Marajó Island (Pará state) situated in the Amazon River delta is the world’s largest fluvial land mass and home to approximately 30,000 equids [1]. Among this equine population are horses that are direct descendants of animals introduced into the region by Portuguese colonizers in the early Eighteenth Century [2]. After transportation from the temperate grasslands of Europe to the Amazon delta region these animals were confronted with a tropical savanna type environment where in addition to different plant nutrient sources they were subjected to harsh hot (30-32oC) and humid (≥ 80%) conditions all year round. Moreover, the area has a high annual rainfall (2.800-3.400mm) with the majority falling between January and May resulting in the case of Marajó Island in more than two thirds of the landmass becoming inundated [3, 4]. Although many equids live as feral in the island, some of them have become essential for transportation and extensive livestock maintenance of cattle and buffalo herds, the most important economic activities in the region [2, 5].
The initial horse population was interbred with some other Portuguese breeds including Arabian and Alter Real and eventually became recognized as the distinctive Marajoara horse. One of the major characteristics of this breed is adaptation to adverse tropical conditions [2, 6]. Although originally geographically isolated by the island environment, these unique horses have been interbred with other horse breeds recently imported into the area, to improve the size, posture and appearance, once the most horse breeders have prioritized the phenotypic characteristics, without taking into account the adaptation to the hostile environment. Unfortunately this practice is becoming a major threat to the genetic integrity of the Marajoara horse breed [2, 7, 8]. Another highly specialized breed found on Marajó Island is the Puruca horse. This was generated by breeding Marajoara horses to Shetland ponies and selecting individuals with a maximum standard height of 1.18 meters and they suffer an even greater threat, because it constitutes a very small population and the crossbreeding has resulted in the progressive replacement of this one to a bigger animal, mischaracterizing the main pattern of the breed [2, 7, 8].
Therefore, the equid population of Marajó Island contains distinctive, rare horse breeds that should be preserved because they possess genetic characteristics uniquely associated with adaptation to the tropical environment of the Amazon River Delta. However, in addition to dilution of these genetic characteristics by interbreeding another major threat to equids inhabiting Marajó Island is Equine Infectious Anemia Virus (EIAV). In a survey conducted on the island of just 294 equids 46.26% were found to be seropositive for this virus [9] suggesting it is endemic in the region.
EIAV, that causes the equine infectious anemia (EIA), is classified within the Lentivirus genus of the Orthoretrovirinae subfamily, family Retroviridae (ICTV, 2021). The virus is equid specific being reported in horses, donkeys and mules where it produces a persistent infection. Although clinical signs following exposure to EIAV are highly variable ranging from a subclinical infection to death, in many cases the disease presents in the form of three distinct clinical phases. These consist of an “acute”stage characterized by pyrexia coupled with thrombocytopenia, a “chronic” phase consisting of recurrent disease episodes where pyrexia plus thrombocytopenia are associated with anemia, edema, depressed neurological reactions together with cachexia and finally an “inapparent” phase where overt disease signs are not observed. Approximately 90% of equids survive the acute and chronic phases and after a period of 8–12 months gradually transition to the asymptomatic state where although blood-associated viral loads are usually low they remain a potential source for viral transmission [11, 12].
EIAV is transmitted mainly via contaminated blood with iatrogenic mechanisms such the reuse of veterinary equipment especially hypodermic syringes being a major source of spread. However, while vertical transmission has been reported, the most important means of spread, in the absence of human intervention, is the feeding behavior of large hematophagous insects including horse flies (Tabanidae family, Tabanus or Hybomitra genus), deer flies (Tabanidae family, Chrysops genus) and perhaps less frequently, stable flies (Muscidae family, Stomoxys genus) [13]. Transmission by hematophagus flies does not involve EIAV replication in insect tissues and so the process is purely mechanical. It occurs when feeding on an infected individual is interrupted causing the fly to seek a second uninfected equid host on which to complete its blood meal. Therefore the risk of transmission is directly related to the number of infected animals, proximity between animals and density of vectors, the latter being determined by environmental factors, including high temperatures and humidity [14–16]. These climatic conditions typify the Amazon Delta region and therefore equid populations in the region are subjected to year round high insect vector populations that consist of many different tabanid species [14, 17].
In the absence of viable vaccines or other effective treatments control of EIAV is dependent on the identification and removal of infected subjects from the population. The latter is either accomplished by life-long quarantine or more often by euthanasia. Although effective at reducing the overall incidence of EIAV in an equid population such methods, especially when involving euthanasia, are not compatible with rare breed preservation where the maintenance of genetic diversity is a priority. However, it has been found that foals born to and raised by EIAV seropositive mares can remain seronegative [18] suggesting the possibility that virally infected mares can be successfully incorporated into breeding programs. Obviously, if verified this observation will have significant implications for the preservation of rare or otherwise valuable equids in areas where EIAV is prevalent. However, the study of McConnico et al. (2000) was conducted in the United States where high insect vector populations are not continually present and the foals were separated early from their mothers [18]. Therefore, a longitudinal study was conducted to determine viral transmission rates in foals born to EIAV feral seropositive mares, under natural weaning conditions, inhabiting Marajó Island, a region with a humid tropical rainforest climate, year round high insect vector populations and different species composition compared to those found in North America.