A review on foot and mouth disease in dairy animals, etiology, pathogenesis and clinical findings

Foot-and-mouth disease (FMD) is an extremely contagious disease of angulated animals and in livestock, it is an important pathogen more than 120 years after it was recognized. The virus of this disease belong to a genus Aphthovirus of the family picornaviridae and present as seven serotypes C, O, A, SAT1, SAT2, SAT3, and Asia1. Transmission of this disease occurs by direct and indirect contact, ingestion and by aerosols. The best multiplication sites of this virus are oral cavity, udder, heart, feet, and oro-pharynx. Specific, sensitive and quick diagnostic tools are required for effective control of this disease. FMD causes fever, salivation, anorexia and vesicular eruption on teats, feet, and mouth. In FMDV (Foot and Mouth disease Virus) endemic areas, annual production losses and vaccination estimated US$6.5-US$21 billion. Currently, the techniques which are used for diagnosis of FMD are Sandwich-ELISA (S-ELISA), Virus Isolation (VI), indirect ELISA (DIVA), real timePCR and Liquid-Phase Blocking ELISA (LPBE) can be used for recognition of antibody against nonstructural proteins. The techniques which are used for quick and specific detection of FMDV are microarray and recombinant antigen-based detection, biosensor, Nucleotide sequencing, phage display and nucleic-acid-based diagnostic for serotyping. This review provides information about the FMD in Pakistan as many cases are reported in Pakistan and lead to morbidity and mortality of many animals. This review helps the reader to handle the cases practically in the field and helps the researchers to analyze the comprehensive picture of the FMD and in future researchers try to minimize such cases after reading this in Pakistan.

Introduction FMD is a major health problem of animals in all over the world [1]. It is the contagious TAD (transboundary animal disease) affecting the cloven-hoofed animals of wildlife and domesticated. Goats, sheep, cattle, pigs, and buffalo are vulnerable species of domesticated animals.
Aphthovirus is an RNA virus that has 7 different serotypes such as C, O, A, SAT1, SAT2, SAT3, and Asia1 as well as over 60 subtypes. There are 60 copies of structural proteins present in the capsid of FMDV. These proteins are VP2, VP3, VP1 and there are 8 nonstructural protein (3A, 3B, 3C, L, 2A, 2B, 2C and 3D) [2,3]. The virus of this disease is also called foot and mouth disease virus. This disease is spread throughout the world but most affected regions are Africa, Asia, and the Middle East. FMD free countries are Australia, Japan and New Zealand [4]. The main route of transmission in ruminants is inhalation of droplets but inoculation with contaminated vaccines, ingestion of infected feed and insemination with contaminated semen can also produce infection. Animals which are effected via the respiratory tract, in the pharyngeal area and in the lungs replication of virus occur followed by viremic spread to other organs and tissue before the beginning of the clinical disease. Then the virus distributed throughout the body, and reach in different sites of the body which favor replication of virus like oral cavity, udder, heart, feet, and oro-pharynx [5]. Symptomatically, FMD is characterized by anorexia, fever, blisters on the mucous membranes particularly on mouth, feet, and udder [6]. By using several techniques we can detect FMD virus or viral antigens. In areas where the disease is endemic, routine vaccination is used. On other hand, disease-free countries never vaccinated their livestock but they preferred animal movement restriction, the slaughter of infected and suspected animals when outbreaks of FMD occur. In livestock, it is the most important disease which has a great effect on the economy because FMD causes not only loss of production but also limit the trade of animals both locally and internationally [7]. The occurrence of death is more in the young animals but in adults it having high morbidity and low mortality [8].  1).

Agent
Common disinfectants having no effect on the virus. It may be found in gross lesions for more than one year to grow only in areas exposed to mechanical trauma or unusual physiological conditions such as feet, the epithelium of the mouth and teats. In young animals, particularly neonates, most of the time virus can cause necrotizing myocarditis and adults may also be infected with some  (Fig. 2).

Clinical signs
When vulnerable animals come in contact with infected animals, clinical signs may appear in 3 to 5 days, although, by natural infection, the incubation period may vary from 2 to 14 days. The severity of clinical signs of the disease depends on the exposure dose, age, and breed of the animal, the strain of the virus, the host species and its degree of immunity. FMD causes fever, excessive salivation, depression and the formation of vesicular type lesions on the mucous membrane of the gums, tongue, dental pad and the surface of udder teats, muzzle, coronary band and interdigital spaces [29,30]. Lesions present on tongue take a few days to heal but which are present on feet and in the nasal cavity may lead to secondary bacterial infection resulting in mucopurulent discharge from the nasal cavity and prolonged lameness [6]. Due to necrotizing myocarditis, young animals may die before showing any vesicles. In lactating cow milk yield also drop due to the formation of vesicles on the skin of teats and udder which leads to mastitis [30]. Clinical signs of FMD (Fig. 3). Morbidity and mortality rate FMD virus can easily transmit to other animals and about 100% exposed animals become infected. The morbidity rate in FMD infected animals is 100%. However, the mortality rate is 20% in young animals and 2% in adults. The morbidity rate depends on sex, species and status of immunity. Self-recovery is the result of immunity against the serotype of the virus. Mostly, FMD occurs due to one type of virus and immunity remain limited against specific serotype, thus no immunity develops against another serotypes, so in endemic areas, it is the main reason behind occurrence of disease [31]. In most extreme cases, mortality in lambs and suckling pigs ranges from 20-75% and it depends on age. Mortality is high in lower age animals like under 4 weeks of age but it decreases rapidly as animal age increase (more than 4 weeks). Most deaths occur due to slaughter policies that involves all suspected animals during an outbreak in endemic and developed countries [32]. Post mortem finding Vesicular lesion and erosion of FMD present on udder, food and mouth. When the vesicles rupture, the formation of the eroded red area and then gray fibrinous covert this part. Color of coting becomes green or yellow then-new epithelium replace this area. Sometimes vesicles are not formed and fluid escape from the epidermis. The appearance of these "Dry" lesions are necrotic instead of vesicular. In the oral cavity of the pig, these Dry lesions are very common [33]. In Ciuca (1929) show that to detect the FMDV and serotype isolates, a direct complement fixation test could be used [37]. The basic concept of this method was based on virus-antibody complexes bind to the guinea pig-derived complement. In the presence of an anti-sheep RBC antibody, if the binding of the virus with an antibody does not occur, free complement cause the lyses of sheep red blood cells (RBC). FMDV antibodies are serotype-specific, so the identification of FMDV serotypes using the direct complement fixation test was possible [38].

ELISA (Enzyme Linked Immunosorbent Assay)
For antibody detection ELIZA is the most suitable test [39]. Although some methods that are based on virus isolation or nucleic acid in samples of tissue or the demonstration of FMD viral antigen or culture products is enough for a positive diagnosis. For the detection of FMD viral antigen, the ELISA using type-specific serological reagents is the preferred. For a faster approach to detect viral antigens, Sandwich ELISA is best but it has low sensitivity. Several researches are conducted to developing alternative assay systems that are more rapid for confirmation of clinical diagnosis and these methods can also perform 'Pen side' [40]. RT

Lab diagnosis
For diagnosis of FMD in an animal, the sample is collected from vesicular fluid or epithelium of suspected animal. In cattle sample of choice are lesions from buccal mucosa, tissue of tongue, wounds of hoofs or feet. Fluid filled vesicular wound in pig from hoof shall of coronary band, snout and tongue shall be collected [1]. Treatment With mild disinfectants, infected animals can be treated topically and some broad-spectrum antibiotics can also be used parentally such as tetracycline [26]. However, the treatment of secondary bacterial infection and proper animal husbandry practices is recommended in endemic countries [9,19]. Control and prevention FMD required good preventive measures at the national and international level. FMD free countries should impose strict regulations on animals and animal products import from FMD infected countries [35]. To control the spread of FMD, vaccinations programs can also be used [1]. Import of animals and animal product from FMD infected countries should be banned when the disease is absent in the country. Preventive measures in case of the outbreak are immediate slaughter the infected animals and there should be no contact of the diseased animals with normal animals [7]. Disease outbreaks FMD has been a considerable concern to many countries and due to fear of an outbreak of this disease, many research institutes were established. In particular, Lindholm Island in Denmark in 1925, the Insel Reims in Germany in 1909 and the Pirbright laboratory in the United Kingdom in 1924 were specifically opened to study on FMD [45]. In Europe, FMD outbreaks occurred sporadically in the early 20th centuries but their occurrence had shocking consequences [46]. Some in Western Europe, the experience of the outbreak was 104 to 105 per year by the early 1950s [47]. Disease controlled by disinfection, the slaughter of infected animals and inhibition of animal movement, at that time. An inactivated vaccine of FMD was developed in 1930 by Waldmann and colleagues in Germany [48]. The virus was inactivated by formalin aluminum hydroxide gel to produce the vaccine. The virus was collected from collecting epithelium, vesicular fluid and from infecting cattle at the slaughterhouse for the Waldmann vaccine [45]. By this method, the needed amount of vaccine was unable to produce, since only limited numbers of animals could be infected. The virus was produced by infecting bovine tongue epithelium, obtained at the time of slaughter of healthy animals that commercialization of FMD vaccine became a reality and this method was developed by Frenkel [49]. In Western Europe, cattle vaccination with this product causes a great reduction in the cases of FMD [47].

De-Population of infected animals
The killing practice of animals which are suspected and affected by FMD in the herd, or in other herds which has been exposed to FMDV by direct animal to animal contact or by indirect contact likely to cause the transmission of the causal pathogen [50]. Disinfection The virus is sensitive to common household bleach of concentration 3%. It is good to disinfect the properties but not a good choice for the disinfection of footpaths and equipment. Vinegar at 4-5% dilution can also be used to kill the virus. New disinfectants like Virkos S (surfactant/ organic acid/ per oxygen molecule combination) have a wide spectrum of activity against FMDV and many other germs [51]. Vaccination Generally Killed trivalent vaccine (contain C, A and O serotypes) are used but It is more common practice to the production of vaccines from the locally isolated virus due to an increase in the number of antigenically dissimilar substrains. In endemic areas, for prophylactive protection, at least two vaccinations are recommended. The current FMD vaccine protects for 6 months. In 21-28 days vaccinated animals attained peak antibody response. Vaccination can also be used to protect specific animals [30]. During an outbreak, prevention of FMD from spreading can be done with the help of emergency vaccinations. There is continuous development of new vaccines but it is labor and difficult to develop a new vaccine whenever a new virus appears [52, 53, 54]. Therefore, many kinds of research have been conducted to develop advanced vaccines that can respond to evolving versions of the virus [55,56]. Current problems with FMD vaccines include antigens losses during the process of purification, unable to maintain the antibodies, low immunogenicity and narrow protection range of the antigens [57]. Economic impact of FMD FMD is a highly contagious Transboundary animal disease which effects the clovenhoofed animals. Due to this character, the virus of FMD causes severe economic loss by restrictions of export trade and high morbidity. There is a rapid transmission of FMD within and between a wide range of susceptible host species, so this is a threat to small-scale farmers as well as multinational livestock industries [58,59]. In many highincome countries, FMD has been eradicated but it is still present in middle and lowincome countries [60]. FMD has low mortality but significant losses of production that may lead to important economic losses [61]. Although research has been conducted to understand the impact of FMD virus [62,63]. Previous studies show that the introduction of FMD virus in FMD free countries cause a severe impact on national economies [64]. The economic impact of FMD can be divided into two components, direct impact and indirect impact. The direct impacts include reduced livestock growth, reduced milk production, problems with fertility, mortality in young stock and indirect impact includes additional cost like vaccination costs and movement control [65,33]. Some governments have been considered the potential role of FMD in bioterrorism due to the relative case with which from outbreaks in endemic countries, highly contagious material can be obtained and severe economic losses in FMD free countries [66,67]. In most parts of Africa, FMD is enzootic and some other countries have managed tools to control the disease [68, 69,70]. A high rate of contact among animals at commercial markets and Extensive movement of livestock in watering points and communal grazing areas can cause the incidence of disease [64, 71, 72]. Conclusion FMD is distributed all over the world. This disease having great economic importance because it not only reduce production but also restricts the trading of animals both locally and internationally. So the following points are recommended on the base of the above conclusions. During the outbreak of FMD, restrict the movement of the animal both across national and international boundaries to prevent the spread of the virus. Proper disposal of manure and dead animals. Government should regularly monitor the occurrence of an outbreak and should take necessary steps as soon as possible. In exiting heard, introduction of new animals should controlled. Controls on movement of livestock, vehicles and equipment's are necessary. Key elements of effective disease management are information on the epidemiology and rapid diagnosis. Highly infected areas should be considered during control program. Periodic and Regular mass vaccination of animals (2 times in a year is compulsory).