Common Pigweed (Amaranthus retroflexus) is an insect and wind-pollinated plant that grows 20 to 150 cm in height. It is commonly known as an allergenic weed worldwide due to its capability to produce enormous number of pollen. This plant is abundantly found in pasturelands, farming fields, and grounds. The pollination period of this plant starts in August and lasts until October. It is extensively found in countries like Saudi Arabia, India, Iran, the US, Australia, etc. Pigweed pollen sensitivity may vary across geographical regions, depending on the tested population's diversity. It is considered one of the primary risk factors responsible for inducing respiratory allergies (like allergic rhinitis, allergic rhino-conjunctivitis, and asthma) in sensitive individuals. Pigweed pollen allergens Ama r 1 and Ama r 2 have been identified and listed in the IUIS database. Additionally, Ama r 3, a polcalcin allergenic protein from this pollen, has also been recognized by a study. Pigweed pollen has exhibited cross-reactivity with pollen from other species like lamb’s quarters, Russian thistle, Mesquite, Needle bush, and Burning bush.

Worldwide distribution

The Amaranthaceae family pollens have been determined as a major cause of pollinosis in countries of the northern hemisphere like Asia, Europe, and the US. Specifically, common pigweed pollen from the Amaranthus genus is considered a major cause of respiratory allergy in the semi-desert regions around the world (8).

In an aeroallergen study conducted on 299 allergic rhinitis patients in Ahvaz, Iran, weed pollen was the most prevalent (89%) outdoor allergen. Among the weed allergens, 70.9% of the patients showed positive skin prick test (SPT) and total IgE to pigweed allergen (9).

A US-based study conducted on 562 military children (≤18 years) with rhinitis underwent SPT with an 8-test screening panel (various allergen mixes) (n=209) or a 51-aeroallergen standard panel (n=345). 8 patients were excluded from the study based on positive diluent or negative histamine control. 80.3% (277/345) of the patients showed positive SPT towards at least 1 aeroallergen. 27% of the 345 patients exhibited positive SPT towards pigweed allergen. (10).

In a retrospective review conducted in Northern Mexico on 247 allergic patients to determine the prevalence of hypersensitivity to common allergens, 5.4% of the individuals were found sensitive to pigweed pollen (11).

According to an aeroallergen sensitization study conducted in Thailand on 100 patients with allergic rhinitis, 16% of them showed positive SPT to pigweed allergen (12).

As per a study in Iran, 68.8 % of the total participating patients (n=306) and 85.6% of the atopic patients suffering from allergic rhinitis showed positive SPT and total IgE towards pigweed allergen in a study conducted in Iran (13).

Worldwide distribution

Common pigweed pollen is prevalent worldwide in various climatic conditions. It is one of the most allergenic weeds and considered a major risk factor for the development of respiratory allergy in temperate as well as dry areas such as Saudi Arabia, India, Iran, Kuwait and western parts of the United States, and the tropical regions of Africa, Australia, and the Mediterranean countries (2, 5, 6). It is also found in Pakistan, China, Russia, and Canada (4).

Main

The main route of exposure for common pigweed pollen is the airway (9, 13).

Aeroallergens such as pollens are the primary reason for the development of allergic rhinitis. Allergic reaction induced by allergenic pollen such as pigweed pollen is characterized by sneezing, itching, runny nose, and nasal congestion. Allergic rhinitis, as well as asthma, are considered the most common allergic conditions. They are regarded as a significant public health issue throughout the world. They also pose a significant burden on the patients' quality of life and further leads to increase in cost of healthcare treatment (13, 14).

Allergic rhinitis (AR) and allergic rhino-conjunctivitis (ARC)

In an Iranian study conducted on 306 AR patients to determine the prevalence of various aeroallergens, 74% (226/306) of these patients were also suffering from ARC. The study revealed weed species as the most prevalent category of aeroallergen. As per the survey, positive SPT to pigweed was seen among 85.6% of the atopic individuals with AR (13).

Asthma

A prospective study was conducted on 189 patients who have asthma, AR, and ARC in Venezuela. 83.6% had moderate severity among 49 of the patients who have asthma. The results showed serum IgE antibodies to common pigweed in 8.6% of the patients' sera (14).

Another cross-sectional study was conducted on 1700 schoolchildren in Saudi Arabia. 27.5% (n=468) children among them were diagnosed with bronchial asthma and 20.8% (n=353) had 12-month nocturnal cough (severe asthma). Among these, 3.2% with bronchial asthma and 2% with severe asthma had sensitization to common pigweed allergen (15).

Allergen immunotherapy

In a study conducted on 51 patients with ARC, sublingual immunotherapy containing pigweed showed significant improvement (p< 0.05) while controlling allergic symptoms (17).

Prevention strategies

Avoidance

Avoiding exposure to a particular allergen can be the first step towards allergic rhinitis management (13).

One of the main measures that can be taken is to cut down the pollen amounts in the respiratory system. During pollen season, allergic patients are suggested to stay indoors or wear masks outdoors to keep away from pollen allergens. In worsening conditions, patients may be recommended to move or live in a non-pollen area (16).

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Christian Fischer

Last reviewed: January 2021