Etiology and epizootiological features of swine erysipelas spread

Swine erysipelas is an infectious disease characterized in its acute course by septicemia and inflammatory skin erythema, and in its chronic form by endocarditis and arthritis.

The causative agent — bacteria are non-motile bacteria that do not form spores or capsules. It belongs to ubiquitous microorganisms.

The resistance of the erysipelas causative agent in the external environment is high; it persists for many months in soil, water, carcasses, and liquid manure. Salting and smoking of pork products do not kill it. Erysipelas bacteria are sensitive to high temperatures, certain antibiotics, and disinfectants, including solutions of sodium hydroxide and formaldehyde (2%), chlorinated lime (10%), phenol (3%), and others in standard concentrations.

Epizootiological data. Erysipelas is the most widespread infectious disease of pigs worldwide. It is registered almost everywhere as sporadic cases or epizootic outbreaks. Pigs aged 3-12 months are more frequently affected. The resistance of suckling piglets up to 40 days is associated with passive immunity, and in adult animals — with age-related natural resistance and latent infection.

Erysipelas occurs sporadically among horses, cattle, sheep, dogs, many wild mammals, and humans can also contract erysipelas. Microbe carriage is widespread among many species of domestic and wild animals, especially among pigs, rodents, and poultry, which provides a constant reservoir of the erysipelas causative agent.

The source of the erysipelas causative agent is overtly sick pigs excreting microbes with urine and feces, and clinically healthy pigs that are carriers. As a result, epizootic outbreaks of erysipelas on farms often arise endogenously, without the introduction of the pathogen from the outside.

Transmission factors for the erysipelas causative agent include infected grooming tools, feed and water, animal slaughter products, carcasses, soil, etc. The pathogen is most often carried by rodents, biting flies, and birds. The main route of pathogen spread is alimentary, less often transmissive and contact.

Erysipelas, being a soil-borne infection, has a pronounced spring-summer seasonality and occurs more often among replacement and fattening young stock. Morbidity usually does not exceed 20-30%, mortality – 55-80%.

Course and symptoms. Incubation period is usually 2-5 days. Fulminant, acute, subacute, and chronic courses are distinguished, as well as septic, cutaneous (urticaria), and latent forms.

The fulminant course is registered relatively rarely, mainly in fattening shoats aged 7-10 months kept in poor zoohygienic conditions or during transportation. It manifests as sharp depression, hyperthermia, and rapidly progressing heart failure without the appearance of red spots on the skin, ending in the death of the animal within a few hours.

The acute course is most typical for the septic form of erysipelas; it is often registered at the beginning of an epizootic outbreak. The disease begins with a depression of the general state and a sudden rise in body temperature to 42°C and above. Sick animals isolate themselves from the general group, lie down more; move reluctantly, refuse feed, constipation, chills, heart failure, pulmonary edema, and skin cyanosis appear. Erythematous spots of pale pink, and subsequently dark red color, appear in the submandibular region, neck, and abdominal wall. The disease lasts 2-4 days and ends lethally without assistance.

The subacute course of erysipelas manifests relatively milder in the cutaneous form (urticaria), which is characterized by a temperature rise to 41°C and above, and the formation after 1-2 days on the skin of the head and trunk, less often on other parts of the body, of dense inflamed swellings of square, rhombic, and less often round shape. In most cases, urticaria proceeds benignly for 7-12 days, and upon the animal's recovery, the spots gradually fade and disappear.

Immunity. A live vaccine against swine erysipelas from the BP-2 strain is used for immunization.

Prevention and control measures. For prophylactic purposes, all piglets are subjected to vaccination according to instructions. When erysipelas appears on a farm, restrictions are imposed on:

• export and import of pigs;
• export of non-disinfected meat and feed.

The affected population is thoroughly clinically examined, and the sick are immediately isolated and treated. Clinically healthy pigs in the affected pigsty (unit) are vaccinated against erysipelas. If necessary, emergency slaughter of pigs sick with erysipelas is carried out in a specially equipped place, and the meat is allowed for human consumption only after boiling. After each case of isolating pigs sick with erysipelas, the pens are cleaned, washed, and disinfected.

Classical swine fever (hereinafter CSF) is a highly contagious viral disease affecting pigs of all ages, characterized in the acute course by a severe septic process with phenomena of hemorrhagic diathesis, a constant type of fever, and in subacute and chronic cases, it is often complicated by a secondary infection, which manifests as croupous pneumonia and croupous-diphtheritic colitis.

Under natural conditions, only domestic and wild pigs are susceptible to the swine fever virus. Swine fever is a particularly dangerous disease. Humans can be carriers of the infection, but they do not develop the disease. In the absence of proper veterinary and preventive measures in pig farming, very large losses of the swine population are inevitable due to mass mortality of livestock and the need for compulsory culling of both sick and healthy animals. Significant losses also arise from the reduction in the market value of meat, as well as the costs of implementing quarantine measures and eradicating the disease.

The causative agent of the disease is an RNA-containing virus of the genus Pestivirus, which is highly infectious when present in the blood, tissues, and internal organs of sick animals. Based on virulence (degree of contagiousness), three variants of the virus are distinguished—A, B, and C:

Resistance. In terms of resistance to chemical disinfectants, the swine fever virus is classified as resistant (group 2). It is resistant to drying and low temperatures. It does not lose its virulence in pigsties for a year, in frozen meat for more than 4 years, in fresh chilled meat for 45-71 days, in salted meat for more than 6 months, in smoked products for 3 months, and in salted meat for up to 315 days. Heating to 70-80°C kills the virus within one hour. Direct sunlight kills the CSF pathogen on the soil surface within 1-2 weeks; the virus persists in carcasses and manure for 3-5 days; and in a dried state in contaminated yards and pigsties, it lasts up to a year. Disinfectants kill the virus: 2-3% solution of caustic alkali, 15-20% lime milk from freshly slaked lime or chlorinated lime, 5% sulfuric-carbolic solution, 0.3-0.5% dezkoten solution, 5% bional solution, 3% phospar solution, 1% iodez, and other modern disinfectants.

Epizootiological data. The source of the disease is sick pigs or recovered swine fever carriers. The virus is shed into the external environment from sick and recovered pigs via all secretions and excretions.

Carcasses of compulsorily slaughtered pigs, offal, cadavers, and untreated slaughterhouse or kitchen waste can be a source of swine fever infection. The virus is transmitted through grooming tools, feed, water sources, vehicles, clothing, footwear of staff, and surfaces contaminated with the excretions of sick animals and virus carriers. Dogs, poultry, rodents, flies, and even earthworms can play a role in the spread of swine fever. Infection with CSF occurs through the digestive tract, as well as through the respiratory system and damaged skin. Swine fever can occur at any time of the year; however, it is most frequently recorded in autumn, when mass movement, sale, and slaughter of pigs take place.

In new outbreaks where a non-immune population is present, the epizootic process proceeds intensively in the form of epizootics. When the pathogen is introduced by virus carriers into a farm, only a few animals initially become ill, but within 10-14 days, almost the entire swine population is affected. If the virus is introduced via infected feed, a large portion or almost all of the animals fall ill within 2-3 days. The morbidity rate of swine fever reaches 95-100%, and the lethality rate is 60-100%.

Course and symptoms of the disease. The incubation period lasts 1-9 days, rarely extending to 2-3 weeks. The course of the disease is acute, subacute, and chronic. In suckling piglets, the disease often follows a peracute course.

At the beginning, the body temperature rises to 40-41°C, and anorexia, chills, vomiting, and constipation followed by diarrhea (sometimes bloody) and conjunctivitis develop. Pustules filled with yellow exudate appear on the skin, and a little later—petechial hemorrhages, which subsequently merge to form dark crimson spots that do not disappear upon pressure. Pregnant sows abort. Mucopurulent rhinitis often occurs; some animals exhibit nosebleeds, uncoordinated movements, tremors of the limbs, convulsions, and epileptic seizures. When the lungs are affected, croupous pneumonia develops, manifesting as spasmodic coughing, difficult wheezing, and chest pain.

In piglets, a peracute course of CSF is possible, occurring as fever, depression, and bouts of vomiting. Piglets die within 1-2 days with symptoms of epilepsy or in a comatose state.

Towards the end of a CSF outbreak, as a result of increased immunity in pigs and decreased virulence of the virus, the fever takes on a subacute, protracted, and chronic course. In the subacute course of CSF, the disease lasts 2-3 weeks, and the typical signs of CSF are less pronounced. Sick animals exhibit periodic spikes in body temperature, and due to croupous-necrotic enterocolitis, they experience constipation alternating with diarrhea, and an impaired appetite.

Lung damage results in coughing, dyspnea, nasal discharge, and pneumonia. To facilitate breathing and reduce pleural pain, piglets attempt to lie on their chests or adopt a "sitting dog" posture. The livestock animals weaken, lose weight, and most of them die.

In the chronic course of Classical Swine Fever (CSF), the disease persists for up to 2 months. Piglets experience periodic diarrhea, coughing, and fluctuating appetite. The skin appears wrinkled and becomes covered in eczematous scabs. The disease usually leads to mortality. Surviving emaciated livestock animals can remain CSF virus carriers for up to one year.

Pathological changes. The skin of the ears, neck, abdominal wall, and inner thighs is dark purple. The entire skin surface is dotted with single or multiple hemorrhages of various shapes and sizes. Lymph nodes are enlarged, dark to black-red in color; a characteristic marbled pattern is visible on cross-section. The lungs are mottled and succulent; single hemorrhages and sometimes hemorrhagic infarctions are seen on cross-section.

Diagnosis and vaccine prophylaxis of Classical Swine Fever

During the subacute course of Classical Swine Fever (CSF), livestock animals develop croupous-hemorrhagic gastritis and enteritis. The chronic form is characterized by croupous-diphtheritic and ulcerative-necrotic lesions, which are most distinct in the cecum and colon. The intestinal mucosa becomes diffusely hyperemic and covered with layered scabs — plague "buttons" and follicular ulcers. Intestinal pathologies are most severe in piglets aged 2–4 months and older.

Diagnosis of CSF is established comprehensively, considering epizootological, clinical, and pathological data. The laboratory component includes mandatory virus detection using immunofluorescence (IF), polymerase chain reaction (PCR), and a bioassay on non-immune piglets.

After recovery, surviving pigs develop non-sterile immunity. For planned active immunization of the herd, three preparations are used: dry cell-culture virus vaccine KS, dry cell-culture virus vaccine LK-VNIIViM, and dry lapinized virus vaccine SINLAK. A single vaccination creates an immune barrier in adult livestock animals for a period of at least 1 year. For the vaccination of wild boars via oral, intramuscular, or aerosol routes, an inactivated dry cell-culture virus vaccine from the LK-K strain has been developed.

• Age of initial piglet vaccination — 2 months
• Repeat piglet vaccination — after 10 days
• Herd revaccination — after 1 year (annually)
• Immunity duration in adults — at least 1 year
• Post-vaccination observation in the outbreak area — 20 days

Quarantine measures and eradication of infection foci

Upon the first suspicion of a swine fever outbreak, sick livestock animals are immediately isolated, and all movement within the herd is completely halted. Facilities and affected pens are disinfected with a 2–3% caustic soda solution. To assess the situation, the veterinary service organizes an epizootological analysis, a head-count examination, and mandatory thermometry of the pigs.

Following official confirmation of the diagnosis, the farm is placed under quarantine and an at-risk zone is established. Under quarantine conditions, veterinary legislation prohibits:

• the import and export of pigs, as well as the transport of raw pork and emergency slaughter products;
• regrouping of the swine population within the farm without coordination with a veterinary specialist;
• slaughter of livestock animals without the permission of a veterinarian or veterinary paramedic;
• departure of any transport and exit of personnel without sanitary treatment of outer clothing and footwear;
• the sale of pigs, raw meat, lard, and slaughter products in markets and bazaars within the quarantine and at-risk zones.

The strategy for rehabilitation measures depends on the farm's production type. In fattening, subsidiary, and kitchen-waste farms, the entire swine population of affected barns is slaughtered, and subsequent restocking is conducted only with vaccinated livestock animals. In breeding centers and pedigree farms, daily inspection and thermometry are performed: all sick, suspicious, and emaciated pigs, as well as individuals with pulmonary or intestinal pathologies, are slaughtered. The remaining livestock animals with normal temperatures are vaccinated and isolated for observation for 20 days — if their temperature rises or they refuse feed, they are also sent for slaughter. The carcasses of dead pigs are incinerated.

In clinical practice, it is important to distinguish CSF from African Swine Fever (Montgomery disease, ASF). ASF is a highly dangerous, highly contagious viral disease, accompanied by fever, skin cyanosis, extensive internal hemorrhages, and a nearly 100% mortality rate. The ASF virus poses no danger to humans.

Characteristics of the pathogen, resistance, and epizootiology of ASF

African swine fever (ASF) was first recorded in the early 20th century in African countries following the importation of domestic pigs from Europe. The disease occurred with mass mortality of the livestock (nearly 100% of cases) and, based on clinical and pathological signs, resembled classical swine fever. Between 1909 and 1915, 15 outbreaks of the infection were recorded in East Africa. Further observations showed that the disease is widespread south of the Sahara Desert, and infection in pigs occurred primarily through contact with wild African species.

Subsequently, the introduction of the virus through port areas led to major epizootic outbreaks in 1977, and starting from 2007, the infection covered the European part of the country, spreading as far as the Irkutsk region by 2017. The economic damage from ASF is enormous: it consists of the total culling of pigs in epizootic centers, the costs of veterinary and sanitary operations, and strict restrictive quarantine measures.

The causative agent of the disease is a DNA-containing virus with a complex structure, having several antigenic types. The virus is characterized by extremely high resistance to physicochemical environmental factors. It remains stable at pH levels from 4 to 13, is sensitive to fat solvents (ether, chloroform), and is completely inactivated by heating to 60 °C for 30 minutes.

• Incubation period — 2–20 days (usually 2–5)
• Body temperature during fever — up to 41–42 °C
• pH resistance — from 4 to 13
• Inactivation by heating — 60 °C for 30 minutes
• Mortality in peracute course — about 100%

Domestic pigs of all breeds and ages in any season are susceptible to the ASF virus, as well as wild boars and African pigs (warthogs, bushpigs, and giant forest hogs). During alimentary infection, the virus primarily infects monocytes and macrophages of the tonsils and submandibular lymph nodes. Then, through the blood and lymph flow, the pathogen is carried to the spleen, bone marrow, lymph nodes, lungs, liver, and kidneys, causing dystrophic and necrotic changes in them. Damage to the vascular endothelium leads to extensive hemorrhages in internal organs and tissues.

Clinical forms of the course and pathological picture

The incubation period of the disease ranges from 2 to 20 days (in most cases — 2–5 days) and directly depends on the virulence of the strain and the natural resistance of the animals. Clinically, ASF can be peracute, acute, subacute, chronic, or latent (asymptomatic).

In the peracute course, sudden death of animals without prior symptoms is possible, or a sharp rise in temperature up to 41–42 °C, rapid breathing, and reddening of the skin. Pigs die 1–3 days after the appearance of the first signs; mortality reaches 100%. Acute and subacute courses are observed most often: the disease begins with a fever of up to 42 °C, while the animals' appetite is maintained almost until the very end and disappears only 1–2 days before death.

Animals that recover from the subacute form survive but become lifelong virus carriers. The chronic form lasts from 2 to 10 months and is characterized by emaciation, growth retardation, signs of pneumonia, lesions of joints, skin, and eyes; the mortality rate is 50–60%. A latent course with asymptomatic carriage is characteristic of wild African pigs and has also been detected in domestic and wild pigs in Portugal, Spain, and Italy.

A primary diagnosis of African swine fever is suspected upon the appearance of fever and hemorrhagic diathesis in animals previously vaccinated against classical swine fever. Definitive laboratory confirmation is carried out exclusively by specialized research institutions under conditions of a strict sanitary regime. To prevent the introduction of the infection, the import of domestic and wild animals, their meat products, and feed from affected regions is prohibited.

Upon confirmation of the disease, an epizootic focus, as well as first and second threatened zones, are immediately designated around the infected site. In the first zone, the entire pig herd is sent for slaughter in compliance with veterinary and sanitary regulations, and the resulting meat is permitted to be processed only into cooked sausages, cooked-smoked sausages, or canned goods for use within the disadvantaged territory. In the second zone, veterinary supervision is strengthened, and pigs are vaccinated according to a plan against CSF and erysipelas.

• First threatened zone — 5–20 km
• Second threatened zone — 100–150 km
• Quarantine lifting — after 30 days
• Post-quarantine restrictions — 6 months
• Restocking of livestock — after 1 year

1. Destroy the entire pig herd in the epizootic focus using a bloodless method.
2. Burn animal carcasses or bury them to a depth of at least two meters.
3. Carry out triple disinfection of the premises using chlorine-based preparations.

The quarantine is lifted 30 days after the slaughter of animals in the focus and the first zone. After this, restrictions on the movement of livestock remain in effect for 6 months, and restocking the farm with pigs in the former focus and the first zone is permitted only after 1 year.

Etiology and stability of the equine strangles pathogen

Strangles is an acute infectious disease of horses, manifested by fever, purulent-catarrhal inflammation of the mucous membranes of the nasal cavity and pharynx, as well as abscessation of the submandibular lymph nodes. The pathogen is *Streptococcus equi*, a round-shaped bacterium that forms long, twisted chains in pus smears. The microorganism produces hemotoxin, leukotoxin, and aggressins.

The strangles pathogen is sensitive to physical and chemical exposure: when heated to 75 °C, it dies within 1 hour, and when boiled — instantly. A 5% solution of carbolic acid and formalin preparations are used for disinfection and sanitization of premises.