Feline Immunodeficiency Virus Is Concentrated in Milk Early in Lactation

We studied mother-to-offspring transmission of feline immunodeficiency virus (FIV), focusing on milk-borne virus transmission in order to assess its similarities to perinatal HIV transmission. We also attempted to evaluate the influence of intragestational treatment with 9-[2-(phosphono-methoxy)-propyl]adenine (PMPA) on virus transmission to offspring. Eleven female cats (queens), chronically infected with FIV-B-2542 and bred to an FIV-negative male, produced a total of 25 viable and 18 nonviable term kittens. Overall, the vertical transmission rate by untreated queens was 22%, similar to that for HIV, which unfortunately precluded adequate assessment of PMPA efficacy. However, at delivery 9 of 10 queens (90%) had higher viral RNA loads in milk (4 × 10⁴ to 4 × 10⁸ viral copies/ml) than in plasma (5 × 10³ to 2.5 × 10⁶ viral copies/ml). Conversely, 10 of 11 queens (91%) had lower proviral loads in milk cells (0 to 10² proviral copies/μg DNA) than blood cells (10² to 10⁴ proviral copies/μg DNA). Thus, FIV is concentrated in early milk despite relatively low proviral loads in milk cells, suggesting that virus may be actively secreted by the mammary gland for dissemination to offspring. FIV provides a model for the study of milk-borne lentivirus transmission and assessment of strategies to reduce postnatal HIV vertical transmission.