Feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) infections are found in cats worldwide. Both infections are associated with a variety of clinical signs and can impact quality of life and longevity.
This document is an update of the 2008 American Association of Feline Practitioners\' feline retrovirus management guidelines and represents current knowledge on pathogenesis, diagnosis, prevention and treatment of retrovirus infections in cats.
Although vaccines are available for FeLV in many countries and for FIV in some countries, identification of infected cats remains an important factor for preventing new infections. The retrovirus status of every cat at risk of infection should be known. Cats should be tested as soon as possible after they are acquired, following exposure to an infected cat or a cat of unknown infection status, prior to vaccination against FeLV or FIV, and whenever clinical illness occurs. It might not be possible to determine a cat\'s infection status based on testing at a single point in time; repeat testing using different methods could be required. Although FeLV and FIV infections can be associated with clinical disease, some infected cats, especially those infected with FIV, can live for many years with good quality of life.
There is a paucity of data evaluating treatments for infected cats, especially antiretroviral and immunomodulatory drugs. Management of infected cats is focused on effective preventive healthcare strategies, and prompt identification and treatment of illness, as well as limiting the spread of infection.

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Xenotransplantation using porcine cells, tissues, or organs may offer a potential solution for the shortage of allogeneic human organs. Prior to the clinical use of porcine xenotransplants, three main hurdles must be overcome: immunologic rejection, physiologic incompatibility, and risk of transmission of porcine pathogens. Designated pathogen-free breeding of pigs can prevent transmission of most porcine microbes. However, this is not possible in the case of porcine endogenous retroviruses (PERV), which are integrated in the pig genome and can infect human cells in vitro. In order to assess the probability of transmission of PERV, a careful screening of the source pig herd is recommended. Two types of PERV are present in pigs, human-tropic PERV-A and PERV-B, which are both present in the genome of all pigs, and PERV-C, which is not ubiquitous and infects only pig cells. In addition to these viruses, recombinant PERV-A/C viruses have recently been described that (i) are able to infect human cells; (ii) are characterized by high titre replication; and (iii) are associated with proviruses that are de novo integrated in the DNA of somatic pig cells, but not yet in the pig germ line. The risks presented by PERV-A/C recombinant viruses could easily be eliminated by using pigs not containing PERV-C in their germ line, thereby effectively preventing recombination with PERV-A. Selection of PERV-C-free animals, if possible, therefore reduces the risk of PERV-A/C transmission to humans. Although it is unclear whether PERV-C may be transmitted in vivo from pig-to-pig, an infection of PERV-C-free animals with this virus may be prevented. To select pigs with low-level expression of PERV-A and PERV-B, it is recommended to apply assays based on real-time reverse transcriptase polymerase chain reaction (RT-PCR), which enables discrimination between pigs with high-level expression and low-level expression. Screening xenotransplant recipients for PERV transmission can be done in a number of ways. Provirus integration and PERV expression could theoretically be detected in peripheral blood mononuclear cells using PCR and RT-PCR. However, as the cells in which PERV replicates are still unknown, it is unclear whether this will be a reliable approach. Applying sufficiently sensitive assays to differentiate between transmission and chimerism is recommended. As can be commonly observed after retrovirus infection, the detection of virus-specific antibodies may indicate infection; however, the possibility of abortive infection, antigen exposure without infection, or cross-reactive response must be considered and explored as alternative explanations. On the other hand, it remains unclear whether the absence of specific antibodies indicates the absence of such infection, in particular if recipients of a xenotransplantation product are under chronic immunosuppression that could prevent antibody formation. Antibodies may be detected by Western blot or ELISA, using purified virus or recombinant viral proteins as antigens. Finally, it may be possible to detect cross-species PERV transmission by evaluating cells from the recipient for their in vitro potential of transmission to specified target cells (the human renal epithelial 293 cell line being the best example). There is no in vivo animal model for cross-species PERV transmission, and therefore it is not possible to validate monitoring assays for PERV transmission in an in vivo situation. Finally, virus safety of xenotransplantation is a fast-developing field, and new experimental findings will change existing strategies and introduce new ones.

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Although several sets of treatment guidelines recommend when antiretroviral therapy should begin, no overall consensus has emerged. Factors that must be weighed in deciding when to start include not only CD4+ cell count and viral load but also the stage and tempo of the disease and the commitment of the patient. Evidence suggests that a first-line, triple-nucleoside regimen may not be the best option, especially for people with more advanced disease. Although deciding between a protease inhibitor (PI) regimen and a non-nucleoside regimen is more difficult, some data indicate that PIs may exert beneficial effects not seen with other antiretrovirals. Simplifying regimens by combining agents with longer half-lives, or by combining two PIs, can make antiretroviral therapy easier to take and more tolerable. But no current regimen will be durable without the utmost adherence by the patient.

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The authors are members of a working group that formulated guidelines to minimize transmission of simian immunodeficiency virus (SIV) infection to man. Biosafety level (BSL) 2 standards are recommended for handling of clinical specimens and housing of SIV inoculated animals. Manipulation of SIV preparations may be performed in a BSL 2 facility with additional BSL 3 practices and equipment; for large volume or concentrated preparations of SIV BSL 3 containment is necessary. Written policies regarding management and testing of workers exposed to SIV are recommended.