Although retroviral diseases in cats have diverse manifestations, most of them are associated with immunosuppression, anaemia and lymphoma [6, 7, 10]. A low packed cell volume (PCV), detected in this study, is a maker of anaemia, which is a typical symptom of FeLV-infected cats. This study is the first to show a significant association between WSH and FeLV infection. The specificity and positive predictive value of WSH to detect the presence of FeLV infection (presence of FeLV antigen in the blood) were high, indicating that sinus hair changes may be a distinct external marker for FeLV-infected cats. Another important feline retroviral disease, feline immunodeficiency virus (FIV) infection [7], was not associated with WSH.
FeLV infection is commonly diagnosed using blood-based antigen detection [11]. However, because the symptoms of FeLV infection vary and the disease progresses slowly [10], it is difficult for owners to notice abnormalities in their cats. In addition, cats are often kept in large numbers, and detecting infection at an early stage is important to prevent its spread. Therefore, the findings of this study, i.e., that the easily observable sinus hairs can be an external finding suggestive of FeLV infection, are an important discovery for the control of FeLV. However, the sensitivity was slightly low at 68.4%. This could be attributed to the detection of WSH in some of the FeLV-negative cases (six cases). These six cases were elderly and had chronic renal failure (two cases), hemobartonellosis (one case), and non-specific diseases (three cases). The reason for the presence of WSH in these six cases is unknown.
Multivariate analysis showed that FeLV infection was associated with lethargy and appetite, sex (lower incidence in castrated males than in intact males), and weight. It was speculated that the general condition and tendency toward to become anaemic affected the nutritional status and activity, which enhanced the negative effects of FeLV infection. Regarding the differences in sex, FeLV is more common among stray cats [12, 13], and intact males are at a greater risk of horizontal infection due to fighting. Regarding body weight (BW) (heavier weight was associated with increased incidence of FeLV), the effect of this variable is not clear; however, the population analysed included cats of various ages and breeds, and younger and smaller cats were relatively more likely to be FeLV-negative.
FeLV is transmitted among individuals through body fluids (saliva, tears, urine) [6, 7, 10, 12-14]. The virus-infected cells in the body are mainly blood cells (lymphocytes and monocytes) [15, 16] and the mechanisms of virus extravasation remain unclear. In this study, viral antigens were abundant in epithelial cells of the proboscis tissue, confirming the importance of these cells as a source of viral efflux into saliva [18]. Viral antigens were also detected in sebaceous and sweat glands, suggesting that FeLV may be excreted in exocrine fluids such as sebum and sweat.
The mononuclear cells infiltrating into the dermis consisted of a mixture of T lymphocytes, B lymphocytes, and macrophages (confirmed by immunostaining, data not shown). The significance of these infiltrating cells, i.e., the cause of infiltration, is unknown; however, they may represent a reactive change to virus-infected cells or a complication of mild stomatitis or dermatitis. No neoplastic lymphocyte proliferation (lymphoma) was observed in the cases analysed in the present study.
In this study, FeLV antigens were detected in the follicular epithelial cells of sinus hairs. Three FeLV antigens (p27, gp70 and p15E) were tested by IHC. The viral core protein p27 is a common target for FeLV detection [19, 20]. Among the three antigens, p27 was the most frequently detected. The envelope protein gp70 was also detected. p15E is a transmembrane protein that forms the envelope. Gp85, the precursor protein, is enzymatically cleaved to generate gp70 and p15E. In this study, p15E was detected at low levels, although the reason for the low detection remain unclear. The gp70 antibody used in this study cross-reacts with the precursor protein gp85 [21], whereas the p15E antibody detects only p15E [22], which may explain the difference in the detection levels of the two proteins. Interestingly, p27 was also detected in skeletal muscle fibers in 35% (5 cases) of FeLV-positive cases in blood. There have been no previous reports of FeLV infection in skeletal muscle. However, it has been reported that FeLV-A enters cells using feline thiamine transport protein 1 (feTHTR1), which is distributed throughout the body, as a receptor, and that feTHTR1 is also slightly expressed in skeletal muscles [23]. The significance of this finding remain unknown and is an issue for further study.
In a preliminary study with a few cases, virus-like particles were detected in the epithelium of sinus hair of FeLV-positive cat by ultrastructural analysis (transmission electron microscopy; data not shown). This suggests that the infected viruses replicate in the sinus hair follicular epithelium and produce progeny virion. However, there was no morphological degeneration or necrosis of the cells comprising the sinus hair follicular walls, suggesting that the wavy changes in sinus hairs caused by FeLV infection were not due to injury to the follicular wall. FeLV antigens were also observed in the skin hair follicular walls, although no abnormalities such as alopecia or wavy changes were observed in the whole body coat. This suggests that the effect of FeLV infection in the hair shaft may be limited to sinus hairs. The tendency of WSH to bend at multiple sites suggests that the wavy changes occurred as a result of the intermittent long-term effects of FeLV infection. FeLV infection occurs in various stages with different outcomes [10, 24]. For example, during viraemic stages, viral particles are expelled from the body, whereas at other stages, the viral genome remains in bone marrow cells after transient viremia. The findings of this study in combination with conventional FeLV testing methods may provide insight into various FeLV infection conditions. The detailed mechanism underlying the association of WSH with FeLV infection remains unclear.
A study analyzing the association between viral infection and sinus hair reported the detection of rabies virus in the sinus hair follicular wall of rabies-affected animals [25]. The rabies virus has been detected in Merkel cells, which are specialized sensory receptor cells in the sinus hair wall, and it could be involved in the dense nerve distribution in these cells. Another study reported that murine leukaemia virus can proliferate in the follicular wall of the skin [26]; however, the results were obtained by experimental subcutaneous virus inoculation of newborn mice, and the significance of this is unknown.