All 16 entries tagged Epidemiology
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January 31, 2012
The AHVLA reported today that it has now confirmed the presence of Schmallenberg Virus on 11 sheep farms in the UK. The disease has also been found in Belgium, Germany, and the Netherlands.
What do we know about Schmallenberg virus?
It’s named after the town of Schmallenberg, from where the virus was first isolated. The virus (SBV) is an Orthobunyavirus, a large group of viruses, most of which are spread by biting insects and cause diseases of cattle (Wikipedia has a list, if you’re interested), although a couple of species cause encephalitis in humans. SBV has been found in cattle, sheep, and goats. In adult animals it causes fever, loss of appetite, a reduction in milk production, and sometimes diarrhoea, although the symptoms only last for a few days. More worryingly, though, are the effects on the offspring of infected animals – abortions and stillbirths of young with brain and limb abnormalities (torticollis, arthrogryposis, and hydranencephaly). We saw the clinical signs in adult animals in Europe during the summer, and now we’re seeing the deformed offspring in the UK and the continent.
Is that all?
Pretty much! All the rest is based on behaviour of related viruses, and what we have (and haven’t) observed.
So what are our best guesses about SBV?
- It’s most likely spread by biting insects (this is called vector transmission), probably midges
- It’s probably no risk to human health
- The cases in the UK are likely due to infected vectors blowing over the channel, in a similar manner to bluetongue
- There are probably more infected animals than we are aware of
What don’t we know that we’d like to know?
- Can it infect other species? Other ruminants would be obvious possibilities
- What’s the geographic spread of this disease?
- What insect species is/are competent vectors?
What happens now?
Work is ongoing to try and develop good tests for SBV; particularly a test that would identify animals that have been exposed to it would give us a much better idea of how far the disease has spread and what its prevalence is. Similarly, researchers will be trying to find out the basic epidemiology of SBV, so that control measures can be devised. Animal keepers are being encouraged to look out for deformed newborn, stillborn, or aborted ruminants, and to report them to the relevant authority (AHVLA in the UK). It’s going to be interesting to see how the situation develops; given how mild winter has been so far, we might well expect higher midge numbers this year, which I’d expect to increase the spread of SBV.
If you want to know more, DEFRA is publishing updates on the situation here.
September 01, 2011
Dengue fever is an important viral disease, affecting around 50 million people a year. It’s a vector-borne disease, meaning that people cannot directly infect each other. Rather, mosquitos, most commonly Aedes aegypti, transmit the infection. First, a female mosquito must first bite an infected person, then the dengue virus multiplies within the mosquito (a process taking 7-14 days), then the infected mosquito bites an uninfected person, and transmits dengue through its saliva. Aedes mosquitos live near humans, and tend to bite during the day, making bednet-based strategies much less effective at reducing transmission of dengue. Despite attempts to control Aedes mosquitoes, dengue is not being effectively controlled in tropical countries, and dengue’s range (and the severity of outbreaks) is increasing.
Recent research published in Nature suggests a promising new approach to controlling dengue. Briefly, the researchers have managed to infect mosquitos with a strain of Wolbachia (a type of bacteria) that stops dengue from reproducing in those mosquitos. Better still, the strain seems able to infect over 90% of wild mosquitos when infected mosquitos are released in field conditions.
How does this work? Well, Wolbachia pipientis has a neat mechanism for invading insect populations called cytoplasmic incompatibility. It is maternally transmitted (i.e. infected females pass it to their offspring), but if an infected male mates with an uninfected female, then the resulting embryos die. An infected female, however, can successfully reproduce with both infected and uninfected males. This means that infected females have a reproductive advantage over uninfected females, which helps Wolbachia to invade the mosquito population. Nonetheless, many Wolbachia strains are pathogenic in mosquitos, making the mosquitos shorter-lived or their eggs less viable. Walker and colleagues have developed a strain (derived from one found in Drosophila, the fruit fly beloved of lab biologists) that has relatively little deleterious effects whilst still blocking dengue transmission, and shown that it can invade caged populations of Aedes aegypti. In a related article published in the same edition of Nature, Hoffmann and colleagues take this approach further, and release infected mosquitos into two areas of north-eastern Australia. They were able to infect the vast majority of the wild population of A aegtypi with Wolbachia by 9 roughly-weekly releases of infected mosquitos.
This is exciting stuff, as it means that populations of mosquitos infected with a strain of Wolbachia that blocks transmission of dengue can be established reasonably straightforwardly, with obvious implications for the control of dengue. The next step, of course, is to show that this approach can actually reduce the incidence of disease in the field…
August 24, 2011
There was quite a fanfare when Smallpox was officially eradicated 1979. This year saw the second-ever eradication of an infectious disease, which seems to have been much less widely reported. That disease is Rinderpest. An informal survey at a party the other day suggests that while just about everyone has heard of smallpox, almost no-one knows anything about Rinderpest.
Rinderpest used to be devastating in the UK, but we’ve not had a case here since 1877, which may explain why it’s not as well-known as, say, foot and mouth! It’s a viral disease of cattle, buffalo, and some other even-toed ungulates, from the same family as canine distemper and measles. Rinderpest is a deadly and highly infectious disease, with mortality rates of over 80% in naive populations. It does not, however, survive long in the environment. This means that slaughter, movement control, and import restrictions are an effective battery of measures; these were used to clear rinderpest from much of Europe in the late 19th century.
Globally, however, hygiene measures alone were not sufficient. The development of an effective vaccine was, therefore, a real breakthrough. Effective co-ordinated vaccination campaigns lead by the FAO meant that by the mid 1990s rinderpest was confined to six areas of the world (four in Asia, two in Africa). The Global Rinderpest Eradication Campaign (GREP) was launched in 1994, aiming to eliminate rinderpest by 2010. This was a major effort, involving epidemiology, vaccination, careful surveillance extending for years after the last observed case, and training of local veterinary services, and was very successful – the last known case of rinderpest was found in Kenya in 2001. Another decade of surveillance based on state of the art diagnostic tests revealed no further cases, and so rinderpest was officially declared eradicated in June 2011.
If you want to know more about rinderpest, there’s a good article on the IAH’s website here.
August 23, 2011
Writing about web page http://www.biomedcentral.com/1746-6148/7/31
As a consequence of The BSE outbreak in the late 1980s in the UK, the UK government set up a computerised system for recording the births, deaths, and movements of cattle. Spurred on by the realisation that the 2001 foot and mouth disease outbreak was initially spread around the country by people buying infected animals, these data have been made available to researchers. This means that we now have a comprehensive picture of the lives, locations, and deaths of cattle in the UK for over a decade – some 43 million animals, and 157 million animal movements!
This is a great resource for epidemiologists, as every trade in live animals is risky – if I sell you cattle from my farm, then if there are infectious diseases circulating on my farm, there’s a chance that you’ll buy infected cattle from me, and thus I have transmitted infection from my farm to yours. But, trading animals is essential to the farming industry in the UK, so there will always be a balance to be struck between economics and infectious disease control.
So what do all these data tells us? Well, for the full details, have a look at my recent paper in the journal BMC Veterinary Research, but since it’s open access, I can show you a couple of the figures here. In common with other European countries, British cattle tend to not move that often during their lives, and to not move great distances, although a few cattle really rack up the miles:
Of course, the other thing you can do with a decade’s worth of data is to look at trends across time. If we look at the number of movements per month, you can can clearly see the troughs caused by foot and mouth disease in 2001 and 2007:
(The peak in 2000 is an artifact of “Cattle Count 2000”, not a real surge in movements)
A seasonal trend is pretty clear, too – more animals are moved in the spring and autumn, co-inciding with the main calving times. Beyond that, there isn’t much change over time; if we look at the numbers of cattle each farm moves on and off in a year, there was some increase between 2002 and 2005, but things have rather levelled off since:
(the boxes are median and quartiles, whiskers are the central ninety-five percentiles, the solid lines are the means)
This, actually, shows the value of taking a long-term view: some scientists looked at the changes between 2002 and 2005 and concluded that there was a real increasing trend in movement numbers (and some associated disease-risk measures); we can now see that this was only a short-term issue. These data, as well as providing an insight into the structure of cattle farming in the UK, might well be the basis for a predictive model of livestock movements. That would be really useful in trying to understand the impact of proposed regulatory changes – rather than introducing things like pre-movement testing for tuberculosis and hoping they’re beneficial, we could attempt to model what changes would occur, and so design better regulations to keep the UK cattle industry safe.
You might be wondering where the infectious disease modelling is in this work – how has the UK’s vulnerability to an invading disease of cattle changed in the last decade? I’ve wondered that too, and have been doing some work on it, but that paper’s still with referees, so you’ll have to wait a little longer before I can write about it here…
August 04, 2010
Writing about web page http://www2.warwick.ac.uk/fac/cross_fac/gld/results/papers/deframoveconsult1.pdf
Along with some colleagues from the Governance of Livestock Diseases project, I have responded to DEFRA’s consultation on the livestock movement regulations. Our response should be linked above, with a bit of luck!
Our basic point is that we agree that simplifying the livestock movement regulations, and removing some of the more daft exceptions is a sound idea, but that it’s important to think about what the rules are for whilst revising them. Collecting accurate and comprehensive livestock movement data doesn’t just keep researchers happy, it enables a sound evidence base for disease control policy to be developed, and is vital during a disease outbreak.
I look forward to seeing the outcome of the consultation exercise, and seeing how the new government actually goes about implementing change.
April 26, 2010
DEFRA have launched a consultation on simplifying the livestock movement and animal identification rules. That’s more interesting than it might sound! Our models of disease spread amongst UK livestock depend on data provided by DEFRA on where those livestock are, and when and where they move.
The current movement regulations are overly complex, and exclude some classes of cattle movement that are epidemiologically important – for example, movements to and from common grazing land (where cattle owned by several keepers can meet and potentially transmit disease) are not currently required to be reported to the British Cattle Movement Service. So this consultation has not only the potential to simplify the system somewhat, but also to make more accurate livestock movement data available to epidemiologists. We will be submitting a response, and I’d like to invite any interested parties reading this to consider doing so, too. The deadline is 30 June, 2010.
Researchers studying disease transmission in people also need good data about contacts between individuals. To this end, let me encourage you to go and fill in the Social Contact Survey. It doesn’t take very long, and the more people who do so, the better data we’ll be able to collect. Tell your friends, too ;-)
March 25, 2009
I’ve had a few comments and emails since my previous entry on the Welsh Assembly Government’s proposal to cull badgers. Yesterday, Elin Jones, the Rural Affairs Minister made another announcement about her proposal, so it seems a good time to look again at what is going to happen.
There is still precious little detail in the proposal, and my request for more details on exactly what was going to be done was ignored by the Office of the Chief Veterinary Officer, who did little more than send me the URL of their website by way of reply.
There are two big problems with the planned cull that I didn’t address before. The first is that trapping and shooting is not an effective way to cull out a badger sett. We know from previous research that the harmful effects of badger culling – increased spread of TB in the surrounding area – are at least partly due to partial culls, and would probably be reduced by complete culls. Sadly, there is a shortage of humane ways to completely cull setts.
Secondly, and more significantly, they are rigging the result of their pilot study in North Pembrokeshire. As well as culling badgers (over a period of 4 years), they are going to implement “additional cattle measures” (presumably sensible things like increasing farm biosecurity, buying animals from non-endemic areas, and so on) to try and reduce the spread of TB between cattle. What this means is that the results from the pilot area will be of no value at all in assessing whether or not badger culling has been effective. If TB levels decline in the pilot area, there will be no way of telling whether that was due to the badger culling, or the cattle-based control measures.
You can be sure, though, that if TB levels do decline, it will be trumpeted by the Welsh Assembly Government as evidence that badger culling works – a classic example of policy-based evidence making!
April 15, 2008
The Welsh Assembly Government’s (WAG) Rural Affairs Minister, Elin Jones, has recently announced a £27M program to attempt to eradicate bovine tuberculosis (BTB) from Wales. The most controversial aspect of the proposals is the plan to cull badgers. I’ve had no useful response to my request for details, but there are some sketchy outlines online.
What is not controversial is that the level of BTB is rising, that this has a significant economic impact, and that badgers and cattle can transmit BTB to one another. The role of badgers in the epidemiology of BTB is, however, open to dispute. The Independent Scientific Group on Cattle TB (ISG) concluded after 10 years of research that “badger culling can make no meaningful contribution to cattle TB control in Britain”. Sir David King (then the Chief Scientific Advisor) and colleagues re-evaluated the ISG’s work, and concluded that “the removal of badgers could make a significant contribution to the control of cattle TB in those areas of England where there is a high and persistent incidence of TB in cattle”, although their terms of reference required them to ignore practical and economic considerations(!). The ISG issued a strongly-worded rebuttal to Sir David’s statement. In the light of this, there are several aspects of the Welsh plans that give me cause for concern.
The Human Health Risks
The WAG FAQ on BTB states “The main reason for Government intervention in bovine TB is in order to protect public health as well as animal health. Bovine TB is a zoonotic disease which means that it can infect and cause TB in humans”. This is rather misleading. BTB is caused by Mycobacterium bovis, whilst most TB in humans (at least in the UK) is caused by Mycobacterium tuberculosis. There were around 14 cases of TB in the UK per 100,000 population in 2007, nearly half of which occurred in London. Around 2/3 of TB cases are amongst those not born in the UK, and UK children are no longer routinely given the BCG vaccine against TB. Furthermore, the main route of transmission of BTB from cattle to humans is via unpasteurised milk, and in the UK milk is almost universally pasteurised. There are good reasons to try and control BTB, but the risk to human health is not one of them in the UK
The role of wildlife, and the importance of livestock movements
The WAG FAQ has this to say on badgers: “The [Badger Found Dead Survey] concluded that the prevalence of M. bovis infection in badgers was highest in areas of high cattle prevalence and lowest in areas of low cattle prevalence. The results of this survey are consistent with the hypothesis that the badger is an important component in the epidemiology of bovine tuberculosis in areas of high cattle incidence”. This again is seriously misleading. Cattle and badgers can transmit BTB to one another, so it’s only to be expected that areas with high incidence in one species will have high incidence in the other species. Whilst these data are consistent with the WAG’s hypothesis, they are also consistent with the hypothesis that cattle are the main drivers for BTB infection in badgers! The fact that high BTB levels in cattle are associated with high BTB levels in badgers does not establish causality in either direction.
Slightly bizarrely, the WAG FAQ cites the ISG’s work as supporting their view that there is evidence of TB transmission from badgers to cattle, yet curiously fails to mention the conclusion in the ISG’s 2006 Nature paper that badger culling has negative as well as positive effects on BTB in cattle. Whilst intensive badger culling can reduce BTB incidence in an area, the resulting disruption to badger social groups leads to increased movement of badgers, which results in greater spread of BTB in areas surrounding the cull region.
In addressing a question about TB coming from the movement of infected animals from SW England, the WAG FAQ says “There is no evidence that the increase in cattle numbers is due to movement of cattle from the South West of England.” Assuming that is meant to be a rebuttal of the suggestion, the authors would be advised to consult Gilbert et al’s 2005 Nature paper “Cattle movements and bovine tuberculosis in Great Britain”, which elegantly demonstrates that the best way to predict where BTB is going to spread is to look at where cattle from TB-endemic areas are being moved to, and highlights areas of Wales (especially in the SW of Wales) as being at high risk of importing BTB.
What exactly is proposed?
This is unclear. Whilst the Minister’s statement is clear that some badgers will be killed, the accompanying FAQ says that how many badgers will be culled, where they will be culled, when they will be culled, and what effect this will have on the badger population are all “considerations that will need to be made as part [of] the decision on a badger cull”. This seems to me to be begging the question. If none of the details have been considered yet, how can anyone be sure that killing badgers is the right way to control BTB in Wales? It seems that the policy cart is being set before the scientific horse.
Particularly, the effect on the badger population will be key to the impact on BTB levels of a badger cull – all the evidence is that it’s the increased movement of badgers post-culling that can be responsible for increasing BTB incidence around cull areas. If this cull is not to make matters worse in Wales, this should be being considered before the decision to cull is made.
The documentation available from the Welsh Assembly Government so far paints a worrying picture. Rather than carefully weighing up the evidence on the advisability of culling badgers as part of a TB control strategy, the science has been misinterpreted or misrepresented to bolster a “we will cull badgers” position, before the matter has been considered in any great detail. It might be possible to control BTB in Wales by using a strategy that includes the culling of badgers, but the devil really is in the detail: it’s wrong to just decide willy-nilly to cull badgers as appears to have happened here. To conclude that a badger cull is part of an effective BTB control plan, you need to have a clear plan of action in place – only once you have a proposal for how many badgers you plan to remove from where over what timescale can anyone assess whether it’s likely to increase or decrease BTB incidence in cattle, never mind whether it will do so in a cost-effective manner.
If anyone is to have confidence that badgers are being culled as part of a sound BTB control program, rather than for political reasons, it needs to be much clearer that science is driving policy decisions, not the other way round.
November 15, 2007
Infectious diseases of livestock are in the headlines again; this time it’s highly pathogenic avian influenza (HPAI), specifically H5N1. The thing to stress (and media coverage in the UK has tended not to stress this enough) is that the risk to human health is minimal, and that properly-cooked poultry meat remains safe.
H5N1 HPAI is a serious and highly infectious disease of birds. There is real concern that it might get into the wild bird population in the UK (which would make control tricky), as that would put many free-range birds at risk. That’s why the infected unit has been culled out, and protection and surveillance zones put into place. Currently, however, it isn’t transmitting to other species very readily. The human cases have been in Asia, where people live cheek-by-jowl with large numbers of infected birds.
Poultry keepers need to remain vigilant; if you don’t keep birds, the only thing you should do is keep an eye out for groups of dead birds (more than 3 of the same species or more than 5 of different species in a small area) – if you see these, don’t touch them, but instead call DEFRA on 08459 335577.
More information on the current outbreak may be found at the DEFRA website.
September 19, 2007
The mystery surrounding the month-long gap between FMD cases seems at least partially solved. DEFRA note that the lesions on the most recently-culled infectious premise in Surrey were a couple of weeks old. That farm is just outside the old surveillance zone, and it seems that the farmer failed to spot the FMD symptoms in his animals; this provides an explanation of where the infection we saw on the 12th came from. Very embarrasing for the farmer concerned, of course, but from an epidemiologist’s point of view, somewhat of a relief – we now have a scenario that explains why FMD has re-appeared over a month after the last farm was culled out, and it isn’t one of the nightmare ones I was worried about when I wrote my previous entry.
Separately, a temporary control zone has been established near Solihull. I’ll stick my neck out here, and say that I don’t expect FMD to be confirmed.