dosages of 3 or 6mgkg(-1) at 30 days post-inoculation (PI), or of 3 or 6mgkg(-1) at 45 days PI, or of 6mgkg(-1) at 60 days PI. Cats were treated topically with unit doses providing a minimum dosage of 6mgkg(-1) selamectin at 30 days PI. Of the animals that were treated 30 days PI, some dogs were bathed with water or shampoo between 2 and 96h after treatment, and some cats were bathed with shampoo at 24h after treatment. Between 140 and 199 days PI, the animals were euthanized and examined for adult D. immitis. Adult heartworms developed in all control dogs (geometric mean count, 18.7 worms) and in 88% of control cats (geometric mean count, 2.1 worms). Selamectin was 100% effective in preventing heartworm development in dogs when administered as a single topical dose of 3 or 6mgkg(-1) at 30 days after infection, 3 or 6mgkg(-1) at 45 days after infection, or 6mgkg(-1) at 60 days after infection. Selamectin was 100% effective against heartworm infections in cats when administered as a single topical unit dose of 6mgkg(-1). Bathing with water or shampoo between 2 and 96h after treatment did not reduce the efficacy of selamectin as a heartworm prophylactic in dogs. Likewise, bathing with shampoo at 24h after treatment did not reduce the efficacy of selamectin in cats. These studies demonstrated that, at the recommended dosage and treatment interval, a single topical administration of selamectin was 100% effective in preventing the development of D. immitis in dogs and cats.     

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.     

Immunoblot analysis of specific antigen bands predictable for Dirofilaria immitis infection in cats

Serial sera from four mongrel cats experimentally inoculated with infectious larvae of Dirofilaria immitis were analyzed by immunoblot patterns against a phosphate buffered saline-extract of D. immitis. Antigen-specific protein bands detected indicate that the low molecular weight bands of 36, 32, 22, 19 and 14 kDa, are predictable for positive adult worm infection, suggesting diagnostic usefulness for adult D. immitis infection in cats.     

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.     

Aberrant migration and surgical removal of a heartworm (Dirofilaria immitis) from the femoral artery of a cat

A cat was evaluated for an acute‐onset of right pelvic limb paresis. Thoracic radiographs revealed normal cardiac size and tortuous pulmonary arteries. Abdominal ultrasound identified a heartworm (HW) extending from the caudal abdominal aorta into the right external iliac artery and right femoral artery. The cat was HW‐antigen positive. Echocardiography revealed a HW within the right branch of the main pulmonary artery and evidence of pulmonary hypertension. An agitated‐saline contrast echocardiogram revealed a small right to left intracardiac shunt at the level of the atria. Surgical removal of the HW was performed with no substantial postoperative complications. There was return of blood flow and improved motor function to the limb. The cat remains mildly paretic on the affected limb with no other clinical signs.