What are the Findings at Post-Mortem?
Normal lungs resemble a pink sponge. With maedi visna, the lungs are larger than normal, are tan-coloured, and are heavy and dense to the touch – particularly dorsally (i.e. the top of the lung) (Figure 1). Bacterial pneumonia causes most of its damage on the lower front part (anterio-ventral) of the lung because the bacteria are inhaled. Since maedi visna virus (MVV) infects through the blood stream, it causes damage more equally through the lung. The esophagus and ribs may be imprinted on the lungs because the lungs are enlarged with billions of inflammatory cells. The lymph nodes located in the mediastinum of the thorax (i.e. between the lobes of the lungs) are enlarged. On cut-surface, the lung is cellular appearing with small whitish granules (Figure 2. arrow and Figure 3).
Histologically (under the microscope), one can see a proliferation of immune cells, mostly lymphocytes as well as plasma cells and macrophages around the blood vessels and airways. These clusters of cells are called “lymphoid follicles” and the term for the damage seen is "lymphocytic interstitial" (i.e. between the blood vessels and airways in the interstitium) "pneumonia" (inflammation of the lungs). Virus and provirus can be demonstrated in the lung tissues using special techniques. Interestingly, lambs can develop lung lesions as little as 3 to 4 weeks post-infection. Adults develop changes more slowly.
Figure 1. Lungs from a ewe with weight loss. Note tan colour, heavy appearance and imprint of esophagus.
|Figure 2. Close-up of MV lung shows lobular appearance due to cellular infiltrate at level of bronchioles (arrow).||Figure 3. Cross-section of same lung. Note meaty appearance.|
The Central Nervous System (CNS)
CNS disease shows up as changes in the brain within a few weeks of infection. Presence of lymphoid follicles and meningoencephalitis (inflammation of the brain tissue as well as the lining, i.e. meninges, of the brain), as well as demyelinating lesions in the brain and spinal cord are the predominant changes. Severe lesions only occur in sheep with clinically apparent CNS disease such as paresis and paralysis.
Grossly, the udder and milk look normal but the udder tissue feels much firmer to the touch. Changes are usually uniform but one gland may be worse than the other. Histologically, mastitis presents with lymphoid follicles and lymphocytes cuffing the blood vessels. Virus can be detected both in the macrophages and epithelial cells lining the milk ducts. Experimentally induced mastitis has lesions which peak 8 months post-infection and decreased between 16 and 28 months post-infection. In some cases, lesions will resolve but scar tissue and loss of function may remain. It is not known if this occurs in naturally infected flocks where repeated infection likely occurs. Colostrum and milk contains infected cells as detected by PCR (polymerase chain reaction which detects DNA or RNA). Colostrum contains infected macrophages and epithelial cells and milk contains only infected epithelial cells. Somatic cell counts are elevated by the presence of increased numbers of lymphocytes, specifially CD8+ T cells.