Study shows cows harbor both human and avian flu receptors in their mammary glands

By Pooja Toshniwal PahariaReviewed by Susha Cheriyedath, M.Sc.May 8 2024

In a recent study posted to the bioRxiv preprint* server, researchers used lectin histochemistry to evaluate the in situ expression of influenza A virus (IAV) receptors in the bovine respiratory system, cerebrum, and mammary glands.

Study: The avian and human influenza A virus receptors sialic acid (SA)-α2,3 and SA-α2,6 are widely expressed in the bovine mammary gland. Image Credit: Arie van der Wulp / Shutterstock

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

Background

A highly pathogenic IAV (HPIAV) epidemic has been discovered in the United States (US) dairy cows, a relatively unknown domestic IAV host species. IAV is a negative-sense, single-stranded ribonucleic acid (RNA) virus that has not caused widespread infection in cattle. Cattle are primarily immune to IAV but vulnerable to influenza C and D viruses. 

However, the highly pathogenic avian influenza virus (HPAIV) H5N1 was discovered in Texas and swiftly spread to over 40 herds across eight US states. Clinical indicators include an abrupt decrease in milk production, mastitis, moderate respiratory signs, and neurological symptoms absent in other mammals infected with 2.3.4.4b viruses.

About the study

In the present study, researchers examined the IAV receptor expression in situ in cattle's mammary glands, respiratory tract, and cerebrum, considered less vulnerable to IAV infections.

The researchers obtained specimens of the trachea and lung from two 2.0-month-old beef calves, one of which had severe suppurative tracheitis. They collected the tissues during regular necropsies from various clinical patients at the Veterinary Pathology division of Copenhagen University of Denmark. They obtained two disease-free mammary glands from a nursing dairy cow (four years old). They sourced cerebral specimens from a five-month-old beef calf and a 2.5-year-old dairy cow.

The tissues obtained were fixed with formalin, embedded in paraffin, and sliced into 4.0 to 5.0 µm slices. Researchers evaluated the existence of IAV receptors in cow lungs, cerebrum, and mammary glands. The team detected the human receptor [sialic acid (SA)-α2,6)] using Sambucus Nigra Lectin (SNA), the chicken receptor (SA-α2,3-Gal-β1,4) with Maackia amurensis Lectin I (MAA-I), and the duck receptor (SA-α2,3-Gal-β1,3) with Maackia amurensis Lectin II (MAA-II).

The researchers studied the trachea, bronchi, bronchioles, alveoli, and cerebellar cortex of beef calves and dairy cows and the mammary glandular alveoli and ducts. They performed lectin histochemistry to identify biotinylated MAA-2 (B-1265-1), SA-α2,6, and SA-α2,3. They evaluated the staining intensity of receptors based on their presence in the cells as no staining was seen, 1.0: present in less than 50% of cells examined, and 2.0: present in more than 50% of cells.

Results

Researchers observed that IAV receptors are expressed to varying degrees in beef and dairy cattle's mammary glands, respiratory system, and cerebrum. They observed a high expression of duck and human IAV receptors in the bovine mammary gland, but the chicken receptor dominated the respiratory tract. IAV receptors were modestly abundant in cerebrum neurons.

Human and duck receptors were broadly dispersed in the mammary gland alveolar cells but not the ducts. In the pulmonary system, the researchers identified all receptors in tracheal goblet cells; however, there were fewer human IAV receptor-expressing cells. The chicken IAV receptor was exclusively found on ciliated-type epithelial cellular surfaces in the calf without tracheal irritation. The researchers found only duck and chicken receptors within the beef calf. Endothelial cells and bovine erythrocytes showed positively stained chicken and human receptors.

The chicken IAV receptor was expressed on the respiratory epithelium in the upper respiratory tract and upper portion of the lower respiratory tract, while the human and duck receptors were not or were just partially expressed. The finding contradicts data from individuals and pigs, indicating that bovines are highly resistant to the IAV when exposed via the respiratory route.

Human and duck receptors were abundant in mammary glands, which lacked chicken receptors. This indicates that the elevated viral load in milk obtained from HPAI H5N1-infected cows was due to viral replication in local sites. The avian IAV receptor is also abundant in human conjunctival and corneal cells, which is why conjunctivitis is the most common clinical manifestation in an individual infected with Texas dairy cows.  The lack of duck, chicken, or human receptors in the mammary glandular ducts makes ascending infections in mammary glands more challenging to treat. The researchers noted limited receptor expression in cattle cerebrums, indicating that the highly pathogenic IAV-infected cows had no neurological symptoms.

The study showed that high quantities of the H5N1 virus in infected bovine milk are connected to cattle, indicating them as a potential source of new IAVs. The H5N1 transmission mechanism and pathophysiology in cows are unknown, as are the levels of viremia in HPAIV-infected cows. The study also sheds light on IAV host receptors in cattle, with duck receptor expression in mammary glands consistent with widespread infections in the United States. Human and avian receptor presence in the mammary glands indicates sensitivity to swine, human, and avian-origin viruses. Further research is required to improve understanding of the pathology and epidemiology of IAV infections in cattle and other ruminants.

1/ New #H5N1 pre-print ⬆️ the threat level for the ongoing outbreak in dairy cows.

Researchers found that cow mammary glands contain the same kind of mixed flu receptors seen in pigs.

This mix of receptors is why scientists call pigs "evolutionary labs for flu host switching". pic.twitter.com/N1dBKEn2W0

— Sam Scarpino (@svscarpino) May 6, 2024

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources