Infectious Diseases

NOTIFIABLE INFECTIOUS DISEASES

The infectious diseases discussed in this section are those included in the OIE’s list of notifiable diseases. Information on the diseases can be found on the OIE website.

Each country will have local regulations for the reporting of these diseases and it is important to be aware of them and of the protocols for notification.

African Horse Sickness (AHS)

The infectious agent is an orbivirus and transmission is by midges.

AHS is usually subclinical but mild clinical signs can be seen in the donkey.

• Asian and European donkeys are moderately susceptible, with 5—10% mortality.
• African donkeys, where the disease is endemic, are often described as relatively resistant and considered as reservoir hosts. However, they have been shown to become viraemic following inoculation with virulent AHS virus, and have been seen to exhibit a mild form of the disease, ‘horse sickness fever’.
• Where clinical signs are seen, they are typically:
  • high fever
  • weakness
  • dyspnoea
  • extensive oedema of the supraorbital fossae and the eyelids.
• Congestion and haemorrhage of conjunctivae were also observed in donkeys in Kenya which were serologically positive for AHS virus.
• Mortality of donkeys with typical clinical signs of the pulmonary form of AHS, characterised by dyspnoea and terminal frothy nasal exudate, has been seen in Ethiopia.
   

Equine Herpes virus (EHV)

A similar range of herpes viruses affect donkeys as those in horses and ponies.

Clinical signs are similar to those seen in horses and ponies.

Infection can be treated as in horses and ponies, but with some differences.

Equine Influenza

Equine Influenza presents with the same clinical signs seen in horses and ponies. However, there are reports that donkeys may be more severely affected.

Equine Infectious Anaemia (EIA)

The infectious agent is a lentivirus and transmission is by biting insects.

EIA is usually subclinical in the donkey.

There is limited information on the epidemiology of EIA in donkeys.

Equine Viral Arteritis (EVA)

The infectious agent is an arterivirus and transmission is via secretions from the respiratory or reproductive systems.

EVA is usually subclinical but mild clinical signs may be seen in the donkey.

• There is one major serotype, but different isolates have been recognised in donkeys, mules and horses.
• Studies have demonstrated that donkeys can be clinically affected, although signs are not as severe as in horses and ponies.
• Where clinical signs are evident, they are typically:
  • fever
  • depression
  • ocular and nasal discharge
  • conjunctivitis.
• Respiratory and venereal routes of transmission of EVA have been reported in donkeys.
• An asinine virus closely related to the equine arteritis virus has been isolated from donkey semen.
• Affected male donkeys may become long-term carriers and may shed the virus in their semen, serving as reservoirs for the virus.

Rabies

The infectious agent is a lyssavirus and transmission is via the bite of an infected animal.

Donkeys typically show the encephalitic or paralytic form.

• Working donkeys usually become infected with rabies from the bites of infected carnivores, in particular dogs, jackals, hyenas and foxes.
• Working donkeys are more at risk because they are often kept outside at night or allowed to wander freely.
• Vampire bat-transmitted rabies in donkeys has been reported in Mexico and reports from Latin America suggest that donkeys are more likely than horses and ponies to be attacked by vampire bats.
• Donkeys may show vague, non-specific signs, such as depression, anorexia, apparent choke (oesophageal obstruction) and colic in the early stages of the disease.
• ‘Furious’ (neurological) signs are common in the advanced stage of the disease and are characterised by aggressive behaviours: trying to bite other animals or handlers, baring teeth, teeth grinding, self-mutilation and eating foreign bodies.
• Donkeys may die within a few days of the onset of clinical signs, but progression can be slower (up to two weeks) in some cases.

Personal protection of any handlers, clinicians or other professionals is of utmost importance and must always be the primary consideration when examining or carrying out procedures on a donkey suspected of being infected with rabies.

Glanders

The infectious agent is Burkholderia and it is found in respiratory secretions and in exudate from nodular abscesses, where these are found in the skin. Transmission is by direct contact, fomites and environmental contamination.

Clinical signs in the donkey are:

• fever
• nasal discharge
• enlarged lymph nodes with/without discharge
• coughing.

Donkeys are reported to be the most susceptible equid species.

• They will often develop the acute form of the disease and it is often fatal.
• There have been reports of outbreaks of Glanders in donkeys.

Trypanosomiasis

Trypanosoma brucei equiperdum, T. b. evansi, T. vivax, T. congolense and T. brucei are the most common trypanosomes infecting donkeys. Transmission is by biting insects, with the exception of T. brucei, which is sexually transmitted.

Trypanosomiasis is usually subclinical or chronic in donkeys.

Signs vary according to the species of trypanosomes involved, the virulence of the strain and the general health status of the animals, including level of stress, work, pregnancy status and any concurrent diseases.

Trypanosomes affecting donkeys:
 

• Donkeys are generally considered more resistant to trypanosomiasis than horses and ponies.
• Studies in sub-Saharan Africa, where tsetse-transmitted trypanosomiasis is endemic, have shown a high prevalence of infection and clinical cases of T. congolense, T. b. brucei and T. vivax (African Animal Trypanosomiasis; ATT) in working donkeys.
• Mixed infections with two or more Trypanosoma spp. are common findings in donkeys and are often fatal.
• Donkeys suffer from both patent and subclinical infections. AAT is claimed as the major health constraint of working donkeys in tsetse-belt regions of Ethiopia and Kenya.

T. b. brucei:

• Recent studies in Gambia show an increasing incidence of the neurological form.
• The disease is often fatal.
• Clinical signs are deteriorating cerebral function and cranial nerve abnormalities with diffuse lymphocytic-plasmacytic meningo-encephalomyelitis.

T. b. evansi (Surra):

• Surra is often reported in donkeys with a high infection prevalence and sometimes as an outbreak.
• Both acute and chronic infection with T. b. evansi have been reported in donkeys.
• Clinical signs are associated with:
  • high levels of parasitaemia
  • anaemia
  • meningoencephalitis and
  • follicular hyperplasia of lymph nodes and the spleen.

T. b. equiperdum (Dourine):

• Dourine has been reported in donkeys and typical clinical cases of Dourine are seen in the highlands of Ethiopia, where it is endemic.
• Dourine in donkeys and mules is mild to asymptomatic and may produce variable signs.
• Clinical signs, when evident, may include:
  • anaemia
  • oedema in the genitals (although this is often not obvious)
  • skin plaques (only rarely seen).

There are differences in diagnosis and treatment to that of the horse and pony.

• Many blood tests designed for horses are less effective in donkeys as the donkeys seems to have higher levels of a complement factor. This makes the complement fixation test difficult to interpret. Over 50% of samples from donkeys have anti-complementary levels. This can be reduced by dilution of the serum by half and heat inactivation at 60— 630°C.
• Although trypanocidal drugs are effective in donkeys, adverse reactions are also recorded. Clinicians are therefore advised to consult local regulations and manufacturers’ recommendations prior to treatment.
• Drug resistance is becoming a major problem and this must be taken into consideration when treating donkeys and mules in endemic areas.

Equine Piroplasmosis (EP)

The infectious agent is Babesia spp and transmission is by ticks.
EP is usually subclinical in donkeys unless other stress factors are concurrent.

• Concurrent infection with Theileria equi and Babesia caballi is common in donkeys and is endemic in most tropical and subtropical regions.
• Although EP is usually subclincial in the donkey, clinical signs may be seen and include:
  • congested mucous membranes
  • lacrimation
  • depression
  • fever.
• Once infected, donkeys usually remain asymptomatic carriers with positive antibody titres throughout life.
• Stress factors, concurrent disease and re-introduction to a region where piroplasmosis is endemic after a period of absence may precipitate clinical infection in the donkey. Stress from poormanagement, inadequate veterinary care, poor nutrition and overwork may exacerbate the impact of infection in working donkeys.
• Treatment with imidocarb diproprionate has been found to be effective.

Others

Anthrax, Brucellosis, West Nile Fever, Equine encephalomyelitis and Contagious Equine Metritis are all found in donkeys and treated as in horses and ponies.

OTHER INFECTIOUS DISEASES

The majority of the infectious diseases show clinical signs in donkeys similar to those seen in horses and ponies and are treated in the same way.

• Diseases that typically show signs of diarrhoea in the horse and pony may not present with diarrhoea in the donkey. Examples are Salmonella and Clostridial enterocolitis.
• Donkeys are rarely vaccinated effectively and may therefore be at an increased risk of infection; tetanus is an example.

Tetanus

Tetanus is a risk where donkeys have not been vaccinated effectively. This is frequently the case and treatment with tetanus antitoxin, must always be an important consideration where wounds are present.

Diagnosis and treatment is as in horses and ponies.

Strangles

Strangles is found in donkeys and is diagnosed and treated as in horses and ponies.

Commercially available serological tests for S. equi have not been validated in donkeys and therefore should be interpreted with caution.
 

Salmonella

Salmonella affects donkeys similarly to horses and ponies, with the exception that diarrhoea is not typically seen as a presenting sign in the donkey.

PREVENTION

Measures to control or prevent infectious diseases in donkeys are often lacking or ineffective.

All clinicians and animal health professionals play an important role in putting in place appropriate measures to minimise the risk of infectious diseases to donkeys under their care.

The principles of best practice that apply to other equids and livestock in general can be used as guidelines.

• Although many vaccines are not licensed for donkeys and data on efficacy is often lacking, effective vaccination programmes can be developed.
• Reliable accurate diagnosis is essential for treatment, effective control and disease surveillance.
• Even when resources are limited, effective biosecurity measures can be implemented.
• Isolating infected animals.
• Quarantine of newly introduced equids for a period that allows incubation of the at-risk diseases’, with close monitoring for disease during this period.
• Good owner education and support is an essential yet often neglected part of effective disease control.
• Surveillance of notifiable diseases is carried out by the World Organisation for Animal Health (OIE).

• Information on specific diseases and biosecurity can often be found online on professional or government websites.

For zoonotic diseases, consideration must be given to the potential impact on any person coming into contact with the animal(s), infected materials and/or vectors.

All clinicians and animal health workers are responsible for ensuring the safety and protection of all people involved with animals that might be infected with zoonotic disease.

Precautions must be considered:

• personal protective equipment should be used where appropriate
• personnel should be vaccinated where appropriate and possible
• insect repellents and barriers should be used where appropriate
• prophylaxis may be necessary.