Actinobacillus spp. infections

Actinobacillus spp. cause a variety of diseases in livestock (Table 1), although most are relatively uncommon in southern Africa. Of these diseases, epididymitis and orchitis in rams (caused by A. seminis), sleepy foal disease (caused by A. equuli), porcine pleuropneumonia (caused by A. pleuropneumoniae) and pyogranulomatous lesions of the soft tissues of the head — in particular the oral cavity or the forestomachs of cattle, sheep and goats — (caused by A. lignieresii) occur most commonly.¹

Newly described Actinobacillus spp. in livestock have not as yet had specific diseases ascribed to them, and have only been regarded as opportunists thus far. They are A. minor, A. porcinus and A. indolicus from the respiratory tract of pigs,¹² A. arthritidis in horses⁴ and A. succinogenes from the bovine rumen.⁷ Actinobacillus rossii has been associated with porcine abortion.¹⁵

Actinobacillus spp. are closely related to bacteria of two other genera: Pasteurella and Haemophilus. These three genera are grouped together in the family Pasteurellaceae, the members of which are obligate parasites of vertebrates.^{9, 11} They particularly favour the mucous membranes of the respiratory tract, but some species also infect the genital and alimentary tracts. Pathogenic strains of these bacteria are part of the normal microbial flora on the mucous membranes of healthy animals, albeit in low numbers.¹⁰ Bacteria that belong to the Pasteurellaceae grow slowly and are easily overgrown by the other bacteria in the normal flora that populate the mucous membranes. Selective media are therefore required for their isolation.

Actinobacillus spp. are labile in the environment, and are particularly susceptible to desiccation.²

Actinobacillus spp. are Gram-negative, non-motile, pleomorphic rods interspersed with coccal forms, giving a ‘Morse code’ appearance. They ferment sugars, producing acid but no gas, and reduce nitrates to nitrites.¹⁴ They are indole-negative.

Actinobacillus equuli, A. lignieresii, A. rossii and A. suis grow readily on ordinary bacterial growth media.¹⁴ Growth in primary cultures made from lesions is often sparse, and the addition of blood or serum to the media, and incubation in an atmosphere consisting of 5 to 10 per cent CO₂ in air aids isolation.¹⁴ However, A. pleuropneumoniae requires a Staphylococcus feeder culture, or a nicotinamide adenine dinucleotide (NAD) supplement in media intended for its growth.⁸

Actinobacillus colonies are small, greyish and may be ‘sticky’ on primary isolation. If touched with a wire loop, a mucoid string is formed between the loop and the colony. This characteristic is most marked in A. equuli,¹⁴ but it may be lost on repeated subculture. When a broth culture of A. equuli is swirled, the growth at the bottom of the tube twists in a mucoid, ropy spiral.^{6, 14}

Distinguishing biochemical characteristics of the Actinobacillus spp. are rather variable (Table 2) and many isolates are not easily identified.^{11, 13, 14} This is probably because more species exist than are currently recognized.¹⁶

Isolates previously described as A. suis but isolated from horses, are now known as Actinobacillus equuli subspecies haemolyticus.

As the antigens of Actinobacillus spp. are complex, the results of serotyping using different methods may produce conflicting results. For this reason the results of serotyping should be interpreted with care.¹⁴

Table 1 Diseases caused by Actinobacillus spp. in livestock

Table 2 Biochemical characteristics of Actinobacillus spp.

d = 11–89 per cent of species are positive

References

1. BIBERSTEIN, E.L., 1981. Haemophilus-Pasteurella-Actinobacillus: Their significance in veterinary medicine. In: kilian, m., frederiksen, w. & biberstein, e.l., (eds). Haemophilus, Pasteurella and Actinobacillus. London, New York, Toronto, Sydney, San Francisco: Academic Press Inc.
2. BIBERSTEIN, E.L., 1990. Our understanding of the Pasteurellaceae. Canadian Journal of Veterinary Research, 54, Supplement, S78–S82.
3. BISGAARD, M., PRECHULLA, K., YING, Y-T., FREDERIKSEN, W. & MANNHEIM, W., 1984. Prevalence of organisms described as Actinobacillus suis or haemolytic Actinobacillus equuli in the oral cavity of horses. Comparative investigations of strains obtained and porcine strains of A. suis sensu stricto. Acta Pathologica et Microbiologica Scandinavica Section B...