12 Sept 2016
Alan Murphy discusses the importance of being well informed and prepared when faced with novel conditions in livestock, looking at their occurrence and types.
Alan Murphy
Job Title
Lumpy skin disease.
The diagnosis and control of infectious disease presents a daily challenge to the livestock practitioner. When considering the pathogens involved, the term endemic disease is applied – these being conditions that occur either continuously or recurrently in a particular geographical region.
How should practitioners ensure they are well informed and prepared when faced with a new or emerging infectious disease? The latter is considered to be a clinical syndrome known about for some time, but is either rapidly increasing in incidence or geographical range.
New diseases are those that have never been previously described in any population. These quickly become categorised as emerging as spread within a susceptible population occurs. While the occurrence of new conditions is not that common, the arrival of Schmallenberg virus in the UK in 2012 is still fresh in the memory of the livestock industry1. This disease had spread rapidly through Europe due to vector transmission.
In the UK, this was another timely reminder of the challenges faced from vector-borne disease during the bluetongue virus (BTV) serotype eight outbreaks of 2007. This event presented a substantial challenge, not only in terms of the impact of the disease itself, but the control measures instigated to eliminate a notifiable disease. In light of the disease situation in France, it is assumed BTV will reappear in the UK in 2016. Readers are directed to the comprehensive risk assessment produced in February 2016 by the UK Government2.
But what are the drivers for the emergence of infectious disease? The factors involved are pertinent to all disease conditions, in fact.
Panel 1 highlights the key areas that determine the occurrence of new and emerging infectious disease.
From the factors listed in Panel 1, two key points are considered to represent the most likely route of entry for pathogens into the UK. These are, firstly, the spread of insect vectors and, secondly, the movement of either infected livestock or their products.
The size of this article limits the number of conditions that can be discussed. As such, the author has presented those he considers to represent either the greatest or most immediate threat to UK ruminant and porcine populations. The majority are notifiable, demonstrating the significance of any potential incursion. It is also significant that of the conditions reviewed, all are viruses bar one and those involving insect vectors are well represented. As with all conditions where notifiable disease is suspected, involvement of the APHA is mandatory.
The operation of a robust and reliable surveillance system is fundamental in the early detection of both new and emerging disease. Such systems need to be a collaborative exercise between all stakeholders involved. With the financial constraints faced by the Government in recent years, the role of the private practitioner in front-line surveillance has never been more critical.
As such, an awareness of the clinical signs and differential diagnosis for the diseases highlighted in this article is of fundamental importance. However, textbook descriptions of outbreaks do not always apply and, when the size of the herd is one, as in the situation with a pet pig, the ability to consider herd epidemiological principles can be challenging.
When discussing protozoa in UK cattle, the endemic syndromes recognised to date are babesiosis, neosporosis, coccidiosis, cryptosporidiosis and sarcocystosis. In other areas of the world, the role of agents, such as Trypanosoma, Theileria and Besnoitia, are well documented.
In 2010, the European Food Safety Authority (EFSA) delivered a paper, whereby it identified besnoitiosis as an emerging threat in Europe3. This followed the 2009 World Association for the Advancement of Veterinary Parasitology Conference, where besnoitiosis had also been identified as an emerging disease. The EFSA paper highlighted the need for detailed surveillance of the disease, improved diagnostic tools and greater knowledge regarding routes of transmission and risk factors associated with disease.
The protozoan Besnoitia besnoiti is known to cause considerable economic loss due primarily to its impact on production. It is believed transmission occurs primarily via biting flies, though horizontal spread from open skin lesions is also proposed. Following infection, the tachyzoites proliferate in macrophages, fibroblasts and endothelial cells, resulting in vasculitis4. Clinically, disease is typically divided into two stages – the acute and chronic. Many infected animals are asymptomatic, but others display severe symptoms. In the acute phase, these include depression, pyrexia, ventral oedema and lymphadenopathy.
Chronic cases are characterised by skin changes, alluding to the term elephant skin syndrome, by which the disease is also known. The skin becomes thickened with alopecia, hyperkeratosis and scleroderma. An established phenomenon is the presence of parasitic cysts in the conjunctival mucosa, described typically as having the appearance of sugar grains5. The morbidity is variable, given the asymptomatic status of animals in infected herds, while mortality is usually very low. In endemic areas, chronic disease can see a loss of condition, lower market value and damage to the hide. In bulls, sterility may occur as a result of infection.
Diagnosis is achieved by means of cytology and histopathology in the UK, although serological and PCR methods are available elsewhere and are in development. While the EFSA paper of 2010 brought this condition to the attention of the UK livestock industry, the author is unaware of any confirmed diagnosis in the UK to date. However, in Ireland, the disease has been confirmed6 in 2015 and, given the risks posed by asymptomatic carriers, it poses a significant threat to UK livestock.
Lumpy skin disease is a viral condition regarded as exotic to the EU, but significant concerns arose regarding potential incursions from states neighbouring the EU7. The virus belongs to the Poxviridae family and it is closely related to sheep and goat pox. While endemic in a large number of African and Asian countries, it has, in recent years, become established in the Middle East.
With the political and humanitarian situations throughout this region, the control and eradication of the disease has been challenging. Outbreaks have subsequently been recorded in south-east Europe, with Greece, Bulgaria and, most recently, Montenegro, recording outbreaks8.
The disease is one of great economic importance in terms or production loss. An array of clinical signs are described, with acutely infected animals having severe pyrexia, inappetence, abortion and reduced production if lactating. Mortality is usually low, but the impact on affected animals is profound.
The nodular skin lesions, after which the disease is named, appear within one to two days of pyrexia developing. These are typically 0.5cm to 5cm in diameter, with the predilection sites being the head, neck, perineum, genitalia and udder. Within the carcase, these lesions are replicated in the mucous membranes of the alimentary tract and lungs. The morbidity is highly variable, while mortality is low normally. In some outbreaks, however, the mortality figure can be significant, with figures of more than 50 per cent reported.
Transmission of the virus is primarily by biting insects, although the role of ticks is still to be fully established. Indirect transfer is also considered to be of importance, with fomites containing infected animal secretions a recognised risk. Spread of disease, as outlined earlier in this article, is considered to be due to movement of infected animals and vectors. As a notifiable disease, any clinical suspects must be reported to the APHA. To date, a small number of cases have been presented as report cases, but investigation by the APHA has negated these (publication in press). The risk to the UK is still regarded as low, but the emergence of the disease in eastern Europe poses concerns.
Rinderpest was officially declared as eradicated by the World Organisation for Animal Health in 2011. Peste des petits ruminants (PPR) is caused by infection with a closely-related Morbillivirus and is colloquially described as pseudo-rinderpest, or goat plague.
PPR is a recognised cause of significant disease and production loss in many areas of the world, including large parts of Africa and the Middle East. As with lumpy skin disease, a steady spread of the pathogen has occurred towards eastern Europe, with outbreaks recorded in Turkey.
The principal means of virus spread is by direct transmission from infected animals, in the form of secretions loaded with virus9. The clinical presentation is typically of one starting with acute pyrexia, followed by reddening of the ocular and nasal conjunctiva. A severe nasal and ocular discharge then develops, followed by widespread necrosis of the mucosa throughout the alimentary tract. Frank necrosis may be seen in conjunction with diarrhoea.
The affected animals will be markedly depressed, with abortion in pregnant animals. Both morbidity and mortality are very variable, but can rise up to 100 per cent, depending on the age of the animals, secondary infections and the viral strain. Increased mortality is reported in younger animals. As a notifiable disease, any clinical suspicion needs to be reported to the APHA.
The UK pig industry is regarded as being at considerable risk from a potential incursion of either emerging or notifiable disease. While the issues faced by the national herd are numerous, the threat of such additional problems, such as those outlined later, would only serve to compromise the industry further.
An additional feature to also note is the number of registered pig owners is increasing. Rather than being units on a commercial scale, however, it is a marked expansion in smallholder and pet pig ownership. These represent fresh challenges to any veterinary surgeon responsible for their care.
In 2000, the UK controlled and eradicated the last recorded incursion of classical swine fever (CSF). When faced with an episode of haemorrhagic diathesis, a predisposition to abnormal haemostasis, a number of potential aetiologies, including notifiable disease, should be considered. These include porcine reproductive and respiratory syndrome (PRRS), various bacteria and, less commonly, toxin ingestion, such as rodenticides or coal tar. The clinical and gross pathological findings in CSF and African swine fever (ASF) are considered indistinguishable10.
ASF infects monocytes and macrophages, leading to their activation and a breakdown on the regulation of cytokine activation pathways. This leads to impaired immune function and haemostasis. When infected, the mortality can vary depending on the strain involved. The clinical signs are of profound pyrexia, depression and nasal discharge. A wide variety of lesions may be found at postmortem and these are dependent on the strain involved.
ASF has never been recorded in the UK to date. However, it is considered to have become established in the wild boar population of Russia and cases have been reported in eastern Europe. Given the virus can persist in pig products, there is a risk it could be introduced into the UK by indirect means.
PRRS has been a recognised cause of immune suppression for many years. Its role has always been considered as one of facilitating concurrent infections, by impairing the host’s defences. The virus invades macrophages, where it multiplies and lyses the macrophage. More recently, however, the disease picture has altered.
In Europe, it is generally considered PRRS has been, on the whole, well managed. But in the US and Asia, mutation of the virus has seen what were effective control methods having reduced impact. These new strains pose a significant threat to the UK. Although not notifiable, the use of robust diagnostic tests is needed to ensure potential incursions are identified as quickly as possible and effective action taken11.
On 18 December 2015, this disease was added to the list of notifiable diseases in England, with the same status applied to it from March 2016 in Scotland. In Wales, however, the condition has not been designated notifiable to date.
The potential threat from this disease, caused by an alphacoronavirus, was clearly demonstrated when it was diagnosed in the US in 2013. The virus infects all naïve pigs, with profound mortality in neonatal animals. The clinical signs in this age group are of severe diarrhoea, dehydration and death. In some cases, vomiting may also be seen. The morbidity and mortality can reach 100 per cent. As the age of the infected pig increases, the mortality pattern reduces markedly12.
The disease has been known about since the early 1970s, having been first detected in the UK. Over the following two decades, the incidence of clinical disease reduced with serology indicating a low prevalence of disease9. In Asia, however, the disease has become an increasing problem and it is from here the 2013 US epidemic is believed to have originated. Infection is by the faeco-oral route, with indirect transmission of key importance. The application of robust biosecurity has been fundamental in controlling the spread of this disease.
The reliance on textbooks and the written word is reducing. Through electronic media, veterinary surgeons now have access to a phenomenal amount of robust and reliable information on the subject of new and emerging disease. With regards to notifiable disease, Gov.uk is a tremendous asset. From detailed information on notifiable disease through to the APHA pages and material, such as their quarterly reports, it can be described as really essential reading.
The text alert system created by the Government was in response to practitioners’ pleas for greater communication and speed in regards to disease updates. This has worked well and the potential for a broader, dynamic system looking at non-notifiable disease incidents of significance is worth considering. Involvement or ownership by the specialist divisions of the BVA could be a potential way forward.
Other sites, such as those of the EFSA and the OIE, give very detailed information and present perspectives for different countries and continents. Also, although a number of other sources have not been mentioned, it would be remiss not to highlight www.promedmail.org – for anyone with an inquisitive mind and appetite for a global view of one health, this is a must.
Livestock industries across the world will always face new challenges – from both infectious and non-infectious sources.
In the UK, ensuring a robust surveillance system, complemented with effective diagnostic testing, will ensure new and emerging infectious threats can be identified in a timely manner. This is assuming they fall out with the jurisdiction of the Government and notifiable disease. As an island, the UK should be at an advantage in controlling potential threats. However failures in biosecurity, importation of infected animals/products or the movement of insect vectors may put sections of the industry at risk.
The author would like to thank numerous colleagues for their contributions and advice in the creation of this article.
