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From the Pet Health Library @ veterinarypartner.com

Written by Wendy C. Brooks, DVM

Heartworm in LA

Los Angeles may have riots, earthquakes, mudslides, and fires but it seems to have gotten off easy in terms of parasites. Whipworms and hookworms that plague dogs and puppies in other states are rare here. Ticks are few and far between (with some regional exceptions) and Lyme disease is not felt to occur with any significance south of Bakersfield. We don't have mosquitoes and we don't have heartworm.

Or so we have always thought.

And because we have been so secure in our belief that heartworm infection does not occur here, we do not test routinely for it nor do we strongly encourage use of the monthly preventives.

Have we been kidding ourselves or are we really safe?

What Happened in Salt Lake City, Utah

It seems very basic to say that mosquitoes do not live in the desert. Mosquitoes like the humid muggy summers that most Angelenos have left behind. For mosquitoes to suddenly establish in Los Angeles, it would seem that a major climatic change would be needed and no one is expecting anything like that to happen, at least not in this lifetime. But is a major climatic change really necessary to change an area's heartworm status?  Salt Lake City, like Los Angeles, was relatively free from mosquitoes and was considered a non-endemic area for heartworm until a city beautification project led to the planting of new trees throughout the city. The following year, these trees were pruned for the first time, leading to thousands of knot-holes in trees throughout Salt Lake City. This suited Aedes sierrensis, the tree hole mosquito, just fine and soon heartworm cases began appearing. Salt Lake City is now considered as endemic an area for heartworm just like Texas, Louisiana, Florida or any other place we associate with heartworm disease.

Planting trees through a city is hardly a major climatic event. On the contrary, it is a typical man-made environmental change and it was enough to establish heartworm and its mosquito vector in a new area.

Could it Happen in Los Angeles?

The real question is "Has it already happened here?" or "Is it happening now and can we stop it?" During1996 to 1998, Dr. Jerry Theis at the U.C. Davis Veterinary School conducted a survey to find out what the real situation in Los Angeles County truly is.

A total of 4,350 dogs in 103 Los Angeles County cities from 21 participating animal hospitals were tested. The following questions were answered.

What is the incidence of heartworm infection in Los Angeles County?

Out of the 4,350 dogs, a total of 18 positive dogs were identified. While positive individuals were located all over Los Angeles County, the incidence of infection was approximately THREE TIMES HIGHER in West Hills, Woodland Hills and Canoga Park than in any other L.A. County region. Overall, in L.A. County the incidence of infection was one in 250.

AHWLA1.gif B HWLA2.gif

A) Heartworm positive dogs in So.Cal. -1995

B) Heartworm positive dogs in So.Cal. -More recent years

Were dogs that had a history of travel at higher risk?

Two surprising findings came from this study regarding travel history. The first surprise was that 63% of dogs had a history of travel outside of L.A. County at some time in their lives.

Previous surveys of local veterinarians had indicated that L.A. veterinarians are under the impression that they are recommending heartworm prevention for their canine patients that travel. These findings indicate that a great deal of travel is taking place without veterinary consultation and that many dog owners are taking their pets into heartworm areas without heartworm warning or protection.

The second surprise was that in L.A. County travel did not pose an additional risk for infection. Dogs that had never traveled out of L.A. County were infected as commonly as their traveling counterparts.

Were dogs living primarily indoors protected from infection?

Many people believe that mosquitoes are an outdoor hazard only and that their dog is safe if kept primarily indoors. In fact, 50% of the infected animals were described by their owners as "always indoors." Indoor dogs had just as much risk as outdoor dogs.

Is the infection rate expected to increase?

Some species of mosquitoes will come inside through unscreened windows or other open areas. The L.A. County heartworm vector (Culiseta incidens) primarily breeds in flowerpots and swimming pools. We do not yet know enough about this mosquito's behavior to know if it is actually coming indoors or what time of day or night it might be doing so.

At this time, this is the only mosquito species that we know of in L.A. County that is able to transmit heartworm. Luckily, it is not an efficient vector (for every 10 larval heartworms this mosquito picks up from an infected dog, only one is transmitted to the next dog). This keeps the heartworm population from rapidly expanding.

More construction of homes means more gardens and more swimming pools and thus more Culiseta incidens mosquitoes. Climate plays a crucial role as well. If there are floods in the winter, the mosquito season is longer. If there is a drought, the mosquito populations plummet. There is certainly potential for an increasing heartworm problem in Los Angeles County, especially since awareness of the general population is relatively poor.

What other risk factors were reviewed in the study?

The effect of gender was reviewed but male and female dogs were found to be at equal risk. Age was also reviewed, but the age of the dog (considering that detectable infection is not physically possible in dogs under age 7 months) was not a risk factor.
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Now that we have actually looked for heartworm infection in this "non-endemic area," we know that there is some heartworm here after all and that there is reason to expect that it could become more common. We have found that a short visit to a heartworm endemic area might be relatively low risk but since low risk can be become no risk with a simple topical or treat, it is important not to ignore this classical infection.

Date Published: 1/1/2001
Date Reviewed/Revised: 12/09/2009