Feline immunodeficiency virus (FIV) and feline leukemia virus (FeLV) infections occur in cats worldwide and are associated with several disease syndromes. Prevalence of both infections is highly variable among countries and regions. Prevalence of FIV infection ranges from 2 to 44% worldwide (1–7). In Europe, prevalence is highly variable with up to 30% of cats being infected in countries with large free roaming feline populations such as Italy (8–11). In the US, prevalence ranges from 3% in the overall population to 18% in sick cats (1). In Asia, prevalence varies from 6 to 44% (7, 12–16). FeLV prevalence has been reported in <1 to 31% of cats worldwide, from <1 to 15% in Europe, 2 to 3% in the United States, 3 to 28% in South America and <1 to 24.5% in Asia, Australia and New Zealand (1, 8–11, 14, 15, 17–33). In previous studies, prevalences for FIV and FeLV infection in Asia were higher than in Europe or the United States (1, 13, 14, 16, 17). It was suggested that this might be due to different cat living conditions and vaccination strategies. Cats in Thailand mostly have outdoor access and contact to many other cats of which a high number are strays (16). In 2016, the estimated number of cats in Bangkok was about 4,00,000, of which about 90,000 were strays (32). Free roaming cats are at high risk for retrovirus infections (17, 34). FeLV and FIV vaccines are available in Thailand but are not commonly used.

There is limited information on FIV and FeLV infection prevalence in Thailand, and available studies either included only a low number of cats or investigated cats in only small areas (12, 15, 16, 35–40) (Table 1). Most of these studies looked at cats from Bangkok and its vicinities. In addition, these studies mostly included sick cats (12, 15, 16, 36, 37) (Table 1). So far, there is only one study in a limited region of Northeast Thailand (Khon Kaen city) that evaluated clinically healthy cats in 2018 (40) and only one study investigating risk factors for FIV and FeLV infection in Thailand (15). One recent study looked at risk factors for FeLV only, again in a limited region (Bangkok and Chiang Mai) (35). Thus, the present study aimed to determine the prevalence of FeLV and FIV in healthy outdoor cats in North, Northeast, and Central Thailand. In addition, risk factors for FeLV and FIV infection in Thailand were evaluated.

The present study investigated the prevalence of FIV and FeLV in 260 clinically healthy cats in North, Northeast, and Central Thailand. A FIV prevalence of 4.2% (11/260) was detected. Previous studies only investigated cats from Bangkok and its vicinities for FIV infection, and prevalence of up to 40.1% was described (12, 15, 16, 37, 38, 40) (Table 1). Prevalence of FIV infection in the present study was much lower than in most previous reports, potentially due to different study populations, such as concerning health status, sample locations, and living conditions. In the present study, healthy cats in a larger region were included whereas in earlier studies, cats were presented to different veterinary clinics with clinical signs in Thailand (12, 15, 16, 37, 38) (Table 1). A significantly higher prevalence for FIV infection in sick cats was determined in studies in other countries before which might explain the difference (14, 37, 41). In a recent study, Aiyaranoi et al. (2018) investigated cats from Khon Kaen city, which is located in Northeast of Thailand and reported an prevalence of 6.1% (40). The cat populations of the present study and of the study in Khon Kaen were very similar and comparable regarding age and gender. Furthermore, both studies included only healthy cats. In the present study, a prevalence of 6.3% was determined for cats originating from Northeast Thailand, and this is indeed very similar to that of the study of Ayaranoi et al. (40). It should be considered that in the present study, in the region of Amnat Charoen 28.6% (2/7) of sampled cats tested positive, whereas in Nakhon Ratschasima and Udonthani lower prevalences were detected (Table 3). Amnat Charoen is one of the poorest provinces in Thailand and situated at the border to Laos (42). There are no studies available about FIV prevalence in Laos, but high FIV prevalences in cats in Laos could be one explanation for the very high prevalence in Amnat Charoen (15). FIV prevalence of 4.9% was determined in Bangkok and its vicinities in the present study. In contrast, a very high prevalence (40.1%) of FIV infection was detected in the same area in 1994 (38). In a study from 1997, prevalence of FIV infection was 32.1% (12), in 2009, prevalence was 20.1% (15), and a recent study determined a prevalence of 5.4% for cats in Bangkok and its vicinities (16), which is similar to the present study. Therefore, prevalence has considerably decreased in the last 25 years (Table 1). Nowadays, there are several private and governmental organizations that neuter cats in the Bangkok area. As a result there are less intact fighting male cats in this area and transmission of FIV could be reduced (16).

In the binary logistic regression analysis of the present study, cats at the age of 2 years or older had a higher risk to be FIV-infected. This is in accordance with other studies performed in Thailand and other countries worldwide (9, 10, 12, 14, 15, 29, 41, 43), and can be explained by an increasing risk of exposure in older cats. No other risk factor was found to be significantly associated with a higher risk for FIV infection, although in other studies male intact cats were at higher risk to be infected with FIV (10, 15, 29, 41).

In the present study, the FeLV prevalence was 4.2% in North, Northeast, and Central Thailand. So far, there is only one study published on FeLV prevalence with a small study population (119 cats) from Bangkok and Chiang Mai, which is situated in the North of Thailand (35). The authors determined a prevalence of progressive FeLV infection of 14.3 % (35). The number of cats sampled in Chiang Mai was neither specified in this study (35), nor in an overlapping larger study in which cats and dogs from Eastern and Southeast Asia were tested for vector-borne pathogens and ectoparasites (44). There is no study that investigated FeLV prevalence in Northeast Thailand so far. Previous studies in Bangkok and its vicinities mostly included sick cats and even recent studies detected prevalences up to 24.5% (15, 16, 37). One study found a prevalence of 20.9% in sick cats (37) (Table 1). The previous study conducted in Bangkok and Chiang Mai included not only, but also sick cats as the study involved cats with clinical signs (i.e., enlarged lymph nodes and skin abnormalities) from academic institutions and private facilities (44). This could be one explanation for the higher prevalence determined in that study. In one study from 2016 in which only healthy cats were included, prevalence of progressive FeLV infection (4.2%) was comparable to the prevalence determined in the present study (40).

In the present study, only healthy cats were included and this might explain the much lower prevalence when compared to most previous studies. Being ill has been detected as a significant risk factor for FeLV infection in several studies worldwide (45). All FeLV-infected cats in the present study originated either from Bangkok or Lamphun, a city in North Thailand (Table 3). In the other five sample locations, no cat tested positive. Possibly, FeLV is endemic in Bangkok and Lamphun in contrast to other locations. The observed differences might be attributed to variation among geographic regions, cat population densities, lifestyles and control policies and practices among different regions. Fromont et al. (2003) investigated models to predict dynamics of FeLV in cat populations and found that extinction of FeLV is possible in small populations (46). Furthermore, these models showed that FeLV dynamics depend on size of the population and the relationship between host density and the pattern of contact of individual cats (46). Nakumara et al. (2000) even suggested that some cat populations in Asia might be free of FeLV infection; as an example, none of the tested cats in North and South Vietnam was progressively infected with FeLV (13).

The higher prevalence of FeLV in the study of Capozza et al. (2021) could be explained by a higher regional prevalence in Bangkok and North Thailand (where Chiang Mai is situated) which is in accordance to the present study (35). Capozza et al. (2021) found a significant (p < 0.05) association between Thailand as country of cats' origin, adult age, abnormal oral mucosa, and positive FeLV-antigen results (35). In the present study, adult age was not a significant risk factor (p = 0.052) for FeLV infection. As only healthy cats were included, clinical sings could not be evaluated as a potential risk factor.

One limitation of the study is that samples were only tested for FeLV antigen and thus, only cats with progressive infection (and occasionally those with early regressive infection) were detected but most regressively infected and all abortively infected cats were likely missed (47). After a short period of viremia, regressively infected cats only harbor provirus in bone marrow and blood cells, and thus, true FeLV prevalence is always higher than the prevalence of antigen-positive cats. Furthermore, confirmatory testing was not performed and therefore, potentially false positive results cannot be completely ruled out. Especially FeLV polymerase chain reaction would have been very interesting not only as confirmatory test but also to detect regressive FeLV infection. These additional tests should be investigated in further studies.

FIV prevalence of 5.8% and FeLV infection prevalence of 4.2% were detected in healthy cats in North, Northeast, and Central Thailand. Cats had a significantly (p = 0.025) higher risk for FIV infection when they were 2 years or older. This study can help to monitor FIV and FeLV infections in cats in Thailand and develop control strategies such as recommendation for vaccination. Cats in the present study were not vaccinated against FIV and/or FeLV. Both vaccines are available in Thailand, but are not widely used. FIV vaccination is discussed controversially, according to the present study, FeLV infection is still endemic in Thailand although prevalence is decreasing, and therefore, vaccination against FeLV is highly recommended.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The animal study was reviewed and approved by Animal Ethics Committee of the Faculty of Veterinary Science, Chulalongkorn University of Bangkok, Thailand (approval number: 1731042). Written informed consent was obtained from the owners for the participation of their animals in this study.

KH, FS, and MB contributed to conception and design of the study. FS performed the sampling and testing with the help of SL, PJ, and RP. FS organized the data base and wrote the first draft of the manuscript. SR performed the statistical analysis. KH and MB did the proof-reading. All authors contributed to manuscript revision, read, and approved the submitted version.

This research was partially funded by MSD Animal Health, the Netherlands. MSD Animal Health played no role in the collection and interpretation of data, or in the decision to submit the manuscript for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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