Can a single ingredient stop heartworm disease transmission in its tracks?

(Getty Images)Did you know that a flea preventive you have already been recommending for your veterinary patients may provide an extra level of protection you weren't even aware of?

At an industry session sponsored by Ceva Animal Health at the Western Veterinary Conference in March, John McCall, MS, PhD, discussed the findings of his recent study funded in part by Ceva, offering a new perspective on heartworm disease transmission and its primary vector.

As the incidence of heartworm disease has increased from 2013 to 2015 and the geographical distribution has expanded, one thing is clear-heartworm disease is still prevalent. And although we have made great strides against it, the veterinary profession still has yet to control its spread.

The objective of Dr. McCall's study was to answer the question: Does mosquito repellency help in the prevention of heartworm disease transmission?

What to say to clients

If you routinely receive the same objections to topical ectoparasiticide treatment from clients, here are some talking points to help:

• Year-round heartworm prevention is still the primary recommendation from the American Heartworm Association.

• Repellency in various topical ectoparasitic products now provides you an additional argument for why flea, tick and biting insect monthly preventive treatment is warranted.

• When owners question the safety of the product, remind them that permethrin-impregnated bed nets and clothing on humans have been used for decades in malaria-endemic areas to reduce disease transmission.

Study overview

The specific goal of the study was to determine the repellency of a topical product that contains dinotefuran, pyriproxyfen and permethrin (DPP, Vectra 3D-Ceva) and whether it could block the transmission of Dirofilaria immitis microfilariae from dogs to mosquitoes. Yes, read that again, from heartworm-positive dogs to mosquitoes. Hasn't been studied before.

Based on their pretreatment peripheral blood microfilarial counts, six adult heartworm-positive beagles were randomly assigned to either the treatment or control group. The treatment group was treated on day 0 with DPP. Both the treatment and control groups were exposed to young female microfilaria-negative mosquitoes on days -7, 7, 14, 21 and 28 for an hour each session.

Microfilarial counts were performed on peripheral blood samples taken from each of the dogs in both groups immediately before and after each exposure to mosquitoes (except on day 7). Each dog was anesthetized before and during each exposure to the mosquitoes.

During exposure, engorged mosquitoes were collected and dissected so that microfilarial counts could be obtained. After exposure for one hour, the rest of the mosquitoes (live, moribund and dead) were counted.

The mosquitoes were then placed into two different groups:

Live and moribund mosquitoes that were engorged

Live and moribund mosquitoes that were unengorged

These groups were aspirated into two separate containers and placed into an insectary. Sixteen days after being placed into the insectary, remaining live mosquitoes were dissected individually, and L3 counts were obtained.

Experimental findings

After treatment with DPP, the repellency was assessed one hour after exposure to mosquitoes and ranged from 95.8% on day 14 to 100% on day 7. Twenty-four hours after exposure to mosquitoes, insecticidal efficacy was evaluated by counting the number of live mosquitoes; the efficacy ranged from 95.5% on day 21 to 100% on day 7.

All mosquitoes that survived the incubation period were dissected for L3 counts. A total of 22 mosquitoes that fed on the treatment group during the four exposure time points lived for awhile. But all 22 mosquitoes were dead within 72 hours, giving an anti-transmission effect of 100%. Of the 86 mosquitoes that fed on the untreated group during the four exposures, 79 had L3 present 16 days after feeding, with an average of 13.4 L3 per mosquito.

The take-home

Overall, the study found that:

• DPP was more than 95% effective in repelling and killing mosquitoes for 28 days after treatment.

• DPP was 100% effective in blocking the transmission of microfilariae from dogs studied to mosquitoes.

McCall thinks this study shows the effectiveness of a multimodal approach to the prevention of heartworm disease. This approach includes decreasing the mosquito vector population, preventing mosquito biting and killing mosquitoes, in addition to monthly or biannual administration of a macrocyclic lactone heartworm preventive.

This study was performed using DPP. Other topical products with repellency claims could be studied in the future to determine similar efficacy.

Dr. Meghan E. Burns owns Connect Veterinary Consulting. Her expertise includes product and business development, key opinion leader management, medical writing, and marketing.

http://www.connectveterinaryconsulting.com