By: Amanda Smith-Stasinopoulos and Nicole Burcar
Corynebacterium pseudotuberculosis is a gram positive bacterial pathogen that primarily causes caseous lymphadenitis (CLA) in goats and other small ruminants. CLA produces large glandular swelling, “cheesy glands” (Figure 1); that form externally on the skin around lymph nodes or internally within organs. C. pseudotuberculosis lives inside host cells but can survive in harsh environments for extended periods of time despite not being able to form highly resistant spores or move around. C. pseudotuberculosis was first fully described in 1894 by Preisz for its resemblance to Diphtheria bacillus.
Figure 1: Sheep with cheesy glands caused by C. paratuberculosis (Source: Wikimedia commons)
In goats and sheep, C. pseudotuberculosis infects superficial and visceral lymph nodes forming an external or internal infection. Lymph nodes are the sites of immune cell activation and act as filters that detect foreign particles. Bacterial pathogens that target the lymph nodes aim to decrease the hosts’ defense against disease establishment. Affected superficial lymph nodes undergo necrosis, uncontrolled cell death, and form abscesses visible on the skin (Figure 1). The pus-like discharge from the abscesses is caused when the bacteria destroys the lymph node tissue, resulting in inflammation. Inflammation attracts neutrophils (a specific immune cell able to engulf foreign particles characteristically found in these types of infections) and carries the bacteria to other locations. The animal can then transmit the disease. Visceral lymph nodes around the lungs, liver and spleen are also commonly affected.
Animals can obtain bacterial particles from a soil environment through lesions in their skin. There have been cases where infected animals cough on superficial wounds of herd mates and spread the disease by releasing bacterial particles. These superficial wounds can be created by activities such as tagging, shearing or castrating, among other activities. Mechanical transmission through vectors such as flies have been identified in cattle and buffalo.
It may take a number of weeks after infection for the disease to start producing symptoms in the animal, known as an incubation period. Affected animals become less productive, in goats this may result in lower milk production and in sheep lower quality wool. The disease has a life-long prevalence and animals infected may show signs like weight loss and abortion, along with recurring abscesses.
C. pseudotuberculosis can cause disease in a number of other species; in horses it causes ulcerative lymphangitis forming severe swelling in the limbs often causing lameness. Pigeon fever is classified by the presence of external abscesses around the chest and can be found in cattle, camels, pigs, buffaloes and humans. It is not common for humans to be infected, normally those that show symptoms of the disease have been exposed to infected animals or have consumed raw goat milk or meat. Like goats, infected humans have abscesses in the liver and internal lymph nodes.
C. pseudotuberculosis is found worldwide with varying prevalence in different countries. In Brazil up to 75 % of sheep can be affected, other countries have a lower seroprevalence (number of animals with antibodies against the pathogen in their blood) of the disease making the strains less likely to be spread. In Australia 26% of sheep are affected, and in Canada 21% of sheep brought to slaughterhouses had the disease.
Endemic areas where the disease affects horses in the United States are California and Texas. They have a usual annual disease prevalence of 5-10% but have seen times of large incidence increases and sporadic outbreaks. In 2002 and 2003, thousands of horses were reported as being infected with disease caused by C. pseudotuberculosis in Wyoming, Utah, Kentucky and Colorado, where disease prevalence is historically low. Further, small outbreaks have been reported in areas such as Alberta, Canada in 2013 and British Columbia in 2010 with 350 symptomatic cases.
A long-term study (1989-2001) of dairy cattle in Israel identified 3 clinical forms of C. pseudotuberculosis in 827 animals, the more common form produced ulcer abscesses on the skin and resulted in a 16.3% culling rate (animals removed from the herd) of the infected animals. The other forms of the disease caused mastitis in cattle and infected the visceral lymph nodes.
The major factor that contributes to the pathogen’s ability to cause damage and disease is an enzyme called phospholipase D. This enzyme is a toxin that is secreted outside the bacterial cell, which classifies it as an exotoxin. Phospholipase D breaks bonds in the membranes of host cells allowing the pathogen to spread to secondary locations. Phagocytes (like neutrophils or macrophages: cells involved in the immune response that engulf foreign particles) engulf these exotoxins and taxi them to regional lymph nodes, where they produce the characteristic lesion of the disease (Figure 2). Phospholipase D is not degraded during transport because it disrupts the phospholipids that make up the compartment inside the macrophage containing the toxin. At one point it will kill the cell since the cell membrane of the macrophage is also made up of these phospholipids.
Figure 2: Corynebacterium pseudotuberculosis pathogenesis. Source: Amanda Smith-Stasinopoulos.
The surface lipids have cytotoxic effects, meaning they can kill host cells, which also adds to the virulence of C. pseudotuberculosis by preventing the macrophages from killing the pathogen and contribute to the formation of the necrotic lesions.
The bacteria has different strains that are susceptible to different antibiotics but some antibiotics are able to target all strains. The problem with targeting the disease once it has established in the host, is that the bacteria are encapsulated in the lymph nodes making it difficult to avoid causing harm to the host. Antibiotics are therefore not a viable treatment.
In humans infected lymph nodes are surgically removed. Unfortunately, this is not an economically viable option for goat farmers and infected animals are removed from the herd.
The best option for producers is to prevent the disease by vaccination. A common vaccine uses an inactivated version of phospholipase D. With a proper vaccine protocol the number of abscesses and spread is decreased, however the proper protocol is not followed by many farmers. Many vaccines produced for sheep cannot be used to prevent C. pseudotuberculosis disease in goats. Goats need more frequent booster shots than sheep; every 6 months compared to yearly, after a set of 2 initial doses. The vaccine is more effective when secreted bacterial components are injected with an adjuvant that stimulates an immune response. Some vaccines are made with the whole bacteria but these only show 33% effectiveness and a weak immune response.
Baird GJ, Fontaine MC: Corynebacterium pseudotuberculosis and its Role in Ovine Caseous Lymphadenitis. Journal of Comparative Pathology 2007, 137(4):179-210.
Dorella FA, Pacheco LGC, Oliveira SC, Miyoshi A, Azevedo V: Corynebacterium pseudotuberculosis: microbiology, biochemical properties, pathogenesis and molecular studies of virulence. Vet Res 2006, 37(2):201-218.
Guimarães AdS, Dorneles EMS, Andrade GI, Lage AP, Miyoshi A, Azevedo V, Gouveia AMG, Heinemann MB: Molecular characterization of Corynebacterium pseudotuberculosis isolates using ERIC-PCR. Veterinary Microbiology 2011, 153(3):299-306.
Moura-Costa LF, Bahia RC, Carminati R, Vale VLC, Paule BJA, Portela RW, Freire SM, Nascimento I, Schaer R, Barreto LMS et al: Evaluation of the humoral and cellular immune response to different antigens of Corynebacterium pseudotuberculosis in Canindé goats and their potential protection against caseous lymphadenitis. Veterinary Immunology and Immunopathology 2008, 126(1):131-141.
Moussa IM, Ali MS, Hessain AM, Kabli SA, Hemeg HA, Selim SA: Vaccination against Corynebacterium pseudotuberculosis infections controlling caseous lymphadenitis (CLA) and oedematousskin disease. Saudi Journal of Biological Sciences 2016, 23(6):718-723.
Paton, M., Sutherland, S., Rose, I., Hart, R., Mercy, A. and Ellis, T. The spread of Corynebacterium pseudotuberculosis infection to unvaccinated and vaccinated sheep. Australian Veterinary Journal 1995, 72: 266-269. doi:10.1111/j.1751-0813.1995.tb03542.x
Yeruham, I., Elad, D., Friedman, S. and Perl, S. Corynebacterium pseudotuberculosis infection in Israeli dairy cattle. Epidemiology of Infection 2003, 131: 947–955. doi: 10.1017/S095026880300894X
Da Conceição Aquino De Sá, M., Gouveia, GV., Da Costa Krewer, C., Aparecida Veschi, JL., De Mattos-Guaraldi, AL. and Da Mateus Matiuzzi, C. Distribution of PLD and FagA, B, C and D genes in Corynebacterium pseudotuberculosis isolates from sheep and goats with caseus lymphadenitis. Genetics and Molecular Biology 2013, 36(2): 265-268. https://dx.doi.org/10.1590/S1415-47572013005000013
Foley, J.E., Spier, S.J., Mihalyi, J., Drazenovich, N. and Leutenegger, C.M. Molecular epidemiologic features of Corynebacterium pseudotuberculosis isolated from horses. American Journal of Veterinary Research 2004, 65(12): 1734-1737.