THE GENUS HAEMOPHILUS

Haemophilus spp. are assigned to the family Pasteurellaceae and are common commensal organisms of the mucous membranes of animals and humans. They are highly susceptible to desiccation and, in consequence, cannot survive for long periods in the environment. Most species are opportunists in stressed or compromised hosts, but some, such as Haemophilus parasuis in swine, cause more serious disease. Most animals and birds harbor at least one species-specific strain (Table 24-1). Human disease caused by species other than Haemophilus influenzae has, with some exceptions, been considered unusual.

TABLE 24-1 Haemophilus Species of Veterinary Significance

All members of the genus Haemophilus are small, pleomorphic, facultatively anaerobic, nonmotile, gram-negative rods to coccobacilli that may form filaments during periods of environmental stress. The genus name is in reference to the fact that these organisms require factors X (hemin) and/or V (NAD) in blood for growth. Species designated with the prefix “para” require only V factor. Optimal growth is obtained in an atmosphere of 5% to 10% CO₂. The most uniformly satisfactory medium for propagation of Haemophilus species is chocolate agar because it provides both X and V factors. On blood agar, Haemophilus colonies cluster around a Staphylococcus streak line in a phenomenon called satellitism. Phenotypic characteristics allow differentiation of Haemophilus spp. (Table 24-2).

TABLE 24-2 Differential Characteristics of Haemophilus Species

HAEMOPHILUS PARAGALLINARUM

Haemophilus paragallinarum is the agent of infectious coryza, an upper respiratory disease of laying and growing chickens worldwide. Disease is exacerbated by concurrent mycoplasmal or viral infections and, in those situations, mortality may be high. A characteristic swelling of the infraorbital region, oculonasal discharge, swollen wattles, diarrhea, and inappetence are common clinical signs (Figure 24-1). Losses result from decreased feed consumption, which impacts egg and meat yield. Unusual clinical presentations in developing countries include arthritis and septicemia. Lesions observed at necropsy are associated with catarrhal inflammation of the respiratory tract, and include sinusitis with congestion, edema, and sloughing. Disease transmission is via the respiratory route or through contact with contaminated drinking water. Susceptible birds exposed to the agent usually develop clinical signs within 3 days. Birds that recover from acute illness may be a source of infection for young chicks that become susceptible 4 weeks after hatching. Serogroups A, B, and C of H. paragallinarum are currently recognized, with four serovars in groups A and C. Infectious coryza has been infrequently diagnosed in pheasants, guinea fowl, quail, and companion birds. Ducks, turkeys, and pigeons are refractory to experimental infection.

FIGURE 24-1 Chicken infected with Haemophilus paragallinarum. Note swollen infraorbital region and visible purplish discoloration of the skin.

(Courtesy H.M. Opitz.)

There is a paucity of information on the pathogenesis of infectious coryza. Adherence to and colonization of the nasal mucosa is apparently the initial step in pathogenesis. Potential virulence attributes of H. paragallinarum include a polysaccharide capsule, which may mediate attachment of the organism to cilia of the nasal mucosa. The hyaluronic acid component of the capsule and the lipopolysaccharide in the cell wall may contribute to pathogenesis, but these interactions with the host have not been clearly defined. The principal lesion is acute catarrhal inflammation of the upper respiratory tract, primarily the nasal cavity and paranasal sinus. Mast cell infiltration into the lamina propria of the mucous membrane of the nasal cavity follows. Mast cells may be responsible for the clinical signs of infectious coryza, through the activation of inflammatory mediators.

Diagnosis of infectious coryza is based on the characteristic clinical signs (facial swelling), recovery of the bacterium from clinical materials, and results of serologic testing. Although fastidious, H. paragallinarum is easily recovered from the sinuses of affected birds in the acute stage of infection, by bacteriologic culture on chocolate agar. The strain characterized originally apparently required both X and V factors, but the current representatives of H. paragallinarum require only V factor, and V-factor independent strains have been recovered from chickens in South Africa. Differentiation of the pathogenic H. paragallinarum from Haemophilus-like commensals of the avian upper respiratory tract can be based on phenotypic differences (Table 24-3). Several serologic assays allow detection of antibodies to H. paragallinarum, and the test of choice appears to be hemagglutination inhibition, which detects both vaccinal titers and those resulting from infection. A polymerase chain reaction (PCR) test allows rapid diagnosis of coryza but has limited availability.

TABLE 24-3 Characteristics Useful in the Differentiation of Haemophilus paragallinarum from Avian Haemophilus-like Organisms

A killed vaccine containing serovars A and C has been available for many years for prevention of infectious coryza. A commercial trivalent vaccine containing serotypes A, B, and C has been developed, but outbreaks of coryza have been reported in vaccinated flocks in several countries, suggesting that there may be new serotypes. Macrolides, sulfonamides, or tetracyclines are administered in feed or water in the face of outbreaks, as a control method. These agents are bacteriostatic, so the carrier state is not eliminated and relapses after therapy are not uncommon. Management of disease should include use of coryza-free replacement birds and an all-in/all-out flow of birds. Often it is necessary to depopulate a flock for disease elimination.

HAEMOPHILUS PARASUIS

In 1910, Glässer described a disease of pigs associated with infection by a gram-negative bacillus and characterized by polyserositis, polysynovitis, and meningitis. Today the organism is ubiquitous in the nasal cavity, tonsil, and trachea of healthy pigs, and can occasionally be isolated from healthy lungs. However, disease caused by H. parasuis has become increasingly significant in swine-producing countries throughout the world, despite changes in production methods. Segregated early weaning (SEW) programs that have been advocated to reduce pathogens are ineffective in controlling H. parasuis colonization because the bacterium is transferred from the sow to her piglets before 10 days of age. Morbidity and mortality can be especially high in conventional herds infected with porcine reproductive and respiratory syndrome (PRRS) virus, and in naïve swine populations, such as those that are specific pathogen free (SPF) or raised by SEW. Survivors grow poorly and add to production losses.

More than 15 serotypes of H. parasuis have been described, based on heat-stable antigens extracted from bacterial cultures and detected either in an agar gel precipitation test or an enzyme immunoassay. Up to 25% of isolates remain untypable. Some data suggest that serotypes 1, 5, and 12 through 14 may be more virulent than others. In North America, serotypes 2, 4, 5, 12, 13, and 14 account for approximately 75% of isolates recovered from clinically ill animals.

Haemophilus parasuis is a primary agent in nursery mortality. Disease severity, as well as the age of the affected animals, depends on the health status of the herd and the virulence of the infecting strain. In some herds, disease can occur within a week of weaning, and this reflects a deficiency in maternal immunity. Animals are affected at 4 to 6 weeks postweaning in the majority of herds. Glässer’s disease is characterized by a high fever (107° C), swollen joints, respiratory distress, and central nervous system (CNS) signs. Severe lesions observed at necropsy include fibrinous exudates in the pleura, pericardium, synovia, meninges, brain, and peritoneal cavity (Figures 24-2 and 24-3). Acute pneumonia without polyserositis has been reported in older animals in endemically infected, stable herds. Acute septicemia or arthritis may occur in adult populations, especially sow herds. Other disease manifestations include acute fasciitis and myositis in primary SPF sows, in the absence of lesions of septicemia, pneumonia, or polyserositis.

FIGURE 24-2 Respiratory lesions of Glässer’s disease in a finishing pig.

(Courtesy Raymond E. Reed.)