Is heartworm disease really spreading in Europe?

In Umbria (hilly central region), where only imported cases have been previously reported, the prevalence ranges between 5 and 15%.

“In Umbria (hilly central region), where only imported cases have been previously reported, the prevalence ranges between 5 and 15% (Piergilli-Fioretti et al., 2002, 2003; Genchi et al., 2005; Mortarino et al., 2008).”

Climate and its changes determine the transmission and presence of dirofilariasis in temperate regions.

“Consequently, the climate and its changes determine the transmission and presence of dirofilariasis in temperate regions (Genchi et al., 2005).”

Continuous globalisation and climate warming will probably lead to a further spread and increased circulation of D. immitis and D. repens in the future.

“Continuous globalisation and climate warming will probably lead to a further spread and increased circulation in the future.”

The increase in publications reporting Dirofilaria spp. infections is most likely driven by both increased research and awareness, but also the spread of the parasites.

“This is most likely driven by both increased research and awareness, but also the spread of the parasites [25, 39].”

Microfilariae do not develop to L3 at temperatures below 14 °C.

“Microfilariae do not develop to L3 at temperatures below 14 °C [27].”

Several models showed that the expansion from Southern to Central and Northern Europe (but also in North America) is probable.

“Several models showed that the expansion from Southern to Central and Northern Europe (but also in North America) is probable [27,192,271].”

Dirofilariosis is endemic in the Mediterranean region, affecting southern European countries including Portugal, Spain, France and Italy.

“Dirofilariosis is endemic in the Mediterranean region, affecting southern European countries including Portugal, Spain, France and Italy (Genchi et al., 2005 and Morchón et al., 2012).”

Predictive studies using GIS tools have confirmed a correlation between the actual trend of D. immitis spread and temperature increase into previously infection-free areas.

“Different predictive studies, conducted using Geographical Information Systems (GIS) tools [15, 16], confirmed a correlation between the actual trend of D. immitis spread and the temperature increase into previously infection free areas”

Both the heartworm predictive model and the Dirofilaria development units show parts of eastern Austria as suitable for the introduction and/or establishment of D. repens as well as D. immitis.

“Both the heartworm predictive model (based on growing degree days) and the Dirofilaria development units show parts of eastern Austria (the regions where most D. repens cases in dogs, the findings in mosquitoes, and the human case were documented) as suitable for the introduction and/or establishment of D. repens as well as D. immitis [24].”

Several models have shown that the expansion of Dirofilaria from southern to central and northern Europe, up to 50° N in the case of D. immitis, is most probable.

“Several models have shown that the expansion from southern to central and northern Europe (up to 50° N in the case of D. immitis) is most probable.”

The chance that Dirofilaria spp. were introduced to Hungary with infected mosquitoes is very small because the movement and ranges of these insects are limited.

“The chance that Dirofilaria spp. were introduced to Hungary with infected mosquitoes is very small, because the movement and ranges of these insects are limited (Genchi et al. 2005; Tsuda et al. 2008; Zittra et al. 2015).”

The higher estimate of Dirofilaria generations per year compared to Genchi et al. can be explained by increasing temperature due to global warming.

“We obtained a higher estimate for the number of Dirofilaria generations per year. This can be explained by the increasing temperature due to global warming.”

Cases of human dirofilariosis by Dirofilaria immitis have been recently described in Italy, Greece and Spain, and this trend is at an increase in Europe, most likely paralleling the spread of infestation in dogs in central and north-eastern countries.

“However, cases of human dirofilariosis by D. immitis have been recently described in Italy, Greece and Spain [89-91] and this trend is at an increase in Europe, most likely paralleling the spread of infestation in dogs in central and north-eastern countries (e.g., south of Switzerland, Czech Republic, Hungary, Serbia and Slovak Republic) [5,7,48-52].”

Heartworm disease is endemic in Mediterranean countries.

“In Europe, heartworm disease is endemic in Mediterranean countries [7,8]”

Dirofilaria transmission is markedly seasonal in Portugal, with peaks in the summer.

“The results obtained by using the forecast model show that Dirofilaria transmission is markedly seasonal in Portugal, with peaks in the summer, confirming data reported by Genchi et al. (2005) and Rinaldi et al. (2007).”

The number of dogs with a travel history to endemic regions or imported from endemic countries is increasing.

“Noteworthy in this context is the increasing number of dogs matching these criteria [4,5].”

The occurrence of D. repens can be considered a model for the estimated future distribution of D. immitis, because D. immitis seems to be spreading mainly to areas where D. repens has already established itself.

“The occurrence of D. repens could be considered a model for the estimated future distribution of D. immitis, as the heartworm seems to be spreading mainly to the areas where D. repens has already established itself [4,8].”

A particularly strong increase in the number of D. immitis infections is currently observed in Eastern Europe.

“A particularly strong increase in the number of infections is currently observed in Eastern Europe [4,23].”

In Europe, D. immitis is predominantly found in southern countries such as France, Greece, Italy, Portugal and Spain.

“In Europe, the parasite is predominantly found in southern countries, such as France, Greece, Italy, Portugal and Spain.”

In Austria, the endemisation of D. repens already appears to be concluded and could be considered a model for the expected future distribution of D. immitis, as the heartworm seems to be spreading mainly to the areas where D. repens has already established itself.

“In Austria, the endemisation of D. repens already appears to be concluded and could be considered a model for the expected future distribution of D. immitis, as the heartworm seems to be spreading mainly to the areas where D. repens has already established itself [1,4,8,9].”

The largest endemic region in Europe is the Padan Plain in Northern Italy, where canine dirofilariosis can be found with a prevalence of 50-80%.

“The largest endemic region in Europe is the Padan Plain in Northern Italy, where canine dirofilariosis can be found with a prevalence of 50–80% (Genchi et al. 2005).”

While D. repens is known to occur in dogs throughout Hungary, canine cardiopulmonary dirofilariosis has so far been documented in only a few individual cases.

“While D. repens is known to occur in dogs throughout Hungary, canine cardiopulmonary dirofilariosis has so far been documented in only a few individual cases (Genchi et al. 2005; Fok 2007; Jacsó et al. 2009; Tolnai et al. 2014).”

D. immitis is establishing itself more slowly in areas where D. repens is already present.

“It also appears that D. immitis is establishing itself more slowly in areas where D. repens is already present (Genchi et al. 2005; Capelli et al. 2018).”

D. repens has spread even faster to Northern Europe than D. immitis, which could be due to the more frequent subclinical infections and therefore undiagnosed dogs can serve as reservoirs and carriers.

“D. repens has spread even faster to Northern Europe than D. immitis, which could be due to the more frequent subclinical infections and therefore undiagnosed dogs can serve as reservoirs and carriers.”

Our results are in agreement with predictions of occurrence and seasonality of Dirofilaria spp., with peaks of infection in Summer (June to September).

“Our results are in agreement with predictions of occurrence and seasonality of Dirofilaria spp., with peaks of infection in Summer, from June to September”

Dirofilaria immitis and D. repens are traditionally endemic in Southern and Eastern Europe.

“Dirofilaria immitis and D. repens are traditionally endemic in Southern and Eastern Europe”

Epidemiological studies and recent clinical reports described a significant prevalence of autochthonous infections of dogs with D. immitis and/or D. repens in central and northern European countries where canine dirofilariasis was previously not reported or only sporadically found.

“Epidemiological studies and recent clinical reports on the expansion and emergence of dirofilariasis described a significant prevalence of autochthonous infections of dogs with D. immitis and/or D. repens in central and northern European countries where canine dirofilariasis was previously not reported or where cases were only sporadically found”

Climate and its changes determine the presence and dynamics of transmission of dirofilariasis in temperate regions.

“Consequently, the climate and its changes determine the presence and dynamics of transmission of dirofilariasis in temperate regions (149).”

In the decade 2000 to 2010, subcutaneous/ocular dirofilariasis expanded from southern to central and northern Europe.

“In this decade (2000 to 2010), subcutaneous/ocular dirofilariasis expanded from southern to central and northern Europe (149,391,393).”

Studies in Europe have allowed determination of the number of annual D. immitis generations and the length of periods of high infection risk throughout several regions of the continent.

“Studies in Europe have allowed the determination of the number of annual D. immitis generations and the length of periods of high infection risk throughout several regions of the continent (Fig. 7) (148, 149, 150).”

Initial work in this area occurred in Canada and the US, and this has since led to studies on prediction of different transmission seasons in Europe, the United Kingdom and Argentina.

“Initial work in this area occurred in Canada and the US [ 26 , 27 ], and this has since led to studies on the prediction of the different transmission seasons in Europe [ 53 ], the United Kingdom [ 54 ] and Argentina [ 55 ].”