Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066





We may have riots, earthquakes, mudslides, and fires but we seem to have gotten off easy with parasites. Whipworms and hookworms that plague dogs and puppies in other states are much less common here. Ticks do not seem to thrive in our urban concrete jungle and Lyme disease is unusual south of Bakersfield and without a heavy mosquito population, heartworm has never gotten a foothold in our county.

Or so we have always thought.

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





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 as 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. In fact, Aedes sierrensis is now endemic in Los Angeles County.

But speaking of major climatic events, southern California has just come out of a severe drought. The dry hot weather suppressed the mosquito species that need humidity but gave a marked advantage to three more Aedes species: Aedes albopictus, Aedes aegypti and Aedes notoscriptus. Two of these are known vectors of heartworm disease (see below).

Aedes sierrensisAedes sierrensis
(Photocredit: Marin / Sonoma
Mosquito & Vector Control District)
Aedes albopictusAedes albopictus
(Photocredit: CDC Public Health Image Library)

Aedes aegyptiAedes aegypti
(Photocredit: CDC Public Health Image Library)

Aedes notoscriptusAedes notoscriptus
(Photocredit: John Tann via Wikimedia Commons)



The real question is "Has it already happened here?" or "Is it happening now and can we stop it?” During the years 1996 - 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 less comprehensive study from the health department followed from 2005-2010.

Dr. Jerry Theis

Dr. Jerry Theis

In the Theis study, a total of 4,350 dogs in 103 Los Angeles County cities from 21 participating animal hospitals (including ours) were tested. In the health department study, veterinarians in Los Angeles County were asked to report heartworm positive animals seen at their clinics. Here, 67 heartworm positive dogs were identified and background information about them was collected. Adding to this is the information collected by the Los Angeles County Health Department where heartworm has been tracked since 2005. In 2015, reporting heartworm infections went from being voluntary by the public and veterinarians to being mandatory from reference laboratories. Once the local laboratories were required to report infection, confirmed cases tripled in number making for 257 cases of heartworm between 2005 and 2015 (239 dogs and 18 cats). Here is what we have learned:



Out of the 4,350 dogs tested in 1996-98, 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. This was before Aedes sierrensis came to town.

The health department studies did not compare numbers of dogs testing positive to numbers of dogs testing negative but in doing their research, the health department looked at the heartworm incidence in coyotes. Coyotes are killed and trapped all over the state of California and, since they never receive heartworm preventive medications of any kind in the wild, the incidence of heartworm in a local coyote population can be extrapolated to the incidence in dogs not receiving preventive. In this graphic taken from the study it is easy to see that some areas of Los Angeles County show coyote populations with over 20% infection rates! Clearly, the canine population in Los Angeles County is vulnerable given that the use of heartworm preventive medications is uncommon.


Modeling the distribution and abundance of the non-native parasite, canine heartworm, in California coyotes
From “Modeling the distribution and abundance of the non-native parasite,
canine heartworm, in California coyotes”  (Sacks et al, 2004)

The coyote is the main wildlife reservoir for heartworm in our areaThe coyote is the main wildlife reservoir
for heartworm in our area


Two surprising findings came from the This study regarding travel history of dogs in general. The first surprise was that 63% of all dogs tested 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 only 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 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. In the 2005-2010 data, only a slight increase in incidence was seen in traveling dogs (of the positive dogs reported, 60% had traveled while 40% had not.) Before this was actually studied, it had been commonly held that a dog would have to travel in order to get infected. Clearly this is not the case.



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.



Some species of mosquitoes will actively come inside through unscreened windows or other open areas. One of the two L.A. County heartworm vectors (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.

Mosquito feeding

For a long time, this was the only mosquito species 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.

Aedes sierrensis is unfortunately a very efficient vector of heartworm. The mosquito itself typically lives a full month longer than other mosquito species giving it extra time to spread any heartworm larvae it carries around. It breeds in tree knotholes so where ever there are trees, there is likely to be Aedes sierrensis. Further, Aedes sierrensis is an aggressive feeder on dogs (and humans). It seems at this point, it is just a matter of time before the heartworm population follows the introduction of this very effective vector; there is certainly potential for an increasing heartworm problem in Los Angeles County, especially since awareness of the general population is relatively poor.

Other newer mosquitoes in our community include Aedes albopictus, the Asian Tiger Mosquito, which came to Southern California via Hawaiian and Asian bamboo shipments.  This mosquito is an aggressively feeding pest capable of spreading numerous exotic diseases to humans and it would no doubt do so if such diseases were  present here and it also is an efficient vector of heartworm in dogs. The San Gabriel Valley is known to have infested areas.

In September of 2014, Aedes notoscriptus, the "Australian Mozzie" was found in Montebello and Monterey Park. This mosquito was not previously known in Los Angeles County (or even in North America) but is an important vector (transmitter) of heartworm as well as some unpleasant human diseases in Australia.


Aedes albopictusAedes albopictus
(Photocredit: CDC Public Health Image Library)
Aedes notoscriptusAedes notoscriptus
(Photocredit: John Tann via Wikimedia Commons)

The L.A. County Health Department has asked that black and white mosquito sightings be reported to them and that mosquito bites occurring during the daytime be reported. This can be done at


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.

Now that we have actually looked for heartworm infection in our "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 suddenly. 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.

To view the Los Angeles County Public Health page on Heartworm which includes a map of reported cases, visit:


 Page last updated: 9/28/2019