A paper that will be published in the September edition of Emerging Infectious Diseases (Mead et al 2011) talks about the potential for dogs to act as indicators of Lyme disease activity and risk for people. The use of animals as sentinels for human disease is well established. Sometimes it’s because animals are more readily affected. Sometimes it’s because the disease is easier to diagnose in animals. Sometimes it’s because getting access to samples from animals is easier than from humans.

Because of the distribution of ticks that transmit the causative bacterium (Borrelia burgdorferi) and wildlife that act as the reservoir, the occurrence of Lyme disease is highly variable geographically. Knowing the amount of Lyme disease activity in a specific region is important for understanding the role of this disease in humans, and for implementing preventive measures.

Like people, dogs can get Lyme disease. Dogs are NOT sources of human infection, but since both dogs and people get Lyme disease the same way (from Ixodes ticks), infections in dogs can indicate the potential for infections in people (and vice versa). Since dogs may be more prone to being exposed to ticks, and since ticks are most likely to stay on dogs for the 24 hours or so that is required for ticks to transmit the bacterium, dogs may be more likely to be exposed to Borrelia spp. in endemic regions.

That’s the reasoning in the Mead paper which suggests that dogs, because of their potentially greater chance of exposure and tendency to produce a good immune response after exposure, might be good indicators of human Lyme disease risk. To examine this premise, the authors compared data about B. burgdorferi antibody levels in dogs to human infection data. (Note: These are two different things: In dogs, they looked at antibodies, which indicate exposure but not necessarily disease. In people, they looked at disease. It’s not inappropriate to compare the two, but you have to be aware of what they were comparing).

Overall, they showed (not surprisingly) that there was a relationship between antibody levels in dogs and Lyme disease in people. Some key findings were:

When the percentage of dogs with antibodies against B. burgdorferi was <1%, there were extremely low rates of disease in people in the area.

  • This makes sense since it would indicate that there’s little or no transmission occurring in the area. Low levels of B. burgdorferi in dogs or people don’t necessarily indicate that Lyme disease is active in the area, since some cases could have been acquired during travel, and false positive results are possible with current testing.

The risk of disease is generally low to non-existent outside the highly Lyme-disease endemic areas: Northeast, mid-Atlantic and upper Midwest regions of the US.

  • These are the areas where ticks capable of spreading the bacterium are common and where the bacterium is resident in wildlife, so that’s not surprising.

Where 5% of more dogs had B. burgdorferi antibodies in their blood, there was always an above-average incidence of Lyme disease in people in the state, with a lesser association at the county level.

  • Again, this makes sense. If most dogs are exposed, more people are going to be exposed, and more people will develop disease.

In 15% of counties where dogs had a >5% rate of antibodies, people did not have above average disease rates. However, in half of them, the incidence of disease in people increased to above average in the following 3 years.

  • This is quite interesting and perhaps the most important finding of this study. It suggests that monitoring rates in dogs may predict trends in people.

What does this all mean? Well, a lot of these results would be expected based on what we know about Lyme disease. However, the apparent close linkage between human disease rates and dog antibody rates, and particularly the potential that dog rates could predict human rates, is intriguing and could be useful. By routinely monitoring for antibodies in dogs, areas where Lyme disease might be on the rise or might be emerging in people could be identified, leading to more focused educational efforts directed at both the public and healthcare personnel. Getting the dog data (or at least getting good dog data) is perhaps the problem, since testing would need to be done on a subset of the dog population that’s not biased and is of adequate size to say something useful. There are ways to do this, but it takes time and money to do it right. Taking advantage of blood samples collected for heartworm testing is one possible approach, but careful thought needs to go into what could be done and whether it would be better than more intensive surveillance of humans or ticks.

China has a huge canine rabies problem, with thousands of human rabies deaths each year. There are several reasons for this, including large feral dog populations, inadequate vaccination of pet dogs, differences in approach and access to veterinary care for pets, inadequate education regarding dog bite prevention, and presumably inadequate education of people and/or healthcare personnel regarding when and how to seek proper post-exposure treatment.

Periodically, the knee-jerk reaction of dog culling rears its head in China, despite the ineffectiveness of culling alone as a rabies control tool.

Recently, authorities in Guangdong province have banned ownership of dogs (in most situations) and given residents until August 26 to get rid of their pets. Dogs remaining at that time will be put down, except for dogs that are used to protect property worth ~ $750 000 (or more). Those dogs must be vaccinated and "kept locked up." (Why the same things (i.e. vaccination and confinement) can’t be done with any other pet dog is unclear, since being owned by a rich person doesn’t make a dog less susceptible to rabies.)

An expert from the Chinese Center for Disease Control summed up the issue nicely: "This [ban] is not scientific, not humane, and it will not last long. In short term, maybe it could be effective, but after that, people still want to keep dogs."

Culls don’t work well. A cull can decrease the population of concern for a time, but it’s extremely unlikely that it would reduce the population enough to have any longterm effect. Dogs can reproduce quickly and replace the culled animals in a short period of time.

What would make more sense?

  • Widespread vaccination of pet dogs, to reduce the risk of exposure of people from pets that get infected from feral dogs.
  • Widespread vaccination of feral dogs, to reduce exposure of people and pet dogs. Achieving high vaccination rates (>70%) in the feral population is a critical control measure, but can be very challenging.
  • Education of people about dog bite prevention.
  • Education of people about dog bite care, particularly ensuring that they seek post-exposure treatment if bitten by a feral dog or a dog of unknown rabies status that can’t be quarantined for 10 days to ensure it’s not rabid.
  • Education of healthcare personnel so that everyone who needs post-exposure treatment gets it (and gets it done right).
  • Education of public health personnel to ensure that the two points above get done right.
  • Ensuring adequate supply of good quality rabies vaccine and antibody for post-exposure treatment.

Yes, these measures require more work than a cull, and to some degree they also require a culture shift in the approach to keeping pets, but if China really wants to start preventing the thousands of rabies deaths that occur annually, that’s what needs to be done.

A recent paper about toxoplasmosis in the journal Biology Letters (Thomas et al. 2011) has attracted a lot of attention because it reports a possible link between this cat-associated parasite and brain cancer in people.

Toxoplasma gondii is a protozoal parasite. Cats are the natural host and can (but rarely do) pass the parasite oocysts (eggs) in their feces. People can be infected by ingestion of Toxoplasma oocysts that have been outside of the cat long enough (24 hours or more) to develop into an infectious form. Human Toxoplasma exposure is common, although this is typically not acquired directly from pet cats.  Adult cats rarely shed oocysts, whereas exposure from contaminated soil (e.g. from working in a garden and not washing ones hands afterward) or undercooked meat of various kinds are also important sources in different areas. Almost always, when a person gets infected, disease does not occur. Rather, the immune system contains the parasite, but is unable to kill it completely.  The parasite therefore often enters a dormant state forming small cysts in various body tissues, including in the brain.

Most concern regarding Toxoplasma involves pregnant women who have not been previously exposed to the parasite (and therefore have no antibodies to fight it off quickly). If they become infected during pregnancy, serious infection of the fetus can occur. Another problem in people is Toxoplasma encephalitis, a severe brain condition that occurs most commonly in people with compromised immune systems (e.g. HIV patients), typically from re-activation of dormant Toxoplasma. 

The recent paper by Thomas et al raises an additional concern: brain cancer. The basic premise of the paper is that some types of persistent infections might be able to induce mutations at the cellular level, resulting in cancer. Since Toxoplasma can cause longterm but dormant infection in the brain, they investigated a possible linkage.

The researchers took international statistics about malignant brain cancer from the International Agency for Research on Cancer database to determine brain cancer rates in different countries. They then compared these to Toxoplasma antibody rate data from 37 countries (from a 2006 paper that collected data from other papers). They found that the prevalence of antibodies against Toxoplasma in a country was related to the prevalence of brain cancer in the country (that’s an oversimplification of what they did, since they did various kinds of statistical analyses, but that was their basic conclusion).

Some things to consider, though, before you ask for an MRI because you’re a cat owner:

  • This is a pretty superficial study. When I started reading it, I assumed they had done a case-control study, whereby they took a group of people with brain tumours and an equivalent group of people without brain tumours, and compared Toxoplasma antibody rates. That’s not what was done. Rather, what they are essentially saying is that the incidence of brain tumours is higher in countries with a higher incidence of Toxoplasma antibodies in people. There are potentially a lot of other factors that might be involved. For example, they also showed that the incidence of brain cancer increased with increased gross domestic product (GDP) of the country, i.e. brain cancer rates are higher in wealthier countries. This could be because brain cancers are most often diagnosed as a result of more advanced medical care or that there are other lifestyle issues that might be risk factors for cancer. Toxoplasma antibody rates may be higher in high GDP countries (possibly because more people own cats, or for a number of other reasons) and thus the statistics shown an association between brain cancer and Toxoplasma antibodies.  It does NOT mean that Toxoplasma infection is necessarily causing brain cancer.  As the authors say, "This leaves open the possibility that brain cancers and T. gondii are both affected by a third correlated factor."
  • This study included all brain tumours. It’s hard to believe that Toxoplasma would be a risk factor for all of the various brain tumour types, if there is actually an association.
  • The Toxoplasma prevalence data weren’t obtained in a standard manner by the researchers. Rather, they took data from a paper written in 2006. That paper obtained the data from various other published papers, some decades old. It doesn’t mean that the data are not valid, but using data from studies that collected the information in different ways and from vastly different time periods creates some weaknesses.
  • Studies that are looking at a subject from a high level don’t necessarily tell you what the risk is at the individual level. As the authors state "…analyses of data aggregated at the population level may not pertain to individual risk." (In epidemiology, this is called the ecological fallacy.)
  • A large percentage of the human population (~1/3) has antibodies to Toxoplasma, indicating previous infection. That’s a huge number, yet the incidence of brain cancer is very low overall. Certainly, even if there is a true association, the likelihood of an individual developing cancer from Toxoplasma is very low.
  • Pet cats rarely shed Toxoplasma. They usually only shed for a couple of weeks of their life, typically when they’re kittens. Also, the Toxoplama oocysts must live in the environment for 24 hours or longer to become infectious and a person has to ingest them to get infected. Therefore basic hygiene practices should reduce the chance of exposure even if a cat is shedding oocysts.

This is an example of a study that is great for generating questions that need to be answered, but not answering the questions. They’ve shown a crude association between toxoplasmosis and brain cancer. What that means is more detailed study is needed to see if the association is real (i.e. not the result of some other correlated factor) and furthermore to try to determine if the association is actually causative. Unfortunately, the press and internet don’t understand that aspect and freak people out with statements such as "Cat parasite linked to brain cancer. A parasite spread by cats could almost double their owner’s chance of developing brain cancer, research suggests."

One of the paper’s concluding statements sums it up nicely: "Clearly, further research is necessary to determine the proximate links between T. gondii and different types of brain tumours and to investigate a mechanism of action." That means we need to work to determine what the true nature of the association is, and not panic at the possibilities.

Close to a dozen dogs in the Big Bay area of Michigan’s upper peninsula have been diagnosed with blastomycosis, an uncommon but regionally important disease. Blastomycosis is caused by a fungus, Blastomyces dermatitidis. It’s a dimorphic fungus, meaning it exists in 2 forms:

  • Normally, it lives in the environment in the mold form. This is the infectious form to which dogs (and people) can be exposed via inhalation, ingestion or contamination of wounds.
  • The other form is the yeast form. This develops from the mold form once it gets into the body, and this is what causes disease.

Dogs that are infected with blasto are of minimal risk to others since they are carrying the yeast form, and the yeast form is not transmissible under normal conditions. There is only a risk of infection in rare situations, such as a bite from a dog that has the yeast form in its mouth, or if someone sticks themselves with a needle that was used to sample an infected site. The main issue with finding blasto in a dog is that it is an indication that the fungus is present in the environment in the area where the dog has been in the past few months.  That means people who went to those areas may have also been exposed.

Knowing where blasto is present is important for diagnosing disease in people and animals. Blasto is also a great example of a disease when getting a travel history can be critical for diagnosis. In some regions, blasto is most common where people tend to vacation or have cottages. If a veterinarian doesn’t know that a dog has visited a high-risk area, blasto may not be considered. Not asking about travel history (or not getting a clear answer) can significantly impact the ability to diagnose this disease, and early diagnosis and treatment are critical for getting a good response.

If you live in an area where blasto is present, avoiding it can be tough. Staying away from areas that have been associated with the fungus can help, but defining this is difficult because of poor reporting and the long incubation period. Staying away from soil is pretty tough to do as a routine measure, so people living in endemic areas have to be aware of the disease and ensure that proper veterinary care is provided if there are early signs of infection (e.g. respiratory disease, skin lesion, unexplained weight loss). People who travel to areas where blasto is present should make sure their veterinarian knows about the potential for blasto exposure in any animals that may travel with them.

Dusty, the dog who tested positive for Hendra virus exposure on a Queensland farm, has been euthanized. The owners confirmed through their Member of Parliament that their pet had been euthanized voluntarily, rather than waiting for Biosecurity Queensland to compel them to do so.

Based on publicly available information, this seems like an illogical and unnecessary response, as well as an unethical approach by the government because it did not explain the implications of a positive result when they ask for voluntary testing of the dog.

The family said Dusty was euthanized because "most recent blood tests confirmed that he carried Hendra virus antibodies, which meant he was able to shed the virus to other animals." However, that’s false. If government officials told them that, that’s incredibly frustrating, disappointing and concerning. Antibodies have nothing to do with being infectious. I carry antibodies in my blood to a wide range of viral diseases that I’ve had over the course of my life. That doesn’t mean that I still have the viruses in m. The presence of antibodies simply means the body has been exposed and mounted an immune response.

All information that I’ve seen so far indicates that there’s no evidence that the dog was shedding virus and therefore no evidence that it posed a potential infectious risk. Furthermore, an experimental study from 1994 showed that infected dogs did not shed the virus. So, unless there are new data that aren’t being released, euthanasia is a completely unnecessary and illogical response, probably based more on fear of liability than any evidence or reasonable assessment of risk. It would have been better to quarantine and monitor the dog, to prevent unnecessary euthanasia and to learn more about this virus in dogs.

If evidence indicating a true risk of transmission of Hendra virus from pets exists, this information needs to be released so that other pet owners in Queensland know what the situation is. If not, more common sense needs to be used when dealing with this disease in such animals.

(click image for source)

This Worms & Germs blog entry was originally posted on equIDblog on 01-Aug-11.

Sunday’s Toronto Sun contained an article entitled "Woman’s hand disfigured by cat attack" with the compulsory gross picture. The story is about Brenda Sims, who took in a feral cat, was then bitten by the cat, and then developed severe complications from the bite. The situation is a reminder of the potential problems encountered when dealing with feral animals (cats included) and complications that can occur from any cat bite.

Back in April, Ms. Sims took in a young male cat that she described as "five pounds of pure disease."  The cat, named Cheech, had been taken off the streets and was clearly not someone’s lost pet. Cheech began following her around the house, growling and hissing. Not surprisingly, the well-meaning woman was eventually bitten. 

Ms. Sims has had multiple surgeries since then, including one that removed a large section of tissue from her hand and replaced it with some taken from her thigh. She’s been largely unable to work since the injury and faces long term problems with function and appearance of her hand.

Ms. Sims is warning people about the dangers of feral cats. "It’s like taking a tiger out of the jungle and into your home, and expecting it to be all good, and be a house cat..."

Dr. John Embil, Director of Infection Prevention and Control for the Winnipeg Regional Health Authority echoes her warning about feral cats, adding that feral cats can transmit rabies or severe bacterial infections such as Pasteurella multocida through bites. That’s very true and a good reminder, but it perhaps overstates the importance of "feral." Any cat can carry Pasteurella multocida, along with many other problematic bacteria, in its mouth. A bite from any cat – domestic or feral – can cause serious infections. While an individual feral cat may be more likely to bite than am individual pet cat, most cat bites and cat bite infections are presumably from pet cats because people have more contact with them.

Dr. Embil highlights the problem with cat bites: "The concern we have with cat bites is that the teeth are very long, sharp and pointy. And those teeth can puncture deeply. You can get serious infections." Cat bites can be similar to injecting a bacterial cocktail deep into the tissues, not surprisingly resulting in infection in many cases.

Some take home messages:

  • Leave rescuing feral cats to people with experience.
  • If you want to rescue a cat, get it from a shelter, humane society or other reputable and experienced organization. They will have assessed the cat’s behaviour and health status, which will decrease the chance that you’ll get an aggressive and/or sick cat.
  • If a cat constantly stalks you around the house looking like it’s going to attack (and not like it wants to play), it probably will attack and it shouldn’t be in your house.
  • Any cat bite can cause severe complications. Every cat bite should be taken seriously and medical advice should be sought. Bites over the hands, feet, joints, tendons, genitals or in people with compromised immune systems typically require antibiotics to help prevent infection.

More information about cat bites can be found on the Worms & Germs Resources page.

Neil Fearon and his family have lost three horses to Hendra virus, and are concerned about one other. They are now dealing with the implications of their dog, a Kelpie named Dusty, having tested positive for Hendra virus antibodies in its blood. As I mentioned in yesterday’s post, the presence of antibodies in the blood of this dog, detected during voluntary testing as part of the outbreak response, only indicates that the dog was exposed to the virus. Viral shedding was not identified, suggesting that the exposure was a prior event and that an active infection was not present. Despite this, government authorities are requiring that the Hendra antibody-positive dog be euthanized.

Poor communication and mixed messages are often the cause of problems during outbreak management, and this seems to be the case here. Based on the news reports, there are some pretty concerning issues.

Testing of the dog was voluntary and the owner was not notified that euthanasia would be required if the dog tested positive.

  • This is rather unethical. People need to understand the implications of outbreak control measures. It’s not fair to have such an aggressive response to a voluntary test without proper notification.

Mixed messages are being given about the risk the dog poses to the family.

  • Authorities want to euthanize the dog, indicating they must believe there is some risk. However, the owner is very concerned about his 11-year-old son who has slept with the dog in his bed for the last few weeks. Yet, ABC news indicates authorities reassured Mr. Fearon that the risks are minimal. If the risks are minimal from that type of prolonged, close contact during the period when the dog may have been actively infected, it’s hard to justify euthanasia after the fact on the basis of the dog posing a risk to people or animals (especially when the virus is endemic in the bat population in the area).
  • Why euthanasia is being required seems to be unclear. While fear of Hendra virus shedding makes the most sense, Queensland’s chief vet has stated that the dog will be euthanized as a precaution because "As a result of that infection, it may make it aggressive." It seems rather strange to euthanize a dog because of concern that an infection (which may not be active) might cause aggression, with no evidence that disease will occur or that it can cause aggression in dogs. Quarantine and observation would make more sense. There are a lot more dogs that are prone to aggression wandering around Australia.

This type of action drives things underground.

  • When overly-aggressive actions are used, and people either don’t agree with them or don’t understand them, faith in the system decreases. What’s the likelihood that people are going to allow their pets to be tested now? I assume it’s a lot lower now that they’ve seen what will happen. So, the ability to determine exposure of other species and the potential risks from other species will be impacted.

Hendra virus is not something with which to play around. It’s a very serious disease and one must err on the side of caution. How far you err on the side of caution is the question, and it’s a hard thing to determine. It’s easy to be very strict when setting rules, and fear of liability or fear of making a subjective decision often override logical thought and discussion.

As a somewhat informed outsider, I have a hard time supporting mandatory euthanasia for a dog that has evidence of previous infection but no evidence of active viral shedding. Yes, no test for virus shedding is 100%, but a pretty high level of assurance can be obtained and the dog can be quarantined for further testing. There’s no indication from laboratory studies that I know of that dogs (or other non-bat species) can become longterm carriers of the virus. The owners should be involved in the decision making process and be given enough information to understand the implications of keeping the dog, the risks that might be present, and what they can do to reduce the risks. Government authorities need to clearly state their concerns and the evidence supporting them. With that, it’s easier to make a logical plan that protects the public but is also appropriate for the animal and its owners. If the risk is deemed to be real and/or the owners are not willing to accept some degree of risk, then euthanasia is reasonable.

"Kill the dog" is an easy knee-jerk response. I simply don’t see the evidence supporting it. Is it possible that authorities have a true reason to be concerned? Sure, but if so, that indicates another communication problem. If there is really evidence that this dog is a concern, this needs to be clearly communicated so people understand what’s happening and why such drastic actions are being taken.

(click image for source)

This Worms & Germs blog entry was originally posted on equIDblog on 27-Jul-11.

Adding a new twist to the already very concerning situation in Australia, Hendra virus infection has now also been identified in a dog. It’s been a bad year for Hendra virus in Australia, with larger numbers of cases of this highly fatal disease in horses in a geographic range that seems to be expanding. Spread by flying foxes (fruit bats), Hendra virus predominantly infects horses, but can be transmitted to people working with infectedhorses.

The Australian Animal Health Laboratory in Geelong has now announced diagnosis of Hendra virus infection in a dog. The dog is from a quarantined farm in Queensland where the virus has been identified in a horse. The dog was healthy and was tested as part of a standard policy to test dogs and cats on infected farms. It’s great to see this approach being used, since it helps identify other potential sources proactively – something that is often overlooked in outbreak investigations that focus only on the main species that are already known to be involved.

In this case, the dog had antibodies against the virus in its blood. That means that it was exposed to the virus and mounted an immune response. It doesn’t indicate whether it was exposed recently or in the past. Two tests for the virus itself were negative, suggesting that the dog’s immune system eliminated the virus (or that the virus isn’t really capable of surviving for long in a dog). This is a good news/bad news scenario.

Bad news:

  • Dogs can be infected. It increases the range of known susceptible species.
  • If dogs can be infected and shed live virus, then they could be a source of infection for other individuals, including people.
  • The dog wasn’t sick. This might sound like strange "bad news," but healthy carriers of infectious diseases are harder to spot and control than ones that are sick.

Good news:

  • The dog wasn’t shedding the virus. That’s critical since if dogs can be infected but not infectious (i.e. if they can carry the virus but not transmit it), then they are of limited concern.
  • They have been testing farm dogs and cats as a routine measure, and this was the first positive. Infection of pets therefore must be relatively uncommon even on farms where the virus is active.
  • The dog wasn’t sick. While it’s only one case and doesn’t guarantee dogs won’t be affected clinically, this might suggest that dogs just occasionally get exposed with no disease. Since it’s highly fatal in other species, that’s a good thing.

What should be done based on this?

  • Probably not much more than should have been done before this finding, but it’s a good reminder about the potential involvement of other species.
  • Dogs and cats should be kept away from fruit bat roosting sites.
  • Dogs and cats should be kept away from infected horses.
  • If a farm is quarantined because of Hendra virus, dogs and cats should be tested and quarantined. Quarantining the animal while testing is underway helps reduce the risk of an infectious dog or cat (should that occur) transmiting the virus to people on the farm, or wandering away and exposing other people or animals.
  • Animals of any type in areas where Hendra virus is active that get sick with signs that could possibly be consistent with Hendra virus infection should be tested.

This should also be taken as yet another reminder that infectious diseases are unpredictable. Considering the potential involvement of different species in a proactive manner as was done here is critical.

Image: Bay Horse and White Dog by George Stubbs (1724-1806)

This Worms & Germs blog entry was originally posted on equIDblog on 26-Jul-11.

A 73-year-old New Jersey woman has died of rabies after being bitten by an infected dog. The woman was visiting Haiti in April when she was bitten, and she developed signs of neurological disease in late June. Family members and healthcare workers are being assessed to determine whether they may have been exposed to rabies during care of the woman. If so, post-exposure treatment would be started.

Rabies is a devastating but almost 100% preventable disease. While rare in most developed countries, canine rabies is a huge problem internationally and kills tens of thousands of people every year. The main reason it kills so many people is because of inadequate access to proper post-exposure treatment or failure to seek medical care. Timely access to post-exposure care can virtually guarantee that a person won’t get rabies.

Why this woman didn’t get post-exposure prophylaxis (I’m making the assumption that she didn’t) isn’t reported, and it could be because of patient or healthcare factors such as:

  • Assuming a minor bite isn’t a big deal.
  • Not thinking about the potential for rabies.
  • No access to adequate heatlhcare.
  • The physician not thinking about rabies.
  • Inadequate or no supply of rabies vaccine (for post-exposure treatment).

All of these problems can occur.  Education of the public and medical personnel, as well as ensuring adequate access to rabies vaccine, are critical to prevention.

More information about rabies can be found in the Worms & Germs Resources page.

As Australia faces a particularly bad year for Hendra virus, with possible expansion of the range of this serious disease, there have been calls for a mass cull of flying foxes (fruit bats).  These bats are the reservoir of the virus but also a protected species. The virus lives in the bats and is spread mainly through their urine. Horses that are exposed to bat urine or feces (e.g. grazing under a tree where bats are roosting) can become infected and then serve as a source of human infection. Being a highly fatal disease for which there is no available vaccine, looking at ways to reduce exposure to the virus is critical.  When you have a wildlife-associated disease, questions about trying to eliminate the wildlife source often arise.  Any discussion of culling wildlife leads to intense debate, and this situation is no different. Some people support culling bats in areas around people and horses, while others are opposed on various grounds, including a lack of evidence that it will be effective.

Can fruit bat numbers really be decreased? A lot of bats would have to be killed to have a significant impact on the population. Bats can reproduce quickly and migrate readily, therefore a single cull may have only a limited and short-term effect. A good understanding of the dynamics of the bat population is required to determine how many would need to be killed in a given area to have any significant impact. As Biosecurity Queensland‘s chief veterinarian RIck Symons stated "Culling is against government policy. I believe in terms of biosecurity it’s counterproductive, because it does stress flying foxes and they’re more likely to excrete (the virus). It could be filled by another bat colony the next day and if you’re moving them on, you’re moving it on to somebody else and it’s somebody else’s problem, so that is not the solution."

Will a cull actually achieve anything? Even if effective at reducing bat numbers (probably just in the short term), culls don’t necessarily have an impact on disease rates. All bats would not be eliminated, and it’s unclear whether there is a critical mass of bats that is required to transmit infection or whether a small number of bats distributed across the same region would be as likely to result in infections. Small or temporary decreases in bat numbers may have no effect.

What unintended consequences might occur if a cull is effective at reducing bat numbers? Removing an animal from any ecosystem has an effect, and it’s important to be confident that that effect isn’t accompanied by problems of its own. I don’t know enough about fruit bat ecology to say much here, but if this species is greatly reduced, are there other species that will come and occupy that ecological niche, and might they be associated with problems of their own? Careful scientific study can help to figure this out in theory, but you can never be certain.

Are other control measures, such as removing roosting sites from pastures and other bat avoidance measures, being adequately used? Culls should only be considered when other measures have failed, but it can be difficult to ensure or enforce compliance with these other measures. Certainly, people in endemic areas should remove trees in which bats roost from pastures. However, not all Hendra cases are associated with identifiable roosting sites.  For example, one affected Queensland farm does not have any fruit bats residing on the property, but it lies along a common flight path for the bats.

It’s easy to talk about avoiding a cull when you’re not in the heart of the Hendra epidemic, and I understand the reasoning behind the calls for a cull. Hendra is a devastating disease that’s a threat to both horse and human health, and it’s unpredictable – and that’s scarey for a lot of folks. People that have been exposed face an incredibly stressful period while they wait and see if they’ve been infected with a virus that kills in ~50% of cases. A vaccine is probably still a couple of years away, leaving a period of continued risk and stress. With such a serious disease, considering culling is reasonable. However, it can’t be a knee-jerk reaction to public outcry. It needs to be based on sound science to ensure that if it’s used, it will be effective. The impact on this protected species also can’t be ignored.

This Worms & Germs blog entry was originally posted on equIDblog on 19-Jul-11.