Caseous abscessation of lymph nodes and internal organs caused by Corynebacterium pseudotuberculosis occurs worldwide and is widely distributed throughout North America. It is an important endemic infection in regions with large sheep and goat populations. Economic losses result mostly from condemnation and trim of infected carcasses and devaluation of hides. Caseous lymphadenitis is also a cause of ill-thrift and sudden death in animals with internal abscesses. However, producers often report that the major impact in the flock is from disagreeable aesthetics, which may result in loss of breeding stock sales as well as early culling. Although principally an infection of sheep and goats, sporadic disease also occurs in horses and cattle ( see Corynebacterium pseudotuberculosis Infection of Horses and Cattle), camelids, water buffalo, wild ruminants, primates, pigs, and fowl. It rarely causes regional lymphadenitis in humans.
Etiology and Pathogenesis:
The small gram-positive rod is a facultative intracellular parasite that is found on fomites and in soil and manure contaminated with purulent exudate. Two biotypes have been identified based on the ability of the bacteria to reduce nitrate: a nitrate-negative group that infects sheep and goats, and a nitrate-positive group that infects horses. Isolates from cattle are a heterogeneous group. All strains produce an antigenically similar exotoxin with enzymatic activity (phospholipase D) that appears to be leukotoxic and that can damage endothelial cells and promote spread from the initial site of infection to regional lymph nodes and visceral organs. The chemical composition of its cell wall (high lipid content) enables the organism to resist being killed by phagocytes and to maintain chronic infection despite a good immune response.
Infection occurs after C pseudotuberculosis penetrates through unbroken or abraded skin or through mucous membranes. Most infections occur through wounds contaminated with purulent exudate from ruptured external and pulmonary abscesses. Contaminated dipping vats and shearing, handling, and feeding equipment are responsible for spread of the organism, along with confinement housing at high stocking densities. The pus contains large numbers of bacteria that can survive for months in hay, shavings, and soil. The disease is most often introduced into a flock by entry of an apparently healthy carrier from an infected flock, by contact on shared pastures, or via contaminated fomites such as shearing equipment.
Clinical Findings:
Caseous lymphadenitis is a chronic, recurring disease. A slowly enlarging, localized, and nonpainful abscess may develop either at the point of entry into the skin or in the regional lymph node (superficial or external form), from which it may spread via the blood or lymphatic system and cause abscessation of internal lymph nodes or organs (visceral or internal form). Initial infection may cause no clinical signs or may be accompanied by high fever, anorexia, anemia, and cellulitis at the infection site. Superficial abscesses enlarge and may rupture and discharge infectious pus. In sheep on range, most superficial abscesses develop in the prescapular and prefemoral region, with transmission probably occurring when the flock is gathered for group treatments such as shearing. In housed goats and sheep, superficial abscesses develop mainly in the head and neck region, likely due to transmission by contaminated feed, feeders, and other fomites. Animals with superficial abscesses show no obvious ill effects unless the location of the abscess interferes with functions such as swallowing or breathing. Abscessation may recur at the same site. Internal abscesses should be considered as a potential diagnosis for “thin ewe syndrome,” in which an adult small ruminant loses condition in the face of adequate nutrition. The visceral form is usually more extensive in sheep than in goats, mostly involving abscesses of the pulmonary parenchyma and mediastinal lymph nodes. Other less common manifestations include caseous bronchopneumonia, arthritis, abortion, CNS abscessation often occurring in the pituitary gland, abscessation of a vertebral body causing paresis or paralysis, scrotal abscessation, and rarely, mastitis. Scrotal (inguinal) abscesses can cause temporary infertility in rams. The incidence of abscesses steadily increases with age; clinical disease is more prevalent in adults, and up to 40% of animals in a flock can have superficial abscesses.
Caseous lymphadenitis kidney lesion, goat
Caseous lymphadenitis, goat
Lesions: The typical gross lesion is a discrete abscess distended by thick and often dry, greenish yellow or white, purulent exudate. In sheep, the abscess often has the classically described laminated “onion-ring” appearance in cross section, with concentric fibrous layers separated by inspissated caseous exudate. In goats, the exudate is usually soft and pasty.
Diagnosis:
The diagnosis can usually be based on clinical signs and flock history. For definitive diagnosis, an aspirate of an intact abscess should be submitted for bacteriologic examination; C pseudotuberculosis can easily be isolated, although it may be recovered in mixed culture with other pyogenic organisms. Suppurative lymphadenitis and abscesses can also be caused by various other pyogenic organisms, such as Arcanobacter (Actinomyces) pyogenes , Staphylococcus aureus , Pasteurella multocida , and occasionally anaerobes such as Fusobacterium necrophorum . Scrotal abscesses may be confused with orchitis due to A pyogenes or with epididymitis due to Histophilus sp , Brucella ovis , or other pyogenic organisms. The differential diagnosis for emaciated animals also should include ovine progressive pneumonia (maedi, visna), caprine arthritis and encephalitis, paratuberculosis (Johne’s disease), dental disease, and parasitism.
Serologic testing is available commercially. Most tests detect antibodies to the phospholipase D exotoxin. A positive test should be interpreted as exposure to the exotoxin and may indicate active infection. Infected animals may have a false-negative result, likely because the infection is well walled off in abscesses, or if they are severely debilitated. Colostral titers usually disappear by 3-6 mo of age, so serologic testing of lambs or kids <6 mo old should be interpreted with caution. Vaccinated animals will test positive and should not be included in a serologic program. Goats tend to have higher titers than sheep.
Treatment and Control:
Although the organism is susceptible to penicillin, treatment with antibiotics is usually not attempted because the formation of abscesses limits their penetration and effectiveness. Therefore, prophylactic and therapeutic treatment will not eliminate C pseudotuberculosis from infected flocks or individuals. Abscesses frequently recur after draining or attempted surgical excision. The practice of injecting abscesses with formalin should be discouraged as this practice is painful to the animal and may leave a residue of a carcinogenic compound in a food-producing animal.
Prevention is based on reducing transmission of the organism from infected to susceptible animals. Emaciated animals and those with recurring abscesses should be culled. When animals are too valuable to cull, those with developing abscesses should be isolated, and the abscesses lanced and flushed with an iodine solution. Any lancing of abscesses should be done so that the purulent material can be collected and environmental contamination prevented. Young animals should be raised in isolation from older, infected animals. Older animals and those with abscesses should be shorn last, and equipment should be disinfected whenever it is contaminated with draining exudate. Skin wounds from shearing or fighting should be treated topically with iodine and sutured if necessary.
Commercial vaccines, currently only licensed for use in sheep, reduce the incidence and prevalence of caseous lymphadenitis within a flock, but they neither prevent all new infections nor cure animals already infected. Currently, all contain phospholipase D toxoid, and some also contain killed whole bacterial cells. Usually, Clostridium tetani and C perfringens type D, along with other clostridial antigens, are included in the vaccine. Vaccination should start in young stock after colostral immunity has waned (~3 mo of age). Colostral immunity can be improved by administering a booster to pregnant ewes and does 1 mo prior to lambing/kidding. A primary series (2 injections, 4 wk apart) is required in young stock or previously unvaccinated adults; repeat vaccination at least annually will help reduce the prevalence of disease. Vaccination will not cure existing infections, so it is important to ensure that animals are well vaccinated prior to exposure. Evidence suggests that increasing the frequency of vaccination to every 4-6 mo may be of benefit in flocks where exposure is high (eg, confinement housed for at least part of the year). These vaccines should be used with caution in potentially infected goats, as adverse reactions are sometimes reported.
An effective control program must involve vaccinating, culling, and reducing exposure to potentially contaminated fomites, such as shearing blades, dipping fluids, feeders, and feed. Eradication is difficult and requires strict biosecurity and vaccination protocols and rigorous culling of all infected animals. Once the disease is at a low prevalence, vaccination should be stopped and all seropositive, nonvaccinated animals culled. Prevention of disease entry into a clean flock is based on serologic screening and isolation of incoming animals. Seropostive unvaccinated animals should not be accepted into the flock.
If management allows, removal of young stock at birth and rearing separately from the infected flock has been shown to be an effective means of eradication. The infected flock is then culled as quickly as economics allow. Prevention of caseous lymphadenitis in an uninfected flock involves shearing biosecurity and purchase only of uninfected replacement stock. Producers should purchase their own shearing equipment and not share it with other flocks. Shearers should arrive at the flock wearing clean footwear and clothing. Wool bags, shearing boards, moccasins, hands, clothing, and all parts of clippers can be a source of bacteria. If there is a break in biosecurity, disease usually shows up 1-3 mo after exposure, or longer if the initial abscesses are internal or are missed.
Taken from the Merck Veterinary Manual
