Actinobacillosis in animals can be caused by several different species of gram-negative coccobacilli of the genus Actinobacillus, belonging to the family Pasteurellaceae. Actinobacillus spp are generally opportunistic, causing disease when breaks in the mucosa or skin allow entry of the bacteria into protected sites.
Depending on the species of animal infected and the species of Actinobacillus, clinical signs of infection can include septicemia, localized pyogranulomatous abscesses, pleuropneumonia, epididymitis/orchitis, and arthritis.
Actinobacillus spp can be identified via culture or PCR assay, supported by serological and histological examination of biopsied lesions.
Treatment of actinobacillosis relies on antimicrobials for swine and horses, and on sodium iodide for ruminants. Surgical debulking, cryotherapy, or adjunctive antimicrobial use might be required in severe or refractory cases.
Etiology of Actinobacillosis
The genus Actinobacillus includes gram-negative coccobacilli, many of which reside as normal flora of the upper respiratory tract, oral cavity, and reproductive tract, that occasionally cause disease. Of the > 22 different bacterial species in this genus, 5 (A pleuropneumoniae, A suis, A equuli, A lignieresii, and A seminis) are regularly associated with disease in animals.
Clinical Findings of Actinobacillosis
Clinical Findings of Infection by Actinobacillus pleuropneumoniae
Actinobacillus pleuropneumoniae (formerly Haemophilus pleuropneumoniae or Haemophilus parahaemolyticus) causes contagious pleuropneumonia in pigs. It is one of the most economically important causes of respiratory disease worldwide in swine production systems (1).
Disease caused by A pleuropneumoniae ranges from acute, severe fibrinous pleuropneumonia with high morbidity and mortality rates to subacute or chronic infection with pleuritis, slow growth, and pulmonary abscessation (see lung lesions image).
Courtesy of Dr. Grace VanHoy.
Immune complexes that form as a result of the host animal's response to infection by A pleuropneumoniae can damage endothelial cells, resulting in vasculitis and thrombosis, with edema, necrosis, infarction, and hemorrhage. In naive herds, all age groups can be affected; however, severe disease usually occurs in pigs < 5 months old.
A pleuropneumoniae can be part of the normal mucosal flora in pigs, cattle, and sheep, and it has reportedly caused disease only rarely in horses (2).
Clinical Findings of Infection by Actinobacillus suis
Actinobacillus suis is part of the normal flora of the oral cavity of pigs. It causes septicemia in young pigs and causes arthritis, pneumonia, and pericarditis in older pigs. A suis can also cause septicemia, arthritis, pneumonia, and purulent nephritis in foals.
A suis can cross-react with A pleuropneumoniae on serological tests.
Disease caused by A suis follows a break in the integrity of the oral mucosa and can be associated with immunosuppression. Outbreaks typically occur in naive populations.
Clinical Findings of Infection by Actinobacillus equuli
The natural host of Actinobacillus equuli is the horse, and infections occur in both foals and adult horses.
Disease caused by A equuli in foals can manifest as diarrhea, meningitis, pneumonia, purulent nephritis, or septic polyarthritis (sleepy foal disease or joint ill); see A equuli infection image. Infection in foals can be acquired through a contaminated umbilicus, via translocation from the GI tract, or by inhalation. Predisposing factors include prematurity/dysmaturity, maladjustment syndrome, and failure of transfer of passive immunity.
Courtesy of Dr. Grace VanHoy.
Abortions, septicemia, nephritis, peritonitis, and endocarditis can result from A equuli infection in adult horses. A equuli peritonitis can be associated with translocation of bacteria from the GI tract or with migration of Strongylus spp; affected horses often present with mild to moderate colic and fever.
Clinical Findings of Infection by Actinobacillus arthritidis
Actinobacillus arthritidis (previously classified as Bisgaard taxon 9) has been isolated on rare occasions from horses with arthritis and septicemia.
Clinical Findings of Infection by Actinobacillus lignieresii
Actinobacillus lignieresii can cause tumorlike lingual abscesses, usually referred to as wooden tongue. Wooden tongue occurs primarily in cattle but also in sheep, horses, pigs, and dogs, and it can rarely affect chickens.
A lignieresii can also cause pyogranulomatous lesions in soft tissues associated with the head, neck, limbs (see atypical presentation image), and, occasionally, the lungs, pleura, udder, and subcutaneous tissue; many cases do not involve the tongue (see lung and heart lesion images).
Courtesy of Dr. Calvin Price.
Courtesy of Dr. Grace VanHoy.
A lignieresii is part of the normal mucosal flora of the upper GI tract and causes disease when it gains access to adjacent soft tissue via penetrating wounds. It causes localized pyogranulomas that can resemble tumors, and it can spread via the lymphatics to other tissues (see submandibular swelling image).
Courtesy of Dr. Sameeh M. Abutarbush.
The primary lesion associated with wooden tongue in cattle is a very hard, diffusely swollen tongue. Tongue swelling leads to excessive salivation, an inability to prehend feed normally, and sometimes a visibly enlarged tongue that protrudes from the mouth (see wooden tongue image). On palpation, the tongue feels hard and the animal exhibits pain; in addition, cows with wooden tongue often have firm or fluctuant swelling in the intermandibular space.
Courtesy of Dr. Geof Smith.
Wooden tongue is found worldwide but is sporadic and thus difficult to prevent. Herd outbreaks are also possible and are generally associated with consumption of coarse, abrasive feeds that predispose cows to the formation of oral lesions.
Clinical Findings of Infection by Other Actinobacillus spp
Actinobacillus seminis is an occasional cause of polyarthritis in young lambs and of placentitis and abortion in ewes. A seminis–induced actinobacillosis, closely resembling brucellosis, is an important differential diagnosis for infertility, subfertility, and epididymitis/orchitis in adult rams (3). Young rams are often infected and subclinical, with disease manifesting in maturity.
Actinobacillus ureae has caused upper respiratory tract infections in humans and sepsis and abortions in pigs.
Actinobacillus actinoides has occasionally been associated with suppurative pneumonia in calves and seminal vesiculitis in bulls.
Diagnosis of Actinobacillosis
Culture
PCR assay
ELISA
Actinobacillus pleuropneumoniae and Actinobacillus suis in swine are identified via culture of the organism from nasal swabs ante mortem or from lung tissue at necropsy. Molecular techniques, such as PCR assay and ELISA that detect the presence of these organisms in tissue samples, have also been developed; however, A pleuropneumoniae and A suis can cross-react on ELISA.
In foals and adult horses, several other bacteria can cause the same disease syndromes that Actinobacillus equuli causes, so definitive diagnosis is important, and it requires isolating the bacteria in culture or PCR assay.
In cattle, diagnosis of actinobacillosis requires culture and biopsy of the lesion. Pus from the abscesses crushed between two glass slides might show clublike spicules of calcium phosphate, giving the appearance of sulfur granules < 1 mm in diameter, which are actually accumulations of dead bacteria, proteins, calcium phosphate, and immune cells.
Pearls & Pitfalls
|
No reliable serological tests are available for actinobacillosis, and hematologic and serum biochemical findings are generally normal. Gross pathology of wooden tongue generally reveals a firm, pale tongue containing multifocal nodules (see tongue abscesses image). These nodules are often filled with thick, yellow-white pus. Histologically, the primary lesion is a granulomatous abscess.
Courtesy of Dr. John Prescott.
Identification of Actinobacillus seminis in rams can be based on serological testing, culture of semen (however, rams might not shed the bacteria consistently in semen), and multiplex PCR assay of semen, which includes tests for Brucella ovis, A seminis, and Histophilus somni. Screening programs for early detection in young rams are an important control measure in countries with a high prevalence of A seminis.
Treatment of Actinobacillosis
Antimicrobials and supportive care in swine and horses
Sodium iodide in ruminants
In swine, respiratory diseases are treated with antimicrobials, including penicillin, tetracycline, spectinomycin, cephalosporins, and fluoroquinolones.
In horses, Actinobacillus equuli infections can be treated with chloramphenicol, gentamicin, or third-generation cephalosporins, depending on the nature of the infection and the ability to achieve therapeutic concentrations at the site of infection.
When chloramphenicol is used, it should not be crushed, handlers should wear gloves, and pregnant women should avoid handling it. Chloramphenicol exposure has been associated with non–dose-related aplastic anemia experienced in some humans exposed to this drug.
Beta-lactam antimicrobials and sulfonamides have been recommended in the past for treating actinobacillosis in horses; however, widespread resistance to both of these antimicrobials has been reported. Antimicrobial susceptibility testing is recommended in clinical cases.
Sodium iodide is the treatment of choice for actinobacillosis in ruminants. IV sodium iodide (70 mg/kg of a 10–20% solution) is administered once and then repeated 1–2 times at 7- to 10-day intervals. Although the label for IV sodium iodide notes that the product should not be used in pregnant animals, the risk of abortion has been found to be minimal (4). Alternatively, organic iodide can be administered at 28 g (1 ounce) per 450 kg, PO, mixed in feed daily after an initial IV dose of sodium iodide. If clinical signs of iodine toxicosis develop (including dandruff, diarrhea, anorexia, coughing, and excessive lacrimation), iodine administration should be discontinued.
Clinical improvement in ruminants with actinobacillosis is often dramatic and occurs within 48 hours after treatment with sodium iodide, and when only the tongue is involved, treatment is usually successful. Systemic antimicrobial agents, such as ceftiofur, penicillin, ampicillin, florfenicol, and tetracyclines at the appropriate labeled dose, route, frequency, and duration, can be effective and are recommended primarily in severe cases or in cases refractory to sodium iodide treatment.
Surgical debulking with or without cryotherapy of lesions, especially if they interfere with airflow, might be necessary. This is particularly true when there are large granulomatous masses that do not respond to medical treatment.
In the US, all antimicrobial treatment of food-producing species should follow appropriate labeled dosage, route, frequency, and duration and should be supported by the Food Animal Residue Avoidance Databank (FARAD) and the Animal Medicinal Drug Use Clarification Act (AMDUCA). Following appropriate withdrawal recommendations for antimicrobial use in food-producing species (including animals not intended for human consumption) is required by law in the US.
Prevention of Actinobacillosis
Control of Actinobacillus pleuropneumoniae in swine focuses on good management, including maintaining all-in all-out closed herds, testing, quarantine and elimination of predisposing factors such as overstocking, ensuring proper ventilation, and eliminating or controlling other diseases that can cause immunosuppression.
In addition, cross-protective vaccination for all serotypes is recommended for piglets in at-risk or endemic herds. Eradication might be possible with depopulation.
Control of Actinobacillus suis is similar, and Actinobacillus pleuropneumoniae vaccination might provide some cross-protection.
In ruminants, actinobacillosis is prevented primarily by avoidance of coarse, stemmy feedstuffs and pastures full of hard, penetrating plant awns (ie, foxtails or thistles).
In rams, infection by Actinobacillus seminis can be prevented by careful surveillance and periodic testing, removal of carriers from the flock, and separation of juvenile and mature rams.
Key Points
Actinobacillus spp cause a variety of diseases in animals.
A pleuropneumoniae can cause severe respiratory disease in swine that often results in high morbidity and mortality rates.
A equuli can cause septicemia and pneumonia in foals, and peritonitis in adult horses.
A lignieresii causes "wooden tongue" in ruminants—abscesses in the tongue that hinder the ability of the animal to eat.
Typical treatment includes antimicrobials in swine and horses, and sodium iodide in ruminants.
For More Information
Matthews S, Dart AJ, Dowling BA, Hodgson JL, Hodgson DR. Peritonitis associated with Actinobacillus equuli in horses: 51 cases. Aust Vet J. 2001;79(8):536-569.
Smith BP, Van Metre DC, Pusterla N, eds. Large Animal Internal Medicine. 6th ed. Elsevier Mosby; 2020.
Divers TJ, Peek SF. Rebhun's Diseases of Dairy Cattle. 3rd ed. Elsevier; 2018.
Also see pet owner content regarding actinobacillosis in horses and dogs.
References
Soto Perezchica MM, Guerrero Barrera AL, Avelar Gonzalez FJ, Quezada Tristan T, Macias Marin O. Actinobacillus pleuropneumoniae, surface proteins and virulence: a review. Front Vet Sci. 2023;10:1276712. doi:10.3389/fvets.2023.1276712
Layman QD, Rezabek GB, Ramachandran A, Love BC, Confer AW. A retrospective study of equine actinobacillosis cases: 1999–2011. J Vet Diagn Invest. 2014;26(3):365-375. doi:10.1177/1040638714531766
Al-Katib WA, Dennis SM. Ovine genital actinobacillosis: a review. N Z Vet J. 2009;57(6):352-358. doi:10.1080/00480169.2009.64722
Riemann HP, Willeberg PW, Farver T. Failure to cause abortion in cows with intravenous sodium iodide treatment. J Am Vet Med Assoc. 1978;172(10):1147. doi:10.2460/javma.1978.172.10.1147
