Control of mosquito transmitted heartworm of dogs on a community wide basis can be achieved.

The Intermediate Valley and Foothill Plant Climate Zones of Napa, Sonoma, Lake and Mendocino counties are ideally suited for this pilot control study. Practicing veterinarians in those counties have agreed to participate. Mosquito control personnel have also agreed to contribute to the pilot study and the prevalence of infections in unprotected outdoor dogs in those 3 counties ranges from 14 to 42 infected dogs per thousand depending upon the plant climate zone the dog lives in (see map).

I have already demonstrated in a retrospective study conducted over 9 years that in the intermediate valleys plant climate zone for every 1% increase in the number of dogs receiving heartworm preventive medicine the prevalence of heartworm in dogs living in the same community but not receiving prophylaxis declined by 3.5 to 9%. During the period from 1983 through 1991, the use of heartworm prophylaxis increased from 3.9% of the dogs surveyed to 35% in the intermediate valleys of central California. During that same time, the prevalence of heartworm in non-prophylacted dogs in the same cities of the intermediate plant climate zone declined from 14 per thousand dogs examined to 2 per thousand examined. From 1987 through 1990, the intermediate valleys plant climate zone received less than normal rainfall and the mosquito population declined in each of those years. Analysis showed that when rainfall is less than 36 cm/yr the decline in heartworm infections among non prophylacted dogs occurs more rapidly than it does when rainfall is greater than 36 cm/yr. This means that prophylaxis and drought conditions synergize to reduce transmission of heartworm by making more dogs refractory to infection and reducing mosquito vector abundance.

The pilot study will necessitate collection of the following information from the 3 study counties.

1. An accurate dog census from urban and rural communities.
2. Testing of dogs for heartworm and treating infected dogs.
3. Increasing the number of dogs receiving prophylaxis.
4. Examining mosquitoes for infection with heartworm larvae.
5. Determining the crude death rate of dogs in the study area as well as the number dying of heartworm.

Using the values obtained, it is possible to calculate the reproductive success of heartworm by the following relationships:

Ro = reproductive success
$ = transmission rate
N = susceptible dogs
" = number of dogs dying due to heartworm over a unit of time
µ = number of dogs dying from other causes over same unit of time
< = number of dogs cured of heartworm

When Ro is less than 1 the parasite (heartworm in this case) will not be replacing itself and if Ro can be kept below 1 the parasite will die out of the dog population. Now the problem is that an adult heartworm can live 5-7 years in an infected untreated dog, so since every community has some irresponsible pet owners who will not get their dogs tested and treated if infected, the control program has to be sustained long enough to either have the worms in the dogs whose owners will not participate die of old age or the dogs die themselves. This time period will be about 10 years. It requires a sustained effort to control filarial worms because they have such a long life expectancy in their vertebrate host.

Although one U.S. pharmaceutical company has donated millions of doses of the same drug used to prevent heartworm to several African countries over the last 10 years to help control a filarial worm of humans, they are not interested in supporting a lengthy control program in dogs in their home country. Likewise, two other veterinary pharmaceutical companies that produce heartworm prevention drugs are not willing to fund even a part of the 10 year control program.

So as with many problems, the technology to get the job done is available but the funding to pay for the application of the technology is wanting.

The cost benefit to responsible dog owners is clearly on the side of prevention. The medical expense for treating a dog for heartworm is between 750 and 1,000 dollars depending upon the necessity and length of hospitalization and nursing care. The preventive oral medication given once/month that is 99% effective in preventing infection costs, for a medium sized dog, about 54 dollars per year. A dog owner can, therefore, protect their dog for 10 years (the life expectancy of many medium sized breeds) and still be 210 dollars to the good for what it can cost them to treat the same dog just once (540 vs. 750 dollars). And this says nothing about the permanent damage heartworm infections do to the dog's lungs and heart, which prevention keeps from happening, whereas treatment for the worms will not repair the damage to the dog's organs. 

Project Donation Progress:
(goal: $25,000.00 per county)

* as of 8-24-00

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