Equine species (Horses, Donkey, Zebra etc.) is an important part of our ecosystem who helped us in wars, setting up civilizations, hunting, acted as fast means of transport and winning horse race Satta in India, donkeys who helped us in carrying loads and loads of burden. Apart from the above-mentioned roles played by them, they are also companion animals. Their health and safety are equally important as your pet dog or cat. Hence, diagnosing different equine diseases is of utmost importance to implement a correct treatment plan. The deadliest diseases which affect equine species are: –
- African Horse Sickness (AHS)
- Potomac Horse Fever
- Equine Herpesvirus
Let’s talk about each of them in detail.
African Horse Sickness
This equine disease is caused because of the African horse sickness virus which is a member of the Orbivirus genus from the Reoviridae family. It can infect various equids like donkeys, mules, horses. Although it is known to infect the equine species, a study found that carnivores feeding on the infected animal can also acquire the disease. The common symptoms include high fever, frothy discharge from the nostril, coughing and oedema near the eyes and lips. This virus attacks the cardiac and the respiratory system which often results in death. The mortality rate of AHS is almost 95%.
Diagnosis – This particular sickness is most commonly diagnosed using virological studies. AHSV is isolated from various tissues of the live animal. The sample for virus isolation is usually blood which is collected in heparin (anti-coagulant). The AHSV antigen can be detected using an immunological technique called enzyme-linked immunosorbent assay. All of these assays make use of anti-AHSV antisera coated plates to capture the viral epitope. Then the bound virus can be detected using different antibodies. Cytopathic effects are usually demonstrated using VERO cell lines. RT-PCR can also be used which involves purification of the viral dsRNA, selection of suitable primers, complementary DNA synthesis and finally performing PCR. Another method that can be utilized is in situ hybridization of the virus’ nucleic acid. For example – sequences with less than 10% divergence within the AHS serogroup and which differ from the related sequences of the Orbivirus by 40% are a good nominee for hybridization.
Potomac Horse Fever
PHF, one of the deadliest equine diseases is caused by Neorickettsia risticii which results in enterocolitis (inflammation of digestive tract). The symptoms include fever, diarrhoea and anorexia. Furthermore, the infected enterocytes in the small and large intestines result in acute colitis which is a primary clinical sign of PHF. This gram-negative organism lives as an endosymbiont ( lives inside the body/cells of another organism) to flatworms. These flatworms then live inside aquatic snails. When the temperature of the water rises, the snails release the flatworms which can be ingested by horses when drinking water.
Diagnosis – The diagnosis is depended on the appearance of clinical signs and degree of lesions. One of the most promising diagnostic /detection methods is to isolate and identify Neorickettsia risticii from the blood or faeces of the infected horses. Using an electron microscope and Giemsa stain we can demonstrate the presence of the organism in dried blood smears. The detection can also be performed by using PCR. The specific TaqMan assay makes use of the endogenous nuclease activity of Taq DNA polymerase. This digests the TaqMan probe which hybridizes the amplicon. When the probe is hybridized and when the reporter dye and quencher dye are close by to each other it leads to suppression of reporter dye’s fluorescence. During amplification, both the dyes are separated due to cleavage. This makes the reporter dye fluoresce. The intensity of fluorescence is directly proportional to the amount of target DNA. Another method is by using the principle of indirect fluorescent antibody test which catches hold of antibodies in sera that are produced when infected with Neorickettsia risticii. A rapid increase in antibody titre can be paired with infection but this test does not necessarily indicate a recent infection. IFA positive results only indicate that the horse was infected/exposed to the bacteria sometime in its lifetime. Hence rt- PCR is preferred.
EHV is linear ds DNA viruses that belong to the family Herpesviridae. It is considered one of the most contagious equine diseases which includes respiratory tract inflammation, paralysis, abortion in a pregnant mare. EH, type 1 and EH type 4 are most clinically important which leads to the following symptoms – fever, nasal discharge, cough and lethargy. Equine herpesvirus is so contagious that it can spread through aerosols and direct physical contact. It has been estimated that most of the horses had this infection at least once in their lifetime. The incubation period of the virus can be anywhere between 2 to 10 days. After which the horses develop a mild fever and start showing the symptoms mentioned above. EH1 and EH4 have different mechanisms – EH1 attacks the endothelium layer – nasal mucosa, lungs, placenta and CNS while EH 4 attacks respiratory epithelium along with lymph nodes.
Diagnosis- Similar to the diagnostic methods used above to detect AHSV and PHF, we can make use of virus isolation and PCR based techniques. Samples that are required for the detection includes nasal swab and nasopharyngeal lavage collection.
PCR based methods are most commonly used for the detection of viruses or bacteria in the sera as they are highly sensitive and does not require a huge amount of sample. Moreover, PCR is considered to be faster than viral isolation hence it is rapid.
However, when it comes to prevention, many vaccines have been developed and many are under development (clinical trials). In each case, it is important to isolate and quarantine the animal when the disease is suspected.