**Preface to "Veterinary Microbiology & Parasitology"**

Numerous pathogens, both microscopic and macroscopic, affect animal species of veterinary relevance, including companion animals, livestock, and exotic animals. The term pathogen was originally referred to infectious microorganisms, such as viruses and viroids, bacteria, fungi, and protozoans. While prions do not contain genetic materials, they are transmissible and are often classified as pathogens. The modern concept of pathogens includes both infectious microbes and macroscopic parasites. The scientific disciplines that study microscopic organisms of veterinary significance are generally classified as veterinary microbiology, while parasitology refers to the scientific study of parasites and their hosts. However, the boundaries between these two scientific disciplines are not always rigidly defined. For example, some single-cell microscopic protozoan pathogens behave much like infectious microorganisms, but they belong to eukaryotic phyla and are often the subject of parasitology studies.

Animal pathogens affect animal health and wellbeing and production efficiency. These pathogens also have a considerate impact on social economics, food safety and security, and human health. Numerous historical events have demonstrated the extreme negative potential of infectious pathogens on animal health and wellbeing. For example, the recent high pathogenic avian influenza outbreak in the U.S. resulted in the depopulation of 7.5 million turkeys and 42 million chickens. The birds infected by high-risk virus pathogens had extremely high morbidity and mortality; and associated economic losses reached billions of dollars. In addition to infectious viral agents, many parasite species are deleterious to human and animal health and agricultural efficiency. Approximately 70% of production animals in developing countries are estimated to experience severe parasite transmission. In ruminants, parasitic infections result in a liveweight reduction up to 14% and milk yield reduction up to 2.2 kg milk/cow per day. Parasitism is not just a disease affecting productivity, but also a leading cause of mortality in young animals. The annual losses of crop yield due to parasitic nematodes are approximated 12% of the total, over 100 billion USD. As a result, parasitism represents one of the key factors threatening global food availability.

Studies of pathogens of veterinary significance do not just result in a better understanding of animal diseases and promote animal welfare, but also inspire the discovery of new human pathogens and improve human health. It is the pioneer work in dissecting etiology of avian sarcoma that led to the discovery of multiple important human oncogenes, such as MYC and JUN. The causal association between rotaviruses and diarrhea was first discovered in calves, long before they were identified as main causal agents for diarrhea in infants and children around the globe. Unfortunately, the significance and importance of many studies on animal pathogens and their profound impact on medicine and public health are not widely appreciated outside the veterinary community.

Infectious diseases that originate from both domestical animals and wildlife represent one of the greatest threats to human health, as evidenced by the devastating consequence of the COVID-19 pandemic. Each year, over 1 billion cases of human illness are attributable to zoonotic pathogens. As a result, identifying wild reservoirs of zoonotic pathogens has been an urgent public health priority. Recent studies show that domesticated species harbor approximately 84 times more zoonotic viruses than wild species. Eight of the top 10 mammalian species with the highest number of zoonotic viruses are domestical, such as pigs, cattle, and horses, each with 31 zoonotic viruses, followed by sheep, dogs, goats, cats, and camels. Moreover, many of animal parasites are also zoonotic, constituting an additional burden on human health.

Furthermore, the rapid emergence and spread of drug resistant pathogen strains are alarming. Climate changes will undoubtedly alter the interactions between animals and between animals and humans, which will have a considerate impact on the transmission rate of existing pathogens and the emergence of new pathogens or the reemergence of old pathogens.

This special collection covers a broad range of research topics, which likely represent frontiers in the contemporary veterinary microbiology and parasitology. The host species under investigation included wild animals (rodents), companion animals, such as dogs and horses, and livestock species, such as camels, poultry, ruminants, and swine. Almost all major pathogen types, including viruses, bacteria, mites and flies, protozoans, and helminths, have been discussed. The tools and approaches used range from epidemiological survey, immunology, to cutting-edge microbiome studies and metabolomics. This collection provides a broad knowledge base that will encourage dialogue and enhance interactions across the wide distribution of the research community in microbiology and parasitology.

The highest proportion of zoonotic viruses have been identified among species in the order Rodentia, followed by Chiropetra, and Primates. Many rodent species are hyper-reservoirs, carrying between 2 and 11 zoonotic pathogens, including viral pathogens and zoonotic helminths. Kalogianni and colleagues present a review article on Maedi-visna (MV) in sheep, a disease caused by maedi-visna virus, a small ruminant lentivirus (SRLV). The predominant clinical manifestations of MV are pneumonia and mastitis. MV has a worldwide distribution with distinct viral transmission patterns. Nevertheless, the prevalence rate of MV has been increasing globally. Currently, gaps in understanding the epizootiology of MV, the continuous mutation of existing and the emergence of new SRLV strains, lack of an effective detection protocol, and the inefficiency of preventive measures render the elimination of MV an unrealistic target. Therefore, modifications on the existing MV surveillance and control schemes on an evidentiary basis are necessary. Updated control schemes require the development of diagnostic protocols for the early and definitive diagnosis of SRLV infections. These authors summarize the current knowledge in the epizootiology and control of MV in dairy sheep and describe the research framework to close existing knowledge gaps. In another report, Piegari and colleagues conduct a pathological and microbiological evaluation of "sudden and unexpected death (SUD)" cases in young dogs. They conclude that viral infection with Canine parvovirus type 2 is the most common cause of SUD.

When pathogenic bacteria, such as staphylococci, streptococci, Gram-negative bacilli, and anaerobes, enter the animal's body and multiply, they directly disrupt the normal function of the cells, tissues, or organs involved. The toxins released by these infectious agents can also result in harmful effects, affecting animal health and wellbeing. Furthermore, numerous bacteria are zoonotic. In this collection, at least two bacterial-related diseases are discussed. Anaplasma phagocytophilum, a Gram-negative bacterium transmitted by ticks, is the causal agent of pasture fever, a disease affecting domestic ruminants, such as cattle, sheep, and goats. This pathogen can also infect humans in temperate and tropical regions. Atkas and colleagues present a PCR-based diagnostic technique that can be used to distinguish two variants of this important pathogen. The phylogeny and prevalence of A. phagocytophilum and related variants were then investigated in the Mediterranean region of Turkey. Their findings highlight the importance of detection of variants and provide a basic took kit for studying molecular epidemiology of caprine and ovine anaplasmosis. Bovine digital dermatitis (BDD), first reported in 1974, is arguably the most important foot infection causing lameness in cattle, which has a serious impact on animal welfare and productivity. Treponema species have been considered as the primary causal agent. To understand BDD pathogenesis, Espiritu and colleagues compared the microbial composition and diversity between BDD lesions and normal cattle skin using 16S rRNA gene-based sequencing. These authors conclude that rich microbial diversity and the overabundance of opportunistic bacterial pathogens are likely associated with BDD.

Three articles in this collection discuss the biology of mites and flies. Oestrus ovis, the sheep bot fly, is an obligate parasite having a worldwide distribution with the high prevalence rates in Mediterranean and West African countries. While generally not considered a life-threatening parasite, this species can cause significant losses in productivity, particularly in tropical and Mediterranean regions. Metwally and colleagues investigated the prevalence rate of this species in small ruminants in Saudi Arabia and found that the infestation rate is two times higher in goats than in sheep. Furthermore, male hosts tend to have a significantly higher infestation rate than females. Dr. Arun De's group presents a report to examine host-parasite interactions and genetic characterization of Sarcoptes scabiei, a burrowing mite affecting humans and many other mammals. The disease scabies is one of the earliest diseases with a known cause recorded in the Bible. This group evaluated the effect of S. scabiei infestation on host physiology with special emphasis on serum biochemical parameters, lipid profile, oxidant/antioxidant balance, stress parameters, and immune responses using in a porcine model. They found that S. scabiei triggered stress response, elevated levels of serum cortisol and heat shock proteins, increased the serum concentration of immunoglobulins as well as IL-2, IFN-gamma, IL-1-beta, and IL-4 cytokine expression. In the third article, Koziatek-Sadłowska and Sokoł studied Dermanyssus gallinae, a parasitic poultry mite and aimed to determine its effect ´ on the development of post-vaccination immune responses in layer hens. They found that D. gallinae infestation may inhibit humoral immune responses since the percentages of B cells and Th cells were negatively correlated with the number of mites. However, further research is needed to determine whether D. gallinae suppresses the production of vaccine-induced antibodies.

At least six original research articles in this collection study parasites, from intracellular protozoans, hookworms, roundworms, to tapeworms. Islam et al. (2021) conducts a systematic review on rodent helminths and potential threats of helminth parasites on public health in the Middle East using meta-analysis. These authors identified 44 rodent species as primary hosts for helminth infestation. The study detected 22 species of helminths that are of zoonotic importance, such as Capillaria hepatica, Cysticercus fasciolaris, Hymenolepis diminuta, and Hymenolepis nana. This study provides baseline data on rodent helminths at the human-animal interface in Middle East countries, which should facilitate the development of efficient rodent control programs in the region. The Metwally team surveyed two species of intracellular protozoan parasites, Sarcocystis cameli and S. camelicanis in Camelus dromedarius samples obtained in Saudi Arabia and for the first time identified the presence of S. camelicanis in this country. Slater and colleagues compared the intestinal microbiome and volatile organic compounds of colonic contents obtained from healthy horses and those infected with varying levels of the tapeworm, Anoplocephala perfoliate, an equine parasite causing abdominal pain. Their findings show that a general reduction in certain beneficial bacteria in horses with tapeworm infection, indicating a possible negative consequence of parasitic infection. The Hedberg-Alm team presents a case control study on parasite prevalence in Swedish horses and parasite management practice by horse owners. Their data demonstrate a high level of exposure of Swedish horses to Strongylus vulgaris, arguably the most pathogenic equine internal parasite. The findings also suggest an urgent need for education in the use of fecal parasite egg counts and pasture management, such as reducing stocking intensity and frequent removal of fecal matters. The teams led by Dr. Saruda Tiwananthagorn presents two original papers in this collection. They first examined geographical and spatial distribution of two nematode species in community dogs, Dirofilaria immitis and Brugia pahangi, which is a serious public health concern for humans, dogs, cats, and wildlife species, particularly in southeast Asian countries. They detected Brugia pahangi infection in dogs for the first time in Thailand. Spatial distribution patterns of these two species differ: at a higher altitude between 400 and 800m, B. pahangi infections significantly outnumber D. immitis infection while the overall prevalence rate of the former is lower than the latter. In the second paper, the prevalence of Ancylostoma ceylanicum, a zoonotic hookworm, was carefully examined in dogs and soil samples from Thailand and Asian countries. Genetics and diversity of this species was also evaluated; and nine haplotypes were identified from the Thai hookworm population. These authors conclude that increased public awareness and proper deworming programs are urgently needed to reduce the risk of the transmission of this important zoonotic disease. Dominika Mravcˇakov ´ a and colleagues tested ´ anthelmintic activities of two medicinal plants, wormwood and mallow, against the barber's pole worm, Haemonchus contortus, in both in vitro and in vivo settings. The aqueous extracts of both plants exhibited strong parasiticidal effects in vitro. However, in sheep, the mean fecal egg counts did not differ significantly between the groups with and without plant supplements. While their study demonstrated the potential of medicinal plant-based alternatives to overcome the anthelmintic resistance, more research is warranted.

The Zaragoza-Bastida team evaluated the antibacterial and hemolytic activity of two rattlesnake venoms against Pseudomonas aeruginosa, an important bacterial pathogen that affects both animals and humans. These authors presented evidence for the first time that venoms from rattlesnakes contain bioactive compounds with bactericidal activity against Pseudomonas aeruginosa and can be developed as alternatives to antibiotics.

We strongly believe that this special collection has made a valuable contribution to the veterinary literature. The findings from these studies provide novel insights into host-pathogen interactions, which should facilitate the development of vaccines and alternative pathogen control strategies. We wish to express our gratitude to all contributing authors for their outstanding research efforts. Without their hard work, this collection will not come into existence. Finally, we want to thank the editorial board members and staff of Animals for their support to this special issue.

> **Robert W. Li** *Editor*

*Review*
