The virus of the epizootic hemorrhagic disease (MHE) will inevitably circulate from this summer or in the fall in French cattle herds. This viral disease transmitted by a stingy insect of the genre Culicoids Presents a lot of similarities with the FCO, and at the same time, there are a lot of things that we do not yet know about it.
“The MHE is due to an orbivirus, for which seven serotypes are known. A single serotype circulates in Europe: the serotype 8 ”, presented Théo Chomarat, from theToulouse National Veterinary School (ENVT), during the National Days of GTV (Veterinary Technical Groups) in May.
The serotype 8 was identified in the 1980s in Australia, then it was no longer found for forty years and we do not know why he was reappeared in 2021 in Tunisia. From there, he proliferated and spread north. The MHE was indeed detected in Sicily, Sardinia and Spain in October 2022. In the Pyrénées-Atlantiques and the Hautes-Pyrénées, it arrived in September 2023.
Between 0 and 10 % mortality per herd
In most cases, the disease is asymptomatic. A Spanish study in 2023 in seven dairy farms (out of 3,000 cattle) showed that the MHE is subclinical according to aging in 67 to 98 % of cases. “We do not have this type of data for France, but the discovery of the MHE in certain herds was done in a fortuitous manner during health checks for export. In dairy herd, sometimes the only observable sign was a slight drop in global dairy production ”explains Théo Chomarat.
And in the most serious cases, the death of cattle is possible in a few days. In Spain, the mortality Average was evaluated between 2 and 3 % of sick cattle. In France, studies of the GDS of the Pyrénées-Atlantiques and Hautes-Pyrénées have shown that mortality could vary depending on the farms from 0 to 10 %.
You can have sick cattle for one or two days with just one ulcer on the tongue, while other animals will combine all clinical signs for three to four weeks.
“During infection during gestation, abortions, or even death can be observed, especially if the infection takes place at the end of gestation. We also highlighted the birth of puny calves, which have trouble starting and which do not suckle ”continues Théo Chomarat. It is assumed for these cases of “puny calves” that the mother was contaminated when she was two or three months of gestation (as is the case for the FCO).
A transmission of the mother to the fetus
“The vertical transmission of the mother to the fetus is possible, we have identified this in two cases. »» The Toulouse National Veterinary School is studying for two years, and has proven that an excretion of the virus is possible in the seed by some bulls for four weeks post-infection.
“We also observed, as the Spanish had done in 2023, the existence of a chronic form of the MHE which had never been described before”learns Théo Chomarat. This form can affect 0 to 14 % of sick cattle depending on the farms. “Some are those who have suffered from a severe acute phase and who cannot recover. They gradually get on each other and end up being euthanized. Renal nephropathies and insufficiency could be secondary complications in the acute phase. Another category of animals is classified in this chronic form of MHE: those who suffer from delayed complications of the MHE, among which there are boxes with lesions of the horn, the sole.»
Why is there such diversity in the intensity of MHE forms? “The Spanish study of 2023 has shown that the older a cow, the more likely it may be sick with the MHE, and that clinical cases before 2 years old are rare. It can also be assumed that intercurrent diseases can play a role as well as insufficient mineral and vitamin diet. There may also be genetic factors and farming driving factors ”advance Théo Chomarat.
A period of infectiousness of ten days to four weeks
It is impossible to distinguish clinically and epidemiologically the MHE of the FCO. “In case of doubt, you must send a blood sample for a PCR laboratory analysis to search for both diseases”explains Théo Chomarat, of the national veterinary school of Toulouse.
The infectiousness period – period during which cattle have in their blood virus which is capable of multiplying and infecting another cattle – has an estimated duration between ten days and four weeks. Beyond four weeks, the result of a PCR is always positive but the virus is no longer strongly capable of multiplying. Then, the quantity of viruses in the blood drops slowly and four to eight months after the infection, it arrives below the detection threshold by PCR.
The PCR test does not allow the difference between vaccinated animals and those who have been sick.
For infected animals, antibodies are detectable in the blood ten to fourteen days after the infection and their persistence probably lasts for several months, even several years.
A study is underway at the Toulouse veterinary school on the detection of MHE antibodies in tank milk. This could allow national surveillance of the virus circulation. This method would be fast, precise and inexpensive.
Ulcers in the mouth and digestive tract
When clinical signs are present, the MHE is characterized by fever beyond 40 ° C with abatement, anorexia and the absence of rumination. During acute shapes, we especially observe signs at the head of the animal. The muffle can crack with ulcers and necrosis (skin that is a hit at the level of the muffle and desquame, leaving the mucous membrane). Many ulcers in the mouth and digestive system are found. “The most frequent is on the upper gum. You can also find ulcers in the pharynx, the esophagus and even in the pre-estomacs. »» These ulcers prevent the cattle from drinking and eating normally and can last up to three weeks. Hence strong dehydration and potentially renal failure.
We can also find signs on the skin of trees (congestion, ulceration) and sometimes locomotor signs (steep or hesitant approach), lesions of the feet ranging from simple inflammation and congestion to, quite frequently, ulcers in the interdigital space.
