Casual Transmission of FIV?
1. Introduction
2. Studies of Mixed Households
3. FIV Survival Outside the Body
4. Potential Modes of Transmission
5. Productive Infection and Cryptic Infection
6. Conclusions
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1. Introduction
More often than not, cats infected with FIV are rescued cats, and a common dilemma faced by their
rescuers is that they already have other cats in their household who are not infected with FIV. Looking to
their vets for advice, they are told that even a stable, friendly multi-cat household is no place for an FIV+
cat if it also contains FIV- cats. Looking to rescue groups for advice, they are told that “mixed” households
of FIV+ and FIV- cats are perfectly workable and that they should put their mind at ease about an FIV+ cat
into such a household. Good intentions on everyone’s part aside, both sources of advice have agendas to
be protected. Vets do not want to expose themselves to blame (or even legal jeopardy) if an uninfected
cat becomes infected because of their advice. Rescue organizations want to adopt out FIV+ cats to
caring owners.
Vets and rescue organizations alike accept the view that FIV is transmitted principally by bite wounds,
which allow virus in the saliva of infected cats to come in contact with the blood of an uninfected cat.
While aggressive encounters between newcomers and cats in an established household are always a
possibility, actual fighting-with-biting between cats in a stable, “friendly” group is not usual. Careful, skillful integration of FIV+ newcomers into existing households of one or more FIV- cats can solve the problem of
transmission through fighting in the overwhelming number of instances. What worries many would-be
adopters is whether a newcomer might transmit its infection by means other than fighting and biting–in the
course of casual play, through common access to food and water dishes or litter boxes, or by engaging in
mutual grooming. In other words, “casual transmission” through routine interaction.
The cause of sorting through such worries is not advanced by confusing FIV, the feline immunodeficiency
virus, with its retroviral fellow traveler FeLV, the feline leukemia virus. It is well established that FeLV can
be transmitted through casual interaction of cats within a household. FIV, however, is its own separate
issue, and what follows is an attempt to shine some light on this issue that is troubling to so
many.
2. Studies of “Mixed” Households
There have been numerous scientific studies of FIV transmission within stable social groups. Some set
out to study this very issue; other’s simply recorded data in the course of studying another issue or a
range of issues. Following is a table recording the findings of these studies, modified from its appearance
in a 2000 publication by Addie et al to include both its own 2000 data and that of an additional study
[Dandakar] from 1992 that was not included in the original tabulation. The columns (left to right) record the
number of FIV+ in a social group being studied, the number of FIV- cats living with them at the start of the
study period, the number of FIV- who became FIV+, the number of groups for which information was
recorded, and the published study.
# infected......# in contact......# of in-contact infected.......FIV-infected households.......Reference
N .......................14..........................0..............................................1..............................................Yamamoto (1988)
16......................31..........................0..............................................16............................................Shelton (1989)
4........................68..........................0..............................................1..............................................Shelton (1990)
N.......................34..........................1...............................................N.............................................Sparger (1989)
9........................7...........................2...............................................1..............................................Pedersen (1987)
N.......................N...........................3-67%.......................................N.............................................Hosie (1989a)
N.......................27.........................14..............................................N.............................................Hopper (1989)
N.......................11.........................11..............................................1..............................................Hosie (1989b)
N.......................20.........................1................................................1..............................................Dandakar (1992)
8.......................18..........................6 or7 ........................................1..............................................Addie (2000)
*N=number not provided
In all studies where the information is provided (save one, Shelton, 1989, which was a retrospective study
of accumulated data), one social group was the focus of study. The length of observation varied from less
than a year to ten years in the case of the most recent [Addie, 2000] study. Even a cursory glance at the
table will suggest that study outcomes have been wildly inconsistent. Three of the studies found no
evidence at all of transmission. Two found a very low level (1 of 20; 1 of 34). The rest of the studies found
levels in a range that might be characterized as “significant” at one end and “inevitable” at the other. In trying to explain inconsistency of outcomes, one study author hazarded differences in the virulence
of viral strains, the level of viral shedding by symptomatic cats, susceptibility to infection, and/or patterns of
aggression among cats in the same household [Hosie, 1989b]. To these might be added the inherent
limitations in the design of such studies. Cats cannot be monitored 24 hours a day for years on end to
eliminate the possibility of bites going unnoticed. Some of the studies involved colonies subject to very
loose observation. Even the 2000 study relied on a caretaker’s report, which included characterizations
such as, “. . . the cats displayed little aggression, with the exception of occasional paw-flailing involving
cats D, E and W.” That study–certainly one of the most troubling of the group–was further complicated by
involvement of cats with FeLV and with positive corona virus titers (some of whom contracted FIP). In
such a situation, the effect of a pre-existing infection with one type of virus on the contracting of another
can only be guessed at.
3. FIV Survival Outside the Body
Casual transmission of FIV by its very nature implies a period of delay, however brief, during which the
virus is outside the infected carrier, but not yet inside an uninfected cat. It has never been established in
HIV research that an intermediate host, such as a mosquito or flea, can transmit productive infection
[Webb], and the same is believed to be the case with FIV. A number of informational sources [e.g.,
Vet.cornell] have placed the viability of the feline immunodeficiency virus outside a body at “several hours.”
It is not clear (though it is certainly possible) that this figure has been empirically arrived at by laboratory
study. It is possible that the figure has been borrowed from studies of HIV. Various HIV resources mirror
the general figure of “a few hours,” although one, The Body, states, “Transmission must go directly from
one person to the other very quickly . . . the virus does not survive more than a few minutes outside the
body” [Sowadsky, italics added]. All reports of survivability must be qualified by the nature of the infecting
medium and the environment to which it is exposed. It has been reported, for instance, that bloodborne
HIV in a syringe can survive outside the human body for periods up to several weeks; viability is
“influenced by virus titre, volume of blood, ambient temperature, exposure to sunlight and humidity”
[Thompson]. While cause for concern among human drug users, this is of little practical relevance to
felines, although the variables cited undoubtedly play some role in an exposed environment as they do in
a hypodermic syringe. FIV, like HIV, is not particularly sensitive to ordinary room temperature, but is very
sensitive to such variables as pH level [AIDSMap], a fact which accounts, for instance, for inactivation by
stomach acid.
The question of most concern to cat owners is whether virus deposited within the environment, particularly
on food or water dishes, in litter boxes, or on the external cat can remain viable long enough to be a
potential source of infection. The answer is yes in some circumstances, although it is well established that
FIV has nothing remotely approaching the potential for lengthy environmental contamination of common
feline upper respiratory viruses such as herpes or calici. Moreover, any discussion of actual infectability
must take into account the dose of virus necessary to establish infection and the chance that the virus will
reach target cells [AIDSmap]. In other words, exposure to viable virus does not equate with actual
infection.
4. Potential Modes of Transmission
Viral “shedding” in an FIV+ carrier occurs in many of the same ways as in HIV+. HIV is present in saliva
and nasal secretions, though in small amounts by comparison with FIV, for which saliva is the primary
medium of transmission. Feline saliva can be shed through drooling and either self-grooming or grooming
of another cat; nasal secretions can be discharged by sneezing. Blood, of course, contains both viruses
and can be shed. Drying does not seem to affect the infectivity of HIV, and the same is probably true of
FIV [AIDSMap]. Urine and Feces both contain HIV and FIV, although neither has been regarded as a
source of HIV infection requiring special precautions for, for instance, healthcare workers.
With one possible exception, the hypothetical mode of entry of FIV as a result of casual interaction is oral.
The exception is “play biting.” Although the phrase “deep bite wound” has been treated as Scripture by individuals and organizations wishing to de-emphasize casual interaction as a mode of transmission,
common sense suggests that any bite capable of drawing blood is potentially capable of transmitting
infection. Bites of any sort are not regarded as modes of HIV transmission, but this is obviously not true of
felines and FIV. There has never been a published study of cats to determine what level of
puncture is necessary to produce infection, and even if there had been, the amount of virus (which will
vary with the disease state of the infecting cat) would certainly have to be factored in. Because they are
covered with fur, cats present great difficulty when it comes to inspecting for minor bite wounds that were
perhaps not delivered with serious intentions. Human suffer inconsequential breaches of the skin by teeth
easily when playing with overenthusiastic cats. The extent to which fur prevents such breaches in cats
playing with one another is hard to know.
Oral transmission of FIV has been demonstrated both in and out of the laboratory. Kittens, for instance,
can acquire FIV through lactation [Sellon]. Oral infusions in the laboratory have established productive
infection. One discovery, though, was that strains of virus differ markedly in infectability. Oral
transmission of cell-associated strains of FIV (those capable of spreading by direct cell-to-cell contact) is
inefficient, “requiring exposure of up to 10X greater virus inoculum than other mucosal surfaces for
systemic infection. In contrast, the transmission of cell-free inoculum using a C clade FIV is reported to be
more efficient across the vaginal and oral mucosa than rectal” [Burkhard, 2003]. Potential target cells and
tissues in transmucosal FIV infection are the tonsils (lymphoid tissue in commerce with retropharyngeal
and other lymph nodes) [Obert] or dendritic cells of the squamous oral mucosa [Sellon]
It has been speculated that oral disease causing bleeding gums or tissue breaks might be a particular risk
factor for oral transmission of FIV because it would provide direct access to the blood stream or to
infectable cells of the oral mucosa. Infection despite an intact oral mucosa, while possible, is
dependent on several factors. One, as already noted, is viral strain. Some strains of virus are simply
more efficient at infecting via mucosal surfaces than others [Burkhard, 2002]. A second is the amount of
virus. “Up to 10,000 fold more virus is required to infect by mucosal routes as compared to parenteral
routes (intravenous, intraperitoneal, or subcutaneous)” [Burkhard, 2003]. This may be the single most
telling reason why casual transmission of FIV is not routinely reported. To put the matter in
perspective, the milk of infected mothers contains substantial amounts of virus, and neonates consume
substantial amounts of milk. Compare this situation as a scenario for possible infection with hypothetical
scenarios for casual transmission: trace amounts of saliva deposited on water or food dish, whatever
modest amounts of saliva might be deposited in the act of grooming or sneezing, paws that have just been in a
common litterbox being licked. The obstacles to infection are not inconsiderable.
5. Productive Infection and Cryptic Infection
FIV lore, at both the scientific and individual anecdotal level, is filled with odd events that challenge
received wisdom and defy easy explanation. One of these is recorded in the study by Dandekar et al
(1992), in which 1 of 20 uninfected cats cohabiting with FIV+ cat became productively infected and
developed signs of disease. 10 of the remaining 19 produced an unexpected finding when bone marrow
and peripheral blood were subjected to PCR (polymerase chain reaction) for the presence FIV DNA.
Despite testing negative for FIV antibodies, they tested positive for the presence of FIV genomic material.
Moreover, “FIV RNA transcripts were detected in concanavalin A-stimulated PBMC cultures from 4 of 10
FIV DNA-positive, seronegative cats by in situ hybridization. PBMC from three of four naive SPF cats
acquired FIV nucleic acids after the cats were transfused with blood and bone marrow from FIV
genome-positive, seronegative donors. Three of five FIV-seronegative cats housed for years with naturally
FIV-infected cats in a private household were also found to harbor FIV DNA, indicating that the same
phenomenon occurred in the field. These findings demonstrate that cats living in close contact with
FIV-infected seropositive cats can acquire FIV nucleic acids without developing detectable levels of serum
antibodies or disease” [Dandekar]. Just as strangely, the cats negative for antibodies, although they
scored much better on absolute counts, percentages, and ratios (a common benchmark of retroviral
infections) of the CD4+ and CD8+ T cells typically lost to chronic infection, did not score as well as dna
negative seronegative cats.
Three explanations are suggested for this confounding outcome: (1) seroconversion (and antibody
production) were delayed but would eventually take place at some unspecified point in the future; (2) the
FIV genome was replication defective, failing to establish clinically significant disease course or to evoke
an immune response; (3) a strong local immune response at the point of infection resulted in containment
of the viral genome in cells at a level insufficient to stimulate antibody production. Since there was no
follow-up report, the validity of option (1) can’t be confirmed or refuted. As for option (3), a similar
phenomenon has been described in vertically (i.e., maternally) infected kittens [O’Neil]. In that case as
well the clinical significance (if any) of such “contained” infection was unclear.
What seems somewhat clearer is that casual exposure to FIV via relatively tiny doses of virus following
ingestion of infective saliva, feces, or urine can result in viral entrance to feline cells. As already noted, a
number of variables such as route of transmission (particularly if mucosal), quantity of virus, and
nature of viral strain may play key roles in determining whether productive infection and actual antibody
production occurs.
6. Conclusions
In recent years, the guidelines on FIV+ cats in multi-cat households provided by veterinary organizations
have struck somewhat different notes. The AAFP has taken the following stance:
Generally, cats in households with stable social structures where housemates do not fight are at a low-risk for acquiring FIV
infection, but a high rate of transmission within a household without observed fighting has been reported (Addie et al 2000). Therefore, separation of infected cats from uninfected housemates is recommended to eliminate the potential for FIV transmission. If separation is not possible, and to reduce the risk of territorial aggression, no new cats should be introduced in the household.
[Levy]
The European Advisory Board on Cat Diseases (ABCD) has adopted a somewhat more nuanced stance:
A number of factors can influence the risk of transmission of FIV between cats within a household. . . . FIV is mainly transmitted
through biting and fighting, and if no fights occur due to the stability of social structures, FIV will probably not be transmitted. In follow-up studies of households with FIV-infected cats, few additional cats became FIV-positive over time; some households exist in which no transmission has occurred over many years. It is advisable that all cats in these households be neutered, and it is crucial not to introduce new cats, as this might lead to fights and hence transmission may occur, even between cats that have lived peacefully together for a long time. [European]
The difference in emphasis seems to owe less to available scientific evidence than to the weight given to
it. It is unclear what “follow-up studies” are being referred to in the ABCD Guidelines: private inquiries into events recorded in existing studies subsequent to their end point? the quite substantial
advisory board relying on the clinical experiences of its own members and their contacts? Certainly there are many
private households and rescue organizations which insist that they have not seen casual transmission
occur. There are also occasional, anecdotal reports by people who say that they believe it has. It is
difficult in some cases to eliminate all other possible avenues of infection, but the impression is that at
least some of these claims are accurate. And the existing scientific literature suggests that there are
grounds for believing that it does occur.
Anyone choosing to integrate an FIV+ cat within an existing household of cats where some or all are FIV-
should do so with an understanding that there is some level of risk of transmission. That risk may rest, in
part, on factors that the owner is never likely to be in a position to know: the particular level of viremia in
the FIV+ cat, the particular strain of virus, etc. Exactly how great the risk is in any given case – or
generally – is a very difficult calculation. A majority of the time it is probably small. But anyone
contemplating a mixed household should nevertheless do so with open eyes.
__________________________________________________________
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