Heartworms are large parasites that live in the heart and major arteries of dogs.  The adult worms produce larvae that circulate in the blood stream.  The larvae are picked up by mosquitoes when they feed on infected dogs and are then transported by the mosquitoes to new dogs.  Until the early 1960s, heartworm was known in America only in the Deep South.   In the 1960s, it spread up the Mississippi Valley and now occurs throughout the United States wherever mosquitoes are present and temperatures are sufficiently warm.

In the 1990's, the University of California at Davis conducted a study in which over 51,000 dogs in Northern California were tested for heartworm infection.  The incidence of heartworm in San Joaquin county was very low, with one positive dog for every 40 to 100 dogs tested.  However, infection rates in Amador and El Dorado counties were high, with one positive dog for every 6 to 20 dogs tested.  Infection rates were also high in parts of the Sacramento Valley (especially in the north) and in coastal valleys where moisture and temperature conditions are favorable to mosquitoes. 

Because the heartworm infection rate is increasing and due to the proximity of areas with a high rate of infection, we recommend that all dogs be on heartworm preventative, starting from mid April and ending in October.  Prior to initiating preventative medication, dogs should be tested for heartworm to establish that they are not already infected.

There is new information that cats are more affected by heartworm disease than previously thought (see Feline Heartworm).  We do not believe that feline heartworm disease is a significant problem in San Joaquin County and currently do not recommend heartworm preventative in cats.