Earlier this year, the first case of avian influenza in US dairy cattle had been reported. The virus spread through milk and contaminated tools from cow-to-cow. Dr Stefan Steyn, technical and regulatory veterinarian for Afrivet Technical and Training Services, breaks this down in detail.
As South Africa’s temperatures decrease and the flu season is underway, the Northern Hemisphere is starting its journey closer to the sun and hoping its season is ending. Unfortunately, the United States is still battling its own flu outbreak, but this time it is bird flu in their dairy cows!
Avian influenza (bird flu) is an influenza A virus that mainly infects birds and can be divided into two types based on the extent of damage caused to the bird. The first type, low pathogenicity avian influenza (LPAI), causes few clinical signs and is mostly limited to replicating in the digestive tract.
The second pathotype, highly pathogenic AI (HPAI), has the potential to replicate in a larger variety of tissues than those of the respiratory and gastrointestinal tracts, at a higher rate, making birds sicker.
Populations of the virus circulate in wild aquatic birds and spread globally through various bird flyways. Once domestic poultry is infected, the virus propagates and can potentially mutate from an LPAI to an HPAI or reassort with other Influenza A viruses and gain genes needed to replicate in a mammal.
Humans test positive
On 30 March 2024, the USA reported the first avian influenza case in dairy cattle to the World Organisation for Animal Health (WOAH, previously OIE). HPAI H5N1 (Guangdong clade 2.3.4.4b) was confirmed in dairy herds in Kansas and Texas. The origin of the virus was linked to an H5N1 virus isolated from wild birds in November 2023 (WAHIS, 2024).
The WOAH raised concerns in late April due to the increased incidence of H5N1 in mammals over the past few years. H5N1 has caused sporadic infections in humans since being detected in waterfowl in 1996 in China (Abassi, 2023). Up to the beginning of June, there have only been three confirmed human cases.
The first was a dairy worker who contracted viral conjunctivitis whilst working in close contact with infected cows in the milking shed and tested positive for this specific H5N1 strain at the beginning of April. Currently, nine US states have reported outbreaks and 81 affected herds.
The Centre for Disease Control (CDC) has been monitoring 390 people for disease who were in contact with infected herds. Of these, only 44 people showed clinical signs, of which only three (mentioned earlier) tested positive for the virus.
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The virus in cows
Currently, the infection seems to spread between herds (cow-to-cow), with peak virus concentrations in milk. Cows can be infected by being in contact with milk directly or in contact with inanimate objects contaminated by milk (boots, milking machines, hands, etc.). Any secretion from an infected animal has the potential to spread the virus.
Infection incidence has been linked epidemiologically between some dairy and poultry outbreaks. Due to socio-economic factors, the spread between humans is difficult to confirm. Aerosolised milk droplets can spread the virus for very short distances like a cough or a sneeze, but the virus is not airborne.
The virus replicates in the udder, and infected herds show a sudden and severe drop in overall milk production, with the individual’s milk becoming thicker and more concentrated, resembling colostrum. Cattle can show typical disease signs like a drop in water and feed intake, decreased rumen motility, lethargy, fever, and dehydration.
The faeces can be tacky and loose. Infection is commonly seen in older lactating cows, and recovery is usually within 7-10 days. Very few deaths have been reported and these seem to be the exception rather than the rule. Pasteurisation effectively kills the virus. The CDC and World Health Organisation (WHO) strongly recommend not consuming raw milk, and the thorough cooking of meat is described on their websites.
Safety measures
At the date of writing this article, the CDC is confident that the possibility of this virus infecting more humans is low. However, together with the WHO, the WOAH, and the United States Department of Agriculture (USDA), they have advised dairy workers to wear protective clothing whilst interacting with sick cows, as well as an increase in biosecurity on farms.
The latter includes increased hand sanitising and boot disinfection when entering and exiting the milking shed with an approved long-acting disinfection agent. The USDA is enforcing movement controls on lactating animals between states pending a negative PCR result. They advise isolating sick animals and keeping newly acquired animals in quarantine for 30 days before introducing them into herds (USDA, 2024).
The infection in dairy herds is an abnormality that still warrants close monitoring due to the rapid mutation of the virus, however, the probability of it reaching our shores is much less than the other challenges our dairy farmers face every day.
For more information, please visit the WOAH website and follow the links to the respective organisations’ sites.
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