and male mink kits, Mustela visonKits were
vaccinated subcutaneously at 6 weeks of age with 3 portions of the 4-way
combination vaccine including inactivated mink enteritis virus, Clostridium
botulinum Type C bacterin-toxoid and inactivated Pseudomonas aeruginosa
bacterin. Kits were vaccinated at approximately 10-12 weeks of age with the
modified- live canine distemper virus portion of the 4-way combination vaccine
via aerosol spray.
Approximately 10 days later, mortality started with 5- 10 dead kits per day. Mortality was concentrated in 2 of 8 barns and the mahogany mink were primarily affected. Affected mink kits had wet heads and were screaming, seizuring before dying. This producer mixes his own feed on farm and in the last week, had a feed change to fish, chicken and processed meats. Only the chicken is fresh daily, the other protein sources are frozen. As of a week ago, antibiotics (CSP 250) were mixed in the feed. The older mink are fine.
Four mink kits
were submitted for postmortem examination. They were in good body condition
with ample internal and external fat stores. They were mildly to moderately
dehydrated. The lungs were diffusely mottled purple/red and were edematous. The
tracheal lumina did not contain fluid or exudate. Stomachs were generally
either empty or contained brown flecked mucus or brown fluid. The small
intestines were generally empty and only a small amount of gold brown mucoid to
pasty material was in the rectum. Livers were red tinged with yellow. The
spleens were small.
The lung is markedly congested with alveoli containing eosinophilic edema fluid, small amounts of fibrin and red blood cells. Multifocally, small to moderate numbers of macrophages with abundant often foamy cytoplasm, less frequently multinucleated cells and infrequent neutrophils are also collecting in alveolar spaces. Bronchial lumina contain eosinophilic edema fluid, small numbers of red blood cells, macrophages, multinucleate cells, neutrophils and degenerate cells. Many of the bronchi and nearby vessels are rimmed by one to two cell thick lymphohistiocytic cuffs. Numerous large eosinophilic spherical to rectangular inclusions morphologically compatible with viral inclusions are present within the cytoplasm of normal and proliferative bronchial and bronchiolar epithelium and occasionally within macrophages or multinucleated cells. Strong positive staining for CDV is present within bronchiolar epithelium and alveolar macrophages on immunohistochemistry.
1. Marked pulmonary congestion and moderate
2. Proliferative bronchitis, bronchiolitis and multifocal histiocytic pneumonia with eosinophilic intracytoplasmic inclusions.
3. Very mild acute suppurative bronchitis, bronchiolitis and pneumonia
Aleutian disease by CIE testing. Multiple tissues including lung, spleen,
liver, adrenal gland and brain had positive immunohistochemical (IHC) staining
for canine distemper virus (CDV).
Canine distemper virus
The host range of canine
morbillivirus (canine distemper virus, CDV) is broad and species in all
families in the order Carnivora, including Canidae, Mustelidae,
Procyonidae, Hyaenidae, Ursidae , Viverridae and Felidae are
considered susceptible.9 Canine distemper virus causes multisystemic
disease in farmed mink4 with juvenile mink being most susceptible
with a mortality rate of 90% as compared to a more variable rate of 20-90% in
adult mink.9 Outbreaks of CDV infection on commercial mink farms
have been associated with infected raccoons (Procyon lotor) and striped
skunks (Mephitis mephitis) found on the farm premises4. The
transmission of the virus is mainly by aerosol or through direct contact with
or conjunctival fluid as the canine distemper virus does not survive long in
the environment and it can be rapidly in-activated by ultraviolet light,
drying, heat and common disinfectants. However, it is possible for the gloves
used for handling infected mink to act as fomites and transmit sufficient virus
to infect susceptible mink for a short period of time.2
Routine vaccination of kits via subcutaneous injection using a multivalent vaccine including canine distemper virus is typically carried out after weaning.4 Some producers will also revaccinate the mature females and males being retained for breeding at this time (J Mitchell, personal communication). On this farm, the 4-way combination vaccine was split and the mink kits were immunized by subcutaneous injection for mink enteritis virus, Clostridium botulinum Type C and Pseudomonas aeruginosa at 6 weeks of age. Four to six weeks later, the kits were given the canine distemper virus portion of the vaccine by aerosol spray, however, as the vaccine is meant to be given by subcutaneous injection it is likely the mink kits were not sufficiently immunized against canine distemper virus and subsequently developed clinical disease.
At necropsy, the most consistent lesions were identified in the lungs with pro-liferative bronchitis and bronchiolitis and variable numbers of large rectangular eosinophilic intracytoplasmic inclusions identified in bronchial and bronchiolar epithelium. Low numbers of multinucleated cells in alveoli occasionally also contain in-tracytoplasmic inclusions. Small accum-ulations of neutrophils were aggregating in small airways and secondary bacterial bron-chopneumonia is reported to commonly occur.4 The diagnosis of CDV in these individuals was confirmed with IHC.
In experimental infections with the virus given by injection, young mink are reported to develop reddening of the skin progressing to dermatitis, conjunctivitis and nasal dis-charge, but dyspnea is uncommon.4 How-ever, histologically, pulmonary lesions are common and the intracytoplasmic and intranuclear viral inclusions are most often present in the respiratory or bladder ep-ithelium.4 Viral inclusions are not identified in the transitional epithelium of the bladder in this case, despite the tremendous numbers visible in the lung tissues.
Bronchiolar epithelial hyperplasia, marked, with mild histiocytic bronchiolitis,
and epithelial and histiocytic intracytoplasmic viral inclusion bodies.
Canine distemper virus (CDV) is an enveloped, single stranded RNA (ssRNA) virus
of the genus Morbillivirus in the Paramyxoviridae family1-8
Morbilliviruses are highly con-tagious
pathogens that are responsible for some of the most devastating diseases that
affect mammals worldwide. Other members
of this genus include measles virus, peste des petits ruminants virus,
rinderpest virus, phocine distemper virus, and dolphin morbillivirus.6
CDV is one of the most important and ubiquitous infectious diseases of wild and
domestic predators with high morbidity and mortality in imm-unologically naïve
populations.5 As men-tioned by the contributor, mustelids (i.e. ferrets,
minks, and badgers) are particularly sensitive to the virus with mortality up
to 100% in non-vaccinated ferrets.5
Like all paramyxoviruses, CDV contains six structural proteins: nucleocapsid (N), phospho (P), large (L), matrix (M), hem-agglutinin (H), and fusion (F) protein. The viral envelope contains H, used for attachment to the host cell receptor, and F, required for entry and syncytia formation. In this case, syncytial cells are occasionally present within bronchioles and alveoli.1,6
CDV is typically transmitted via inhalation of infected aerosols.3,4 The virus enters macrophages, lymphocytes, and dendritic cells via the signaling lymphocyte activation molecule (SLAM) receptor binding to H viral protein within the first day of infection.1,6 The virus spreads to local lymph nodes and other lymphoid organs within two to five days post infection. Primary viral replication in the lymphoid tissues leads to severe immunosuppression and viremia. About eight to ten days post-infection, CDV disseminates to several epithelial tissues (respiratory, intestinal, and urinary) and the central nervous system. The virus uses epithelial cell receptor nectin-4 to gain entry into epithelial cells.1,4,6
Unfortunately, in this case, the conference participants had difficulty assessing the alveolar septal changes due to poor insufflation of the lung. However, con-ference participants easily identified numerous intracytoplasmic viral inclusions within histiocytes and the markedly hyperplastic bronchiolar epithelium. In addition, some recognized rare intranuclear inclusions. CDV is unique because it causes both intranuclear and intracytoplasmic inclusions.3,7,8 Given that CDV is an ssRNA virus that requires RNA-dependent RNA polymerase complex present only in the cytosol to replicate, the presence of int-ranuclear inclusions is initially con-founding.7 However, CDV infection induces the cellular stress response which causes elevated expression of heat shock proteins. These proteins translocate viral nucleocapsid (N) proteins from the cytoplasm into the nucleus as part of the normal host cell response to cellular stress. Under normal circumstances, the N protein would be too large to pass through nuclear pores.7 N proteins form the basis for nuclear body formation, and propagation of viral nuc-leocapsids. These particles eventually partially, or completely fill the nucleus resulting the in the formation of the intranuclear inclusion body visible by light microscopy.7,8
The contributor mentions the wide range of species now infected by CDV and related viruses. Additionally, lethal outbreaks in rhesus and cynomolgus macaques ori-ginating in China have been documented within the last decade.
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