Lumpy Skin Disease: Pathogenesis of an African capripox virus disease

Lumpy Skin Disease: Pathogenesis of an African capripox virus disease

Suhas K. S. ^1*, Kaveri Jambagi ², Bindu S ³, Dilip Kumar D. N.⁴  

¹ Division of Pharmacology and Toxicology, ICAR-Indian Veterinary Research Institute; India; email id: suhasks92@gmail.com; ORCID : https://orcid.org/0000-0002-7678-736X

²Division of Medicine, ICAR-Indian Veterinary Research Institute; India, email id: kaverijambagi3@gmail.com; ORCID: https://orcid.org/0000-0002-1001-2545

³ Division of Immunology, ICAR-Indian Veterinary Research Institute; email id: bindusmoksha@gmail.com.

⁴ Veterinary Officer, Department of Animal Husbandry and Veterinary Services, Government of Karnataka, email id: dilidnm@gmail.com.

Running Title: Pathogenesis of Lumpy Skin Disease

^*Corresponding author details:

Dr. Suhas K S

Email id: suhasks92@gmail.com

Mobile: +91 9634258882

Abstract

Arthropod-borne viral diseases are equally intimidating to both human and animal well-being in the present-day scenario. Lumpy skin disease is an evolving viral disease that particularly affects cattle and water buffaloes. The lumpy skin disease virus is mechanically passed on from infected hosts to the naïve animals through the bite of insect vectors. Epizootics of the disease occur in the warm and damp seasons which is advantageous for the propagation of vectors. It is a disease of economic importance as the affected animals become unproductive. Numerous investigations have been carried on various aspects of lumpy skin disease in recent times as the virus is quickly spreading to new regions that are free from the disease. By understanding the disease development process at the molecular level, better management strategies can be established to reduce the suffering of the animal as well as to lessen the economic impact faced by livestock owners. However, the exact molecular mechanisms causing the pathogenesis of lumpy skin disease is not well established.  This review attempts to give a clear picture of hematological and biochemical alterations in lumpy skin disease affected animals. Further, it defines oxidative injury and inflammation as a possible mechanism in causing the disease.

Keywords: Hematology, serum biochemistry, vector-borne, biomarker, oxidative stress, inflammation

Introduction

Lumpy skin disease (LSD) alternatively known as “pseudo-urticaria”, “knopvelsiekte”, “Neethling virus disease” and “nodular exanthema of bovines” is an emerging disease of cattle and buffaloes in different parts of the world [1]. The lumpy skin disease virus (LSDV) is classified as a double-stranded DNA virus belonging to the Capripoxvirus genus and Poxviridae family [2]. Arthropod insect vectors like ticks, biting flies, and mosquitoes are involved in the mechanical transmission of this virus. The movement of animals and insect vectors from one place to another is accountable for viral transmission that can take place in both short and long routes. LSD is characterized by the typical development of diffused nodular lesions on different parts of the animal. The affected animal exhibits fever, ocular discharge, enlargement of regional lymph nodes, go off-feed and become emaciated. The occurrence of LSD shows a pattern with outbreaks taking place in seasons with an increased population of insects. It is adjudged as a non-zoonotic disease [3].

It can prove to have a serious impact on the economic status of the country as the disease affects milk, meat, and hide production. It also causes a restriction on international trade [4]. LSD, once restricted to the African sub-continent, is now rapidly spreading to different nations which never encountered the disease earlier. It is an OIE notifiable disease of cattle and buffaloes due to its ability of transboundary spread [5]. Recently, India also reported the disease for the first time. Effective vaccination strategies, curtailing animal movement, vector control, proper biosecurity measures, and surveillance programs can prevent and control LSD occurrence [6].

LSD virus follows the typical pathogenesis sequence of other poxviruses, starting from the viral entry to the development of pox-like lesions. But the molecular mechanisms fundamental to the development of the disease are not well understood. Insights into clinical signs, gross macroscopic lesions, histopathological studies, and analytical quantification of biomarker molecules provide evidence for a better understanding of the disease. Here, we describe the pathogenesis of LSD concerning hematological and serum biochemical alterations in the animals affected by the disease, which provides a clue for knowing the mechanism of disease progression in detail at the molecular level.

Pathogenesis

There appears to be a species difference in the occurrence of LSD. Cattle appear to be more vulnerable to the disease condition than buffaloes. Sheep and goats are relatively resistant to natural infections. Only a few studies describe the detailed pathogenesis of this disease [7]. Broadly, the events of pathogenesis follow a certain progression that comprises of viral inoculation and establishment at the biting place, multiplication in the nearby susceptible cells, viremia, viral transport to different target tissues, and spread of virions to the adjacent organs [8]. 

Soon after the entry into the animal, the virus by evading the host cellular defense mechanism starts to replicate in vulnerable cells which include keratinocytes, hair follicle epithelium, fibroblasts, interstitial macrophages, and pericytes, consequently probing the dermis, sub-cutis, and parenchyma of lymph nodes [9, 10]. There are likelihoods of the development of vasculitis and lymphangitis as a result of viral replication and potential injury to the endothelial cell of the blood vessels and lymphatics respectively. This can further lead to thrombosis and infarction in more severe cases [11, 12].

Viremia is seen usually after 6 days of infection as the virus continues to multiply. Once in the blood, the viral particles spread to various organs and multiply at these secondary sites. The blood monocytes are believed to carry these viral particles to various organs. Peaks of viremia are reflected as intermittent rise and fall in body temperature. The virus exhibits a tropism towards keratinocytes and brings about various pathological changes. Initially, there is hyperplasia and ballooning degeneration, later epidermal micro-vesicles develop and the released chemokines attract inflammatory cells to the site [13].

At around the 7th day, nodular lesions start to appear on the skin of the affected animal. Such lesions are also evidenced in various tissue containing epithelial cells. Later on, the micro-vesicles join each other and form large vesicles, and the lesions begin to ulcerate and exude serous discharge [14]. These nodular sites become necrotic enclosing a typical grey to a pink-colored conical area that gets separated from the adjoining tissue and these lesions are known as ‘sit-fasts’. There are congestion, hemorrhages, and edema in the neighboring zone of tissue. The necrotic zones of tissue are invaded by bacteria in the mean course of time which further complicates the pathogenesis. The lymph nodes become enlarged. The host factors, the viral pathogenic factors, and the environment impact disease progression [5].

The state of viremia usually lasts for about 9 days. After this period the antibodies are raised in the host that tend to stop viremia by neutralizing the LSDV. The affected animal excretes virus in nasal, oral, and conjunctival secretions [9]. Also, the virus spreads and replicates in mucous membranes of the buccal cavity, pharynx as well as in the gastric epithelium of rumen, reticulum, omasum, and abomasum. There is evidence for intrauterine transmission of viral particles and also their excretion in the semen. Also, the mucous membranes of the nasal cavity including turbinate, trachea, and lungs are affected. Nodular growths on the trachea cause stenosis of the airway and bring about respiratory distress that may persist for a few weeks or in some cases even up to a month [15].

As the disease progresses and becomes severe, the incidence of other organs being invaded by the virus increases dramatically. The high viral load in the cutaneous lesions and the bloodstream of the affected animal has a significant role in the transmission of LSD to naïve animals through biting insects. For culture, isolation, identification, and diagnosis of viral particles in the laboratory, active lesions can be taken. Grossly normal tissues usually appear to be free from viruses. At times, the LSD affected animals may show characteristic nodular lesions only on the skin without any involvement of other tissues which may be due to the high affinity of this virus to the integument system of the organism.

As the young animals display an ill-developed immune system, LSD has a more drastic outcome in these compared to the adults of the same species. Besides, the lactating cows and under-fed animals are also highly vulnerable to the disease in natural infections. The viral particles remain relatively stable in the host cells and hence, it takes a long time for the animal to clear-off the infection from the body.

Eventually, affected animals clear the infection and there are no known carrier states for LSD affected animals [16]. Usually, the animals recovering from clinical LSD develop lifelong cell-mediated immunity. The calves born to immune cows acquire maternal antibody and are resistant to the clinical disease for about six months [11, 17]. LSD is considered to have a low mortality rate and a relatively high morbidity rate.

Experimentally, LSD can be produced by inoculating the virus by intravenous, intradermal, and subcutaneous routes. The viral entry from the intravenous route flourishes a severe widespread infection, while the other routes of inoculation may produce localized lesions or no apparent disease at all. Following subcutaneous or intradermal injection of cattle with LSDV, localized swelling at the site of inoculation developed initially, followed by swelling of the regional lymph nodes and an extensive flare-up of skin nodules later [14]. The molecular pathogenesis of LSD can be better understood to a greater degree by knowing the alterations in hematology, serum biochemistry findings, along with clinical signs and lesions in diseased animals.

Clinical signs and pathological lesions 

The LSD affected cattle and buffaloes exhibit recurrent fever, serous discharges from eyes and nose, anorexia, enlarged lymph nodes that become palpable, and the animal becomes lethargic. The animals develop nodular cutaneous lesions that get distributed diffusely throughout the body in the mean course of the disease [14]. These nodules become necrotized and ulcerated to become a site for flystrike. Secondary bacterial infection turns the nodular discharge purulent and worsens the condition. Some animals show oedematic changes in various parts of the body. The animals display a decrease in feeding, ruminating, exploratory, and self-care behavior [18]. There is a development of respiratory distress and dyspnoea due to nodular lesions in the upper respiratory tract, especially in the trachea. ECG examination shows altered cardiac rhythms with changes in amplitude and frequency of heart waves in affected cattle [19]. LSD also affects fertility and causes complications like pneumonia, maggot wounds, mastitis, and orchitis [20].

Hematological alterations in LSD affected animals

The analysis of hematological alterations not only assist in disease diagnosis but also give useful information about the pathogenesis of any disease. In general, most of the studies with LSD affected animals indicate a reduction in RBCs and hemoglobin concentration. Some case investigations have also reported the presence of hemolytic anemia [21]. The reason for this may be due to an alteration in serum phosphorus content in the affected animals which may trigger an inflammatory change and free radical injury to the red blood cell membrane. Macrocytic hypochromic anemia is a feature of this disease. Few studies indicate an increase in hematocrit and RBCs [22]. This may be due to the presence of dehydration. Leukogram of the LSD affected animals suggest a decrease in total leucocytes that are typically seen in viral infection. Other studies have reported leucocytosis, specifically granulocytosis, which may be due to inflammatory changes caused by secondary bacterial infection [23]. The hematological alterations seen in LSD affected animals as reported in different studies are given in Table 1. Serum biochemical alterations in LSD affected animals

Measuring serum enzymes and biomarker molecules is one of the ways for understanding specific tissue injury in the pathogenesis of a disease. Most of the studies indicate an increase in ALT and AST and a decrease in albumin levels in blood suggesting potential damage to the liver in the affected animal. An increase in globulins, especially gamma globulins suggest the involvement of inflammatory change. Also, significant increases in creatinine and BUN reflect kidney injury. Data also reveals damage and involvement of skeletal muscle and cardiac muscle as indicated by an increase in CK and cardiac troponin [21]. All these suggest that the LSD virus disseminates to multiple vital organs like the heart, liver, kidney, and trigger pathogenic changes. There is a possible involvement of inflammatory mediators in bringing about these alterations. The serum biochemical changes as recorded in various studies of LSD affected animals are summarised in Table.2.

Oxidative stress and inflammation in the pathogenesis of LSD

Oxidative stress and inflammation are implicated as the cause of many metabolic and infectious diseases. Cattle infected with LSD showed significant alteration in the oxidative stress markers characterized by a decrease in glutathione (GSH), total antioxidant capacity in plasma, and an increase in lipid peroxidation (MDA) [18, 22, 24].  Similar results were observed in buffaloes which also exhibited a lessening of SOD, CAT, and an increase in MDA [21].

Studies with naturally infected cattle and buffalo also exhibited an increase in inflammatory mediators. Buffaloes showed an increase in the levels of TNF-α, and IL-6 [21]. Cattle also showed an increasing trend of IL-4 and TNF-α levels [22]. One of the studies has also demonstrated an increase in inflammatory markers of acute-phase reaction including haptoglobin (Hp), serum amyloid A (SAA), and neopterin in cattle affected with LSD [25]. Immunological parameters like lymphocyte transformation rate, phagocytic activity, and killing percentage were significantly decreased in affected cattle 2 weeks post-infection [23]. All these findings indicate a strong possibility of inflammatory response and involvement of oxidative stress in LSD pathogenesis. Figure.1 shows the sequence of disease development.

Conclusion

Lumpy skin disease is an economically important arthropod-borne viral disease of cattle and buffalo. Depending on the severity of the disease, various vital organs get affected through the systemic spread of the virus. The pathogenesis at the molecular level can be better understood by studying the characteristic hematological and serum biochemical alterations in LSD, along with clinical signs, gross and microscopic pathological lesions. We conclude that there is an association of inflammatory response and oxidative stress that majorly operate in the pathogenesis of LSD. However, more studies are required to unravel whether the inflammatory response is triggering oxidative stress or the oxidative stress by itself is causing the inflammatory response in the LSD affected cattle and buffaloes.

Funding

The review received no funding.

Conflict of interest

The authors declare no conflicts of interest.

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Table 1: Altered haematological parameters in LSD affected animals

Table 2: Altered serum biochemical parameters in LSD affected animals

Figure 1: Schematic representation of pathogenesis of lumpy skin disease

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