Clinical Features and Epidemiology

The clinical presentation of infectious bronchitis depends on the age, genetic background, and immune status of the bird at the time of infection, route of exposure, nutritional factors (especially levels of calcium in the diet), virulence of the virus strain, and the presence of stressors such as cold temperatures, poor ventilation or secondary bacterial pathogens. Outbreaks may be explosive, with the virus spreading rapidly to involve the entire flock within a few days. The incubation period is typically brief: 18–48 hours. In chicks 1–4 weeks of age, virulent virus strains produce severe respiratory disease, with gasping, coughing, tracheal rales, sneezing, nasal exudate, wet eyes, respiratory distress, and, occasionally, swollen sinuses. Mortality in young chicks is usually 25–30%, but in some outbreaks can be as high as 75%. Less virulent strains cause fewer and milder respiratory signs, and lower morbidity and mortality rates. Infection of young female chicks may result in permanent hypoplasia of the oviduct that is evident later in life as reduced egg production and inferior quality eggs.

When the disease is uncomplicated by opportunistic bacterial superinfection, respiratory signs last for 5–7 days and disappear from the flock in 10–14 days. High mortality can occur in broilers as a result of secondary infection with Escherichia coli or pathogenic mycoplasmas. Egg-laying chickens usually present with reproductive tract involvement that is manifest as a decline or cessation in egg production or, less consistently, respiratory disease. When laying resumes, many eggs are abnormal, including lack of calcified shell, thin shells, and shells with stipples, distortions, dimples, depressions, or ridging; eggs that should be colored are often pale or white, and egg albumen may be watery. In acutely infected birds, the kidneys can be pale and swollen, with urates distending the ureters, and in the chronic phase there can be atrophy of kidney lobules, with large calculi within the ureters (urolithiasis).

Infectious bronchitis virus spreads between birds by aerosol and by ingestion of food contaminated with feces. In the environment, the virus can survive on fomites for several days and possibly for weeks, especially at low environmental temperatures. Outbreaks of infectious bronchitis have declined in recent years as a result of the extensive use of vaccines; however, the disease may occur even in vaccinated flocks when immunity is waning, or upon exposure to variant virus serotypes, with the first variant strains emerging in the 1940s and new variants continue to emerge today. To minimize this risk, most poultry producers obtain 1-day-old chicks from maternal antibody-positive breeders and then spray-vaccinate them with live-attenuated vaccine in the hatchery, with additional boosts by live-attenuated and/or inactivated vaccines. The current trend in “free-range” and backyard poultry production is likely to lead to a resurgence of infectious bronchitis.

Pathogenesis and Pathology

The virus replicates to high titer first in the respiratory tract (ciliated epithelial cells); this is followed by viremia (within 1–2 days of infection), which distributes the virus to many organs. The virus can cause extensive damage to the ovaries, oviduct, and the kidneys, but this is dependent on the properties of individual virus strains. The intestinal tract is another site of primary infection, but damage usually is minimal.

Infectivity declines rapidly, and isolation of virus beyond 7 days after infection is uncommon (except from chicks). Rarely, virus has been reported to persist for up to 14 weeks in cecal tonsils, and has been recovered from the feces for up to 20 weeks after infection. Kidney and intestine are the likely sites of virus persistence.

The most frequent gross pathologic finding is mucosal thickening within the upper and lower respiratory tract, with serous or catarrhal exudate in the nasal passages, trachea, bronchi, and airsacs. In very young chicks, the main bronchi may be blocked with caseous yellow casts. Pneumonia and conjunctivitis occur in some cases. In laying birds, ova can be congested and sometimes ruptured, with free yolk in the abdominal cavity. Desquamation of respiratory epithelium, edema, epithelial hyperplasia, mononuclear cell infiltration of the submucosa, and regeneration occur in various combinations. Repair processes begin after 6–10 days, and are complete in 14–21 days. Some virus strains affect the kidney, causing interstitial nephritis.

Diagnosis

Tracheal swabs or fresh samples of trachea are most useful for virus detection or isolation. Direct immunofluorescence staining of tracheal tissue smears is useful in the diagnosis of early cases before secondary bacterial infection has occurred. For virus isolation, embryonated chicken eggs are inoculated via the allantoic sac route. Infectious bronchitis virus does not typically infect cells in culture, although primary chick kidney cells can propagate the virus. Changes suggestive of the presence of a coronavirus include congestion of the main blood vessels in the chorioallantoic membrane and embryo stunting, curling, clubbing of down, or urate deposits in the mesonephros. Identification of virus in the chorioallantoic membrane is usually done by immunofluorescence or immunohistochemical staining, or in allantoic fluid by serological methods, nucleic acid analysis, or electron microscopy. Isolates are usually typed and subtyped by serologic methods and nucleic acid analyses such as genotype-specific RT-PCR assays.

Immunity, Prevention, and Control

Infection induces IgM, IgG, and IgA antibodies. In immune laying hens, the ovum begins to acquire IgG antibody (some of it virus specific) from the blood about 5 days before the egg is laid. As it becomes surrounded with albumen during passage down the oviduct, the ovum acquires both IgM and IgA antibodies, which are transferred into the amniotic fluid about halfway through development. During the last third of embryonation, IgG enters the circulation from the yolk; antibody can inhibit virus replication at this time. The chick hatches with a circulating IgG level similar to that of the hen. IgG antibody is metabolized with a half-life of approximately 3 days and may persist for 3–4 weeks. The virus may survive until passive immunity declines to a level at which it can replicate again, at which time the chicken mounts an active immune response. However, the correlates of active immunity to infectious bronchitis virus are less certain. Neutralizing antibodies can prevent virus dissemination from the respiratory tract and block secondary infection of the reproductive tract and kidneys. The adaptive transfer of CD8 T lymphocytes protects chicks against infectious bronchitis virus challenge, suggesting a role for cellular immunity as well in protection.

Live-attenuated virus vaccines are widely used to protect meat chickens. These vaccine viruses are derived by serial passage in embryonated chicken eggs. They are administered in drinking water, by coarse spray, or by deposition on the conjunctiva (eye drops). The first vaccination is typically given in the hatchery when birds are 1 day old, and booster vaccination is given at 10–18 days. Passively acquired maternal immunity prevents respiratory infection and disease for the first 7 days. For layers or breeders, live-attenuated vaccines are used for priming, followed by killed oil-adjuvanted booster vaccines, often given repeatedly during the laying cycle. Vaccination breaks occur because of the variable presence of new antigenic variants and existence of several serotypes. Such variants will continue to emerge and spread, posing continuing problems for poultry producers.

Control of infectious bronchitis is difficult because of the presence of persistently infected chickens in some flocks and the continuing emergence of antigenically variant viruses. The domestic chicken is the primary and most important host, but infections and disease have been described in pheasants infected with a closely related coronavirus. Sporadic or individual cases of avian infectious bronchitis virus infection also have been described in peafowl, teal, partridge, and guinea fowl. Avian coronaviruses related to infectious bronchitis virus have also been identified in many wild bird species, but these are typically found in the gastrointestinal tract.

Clinical Features and Epidemiology

Feline infectious peritonitis is a common progressive, debilitating and lethal disease of domestic and wild members of the family Felidae. Disease typically occurs in young or very old cats, or in the context of immune suppression. The initial clinical signs are vague, and affected cats present with anorexia, chronic fever, malaise, and weight loss. Ocular and/or neurological manifestations occur in some individuals. In the classical wet or effusive form of feline infectious peritonitis, these signs are accompanied by progressive abdominal distention from the accumulation of a highly viscous fluid in the peritoneal cavity and rapid disease progression, with death typically within weeks to months. The dry or noneffusive form of the disease, with little or no peritoneal exudate, is more slowly progressive. The wet and dry forms of feline infectious peritonitis are different manifestations of the same infection, and both forms of the disease are characterized by foci of pyogranulomatous inflammation in several organs.

The following is a proposed scenario of fatal feline infectious peritonitis. A kitten suckling a seropositive queen is protected by colostral antibody against enteric coronavirus infection during the first few weeks of life. As maternal antibody wanes, the kitten becomes infected during an episode of maternal shedding of feline enteric coronavirus. The kitten now develops an active immune response, but in most cases not a sterilizing response, and a persistent viral infection of the gut with chronic fecal shedding is established. Virus and antibodies coexist in the kitten, but the infection is modulated by an efficient cellular immune response that keeps virus replication in infected macrophages and monocytes in check. The animal may remain healthy, but becomes susceptible to development of feline infectious peritonitis should it become stressed or immunosuppressed. Viral mutants then emerge, with rapid selection and proliferation of macrophage-tropic variants that cause the development of feline infectious peritonitis.

Pathogenesis and Pathology

The key initiating pathogenic event in feline infectious peritonitis is the productive infection of monocytes and macrophages by genetic variants (mutants) of the original enteric coronavirus. Experimentally, the virulence of strains of feline enteric coronavirus has been correlated with their capability of productive infection of cultured peritoneal macrophages, with avirulent strains infecting fewer macrophages and producing lower virus titers than virulent strains. Avirulent strains are also less able to sustain virus replication and spread between macrophages. Mutations within the spike (S) and, potentially, other proteins alter the tropism of the ubiquitous avirulent feline enteric coronavirus to macrophages, which then allows the virus to spread and ultimately to cause feline infectious peritonitis. The most consistently occurring mutations appear to be within the cleavage-activation and fusion domains of spike, and within the 3C accessory gene. Affected cats typically produce a strong antibody response that is ineffective in eliminating the virus, and cellular immune responses are unable to prevent virus replication in macrophages.

The lesions in feline infectious peritonitis are characteristically centered on small blood vessels, and vascular injury and leakage are central to the pathogenesis of the wet form of the disease. However, there is uncertainty regarding the pathogenetic mechanisms involved, as there is increasing evidence that vascular injury is not simply the result of immune complex deposition in the walls of the affected vessels, as was once proposed. The central role of viral infection of macrophages, however, is clear, and perivascular clusters of virus-infected macrophages are characteristically present in the tissues of cats with both the wet and dry forms of feline infectious peritonitis. Despite the inability of macrophages to prevent virus from replicating in them, infection of macrophages probably leads to their activation, with production of inflammatory mediators including cytokines and arachidonic acid derivatives (leukotrienes and prostaglandins). These mediators probably contribute substantially to the disease process, as these host-response molecules induce changes in vascular permeability and provide chemotactic stimuli for neutrophils and monocytes that further contribute to the inflammatory response. Both intravascular and recently emigrated monocytes and macrophages probably serve as new virus targets, thereby amplifying the infection further. The end result is enhanced local virus production, increased tissue damage, and a strong but ineffective host immune response.

Humoral immunity is not protective, and may actually enhance disease progression. Antibody-dependent enhancement of infection of macrophages is apparently mediated by neutralizing antibodies to the S protein, making vaccine development problematic. Cats that are seropositive to feline enteric coronavirus, either from natural infection or via purified IgG antibodies transfused into uninfected animals, develop an accelerated, fulminant disease when challenged experimentally with virulent feline coronavirus (so-called feline infectious peritonitis virus). Clinical signs and lesions develop earlier, and the mean survival time is reduced as compared with seronegative cats.