Profile of lymphomas related to feline viral leukemia in domestic cats in the northwest and Midwest regions of Rio Grande do SulABSTRACT
Lymphoma is one of the most observed tumors in cats. Animals that test positive for Feline Leukaemia Virus (FeLV) are more predisposed to developing lymphoma. The objective of this study is to examine the profile of feline lymphomas related to Feline Leukaemia Virus, recorded in the north-western and central-western regions of Rio Grande do Sul. Cases of lymphoma referred to university laboratories in the regions, originating from biopsies and necropsies, were included in the period from January 2016 to December 2023. The data were analyzed using statistical tests. The Kruskal-Wallis test reviewed whether there was a relationship between age and tumor location, while the ANOVA test showed a correlation between FeLV and the affected site. The animals with lymphomas, 72.73% were positive for FeLV. Mediastinal and extranodal lymphomas were the most common, and a correlation was observed between FeLV and the affected site. It was concluded that the profile of lymphomas in the regions studied was mediastinal tumors followed by extranodal tumors, mainly in young animals between seven months and two years of age. The most affected sites were the thymus and spinal canal in FeLV-positive animals.
Keywords:
lymphoma; feline; haematopoietic; disease; tumor
RESUMO
O linfoma é um dos tumores mais observados entre os felinos. Animais positivos para imunodeficiência viral felina (FIV) e leucemia viral felina (FeLV) possuem maior predisposição ao desenvolvimento do linfoma. O objetivo deste trabalho foi estudar o perfil de linfomas felinos relacionados a doenças retrovirais registrados nas regiões noroeste e centro-oeste do estado do Rio Grande do Sul. Foram incluídos casos de linfomas, cujas amostras, provenientes de biópsias e necropsias, foram encaminhadas a laboratórios universitários das respectivas regiões, no período de janeiro de 2016 a dezembro de 2023. Os dados foram analisados por meio de testes estatísticos; o Kruskal-Wallis avaliou se existe relação entre idade e local do tumor, o teste de ANOVA evidenciou a correlação entre a FeLV e o local acometido. Dos animais com linfomas, 72,73% eram positivos para FeLV, sendo os mais encontrados os linfomas mediastinal e extranodal. Além disso, observou-se uma correlação entre a FeLV e o local acometido. Conclui-se que o perfil de linfomas nas regiões estudadas foi predominado por tumores de mediastino seguidos de tumores extranodais, ocorrendo principalmente em animais jovens, com idade entre sete meses e dois anos. Os locais mais acometidos foram o timo e o canal medular em animais positivos para a FeLV.
Palavras-chave:
linfoma; felino; retrovirais; doença; tumor
INTRODUCTION
Hematopoietic tumors are the most prevalent in felines (Vail and Pinkerton, 2019), with lymphoma being the most frequently observed among cats and considered one of the leading causes of death among the species (Batista and Griebeler, 2023). The incidence in cats is estimated at 200 cases per 100,000 animals worldwide, being more prevalent in young animals (Daleck and Nardi, 2016).
Animals that test positive for retroviruses such as Feline Leukemia Virus (FeLV) are more predisposed to developing lymphoma when compared to animals that test negative for these viruses (Batista and Griebeler, 2023). A study conducted by Stützer et al., (2011) with 77 animals, 16 of them FeLV positive, revealed that only 20.8% of cats with lymphoma in that study were FeLV positive, and among the most common correlated forms were thymic and multicentric lymphomas, meaning that FeLV would not be solely responsible for most lymphomas. The association of retroviruses with tumor development has been declining with increased vaccination and population control measures, however the prevalence of lymphoma continues to rise (Daleck and Nardi, 2016).
FeLV, in turn, alters cell growth, causing malignant transformation (Daleck and Nardi, 2016). Infection with feline leukemia viruses can evolve through different biological outcomes, modulated by the host's immune response, initial viral load, and the efficiency of the agent's replication. Broadly speaking, there are four main patterns of progression. In abortive infection, the immune response controls viral replication earlier, preventing the virus from reaching the bone marrow, so that no persistent viremia or integration of the provirus into the cellular genome is observed (Hartmann, 2012). In regressive infection, FeLV reaches the bone marrow, and the provirus integrates into the host DNA; however, viremia does not persist, and the provirus remains in a latent state, which can be reactivated under conditions of immunosuppression (Lloret et al., 2017). Progressive infection is characterized by intense and continuous viral replication, resulting in sustained viremia, high viral load and extensive proviral integration, factors that increase the risk of related diseases such as lymphoma (Hartmann, 2012). Finally, in focal or atypical infection, FeLV replication is restricted to certain tissues or specific cell populations, usually without detectable viremia, which makes diagnosis difficult due to the localized distribution of the provirus (Beatty, 2014).
The clinical manifestations of lymphoma vary according to the anatomical type involved, but nonspecific signs such as anorexia, progressive weight loss, lethargy, vomiting, and digestive changes are frequently reported (Batista and Griebeler, 2023). Anatomically, feline lymphoma is classified into five main presentations: gastrointestinal, mediastinal, multicentric, nodal, and extranodal (Vail and Pinkerton, 2019).
The objective of this study is to conduct a retrospective study on the profile of feline lymphomas related to Feline Viral Leukemia registered in the northwest and midwest regions of Rio Grande do Sul.
MATERIAL AND METHODS
All confirmed cases of lymphoma in cats were included in a retrospective study, referred to university veterinary pathology laboratories in the northwest and midwest regions of Rio Grande do Sul, originating from biopsies and necropsies performed by the Animal Pathology Laboratory (LPA) in the city of Cruz Alta, the Veterinary Pathology Laboratory (LPV) in the city of Ijuí, and the Veterinary Diagnostic Consulting Service (SEDIVET) in the city of Santa Maria, from December 2016 to January 2023. The reports were analyzed for: (1) epidemiological data (sex, breed, age, and origin); (2) history of previous diseases (Feline Immunodeficiency Virus (FIV) and/or Feline Leukemia Virus (FeLV)); and (3) anatomical classification of the tumor (alimentary, mediastinal, multicentric, extranodal, nodal). To this end, a comparative table was drawn up containing data on each cat, such as number, sex, age, breed, origin, Feline Immunodeficiency Virus (FIV) and Feline Leukemia Virus (FeLV) and the diagnosis of each educational institution involved. The history of positivity for FIV and FeLV retroviral diseases was based on the information provided by the reports from each laboratory, however, there were no specifications on the forms of diagnosis for either disease, only the record of positivity and/or negativity, so the information provided in the reports was retained. Cases in which epidemiological information was not available were described as not informed (NI) in the statistical table. The data were organized and subsequently submitted to statistical analysis, described below:
The data were organized according to different classification standards. For the general evaluation of anatomical distribution, the total number of cases diagnosed as lymphoma was considered. The age of the animals was categorized as kittens (0-6 months), juniors (7 months-2 years), young adults (3-6 years), adults (7-10 years), seniors (11-14 years), and geriatrics (>15 years), according to the guidelines for feline life stages of the American Association of Feline Practitioners and the American Animal Hospital Association (Cristo et al., 2019). Frequencies were distributed by year of care, type of material received (biopsy or necropsy), sex, age group, and retroviral status (FIV and/or FeLV). The proportions were calculated based on the number of valid cases for each variable. Retroviral status was confirmed retrospectively, using only the information recorded in the clinical and necropsy records of the participating university hospitals. All data available in the medical records were considered in the statistical analysis. It should be noted that the animals were not subjected to immunohistochemistry or PCR for FeLV or FIV, so that retroviral status was based only on previously performed and recorded rapid tests.
Statistical analysis was performed using R software version 4.4.1. Initially, the data were organized and subjected to descriptive analysis, in which measures of trend, dispersion, and frequency of observations per group were calculated. To determine the choice of statistical tests, the data was subjected to tests of normality and homogeneity of variances. Normality was assessed using the Shapiro-Wilk test, while the homogeneity of variances was verified using the Levene test. As the results indicated that the data did not follow a normal distribution (p<0.05), nonparametric tests were applied. The Kruskal-Wallis test was used to compare the groups, to verify statistical differences between independent groups, assessing whether there is a relationship between age and tumor location, separated by age groups. The same test was also used to evaluate whether there is a relationship between sex and tumor location. The ANOVA test for FIV, FeLV, and sample location was performed to assess whether there is a relationship with tumor location.
When significant differences were observed, Dunn's test was applied, with Bonferroni correction to adjust the p-values in multiple comparisons. In addition, an analysis of variance (ANOVA) was performed, followed by Tukey's multiple comparison test. However, due to the data not meeting the assumptions of normality and homogeneity, the ANOVA results were used only descriptively and interpreted with caution.
RESULTS AND DEBATE
Lymphoma has a multifactorial origin, as several factors can contribute to its development, the most common being infection with the Feline Leukemia Virus and the Feline Immunodeficiency Virus (Hartmann and Hofmann-Lehmann, 2020). A total of 86 (100%) cases of lymphoma in cats were analyzed, studied from January 2016 to December 2023, including 17% (15/86) necropsies and 83% (71/86) biopsies. Of these animals, 55% (38/86) were male and 45% (29/86) were female, with all categories of lymphoma showing a higher prevalence in males. However, a retrospective study of the profile of cats diagnosed with lymphoma by Vidor (2019), found a higher number of females with mediastinal lymphoma, while in the extranodal category there was a higher prevalence of males (p=0.011). However, in multicentric and alimentary lymphoma, there was no significant difference.
Regarding retroviruses in animals with lymphoma, 73% (43/86) tested positive for FeLV and only 1.5% (1/86) for FIV. Similar studies, such as that conducted by Vidor (2019), in the city of Porto Alegre, Rio Grande do Sul, found that 62% of cats with lymphoma tested positive for FeLV. Cristo (2019), in his retrospective study in the region of Santa Catarina, SC, found only 56.6% of animals to be positive, demonstrating that there are different casuistic depending on the region of the country, making evident in our study the high concentration of cases in the northwest and Midwest regions of RS. FeLV infection can progress gradually, resulting in persistent viremia, immunosuppression, tumor development, anemia, and eventually death. However, some cats may trigger a partial immune response capable of containing viremia and limiting viral replication, leading to a regressive infection, where only low levels of proviral DNA are detectable (Nesina et al., 2015). When the infection becomes progressive, the virus compromises the immune system and promotes oncogenesis, either by activating proto-oncogenes or by inhibiting genes that act in tumor suppression, a process that occurs due to the integration of proviral DNA into the host genome, leading to insertional mutagenesis (Meichner and Bomhard, 2014).
In cats with lymphoma and FELV infection, the virus integrates into the host cell's DNA, promoting changes in the cell cycle and favoring malignant transformation, predominantly affecting adult animals. In contrast, FIV contributes to oncogenesis by compromising immunosurveillance, hindering the elimination of malignant cells, and is more common in young animals (Couto and Nelson, 2010; Daleck et al., 2008).
Lymphoma is classified based on its anatomical location and related histological and immunophenotypic criteria (Vail, 2013). Some classifications use categories such as extranodal, mediastinal, alimentary, multicentric, nodal, leukemic, and individual forms. Others also combine nodal and extranodal forms into atypical, unclassified, and mixed categories (VAIL, 2013). Following the anatomical classification, mediastinal (29%) and extranodal (28%) lymphomas were the most prevalent in cats in this study (Table 1). In the ANOVA test for FeLV, the p-value was less than 0.05 (0.001195), showing that there is a statistically significant association between FeLV and tumor location. As a result, a close association between FeLV and mediastinal lymphoma (46.5% of cases) was observed in this study. In contrast, a Brazilian study conducted by Cristo (2019) found a higher prevalence of multicentric (24.53%) and mediastinal (20.75%) lymphomas correlated with FeLV.
In mediastinal lymphoma, the thymus and mediastinal and sternal lymph nodes may be affected (Vail et al., 2020), similar to this study, which showed a higher frequency in the thymus (30.30%), followed by the lymph nodes (4.55%) and mediastinum (1.52%). Horta et al., (2020), proposes that FeLV-associated lymphomas are commonly high-grade and affect young cats up to 3 years of age, which corroborates with the data found, whose prevalent age was from 7 months to 2 years (56.3%). Infection usually occurs in the first months of life, and in young cats, viral replication does not occur at an early stage, favoring the development of tumors in this region. In older cats, the thymus undergoes a natural regression process, and resistance to FeLV tends to increase with age. Thus, adult cats with a functional immune system are less likely to develop persistent viremia after exposure to the virus. Therefore, FeLV infection is more common in young cats and those with mediastinal lymphoma (Stützer et al., 2011).
Extranodal lymphomas were the second most common anatomical type found in the current study, accounting for 27.91% (13/86), and were observed in organs such as the spinal canal, kidneys, tonsils, and encephalon, in order of importance. Extranodal forms are those that occur in non-nodal areas, such as the nasal cavity, eyes, central nervous system, kidneys, among others (Couto and Nelson, 2010). In these cases, treatments are enigmatic because they present distinct clinical-pathological characteristics depending on their location (Couto and Nelson, 2010). Lymphoma is the most common spinal tumor in cats, with an occurrence rate of 38.8%, followed by vertebral osteosarcoma, which has an incidence of 16.5%. Given these data, central nervous system lymphoma should be recognized as one of the main neurological diseases affecting cats (Marioni-Henry et al., 2004). The present study found that the spinal cord was the second most affected organ (12.12% [10/86]), behind only the thymus, and was the most frequent site among extranodal lymphomas. Lymphoma has a predilection for lumbar segments, but it can involve all regions of the spinal cord, as described by Mello (2019) in a retrospective study that evaluated the pathological, immunohistochemical (IHC), and etiological characteristics of lymphoma involving the nervous system (NS) in cats in RS. According to the author, SN involvement was observed in 16 (12.2%) of 125 cats with lymphoma, and young cats were the most affected, similar to this study, where the age affected was 7 months to 2 years (40%) for extranodal lymphomas. The kidneys were the second most affected organ in extranodal lymphomas. According to Couto (2001), anemia and enlarged kidneys on palpation may be observed, and in these cases, bilateral affection is common.
Lymphoma in cats has a bimodal age distribution, with two distinct peaks. The first peak occurs around 2 years of age and it is more common in cats that test positive for FeLV. The second peak occurs between 10 and 12 years of age and is predominantly observed in cats that test negative for FeLV. The range age of diagnosis of lymphoma in FeLV-positive cats is around 3 years, while for FeLV-negative cats the range is between 7 and 8 years (COUTO, 2001). The age group of animals diagnosed with lymphoma most commonly observed in the study was 7 months to 2 years, 17/86 (20%), behind only those animals with age not reported in the reports, which accounted for 45% of cases (Table 2). The least affected age group was 0-6 months (2%).
CONCLUSION
In this retrospective study, it was found that lymphomas in cats from the northwest and Midwest regions of Rio Grande do Sul were more frequently of the mediastinal type, followed by the extranodal type, predominantly affecting young animals between seven months and two years of age. Among the organs most affected in animals positive for Feline Leukemia Virus, the thymus and spinal canal stand out. These findings reinforce the importance of regional epidemiological studies to better understand the distribution and factors associated with the occurrence of lymphomas in cats, as well as the influence of retroviral infection on the development of these neoplasms.
ACKNOWLEDGMENTS
This research was supported by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES, http://www.capes.gov.br/). Finance Code 001 and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, http://www.cnpq. br/).
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The research data are available within the article itself.
