First detection of African Swine Fever Virus in Ornithodoros porcinus in Madagascar and new insights into tick distribution and taxonomy

Abstract

Background: African Swine Fever Virus has devastated more than the half of the domestic pig population in Madagascar since its introduction, probably in 1997-1998. One of the hypotheses to explain its persistence on the island is its establishment in local Ornithodoros soft ticks, whose presence has been reported in the past from the north-western coast to the Central Highlands. The aim of the present study was to verify such hypothesis by conducting tick examinations in three distinct zones of pig production in Madagascar where African Swine Fever outbreaks have been regularly reported over the past decade and then to improve our knowledge on the tick distribution and taxonomy.

Results: Ornithodoros ticks were only found in one pig farm in the village of Mahitsy, north-west of Antananarivo in the Central Highlands, whereas the tick seemed to be absent from the two other study zones near Ambatondrazaka and Marovoay. Using 16SrDNA PCR amplification and sequencing, it was confirmed that the collected ticks belonged to the O. porcinus species and is closely related to the O. p. domesticus sub-species Walton, 1962. ASFV was detected in 7.14% (13/182) of the field ticks through the amplification of part of the viral VP72 gene, and their ability to maintain long-term infections was confirmed since all the ticks came from a pig building where no pigs or any other potential vertebrate hosts had been introduced for at least four years.

Conclusions: Considering these results, O. porcinus is a reservoir for ASFV and most likely acts as vector for ASFV in Madagascar, but its apparent restricted distribution may limit its role in the epidemiology of the disease in domestic pigs.

Figure 1

Geographical location of the study zones in Madagascar. (1A) presents the geographical sites where Ornithodoros soft ticks have been previously reported (dark dots with non-underlined names for sites where ticks were found before 1960 and underlined names for those where ticks were recently found). (1B), (1C) and (1D) present specific maps for the Antananarivo, Ambatondrazaka and Marovoay zones, respectively, pinpointing sites where pig sera were positive with the anti-tick ELISA test (dark squares), sites where tick examinations were conducted without finding ticks (grey triangles) and sites where ticks were found (dark crosses).

Figure 2

Infested pig farm in Mahitsy (Madagascar). (2A) presents the building where O. porcinus ticks have been found. This building is typical from the Antananarivo zone with sand or mud soils, mud or concrete brick walls with few or no apertures except the door, and is located at the ground level of a two-storey human dwelling. (2B) presents one of the cracks inside the building where ticks were found and (2C) shows one tick (pointed by a white arrow) and many sloughed shins collected from these cracks.

Figure 3

Phylogeny of Malagasy O. porcinus spp. ticks. This represents a rooted consensus phylogenetic tree of 16 S rDNA sequences from Malagasy O. porcinus spp. ticks collected in pig pens in 2000 and 2006-2008 (underlined names), as well as O. moubata sensu stricto, O. p. porcinus and O. p. domesticus reference sequences (bold names) and some other sequences available in GenBank for the O. moubata complex of species, using the Maximum Likelihood (ML) method, with 1000 bootstraps and random sequence addition. Only bootstraps up to 50% have been indicated and identical sequences have been removed for simplification. Incorrect names given to specimens during field collection are indicated in quotes.