Pasteurella bacteria are ubiquitous microorganisms found in several domestic and wild animal species. In deer, they are found in the upper respiratory tract in healthy and diseased animals. In farmed and wild deer, they are most often associated with outbreaks of septicaemic pasteurellosis (Jones and Hussaini 1982; Eriksen et al. 1999; Malhi et al. 2016), whereas pneumonic pasteurellosis alone is more rarely reported. Pneumonic pasteurellosis, a complex multifactorial disease, is more commonly reported in cattle and sheep as a cause of shipping fever pneumonia (Tweed and Edington 1930). In deer, different forms of stress can lead to pneumonic pasteurellosis, such as rutting in stags, traumatic injuries, other diseases (e.g. Parainfluenza virus 3), extreme weather events, mixing of animals of different origin and transportation, which in some instances lead to mortality. Rough poor forage, including thistles and thorns, can cause oral and pharyngeal mucosal damage and be a trigger factor. Healthy animals that do not harbour the pathogen can be infected by coughed up or exhaled droplets from clinically affected animals.

The disease progression can be slow or quite rapid; clinical signs include inappetence, weight loss, rough coat, increased respiratory rate, open mouth breathing, nasal discharge and depression. Signs from the respiratory system, such as coughing or impaired breathing, are not always noted in early or sub-acute forms. Acute forms can result in mortality within two to four days, whereas sub-acute cases can develop clinical signs several days after being subjected to one or more stressors. Affected animals are usually seen isolating themselves from the herd.

Mycoplasma species are epierythrocytic bacteria that attach to red blood cells in a variety of animals, including cattle, deer and humans. They primarily cause respiratory disease (pneumonia) in young cattle and their role as a disease-causing pathogen in deer has not yet been fully established. Several strains of the bacterium have been identified. Surveillance in domestic and wild ungulates has shown that a large proportion of populations and herds harbour Mycoplasma spp. but in many instances without causing disease. Mycoplasma ovis is regarded as an emerging/re-emerging zoonotic agent that has commonly been found in sheep and goats. It has also been detected in diseased captive white-tailed deer (Boes et al. 2012). Similar Mycoplasma spp. have been reported in captive reindeer (Stoffregen et al 2006), wild sika deer (Watanabe et al. 2010) and wild South American deer (brocket and pampas spp.; André et al. 2020). Mycoplasma bovis is a common cause of respiratory disease in cattle (mainly pneumonia in calves); it has also been found in white-tailed deer with pneumonia (Dyer et al. 2004; Register et al. 2019). Mycoplasma ovipneumoniae is a common cause of pneumonia in sheep and goats; it has also been reported as a contributing cause of pneumonia in barren ground caribou in Alaska (Rovani et al. 2019). Transmission occurs via biting flies (M. ovis), direct contact (M. bovis, M. ovipneumoniae) and via contaminated fomites (M. ovis).

Primarily lethargy and anaemia, but various signs from the respiratory tract can occur, such as coughing, nasal discharge and increased respiratory rate. The signs usually worsen where there is co-infection with other bacteria and/or viruses.

Most other diseases are related to respiratory disease.

Polymerase chain reaction (PCR) of nasal swabs or lung tissue sometimes combined with other methods to facilitate identification such as denaturing gradient gel electrophoresis. Mycoplasma spp. are highly fastidious and difficult to culture and slow growing. Histology may be supportive.

Bovine herpesvirus 1 (BoHV-1) occurs worldwide in cattle and is the causative agent of infectious bovine rhinotracheitis (IBR) in the respiratory system or infectious pustular vulvovaginitis (IPV) and balanoposthitis (IBP) in the reproductive system. Transmission usually occurs via direct contact (nose to nose or during mating) and limited airborne transmission has been reported. For IBR, the natural portal of entry is in the mucous membrane of the upper respiratory tract.

Reputedly, wild deer are exposed to BoHV-1, although rarely (Graham et al. 2017). Studies in captive deer are rare and it has been reported that red deer were susceptible to the virus, although without showing clinical signs or seroconversion (Reid et al. 1986). Captive red deer were reported to become infected with BoHV-1 in an experimental setting where they shed the virus and seroconverted, although with mild or no clinical signs (Mollema et al. 2005). A Danish experimental study (Rønsholt et al. 1987), aiming to investigate the transmission of BoHV-1 from infected deer to uninfected deer and cattle, revealed transmission of the virus, with subsequent development of respiratory disease and mortality, in one of three deer, although the identified virus itself differed slightly from BoHV-1.

A previously unidentified cervid-specific herpesvirus was identified and named cervid herpesvirus 1 (CvHv-1). This alphaherpesvirus has been frequently reported in serological surveys of wild and captive deer (Thiry et al. 2007, Rola et al. 2017). The virus primarily causes ocular disease (Inglis et al. 1983), see also chapter 27. Another related virus, cervid herpesvirus 2 (CvHv-2) and elk herpesvirus 1 (ElkHV-1), can cause subclinical genital infections in reindeer and elk, respectively (Ek-Kommonen et al. 1982, Deregt et al. 2000). CvHV-2 also causes infection of the respiratory tract in reindeer. Infection with BoHV-1 and CvHV-1 can predispose the respiratory tract to secondary bacterial infections (Muylkens et al. 2007).

Given that only experimental infections have been able to show that cattle may be infected with CvHV-1 and deer with BoHV-1, it is considered unlikely that deer pose a significant risk to cattle and IBR. Non-specific cross-serological reactions can make screening and diagnosis problematic in deer.

Signs from the respiratory tract are usually mild and include nasal discharge and elevated body temperature. Immunologically compromised and stressed animals (e.g. following transport, severe climatic conditions, newly introduced animals in the herd) can show moderate to severe signs from the respiratory tract, such as heavier nasal discharge, inappetence, pyrexia, lethargy and subsequent mortality. Corneal opacity and keratitis has been seen in the United Kingdom.

Malignant catarrhal fever (MCF) is a fatal disease affecting the lymphatic system of deer and other ungulate species including cattle and bison (Russell et al. 2009; see also Chapter 17). Affected animals are infected by other ungulate species such as sheep, goats, wildebeest and ibex, where the reservoir hosts are unaffected and where different host species transmit different types of MCF-causing viruses. One main sign in affected animals is nasal discharge, making the disease a part of the respiratory system.

Père David’s deer (Elaphurus davidianus) are known to be highly susceptible to the virus. In addition, experimental infection from domestic sheep to Japanese/sika deer causing disease and mortality has been reported (Imai et al. 2001). Reports of MCF are found for various wild and captive deer species, such as white-tailed deer (Li et al. 2000), red brocket deer (Driemeier et al. 2002) and red deer (Vikøren et al. 2006). In the United Kingdom, MCF has been reported by Animal and Plant Health Agency in enclosed red deer, reindeer and sika deer, and in wild roe deer.

Nasal and ocular discharge, fever, inappetence and subsequent mortality.

Adenoviral haemorrhagic disease (AHD) and epizootic haemorrhagic disease.

Primarily clinical signs, followed by postmortem examination with histopathology. Analysis of virus-specific antibodies in blood or DNA (PCR) in blood or tissue samples.

As the disease is fatal, animals with obvious MCF signs should be euthanized for welfare reasons. Future precautions should include avoiding contact with possible sources of infection including sheep (the source of OHV-2), goats (CPHv-2 and 3), ibex (ibex-MCFV) and wildebeest (the source of AlHV-1), also potentially moufflon.

Bovine parainfluenza viruses and specifically type 3 (BPIV-3) occur worldwide and in several ungulate species, including deer. In cattle, it is a common cause of disease in the respiratory tract, without causing systemic disease. In young animals, signs are generally more severe than in adults (Stevenson and Hore 1970). Co-infections with other viruses worsen the signs; being infected can predispose animals to secondary bacterial infections (Niewiesk and Oglesbee 2022). Deer (e.g. Giovannini et al. 1988; Dubay et al. 2015; Roic et al. 2018) have been found to have antibodies against the virus; the virus has been isolated from nasal swabs in some species (fallow and mule deer, Thorsen et al. 1977). Aerosol transmission is most common for BPIV-3, although fomites are also suggested as a transmission factor. Mortality has been reported in captive fallow deer where calves were suspected to have been infected by cattle housed in the same barn (Dastjerdi et al. 2022a). The same study also revealed infection with a coronavirus similar to bovine coronavirus, although it was not suspected to be the primary or a contributing factor to the signs and subsequent mortality.

Early signs include coughing, nasal discharge and increased respiratory rate, these are most often observed in young animals. In later stages, inappetence followed by lethargy can be observed. Mortality can occur. The severity of the signs can, like most other infections, vary depending on the general health of the herd and individuals, including parasite load and stress factors.

Several other diseases of the respiratory tract, including lungworm, cervid herpesvirus and bacterial pneumonia.

Virus isolation (PCR) from nasal swabs. Postmortem examination of the respiratory tract in particular, followed by histopathology.