3 RESULTS AND DISCUSSION
A total of 1,326 samples collected from 16 cities (Figure. 1) geographically located from the north to the south of China ( within the east longitude of 106°55′ to 126°53′ and north latitude of 20°1′ to 45.8′) were detailed in Table 2, and 79.9% (1060/1326) of cats were exposed to at least one pathogen of the six viruses.
In details, the positive rates of cat exposure to FeLV, FPV, FHV-1, FCV, FIV and FIPV were 59.6%, 19.2%, 16.3%, 14.2%, 1.5% and 0.5%, respectively (Figure 2). The highest ratio of FeLV infection may be caused by lacking the utilization of FeLV vaccine in China. The prevalence vary widely depending on the geographical location (Gleich, Krieger, & Hartmann, 2009). A previous report showed a FeLV positive ratio of 11.33% in Gansu province of northwest China, which was lower than that in our study. Highly variable prevalence (1%~12.2%) of FeLV infection in cats have been reported worldwide (Bande et al., 2012; Chang et al., 2010; Dorny et al., 2002; Gates, Vigeant, & Dale, 2017; Malik et al., 1997). The geographical location, sample quantity and populations may contribute to the variation (Chang et al., 2010). The prevalence of FeLV is 52.5% in male and 40.1% in female in this study, which is consistent with the results of a higher prevalence in male than female in previous reports (Gleich et al., 2009; Levy, Scott, Lachtara, & Crawford, 2006; Norris et al., 2007).The positivity of FeLV infection was higher in male cats than that in female cats, which might attribute to the greater risk of bite wounds in males caused by higher aggressivity.
FPV clinically important in cats with high mortality. Here, the FPV positivity in China is 19.2% from 2016 to 2019, which is lower than that (37.1%) in a previous report of Northeast China from 2016 to 2017 (Niu et al., 2018). Yet, little is known about the prevalence of FPV of cat in other parts of China and other countries.
As the main pathogens of upper respiratory tract infection in cats (Binns et al., 2000), the prevalence of FHV-1 and FCV (16.3% and 14.2%, respectively) in our study were lower than that reported in Beijing (26.3% and 46.3%, respectively) (Xu et al., 2017) and in Switzerland (20% and 45% in suspect population, respectively) (Berger et al., 2015).
FIV is distributed worldwide, and the prevalence of FIV was 2.5% and 5.4% to 31.1% in North America and Asian (Bande et al., 2012; Eckstrand, Sparger, & Murphy, 2017; Levy et al., 2006; Nakamura et al., 2010; Phillips, Lamont, Konings, Shacklett, & Elder, 1992), while it is higher than that in our study (1.5%). Different detection methods may contribute to the difference. The false-positive of antibody tests may own to maternal antibodies or previous FIV vaccination, as the maternal antibodies may persist for 6 months or longer (Barr, Pough, Jacobson, & Scott, 1991).
FIPV, one of the feline coronavirus (FCoV), is the pathogen of FIP, a fatal, immune-mediated disease in wild and domestic cats. FCoVs are classified into two serotypes, type I and type II. There are two types of FCoVs, FECV and FIPV, which classified based on pathogenicity. FECVs produce mild enteric infections (Pedersen, 1983), and can be converted into FIPV, and they exist in both serotypes I and II (Tekes & Thiel, 2016). Previous study showed a significant higher prevalence rate (74.6%) for FIPV in China (Li et al., 2019) than than in our study (0.5%), which may be caused by the limited sampling location.
In all the 1,326 samples, 1,060 (79.9%, 1060/1326) were positive for at least one viral pathogen in nucleotide detection. As shown in Figure 3, 64.34% (682/1060) of cats were infected with single type of virus, and 35.66% (378/1060) of cats were mixed infection. The high complexity of mixed infection, more than 10 kinds of mixed infection patterns as detected, indicated the complexity of viral infectious diseases of cats, it is a challenge for the diagnose of cat infectious diseases in China.
In our study, the correlation between positive rate of cats and specific factors, including age, weather and geographical location, were analyzed. The prevalence of FeLV, FHV-1, FPV and FIV was highly seasonal (Table 3). The prevalence of FHV-1, FPV and FIV in cold seasons (spring and winter) was higher than that in warm seasons (summer and autumn), while an opposite pattern of the prevalence in FeLV could be found. The positive rates of FPV, FHV-1, and FIV infections of cats in north China (FPV, 21.2%; FHV-1, 18.4%; FIV, 1.9%) were higher than that in south China (FPV, 10.7%; FHV-1, 7.5%; FIV, 0.0%), except that the FeLV showed an opposite pattern of infection (north China, 51.8%; south China, 92.5%). As shown in Table 4, the prevalence of FCV was the same in north and south China (14.2%). FPV infection was age-related in cats (age <12 months), while the prevalence of FeLV, FHV-1, FCV was age-irrelevant (Table 5), which varies from previous reports in New Zealand, where the susceptibility of FeLV decreased significantly with age (Luckman & Gates, 2017).
A total of 316 serum samples of cats were collected from four cities of China (Beijing, Tianjin, Qingdao and Zhengzhou). Antibodies against FPV were detected by HI assay, and FCV and FHV-1 antibodies were detected by virus neutralization (VN) assay. The protective titers were settled as 1:40, 1:32, and 1:16 for FPV, FCV, and FHV-1, respectively (Reese et al., 2008). As shown in Figure 4, a total of 267 serum samples of vaccinated cats (Feline Rhinotracheitis- Calici-Panleukopenia Vaccine, Killed Virus) were analyzed by HI assay for FPV antibodies detection, and the protective rate was 83.9% (224/267).
As shown in Table 6, the seroprevalences of FCV and FHV-1 were 58.3% (151/259) and 44.0% (88/200) by VN assay, respectively. The lower seroprevalences in vaccinated cats indicated an insufficient potency based on current commercial vaccines to against the challenge of wild strains of FPV, FCV, and FHV-1. The abroad commercial vaccines utilized in China could not resist the infection of the domestic wild strains. In the 39 serum samples of unvaccinated cats, the seroprevalences of FHV-1, FCV and FPV were 58.6% (17/29), 82.4% (28/34), and 76.9% (30/39), respectively (Table 6), higher than that reported in Milan (37.1%, 85.4%, and 45.7%, respectively.) (Dall’Ara et al., 2019). Another study in Beijing found that 47.6% of cats were FHV-1 positive by ELISA, but the vaccination status of these cats was unknown (Wang et al., 2014). The prevalence of FHV-1 and FPV in Costa Rica were 71.9% and 92.8%, while only 25% and 16.5% of them were previously vaccinated with FHV-1 and FPV, respectively (Blanco, Prendas, Corte, Jimenez, & Dolz, 2009). The seroprevalences of FHV-1, FCV and FPV were higher than the antigen positive rates, which might be caused by the recovery of cat and then the undetectable of the antigens of FHV-1, FCV and FPV.
In conclusion, six main viral infectious diseases were investigated in cat population of China, the most widespread virus in cat population is FeLV, followed by FPV, FHV, FCV, FIV and FIPV. The complexity of cat mix-infection in China suggested the big challenge for the diagnosis and treatment of these diseases. Our data also revealed that insufficient potency by immunization of current commercial vaccines could not give full protection to wild strains infections of FHV-1, FPV and FCV in China, demonstrating the urgency of improvement of immune strategies and the development of new vaccines.