Spread of Dirofilariae (Spirurida, Onchocercidae) in the Natural Populations of Malaria Mosquitos (Diptera, Culicidae) in Tomsk Ob River Region

Abstract

Dirofilariasis is a dangerous transmissible disease of carnivores and humans caused by two species of parasitic nematodes - Dirofilaria repens Railliet et Henry and D. immitis Leidy. The intermediate hosts of Dirofilariae are blood-sucking mosquitoes. Both species are typical of the mild climate of southern Europe, but in recent decades the parasites have spread to the north and northeast due to the global climate change. They have been registered in Western Siberia since 1989, and diro-filariasis cases in both animals and humans cause concern. To assess prospects for the spread of dirofilariasis and develop strategies to combat this disease, one has to determine the main mosquito vectors. Malaria mosquitos are the significant dirofilaria vectors in Europe, but there is a shortage of information on potential dirofilariasis vectors in Siberia. This paper is meant to estimate how extensive the invasion of different species of the malaria mosquitos D. repens and D. immitis is in the territory of Tomsk Region due to the global climate change. 64 samples of mosquitos from 36 localities of Tomsk Region were taken during the summer seasons of 2018 to 2020. Mosquitos were caught in stables, dissected to determine invasion via light microscopy. The species of malaria mosquitos and dirofilariae were identified through an ITS2 PCR-RFLP analysis (Artemov et al., 2021) and COI site-specific PCR (Rishniw et al., 1998), respectively. In total, the authors analyzed 7,591 female malaria mosquitos and 233 female non-malaria mosquitos, where 159 and 1 of them were infected with dirofilariae, respectively. They registered invasion in 44 samples from 21 localities, including the northernmost one - Strezhevoy town (60°44N, see Figure 1). Dirofilariasis is a common helminthiasis in Tomsk Region. The average invasion extensiveness is 2.1+0.2% in the region. In the natural populations of malaria mosquitos on the right bank of the River Ob, the invasion extensiveness is 3.7 times higher vs. the left bank of the Ob, despite the high transport accessibility of this bank (see Figure 2). Invasion extensiveness decreases from the south to the north, but the local conditions might disrupt this trend (see Table 3). Three species of malaria mosquitos - Anopheles daciae, An. messeae and An. beklemishevi (but not An. claviger) - are dirofilariasis vectors in Tomsk Region (see Table 1). South of 57° N, only D. repens and one mixed invasion case were found in An. daciae being the dominant species in this area. However, all the three species mainly invaded with D. repens were detected as vectors north of 57° N. One case of An. messeae invasion with the nematode D. immitis and mixed invasions of An. daciae and An. beklemishevi were found in Kolpashevo agglomeration. All the invasions with D. immitis (including mixed ones) were observed on the right bank of the Ob (see Table 2). The extensiveness of An. messeae and An. daciae invasion were significantly higher vs. An. beklemishevi, despite its domination north of 57° N. The northernmost location where an infective female of An. beklemishevi invaded with D. repens was found is Bolshaya Griva village at 59° N. Approximately 6.5% of the infected mosquitos had superinvasion - re-invasion with dirofilariae (see Figure 3). Thus, malaria mosquitos play a significant role in the circulation and, probably, the spread of dirofilariasis in Tomsk Region. Conclusion: 1. Dirofilariae are widespread within Tomsk Region. The northernmost border of the Dirofilaria repens spread is 60° 44' and that of D. immitis - 58° 32' N. The northernmost border of the D. repens transmission is 59° N. 2. The contribution made by malaria mosquitos to the spread of dirofilariasis in Tomsk Region is as high as that of other blood-sucking mosquitos. 3. Anopheles daciae, An. messeae and An. beklemishevi are the vectors of Dirofilaria repens and D. immitis. Anopheles beklemishevi get infected less often than the two remaining species. 4. The invasion extensiveness of malaria mosquitos is higher on the right bank of the River Ob. 5. Mixed invasions and superinvasions of malaria mosquitos with dirofilariae are frequent phenomena for Tomsk Region. The article contains 4 Figures, 3 Tables, 55 References. The Authors declare no conflict of interest.