Clinical prophylactic activity of melarsomine dihydrochloride (RM 340) against Dirofilaria immitis in heartworm-naive beagles exposed to natural infection in three southeastern states

Examination of the "susceptibility gap" in the treatment of canine heartworm infection

BACKGROUND

The "susceptibility gap" in a dog diagnosed with adult heartworms has been defined as the period of time in which some Dirofilaria immitis stages are not susceptible to treatment with either macrocyclic lactones or melarsomine dihydrochloride. This was previously defined within the American Heartworm Society guidelines as a period of about 3 months "as per product labels." It can be postulated, however, that a susceptibility gap does not exist with the combination of continued macrocyclic lactone therapy coupled with a three-dose melarsomine dihydrochloride protocol where the first intramuscular treatment is near the time of first diagnosis.

DISCUSSION

Melarsomine dihydrochloride was originally also investigated as a "preventive" as well as a treatment for adult heartworm infection where it would be given to dogs by intramuscular injection every 4 months; therefore, there was early interest in its ability to kill younger worms. A single intramuscular injection of 2.5 mg melarsomine dihydrochloride/kg has an efficacy of 82.1% against 4-month-old worms. When it was given to dogs with older heartworms, 7 and 12 months of age, a single injection was only 55.6% and 51.7% effective, respectively. Thiacetarsamide has been shown to be 99.7% effective against 2-month-old heartworms and other work has shown that melarsomine dihydrochloride is 100% efficacious against these younger forms. With the development and US Food and Drug Administration (FDA) approval of spinosad + milbemycin oxime (Trifexis®, Elanco), milbemycin oxime + praziquantel (Interceptor® Plus, Novartis, now Elanco), and milbemycin oxime + lufenuron + praziquantel (Sentinel® Spectrum®, Novartis, now Virbac), it was shown that repeated treatments of dogs with milbemycin oxime also has efficacy against 3-month-old heartworms. Thus, no improvement in efficacy is expected with a delay in initiating therapy with both melarsomine dihydrochloride and macrocyclic lactones, even with the presence of younger heartworms. Starting treatment at diagnosis appears to be acceptable for maximal heartworm clearance based on published data. Delaying treatment has the disadvantage of allowing disease progression and continued heartworm growth.

CONCLUSIONS

The collective data that has been reviewed indicates that continued macrocyclic lactone administration with two additional injections of melarsomine dihydrochloride a month later will protect dogs against all heartworm stages, including those heartworms 2 months of age or younger at diagnosis, when both treatments are started upon diagnosis.

The animal trypanosomiases and their chemotherapy: a review

Pathogenic animal trypanosomes affecting livestock have represented a major constraint to agricultural development in Africa for centuries, and their negative economic impact is increasing in South America and Asia. Chemotherapy and chemoprophylaxis represent the main means of control. However, research into new trypanocides has remained inadequate for decades, leading to a situation where the few compounds available are losing efficacy due to the emergence of drug-resistant parasites. In this review, we provide a comprehensive overview of the current options available for the treatment and prophylaxis of the animal trypanosomiases, with a special focus on the problem of resistance. The key issues surrounding the main economically important animal trypanosome species and the diseases they cause are also presented. As new investment becomes available to develop improved tools to control the animal trypanosomiases, we stress that efforts should be directed towards a better understanding of the biology of the relevant parasite species and strains, to identify new drug targets and interrogate resistance mechanisms.

Experimental Dirofilaria immitis infection in dogs: effects of doxycycline and Advantage Multi® administration on immature adult parasites

To better understand the efficacy of doxycycline and 10% imidacloprid+2.5% moxidectin (Advantage Multi(®); Bayer Animal Health, Shawnee Mission, Kansas) on immature adult Dirofilaria immitis parasites and the results of antigen tests, 12 healthy, randomly selected dogs were experimentally infected with D. immitis and monitored for 407 days. Two dogs in each of three subgroups of four dogs were each infected with six (total of 6 dogs) or 12 (total of 6 dogs) D. immitis infective third-stage larvae (L3) obtained from infected mosquitoes. Doxycycline (10mg/kg per os twice daily×30 days) and 10% imidacloprid+2.5% moxidectin (1ml/kg by topical application every 30 days) treatment was initiated at 105 (Group A) and 149 (Group B) days post infection (PI) in two groups. One subgroup of two dogs given 6 L3 and one subgroup of two dogs given 12 L3 remained as untreated controls (GroupC). Serum obtained regularly throughout the study was evaluated by ELISA (PetChek(®) Heartworm-PF Antigen Test, IDEXX Laboratories, Inc.) for D. immitis adult circulating antigens. Six of the eight dogs in the treated groups had detectable antigenemia starting between 148 and 240 days post infection, but antigen was not detected in any treated dog at the end of the study. In the control subgroups, the dogs that received 6 L3 had no detectable antigen while the two dogs that received 12 L3 had detectable antigen beginning on Day 180 that persisted until the end of the study. None of the infected dogs had evidence of circulating microfilariae. At necropsy, no heartworms were recovered from the treated dogs, but all dogs in the untreated group had viable adult heartworms. These results indicate that early immature adult worms (3.5 and 5 months of age) of D. immitis were susceptible to a combined treatment regimen of doxycycline and 10% imidacloprid+2.5% moxidectin.

First analysis of the secretome of the canine heartworm, Dirofilaria immitis

Background

The characterization of proteins released from filariae is an important step in addressing many of the needs in the diagnosis and treatment of these clinically important parasites, as well as contributing to a clearer understanding of their biology. This report describes findings on the proteins released during in vitro cultivation of adult Dirofilaria immitis , the causative agent of canine and feline heartworm disease. Differences in protein secretion among nematodes in vivo may relate to the ecological niche of each parasite and the pathological changes that they induce.

Methods

The proteins in the secretions of cultured adult worms were run on Tris-Glycine gels, bands separated and peptides from each band analysed by ultra mass spectrometry and compared with a FastA dataset of predicted tryptic peptides derived from a genome sequence of D. immitis.

Results

This study identified 110 proteins. Of these proteins, 52 were unique to D. immitis . A total of 23 (44%) were recognized as proteins likely to be secreted. Although these proteins were unique, the motifs were conserved compared with proteins secreted by other nematodes.

Conclusion

The present data indicate that D. immitis secretes proteins that are unique to this species, when compared with Brugia malayi. The two major functional groups of molecules represented were those representing cellular and of metabolic processes. Unique proteins might be important for maintaining an infection in the host environment, intimately involved in the pathogenesis of disease and may also provide new tools for the diagnosis of heartworm infection.