Results of a new computer model suggest that sterilization via vasectomy and hysterectomy is more effective than traditional spay/neuter at reducing the population of community cats. But the work raises several questions about the model’s validity—and more troubling ones about its implications for animal welfare.
Since starting this blog a little more than three years ago, I’ve been describing TNR as a compromise—but the best option we’ve got in most circumstances. But what if there’s a better option, a non-lethal method for managing the population of stray, abandoned, and feral cats that reduces their numbers more quickly?
According to a team of researchers at Tufts University, the answer is trap-vasectomy-hysterectomy-release, or TVHR. By eliminating the possibility for reproduction while leaving the cats “hormonally intact,” this method takes advantage of biological and behavioral characteristics not found in cats subject to traditional spay/neuter surgery,* thereby outperforming TNR in reducing colony size.
Or at least that’s what their computer model predicts.
What doesn’t require a computer model to predict, though—what we already know—is that hormonally intact cats are at greater risk of FeLV and FIV infection,  more likely to roam, resulting in disappearance and/or death,  and more prone to the nuisance behaviors [3, 4] that so often result in their impoundment and death. [5–7]
OK, so who’s in?
“The simulation showed that to reduce the population by a quarter, 57 percent of the cats in a colony [of 200] had to be removed by lethal means or captured, neutered and released. TVHR, however, could reduce the population by half [in 6,000 days] with an annual capture rate of 35 percent and at that rate could completely eliminate the colony within 11 years. (TNR required capturing 82 percent of the cats in order to eliminate the colony in 11 years.)” 
Co-authors Robert J. McCarthy, clinical associate professor at the Cummings School of Veterinary Medicine, J. Michael Reed, professor of biology in the Tufts’ School of Arts and Sciences, and Stephen H. Levine, professor of civil and environmental engineering at Tufts University School of Engineering, suggest that TVHR ought to be “recommended as a humane and more effective method of decreasing population size”  for island eradications.
In fact, it would be tough to come up with a less humane method for such situations, where the traditional “management toolbox” has typically included disease, poison, hunting, and trapping.  But how does TVHR compare to TNR?
Well, it’s a matter of some debate whether TVHR is more effective. (Or more humane, for that matter—I’ll come back to that shortly.)
Let me begin by saying that I don’t have a detailed understanding of the model’s various inputs and all of its assumptions. That said, I am quite familiar with much of the research upon which McCarthy et al. drew in its development.
Chief among my concerns is their heavy reliance upon an area of research—examining the effects of sterilization on the social structure and corresponding behavioral changes—in which relatively little work has been conducted. As the Alliance for Contraception in Cats & Dogs notes in a statement (PDF available on the organization’s website) issued last Monday:
“The evidence for the success of TVHR in this study depends on assumptions related to the reproductive physiology and behavioral ecology of domestic cats that, in our view, do not seem to be supported by available empirical evidence and remain to be better understood.” 
And, although McCarthy et al. note that “kitten survival rate is a separate and important consideration when comparing methods of population control and in fact may be the major factor limiting population size”  their model fails to take into account the removal of kittens for adoption—standard practice for TNR practitioners in the early stages of colony management. While I don’t know of any research to examine this directly, I suspect there’s a much better chance of pulling kittens from a typical TNR colony than from a hormonally intact—and therefore relatively unstable—colony. (And the difference would be even greater comparing TNR to lethal methods, the implementation of which is typically complaint-driven and therefore focused largely on adult cats.)
The impact of this one oversight is significant. When the Tufts team (based, again, on some pretty thin evidence) modeled the effect of kittens being much more likely to survive among neutered cats, they found that “TNR performed progressively worse and could be counterproductive, such that population size increased, compared with no intervention at all.” 
So, now TNR is worse than looking the other way?
The authors will have a very difficult time persuading anybody who’s actually done it. Or any community that hasn’t.
Another dubious assumption has to do with trapping efficiency—the Tufts team assumes that there’s no difference between TVHR and TNR.** But, as others (whose work McCarthy et al. cite) have demonstrated, “immigration rates were significantly higher and emigration rates significantly lower in the neutered groups than in the unneutered groups.”  This suggests that the likelihood of trapping both the trap-shy holdouts and a colony’s new arrivals is going to be easier under the TNR model.
One wonders, in light of these many issues, just how well the model would have predicted the outcomes of some of the more extensive TNR studies [2, 13], or even the less extensive study that figures so prominently in the Tufts model. [4, 12]
Frankly, I don’t think any of this other research would validate what McCarthy et al. are predicting.
Animal Welfare, Anybody?
If the cats are difficult to trap under a TVHR model, the trappers may be even tougher to come by.
How many TNR trappers and colony caretakers—folks whose dedication and resiliency is truly remarkable—are going to sign up for TVHR, with its numerous undesirable tradeoffs? As anybody in the trenches will tell you, TNR is, under the best of circumstances, already fraught with worry and loss.
With TVHR, there’s more to worry about and the losses are greater.
Cats roam further and more often, and their spraying, yowling, and fighting are more likely to attract the attention of unsympathetic neighbors—resulting in a one-way trip to the shelter. (And this assumes the caretaker is willing to put up with these same behaviors.)
It’s been shown that FeLV and FIV infection rates among TNR colony cats are no different than rates for pet cats. [14, 15] But if the results of a three-year study at the Rio de Janeiro zoo investigating the “impact of hysterectomy of adults, with conservation of the gonads, as a means of population control”  are any indication, infection rates under the TVHR model would likely skyrocket.
Over the course of the research, 75 free-roaming cats were examined. In 2001, no FeLV was detected; in 2004, 13 of 33 cats (39.4 percent) tested positive. And the rate of FIV infection increased from 21 percent (7/39) in 2001 to 75.8 percent (25/33) three years later. [1, 17]
Granted, vaccines are available to protect against FeLV and some strains of FIV, but their cost and corresponding administration can be a significant burden on organizations that are often already struggling with limited resources.*** And cats vaccinated against FIV will subsequently test positive for the virus, greatly complicating colony management.
But, while McCarthy et al. acknowledge these shortcomings, they fail to recognize their real-world implications (or theoretical ones, in the form of reduced trapping efficiency). “Vasectomy allows persistent undesirable male behaviors such as fighting, vocalization, and urine marking,” the authors explain, “and in some situations, elimination of these behaviors is the main impetus for the control program itself.
“Fortunately, in most feral cat populations worldwide, this is not the case, and if vasectomy results in a more rapid and persistent population decline, negative behaviors may be acceptable in the short term.” 
So who’s the intended audience here?
Given that the “short term” is something like 11 years, even with an ambitious 35 percent annual capture rate, I think TVHR would be a very tough sell in most U.S. locations. Indeed, McCarthy et al. acknowledge as much, noting that “the popularity of TNR in the U.S. has been in part due to a goal of maximizing feral cats’ quality of life (e.g., extended life span, vaccinations, assessment for infectious disease) and reducing undesirable behaviors such as aggression and vocalization while still eliminating colonies over time.” 
But quality of life for these cats doesn’t seem to be much of a concern for the Tufts team, who make no mention of the obvious and inevitable negative consequences of TVHR in their press release. As a result, neither did the Los Angeles Times, among others, in their coverage of the story.
“Sterilizing feral cats without removing their sexual organs,” proclaimed the lede, “would do more to control their population than spaying and neutering, according to a new study.”  (The paper also failed to acknowledge the inclusion of lethal control—which, according to the results, was also inferior to TVHR—as one of the three methods modeled.)
• • •
Having read the JAVMA article and the press release, it’s difficult not to see this work—interesting in its own way, to be sure—as just another agenda-driven attempt to undermine TNR. And I say that not merely because of the way it’s been distorted (just as the authors intended, I suspect) in the media.
No, McCarthy et al. tipped their hand six weeks ago in a synopsis posted on the Tufts website, in which they rationalized their work, in part, by declaring:
“What is clear is that feral and free-roaming cat populations cause tremendous and often irreversible damage to indigenous wildlife populations in both urban and rural environments around the globe.” 
Short on actual evidence, the team noted—correctly, but misleadingly—that “cats are listed by the International Union for Conservation of Nature (IUCN) as one of the 100 worst non-native invasive species.”  They failed to acknowledge that this designation has mostly to do with cats’ impact on wildlife native to oceanic islands. 
Further justification was provided in “the estimated annual environmental and economic cost of feral and free-roaming cats in the United States… $17 billion.”  As I’ve discussed previously, even a cursory examination of this “estimate” raises doubts about its validity, and the work involved in its derivation has been thoroughly discredited in a recent Ecological Economics commentary. 
The final straw, though, came in the paper itself, where McCarthy et al. refer to the mythical—and long-ago myth-busted—“Wisconsin Study.”
All of which are big red flags, raising serious questions about the authors’ creditability on the subject and, frankly, their motivation. One wonders, too, what these three are teaching the next generation of veterinarians, biologists, and engineers.
* A little Anatomy 101: A conventional spay involves the removal of the ovaries and uterus; neutering (or castration) involves removals of the testes. Both procedures lead to a significant reduction in sexual hormones.
** The authors also fall into the familiar trap of assuming that the trapping rate for lethal control purposes—whatever it is—is equal to that of a TNR (or even TVHR) program. Given (1) the popularity of TNR (as demonstrated by participation levels) over lethal roundups, and (2) the limited resources of most municipalities, this seems to be wholly inappropriate.
*** The efficacy of these vaccines is a matter of some debate, but is well beyond the scope of the current discussion.
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