Biology of the virus

Virus properties

Feline immunodeficiency virus (FIV) is a retrovirus of the genus Lentivirus that is closely related to HIV, sharing a similar structure, life cycle and pathogenesis [Miller et al.2000]. However, it is important to emphasise that human beings are not susceptible to FIV infection. It has become clear that FIVs are a large and ancient group of viruses; species-specific strains have been isolated from a variety of non-domestic Felidae, including the puma, lion, leopard, and pallas cat [Barr et al. 1997, Brown et al. 1994, Carpenter et al. 1996, Olmsted et al. 1992].
Lentiviruses such as FIV are complex retroviruses, containing accessory genes in addition to gag, pol and env. The FIV gag gene encodes among others the capsid protein p24 which is important for diagnosis. The pol gene encodes protease, integrase and reverse transcriptase proteins as well as additional enzymes that are important to the virulence of FIV. Both gag and pol are relatively conserved between strains. The env gene encodes the viral glycoprotein (gp120) and the transmembrane protein (gp41), the major determinants of viral diversity amongst isolates [Olmsted et al. 1989].
Five genetically distinct subtypes or clades (designated A to E) have been defined, with considerable sequence diversity (up to 26%) amongst regions of env [Sodora et al. 1994, Kakinuma et al. 1995, Pecoraro et al. 1996]. The majority of viruses identified so far belong to either subtype A or B. Although multiple subtypes have been documented in cats from the same continent, geographic clustering of subtypes is evident (Figure 1). This is important for PCR diagnosis. In the UK, only subtype A viruses are found. In other countries, although subtype A viruses predominate, other clades are present (e.g.Switzerland, Australia, the western United States, northern Japan, Germany and South Africa) [Sodora et al. 1994, Kakinuma et al. 1995, Bachmann et al. 1997, Kann et al.2006]. Subtype B viruses are also distributed worldwide but have been more consistently identified in eastern Japan, Italy, Portugal, and eastern United States. In contrast, subtype C viruses are less common. All of the reported subtype D viruses have arisen from Japan [Kakinuma 1995] and two strains from Argentina have been assigned to subtype E [Pecoraro 1996]

Figure 1: Global distribution of FIV subtypes or clades

The virus survives only minutes outside the host and is susceptible to all disinfectants
including common soap.

Epidemiology

Since FIV was first isolated in 1986 [Pedersen 1987], serological studies have demonstrated that FIV is endemic in domestic cat populations worldwide; the seroprevalence of FIV is highly variable between regions, with estimates of 1 to 14% in cats with no clinical signs and up to 44% in sick cats [Hartmann 1998]. Sick adult cats, male cats and entire cats are most likely to be infected [Hosie et al.1989]. The major route of natural transmission is believed to be via the inoculation of saliva during fighting [Yamamoto et al. 1989]. Vertical transmission and transmission between cats in stable households is relatively uncommon.
The majority of natural FIV infections are acquired by biting, presumably through the inoculation of virus, or virus-infected cells, from the saliva of persistently infected cats. Transmission from mother to kittens may occur but only a proportion of the offspring become persistently infected. The proportion of kittens infected depends on the viral load of the queen during pregnancy and birth. E.g. if the queen is acutely infected up to 70% of the kittens may be infected, but if the queen is clinically normal but chronically infected hardly any kittens will be infected [O’Neil et al. 1995a, 1995b, 1996].
Although neither oronasal nor venereal spread has been documented in nature, cats can be infected by experimental inoculation of virus into the nose, mouth, vagina and rectum [Moench et al. 1993] and virus can be recovered from semen following natural or ABCD Guidelines on Feline Immunodeficiency Virus 5/21 experimental infection [Jordan et al. 1998]. Queens however may still be infected at mating if bitten by an infected tomcat.