INTRODUCTION

Equine influenza is a highly contagious virus of horses which infects the respiratory tract. It is widespread throughout the world, and outbreaks have occurred in Europe, North- and South America, Asia, South-Africa and Australia; only Iceland and New Zealand have never had an outbreak. The disease occurs regularly in Europe and North-America. Equine influenza is self-limiting, and although the disease can be severe in naïve populations, in populations in which equine influenza occurs regularly clinical symptoms are usually not severe and it is rarely fatal.

For the equine industry, equine influenza is of potentially major economic significance, because it is highly contagious. Globalisation has led to an increased mobility of horses. Horses travel by air all over the world for competitions and race meetings, and shuttle stallions travel back and forth between the northern and southern hemisphere for breeding purposes. The three bloodstock sales companies in Ireland export horses to 33 different countries, and the thoroughbred horse industry is worth ca. €1 billion to the Irish economy annually. The Sport Horse industry in Ireland contributes an estimated €700 million to the economy.

Contagious diseases are a serious threat to the horse industry. Outbreaks of equine influenza can cause severe economic losses by closing down the racing industry for a number of months, as occurred in South Africa in 1989, Hong Kong in 1992 and Australia in 2007, and limiting the transportation of horses. Therefore, prevention and control of the disease are vitally important. The main strategy for prevention is vaccination. Vaccination has two objectives: the vaccinated horse must be protected against disease, and the spread of virus must be limited. The most important measure of equine influenza infectivity is virus shedding. This study has examined the effect of vaccination on the incidence and level of virus shedding by combining the data of 23 experimental challenge studies, in which 159 vaccinated horses and 132 non-vaccinated control horses were deliberately exposed to the equine influenza virus.

RESULTS

Virus shedding

The results of this study show that 59 out of 159 (37.1%) vaccinated horses tested positive for virus shedding after deliberate exposure to the equine influenza virus. In the unvaccinated control group, 96.2% of horses shed virus after challenge infection. Therefore, vaccination reduced the risk of a horse becoming infective by 61.4%. However, the high number of vaccinated horses shedding virus is cause for concern, as these horses can spread the disease and cause outbreaks.

Clinical symptoms

The most common clinical signs of equine influenza are fever, nasal discharge and coughing. Symptoms usually appear within 48 hours of infection. Fever is typically the first sign, often with loss of appetite, followed by clear, watery nasal discharge and a deep, dry cough. Clinical signs usually resolve in seven to 14 days; persistent respiratory disease is usually caused by secondary bacterial infection.

As shown in figure 1, vaccination significantly reduced the occurrence of clinical symptoms in vaccinated horses compared to the control group.

Influenza virus shedding begins within 24 hours of infection, whereas clinical symptoms usually begin two or three days post infection. Clinical symptoms did not always coincide with the retrieval of virus from nasal swab material and this may indicate that the symptoms were caused by secondary bacterial infection. This makes clinical symptoms such as coughing and nasal discharge unreliable as an indicator for virus shedding.

However, this study demonstrated that horses with fever are 14 times more likely to shed virus than horses without fever. Therefore a rising temperature may be regarded as a reasonable indicator for virus shedding.

Immunological response

The majority of antibodies produced against the equine influenza virus interfere with the ability of the virus to attach to the host cell. Antibody titres after vaccination correlate with the level of protection provided; protection against virus infection is defined as the amount of circulating antibodies which prevent the horse from getting infected. In clinical protection, the horse may not have sufficient antibodies to prevent infection, but is protected against clinical symptoms. Figure 2 shows a comparison of the mean, minimum and maximum antibody response to ProteqFlu, Equip F and Duvaxyn.

Although Duvaxyn elicited a higher antibody response, there was no significant difference in the efficacy of the vaccines in conferring protection against equine influenza.

In clinical protection, the horse may not have sufficient antibodies to prevent infection and can therefore be sub-clinically infected. Subclinical infection is defined as infection which is asymptomatic: horses which are sub-clinically infected can shed virus without showing any of the clinical symptoms associated with equine influenza infection. Sub-clinically infected horses have been responsible for several major epidemics of equine influenza in the past, such as the epidemic in Australia in 2007.

Diagnostic Techniques

The gold standard for determination of virus shedding is virus isolation, because this is the only method which detects live infectious virus. However, this method is not as sensitive as molecular techniques (PCR). In this study, both virus isolation and PCR data were available for 42 vaccinated horses. Virus shedding post challenge increased from 37.1% by VI to 83.3% by PCR as shown in figure 3.

Another important factor in determining the level of virus shedding is the choice of sampling technique: nasopharyngeal swabs were more efficient than nasal swabs for the detection of virus shedding. Meta-analysis showed that the number of positive samples retrieved by nasopharyngeal swabs was higher than the number of positive samples retrieved by nasal swabs. The sensitivity of the method used to detect the virus in swab material also influences the duration of shedding recorded.

CONCLUSIONS

  • The limitations of vaccination are that it is only mandatory for horses in competition or horses which are transported internationally, vaccine induced immunity only lasts a limited amount of time and vaccine breakdown can occur if the circulating virus strains differ sufficiently from vaccine strains.
  • Equine influenza vaccination offers partial protection against virus shedding, reducing the probability of virus shedding by 61.4% and the incidence of clinical symptoms by 60%.
  • There was no significant difference between the vaccines in their efficacy to confer protection from virus infection or clinical symptoms.
  • Although vaccination reduces the amount of virus and the duration of shedding, equine influenza is highly contagious and as the virus is spread rapidly between horses, vaccinated horses shedding virus can cause outbreaks of the disease. Furthermore, vaccinated horses may be sub-clinically infected and shed viable virus without showing clinical symptoms of the disease.
  • Most of the vaccinated horses in this study were exposed to the virus within a short period of time after receiving a primary course of vaccinations. Therefore the relatively high number of virus shedders is cause for concern, and an additional booster three months after the primary course of vaccination may be advisable for competing horses.
  • For horses travelling internationally it is important to adhere to OIE recommendations and administer an additional booster between 21 and 90 days before travelling.
  • This study shows the importance of routine testing for evidence of equine influenza infectivity and management practises such as monitoring temperature twice daily. This is particularly important for horses which have travelled for competitions or racing or for breeding purposes, as these horses may have been exposed to the virus.

    • Sandra Hurley is graduating student of the University of Limerick’s Equine Science BSc undergraduate programme (2017). Further information on equine-related courses (Certificate, Diploma and Degree) offered by UL can be found on the UL website