Identification, Genetic Characterization, and Pathogenicity of Three Feline Herpesvirus Type 1 Isolates from Domestic Cats in China

(1) Background: Feline herpesvirus (FHV-1) is a significant pathogen in cats, causing respiratory and ocular diseases with consequential economic and welfare implications. (2) Methods: This study aimed to isolate and characterize FHV-1 from clinical samples and assess its pathogenicity. We collected 35 nasal and ocular swabs from cats showing symptoms of upper respiratory tract infection and FHV positivity detected by polymerase chain reaction (PCR). Viral isolation was carried out using feline kidney (F81) cell lines. Confirmation of FHV-1 presence was achieved through PCR detection, sequencing, electron microscopy, and indirect immunofluorescence assay. The isolated strains were further characterized by evaluating their titers, growth kinetics, and genetic characteristics. Additionally, we assessed the pathogenicity of the isolated strains in a feline model, monitoring clinical signs, viral shedding, and histopathological changes. (3) Results: Three strains of FHV-1 were isolated, purified, and identified. The isolated FHV-1 strains exhibited high homology among themselves and with domestic isolates and FHV-1 viruses from around the world. However, they showed varying degrees of virulence, with one strain (FHV-A1) causing severe clinical signs and histopathological lesions. (4) Conclusion: This study advances our understanding of the genetic and pathogenic characteristics of FHV-1 in China. These findings underscore FHV-A1 isolate as a potentially ideal candidate for establishing a challenge model and as a potential vaccine strain for vaccine development.

Advances in the immunoescape mechanisms exploited by alphaherpesviruses

Alphaherpesviruses, categorized as viruses with linear DNA composed of two complementary strands, can potentially to induce diseases in both humans and animals as pathogens. Mature viral particles comprise of a core, capsid, tegument, and envelope. While herpesvirus infection can elicit robust immune and inflammatory reactions in the host, its persistence stems from its prolonged interaction with the host, fostering a diverse array of immunoescape mechanisms. In recent years, significant advancements have been achieved in comprehending the immunoescape tactics employed by alphaherpesviruses, including pseudorabies virus (PRV), herpes simplex virus (HSV), varicella-zoster virus (VZV), feline herpesvirus (FeHV), equine herpesvirus (EHV), and caprine herpesvirus type I (CpHV-1). Researchers have unveiled the intricate adaptive mechanisms existing between viruses and their natural hosts. This review endeavors to illuminate the research advancements concerning the immunoescape mechanisms of alphaherpesviruses by delineating the pertinent proteins and genes involved in virus immunity. It aims to furnish valuable insights for further research on related mechanisms and vaccine development, ultimately contributing to virus control and containment efforts.

Construction and in vitro characterisation of virus-vectored immunocontraceptive candidates derived from felid alphaherpesvirus 1

There is a pressing need for effective feral cat management globally due to overabundant feline populations, disease transmission and their destructive impact on biodiversity. Virus-vectored immunocontraception (VVIC) is an attractive method for cat population management. Virus-vectored immunocontraceptives could be self-disseminating through horizontal transmission of the VVIC in feral cat populations, or they may be modified to act as non-transmissible vaccine-type immunocontraceptives for delivery to individual cats. These later constructs may be particularly attractive for use in owned (pet) cats and stray cats but could also be used for feral cats that are caught, vaccinated, and released. Here, we report the construction of three felid alphaherpesvirus 1 (FHV-1) derived immunocontraceptive candidates containing genes that encode for feline zona pellucida subunit 3 (ZP3) and gonadotropin-releasing hormone (GnRH). Two of the vaccine candidates were engineered to include disruptions to the thymidine kinase viral virulence gene to reduce the ability of the vaccines to be horizontally transmitted. Analysis of in vitro growth characteristics and protein expression are reported, and their potential for use as a population management tool for cats is discussed.

Immunogenicity evaluation of a bivalent vaccine based on a recombinant rabies virus expressing gB protein of FHV-1 in mice and cats

Feline viral rhinotracheitis (FVR) is caused by the feline herpesvirus-1 (FHV-1), which commonly results in upper respiratory symptoms, and can result in death in the kittens and weak cats. Rabies is an infectious disease with zoonotic characteristics highly relevant to public health and also poses a serious threat to cats. Vaccines are the most effective method to control the spread of both FHV-1 and RABV and have the advantage that they produce long-term specific immune responses. In this study, we constructed a bivalent vaccine against FHV-1 and rabies virus (RABV) simultaneously. The vaccine was constructed by cloning FHV-1 gB into a RABV based vector, and the recombinant RABV (SRV9-FHV-gB) expressing the FHV-1 gB protein was rescued. The growth characteristics of SRV9-FHV-gB were analyzed on NA and BSR cells. To assess the immunogenicity of the vaccine, mice and cats were immunized with SRV9-FHV-gB supplemented with Gel02 adjuvant. The SRV9-FHV-gB exhibited the same growth characteristics as the parent virus SRV9 in both BSR cells and NA cells. The safety of SRV9-FHV-gB was evaluated using 5-day-old and 14-day-old suckling mice. The results showed that mice infected with the SRV9-FHV-gB survived for longer than those in the SRV9 group. Mice immunized with inactivated SRV9-FHV-gB produced high titers of specific antibodies against FHV-1 and neutralizing antibodies against RABV. Cats that received three immunizations with SRV9-FHV-gB also produced neutralizing antibodies against both FHV-1 and RABV. This study represents the first time that a bivalent vaccine targeting FHV-1 and RABV has been constructed, laying the foundations and providing inspiration for the development of other multivalent vaccines.

Autophagy up-regulation upon FeHV-1 infection on permissive cells

FeHV-1 is a member of the Herpesviridae family that is distributed worldwide and causes feline viral rhinotracheitis (FVR). Since its relationship with the autophagic process has not yet been elucidated, the aim of this work was to evaluate the autophagy mediated by FeHV-1 and to determine its proviral or antiviral role. Our data showed that autophagy is induced by FeHV-1 in a viral dose and time-dependent manner. Phenotypic changes in LC3/p62 axis (increase of LC3-II and degradation of p62) were detected from 12 h post infection using western blot and immuno-fluorescence assays. In a second step, by using late autophagy inhibitors and inducers, the possible proviral role of autophagy during FeHV-1 infection was investigating by assessing the effects of each chemical in terms of viral yield, cytotoxic effects, and expression of viral glycoproteins. Our findings suggest that late-stage autophagy inhibitors (bafilomycin and chloroquine) have a negative impact on viral replication. Interestingly, we observed an accumulation of gB, a viral protein, when cells were pretreated with bafilomycin, whereas the opposite effect was observed when an autophagy inducer was used. The importance of autophagy during FeHV-1 infection was further supported by the results obtained with ATG5 siRNA. In summary, this study demonstrates FeHV-1-mediated autophagy induction, its proviral role, and the negative impact of late autophagy inhibitors on viral replication.

A review of evidence-based management of infectious ocular surface disease in shelter-housed domestic cats

Infectious ocular surface disease (IOSD) is a common problem in shelter-housed domestic cats and has a widespread negative impact on animal welfare. While the common etiological agents are well-described, addressing IOSD in large groups of animals presents a management challenge to the clinician and logistical challenges to shelter employees. Treatments, diagnostics, and prevention strategies that are effective in privately owned or experimental animals may be impractical or ineffective in the shelter environment. This review article focuses on the relative prevalence of etiological agents in feline IOSD, practical diagnostic testing protocols, prevention strategies, and treatment of IOSD in shelter-housed cats. Discrepancies between experimental laboratory-based studies and clinical trials assessing therapeutics for treatment of feline herpes virus are highlighted. Further high-quality clinical trials are necessary to determine optimal preventative and therapeutic protocols for IOSD in shelter-housed cats.

Modifications of the PI3K/Akt/mTOR axis during FeHV-1 infection in permissive cells

FeHV-1 is the causative agent of infectious rhinotracheitis in cats. The relationship between viral infection and the PI3K/Akt/mTOR pathway, as well as its function in crucial physiological processes like as autophagy, apoptosis or the IFN induction cascade is known for other varicelloviruses. However, there is no information on whether autophagy is activated during FeHV-1 infection nor on how this infection modifies PI3K/Akt/mTOR pathway. In this work, we aim to elucidate the involvement of this pathway during cytolytic infection by FeHV-1 in permissive cell lines. Using a phenotypic approach, the expression of proteins involved in the PI3K/Akt/mTOR pathway was examined by Western blot analysis. The findings demonstrated the lack of modifications in relation to viral dose (except for phospho-mTOR), whereas there were changes in the expression of several markers in relation to time as well as a mismatch in the time of activation of this axis. These results suggest that FeHV-1 may interact independently with different autophagic signaling pathways. In addition, we found an early phosphorylation of Akt, approximately 3 h after infection, without a concomitant decrease in constitutive Akt. This result suggests a possible role for this axis in viral entry. In a second phase, the use of early autophagy inhibitors was examined for viral yield, cytotoxic effects, viral glycoprotein expression, and autophagy markers and resulted in inefficient inhibition of viral replication (12 h post-infection for LY294002 and 48 h post-infection for 3-methyladenine). The same markers were examined during Akt knockdown, and we observed no differences in viral replication. This result could be explained by the presence of a protein kinase in the FeHV-1 genome (encoded by the Us3 gene) that can phosphorylate various Akt substrates as an Akt surrogate, as has already been demonstrated in genetically related viruses (HSV-1, PRV, etc.). For the same reasons, the use of LY294002 at the beginning of infection did not affect FeHV-1-mediated Akt phosphorylation. Our findings highlight changes in the PI3K/Akt/mTOR pathway during FeHV-1 infection, although further research is needed to understand the importance of these changes and how they affect cellular processes and viral propagation.

Generalized and fatal felid alphaherpesvirus-1 natural infection with liver involvement in a feline leukaemia virus-positive adult cat: a case report

Generalized and fatal felid alphaherpesvirus-1 (FeHV-1) natural infection with liver involvement is rarely reported in cats, and the occurrence of herpesvirus viraemia with internal organ histologic lesions in adult cats is unknown. A 1.5-year-old cat, female, mixed breed, positive for feline leukaemia virus (FeLV) presented in a veterinary teaching hospital with sneezing, nasal discharge, anorexia, and diarrhoea after two weeks, evolving to inspiratory dyspnoea. Complete blood count and serum biochemistry analysis showed marked leukopenia and thrombocytopenia. After clinical worsening and lack of treatment response, the cat was euthanized. Pathological findings included hepatic necrosis, fibrinonecrotic tracheitis, and bronchointerstitial pneumonia. Marked amounts of coccobacillary bacteria were observed covering the necrotic tracheal and bronchial mucosa, at the cytoplasm of alveolar macrophages, and free in alveoli lumen, mimicking a primary bacterial tracheitis and pneumonia. Both lung and tracheal bacteria exhibited marked immunolabeling in anti-Escherichia coli immunohistochemistry. In addition, rare epithelial cells of bronchi contained round, eosinophilic, intranuclear viral inclusion bodies (4-7 µm) that marginate the chromatin, characteristic of FeHV-1 infection. Strong multifocal anti-FeHV-1 immunolabeling was observed in necrotic epithelial cells of the liver, trachea, and lungs. Generalized herpesvirus infection with the occurrence of acute hepatic necrosis and severe respiratory illness is a potential differential diagnosis in FeLV-positive cats with respiratory signs. The immunodepression in these cats probably favours a FeHV-1 viraemia in addition to the development of opportunistic bacterial infections, such as Escherichia coli, and it is associated with a poor outcome.

Constrictive bronchiolitis obliterans with a presumptive etiology of preceding feline herpesvirus infection in a cat

Background

Bronchiolar disorders are rarely recognized in cats. Constrictive bronchiolitis obliterans is characterized by concentric peribronchiolar fibrosis and inflammation of the bronchioles, but the underlying causes remain poorly understood in current small animal medicine.

Case presentation

A 9-year-old cat presented with paroxysmal tachypnea, infrequent cough and persistent labor breathing. Thoracic radiography showed lung hyperinflation and bronchointerstitial pattern, and pulmonary function assessment revealed flow limitation in the late-expiratory phase and poor response to short-acting bronchodilator. Dorsally distributed subpleural ground glass opacities with distinct margin and tree-in-bud opacities were observed on lung high-resolution computed tomography. The cat underwent bronchoalveolar lavage (BAL) and showed severe neutrophilic inflammation. Feline herpesvirus was the only pathogen detected in the BAL fluid. Multiple therapeutic attempts were unsuccessful and the cat died 8 weeks after the initial presentation. Necropsy revealed the infiltration of inflammatory cells, obstruction of the bronchiolar lumen, and submucosal concentric fibrosis suggesting constrictive bronchiolitis obliterans. Combining the pre- and post-mortem findings, as well as the time from symptom onset or BAL to necropsy, constrictive bronchiolitis obliterans was possibly triggered by a preceding feline herpesvirus infection in this case.

Conclusions

The history of nonvaccinated status, lower airway neutrophilic inflammation, and presence of feline herpesvirus in the BAL fluid without coexistence of other pathogens led to the presumption that constrictive bronchiolitis obliterans was induced by a preceding feline herpesvirus infection in this cat. The pathological changes of bronchiolitis obliterans induced by a preceding feline herpesvirus infection could be different from that of cats with acute herpesvirus pneumonia, such as intranuclear inclusions would disappear over time and were no longer found 7–10 days after inoculation. The presence of patchy distribution of subpleural ground glass opacities on lung high-resolution computed tomography should raise the suspicion of peribronchiolar fibrosis. Clinical awareness of bronchiolar disorders as a differential diagnosis is important in cats with lung hyperinflation and labored breathing who show poor reversibility to bronchodilator.

Feline herpesvirus infection and pathology in captive snow leopard

Feline herpesvirus type 1 (FHV-1) is a common causative agent of domestic cats’ rhinotracheitis in domestic cats, and it increasingly threatens wild felids worldwide. The endangered snow leopard (Panthera uncia) belongs to the family Felidae, and it is the top predator on the Tibetan Plateau. Here we report the identification and isolation of FHV-1 from three dead captive snow leopards that presented with sneezing and rhinorrhea. To explore the relationship between FHV-1 and their deaths, organs and nasal swabs were collected for histopathology, viral isolation and sequence analysis. The results revealed that all three snow leopards were infected with FHV-1. The first animal died primarily of cerebral infarction and secondary non-suppurative meningoencephalitis that was probably caused by FHV-1. The second animal died mainly of renal failure accompanied by interstitial pneumonia caused by FHV-1. The cause of death for the third animal was likely related to the concurrent reactivation of a latent FHV-1 infection. The gD and gE gene sequence alignment of the isolated FHV-1 isolate strain revealed that the virus likely originated from a domestic cat. It was found that FHV-1 infection can cause different lesions in snow leopards than in domestic cats and is associated with high risk of disease in wild felids. This suggests that there should be increased focus on protecting wild felids against FHV-1 infections originating from domestic cats.

First Report of Isolation and Molecular Characterization of Felid Herpesvirus-1 from Symptomatic Domestic Cats in Egypt

Feline herpesvirus 1 (FHV-1) is one of the main causes of upper respiratory tract infection in cats. Despite its veterinary importance, no previous studies investigated the occurrence of this virus in Egypt. In the present work, a total number of one hundred forty (N = 140) conjunctival and/or oropharyngeal swabs were collected from symptomatic cats during veterinary clinic visits located in two Egyptian provinces. Virus isolation was performed in the Chorioallantoic membranes (CAMs) of 12-days-old SPF eggs. Interestingly, the embryos showed stunting growth and abnormal feathering and infected CAMs showed edematous thickening and cloudiness with characteristic white opaque pock lesions. Polymerase chain reaction (PCR) amplification of the thymidine kinase gene (TK) was successful in 16/140 (11.4%) of the suspected cases. Two of the amplified genes were sequenced and the TK gene sequences of the FHV-1 isolates were highly similar to other reference strains in the GenBank database. Given the above information, the present study represents the first report of feline herpesvirus type 1 (FHV-1) in domestic cats in Egypt. Further studies on the causes of upper respiratory tract infections in cats as well as vaccine efficacy are needed.

Causes and Lesions of Fatal Pneumonia in Domestic Cats

Pneumonia in cats may cause severe lung injury and consequent death. We describe the post-mortem findings and aetiologies of naturally fatal pneumonia in 78 domestic cats, using gross and histopathological examinations, immunohistochemistry and microbiological techniques. Morphological patterns found were bronchopneumonia (27/78), interstitial (15/78), bronchointerstitial (13/78), granulomatous (8/78), aspiration (8/78) and pyogranulomatous (5/78) pneumonia, and pleuropneumonia (2/78). Bacterial pneumonia was identified as the most common cause (32/78), followed by viral (15/28 feline calicivirus, 10/28 felid alphaherpesvirus 1 and 3/28 both viruses), aspiration (8/78), fungal (5/78) and parasitic pneumonia (5/78). Co-infection with feline immunodeficiency virus and feline leukaemia virus was found in 54 cats. Viral infections involved cats of all ages, indicating the importance of investigating viral causes in cats with respiratory diseases, including in adult and ageing cats.

Use of feline herpesvirus as a vaccine vector offers alternative applications for feline health

Herpesviruses are attractive vaccine vector candidates due to their large double stranded DNA genome and latency characteristics. Within the scope of veterinary vaccines, herpesvirus-vectored vaccines have been well studied and commercially available vectored vaccines are used to help prevent diseases in different animal species. Felid alphaherpesvirus 1 (FHV-1) has been characterised as a vector candidate to protect against a range of feline pathogens. In this review we highlight the methods used to construct FHV-1 based vaccines and their outcomes, while also proposing alternative uses for FHV-1 as a viral vector.

Detection and molecular characterisation of feline viruses from swab samples

Feline calicivirus (FCV), feline alphaherpesvirus 1 (FHV-1) and feline panleukopenia virus (FPLV) as well as retroviral agents such as feline leukaemia virus (FeLV) and feline immunodeficiency virus (FIV) are important viral pathogens of cats. The aim of this study was to detect and characterise FHV-1, FPLV, FeLV, FIV and feline foamy virus (FFV) in oropharyngeal, nasal and conjunctival swabs from 93 cats that had been screened for FCV previously. We wanted to determine the possible risk factors for infection with these viruses. The prevalence was found to be 12.9% for FHV-1 and 9.7% for FPLV. FIV was detected only in two samples and FeLV in one sample, whereas the presence of FFV was not demonstrated in any of the clinical samples. The statistical analysis of the results showed that breed, age, health status, and lifestyle are important predisposing factors to FHV-1 (P < 0.05). For FPLV, only clinically unhealthy animals were found to be at risk (P < 0.001). Sequence analysis revealed that the two FIV-positive samples in this study contained different (A and B) subtypes of the virus. This is the first report on the occurrence of subtype A FIV in Turkey.

A retrospective study on the presence of selected infectious agents in lung samples of cats with pneumonia

The causative role of some infectious agents found in cases of feline pneumonia is under debate, because they are also part of the physiological microbiota of the respiratory tract of healthy animals. In this retrospective study, archived formalin-fixed and paraffin-wax-embedded lung samples of 69 severe and lethal cases of pneumonia in cats were examined by immunohistochemistry (IHC) for the detection of nine selected infectious agents: Pasteurella multocida, Bordetella bronchiseptica, Mycoplasma felis, M. gateae, Chlamydia felis, feline herpesvirus type 1, feline coronavirus, canine distemper virus, and Toxoplasma gondii. The intention was to elucidate their immediate involvement in pneumonia formation. Due to the cross-reactivity of the applied antibodies, a species-specific polymerase chain reaction (PCR) for both targeted Mycoplasma species was applied additionally. In the 42 cases (60.9%) positive for at least one pathogen, several agents were present in a high proportion of the samples (P. multocida - 34.8%, B. bronchiseptica - 29.0%), while others were present in a moderate (feline herpesvirus type 1 - 18.8%, M. gateae - 13.0%, M. felis - 10.1%) or low percentage (T. gondii - 1.4%). All samples were negative for C. felis, feline coronavirus and canine distemper virus. Mixed infections of up to four pathogens were more frequent than single infections. Mycoplasma preferably colonised lung tissue damaged by other pathogens because they never occurred as single infections. Pasteurella multocida, B. bronchiseptica, M. felis, feline herpesvirus type 1 and T. gondii showed abundant replication within lung lesions, thus suggesting a prominent role in pneumonia formation.

Characterization of feline herpesvirus-1 deletion mutants in tissue explant cultures

Feline herpesvirus-1 (FHV-1) is the primary cause of viral respiratory and ocular disease in cats. While commercial vaccines can provide clinical protection, they do not protect from infection or prevent latency. Moreover, they are not safe for intranasal administration. Our overall objective is to develop a new mucosal vaccine against FHV-1 disease to address these shortcomings. Feline herpesvirus-1 deletion mutants of glycoprotein C (gC-), gE (gE-), US3-encoded serine/threonine protein kinase (PK-), and both gE and thymidine kinase (gE-TK-) were generated by bacterial artificial chromosome (BAC) mutagenesis. Tracheal tissue explants from eight cats were used to compare the pattern of viral infection and associated tissue damage, as well as virus spread through the basement membrane following inoculation with wild-type virus (WT), and gE-, gE-TK-, PK-, and gC- mutants. Tissues were collected at 24, 48, or 72  hours post-inoculation (hpi) followed by immunohistochemistry (IHC) for FHV-1. Histological changes were graded based on the distribution of virus infected cells and the severity of tissue damage. Inoculations with the WT virus resulted in maximal scores at 72 hpi both at a multiplicity of infection (MOI) of 1 and 0.1. Inoculation with the gE- mutant produced scores similar to scores of explants inoculated with the WT virus at 24 and 48 hpi, but scores were significantly decreased at 72 hpi. Explants inoculated with the gE-TK- mutant showed significantly decreased scores at all time points. Further, the majority of explants inoculated with the PK- mutant resulted in scores of zero at all time points, regardless of MOI. Finally, inoculation with WT resulted in significant stromal invasion below the infected epithelium, while stromal invasion was observed in less than 50 % of the samples following inoculation with gE-, gE-TK-, PK-, or gC- mutants and confined closely to the area surrounding the infected epithelium. In conclusion, the gE-TK- and PK- mutants exhibited significantly reduced virulence, tissue damage and spread to the underlying stroma, suggesting that they may be good vaccine candidates for in vivo testing.

Prevalence of serum antibody titres against feline panleukopenia, herpesvirus and calicivirus infections in stray cats of Milan, Italy

The aim of the study was to determine the seroprevalence of feline panleukopenia virus (FPV), feline herpesvirus type 1 (FHV-1) and feline calicivirus (FCV) in stray colony cats from Milan, Italy. Cats were divided in groups based on age, gender, reproductive status, health status and colony of origin. Blood samples were tested with an in-clinic ELISA test. The possible presence of a link between the antibody titre or the presence of seropositive results and the independent variables (age, gender, reproductive status, health status and colony location) was assessed by means of multinomial and univariate logistic regression models, respectively. Seroprevalence of 85.4% was reported for FCV. The diffusion of the other two pathogens in the cat population was much lower compared to FCV, with 45.7% and 37.1% seroprevalence observed for FPV and FHV-1, respectively. An increase of antibody titres from kitten to senior was generally observed for the three pathogens. Age was a statistically significant variable for FHV-1, with senior cats significantly associated with higher antibody titres and higher percentages of seropositive animals compared to younger age groups. Neutered cats had significantly higher antibody titres and showed significantly higher FHV-1 seroprevalences compared to sexually intact cats. Colonies from two of the nine administrative districts of Milan showed significantly higher FPV seroprevalences compared to the others. No other significant differences were observed. Our results, based on cats belonging to 70 different colonies located in urban areas far from each other, suggest that the three viruses circulate in the feline population of stray cats in Milan. The feline calicivirus represents the most common circulating pathogen, as observed also in other studies worldwide. Finally, our results suggest that stray cats may be not adequately protected against FPV, FHV-1 and FCV and vaccination could be a possible strategic solution, especially for FPV.

Calicivirus co-infections in herpesvirus pneumonia in kittens

Felid herpesvirus-1 (FeHV-1) and feline calicivirus (FCV) are the most important infectious causes of respiratory disease in cats. FeHV-1 and FCV co-infections are common in cats with upper respiratory tract disease, but it is unknown whether such co-infections also occur in cats with pneumonia. This study examined the lungs of naturally infected cats with FeHV-1 pneumonia for FCV co-infection by histopathology and immunohistochemistry. The frequency of FCV (13/21, 62%) in this group of cats suggests that co-infection is common in kittens with FeHV-1 pneumonia. FCV infected macrophages were often found in the lumen of FeHV-1 affected airways. In 8/13 (62%) cats, typical FCV lesions were distant from changes induced by FeHV-1. FCV infection of type II pneumocytes/alveolar macrophages was apparent in histologically unaltered areas. It is likely that damage to airways induced by FeHV-1 facilitates secondary infection with FCV due to reduced mucociliary clearance and impaired immune defences.