Tularemia
Bradford P. Smith
Tularemia, an infectious disease of humans, wild animals, livestock, and pets throughout the northern hemisphere, is caused by Francisella (Pasteurella) tularensis.
There are four major subspecies: tularensis, holarctica, mediaasiatica, and novicida, with tularensis being the most virulent. The organism is a facultative, intracellular, non-spore-forming, gram-negative coccobacillus that survives frozen or in mud and water for a period longer than 1 year, but it only survives for hours in carcasses. The natural hosts are rabbits and rodents, and transmission to livestock occurs chiefly through food, water, ticks, fleas, deerflies, and other insects. Sheep are the most frequently affected livestock species, along with dogs, cats, and humans. Massive epidemics with high mortality in range sheep have been reported.4 One paper describes three late-term abortion outbreaks, with one also including sick/dead ewes, between 1997 and 2007 in Wyoming and South Dakota.Humans are stricken with a plague-like illness when bitten by infected ticks, fleas, or insects and from handling infected rabbits or hares or other infected animal carcasses. The disease has also been called rabbit fever, deer fly fever, and meat cutter’s disease. Sheep shearers may become infected by bites of parasites from the sheep. Disease in horses has also been documented. Oral infection from contaminated water occurs, so fresh water should be provided.4
The disease causes an acute septicemia, with localization and lymphadenopathy and granulomatous lesions in the organs (particularly the liver and spleen). Signs are nonspecific, as expected with bacteremia, and include fever, anorexia, lethargy, and in some cases cough, rapid respiration, or diarrhea. Stiffness and edema of the limbs may be seen.
Exposure to eyes can result in oculoglandular disease.3 The course of disease is usually 2 to 14 days.Agglutination titers in recovered affected sheep range from 40 to 5000. Agglutination titers persist for short periods (21 days) in horses,4 probably because they measure mainly IgM. Diagnosis is based on culture of the organism from blood or organs. Samples must be specially handled and tests done in BSL-3 laboratories.7 Diagnosis can also be made by IFA testing or PCR9 or ELISA.7
Necropsy usually reveals ticks on the carcass. Often, reddened or necrotic areas appear in and under the skin at the site of infected bites. Regional lymph nodes may be swollen and congested. Congestion and edema of the lungs are common. Fetal lesions from abortions have multifocal pinpoint necrosis of the liver, spleen, and lung.5 Differentials include other bacteremias such as Mannheimia (Pasteurella) haemolytica in sheep, Histophilus somni (formerly Haemophilus somnus) in cattle, Mycoplasma mycoides subsp. mycoides in goats, and anthrax in all livestock. Treatment early in the course of infection is effective. Aminoglycosides, tetracyclines, cephalosporins, chloramphenicol, or quinolones all are likely to be beneficial initially until results of antimicrobial susceptibility testing are available. Insecticide removal of ticks from affected animals and herdmates is important. No animal vaccine is currently available, so insect and tick control in endemic areas remains the major prevention. Because oral infection from contaminated water has also been documented, fresh water should be provided.