Mycoplasma hyorhinis infections

International Common Names

• English: enzootic pneumonia of pigs; mycoplasma arthritis and septicemia in pigs; mycoplasma arthritis and septicemia in pigs; mycoplasmal arthritis; mycoplasmal polyserositis; otitis in pigs; otitis media, externa, interna, middle and inner ear infections; polyarthritis in pigs; polyserositis in pigs

M. hyorhinis, first characterized by Switzer (1955), is a common contaminant of the upper respiratory tract of pigs worldwide, occasionally causing polyarthritis and polyserositis in young animals. It is often found in association with the more fastidious and economically more important M. hyopneumoniae, the primary cause of enzootic pneumonia in pigs, and other bacteria such as Actinobacillus spp. where it may exacerbate disease; it may also overgrow M. hyopneumoniae in culture. It has been shown to be a primary cause of acute eustachitis and otitis in piglets. Recently, it has been thought to exacerbate porcine reproductive and respiratory syndrome (PRRS). M. hyorhinis is also a common contaminant of cell cultures which it probably infects via the trypsin; once established in cell cultures it is very difficult to eliminate.

Gross lesions consist of serofibrinous inflammation of the membranes lining the pericardial, pleural and peritoneal cavities and serofibrinous to serosanguineous arthritis. In the chronic stages, there are often lesions in the body cavities and chronic arthritis (Ross, 1992). These features are not pathnomonic and other bacteria including Haemophilus parasuis, Streptococcus suis and Pasteurella multocida may also give similar lesions sometimes in collaboration with M. hyorhinis. It has been suggested that while M. hyorhinis can be regarded as a cause of polyserositis in piglets it requires the synergic presence of other bacteria in order for it to spread from the respiratory tract to the serous cavities (Friis and Feenstra, 1994).

M. hyorhinis has been isolated from the auditory tubes and tympanic cavities of piglets exhibiting otitis and eustachitis, leading to the possibility that the mycoplasma may be the primary cause of these ear infections (Morita et al., 1993; Kazama et al., 1994; Morita et al., 1999). Further evidence of its pathogenicity was shown with experimental infections; intranasal infection of specific pathogen free pigs led to inflammation of the auditory tubes and tympanic cavities (Morita et al., 1999).

Outbreaks of polyserositis generally occur in piglets of 3-10 weeks of age but also occasionally in young adults (Ross, 1993). The incubation period is about one week following infection or after the onset of stress. Clinical disease caused by M. hyorhinis is characterized by laboured breathing, lameness, decreased appetite, elevated temperature (40-40.6°C) and abdominal tenderness; the hair coat may have a roughened appearance. Joint disease is usually transient but can persist for up to 6 months.

Evidence that M. hyorhinis is a primary cause of acute and chronic disease has been provided following intranasal and intrperitoneal infections of young piglets (Whittlestone, 1979; Kobisch, 1983). Clinical signs included hyperthermia, joint swelling, lameness and growth retardation; polyserositis and arthritis were clearly seen but pneumonia and rhinitis were absent. In the acute stages, serofibrinous pericarditis, pleuritis and peritonitis were seen accompanied by hypertrophy of the synovial membranes and an increase in synovial fluid. In the chronic disease, lesions were composed of fibrinoid adhesions and articular erosion. Microscopic changes included fibrinous inflammation of the serous membrane and infitration by mononuclear cells. M. hyorhinis was recovered widely from tissues. A difference in the virulence of field strains has been noted.

Other workers have been able to reproduce lung lesions identical to those caused by M. hyopneumoniae, the major cause of enzootic pneumonia (Gois and Kuksa, 1974; Armstrong et al., 1984)

The persistence of M. hyorhinis antigens in the joint tissues long after mycoplasma isolation ceases to be observed suggests that auto-immune mechanisms may be involved in joint damage; re-inoculation of killed organisms 18 months later can induce further cases of arthritis (Cole et al., 1985).

M. hyorhinis normally inhabits the mucous membranes of the upper respiratory tract and tonsils (Friis and Feenstra, 1994) and is present in an estimated 10% of normal nasal cavities in adult and growing pigs (Ross, 1992). It has occasionally been isolated from the conjunctiva (Friis, 1976) and is frequently found in the joints of fattening pigs where it may play a role in arthritis. Adult carrier pigs transmit the mycoplasma to the litter where it spreads rapidly via aerosol and contact; many of the piglets may have clinically inapparent nasal and tracheobronchial infections. From the respiratory tract, the mycoplasma can spread haematogenously producing characteristic serofibrinous lesions in the pericardial, pleural and peritoneal cavities (Kobisch and Friis, 1996). It has been estimated to infect 2-5% of all pigs (Whittlestone, 1979).

Recently, M. hyorhinis has been implicated in fatal cases of PRRS because of its isolation from severe pulmonary lesions along with viruses thought to be responsible for the syndrome (Kawashima et al., 1996; Kobayashi et al., 1996; Shibata et al., 1996). A synergistic link may occur between M. hyorhinis and Pasteurella multocida; it has been shown that M. hyorhinis had no effect on the pneumonic lesion scores in naturally infected pigs unless P. multocida is present (Falk and Lium, 1990).

It is not possible to gauge the economic effects of M. hyorhinis infections, as generally these diseases are multifactorial.

In vitro studies have demonstrated the effectiveness of a range of antibiotics including (in order of potency) tetracycline, tiamulin, enrofloxacin, tylosin and lincomycin against M. hyorhinis in Denmark (Aarestrup et al., 1998). However, in Japan 10% of strains showed resistance to all 14- and 16-membered macrolide antibiotics tested; tiamulin was the most effective antibiotic (Kobayashi et al., 1996). As with many mycoplasma diseases, those caused by M. hyorhinis are usually refractory to antibiotic treatment, particularly in clinically sick pigs (Kobisch and Friis, 1996); early treatment is recommended.

No vaccines are currently available or considered desirable.