Doi:10.1016/j.vetpar.2004.06.024

Veterinary Parasitology 124 (2004) 259–268 Efficacy of long-term monthly administration ofivermectin on the progress of naturally acquired aClinica Veterinaria Citta` di Pavia, Viale Cremona, Pavia, Italy bCollege of Verinary Medicine, The University of Georgia, Athens, GA, USA cDepartment of Pathobiology School of Veterinary Medicine, University of Pennsylvania, dDipartimento di Patologia Animale, Igiene e Sanita` Pubblica Veterinaria, Sezione di Patologia Generale e Parasitologia, Universita` degli Studi di Milano, Received 21 January 2004; received in revised form 15 June 2004; accepted 21 June 2004 This study was designed to evaluate the efficacy of prolonged monthly ivermectin treatment against Dirofilaria immitis in client-owned dogs with naturally acquired infections and to clinicallymonitor the animal’s response to the slow killing of heartworms, with death of the worms distributedover a period of up to 2 years. A total of 17 male and female dogs of different breeds and ages wereused. Prior to treatment, all of the dogs tested positive for heartworm antigen (Ag) and all but two hadmicrofilariae (mf). The dogs were randomly allocated to one group of seven dogs which received acommercial formulation of ivermectin (minimum, 6 mcg IVM/kg) plus pyrantel (minimum, 5 mg PP/kg) (Heartgard PlusTM Chewables, Merial, Ltd.), another group of seven dogs which received acommercial formulation of IVM (min, 6 mcg/kg) (Heartgard1 Chewables, Merial Ltd.), and a groupof three dogs which served as an untreated controls. All dogs were evaluated prior to initiation oftreatment and thereafter at 3- to 5-month-intervals for mf, Ag, and radiographic and echocardio-graphic findings. All of the 17 dogs, with the exception of two dogs in the IVM group, had circulatingmf of D. immitis prior to the 1st monthly dose, and a few also had mf of Dirofilaria repens. After 4monthly doses, only one dog in the IVM/PP group and two dogs in the IVM group had a patentheartworm infection, and no heartworm mf were seen in the 14 treated dogs thereafter. After 10 * Corresponding author. Tel.: +39 02 5031 8101; fax: +39 02 5031 8095.
E-mail address: [email protected] (C. Genchi).
0304-4017/$ – see front matter # 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.vetpar.2004.06.024 L. Venco et al. / Veterinary Parasitology 124 (2004) 259–268 monthly doses, the number of Ag-positive dogs in both of the treated groups decreased gradually.
Efficacy, based on the reduction in number of Ag-positive dogs, was similar for the IVM/PP and IVMgroups, with overall efficacy scores for the 14 dogs of 21, 21, 43, and 71% after 10, 14, 19, and 24monthly doses, respectively. Two of the seven dogs treated with IVM/PP, one of the seven treated withIVM, and two of the three untreated controls showed echocardiographic evidence of a parasiticburden prior to treatment, and all of these scores had decreased by the end of the study. Only one dog(IVM/PP group) had a cardiovascular pattern of heartworm disease by echocardiography prior totreatment, but this dog’s score increased to two and the scores of two additional dogs increased fromzero to two (IVM group) or three (IVM/PP group) by the end of the study. Only 1 (IVM/PP group) ofthe 17 dogs showed a pulmonary pattern of heartworm disease by radiography prior to treatment, butthis dog’s score increased to three by the end of the study. The radiographic scores of two additionaldogs in the treated groups increased from zero to three (IVM/PP) or two (IVM) by the end of thestudy. Thus, monthly administration of IVM to dogs with clinical, radiographic or echocardiographicevidence of heartworm disease is ill-advised and such treatment of even the asymptomatic dog shouldbe done only with much caution and frequent monitoring by the veterinarian.
# 2004 Elsevier B.V. All rights reserved.
Keywords: Ivermectin; Long-term monthly administration; Heartworm adulticide efficacy; Efficacy; Dog The prophylactic efficacy of various formulations of ivermectin (IVM) at 6 mcg/kg against Dirofilaria immitis infection in dogs has been well documented (). IVM has potent activity against the third and fourth stage larvae of the parasite,interrupting their development into the adult stage. Monthly administration of IVM at thisstandard preventive rate over 12–31 months to experimentally infected dogs produced ahigh level of efficacy against many stages of the parasite, including ‘‘immatures’’ (4–5months of age) and young adults (6–7 months of age). This effect was seen even after theparasites had reached the pulmonary arteries (reach-back efficacy). Efficacy reportedagainst these stages in these studies ranged from 95 to 98% (). The continuous administration of IVM also has been found to be efficacious againstadult parasites 8 months of age that had been transplanted intravenously () 1 month earlier. Results of the latter study demonstrated marginal efficacy (56%)against 8-month-old adult worms after 16 monthly doses; however, all of the remaining liveworms were noticeably abnormal in motility and/or appearance. These findings withexperimental infections are very promising; however, data are not presently available onthe use of protracted, monthly IVM treatments against naturally acquired D. immitisinfections under clinical conditions.
The objective of the present study was to evaluate the efficacy of prolonged IVM treatment against D. immitis in naturally infected dogs and to study the evolution of theclinical process. In addition, the study evaluated the effects of heartworms in a populationof naturally infected dogs and the reaction of an animal subjected to various periods ofpathological action of the parasite. The study also evaluated the effects of the continuousrelease of parasitic antigens subsequent to the administration of IVM and the animal’sability to control the thromboembolic incidents consequent to the death of the parasite.
L. Venco et al. / Veterinary Parasitology 124 (2004) 259–268 Seventeen dogs, including males and females of different breeds and ages, naturally infected with D. immitis were used ). All of the dogs were owned by clients of theCitta` di Pavia Animal Clinic in Pavia (Italy) and were determined to be positive forheartworm infection by detection of adult female D. immitis antigen using a commercialELISA test (PetChek1 Heartworm PF Antigen Test, IDEXX Laboratories Inc.,Westbrook, Maine, USA) and most of the dogs were positive for circulating D. immitismicrofilariae by the Knott test during April 2000. Seven of the dogs were randomlyselected for treatment with a commercial chewable formulation of IVM and pyrantelpamoate (IVM/PP) (Heartgard PlusTM Chewables, Merial Ltd., Duluth, GA, USA).
Seven other dogs received a chewable formulation containing only IVM (Heartgard1Chewables, Merial Ltd., Duluth, GA, USA). Three of the dogs were not treated andserved as untreated controls (UTC). The IVM/PP chewable formulation provided aminimum of 6 mcg IVM and 5 mg pyrantel per kilogram of body weight. The threedosage sizes used were as follows: L. Venco et al. / Veterinary Parasitology 124 (2004) 259–268 The IVM tablet formulation provided a minimum of 6 mcg IVM per kilogram of body weight, and the three available dosage sizes provided the same amounts of IVM as theIVM/PP chewable formulation.
Owners treated dogs monthly with the assigned formulation. The first treatment was administered in May 2000, and the study was concluded in May 2002. The untreatedcontrol dogs received melarsomine dihydrochloride (Immiticide1, Merial Ltd., Duluth,GA, USA), according to the instructions on the label, as an adulticide treatment at the endof the study.
Dogs were returned to the clinic for microfilarial counts, antigen testing, and radiographic and echocardiographic evaluations at 3- to 5-month-intervals. The level ofmicrofilaremia was expressed as the number of microfilarie per milligram. Circulatingantigen levels were expressed as optical density (OD). All sera were assayed for heartwormantigen concentration using a microwell ELISA (PetChek1 HTWM PF Canine HeartwormAntigen Test Kit, IDEXX Laboratories, Westbrook, Maine, USA) and quantitatedspectrophotometrically by a protocol described by the kit manufacturer that measures theOD of each reaction. An OD greater than 0.135 was indicative of heartworm (D. immitis)infection with adult female worms. Scoring of thoracic radiographs (cardiovascular andpulmonary aspect) was similar to that used by except that the numericalvalues are different, i.e. 0: normal, 1: slight alterations, 2: moderate alterations, and 3:marked alterations. Echocardiograms (parasitic load) were scored as 0: none evident, 1:low, 2: moderate, and 3: marked evidence. Cardiovascular aspects of the echocardiogramswere scored as 0: normal, 1: slight alterations, 2: moderate alterations, and 3: markedalterations.
Results of the parasitologic examinations on individual animals are presented in and summarized in Twelve of the 14 treated dogs and all of the three untreatedcontrols had circulating D. immitis mf at the start of the study. Counts ranged from 152 to561 mf/ml for dogs that received IVM/PP, 0–2100 in the group treated with IVM, and 151–5850 in the controls. Two dogs in the IVM/PP group also had circulating microfilarie ofDirofilaria repens (dog ID: DCK, 8 and LUK, 53 mf/ml) prior to initiation of the study.
After 4 monthly administrations of IVM/PP, D. immitis mf counts were reduced by 99%.
Only two of the seven dogs treated with IVM had any heartworm mf after treatment startedand these dogs (ELO and BUK) had counts of 23 and 5 mf/ml, respectively. No circulatingD. immitis mf were detected in any of the dogs in either of the two treated groups starting 7months after initiation of the treatments and continuing through the end of the study.
One dog in the IVM group was positive for D. repens mf 10 months after treatment wasinitiated.
Table 2Heartworm antigen levels (OD values) and microfilarial counts (number mf/ml) for client-owned dogs given ivermectin at 6 mcg/kg once a month for 24 consecutivemonths and the untreated controls a Cut-off: OD 0.135.
b OD values under the cut-off level are shown in italics.
c Number mf/ml of D. repens.
L. Venco et al. / Veterinary Parasitology 124 (2004) 259–268 Table 3Number of treated dogs/total number of dogs positive for heartworm microfilarie and circulating Ag at differenttimes in the study and efficacy values based on reduction in number of antigen-positive dogs and number ofheartworm microfilarie-positive dogs mf: microfilariae and Ag: circulating antigens.
a 2 positive also for D. repens.
b 1 positive for D. repens; 1 positive for both D. immitis and D. repens.
c D. repens.
All of the three untreated control dogs had circulating mf when the study started, but all of them were amicrofilaremic by the 19th month.
All dogs were positive for circulating antigen at the beginning of the study. The range in OD values was 0.473–2.487 in the group treated with IVM/PP, 0.612–2.788 in the grouptreated with IVM, and 1.591–2.136 in the untreated control group. Serology values for dogsin both treated groups were similar to those for the control group until after the 7th monthof treatment. In the group treated with IVM/PP, the number of antigen-positive dogs wasreduced by 28.5% after the tenth and 14th monthly treatments, by 43% after the 19thtreatment, and by 71.5% after the 24th monthly treatment. In dogs treated with IVM alone,the reduction in the level of circulating antigen was 14% after the 10th and 14th monthlytreatments, 43% after the 19th, and 71% following the 24th monthly treatment. All of thethree untreated control dogs remained antigen-positive throughout the study.
Considering that in both treated groups the compound active against D. immitis is the IVM portion and that the owners of all of the dogs in both groups were given the sameinstructions regarding housing, feeding, and management, all results of antigen testing arecombined as shown in Using this pooled data from both treated groups, the L. Venco et al. / Veterinary Parasitology 124 (2004) 259–268 Table 4Comparison of scores for the evaluation of echocardiographic and thoracic radiographic results cumulative reduction was 21% after the tenth and 14th monthly treatments, 43% after the19th treatment, and 71% after the 24th treatment. The dogs in the untreated group hadpositive OD antigen values during the entire study (P = 0.406 by the non-parametric test of The results of radiographic and echocardiographic examinations for individual dogs are presented in Acknowledging the limitations of this method, the parasitic loadmeasured by echocardiography shows a reduction in worm burden in all treated anduntreated animals found positive prior to the 1st monthly treatment and no variation inthose animals found negative prior to the 1st monthly treatment.
At the beginning of the study, the cardiovascular image was considered within normal limits for all the dogs in the three groups, except for dog LUK in the IVM + PP group,which had a score of 1. At the end of the study, this dog (LUK) and ADL (IVM group)showed a marked worsening, which was confirmed by the thoracic radiographs. Dog ZFLin the IVM + PP group showed normal radiographic and echocardiographic patterns at thebeginning of the study; however, the images obtained at the end of the study show a markedworsening on both parameters (score of 3 for both). For all other animals, the radiographic L. Venco et al. / Veterinary Parasitology 124 (2004) 259–268 picture was maintained within normal parameters, with no abnormalities observed at theend of the study.
Based on the reduction in antigen levels, the adulticide efficacy of the IVM was found to be 71% after 24 monthly administrations of IVM, either alone or in association with PP.
These values seem to be lower than those previously reported in experimental studies(). This difference in efficacy wasprobably due to the shorter treatment period and the different method used for assessingefficacy utilized in the present study. In one of the previously quoted experimental studies(), the authors utilized 7-month-old heartworms transplanted via thejugular vein to parasite-free dogs 1 month before treatment started and 16 monthly doseswere given. At the end of the study, 60% of the treated dogs were negative for antigen andthe adult worm burden was reduced by 56%. Under those circumstances, the treatmenteffect was seen on nematodes of the same age and in dogs that had not suffered thecharacteristic pathological effects observed in some natural infections, including thenegative effect on the natural and acquired mechanisms of defense in dogs with long-terminfections. In another study, treatment was started 7 months after inoculation of infective,third-stage larvae. Thus, the adult worm infections were relatively young when monthlytreatment was started. In a somewhat similar study conducted by , 12monthly doses of IVM were 98, 86, 52, and 34% effective against 3.5-, 4.5-, 5.5-, and 6.5-month-old heartworms, respectively. In the present study, it is presumed that the dogs hadexperienced a broad range of pathogenic effects of the parasite at the beginning of thetreatments. In the experimental studies, efficacy calculations were based on the differencein parasitic load between the treated and untreated animals and not on the number ofnegative animals at the parasitological examinations.
The worsening of the radio- and echocardiographic pictures in 3 of the 14 treated dogs was probably due to thromboembolic and immunomediated phenomena triggered by thedeath of the parasites. Worsening of these parameters would be expected, regardless ofwhether the cause of death of the worms was by natural attrition or chemical treatment(McCall et al., 2003). One of the three dogs (ZFL) had a worsened image considered to beespecially alarming. For this reason, we recommend much caution in any case whereprolonged preventive treatments are used as filarial adulticides under clinical conditions.
The use of IVM at the dosage of 6 mcg/kg for prolonged periods of time was in fact recently included, as an adulticide alternative, in the 2002 Guidelines for the Diagnosis,Prevention and Management of Heartworm Infections in Dogs (This treatment is indicated as highly efficacious against pre-cardiac larvaeand young adults (less than 7 months in age). The treatment has to be prolonged for at leasta year, and probably two or more years are needed in order to accomplish completeefficacy. According to McCall and et al. (2001), the older the parasite is at the time of thefirst treatment the slower the death of the parasite. It should be kept in mind that the dog isalso subjected to a progressing persistent infection that is subjected to the effect of asubtherapeutic treatment (). For these reasons, the prolonged L. Venco et al. / Veterinary Parasitology 124 (2004) 259–268 treatment with IVM may not be a substitute for the adulticide treatment with arsenicals forall patients. In fact, the previously quoted Guidelines emphasize that while prolongedtreatment with IVM gradually reduces the number of adult parasites in chronic infections,the effect of this treatment may not be completely clinically efficacious (AmericanHeartworm Society, 2002). Results reported in this paper demonstrate how in the course ofa prolonged treatment to eliminate adult stages of D. immitis it is advisable, before startingthe treatment, to correctly determine the clinical class of the patient based on the severity ofthe infection (). It is also mandatory to limit the patient’sexercise and periodically monitor (preferably at least once every 4–6 months) the clinicalstatus of the patient during the complete treatment until the dog becomes negative for thepresence of D. immitis circulating antigen ( The case of the dog ZFL is meaningful, as this dog presented a normal radiological and echocardiographic picture at the beginning of the study and then progressively becameclinically worse, indicating that the thromboembolic phenomena can be severe, eventhough parasites are being killed over a long period of time. This occurred in spite of thefact that all of the owners of the dogs in this study were instructed to have their dog’sphysical activity drastically restricted by cage confinement. According to and it has been clearly demonstrated that forced rest is themost important measure in the prevention of thromboembolism and pulmonary damageinduced during heartworm infections in dogs.
The three dogs in the untreated control group and most of the treated dogs were normal from a clinical, radiographic, and echocardiographic standpoint 2 years after the initialdiagnosis was made. However, under normal clinical conditions, where this drasticrestriction of physical activities cannot be reasonably controlled, it seems appropriate tohypothesize that the worsening of the cardiopulmonary picture could have happened in amore rapid and serious manner.
Based on these results, the prolonged treatment of dogs infected with D. immitis utilizing IVM at the dosage of 6 mcg/kg appears at this time to be ill-advised in patientsthat present with clinical, radiographic, or echocardiographic alterations at the time theinitial diagnosis is made. The prolonged treatment is also not advisable in infected dogsthat are subjected to very active physical life or sports activities, even though normal for theparameters described herein, until further data are available.
American Heartworm Society, 2001. 2002 guidelines for the diagnosis prevention and management of heartworm (Dirofilaria immitis) infection in dogs. In: Seward, R.L. (Ed.), Proceedings of the Recent Advances inHeartworm Disease Symposium ’01, American Heartworm Society, Batavia, IL, pp. 259–266.
Bowman, D.D., Neumann, N.R., Rawlings, C.E., Stansfield, D.G., Legg, W., 2001. Effects of avermectin on microfilarie in dogs with existing and developing heartworm infections. In: Seward, R.L. (Ed.), Proceedings ofthe Recent Advances in Heartworm Disease Symposium ’01, American Heartworm Society, Batavia, IL, pp.
173–178.
Di Sacco, B., Vezzoni, A., 1992. Clinical classification of heartworm disease for the purpose of adding objectivity to the assessment of therapeutic efficacy of adulticidal drugs in the field. In: Otto, G.H. (Ed.), Proceedings ofHeartworm Symposium ’92, American Heartworm Society, Batavia, IL, pp. 209–214.
L. Venco et al. / Veterinary Parasitology 124 (2004) 259–268 Dillon, A.R., Brawner, W.R., Hanrahan, L., 1995. Influence of number of parasites and exercise on the severity of heartworm disease in dogs. In: Soll, M.D., Knight, D.L. (Eds.), Proceedings of the Heartworm Symposium’95, American Heartworm Society, Batavia, IL, pp. 113.
Friedman, M., 1937. The use of ranks to avoid the assumption of normality implicit in the analysis of variance. J.
Fukami, N., Hagio, M., Okano, S., Watanabe, S., 1998. Influence of exercise on recovery of dogs following heartworm adulticide treatment with melarsomine. In: Seward, R.L. (Ed.), Proceedings of the HeartwormSymposium ’98, American Heartworm Society, Batavia, IL, pp. 225–227.
Guerrero, J., McCall, J., Genchi, C., 2002. The use of macrocyclic lactones in the control and prevention of heartworm and other parasites in dogs and cats. In: Vercruysse, J., Rew, R. (Eds.), Macrocyclic Lactones inAntiparasitic Therapy, CABI Publishing, Oxon, UK, pp. 353–369.
McCall, J.W., 2003. Heartworm-positive dog requires tailored treatment. DVM Best Practices (March Issue on McCall, J.W., McTier, T.L., Ryan, W.G., Gross, S.J., Soll, M.D., 1996. Evaluation of ivermectin and milbemycin oxime efficacy against Dirofilaria immitis infections of three and four months’ duration in dogs. Am. J. Vet.
Res. 57, 1189–1192.
McCall, J.W., McTier, T.L., Supakorndej, N., Ricketts, R., 1995. Clinical prophylactic activity of macrolides on young adult heartworms. In: Soll, M.D., Knight, D.H. (Eds.), Proceedings of the Heartworm Symposium ’95,American Heartworm Society, Batavia, IL, pp. 187–195.
McCall, J.W., Guerrero, J., Roberts, R.E., Supakorndej, N., Mansour, A.E., Dzimianski, M.T., McCall, S.D., 2001.
Further evidence of clinical prophylactic retroactive (reach-back) and adulticidal activity of monthlyadministrations of ivermectin (Heartgard PlusTM) in dogs experimentally infected with heartworms. In:Seward, R.L. (Ed.), Proceedings of the Recent Advances in Heartworm Disease Symposium ’01, AmericanHeartworm Society, Batavia, IL, pp. 189–200.
McCall, J.W., Ryan, W.G., Roberts, R.E., Dzimianski, M.T., 1998. Heartworm adulticidal activity of prophylactic doses of ivermectin (6 mg/kg) plus pyrantel administered monthly to dogs. In: Seward, R.L. (Ed.), Pro-ceedings of the Recent Advances in Heartworm Disease Symposium ’98, American Heartworm Society,Batavia, IL, pp. 209–215.
Rawlings, C.A., Bowman, D.D., Howerth, E.W., Stansfield, D.G., Legg, W., Luempert III, L.G., 2001. Disease response to trickle kill when ivermectin or milbemycin is started during Dirofilaria immitis infection in dogs.
In: Seward, R.L. (Ed.), Recent Advances in Heartworm Disease Symposium ’01, American HeartwormSociety, Batavia, IL, pp. 179–187.
Venco, L., Genchi, C., Vigevani Colson, P., Kramer, L., 2001. Relative utility of echocardiography, radiography, serologic testing and microfilarie counts to predict adult worm burden in dogs naturally infected withheartworm. In: Seward, R.L. (Ed.), Recent Advances in Heartworm Disease Symposium ’01, AmericanHeartworm Society, Batavia, IL, pp. 111–124.

Source: http://www.deibacirubati.it/site/images/pdf/ivermectina.pdf