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Mortality Outbreak in New York State Sheep Flock

Fresh liver from Ewe A

The owner of a 100 ewe flock assembled two years ago in northern New York recently contacted their veterinarian for advice regarding an outbreak.  Five ewes died suddenly over the course of about 3-4 weeks.  The owner performed a necropsy on ewe A, an adult ewe that had died overnight.  The owner noted a lot of “blood tinged” abdominal fluid.  She also observed the liver to be grossly abnormal.  She removed the liver, photographed it, and sent the picture to her veterinarian (see image at right).  She then froze the liver for future testing.   A few days later, the veterinarian was called out to the farm to necropsy ewe B.  This adult ewe also had a grossly abnormal liver with multiple dark red foci and fibrin adhesions to the diaphragm and omentum.  The veterinarian noted that most of the  flock appeared to be in good condition, but the affected ewes looked thinner and the owner noted that some of the affected animals seemed to be “off” for a day or two prior to dying.  This farm did not experience any similar problems last year but the owner also reported that there was a lot of standing water in the pastures this grazing season.

The veterinarian submitted a section of fresh liver from ewe A to the Animal Health Diagnostic Laboratory with a test request for bacterial culture.  Sections of both fresh and fixed liver from ewe B were also submitted with the requests of culture and histopathology.   The veterinarian was also interested in testing for possible caseous lymphadenitis (CLA) caused by Corynebacterium pseudotuberculosis.  Feces from two other animals, ewe C and ewe D, were submitted due to the veterinarian’s concern for flukes and possible Johne’s disease.  Upon arrival, this case was reviewed by one of the Veterinary Support Services Veterinarians and a specific diagnostic plan was established as follows:  Histopathology was requested on the fixed liver.  The fresh liver sections were each submitted for both aerobic and anaerobic cultures as well as parasite identification.  The fecal samples were each submitted for Johne’s culture and fecal quantitative floatation. 

Immature Fascioloides magna removed from liver of Ewe BResults from Parasitology showed Eimeria parva in the feces from Ewe C and Eimeria parva, Eimeria ovinoidalis and Strongyles in the feces from Ewe D.  No fluke eggs were detected in either of the samples.  Interestingly, the parasitologist identified an immature Fascioloides magna in the fresh liver from Ewe B (see image at left) and many cysts containing pus were noted in the liver form Ewe A, but no parasites were recovered. 

Aerobic bacterial culture of the liver samples grew few Escherichia Coli in both ewes and many Corynebacterium pseudotuberculosis in the liver from Ewe A.  Anaerobic bacterial culture grew Clostridium perfringens in both livers and a second Clostridium species that could not be further characterized in the liver from Ewe B.  Johne’s culture on Ewes C and D is still pending. 

The histologic results were equally informative showing characteristic findings of Black disease.  Black disease occurs secondarily to migrating helminth larvae, in this case Fascioloides magna.  The migration of these parasites leads to areas of ischemic necrosis where spores of Clostridium novyi can then germinate and release exotoxins, leading to further hepatocellular necrosis. 

In this case, all of the testing proved important in arriving at a complete overall diagnosis.  The parasite identification coupled with the histopathology findings was crucial to not only diagnose Black disease on this farm but also to identify the specific parasite in question.  Knowing that Fascioloides magna is the causative agent behind this outbreak, appropriate control measures can be implemented based on its unique life cycle.  Also, the culture of Corynebacterium pseudotuberculosis along with the histopathology results of pus filled cysts within the liver parenchyma highlight another issue in this flock.

Fascioloides magna is a trematode with an interesting life cycle.  It is widely scattered over North America.  Within New York State, it tends to be found mainly in the Adirondack Mountain region.  The definitive host is the white-tailed deer.  When eggs are deposited within deer feces into a wet environment, a ciliated larva called a miracidium develops.  At summer temperatures, after a couple of weeks, the miracidium escapes from the egg and swims until it finds a snail.  It will die within 24 hours if it does not find a suitable host snail.  Once inside the snail, it forms a sporocyst which in turn forms germinal balls that each develop into a redia.  The rediae grow until they burst out of the sporocyst and into the tissues of the snail.  The redia produces germinal balls that eventually each develop into cercaria, a tadpole-like larval form.  When fully developed in a month or two of summer temperatures, the cercaria leaves the redia and exits the snail into the surrounding water.  It then encysts on the surface of nearby water plants and loses its tail, becoming a metacercaria.  This is the stage that infects sheep and other grazers when consumed.  Within the host’s gastrointestinal tract, the immature fluke, now called a marita, migrates through the intestinal wall to the liver.  In white-tailed deer, these flukes mature and produce eggs that are then excreted through the bile ducts to the gastrointestinal tract.  In sheep and goats, however, these maritas never mature and, instead, continue to wander throughout the liver parenchyma, causing extensive damage and creating a prime environment for the multiplication of Clostridium novyi, leading to the rapidly fatal Black disease.  Therefore, F. magna infection is non-patent in these small ruminants and does not allow for diagnosis by fecal examination, which explains the negative finding of fluke eggs in the fecal exams from this flock.  (Bowman, 2003)

Treatment and control is best aimed at minimizing the optimal environment for F. magna to survive as well as implementing an appropriate vaccination protocol that specifically targets protection against Clostridium novyi.  De-worming is important, but targeting the de-worming protocol toward this parasite is not a practical control measure as the sheep will likely continuously be re-exposed as long as the appropriate wet conditions remain.  Rather, a prevention strategy should aim to avoid exposure of the flock to wet, swampy areas and the snails that inhabit those areas, as well as minimizing pasture exposure to white-tailed deer. 

This case highlights the importance of submitting a variety of samples and taking a multifactorial approach to diagnosing a herd problem.  If we had only received fecal samples without the fresh liver, it would have been impossible to make the diagnosis of a Fascioloides magna infection.  Furthermore, the histopathology along with the bacterial cultures, allowed us to piece together a complete picture of Black disease.

Reference:  Bowman, Dwight. Georgis’ Parasitology for Veterinarians, eighth edition.  St. Louis, Missouri: Elsevier Science, 2003. Pp 115-125.