Increase in the diagnosis of equine infectious anemia over an 11-year period in the western region of Rio Grande do Sul, BrazilABSTRACT
Equine infectious anemia (EIA) is a disease that affects members of the Equidae family. In Brazil, it is detected in all regions, and distribution is associated with environmental factors and socio-cultural aspects. The study aimed to describe and analyze the occurrence of EIA outbreaks between 2009 and 2019 in the western region of Rio Grande do Sul. Data from 125 official reports of focus (113) and suspected (12) cases were analyzed. The majority of cases (102) occurred between 2015 and 2019. July, August, and September accounted for 65% of the records, and the diagnosis was made as a requirement for transporting the animals. In all, 2071 animals were on the focus properties, and 242 horses were identified as positive. Of these, 237 were euthanized, and five vanished from the properties. Properties with up to 10 horses were the most affected (71.7%, 81/113), and these herds contained 41.1% of the positive animals (104/242). Properties located in the peri-urban region of the cities accounted for more than 60% of the cases. One outbreak was diagnosed in horses smuggled from Argentina. The results characterize the occurrence of EIA in the region and demonstrate the maintenance and spread of the virus's circulation among herds.
Keywords
horse; lentivirus; retrovirus; transboundary disease; outbreak
RESUMO
A anemia infecciosa equina (AIE) é uma doença dos membros da família Equidae. No Brasil, a AIE é detectada em todas as regiões e a distribuição está associada com fatores ambientais e aspectos sócio-culturais. O objetivo do estudo foi descrever e analisar a ocorrência de surtos de AIE entre os anos de 2009 e 2019 que foram registrados na região oeste do Rio Grande do Sul. Dados oficiais de 125 registros de focos (113) ou casos suspeitos foram analisados. A maioria dos casos (102) ocorreu entre os anos de 2015 e 2019. Nos meses de julho, agosto e setembro foram registrados 65% dos casos e o diagnóstico foi realizado como requisito para o transporte dos animais. O rebanho equino das propriedades afetadas era de 2071 cavalos, sendo que 242 foram positivos. Entre esses animais, 237 foram eutanasiados e cinco cavalos desapareceram das propriedades. Propriedades com até 10 cavalos (71,7%, 81/113) e localizadas na região periurbana (60%) foram as mais afetadas e esses rebanhos possuíam 41,1% dos positivos (104/242). Um foco foi identificado em cavalos contrabandeados da Argentina. Os resultados caracterizam a ocorrência da AIE na região, demonstram a manutenção e disseminação do vírus nos rebanhos.
Palavras-chave
cavalo; lentivirus; retrovírus; doença transfronteiriça; surto
1. Introduction
Equine infectious anemia (EIA) is a viral disease distributed worldwide (Cook et al., 2013). The EIA virus (EIAV - equine infectious anemia virus) belongs to the species of Lentivirus equinfane, Retroviridae family, genus Lentivirus, and produces persistent infection in the host (Coffin et al., 2021). EIAV preferentially infects monocytes and macrophages, but only in macrophages support productive viral replication, and the liver, spleen, and kidney are considered reservoirs of the virus (Harrold et al., 2000). The sources of infection are animals with acute or chronic infection or healthy animal carriers. Mechanical transmission by hematophagous insects (Tabanidae and Stomoxys calcitrans) is the main form of natural transmission. However, the iatrogenic route (syringes, needles, and blood transfusion) is more efficient (Williams et al., 1981; Issel & Foil, 2015).
Clinical signs vary according to the species, the host's immune status, the infecting viral load, and the virulence of the sample (Cook et al., 2013). In horses, the acute phase, which usually occurs between five and 30 days after infection, is characterized by viremia and thrombocytopenia associated with fever, lethargy, inappetence, and, in the most severe cases, hemolytic anemia and petechiae. In the chronic phase, recurrent episodes of viremia are observed, manifested by weight loss, edema, anemia, hemorrhages, and occasionally neurological signs. Horses that survive these conditions (~90%) become inapparent carriers of the infection, with no clinical signs and low levels of viremia (Cook et al., 2013).
The association of persistent infection and the presence of specific antibodies (anti-p26) makes it possible to use the agar gel immunodiffusion serological test (AGID or Coggins test) for diagnosis (Issel & Cook, 1993). AGID detects seroconversion between three and four weeks after infection and can be undetectable for up to 150 days in rare cases (Issel & Cook, 1993; More et al., 2008). In Brazil, EIAV infection was first diagnosed in 1966 in Rio de Janeiro; in 1974, the Ministry of Agriculture and Livestock (MAPA – Ministério da Agricultura e Pecuária) instituted the AGID test for diagnosing EIA (Brasil, 1988). Official data shows that the infection is present in all regions of the country, with variations in frequency (MAPA, 2016). The highest prevalence rates are found in areas with hot and humid weather, with abundant vectors (horseflies and stable flies), such as in the Pantanal region (Barros & Foil, 2007).
In Brazil, MAPA coordinates the National Equine Health Program (Programa Nacional de Sanidade Equina - PNSE), which determines the guidelines for the control and prevention of EIA based on the international regulations of the World Organization of Animal Health. The PNSE establishes serological diagnosis (AGID and ELISA), control of the movement of seronegative animals, and euthanasia or isolation of positive animals (Brasil, 2004). The national program serves as a parameter for state legislation, which adopts measures according to the importance of horse breeding and the infection situation. Rio Grande do Sul (RS) is the southernmost state in Brazil and has a significant equine population, raising animals for work, sport, and leisure. The state's equine herd is approximately 520,000 animals, representing 8.5% of the national population (Costa et al., 2013; Oliveira et al., 2015).
The State Equine Health Program (Programa Estadual de Sanidade Equina - PESE) is the RS State equine sanitary program, and all properties and animals must be registered with the Agricultural Defense System of the Secretariat of Agriculture, Livestock, Supply, and Rural Development (SDA/SEAPDR) (Brasil, 2004; Rio Grande do Sul, 2010; Rio Grande do Sul, 2015). It also stipulates that equine movements may only occur after the Animal Transportation Certificate (GTA – Guia de Transporte Animal) has been issued by the SDA/SEAPDR, preceded by the presentation of a negative serological AGID test for EIA carried out in a laboratory accredited by MAPA. The serological test is valid for 180 days for intrastate transit and 60 days for interstate transit (Rio Grande do Sul, 2010). A property with a positive diagnosis is defined as a focus. The property is closed, and all susceptible animals have been tested by the Official Veterinary Service (OVS). The movement of animals will resume once two negative serological tests have been obtained at intervals of 30 to 60 days. All positive animals are isolated and euthanized (Rio Grande do Sul, 2010).
Historically, EIA in RS has had a low prevalence and has been diagnosed mainly through passive surveillance, with the requirement of a negative test for animal transportation (Rebelatto et al., 1996; Barzoni et al., 2018). A serological survey in 2014 determined that the prevalence of EIA was 0.3% (Seapa, 2014). However, in the following years, an increase in the number of cases associated with some areas of RS was observed (Machado et al., 2021; Cardenas et al., 2022). The western region of the state is characterized by a high concentration of horses, which are used for work on cattle and sheep farms, reproduction, sports, and leisure. It is also one of the regions of RS that concentrates the majority of EIA outbreaks (Barzoni et al., 2018; Cardenas et al., 2022). Thus, this study aimed to characterize the outbreaks of EIA from 2009 to 2019 in the western region of Rio Grande do Sul, Brazil.
2. Material and methods
2.1. Study region
We analyzed EIA notifications that occurred between 2009 and 2019 in the municipalities of São Borja, Itaqui, Maçambará, Uruguaiana, Barra do Quaraí, and Quaraí, in the state of Rio Grande do Sul, Brazil. For this purpose, the official notification forms (Form-In, Form-Com, and Form-AIE) for focus or suspected properties were obtained from the Department of Agriculture, Livestock and Supply of the state (SEAPA/RS). In addition, part of the data previously published by Barzoni et al. (2018) and Jardim et al. (2023) was included in the study. All information that could identify specific properties or herds was omitted.
2.2. Tabulation and descriptive epidemiology
The information regarding the municipality, date, location, characteristics of property, number of animals, number of positive animals, presence of clinical signs, request for counter-test and retest, and number of sanitizations was reviewed and tabulated in Microsoft Excel® spreadsheets. Based on the information obtained, the focus was characterized according to time distribution, municipality, herd size, type of property, reason for requesting the test, susceptible species, infected animals, clinical signs, and sanitation measures. Absolute frequencies were obtained by descriptive statistical analysis.
2.3. Spatial analysis
A Kernel density analysis map was produced using a search radius (or radius of influence) of 10km as a parameter. After constructing the map, it was possible to find areas with a higher concentration of points (hotspots). The map was generated through exploratory interpolation carried out by the QGIS software itself in version 3.34.9 (QGIS, 2024).
3. Results
During the eleven years (2009 to 2019), 125 notifications of suspected cases or focus of EIA were recorded and analyzed in municipalities in western Rio Grande do Sul, Brazil (Figure 1). Among the records, 12 were classified as investigations and 113 as focus. Investigations refer to properties with an established link to a focus (physical contact with EIA-positive horses). Records of these situations occurred in São Borja (11) and Uruguaiana (1). Serum samples from 44 horses on these properties were collected and tested by the Official Veterinary Service, and no cases of EIA were diagnosed.
Geographical distribution and Kernel density of Equine Infectious Anemia focuses on Rio Grande do Sul State, Brazil. Each outbreak was considered a point, and a radius of 10 km was used. The black dots indicate the cities and urban perimeters. The municipalities of São Borja (A – 57 focus), Maçambará (B – 1 focus), Itaqui (C – 29 focuses), Uruguaiana (D – 19 focuses), Barra do Quaraí (E – 4 focuses), and Quaraí (F – 3 focuses). Note that in São Borja, Itaqui, and Uruguaiana, there is a high concentration of outbreaks in the peri-urban region.
A focus was defined as a property where at least one horse was diagnosed positive for EIA by the AGID test. These situations were concentrated in São Borja (50.4%), Itaqui (25.7%), and Uruguaiana (16.8%). The municipalities of Barra do Quaraí (3.5%), Quaraí (2.7%), and Maçambará (0.9%) together accounted for less than 8% of records. Between 2009 and 2014, there were 11 registries, and in 2015 and 2019, there were 102 focus events (Figure 2a). Analysis of the monthly distribution showed that 74 focus events (65.5%) occurred in July, August, and September (Figure 2b).
Temporal distribution of Equine Infectious Anemia focuses recorded in the western region of Rio Grande do Sul, Brazil. Annual (A) and monthly (B) registration of cases.
In focus events, the total susceptible equine herd was 2,071 animals, of which 2,040 were horses (98.5%), 26 were donkeys (1.3%), and five were mules (0.2%). In this population, 242 horses (11.7%) tested positive for EIA. The characterization of the properties in terms of herd size and number of infected animals in each category is shown in Table 1. Herds with up to ten animals were the category of properties with the highest frequency of positive diagnoses (71.7%). This type of breeding accounted for 41.1% of the EIA-positive animals (104/242) (Table 2). Of the 113 EIA-positive properties, 79 (69.9%) were registered with the SDA/SEAPA, and the other 34 (30.1%) were irregular. Only one property investigated was not registered with the OVS.
Characterization of herds and location of equine infectious anemia focuses diagnosed between 2009 and 2019 in the western region of Rio Grande do Sul State, Brazil.
Characterization of Equine Infectious Anemia records by municipalities in the western region of the State of Rio Grande do Sul, Brazil, from 2009 to 2019.
In 37 focus locations, there were 45 animals, and all of them were infected. The herd on these properties consisted of just one (32), two (2), or three (3) horses. The property with the largest herd had 767 animals and only one positive horse (0.1%). The highest prevalence rates in herds with more than five animals (19/58; 27/63, 12/15, 6/7, 8/16, 5/8, and 19/39) were 32.8%, 42.9%, 80%, 85.7%, 50%, 62.5%, and 48.7%, respectively.
The reason for carrying out the serological test for EIA was the transit of animals in 65.5% of the outbreaks (74/113), because there was a link with an outbreak in 25.7% of the cases (29/113), and, for ten outbreaks, it was not possible to determine. No diagnosis was made as a routine examination or on clinical suspicion. The properties with positive cases were classified as breeding farms (28/24.8%), military units (1/0.9%), and "other" in 84 situations (75.2%). The official documents (MAPA) use "other" to describe any other properties that can not be classified in the previous categories. Here, this property's classification defines small properties in the peri-urban area with few animals (> 5 horses). No focus occurred on stud farms, jockey clubs, or equestrian societies. The localization of EIA+ properties indicated that 68 (60.2%) were in the periurban region and 45 (39.8%) were in rural areas. One outbreak was identified in animals smuggled into Brazil from Argentina, and seven horses were found abandoned on the banks of the river Uruguay. After AGID testing, two animals were seroreactive.
After the initial diagnosis, on 37 occasions, retesting was requested by the producers and carried out by the OVS, and the positive result was confirmed. Sanitation and testing of all susceptible animals were carried out in 77 herds, and the procedures ranged from two to seven. The euthanasia of positive animals must be followed by sanitation measures, where the OVS carries out tests on all susceptible animals on the property. Interestingly, in 36 outbreaks, sanitation was carried out only once, or did not need to be done because the entire herd was eliminated. In three cases, the owners reported that the animals (6) had been stolen; one horse was found after 70 days, and the other five remained missing. All the other 237 horses were euthanized.
4. Discussion
The retrospective analysis of the records of EIA-positive properties diagnostics between 2009 and 2019 showed the occurrence and increased frequency of cases in the last four years of the study. EIAV infection is a cause for concern for the production chain because, in addition to the costs of health tests for transporting the animals, there is a risk of the property being closed and infected animals being euthanized (Machado et al., 2021). Additionally, there is the loss of animals of genetic or emotional value, the devaluation of farms, and the imposition of trade barriers (Almeida et al., 2017; Machado et al., 2021).
The adaptability of EIAV to the host produces non-specific clinical manifestations, from which, in most cases, the animal recovers and becomes a carrier of the infection and a permanent source of the virus. The combination of these characteristics contributes to the suspicion of infection being overlooked by breeders and clinicians (Cook et al., 2013). The association of the health condition and the slight production losses contributes to the circulation of carrier animals between herds and low adherence by producers to diagnostic testing for investigation of suspected cases or herd monitoring (Almeida et al., 2017). However, understanding the pathogenesis of EIAV and recognizing the pattern of infection in a given population is necessary. In this way, breeders and the OVS can direct efforts towards controlling the infection and preventing new cases.
The equine herd in Rio Grande do Sul has approximately 520,000 animals, primarily horses, and less than 2% of them are mules and donkeys. The distribution of horses and breeds is heterogeneous across the state (Costa et al., 2013). The highest concentration of animals (92.2%) is found in herds of up to ten animals, and the most significant number of horses is located in the southern and western regions of the state. Extensive cattle and sheep farming, which uses mixed-breed or Crioulo horses for work, predominates in these regions. These animals occasionally participate in local cultural and sporting events (parade and rodeo) (Costa et al., 2013).
Among the affected herds observed in the study, the majority belonged to small producers located on the outskirts of towns or in stables with animals belonging to several owners. These herds do not have the permanent care of a veterinary clinician and are often transported illegally. This breeding profile allows the circulation and maintenance of EIAV in this subpopulation, characteristics that have been previously demonstrated (Barzoni et al., 2018; Jardim et al., 2023). Purebred farms (Crioulo, Thoroughbred, Quarter Horse, Brazilian sport horse) have animals with high zootechnical and commercial value; they take part in equestrian events and competitions, where they are regularly tested, and producers adopt health control measures (Oliveira et al., 2015; Almeida et al., 2017). In this way, these herds are relatively closed and have contact mainly with other animals of similar aptitude and value, which reduces the likelihood of EIAV and other pathogens emerging. Only one horse tested positive in the outbreak that occurred in the largest herd (767 animals). According to unofficial information, the animal had been transported illegally without an IDGA test and was tested at the destination property. This type of horse movement is relatively common in the region and may contribute to the infection's spread.
In the region studied, in addition to using horses for work on farms or leisure, many of these animals participate in a parade of riders held in September. The parade brings together more than 1,000 animals in some municipalities in RS, with up to 5,000 equines. The OVS supervises this event, which requires a GTA and an EIA negative test for the animals to participate. The animals are prepared, and health checks are carried out in the months leading up to the event. This movement of animals is demonstrated by the higher number of GTAs issued during this period and, consequently, by the concentration of EIA-positive properties (Machado et al., 2021). Evaluation of the monthly distribution of positive EIA diagnoses showed that the highest concentration of cases occurred in July, August, and September. Therefore, passive surveillance is an essential tool for diagnosing the infection, although the results may be more related to the increase in tests carried out than to a real increase in the number of cases.
EIA was first recorded in RS in 1968 (Brasil, 1988), and its occurrence has consistently remained low (Rebelatto et al., 1996; Barzoni et al., 2018). In 2014, the serological survey on 341 properties registered with the SDA tested 1,010 animals, none of which tested positive (Seapa, 2014). The survey results led to some changes in legislation, such as extending the validity of the serological test from 60 to 180 days for transit within the state and applying sanctions on owners who move animals without a GTA and without registering with the SDA (Machado et al., 2021).
The change in legislation led to an increase in the official registration of breeders and animals (Machado et al., 2021). However, after 2015, there was an increase in the registration of infection and the concentration of EIA cases in four regions of the state, including the western border region of RS (Barzoni et al., 2018; Machado et al., 2021; Cardenas et al., 2022). The increase in recorded cases was attributed to an increased number of registered breeders and, consequently, more animal monitoring by passive surveillance of transported animals (Machado et al., 2021). However, the official serological survey only considered properties registered with the SDA and, as demonstrated by Barzoni et al. (2018), more than 50% of outbreaks are recorded in producers without official registration. This producer profile was also observed among the outbreaks studied. Therefore, it is possible to demonstrate that EIAV is present in this equine population and to hypothesize that the association of the lack of sanitary measures (periodic examinations and reuse of syringes and needles) with illegal movement enables the agent to be maintained and spread. Greater sanitary control after the legislation change meant more cases were diagnosed, and the virus was circulating in this subpopulation imperceptibly (Machado et al., 2021). However, maintaining the way animals are raised and extending the validity of the serological test contribute to the presence of EIAV-positive animals.
The spread of EIAV between herds occurs preferably through introducing a healthy virus-carrying animal. Transmission via mechanical vectors (Tabanus spp. and Stomoxys calcitrans) occurs mainly in tropical regions, where vectors are abundant in the hot and humid months. In temperate areas, the presence of vectors is reduced, and this transmission is less relevant (Barros & Foil, 2007). However, the iatrogenic route (syringes, contaminated needles) is the most effective form of transmission and has the potential to spread the virus to a large number of animals in the same herd (Williams et al., 1981). Investigation into the source of infection and the form of transmission has not been determined for most cases. Due to the poor sanitary characteristics of small farms and the high prevalence in some farms, it is suggested that the iatrogenic route is the most important form of transmission in the herds studied. Mechanical insect transmission should be considered, but it is less relevant to the region. The presence of vectors in the region evaluated is markedly seasonal and occurs between November and March (Lucas et al., 2020).
MAPA regulates EIA control strategies in Brazil, and each state adopts measures according to its internal situation. This strategy has proven efficient in controlling and maintaining rates, but is considered deficient in eradicating the infection. The Brazilian equine herd is very diverse, and each region has different ecological and socio-cultural conditions, which, combined with the subpopulations of horses used for other purposes, contribute to the segmentation of these populations (More et al., 2008; Almeida et al., 2017; Barzoni et al., 2018). Therefore, measures to tackle EIA must evolve towards compartmentalizing actions for a given region or subpopulation of animals. These actions must include active surveillance, investigation of focus links, and health education for producers and veterinarians.
5. Conclusions
The western region of Rio Grande do Sul concentrates a considerable portion of the EIA focus identified in the state. The results showed that the virus is present and circulates in small herds, where passive surveillance for events is the primary way of diagnosing cases. Between 2015 and 2019, there was a considerable increase in registrations in the region, especially in the municipality of São Borja. The occurrence of EIA is well above the state's rates as a whole and can be considered alarming. Infected animals appear healthy and circulate without clinical suspicion of their condition. In this way, the region has socio-cultural, environmental, and herd conditions that contribute to the maintenance and spread of the virus. Therefore, control and prevention actions should be directed towards this region, considering its particularities.
Acknowledgments
The authors thank the Secretaria de Agricultura, Pecuária e Desenvolvimento Rural do Estado of the State of Rio Grande do Sul (SEAPDR/RS), Brazil, for providing data and information. JCJS received a doctoral scholarship from “Coordenação de Aperfeiçoamento de Pessoal de Nível Superior” (CAPES), and MELM is the recipient of a scientific initiation scholarship – PIBIC/CNPq. This work was financed with resources from the Laboratório de Virologia/UNIPAMPA, PROPPI/UNIPAMPA (Pró-Reitoria de Pós-Graduação, Pesquisa e Inovação, Universidade Federal do Pampa), and by the CAPES - Finance Code 001.
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Edited by
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Editor:
Sérgio Santos de Azevedo
Publication Dates
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Publication in this collection
07 July 2025 -
Date of issue
2025
History
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Received
22 Feb 2025 -
Accepted
12 May 2025
