Dirofilaria immitis and its clinical aspect, heartworm disease, can be found in tropical and temperate regions of the world, with the highest known prevalence in the USA, South America, Japan, Australia and Italy.

In the USA, the spread of the heartworm disease was monitored extensively over the last years. In a matter of 50 years, the disease spread from a well restricted south-eastern coastal area of the United States to the point where it is believed now that autochthonous transmission may have occurred in all 49 states of the USA and most of the Canadian provinces. This might take as an example of how a vector-borne disease can rapidly engulf a continent in a rather short period – even in the face of highly successful pharmaceutical agents for its prevention and treatment. Major reasons for this dramatic development have been attributed to the increased transport of dogs from southern areas for hunting and breeding, the movement of dogs belonging to army personnel and to normal citizens and the burgeoning wild canids population, like the Californian coyote.

The concern about heartworm spread was faced by the US humane societies and shelter systems after hurricane Katrina struck New Orleans, Lousiana in 2006. It was discovered that up to 80% of the dogs in the shelters of New Orleans were heartworm-positive, and it is estimated that some 5,000 to 10,000 dogs or more were shipped to other parts of the nation to different shelter systems. Some of these had trouble accepting the dogs, because donors and communities were upset that infected dogs were placing other dogs at risk in areas with low levels of heartworm and many local dogs without prevention.

In Europe, heartworm still appears to be confined, for the most part, to southern European countries like Portugal, Spain, France, Italy, Slovenia, Bulgaria, Romania, Greece and Turkey. Infections in Serbia and Croatia have increased from sporadic to occurring (CVBD Occurrence Map Europe). The largest European endemic area is localized in Northern Italy in the Po lowlands with prevalences in dogs up to 50 or even 80%. As pet travel with their owner increases, there may be an increased risk of spread.

Further information

• Bowman, D.D.: Spread of companion animal vector-borne parasitic disease in the US and Europe: Concerns relative to travel, national disasters, shelter-source animals and wildlife. Proceedings of the 2nd Int. CVBD Symposium, 2007, 16-23 (Download)
• CVBD Occurrence Map Europe