Heartworm Disease ( Dirofilaria immitis) and Their Vectors in Europe – New Distribution Trends

The mosquito Aedes (Stegomyia) albopictus (Skuse) (Diptera: Culicidae), originally indigenous to South-east Asia, islands of the Western Pacific and Indian Ocean, has spread during recent decades to Africa, the mid-east, Europe and the Americas (north and south) after extending its range eastwards across Pacific islands during the early 20th century. The majority of introductions are apparently due to transportation of dormant eggs in tyres. Among public health authorities in the newly infested countries and those threatened with the introduction, there has been much concern that Ae. albopictus would lead to serious outbreaks of arbovirus diseases (Ae. albopictus is a competent vector for at least 22 arboviruses), notably dengue (all four serotypes) more commonly transmitted by Aedes (Stegomyia) aegypti (L.). Results of many laboratory studies have shown that many arboviruses are readily transmitted by Ae. albopictus to laboratory animals and birds, and have frequently been isolated from wild-caught mosquitoes of this species, particularly in the Americas. As Ae. albopictus continues to spread, displacing Ae. aegypti in some areas, and is anthropophilic throughout its range, it is important to review the literature and attempt to predict whether the medical risks are as great as have been expressed in scientific journals and the popular press. Examination of the extensive literature indicates that Ae. albopictus probably serves as a maintenance vector of dengue in rural areas of dengue-endemic countries of South-east Asia and Pacific islands. Also Ae. albopictus transmits dog heartworm Dirofilaria immitis (Leidy) (Spirurida: Onchocercidae) in South-east Asia, south-eastern U.S.A. and both D. immitis and Dirofilaria repens (Raillet & Henry) in Italy. Despite the frequent isolation of dengue viruses from wild-caught mosquitoes, there is no evidence that Ae. albopictus is an important urban vector of dengue, except in a limited number of countries where Ae. aegypti is absent, i.e. parts of China, the Seychelles, historically in Japan and most recently in Hawaii. Further research is needed on the dynamics of the interaction between Ae. albopictus and other Stegomyia species. Surveillance must also be maintained on the vectorial role of Ae. albopictus in countries endemic for dengue and other arboviruses (e.g. Chikungunya, EEE, Ross River, WNV, LaCrosse and other California group viruses), for which it would be competent and ecologically suited to serve as a bridge vector.

Heartworm disease due to Dirofilaria immitis continues to cause severe disease and even death in dogs and other animals in many parts of the world, even though safe, highly effective and convenient preventatives have been available for the past two decades. Moreover, the parasite and vector mosquitoes continue to spread into areas where they have not been reported previously. Heartworm societies have been established in the USA and Japan and the First European Dirofilaria Days (FEDD) Conference was held in Zagreb, Croatia, in February of 2007. These organizations promote awareness, encourage research and provide updated guidelines for the diagnosis, treatment and prevention of heartworm disease. The chapter begins with a review of the biology and life cycle of the parasite. It continues with the prevalence and distribution of the disease in domestic and wild animals, with emphasis on more recent data on the spreading of the disease and the use of molecular biology techniques in vector studies. The section on pathogenesis and immunology also includes a discussion of the current knowledge of the potential role of the Wolbachia endosymbiont in inflammatory and immune responses to D. immitis infection, diagnostic use of specific immune responses to the bacteria, immunomodulatory activity and antibiotic treatment of infected animals. Canine, feline and ferret heartworm disease are updated with regard to the clinical presentation, diagnosis, prevention, therapy and management of the disease, with special emphasis on the recently described Heartworm Associated Respiratory Disease (HARD) Syndrome in cats. The section devoted to heartworm infection in humans also includes notes on other epizootic filariae, particularly D. repens in humans in Europe. The chapter concludes with a discussion on emerging strategies in heartworm treatment and control, highlighting the potential role of tetracycline antibiotics in adulticidal therapy.