The complexities of equine respiratory infections

01 November 2015, at 12:00am

MARION McCULLAGH continues her reports from September’s BEVA congress with a review of two of the presentations on infectious respiratory diseases and methods of controlling them

IN the Thursday morning session at the BEVA congress on “Infectious diseases”, Dr Nicola Pusterla of the University of California, Davis described the maze of respiratory disease epidemiology.

His presentation was entitled “Ongoing challenges of common and less common equine respiratory pathogens”.

Upper respiratory tract disease (IURD) in horses is widespread and occurs commonly; outbreaks are seen most often in late autumn, winter and spring when there are high concentrations of young susceptible horses.

IURD is highly contagious: it spreads quickly by means of aerosol, droplets and fomites. The respiratory pathogens have a short incubation time and morbidity can reach 100% while mortality is low. Many factors such as age, immune status and the environment influence each individual’s response to the disease situation.

IURD assessment involves history, clinical examination, blood counts and antigen detection using nasal swabs. Viral disease shows up as mild anaemia and lymphopaenia during the acute phase while strangles and other bacterial diseases cause raised fibrinogen and leucocyte count at the time that the horse has a nasal discharge.

Quantitative polymerase chain reaction (qPCR) is the most effective diagnostic test. It is fast, sensitive, reliable and cost-effective. Nasopharyngeal swabs and guttural pouch lavage may be needed in investigation of equine flu (EIV) and strangles.

Biosurveillance carried out by the OIE showed that EIV exists in North America, Europe, South Africa and Japan only as the clade 1 strain of the Florida sublineage (H3N8). Clade 2 is present in Europe, India and China. Clade 2 showed up in three imported horses in the USA.

Horses are always on the move, globally, and they carry their diseases with them. A vaccinated horse can shed virus without showing clinical signs.

In the USA, EIV is seen in older horses and previously vaccinated horses. Regular re-vaccination with relevant virus at the recommended interval is essential to protect the population.

Major economic loss

Foals, weanlings and yearlings affected by Equine herpesvirus-1 (EHV- 1) and EHV-4 cause big economic loss. These important, ubiquitous viruses also cause clinical disease in adult animals. Infection with these Alphaherpes viruses can be followed by life-long latency with virus reactivation and shedding which may explain how disease outbreaks happen in closed populations.

EHV-1 and EHV-4 may be backed up by the immunosuppressive effects of EHV-2 and EHV-5 which are widespread in the horse population. EHV-2 and EHV-5 are optimally adapted to their host: they are found in both sick and healthy horses, in several different genetic variants. EHV-1 has been genetically classified into two strains: neuropathogenic D 752 and non-neuropathogenic N 752 but, in spite of the genotype, EHV-1 can cause the full spectrum of clinical syndromes: rhinopneumonitis, abortion, neonatal death and myeloencephalopathy.

Equine rhinitis A and B viruses (ERAV, ERBV) are hard to detect but they are widespread in the equine population. They can infect both upper and lower airways. Given to seronegative horses they cause seromucoid nasal discharge, coughing, lymphadenopathy, anorexia and pyrexia, sometimes with swelling of the lower limbs.

ERVs are hard to diagnose. Methods used are virus isolation, qPCR and rising antibody titre through virus neutralisation in acute and convalescent serum. This is complicated by the very short shedding time for these viruses after the appearance of clinical signs. It has been found, however, that the virus can be shed in the urine of twoand three-year-old racehorses. This may be a significant source of infection for other susceptible horses.

Preventing strangles

Philip Ivens of Buckingham Equine Medical Referrals outlined “Strangles prevention: treating carriers and vaccine use, now and in the future”.

Strangles is the most commonly diagnosed infectious respiratory disease in the UK horse population. The obvious means of spread is from discharge from the horse’s nose or from an abscess, from clinical case to non-immune companion.

Bacteria can also be spread indirectly by fomites such as people, tack and stable equipment. Much more troublesome is disease transmission by symptomless carrier horses. Short-term carrriers shed bacteria for up to four to six weeks after their clinical signs have gone.

This state does not warrant treatment but about 10% of these animals can go on to become longterm carriers. These animals can infect any susceptible horse so that clinical disease can recur in the premises of the original outbreak or be spread to other yards. So, identifying long-term carriers is vital for disease control.

The guttural pouch (GP) is the home of the bacteria that spread disease from the symptomless carrier. In detecting such animals, direct endoscopic visualisation of both GPs is followed by guttural pouch lavage (GPL) with sterile saline. This is sent for qPCR and culture. Alternatively, three nasopharyngeal swabs or serology can be used before undertaking GPL.

The bacteria may be present as a microscopic film on the surface of the GP or there may be flakes of mucopus or chondroids or even large accumulations of inspissated pus.

Fragile bacterium

As pathogens go, Streptococcus equi is a fairly fragile bacterium. If there are only small flakes of pus, instillation of penicillin/gelatin mixture followed by GPL one or two weeks later may resolve the situation.

Persistent cases may need repeated saline lavage, possibly using an indwelling Foley catheter, combined with systemic penicillin treatment. Chondroids need physical removal. This can be done by using a helical basket which is passed down the biopsy channel of the endoscope. Surgical intervention may be needed in cases where there are large quantities of pus.

In Europe, vaccination to prevent strangles relies on Equilis StrepE which can be used in animals from four months of age; 0.2ml is applied submucosally in the lip. There may be transient fever and sometimes an abscess forms at the site of application. Owners need to be warned of these sequelae.

Vaccination is most effective when it is done on a herd basis; this has been carried out successfully in Holland and Italy. Many owners prefer to rely on biosecurity even though this may be breached by the arrival of a symptomless carrier and it will involve 21 days’ quarantine for new arrivals.