Foot-and-Mouth Disease in Deer: implications for the policy of control and eradication of the disease
Paul Sutmoller* and Paul Gibbs**
*Animal Health Consultant, 1502 Largo Rd., Virginia 23233, USA,
** College of Veterinary Medicine, University of Florida, Gainesville, Florida 32611, USA, e-mail: email@example.com
The continuing epidemic of foot-and-mouth disease (FMD) in the United Kingdom (UK) has led to veterinarians and others questioning whether wildlife species (particularly deer) are perhaps playing a more significant role in the maintenance of the epidemic than is currently acknowledged. Similar concerns concerning deer are being expressed in the Netherlands, which has also had a FMD epidemic in 2001 (5).
Many wildlife species are known to be susceptible to FMD virus, but it has been very difficult to assess their role in transmission of disease to domestic livestock (6). The general opinion is that wildlife species are not a significant factor in maintaining a FMD epidemic. While the African Cape Buffalo is believed to maintain FMD virus in the absence of infection in domestic livestock in Southern Africa, no comparable situation has been discovered in any other wildlife species elsewhere in the world.
Descriptions of FMD in deer were already made during the 1924-1926 extensive outbreak of the disease in California, USA. This outbreak not only affected livestock, but spread to the deer population in the Stanislaus National Forest. In those years some 22,000 deer were killed, of which 10% showed typical FMD lesions (4).
In the years 1973-74, FMD in deer was studied by a group of investigators of the Animal Virus Research Institute in the UK (1,2,3), presently the World Reference Laboratory for FMD and by a group of researchers of the Plum Island Animal Disease Laboratory (4). The publications of both groups present a very complete picture of FMD in deer.
McVicar et al. (4) reported as follows: .. white-tailed deer were clearly susceptible to infection with this strain of FMD virus both by intranasal inoculation and by contact exposure. The clinical syndrome in deer was somewhat variable but, in general, severity of the disease was intermediate between that seen in cattle and in sheep and goats exposed and housed under similar conditions.
The lack of knowledge about the susceptibility of deer during the extensive 1967-68 FMD outbreak in the UK prompted the UK group to first investigate this aspect of the disease for three of the in the UK existing deer species (1). The conclusions from that work were: All three species were susceptible to infection with FMD virus. Clinical disease was mild or inapparent in the red and fallow deer but was more severe in the roe deer. The appearance and distribution of the lesions were similar to those in sheep (1).
This research was followed by a more detailed investigation on the behavior of FMD virus in infected deer. The results showed that, in general, FMD virus acts very similar in deer, cattle and sheep with regard to the amount of virus in the blood as in excretion of virus and the carrier status (2).
Finally, Gibbs et al. (3) carried out a large experiment that studied the full infectious cycle. It was shown that cattle can infect deer, that these deer in turn can infect cattle and sheep, and that these species can transmit FMD back to deer. These investigation also confirmed all earlier observations: This study and earlier observations . have established that the five species of deer seen in the British countryside are susceptible to FMD. .. the smaller species .. both develop severe disease and death is not uncommon during the acute stage of the disease(underlining added) .
The authors speculated that deer were unlikely to play an important role in the survival and transmission of the virus during a FMD epidemic, because of the natural behavior of free-living deer in the UK: none of the five species are in general likely to come in close contact with domestic livestock except in some areas of common grazing such as the New Forest, Hampshire. However, it must be remembered that the larger deer often intergraze with cattle and sheep in parks and in these conditions transmission of FMD between the species could occur (3).
In the intervening period between the above mentioned studies in the 1970s and 2001, the deer populations of the UK and countries, such as the Netherlands, have risen significantly. In this paper we will therefore examine the validity of the earlier conclusions on the role of deer in the epidemiology of FMD in Western Europe.
Implications of FMD infection of deer for FMD control
Two countries in Western Europe with recent FMD outbreaks and with a substantial deer population are the Netherlands and the United Kingdom.
In the Netherlands FMD was confirmed in February of this year. Control of the outbreak was originally by the stamping-out policy, that is the killing and destruction of all susceptible livestock on the infected farms and their immediate contacts. When it seemed that the outbreak was running out of control, vaccination of all susceptible species was applied in a zone with about 250,000 of such animals. They were mostly cattle and sheep, some goats and pigs. The vaccination stopped the outbreak within a few days, but for fear of carriers among the livestock population all vaccinated animals were killed and destroyed in the course of about a month. The mass killing of these vaccinated healthy animals caused tremendous hardship for the communities that were stricken.
Livestock, roe deer and farmers live in close contact in the infected zone in the Netherlands. According to the official opinion, vaccinated livestock could become easily infected and thus become carriers of FMD virus. This was the reason for the killing of large numbers of healthy vaccinated cattle and vaccinated sheep. However, deer roaming between farm premises in areas that experience FMD outbreaks should at least have to be considered as being potentially contaminated.
In any case, there are two possible scenarios with regard to FMD in roe deer:
(a) either deer in the infected zone in the Netherlands are not infected; or,
(b) deer in the infected zone in the Netherlands are infected.
Scenario 1: Deer not infected
a) FMD vaccination of livestock in it self does not produce FMD virus carriers. Vaccinated livestock must be exposed to FMD virus - before or after vaccination - in order to become a carrier. Exposure may be by contact with infectious secretions, or inhalation of infectious aerosols, from animals with FMD.
b) Roe deer range in the vaccinated area, cross fields between farms, visit premises and yards with animal feed and slurry, etc. Thus, susceptible deer act as sentinels for FMD virus.
c) If FMD virus is present in the environment the chance that free roaming roe deer become infected is many magnitudes greater than the chance of exposure to infection of livestock in pens or stables. The risk that stabled vaccinated livestock becomes infected is close to zero. In the absence of a FMD virus source there cannot be any carrier among the vaccinated livestock.
Scenario 2: Deer infected
a) If FMD has infected the deer population the Veterinary Authority will have a difficult task to control the disease in those species.
b) FMD in deer will run its natural course, and after several months the outbreak is likely to burn out, similarly to what happened in the early days with FMD in cattle, before the introduction of the mass vaccination, or, lately, with FMD in wildlife in sub-Sahara Africa.
c) After the outbreak peters out the virus will probably still be present in the area for some length of time. Re-population of the area with susceptible livestock would be risky. Therefore, all livestock in the area should be vaccinated or re-vaccinated, preferably within three months to obtain an optimum population immunity. Re-population of the area with vaccinated livestock does not need to wait for the infection to peter out in deer.
d) The official opinion that FMD infected roe deer constitute a low risk, because sick animals hide and probably die, is not valid. Like cattle or sheep, susceptible deer are very infectious prior to the development of lesions while they still actively move and graze. Also deer with sub-clinical or minor lesions will still roam around.
Conclusions: In both scenarios the killing of healthy vaccinated livestock is senseless for the control of FMD. In this case of scenario 2 (deer in the area infected), a vaccination zone with solidly immune livestock is the only way to control FMD.
The FMD outbreak in the UK is dealt with by the slaughter and destruction of all susceptible livestock on infected and suspected premises and the culling of all susceptible sheep and pigs on contiguous premises within a 3-km radius. This policy has lead to the destruction of more than 3,500,000 cattle, sheep, goats and pigs. Here again one can question the wisdom of such actions. If deer are roaming and grazing in the infected area there is a good probability that the deer population becomes infected. In that case, the destruction of livestock on contiguous farms does not make much sense. If the deer population is not infected, one can ligitimately ask: Has destruction of livestock at this large scale become an overkill?.
Ecological conditions in several counties have changed since the 1960s, in particular wild deer densities have increased significantly in Europe and in the USA. Thus previous assessments of Gibbs et al. (3) of the role of wildlife may no longer be relevant.
There is anecdotal evidence of infection in wild deer, but there is a lack of scientific data to confirm or refute the presence of wildlife infection.
Like has been done for wildlife in many parts of Africa, a thorough field and laboratory investigation can proof beyond doubt the presence or absence of FMD in deer populations. In the absence of a scientifically sound epidemiological survey of deer and other ungulate populations in the FMD recent infected areas, this uncertainty must be a cause of great concern for all stakeholders such as decision makers, farmer and agricultural industries, wildlife managers and trading partners. If indeed FMD were present in wildlife in some of Western Europe that would be of potential consequences for other countries around the world.
The above arguments underline the urgent need for a reappraisal of the role of wildlife in the epidemiology, prevention, control and eradication of FMD. This revision must be based on risk analysis using scientific facts, not fiction.
1. Forman, A.J.& Gibbs, E.P.J. Studies with Foot and Mouth Disease virus in British deer (Red, Fallow and Roe) I. Clinical Disease. J. Comp. Path. Vol 84, 215- 220, 1974.
2. Forman, A.J., Gibbs, E.P.J., Baber, D.J., Herniman, K.A.J. & Barnett, I.T. Studies with Foot and Mouth Disease virus in British deer (Red, Fallow and Roe) II Recovery of virus and serological response. J. Comp. Path. Vol 84, 221- 228, 1974.
3. Gibbs, E.P.J., Herniman, K.A.J., Lawman, M.J.P. & Sellers, R.F.- Foot and Mouth Disease in British deer: Transmission of virus to cattle, sheep and deer. Veterinary Record June 28th ,1974, 558-563.
4. McVicar, J.W., Sutmoller, P. Ferris, D.H., & Campbell, C.H. Foot and Mouth Disease in white-tailed deer: clinical signs and transmission in the laboratory. Proceedings of the 78th Annual Mgt USAnimal Health Association 1974.
5. Sutmoller, P. - Verdenking van MKZ onder Nederlands wild. Tijdschr. Diergeneesk. 126, 12, 2001, 434-435.
6. Thompson, G.R., Bengis, R.G. & Brown, C. Picornavirus infections in Infectious Diseases of Wild Mammals 3rd Ed. Edited by E.S. Williams and I.K. Baker. Iowa State Univ. Press 2001.